US20210151569A1 - Composite oxide semiconductor and method for manufacturing the same - Google Patents

Composite oxide semiconductor and method for manufacturing the same Download PDF

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Publication number
US20210151569A1
US20210151569A1 US17/136,230 US202017136230A US2021151569A1 US 20210151569 A1 US20210151569 A1 US 20210151569A1 US 202017136230 A US202017136230 A US 202017136230A US 2021151569 A1 US2021151569 A1 US 2021151569A1
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Prior art keywords
oxide semiconductor
film
transistor
region
insulating film
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US17/136,230
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Shunpei Yamazaki
Yasutaka NAKAZAWA
Masashi Oota
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Semiconductor Energy Laboratory Co Ltd
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Semiconductor Energy Laboratory Co Ltd
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Priority to US17/136,230 priority Critical patent/US20210151569A1/en
Publication of US20210151569A1 publication Critical patent/US20210151569A1/en
Priority to US18/213,311 priority patent/US20230335598A1/en
Abandoned legal-status Critical Current

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Definitions

  • One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device.
  • one embodiment of the present invention is not limited to the above technical field.
  • the technical field of one embodiment of the invention disclosed in this specification and the like relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof.
  • a semiconductor device generally means a device that can function by utilizing semiconductor characteristics.
  • a semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each one embodiment of a semiconductor device.
  • An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.
  • transistors are used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device).
  • IC integrated circuit
  • display device display device
  • a semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor.
  • oxide semiconductor has been attracting attention.
  • a semiconductor device achieving high field-effect mobility (simply referred to as mobility or ⁇ FE in some cases) with such a structure that a plurality of oxide semiconductor layers are stacked, the oxide semiconductor layer serving as a channel in the plurality of oxide semiconductor layers contains indium and gallium, and the proportion of indium is higher than the proportion of gallium (see Patent Document 1).
  • the field-effect mobility of a transistor that uses an oxide semiconductor film as a channel region is preferably as high as possible.
  • the transistor has a problem with its characteristics, that is, the transistor tends to be normally on. Note that “normally on” means a state where a channel exists without application of a voltage to a gate electrode and current flows through the transistor.
  • oxygen vacancies which are formed in the oxide semiconductor film adversely affect the transistor characteristics.
  • oxygen vacancies formed in the oxide semiconductor film are bonded with hydrogen to serve as carrier supply sources.
  • the carrier supply sources generated in the oxide semiconductor film cause a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor including the oxide semiconductor film.
  • the amount of oxygen vacancies in the oxide semiconductor film is too large, for example, the threshold voltage of the transistor is shifted in the negative direction, and the transistor has normally-on characteristics.
  • the amount of oxygen vacancies is preferably small or the amount with which the normally-on characteristics are not exhibited.
  • an object of one embodiment of the present invention is to improve field-effect mobility and reliability of a transistor including an oxide semiconductor film. Another object of one embodiment of the present invention is to prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and to improve the reliability of the transistor. Another object of one embodiment of the present invention is to provide a semiconductor device with reduced power consumption. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device.
  • One embodiment of the present invention is a composite oxide semiconductor in which a first region and a second region are mixed.
  • the first region includes a plurality of first clusters containing one or more of indium (In), zinc (Zn), and oxygen (O) as a main component.
  • the second region includes a plurality of second clusters containing one or more of indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen as a main component.
  • the first region includes a portion in which the plurality of first clusters are connected to each other, and the second region includes a portion in which the plurality of second clusters are connected to each other.
  • the first region preferably exists being surrounded by the second region.
  • the first clusters preferably have higher conductivity than the second clusters, and the second clusters preferably have a higher semiconductor property than the first clusters.
  • the composite oxide semiconductor preferably includes a portion in which each of the first clusters has a size of greater than or equal to 0.5 nm and less than or equal to 1.5 nm.
  • One embodiment of the present invention is a semiconductor device including a semiconductor layer, a gate, and a gate insulating layer.
  • the semiconductor layer includes the composite oxide semiconductor described in any of the above embodiments.
  • a maximum value of field-effect mobility be greater than or equal to 100 cm 2 /Vs and less than or equal to 200 cm 2 /Vs in a range in which a gate voltage is higher than 0 V and lower than or equal to 10 V and a drain voltage is higher than 0 V and lower than or equal to 20 V.
  • Another embodiment of the present invention is a method for manufacturing a composite oxide semiconductor, which includes a first step of placing a substrate in a deposition chamber; a second step of introducing one or both of an argon gas and an oxygen gas into the deposition chamber; a third step of applying voltage to a target containing indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen; and a fourth step of depositing the composite oxide semiconductor from the target onto the substrate.
  • the fourth step includes a first step in which the element M and the zinc are preferentially sputtered from the target; and a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target.
  • Another embodiment of the present invention is a method for manufacturing a composite oxide semiconductor, which includes a first step of placing a substrate in a deposition chamber; a second step of introducing, into the deposition chamber, a deposition gas containing an argon gas and not containing an oxygen gas; a third step of applying voltage to a target containing indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen; and a fourth step of depositing the composite oxide semiconductor from the target onto the substrate.
  • the fourth step includes a first step in which the element M and the zinc are preferentially sputtered from the target; and a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target.
  • the substrate not be heated intentionally.
  • Another embodiment of the present invention is a display device which includes the semiconductor device of any one of the above-described embodiments, and a display element.
  • Another embodiment of the present invention is a display module which includes the display device and a touch sensor.
  • Another embodiment of the present invention is an electronic device which includes the semiconductor device of any one of the above-described embodiments, the above-described display device, or the above-described display module; and an operation key or a battery.
  • One embodiment of the present invention can improve field-effect mobility and reliability of a transistor including an oxide semiconductor film.
  • One embodiment of the present invention can prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and improve the reliability of the transistor.
  • One embodiment of the present invention can provide a semiconductor device with reduced power consumption.
  • One embodiment of the present invention can provide a novel semiconductor device.
  • One embodiment of the present invention can provide a novel display device.
  • FIGS. 1A and 1B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIG. 2 illustrates atomic ratios of an oxide semiconductor.
  • FIGS. 3A and 3B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 4A and 4B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 5A and 5B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 6A and 6B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 7A and 7B illustrate a sputtering apparatus.
  • FIG. 8 is a process flow chart illustrating a method for manufacturing a composite oxide semiconductor.
  • FIGS. 9A to 9C illustrate a cross section of the vicinity of a target.
  • FIG. 10 illustrates an analysis method
  • FIG. 11 shows SEM observation and EDX mapping images.
  • FIG. 12 shows SEM observation and EDX mapping images.
  • FIGS. 13A and 13B show compositions of a sample.
  • FIG. 14 shows compositions of a sample.
  • FIG. 15 shows SEM observation and EDX mapping images.
  • FIG. 16 shows SEM observation and EDX mapping images.
  • FIG. 17 shows SEM observation and EDX mapping images.
  • FIG. 18 shows SEM observation and EDX mapping images.
  • FIG. 19 shows SEM observation and EDX mapping images.
  • FIG. 20 shows SEM observation and EDX mapping images.
  • FIGS. 21A to 21C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIG. 22 is a cross-sectional view illustrating a semiconductor device.
  • FIGS. 23A and 23B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 24A and 24B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 25A and 25B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 26A and 26B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 27A and 27B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 28A and 28B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 29A and 29B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 30A and 30B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 31A to 31C each illustrate a band structure.
  • FIGS. 32A to 32C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 33A to 33C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 34A to 34C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 35A to 35C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 36A and 36B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 37A and 37B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 38A to 38C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIG. 39 is a top view illustrating one mode of a display device.
  • FIG. 40 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 41 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 42 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 43 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 44 is a cross-sectional view illustrating one mode of a display device.
  • FIGS. 45A to 45D are cross-sectional views illustrating a method for forming an EL layer.
  • FIG. 46 is a conceptual diagram illustrating a droplet discharge apparatus.
  • FIGS. 47A to 47C are a block diagram and circuit diagrams illustrating a display device.
  • FIG. 48 illustrates a display module
  • FIGS. 49A to 49E illustrate electronic devices.
  • FIGS. 50A to 50G illustrate electronic devices.
  • FIGS. 51A and 51B are perspective views illustrating a display device.
  • FIGS. 52A to 52E are a HAADF-STEM image and EDX mapping images of a plane.
  • FIGS. 53A to 53E are a HAADF-STEM image and EDX mapping images of a cross section.
  • FIG. 54 shows XRD analysis results.
  • FIG. 55 shows I d -V g characteristics of a transistor.
  • FIG. 56 shows I d -V g characteristics of a transistor.
  • FIG. 57 is a cross-sectional STEM image of a transistor.
  • a transistor is an element having at least three terminals of a gate, a drain, and a source.
  • the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel region.
  • a drain a drain terminal, a drain region, or a drain electrode
  • a source a source terminal, a source region, or a source electrode
  • source and drain functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.
  • the expression “electrically connected” includes the case where components are connected through an “object having any electric function”.
  • an “object having any electric function” there is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object.
  • Examples of an “object having any electric function” include a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.
  • parallel means that the angle formed between two straight lines is greater than or equal to ⁇ 10° and less than or equal to 10°, and accordingly also covers the case where the angle is greater than or equal to ⁇ 5° and less than or equal to 5°.
  • perpendicular means that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also covers the case where the angle is greater than or equal to 85° and less than or equal to 95°.
  • the terms “film” and “layer” can be interchanged with each other.
  • the term “conductive layer” can be changed into the term “conductive film” in some cases.
  • the term “insulating film” can be changed into the term “insulating layer” in some cases.
  • the off-state current in this specification and the like refers to a drain current of a transistor in an off state (also referred to as non-conduction state and cutoff state).
  • the off state of an n-channel transistor means that a voltage (V gs ) between its gate and source is lower than the threshold voltage (Vth), and the off state of a p-channel transistor means that the gate-source voltage V gs is higher than the threshold voltage Vth.
  • the off-state current of an n-channel transistor sometimes refers to a drain current that flows when the gate-source voltage V gs is lower than the threshold voltage Vth.
  • the off-state current of a transistor depends on V gs in some cases.
  • the off-state current of a transistor is lower than or equal to I may mean “there is V gs with which the off-state current of the transistor becomes lower than or equal to I”.
  • the off-state current of a transistor means “the off-state current in an off state at predetermined V gs ”, “the off-state current in an off state at V gs in a predetermined range”, “the off-state current in an off state at V gs with which sufficiently reduced off-state current is obtained”, or the like.
  • the assumption is made of an n-channel transistor where the threshold voltage Vth is 0.5 V and the drain current is 1 ⁇ 10 ⁇ 9 A at V gs of 0.5 V, 1 ⁇ 10 ⁇ 13 A at V gs of 0.1 V, 1 ⁇ 10 ⁇ 19 A at V gs of ⁇ 0.5 V, and 1 ⁇ 10 ⁇ 22 A at V gs of ⁇ 0.8 V.
  • the drain current of the transistor is 1 ⁇ 10 ⁇ 19 A or lower at V gs of ⁇ 0.5 V or at V gs in the range of ⁇ 0.8 V to ⁇ 0.5 V; therefore, it can be said that the off-state current of the transistor is 1 ⁇ 10 ⁇ 19 A or lower. Since there is V gs at which the drain current of the transistor is 1 ⁇ 10 ⁇ 22 A or lower, it may be said that the off-state current of the transistor is 1 ⁇ 10 ⁇ 22 A or lower.
  • the off-state current of a transistor with a channel width W is sometimes represented by a current value in relation to the channel width W or by a current value per given channel width (e.g., 1 ⁇ m). In the latter case, the off-state current may be expressed in the unit with the dimension of current per length (e.g., A/ ⁇ m).
  • the off-state current of a transistor depends on temperature in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at room temperature, 60° C., 85° C., 95° C., or 125° C. Alternatively, the off-state current may be an off-state current at a temperature at which the required reliability of a semiconductor device or the like including the transistor is ensured or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.).
  • an off-state current of a transistor is lower than or equal to I may refer to a situation where there is V gs at which the off-state current of a transistor is lower than or equal to I at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature at which the required reliability of a semiconductor device or the like including the transistor is ensured, or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.).
  • the off-state current of a transistor depends on voltage V ds between its drain and source in some cases.
  • the off-state current in this specification may be an off-state current at V ds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V.
  • the off-state current may be an off-state current at V ds at which the required reliability of a semiconductor device or the like including the transistor is ensured or V ds at which the semiconductor device or the like including the transistor is used.
  • an off-state current of a transistor is lower than or equal to I may refer to a situation where there is V gs at which the off-state current of a transistor is lower than or equal to I at V ds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, V ds at which the required reliability of a semiconductor device or the like including the transistor is ensured, or V ds at which the semiconductor device or the like including the transistor is used.
  • off-state current a drain may be replaced with a source. That is, the off-state current sometimes refers to current that flows through a source of a transistor in an off state.
  • the term “leakage current” sometimes expresses the same meaning as off-state current.
  • the off-state current sometimes refers to current that flows between a source and a drain when a transistor is off, for example.
  • the threshold voltage of a transistor refers to a gate voltage (V g ) at which a channel is formed in the transistor.
  • the threshold voltage of a transistor may refer to a gate voltage (V g ) at which the value of I d [A] ⁇ L [ ⁇ m]/W [ ⁇ m] is 1 ⁇ 10 ⁇ 9 [A] where L is channel length and W is channel width.
  • a “semiconductor” can have characteristics of an “insulator” when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called an “insulator” in some cases. Similarly, an “insulator” in this specification and the like can be called a “semiconductor” in some cases. An “insulator” in this specification and the like can be called a “semi-insulator” in some cases.
  • a “semiconductor” can have characteristics of a “conductor” when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called a “conductor” in some cases. Similarly, a “conductor” in this specification and the like can be called a “semiconductor” in some cases.
  • an impurity in a semiconductor refers to an element that is not a main component of the semiconductor.
  • an element with a concentration of lower than 0.1 atomic % is an impurity.
  • the semiconductor contains an impurity, the density of states (DOS) may be formed therein, the carrier mobility may be decreased, or the crystallinity may be decreased, for example.
  • the impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specific examples include hydrogen (also included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen.
  • oxygen vacancies may be formed by entry of impurities such as hydrogen, for example.
  • impurities such as hydrogen, for example.
  • examples of the impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.
  • an oxide semiconductor film that includes a composite oxide semiconductor of one embodiment of the present invention and a semiconductor device that includes this oxide semiconductor film are described.
  • Indium is preferably contained in an oxide semiconductor film.
  • indium and zinc are preferably contained.
  • aluminum, gallium, yttrium, tin, or the like is preferably contained.
  • one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained.
  • an oxide semiconductor film contains indium, an element M, and zinc
  • the element M is aluminum, gallium, yttrium, tin, or the like.
  • Other elements that can be used as the element M include boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium. Note that two or more of the above elements can be used in combination as the element M.
  • the terms of the atomic ratio of indium, the element M, and zinc contained in the oxide semiconductor film are denoted by [In], [M], and [Zn], respectively.
  • FIGS. 1A and 1B are schematic views of an oxide semiconductor film that includes a composite oxide semiconductor of one embodiment of the present invention.
  • FIG. 1A is a schematic view of a top surface of an oxide semiconductor film (a-b plane direction), and FIG. 1B is a schematic view of a cross section of the oxide semiconductor film (c-axis direction) formed over a substrate Sub.
  • FIGS. 1A and 1B illustrate an example in which the oxide semiconductor film is formed over the substrate; however, one embodiment of the present invention is not limited to this example and an insulating film such as a base film or an interlayer film or another semiconductor film such as an oxide semiconductor film may be formed between the substrate and the oxide semiconductor film.
  • an insulating film such as a base film or an interlayer film or another semiconductor film such as an oxide semiconductor film may be formed between the substrate and the oxide semiconductor film.
  • the oxide semiconductor film of one embodiment of the present invention is a composite oxide semiconductor having a structure in which Region A 1 and Region B 1 are mixed as shown in FIGS. 1A and 1B . Therefore, in the following description, the oxide semiconductor film is referred to as a composite oxide semiconductor in some cases.
  • Region A 1 when the atomic ratio of In to the element M in Region A 1 is greater than the atomic ratio of In to the element M in Region B 1 , Region A 1 has higher In concentration than Region B 1 . Therefore, in this specification, Region A 1 is also referred to as an In-rich region, and Region B 1 is also referred to as an In-poor region.
  • the In concentration in Region A 1 is 1.1 or more times, preferably 2 to 10 times that in Region B 1 .
  • Region A 1 is an oxide containing at least In and does not necessarily contain the element M and Zn.
  • a phase diagram in FIG. 2 can be used to show the atomic ratio of elements in the case where Region A 1 in the composite oxide semiconductor contains In, the element M, and Zn.
  • the atomic ratio of In to the element M and Zn is denoted by x:y:z. This atomic ratio can be shown as coordinates (x:y:z) in FIG. 2 . Note that the proportion of oxygen atoms is not illustrated in FIG. 2 .
  • Region A 2 in FIG. 2 represents an example of a preferred range of atomic ratios of indium to the element M and zinc contained in Region A 1 .
  • Region B 2 in FIG. 2 represents an example of a preferred range of atomic ratios of indium to the element M and zinc contained in Region B 1 .
  • Region A 2 with high In concentrations provides higher conductivity than Region B 2 and has a function of increasing carrier mobility of the oxide semiconductor film (or field-effect mobility of a transistor). Therefore, the on-state current and carrier mobility of a transistor using an oxide semiconductor film including Region A 1 can be increased.
  • Region B 2 with low In concentrations provides lower conductivity than Region A 2 and has a function of decreasing leakage current of the oxide semiconductor film or a transistor. Therefore, the off-state current of a transistor using an oxide semiconductor film including Region B 1 can be decreased.
  • Region A 1 and Region B 1 form a composite. That is, carrier movement occurs easily in Region A 1 , whereas carrier movement does not occur easily in Region B 1 . Therefore, an oxide semiconductor of one embodiment of the present invention can be used as a material with high carrier mobility, excellent switching characteristics, and favorable semiconductor characteristics.
  • Region A 1 has a lower semiconductor property and higher conductivity than Region B 1 .
  • Region B 1 has a higher semiconductor property and lower conductivity than Region A 1 .
  • a high semiconductor property means a wide band gap, favorable switching characteristics, being close to an i-type semiconductor, or the like.
  • a plurality of Regions A 1 are present in particulate form (in cluster form) in the a-b plane direction and the c-axis direction as shown in FIGS. 1A and 1B .
  • clusters may be distributed unevenly and irregularly.
  • a plurality of clusters overlap with each other or are connected to each other in some cases.
  • shapes each including a cluster overlapping with another cluster are connected to each other, so that Region A 1 is observed to extend in a cloud-like manner.
  • the clusters included in Region A 1 have a lower semiconductor property and higher conductivity than the clusters included in Region B 1 .
  • the clusters included in Region B 1 have a higher semiconductor property and lower conductivity than the clusters included in Region A 1 .
  • a first region with a high In concentration and a second region with a low In concentration are connected in a cloud-like manner.
  • the first region where In is distributed at a high concentration and the second region where In is not distributed at a high concentration are connected in a cloud-like manner.
  • Regions A 1 are connected to each other in the a-b plane direction, so that Regions A 1 can serve as a current path. Accordingly, the oxide semiconductor film can have increased conductivity and a transistor using this oxide semiconductor film can have increased field-effect mobility.
  • Regions B 1 shown in FIGS. 1A and 1B are scattered in Regions A 1 . Therefore, Region B 1 can exist in a state of being sandwiched three-dimensionally by Regions A 1 . In other words, Region B 1 can exist in a state of being surrounded by Region A 1 . That is, Region B 1 is enclosed by Region A 1 .
  • Region B 1 includes a cluster (also referred to as a second cluster) that is different from a cluster (also referred to as a first cluster) included in Region A 1 .
  • Region B 1 includes a plurality of second clusters and has a portion where the plurality of second clusters are connected to each other.
  • the first cluster included in Region A 1 includes a portion where the first cluster and another first cluster are connected to each other in a cloud-like manner
  • the second cluster included in Region B 1 includes a portion where the second cluster and another second cluster are connected to each other in a cloud-like manner.
  • the proportion of scattered Regions A 1 can be adjusted by changing the formation conditions or composition of the composite oxide semiconductor. For example, it is possible to form a composite oxide semiconductor with a low proportion of Regions A 1 or a composite oxide semiconductor with a high proportion of Regions A 1 . In a composite oxide semiconductor of one embodiment of the present invention, the proportion of Regions A 1 is not always lower than that of Regions B 1 . In a composite oxide semiconductor with an extremely high proportion of Regions A 1 , depending on the observation range, Region B 1 is sometimes formed in Region A 1 . The size of the particulate region of Region A 1 can be appropriately adjusted by changing, for example, the formation conditions or composition of the composite oxide semiconductor.
  • FIGS. 3A and 3B show a composite oxide semiconductor that has a lower proportion of Regions A 1 and a higher proportion of Regions B 1 than the composite oxide semiconductor shown in FIGS. 1A and 1B .
  • FIGS. 4A and 4B show a composite oxide semiconductor that has a lower proportion of Regions A 1 and a higher proportion of Regions B 1 than the composite oxide semiconductor shown in FIGS. 3A and 3B . Because of the low proportion of Regions A 1 , the first clusters scattered without overlapping with each other may be included as shown in FIGS. 4A and 4B .
  • Regions A 1 when all of Regions A 1 are connected in the a-b plane direction, the switching characteristics of a transistor deteriorate in some cases. For example, the off-state current of the transistor might increase. It is thus preferable that as shown in FIGS. 3A and 3B and FIGS. 4A and 4B , Regions A 1 be scattered in Regions B 1 . Therefore, Region A 1 can exist in a state of being sandwiched three-dimensionally by Regions B 1 . In other words, Region A 1 can exist in a state of being surrounded by Region B 1 . That is, Region A 1 is enclosed by Region B 1 . As a result, the switching characteristics of the transistor can be improved. Specifically, the off-state current can be reduced.
  • Region A 1 and Region B 1 are not clearly observed.
  • the sizes of Region A 1 and Region B 1 can be measured with energy dispersive X-ray spectroscopy (EDX) mapping images obtained by EDX.
  • EDX energy dispersive X-ray spectroscopy
  • the diameter of a cluster in Region A 1 is greater than or equal to 0.1 nm and less than or equal to 2.5 nm in the EDX mapping image of a cross-sectional photograph or a plan-view photograph in some cases.
  • the diameter of the cluster is preferably greater than or equal to 0.5 nm and less than or equal to 1.5 nm.
  • an oxide semiconductor of one embodiment of the present invention is a composite oxide semiconductor in which Region A 1 and Region B 1 are mixed and have different functions that are complementary.
  • an oxide semiconductor of one embodiment of the present invention is an In—Ga—Zn oxide (hereinafter referred to as IGZO), in which Ga is used as the element M
  • IGZO In—Ga—Zn oxide
  • C/IGZO complementary IGZO
  • Region A 1 and Region B 1 are stacked in a layered manner, for example, interaction does not take place or is unlikely to take place between Region A 1 and Region B 1 , so that the function of Region A 1 and that of Region B 1 are independently performed in some cases. In that case, even when the carrier mobility is increased owing to Regions A 1 , the off-state current of the transistor might be increased. Therefore, in the case where the above-described composite oxide semiconductor or C/IGZO is used, a function of achieving high carrier mobility and a function of achieving excellent switching characteristics can be obtained at the same time. This is an advantageous effect obtained by using a composite oxide semiconductor of one embodiment of the present invention.
  • the oxide semiconductor is deposited with a sputtering apparatus
  • a film having an atomic ratio deviated from the atomic ratio of the target is formed.
  • [Zn] in the atomic ratio of a deposited film is smaller than that in the atomic ratio of the target in some cases depending on the substrate temperature during deposition.
  • the characteristics of the composite oxide semiconductor of one embodiment of the present invention are not uniquely determined by the atomic ratio. Therefore, the illustrated regions represent preferred atomic ratios of Region A 1 and Region B 1 of the composite oxide semiconductor; a boundary therebetween is not clear.
  • Region B 1 may have crystallinity.
  • Region B 1 preferably includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) described later.
  • CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that the distortion in the CAAC-OS is a portion where the direction of a lattice arrangement changes between a region with a regular lattice arrangement and another region with a regular lattice arrangement.
  • a plurality of nanocrystals that are included in Region B 1 are schematically shown by dashed lines.
  • the shape of the nanocrystal is basically hexagon. However, the shape is not always a regular hexagon and is a non-regular hexagon in some cases.
  • a polygonal nanocrystal such as a pentagonal nanocrystal or a heptagonal nanocrystal is included in some cases.
  • FIG. 5B schematically shows that nanocrystals have c-axis alignment and the c-axes are aligned in a direction substantially perpendicular to a surface over which the CAAC-OS film is formed (also referred to as a formation surface) or the top surface of the CAAC-OS film.
  • the CAAC-OS has a layered crystal structure (also referred to as a layered structure) having c-axis alignment and includes a layer containing indium and oxygen (hereinafter referred to as an In layer) and a layer containing the element M, zinc, and oxygen (hereinafter referred to as an (M,Zn) layer) that are stacked.
  • the layer can also be referred to as an (In,M,Zn) layer. In that case, the In layer and the (In,M,Zn) layer are stacked in the layered structure.
  • FIGS. 6A and 6B show an example in which the proportion of Regions A 1 is low and the proportion of Regions B 1 is high as compared to those in FIGS. 5A and 5B .
  • Oxide semiconductors have various structures and various properties.
  • An oxide semiconductor of one embodiment of the present invention may be a composite oxide semiconductor including two or more of the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, an a-like OS described later, an nc-OS described later, and a CAAC-OS described later. Region A 1 and Region B 1 may have different crystallinities.
  • Region B 1 is preferably non-single-crystal.
  • Region B 1 preferably includes a CAAC-OS.
  • Region B 1 does not necessarily include only a CAAC-OS and may also include a region of a polycrystalline oxide semiconductor, an nc-OS, or the like.
  • the CAAC-OS is an oxide semiconductor with high crystallinity.
  • a reduction in electron mobility due to a grain boundary is less likely to occur because a clear grain boundary cannot be observed. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has few impurities and defects (e.g., oxygen vacancies).
  • a composite oxide semiconductor is physically stable; thus, a composite oxide semiconductor which is resistant to heat and has high reliability can be provided.
  • the transistor With the use of the composite oxide semiconductor in a transistor, the transistor can have high carrier mobility and high switching characteristics. In addition, the transistor can have high reliability.
  • An oxide semiconductor film with low carrier density is preferably used for the transistor.
  • an oxide semiconductor film whose carrier density is lower than 8 ⁇ 10 11 /cm 3 , preferably lower than 1 ⁇ 10 11 /cm 3 , or further preferably lower than 1 ⁇ 10 10 /cm 3 , and greater than or equal to 1 ⁇ 10 ⁇ 9 /cm 3 is used as the oxide semiconductor film.
  • the impurity concentration in the oxide semiconductor film is reduced so that the density of defect states can be reduced.
  • a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state.
  • a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density.
  • a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases.
  • the transistor whose channel region is formed in the oxide semiconductor having a high density of trap states has unstable electrical characteristics in some cases.
  • the concentration of impurities in the oxide semiconductor film is effective to reduce the concentration of impurities in the oxide semiconductor film.
  • the concentration of impurities in a film that is adjacent to the oxide semiconductor film is preferably reduced.
  • the impurities hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, and the like are given.
  • the concentration of silicon or carbon in the oxide semiconductor and around an interface with the oxide semiconductor is set lower than or equal to 2 ⁇ 10 18 atoms/cm 3 , and preferably lower than or equal to 2 ⁇ 10 17 atoms/cm 3 .
  • the oxide semiconductor film contains alkali metal or alkaline earth metal
  • defect states are formed and carriers are generated, in some cases.
  • a transistor including an oxide semiconductor film which contains alkali metal or alkaline earth metal is likely to be normally on. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film.
  • the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film measured by SIMS is set lower than or equal to 1 ⁇ 10 18 atoms/cm 3 , and preferably lower than or equal to 2 ⁇ 10 16 atoms/cm 3 .
  • the oxide semiconductor film contains nitrogen
  • the oxide semiconductor film easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density.
  • a transistor whose semiconductor includes an oxide semiconductor that contains nitrogen is likely to be normally-on.
  • nitrogen in the oxide semiconductor is preferably reduced as much as possible; the nitrogen concentration measured by SIMS is set, for example, lower than 5 ⁇ 10 19 atoms/cm 3 , preferably lower than or equal to 5 ⁇ 10 18 atoms/cm 3 , further preferably lower than or equal to 1 ⁇ 10 18 atoms/cm 3 , and still further preferably lower than or equal to 5 ⁇ 10 17 atoms/cm 3 .
  • Hydrogen contained in an oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy (V o ), in some cases. Due to entry of hydrogen into the oxygen vacancy (V o ), an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen in the oxide semiconductor be reduced as much as possible.
  • the hydrogen concentration measured by SIMS in the oxide semiconductor is lower than 1 ⁇ 10 20 atoms/cm 3 , preferably lower than 1 ⁇ 10 19 atoms/cm 3 , further preferably lower than 5 ⁇ 10 18 atoms/cm 3 , and still further preferably lower than 1 ⁇ 10 18 atoms/cm 3 .
  • the oxygen vacancies (V o ) in the oxide semiconductor film can be reduced by introduction of oxygen into the oxide semiconductor film. That is, the oxygen vacancies (V o ) in the oxide semiconductor film disappear when the oxygen vacancies (V o ) are filled with oxygen. Accordingly, diffusion of oxygen in the oxide semiconductor film can reduce the oxygen vacancies (V o ) in a transistor and improve the reliability of the transistor.
  • an oxide in which oxygen content is higher than that in the stoichiometric composition is provided in contact with the oxide semiconductor film. That is, in the oxide, a region including oxygen in excess of that in the stoichiometric composition (hereinafter also referred to as an excess oxygen region) is preferably formed.
  • an oxide including an excess oxygen region is provided in a base film, an interlayer film, or the like in the vicinity of the transistor, whereby oxygen vacancies in the transistor are reduced, and the reliability can be improved.
  • the transistor When an oxide semiconductor film with sufficiently reduced impurity concentration is used for a channel formation region in a transistor, the transistor can have stable electrical characteristics.
  • One embodiment of the present invention is a semiconductor device including a transistor.
  • the transistor includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a second insulating film over the oxide semiconductor film, a second gate electrode over the second insulating film, and a third insulating film over the oxide semiconductor film and the second gate electrode.
  • the oxide semiconductor film includes a channel region overlapping with the first or second gate electrode, a source region in contact with the third insulating film, and a drain region in contact with the third insulating film.
  • the first gate electrode and the second gate electrode are electrically connected to each other.
  • the above transistor includes, in terms of its electrical characteristics, a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 10 cm 2 /Vs and smaller than 200 cm 2 /Vs, a second region where the threshold voltage is higher than or equal to ⁇ 1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1 ⁇ 10 ⁇ 12 A/cm 2 .
  • the above transistor includes, in terms of its electrical characteristics, a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 60 cm 2 /Vs and smaller than 200 cm 2 /Vs, a second region where the threshold voltage is higher than or equal to ⁇ 1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1 ⁇ 10 ⁇ 12 A/cm 2 .
  • the semiconductor device of one embodiment of the present invention is a transistor which includes an oxide semiconductor film in its channel region and whose field-effect mobility, threshold voltage, off-state current, and S value are extremely favorable.
  • a semiconductor device can be favorably used as a transistor in a pixel of a liquid crystal display or an organic EL display or a transistor in a driver circuit of a liquid crystal display or an organic EL display, for example.
  • a composite oxide semiconductor of one embodiment of the present invention can be formed with a sputtering apparatus.
  • FIG. 7A is a cross-sectional view of a deposition chamber 2501 of the sputtering apparatus.
  • FIG. 7B is a plan view of a magnet unit 2530 a and a magnet unit 2530 b of the sputtering apparatus.
  • the deposition chamber 2501 illustrated in FIG. 7A includes a target holder 2520 a , a target holder 2520 b , a backing plate 2510 a , a backing plate 2510 b , a target 2502 a , a target 2502 b , a member 2542 , and a substrate holder 2570 .
  • the target 2502 a is placed over the backing plate 2510 a .
  • the backing plate 2510 a is placed over the target holder 2520 a .
  • the magnet unit 2530 a is placed under the target 2502 a with the backing plate 2510 a therebetween.
  • the target 2502 b is placed over the backing plate 2510 b .
  • the backing plate 2510 b is placed over the target holder 2520 b .
  • the magnet unit 2530 b is placed under the target 2502 b with the backing plate 2510 b therebetween.
  • the magnet unit 2530 a includes a magnet 2530 N 1 , a magnet 2530 N 2 , a magnet 2530 S, and a magnet holder 2532 .
  • the magnet 2530 N 1 , the magnet 2530 N 2 , and the magnet 2530 S are placed over the magnet holder 2532 in the magnet unit 2530 a .
  • the magnet 2530 N 1 , the magnet 2530 N 2 , and the magnet 2530 S are spaced.
  • the magnet unit 2530 b has a structure similar to that of the magnet unit 2530 a .
  • the target 2502 a , the backing plate 2510 a , and the target holder 2520 a are separated from the target 2502 b , the backing plate 2510 b , and the target holder 2520 b by the member 2542 .
  • the member 2542 is preferably an insulator.
  • the member 2542 may be a conductor or a semiconductor.
  • the member 2542 may be a conductor or a semiconductor whose surface is covered with an insulator.
  • the target holder 2520 a and the backing plate 2510 a are fixed to each other with a screw (e.g., a bolt) and have the same potential.
  • the target holder 2520 a has a function of supporting the target 2502 a with the backing plate 2510 a positioned therebetween.
  • the target holder 2520 b and the backing plate 2510 b are fixed to each other with a screw (e.g., a bolt) and have the same potential.
  • the target holder 2520 b has a function of supporting the target 2502 b with the backing plate 2510 b positioned therebetween.
  • the backing plate 2510 a has a function of fixing the target 2502 a .
  • the backing plate 2510 b has a function of fixing the target 2502 b.
  • Magnetic lines of force 2580 a and 2580 b formed by the magnet unit 2530 a are illustrated in FIG. 7A .
  • the magnet unit 2530 a has a structure in which the magnet 2530 N 1 having a rectangular or substantially rectangular shape, the magnet 2530 N 2 having a rectangular or substantially rectangular shape, and the magnet 2530 S having a rectangular or substantially rectangular shape are fixed to the magnet holder 2532 .
  • the magnet unit 2530 a can be oscillated horizontally as shown by an arrow in FIG. 7B .
  • the magnet unit 2530 a may be oscillated with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz.
  • the magnetic field over the target 2502 a changes in accordance with oscillation of the magnet unit 2530 a .
  • the region with an intense magnetic field is a high-density plasma region; thus, sputtering of the target 2502 a easily occurs in the vicinity of the region.
  • FIG. 8 is a process flow chart showing a method for manufacturing a composite oxide semiconductor.
  • the composite oxide semiconductor shown in FIGS. 1A and 1B is manufactured through at least first to fourth processes shown in FIG. 8 .
  • the first process includes a step of placing a substrate in a deposition chamber (see Step S 101 in FIG. 8 ).
  • the substrate 2560 is placed on the substrate holder 2570 of the deposition chamber 2501 shown in FIGS. 7A and 7B .
  • the temperature of the substrate 2560 in deposition influences the electrical properties of a composite oxide semiconductor.
  • the lower the substrate temperature in deposition is, the more the field-effect mobility at a low gate voltage (e.g., higher than 0 V and lower than or equal to 2 V) is notably increased in a transistor including the composite oxide semiconductor.
  • the temperature of the substrate 2560 can be set higher than or equal to room temperature (25° C.) and lower than or equal to 200° C., preferably higher than or equal to room temperature and lower than or equal to 170° C., further preferably higher than or equal to room temperature and lower than or equal to 130° C.
  • the substrate temperature in the above range is suitable for the case of using a large glass substrate (e.g., a glass substrate having a size of the 8th generation or the 10th generation described later).
  • the substrate temperature in deposition of a composite oxide semiconductor is room temperature, i.e., the substrate is not heated intentionally, the substrate can be favorably prevented from bending or warping.
  • the substrate 2560 may be cooled with a cooling mechanism or the like provided for the substrate holder 2570 .
  • the temperature of the substrate 2560 is set higher than or equal to 100° C. and lower than or equal to 130° C.
  • water in the composite oxide semiconductor can be removed. Water as an impurity is removed in this manner, whereby the field-effect mobility and the reliability can be improved at the same time.
  • the sputtering apparatus can be prevented from warping due to overheat. Accordingly, semiconductor devices can be manufactured with higher productivity. The productivity is thus stabilized, so that a large-scale production apparatus is easy to employ. Thus, a large display device including a large substrate can be easily manufactured.
  • the temperature of the substrate 2560 is high, water in the composite oxide semiconductor can be more effectively removed and moreover, the composite oxide semiconductor can have increased crystallinity.
  • the temperature of the substrate 2560 is set to higher than or equal to 80° C. and lower than or equal to 200° C., preferably higher than or equal to 100° C. and lower than or equal to 170° C., whereby a composite oxide semiconductor with high crystallinity can be formed.
  • the second process includes a step of introducing a gas into the deposition chamber (see Step S 201 in FIG. 8 ).
  • a gas is introduced into the deposition chamber 2501 in FIGS. 7A and 7B .
  • an argon gas and an oxygen gas are introduced as the gas.
  • an inert gas such as helium, xenon, or krypton can be used.
  • the proportion of oxygen in the whole deposition gas in forming a composite oxide semiconductor using an oxygen gas is referred to as an oxygen flow rate percentage in some cases.
  • the oxygen flow rate percentage can be appropriately set in the range from 0% to 100% inclusive so that favorable characteristics of the composite oxide semiconductor suitable to the uses can be obtained.
  • the oxygen flow rate percentage is set to higher than 0% and lower than or equal to 30%, preferably higher than or equal to 5% and lower than or equal to 30%, further preferably higher than or equal to 7% and lower than or equal to 15% in deposition of the composite oxide semiconductor.
  • the composite oxide semiconductor may be formed without using an oxygen gas, in which case the oxygen flow rate percentage is 0%.
  • the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than 30% and lower than 70%, preferably higher than 30% and lower than or equal to 50%.
  • the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than or equal to 10% and lower than or equal to 50%, preferably higher than or equal to 30% and lower than or equal to 50%.
  • the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than or equal to 70% and lower than or equal to 100%.
  • a composite oxide semiconductor that provides desired electrical characteristics can be deposited.
  • a reduction (an increase) of substrate temperature and a reduction (an increase) of oxygen flow rate percentage contribute to the field-effect mobility to the same degree, in some cases. Therefore, even when the substrate temperature cannot be increased sufficiently with the constraints of an apparatus, for example, a transistor having field-effect mobility substantially the same as that when the substrate temperature is increased can be obtained by increasing the oxygen flow rate percentage.
  • a gas which is highly purified to have a dew point of ⁇ 40° C. or lower, preferably ⁇ 80° C. or lower, further preferably ⁇ 100° C. or lower, still further preferably ⁇ 120° C. or lower is used, whereby entry of moisture or the like into the composite oxide semiconductor can be minimized.
  • the deposition chamber 2501 is preferably evacuated to high vacuum (about 5 ⁇ 10 ⁇ 7 Pa to 1 ⁇ 10 ⁇ 4 Pa) with an entrapment vacuum evacuation pump such as a cryopump so that water or the like, which is an impurity for the composite oxide semiconductor, is removed as much as possible.
  • the third process includes a step of applying voltage to a target (see Step S 301 in FIG. 8 ).
  • a potential applied to a terminal V 1 connected to the target holder 2520 a is lower than a potential applied to a terminal V 2 connected to the substrate holder 2570 .
  • a potential applied to a terminal V 4 connected to the target holder 2520 b is lower than the potential applied to the terminal V 2 connected to the substrate holder 2570 .
  • the potential applied to the terminal V 2 connected to the substrate holder 2570 is a ground potential.
  • a potential applied to a terminal V 3 connected to the magnet holder 2532 is a ground potential.
  • the potentials applied to the terminals V 1 , V 2 , V 3 , and V 4 are not limited to the above potentials. Not all the target holder 2520 , the substrate holder 2570 , and the magnet holder 2532 are necessarily supplied with potentials. For example, the substrate holder 2570 may be electrically floating. Note that it is assumed that a power source capable of controlling a potential applied to the terminal V 1 is electrically connected to the terminal V 1 . As the power source, a DC power source, an AC power source, or an RF power source may be used.
  • a target including indium, the element M (M is Al, Ga, Y, or Sn), zinc, and oxygen is preferably used.
  • M is Al, Ga, Y, or Sn
  • the case of using an In-Ga—Zn metal oxide target is described.
  • the fourth process includes a step in which sputtered particles are ejected from the target and a composite oxide semiconductor is deposited on the substrate (see Step S 401 in FIG. 8 ).
  • an argon gas or an oxygen gas is ionized to be separated into cations and electrons, and plasma is created. Then, the cations in the plasma are accelerated toward the targets 2502 a and 2502 b by the potentials applied to the target holders 2520 a and 2520 b . Sputtered particles are generated when the cations collide with the In-Ga—Zn metal oxide target, and the sputtered particles are deposited on the substrate 2560 .
  • FIGS. 9A to 9C are schematic cross-sectional views of the vicinity of the target 2502 a shown in FIGS. 7A and 7B .
  • FIG. 9A shows the state of the target before use
  • FIG. 9B shows the state of the target before deposition
  • FIG. 9C shows the state of the target during the deposition.
  • the target 2502 a plasma 2190 , a cation 2192 , sputtered particles 2504 a and 2506 a , and the like are shown.
  • a surface of the target 2502 a is relatively flat and its composition (e.g., the composition ratio between In, Ga, and Zn) is uniform.
  • FIG. 9B unevenness is formed and compositional segregation occurs on the surface of the target 2502 a by sputtering performed in advance or the like. The unevenness and the segregation can occur because of plasma (e.g., Ar plasma) generated in the sputtering performed in advance.
  • FIG. 9B illustrates a segregation region 2504 and a segregation region 2506 .
  • the segregation region 2504 is a region containing a large amount of Ga and a large amount of Zn (a Ga,Zn-rich region), and the segregation region 2506 is a region containing a large amount of In (an In-rich region).
  • the segregation region 2504 which contains a large amount of Ga, is formed because the melting point of Ga lower than that of In allows part of Ga to be melted by heat applied to the target 2502 a during the plasma treatment and aggregate.
  • an argon gas or an oxygen gas is ionized and separated into the cations 2192 and electrons (not illustrated), and the plasma 2190 is generated.
  • the cations 2192 in the plasma 2190 are accelerated toward the target 2500 a (here, an In-Ga—Zn oxide target).
  • the cations 2192 collide with the In-Ga—Zn oxide target, whereby the sputtered particles 2504 a and 2506 a are generated and ejected from the In-Ga—Zn oxide target.
  • the sputtered particles 2504 a are ejected from the segregation region 2504 , they form a Ga,Zn-rich cluster in some cases.
  • the sputtered particles 2506 a are ejected from the segregation region 2506 , they form an In-rich cluster in some cases.
  • the sputtered particles 2504 a are preferentially sputtered first from the segregation region 2504 .
  • Ga and Zn which have lower relative atomic masses than In, are preferentially ejected from the In-Ga—Zn oxide target by collision of the cation 2192 with the In-Ga—Zn oxide target.
  • the sputtered particles 2504 a that are ejected are deposited over the substrate, thereby forming Region B 1 illustrated in FIGS. 1A and 1B and other drawings.
  • the sputtered particles 2506 a are sputtered from the segregation region 2506 .
  • the sputtered particles 2506 a collide with Region B 1 that has been formed over the substrate, thereby forming Region A 1 illustrated in FIGS. 1A and 1B and other drawings.
  • the target 2502 a is subjected to sputtering throughout the deposition; thus, generation of the segregation region 2504 and disappearance of the segregation region 2504 occur intermittently.
  • the deposition model including the first step and the second step is repeated, whereby the composite oxide semiconductor that is one embodiment of the present invention and shown in FIGS. 1A and 1B and other drawings can be obtained.
  • the sputtered particles ( 2506 a and 2504 a ) are respectively ejected from the In-rich segregation region 2506 and the Ga,Zn-rich segregation region 2504 to be deposited over the substrate.
  • the In-rich regions are connected to each other in a cloud-like manner over the substrate, so that a composite oxide semiconductor of one embodiment of the present invention as illustrated in FIGS. 1A and 1B can be formed.
  • the In-rich regions are connected to each other in a cloud-like manner. Thanks to this, a transistor using the composite oxide semiconductor has a high on-state current (Ion) and high field-effect mobility ( ⁇ FE).
  • a composite oxide semiconductor of one embodiment of the present invention is formed with the use of an argon gas.
  • the composite oxide semiconductor might contain many oxygen vacancies.
  • shallow defect states also referred to as sDOS
  • the sDOS serves as a carrier trap, resulting in a reduction in on-state current and field-effect mobility.
  • Such oxygen supply can be conducted by, for example, performing heat treatment in an atmosphere that contains oxygen after the formation of the composite oxide semiconductor, or performing plasma treatment in an atmosphere that contains oxygen.
  • an insulating film that is in contact with the composite oxide semiconductor of one embodiment of the present invention or an insulating film in the vicinity of the composite oxide semiconductor is made to contain excess oxygen. Details of the mode in which such an insulating film contains excess oxygen are described in Embodiment 2.
  • the formation method is not limited to a sputtering method; a pulsed laser deposition (PLD) method, a plasma-enhanced chemical vapor deposition (PECVD) method, a thermal chemical vapor deposition (CVD) method, an atomic layer deposition (ALD) method, a vacuum evaporation method, or the like may be used.
  • a thermal CVD method a metal organic chemical vapor deposition (MOCVD) method can be given.
  • the shape of the surface of the sputtering target and distributions of the compositions over the surface were examined.
  • a change in the sputtering target before and after the sputtering was examined.
  • FIG. 10 illustrates fabrication of a sample and an analysis method.
  • a part that is cut from a metal oxide target was used.
  • sputtering was performed on the sample surface.
  • the sputtering was performed for one hour using an argon gas as a deposition gas under a pressure of 0.4 Pa and with a DC power of 200 W.
  • the surface subjected to the sputtering was observed with a SEM and analyzed by SEM-EDX with regard to the composition in the above-described manner.
  • an EDX spectrum of a point is obtained in such a manner that electron beam irradiation is performed on the point in an analysis target region of a sample, and the energy of characteristic X-rays of the sample generated by the irradiation and its frequency are measured.
  • peaks of an EDX spectrum of the point were attributed to electron transition to the L shell in an In atom, electron transition to the L shell in a Ga atom, and electron transition to the L shell in a Zn atom and the K shell in an O atom, and the proportions of the atoms in the point were calculated.
  • An EDX mapping image indicating distributions of the proportions of the atoms can be obtained through the process in an analysis target region of a sample.
  • FIG. 11 shows the SEM image and EDX mapping images of the sample surface before the sputtering
  • FIG. 12 shows the SEM image and EDX mapping images of the sample surface after the sputtering.
  • the EDX mapping images of 0 atoms, Zn atoms, Ga atoms, and In atoms existing in the range of the SEM image are shown.
  • the sample surface is relatively flat.
  • a plurality of grains are observed in the SEM image and it is shown that the sample is polycrystalline.
  • the EDX mapping image shows Zn-rich regions in some parts, it was found that the distribution of each element is substantially uniform.
  • the sputtering formed unevenness on the sample surface. Specifically, a particulate precipitate with a diameter of approximately greater than or equal to 0.1 ⁇ m and less than or equal to 5 ⁇ m was observed on the sample surface. Note that in the EDX mapping images in FIG. 12 , there are points the EDX spectra of which were not obtained owing to the influence of the unevenness on the surface.
  • the EDX mapping images in FIG. 12 show that the 0 atoms, the Zn atoms, and the Ga atoms have distribution reflecting the shape of the sample surface and their proportions vary widely from part to part. Moreover, it is shown that the proportions of the Ga atoms and the Zn atoms tend to be high on the surface of the precipitate. In contrast, the distribution of the In atoms does not reflect the shape of the sample surface and the In atoms are more uniformly distributed than other atoms.
  • FIGS. 13A and 13B show pie graphs showing the ratios between In, Ga, and
  • FIG. 14 the results in FIGS. 13A and 13B are shown in the form of column graphs.
  • the composition ratio is substantially uniform before the sputtering and is substantially the same as that of the target; however, after the sputtering, the composition ratio varies from position to position. Specifically, it was found that the precipitate observed after the sputtering included a region where the amount of In is small and Ga and Zn were segregated.
  • Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor.
  • the non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.
  • oxide semiconductors are classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor.
  • the crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.
  • An amorphous structure is generally thought to be isotropic and have no non-uniform structure, to be metastable and have no fixed atomic arrangement, to have a flexible bond angle, and to have a short-range order but have no long-range order, for example.
  • a stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor.
  • an oxide semiconductor that is not isotropic e.g., an oxide semiconductor that has a periodic structure in a microscopic region
  • an a-like OS which is not isotropic, has an unstable structure that contains a void. Because of its instability, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.
  • a CAAC-OS is one of oxide semiconductors and has a plurality of c-axis aligned crystal parts (also referred to as pellets).
  • the CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has few impurities and defects (e.g., oxygen vacancies).
  • an impurity means an element other than the main components of an oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element.
  • an element specifically, silicon or the like
  • a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.
  • nc-OS Analysis of an nc-OS by XRD is described.
  • a peak indicating orientation does not appear. That is, a crystal of an nc-OS does not have orientation.
  • the nc-OS is an oxide semiconductor that has higher regularity than an amorphous oxide semiconductor.
  • the nc-OS has a lower density of defect states than the a-like OS and the amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS in some cases.
  • An a-like OS has a structure between the structure of an nc-OS and the structure of an amorphous oxide semiconductor.
  • the a-like OS contains a void or a low-density region.
  • the a-like OS has an unstable structure because it contains a void.
  • the a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void.
  • the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition.
  • the density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. It is difficult to deposit an oxide semiconductor having a density lower than 78% of the density of the single crystal oxide semiconductor.
  • the density of single crystal InGaZnO 4 with a rhombohedral crystal structure is 6.357 g/cm 3 .
  • the density of the a-like OS is higher than or equal to 5.0 g/cm 3 and lower than 5.9 g/cm 3 , for example.
  • the density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 5.9 g/cm 3 and lower than 6.3 g/cm 3 , for example.
  • an oxide semiconductor having a certain composition does not exist in a single crystal state
  • single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate a density equivalent to that of a single crystal oxide semiconductor with the desired composition.
  • the density of a single crystal oxide semiconductor having the desired composition may be calculated using a weighted average with respect to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.
  • oxide semiconductors have various structures and various properties.
  • two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS may be mixed.
  • Region A 1 described above is preferably non-single-crystal.
  • Region B 1 is preferably non-single-crystal.
  • Region A 1 and Region B 1 may have different crystallinities.
  • FIG. 21A is a top view of a transistor 100 A.
  • FIG. 21B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 21A .
  • FIG. 21C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 21A .
  • some components such as an insulating film 110 are not illustrated in FIG. 21A .
  • some components are not illustrated in some cases in top views of transistors described below.
  • the direction of dashed-dotted line X 1 -X 2 may be referred to as the channel length (L) direction
  • the direction of dashed-dotted line Y 1 -Y 2 may be referred to as the channel width (W) direction.
  • the transistor 100 A illustrated in FIGS. 21A to 21C includes a conductive film 106 over a substrate 102 , an insulating film 104 over the conductive film 106 , an oxide semiconductor film 108 over the insulating film 104 , the insulating film 110 over the oxide semiconductor film 108 , a conductive film 112 over the insulating film 110 , and an insulating film 116 over the insulating film 104 , the oxide semiconductor film 108 , and the conductive film 112 .
  • the oxide semiconductor film 108 includes a channel region 108 i overlapping with the conductive film 112 , a source region 108 s in contact with the insulating film 116 , and a drain region 108 d in contact with the insulating film 116 .
  • the insulating film 116 contains nitrogen or hydrogen.
  • the insulating film 116 is in contact with the source region 108 s and the drain region 108 d , so that nitrogen or hydrogen that is contained in the insulating film 116 is added to the source region 108 s and the drain region 108 d .
  • the source region 108 s and the drain region 108 d each have a high carrier density when nitrogen or hydrogen is added thereto.
  • the transistor 100 A may further include an insulating film 118 over the insulating film 116 , a conductive film 120 a electrically connected to the source region 108 s through an opening 141 a provided in the insulating films 116 and 118 , and a conductive film 120 b electrically connected to the drain region 108 d through an opening 141 b provided in the insulating films 116 and 118 .
  • an insulating film 122 may be provided over the insulating film 118 , the conductive film 120 a , and the conductive film 120 b .
  • the insulating film 104 may be referred to as a first insulating film
  • the insulating film 110 may be referred to as a second insulating film
  • the insulating film 116 may be referred to as a third insulating film
  • the insulating film 118 may be referred to as a fourth insulating film
  • the insulating film 122 may be referred to as a fifth insulating film.
  • the insulating film 104 functions as a first gate insulating film and the insulating film 110 functions as a second gate insulating film.
  • the insulating films 116 and 118 function as a protective insulating film and the insulating film 122 functions as a planarization film.
  • the insulating film 110 includes an excess oxygen region. Since the insulating film 110 includes the excess oxygen region, excess oxygen can be supplied to the channel region 108 i included in the oxide semiconductor film 108 . As a result, oxygen vacancies that might be formed in the channel region 108 i can be filled with excess oxygen, which can provide a highly reliable semiconductor device.
  • excess oxygen may be supplied to the insulating film 104 that is formed under the oxide semiconductor film 108 .
  • excess oxygen contained in the insulating film 104 might also be supplied to the source region 108 s and the drain region 108 d included in the oxide semiconductor film 108 .
  • the resistance of the source region 108 s and the drain region 108 d might be increased.
  • excess oxygen can be selectively supplied only to the channel region 108 i .
  • the carrier density of the source and drain regions 108 s and 108 d can be selectively increased after excess oxygen is supplied to the channel region 108 i and the source and drain regions 108 s and 108 d , in which case an increase in the resistance of the source and drain regions 108 s and 108 d can be prevented.
  • each of the source region 108 s and the drain region 108 d included in the oxide semiconductor film 108 preferably contains an element that forms an oxygen vacancy or an element that is bonded to an oxygen vacancy.
  • the element that forms an oxygen vacancy or the element that is bonded to an oxygen vacancy include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas.
  • the rare gas element include helium, neon, argon, krypton, and xenon.
  • the element that forms an oxygen vacancy is diffused from the insulating film 116 to the source region 108 s and the drain region 108 d .
  • the element that forms an oxygen vacancy is added to the source region 108 s and the drain region 108 d by impurity addition treatment.
  • An impurity element added to the oxide semiconductor film cuts a bond between a metal element and oxygen in the oxide semiconductor film, so that an oxygen vacancy is formed.
  • an impurity element is added to the oxide semiconductor film, oxygen bonded to a metal element in the oxide semiconductor film is bonded to the impurity element and detached from the metal element, so that an oxygen vacancy is formed.
  • the oxide semiconductor film has a higher carrier density, and thus, the conductivity thereof becomes higher.
  • FIG. 22 is an enlarged view showing the oxide semiconductor film 108 in FIG. 21 B and its vicinity.
  • oxygen (O) released from the insulating film 110 because of heat treatment or the like is supplied to the channel region 108 i , so that oxygen vacancies are reduced.
  • the channel region 108 i can be of an i-type.
  • hydrogen (H) is supplied to the source region 108 s and the drain region 108 d and this hydrogen and an oxygen vacancy are bonded.
  • the source region 108 s and the drain region 108 d can be of an n-type.
  • Examples of the above hydrogen include hydrogen contained in a deposition gas used for forming the insulating film 116 and hydrogen released from the insulating film 116 because of heat treatment or the like.
  • the conductive film 106 illustrated in FIGS. 21B and 21C functions as a first gate electrode and the conductive film 112 functions as a second gate electrode.
  • the conductive film 120 a functions as a source electrode and the conductive film 120 b functions as a drain electrode.
  • an opening 143 is provided in the insulating films 104 and 110 .
  • the conductive film 106 is electrically connected to the conductive film 112 through the opening 143 .
  • the same potential is applied to the conductive film 106 and the conductive film 112 .
  • different potentials may be applied to the conductive film 106 and the conductive film 112 without providing the opening 143 .
  • the conductive film 106 may be used as a light-shielding film without providing the opening 143 .
  • the conductive film 106 is formed using a light-shielding material, light irradiation of the channel region 108 i from the bottom can be reduced.
  • the oxide semiconductor film 108 faces the conductive film 106 functioning as the first gate electrode and the conductive film 112 functioning as the second gate electrode and is positioned between the two conductive films functioning as the gate electrodes.
  • the length of the conductive film 112 in the channel width direction is larger than the length of the oxide semiconductor film 108 in the channel width direction.
  • the whole oxide semiconductor film 108 is covered with the conductive film 112 with the insulating film 110 placed therebetween. Since the conductive film 112 is connected to the conductive film 106 through the opening 143 provided in the insulating films 104 and 110 , a side surface of the oxide semiconductor film 108 in the channel width direction faces the conductive film 112 with the insulating film 110 placed therebetween.
  • the conductive films 106 and 112 are connected to each other through the opening 143 provided in the insulating films 104 and 110 , and the conductive films 106 and 112 surround the oxide semiconductor film 108 with the insulating films 104 and 110 positioned therebetween.
  • Such a structure enables the oxide semiconductor film 108 included in the transistor 100 A to be electrically surrounded by electric fields of the conductive film 106 functioning as the first gate electrode and the conductive film 112 functioning as the second gate electrode.
  • a device structure of a transistor, like that of the transistor 100 A, in which electric fields of a first gate electrode and a second gate electrode electrically surround the oxide semiconductor film 108 in which a channel region is formed can be referred to as a surrounded channel (S-channel) structure.
  • the transistor 100 A can also be referred to as a dual-gate transistor from the number of its gate electrodes.
  • the transistor 100 A Since the transistor 100 A has the S-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 106 or the conductive film 112 ; thus, the current drive capability of the transistor 100 A can be improved and high on-state current characteristics can be obtained. Owing to the high on-state current, it is possible to reduce the size of the transistor 100 A. Furthermore, since the transistor 100 A has a structure in which the oxide semiconductor film 108 is surrounded by the conductive film 106 and the conductive film 112 , the mechanical strength of the transistor 100 A can be increased.
  • an opening different from the opening 143 may be formed on the side of the oxide semiconductor film 108 on which the opening 143 is not formed.
  • the transistor 100 A may be called a top-gate self-aligned (TGSA) FET from the position of the conductive film 112 relative to the oxide semiconductor film 108 or the formation method of the conductive film 112 .
  • TGSA top-gate self-aligned
  • the semiconductor device of one embodiment of the present invention is not limited to the above example and may be a bottom-gate top-contact (BGTC) FET.
  • the substrate 102 can be formed using a material having heat resistance high enough to withstand heat treatment in the manufacturing process.
  • non-alkali glass, soda-lime glass, alkali glass, crystal glass, quartz, sapphire, or the like can be used.
  • an inorganic insulating film may be used.
  • the inorganic insulating film include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and an aluminum oxide film.
  • the non-alkali glass may have a thickness of greater than or equal to 0.2 mm and less than or equal to 0.7 mm, for example.
  • the non-alkali glass may be polished to obtain the above thickness.
  • a large-area glass substrate having any of the following sizes can be used: the 6th generation (1500 mm ⁇ 1850 mm), the 7th generation (1870 mm ⁇ 2200 mm), the 8th generation (2200 mm ⁇ 2400 mm), the 9th generation (2400 mm ⁇ 2800 mm), and the 10th generation (2950 mm ⁇ 3400 mm).
  • a large-sized display device can be manufactured.
  • a single-crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used as the substrate 102 .
  • an inorganic material such as a metal
  • examples of the inorganic material such as a metal include stainless steel and aluminum.
  • an organic material such as a resin, a resin film, or plastic may be used.
  • the resin film include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and a resin having a siloxane bond.
  • a composite material of an inorganic material and an organic material may be used.
  • the composite material include a resin film to which a metal plate or a thin glass plate is bonded, a resin film into which a fibrous or particulate metal or a fibrous or particulate glass is dispersed, and an inorganic material into which a fibrous or particulate resin is dispersed.
  • the substrate 102 can at least support films or layers formed thereover or thereunder and may be one or more of an insulating film, a semiconductor film, and a conductive film.
  • the insulating film 104 can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like as appropriate.
  • the insulating film 104 can be formed to have a single-layer structure or a stacked-layer structure of an oxide insulating film and/or a nitride insulating film.
  • at least a region of the insulating film 104 which is in contact with the oxide semiconductor film 108 is preferably formed using an oxide insulating film.
  • oxygen contained in the insulating film 104 can be moved to the oxide semiconductor film 108 by heat treatment.
  • the thickness of the insulating film 104 can be greater than or equal to 50 nm, greater than or equal to 100 nm and less than or equal to 3000 nm, or greater than or equal to 200 nm and less than or equal to 1000 nm.
  • the amount of oxygen released from the insulating film 104 can be increased, and interface states at the interface between the insulating film 104 and the oxide semiconductor film 108 and oxygen vacancies included in the channel region 108 i of the oxide semiconductor film 108 can be reduced.
  • the insulating film 104 can be formed to have a single-layer structure or stacked-layer structure of a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, a gallium oxide film, a Ga—Zn oxide film, or the like.
  • the insulating film 104 has a stacked-layer structure of a silicon nitride film and a silicon oxynitride film. With the insulating film 104 having such a layered structure including a silicon nitride film as a lower layer and a silicon oxynitride film as an upper layer, oxygen can be efficiently introduced into the oxide semiconductor film 108 .
  • the insulating film 110 has a function of supplying oxygen to the oxide semiconductor film 108 , particularly to the channel region 108 i .
  • the insulating film 110 can be formed to have a single-layer structure or a stacked-layer structure of an oxide insulating film and/or a nitride insulating film, for example.
  • a region which is in the insulating film 110 and in contact with the oxide semiconductor film 108 is preferably formed using at least an oxide insulating film.
  • a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, or a silicon nitride film can be used as the insulating film 110 .
  • the thickness of the insulating film 110 can be greater than or equal to 5 nm and less than or equal to 400 nm, greater than or equal to 5 nm and less than or equal to 300 nm, or greater than or equal to 10 nm and less than or equal to 250 nm.
  • the insulating film 110 have few defects and typically have as few signals observed by electron spin resonance (ESR) spectroscopy as possible.
  • the signals include a signal due to an E center observed at a g-factor of 2.001. Note that the E center is due to the dangling bond of silicon.
  • a silicon oxide film or a silicon oxynitride film whose spin density of a signal due to the E center is lower than or equal to 3 ⁇ 10 17 spins/cm 3 , preferably lower than or equal to 5 ⁇ 10 16 spins/cm 3 may be used.
  • a signal due to nitrogen dioxide (NO 2 ) might be observed in the insulating film 110 .
  • the signal is divided into three signals according to the N nuclear spin; a first signal, a second signal, and a third signal.
  • the first signal is observed at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039.
  • the second signal is observed at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003.
  • the third signal is observed at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966.
  • insulating film 110 It is suitable to use an insulating film whose spin density of a signal due to nitrogen dioxide (NO 2 ) is higher than or equal to 1 ⁇ 10 17 spins/cm 3 and lower than 1 ⁇ 10 18 spins/cm 3 as the insulating film 110 , for example.
  • NO 2 nitrogen dioxide
  • nitrogen oxide (NO x ) such as nitrogen dioxide (NO 2 ) forms a state in the insulating film 110 .
  • the state is positioned in the energy gap of the oxide semiconductor film 108 .
  • nitrogen oxide (NO x ) is diffused to the interface between the insulating film 110 and the oxide semiconductor film 108 , an electron might be trapped by the state on the insulating film 110 side.
  • the trapped electron remains in the vicinity of the interface between the insulating film 110 and the oxide semiconductor film 108 , leading to a positive shift of the threshold voltage of the transistor. Accordingly, the use of a film with a low nitrogen oxide content as the insulating film 110 can reduce a shift of the threshold voltage of the transistor.
  • a silicon oxynitride film As an insulating film that releases a small amount of nitrogen oxide (NO x ), for example, a silicon oxynitride film can be used.
  • the silicon oxynitride film releases more ammonia than nitrogen oxide (NO x ) in thermal desorption spectroscopy (TDS); the typical released amount of ammonia is greater than or equal to 1 ⁇ 10 18 /cm 3 and less than or equal to 5 ⁇ 10 19 /cm 3 .
  • the released amount of ammonia is the total amount of ammonia released by heat treatment in a range of 50° C. to 650° C. or 50° C. to 550° C. in TDS.
  • nitrogen oxide (NO x ) reacts with ammonia and oxygen in heat treatment, the use of an insulating film that releases a large amount of ammonia reduces nitrogen oxide (NO x ).
  • nitrogen concentration in the film is preferably lower than or equal to 6 ⁇ 10 20 atoms/cm 3 .
  • the insulating film 110 may be formed using a high-k material such as hafnium silicate (HfSiO x ), hafnium silicate containing nitrogen (HfSi x O y N z ), hafnium aluminate containing nitrogen (HfAl x O y N z ), or hafnium oxide.
  • a high-k material such as hafnium silicate (HfSiO x ), hafnium silicate containing nitrogen (HfSi x O y N z ), hafnium aluminate containing nitrogen (HfAl x O y N z ), or hafnium oxide.
  • the insulating film 116 contains nitrogen or hydrogen.
  • the insulating film 116 may contain fluorine.
  • a nitride insulating film can be used as the insulating film 116 .
  • the nitride insulating film can be formed using silicon nitride, silicon nitride oxide, silicon oxynitride, silicon nitride fluoride, silicon fluoronitride, or the like.
  • the hydrogen concentration in the insulating film 116 is preferably higher than or equal to 1 ⁇ 10 22 atoms/cm 3 .
  • the insulating film 116 is in contact with the source region 108 s and the drain region 108 d of the oxide semiconductor film 108 .
  • the concentration of an impurity (nitrogen or hydrogen) in the source region 108 s and the drain region 108 d in contact with the insulating film 116 is increased, leading to an increase in the carrier density of the source region 108 s and the drain region 108 d.
  • an oxide insulating film can be used as the insulating film 118 .
  • a layered film of an oxide insulating film and a nitride insulating film can be used as the insulating film 118 .
  • the insulating film 118 can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, or a Ga—Zn oxide.
  • the insulating film 118 preferably functions as a barrier film against hydrogen, water, and the like from the outside.
  • the thickness of the insulating film 118 can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.
  • the insulating film 122 has an insulating property and is formed using an inorganic material or an organic material.
  • the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride.
  • the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.
  • the conductive films 106 , 112 , 120 a , and 120 b can be formed by a sputtering method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, a thermal CVD method, or the like.
  • a metal film having conductivity, a conductive film having a function of reflecting visible light, or a conductive film having a function of transmitting visible light may be used.
  • a material containing a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the metal film having conductivity.
  • a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the metal film having conductivity.
  • an alloy containing any of the above metal elements may be used.
  • the metal film having conductivity specifically a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a titanium nitride film, a two-layer structure in which a copper film is stacked over a tantalum nitride film, a three-layer structure in which a titanium film, a copper film, and a titanium film are stacked in this order, or the like may be used.
  • a conductive film containing a copper element is preferably used because the resistance can be reduced.
  • an alloy film containing copper and manganese is given. The alloy film is favorable because it can be processed by a wet etching method.
  • a tantalum nitride film is preferably used for the conductive films 106 , 112 , 120 a , and 120 b .
  • a tantalum nitride film has conductivity and a high barrier property against copper or hydrogen. Because a tantalum nitride film releases little hydrogen from itself, it can be favorably used as a metal film in contact with the oxide semiconductor film 108 or a metal film in the vicinity of the oxide semiconductor film 108 .
  • a conductive macromolecule or a conductive polymer may be used as the metal film having conductivity.
  • a material containing a metal element selected from gold, silver, copper, and palladium can be used.
  • a conductive film containing a silver element is preferably used because reflectance of visible light can be improved.
  • a material containing an element selected from indium, tin, zinc, gallium, and silicon can be used.
  • an In oxide, a Zn oxide, an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (also referred to as ITSO), an In—Zn oxide, an In-Ga—Zn oxide, or the like can be used.
  • a film containing graphene or graphite may be used as the conductive film having a function of transmitting visible light.
  • the film containing graphene can be formed in the following manner: a film containing graphene oxide is formed and is reduced.
  • a reducing method a method with application of heat, a method using a reducing agent, or the like can be employed.
  • the conductive films 112 , 120 a , and 120 b can be formed by electroless plating.
  • a material that can be deposited by electroless plating for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.
  • a diffusion prevention film may be formed under the conductive film to prevent component elements of the conductive film from diffusing outward.
  • a seed layer that can make the conductive film grow may be formed between the diffusion prevention film and the conductive film.
  • the diffusion prevention film can be formed by sputtering, for example.
  • a tantalum nitride film or a titanium nitride film can be used, for example.
  • the seed layer can be formed by electroless plating.
  • a material similar to the material for the conductive film that can be formed by electroless plating can be used.
  • an oxide semiconductor typified by an In-Ga—Zn oxide may be used for the conductive film 112 .
  • the oxide semiconductor can have a high carrier density when nitrogen or hydrogen is supplied from the insulating film 116 .
  • the oxide semiconductor functions as an oxide conductor (OC). Accordingly, the oxide semiconductor can be used for a gate electrode.
  • the conductive film 112 can have, for example, a single-layer structure of an oxide conductor (OC), a single-layer structure of a metal film, or a stacked-layer structure of an oxide conductor (OC) and a metal film.
  • OC oxide conductor
  • metal film a stacked-layer structure of an oxide conductor (OC) and a metal film.
  • the conductive film 112 has a single-layer structure of a light-shielding metal film or a stacked-layer structure of an oxide conductor (OC) and a light-shielding metal film because the channel region 108 i formed under the conductive film 112 can be shielded from light.
  • OC oxide conductor
  • the conductive film 112 has a stacked-layer structure of an oxide semiconductor or an oxide conductor (OC) and a light-shielding metal film
  • a metal film e.g., a titanium film or a tungsten film
  • the resistance of the oxide semiconductor or the oxide conductor (OC) is reduced by the diffusion of the constituent element of the metal film to the oxide semiconductor or oxide conductor (OC) side
  • the resistance is reduced by damage (e.g., sputtering damage) during the deposition of the metal film
  • the resistance is reduced when oxygen vacancies are formed by the diffusion of oxygen in the oxide semiconductor or the oxide conductor (OC) to the metal film.
  • the thickness of the conductive films 106 , 112 , 120 a , and 120 b can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.
  • FIGS. 23A and 23B are cross-sectional views of a transistor 100 B.
  • FIGS. 24A and 24B are cross-sectional views of a transistor 100 C.
  • FIGS. 25A and 25B are cross-sectional views of a transistor 100 D.
  • the top views of the transistors 100 B, 100 C, and 100 D are not illustrated because they are similar to the top view of the transistor 100 A in FIG. 21A .
  • the transistor 100 B illustrated in FIGS. 23A and 23B is different from the transistor 100 A in the layered structure of the conductive film 112 , the shape of the conductive film 112 , and the shape of the insulating film 110 .
  • the conductive film 112 in the transistor 100 B includes a conductive film 112 _ 1 over the insulating film 110 and the conductive film 112 _ 2 over the conductive film 112 _ 1 .
  • an oxide conductive film is used as the conductive film 112 _ 1 , so that excess oxygen can be added to the insulating film 110 .
  • the oxide conductive film can be formed by a sputtering method in an atmosphere containing an oxygen gas.
  • an oxide film containing indium and tin, an oxide film containing tungsten and indium, an oxide film containing tungsten, indium, and zinc, an oxide film containing titanium and indium, an oxide film containing titanium, indium, and tin, an oxide film containing indium and zinc, an oxide film containing silicon, indium, and tin, or an oxide film containing indium, gallium, and zinc can be used, for example.
  • the conductive film 112 _ 2 is connected to the conductive film 106 through the opening 143 .
  • the shape illustrated in FIG. 23B can be obtained.
  • the structure in which the conductive film 112 _ 2 is connected to the conductive film 106 can decrease the contact resistance between the conductive film 112 and the conductive film 106 .
  • the conductive film 112 and the insulating film 110 in the transistor 100 B have a tapered shape. More specifically, the lower end portion of the conductive film 112 is located outward from the upper end portion of the conductive film 112 . The lower end portion of the insulating film 110 is located outward from the upper end portion of the insulating film 110 . In addition, the lower end portion of the conductive film 112 is formed in substantially the same position as that of the upper end portion of the insulating film 110 .
  • the conductive film 112 and the insulating film 110 of the transistor 100 B are formed to have tapered shapes because the coverage with the insulating film 116 can be high as compared with the case of the transistor 100 A in which the conductive film 112 and the insulating film 110 have rectangular shapes.
  • the other components of the transistor 100 B are similar to those of the transistor 100 A described above and have similar effects.
  • the transistor 100 C illustrated in FIGS. 24A and 24B is different from the transistor 100 A in the layered structure of the conductive film 112 , the shape of the conductive film 112 , and the shape of the insulating film 110 .
  • the conductive film 112 in the transistor 100 C includes the conductive film 112 _ 1 over the insulating film 110 and the conductive film 112 _ 2 over the conductive film 112 _ 1 .
  • a lower end portion of the conductive film 112 _ 1 is located outward from an upper end portion of the conductive film 112 _ 2 .
  • the conductive film 112 _ 1 , the conductive film 112 _ 2 , and the insulating film 110 are processed with one mask, the conductive film 112 _ 2 is processed by a wet etching method, and the conductive film 112 _ 1 and the insulating film 110 are processed by a dry etching method, whereby the above-described structure can be obtained.
  • regions 108 f are formed in the oxide semiconductor film 108 in some cases.
  • the regions 108 f are formed between the channel region 108 i and the source region 108 s and between the channel region 108 i and the drain region 108 d.
  • the regions 108 f function as high-resistance regions or low-resistance regions.
  • the high-resistance regions have the same level of resistance as the channel region 108 i and do not overlap with the conductive film 112 functioning as a gate electrode.
  • the regions 108 f function as offset regions.
  • the regions 108 f functioning as offset regions may each have a length of 1 ⁇ m or less in the channel length (L) direction.
  • the low-resistance regions have a resistance that is lower than that of the channel region 108 i and higher than that of the source region 108 s and the drain region 108 d .
  • the regions 108 f function as lightly doped drain (LDD) regions.
  • LDD lightly doped drain
  • the regions 108 f serve as LDD regions
  • the regions 108 f are formed by supplying one or more of nitrogen, hydrogen, and fluorine from the insulating film 116 to the regions 108 f or by adding an impurity element from above the conductive film 112 _ 1 using the insulating film 110 and the conductive film 112 _ 1 as a mask so that the impurity element is added to the oxide semiconductor film 108 through the conductive film 112 _ 1 and the insulating film 110 .
  • the conductive film 112 _ 2 is connected to the conductive film 106 through the opening 143 .
  • the other components of the transistor 100 C are similar to those of the transistor 100 A described above and have similar effects.
  • the transistor 100 D illustrated in FIGS. 25A and 25B is different from the transistor 100 A in the layered structure of the conductive film 112 , the shape of the conductive film 112 , and the shape of the insulating film 110 .
  • the conductive film 112 in the transistor 100 D includes the conductive film 112 _ 1 over the insulating film 110 and the conductive film 112 _ 2 over the conductive film 112 _ 1 .
  • a lower end portion of the conductive film 112 _ 1 is located outward from a lower end portion of the conductive film 112 _ 2 .
  • a lower end portion of the insulating film 110 is located outward from the lower end portion of the conductive film 112 _ 1 .
  • the conductive film 112 _ 1 , the conductive film 112 _ 2 , and the insulating film 110 are processed with one mask, the conductive film 112 _ 2 and the conductive film 112 _ 1 are processed by a wet etching method, and the insulating film 110 is processed by a dry etching method, whereby the above-described structure can be obtained.
  • the regions 108 f are formed in the oxide semiconductor film 108 in the transistor 100 D, in some cases.
  • the regions 108 f are formed between the channel region 108 i and the source region 108 s and between the channel region 108 i and the drain region 108 d.
  • the conductive film 112 _ 2 is connected to the conductive film 106 through the opening 143 .
  • the other components of the transistor 100 D are similar to those of the transistor 100 A described above and have similar effects.
  • FIGS. 26A and 26B are cross-sectional views of a transistor 100 E.
  • FIGS. 27A and 27B are cross-sectional views of a transistor 100 F.
  • FIGS. 28A and 28B are cross-sectional views of a transistor 100 G.
  • FIGS. 29A and 29B are cross-sectional views of a transistor 100 H.
  • FIGS. 30A and 30B are cross-sectional views of a transistor 100 J.
  • the top views of the transistors 100 E, 100 F, 100 G, 100 H, and 100 J are not illustrated because they are similar to the top view of the transistor 100 A in FIG. 21A .
  • the transistors 100 E, 100 F, 100 G, 100 H, and 100 J are different from the above-described the transistor 100 A in the structure of the oxide semiconductor film 108 .
  • the other components are similar to those of the transistor 100 A and have similar effects.
  • the oxide semiconductor film 108 of the transistor 100 E illustrated in FIGS. 26A and 26B includes an oxide semiconductor film 108 _ 1 over the insulating film 104 , an oxide semiconductor film 108 _ 2 over the oxide semiconductor film 108 _ 1 , and an oxide semiconductor film 1083 over the oxide semiconductor film 1082 .
  • the channel region 108 i , the source region 108 s , and the drain region 108 d each have a three-layer structure of the oxide semiconductor film 108 _ 1 , the oxide semiconductor film 1082 , and the oxide semiconductor film 108 _ 3 .
  • the oxide semiconductor film 108 of the transistor 100 F illustrated in FIGS. 27A and 27B includes the oxide semiconductor film 108 _ 2 over the insulating film 104 , and the oxide semiconductor film 1083 over the oxide semiconductor film 108 _ 2 .
  • the channel region 108 i , the source region 108 s , and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108 _ 2 and the oxide semiconductor film 108 _ 3 .
  • the oxide semiconductor film 108 of the transistor 100 G illustrated in FIGS. 28A and 28B includes the oxide semiconductor film 108 _ 1 over the insulating film 104 , and the oxide semiconductor film 1082 over the oxide semiconductor film 108 _ 1 .
  • the channel region 108 i , the source region 108 s , and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108 _ 1 and the oxide semiconductor film 108 _ 2 .
  • the oxide semiconductor film 108 of the transistor 100 H illustrated in FIGS. 29A and 29B includes the oxide semiconductor film 108 _ 1 over the insulating film 104 , the oxide semiconductor film 1082 over the oxide semiconductor film 1081 , and the oxide semiconductor film 1083 over the oxide semiconductor film 1082 .
  • the channel region 108 i has a three-layer structure of the oxide semiconductor film 108 _ 1 , the oxide semiconductor film 1082 , and the oxide semiconductor film 1083 .
  • the source region 108 s and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108 _ 1 and the oxide semiconductor film 1082 . Note that in the cross section of the transistor 100 H in the channel width (W) direction, the oxide semiconductor film 108 _ 3 covers side surfaces of the oxide semiconductor film 108 _ 1 and the oxide semiconductor film 1082 .
  • the oxide semiconductor film 108 of the transistor 100 J illustrated in FIGS. 30A and 30B includes the oxide semiconductor film 108 _ 2 over the insulating film 104 , and the oxide semiconductor film 1083 over the oxide semiconductor film 108 _ 2 .
  • the channel region 108 i has a two-layer structure of the oxide semiconductor film 108 _ 2 and the oxide semiconductor film 108 _ 3 .
  • the source region 108 s and the drain region 108 d each have a single-layer structure of the oxide semiconductor film 108 _ 2 . Note that in the cross section of the transistor 100 J in the channel width (W) direction, the oxide semiconductor film 108 _ 3 covers side surfaces of the oxide semiconductor film 108 _ 2 .
  • a side surface of the channel region 108 i in the channel width (W) direction or a region in the vicinity of the side surface is easily damaged by processing, resulting in a defect (e.g., oxygen vacancy), or easily contaminated by an impurity attached thereto. Therefore, even when the channel region 108 i is substantially intrinsic, stress such as an electric field applied thereto activates the side surface of the channel region 108 i in the channel width (W) direction or the region in the vicinity of the side surface and turns it into a low-resistance (n-type) region easily.
  • a parasitic channel may be formed because the n-type region serves as a carrier path.
  • the channel region 108 i has a stacked-layer structure and side surfaces of the channel region 108 i in the channel width (W) direction are covered with one layer of the stacked layers.
  • defects on or in the vicinity of the side surfaces of the channel region 108 i can be suppressed or adhesion of an impurity to the side surfaces of the channel region 108 i or to regions in the vicinity of the side surfaces can be reduced.
  • FIGS. 31A to 31C are each a band structure of the channel region 108 i.
  • FIG. 31A shows an example of a band structure in the thickness direction of a stack including the insulating film 104 , the oxide semiconductor films 108 _ 1 , 108 _ 2 , and 108 _ 3 , and the insulating film 110 .
  • FIG. 31B shows an example of a band structure in the thickness direction of a stack including the insulating film 104 , the oxide semiconductor films 108 _ 2 and 108 _ 3 , and the insulating film 110 .
  • FIG. 31C shows an example of a band structure in the thickness direction of a stack including the insulating film 104 , the oxide semiconductor films 108 _ 1 and 108 _ 2 , and the insulating film 110 .
  • the band structures show the conduction band minimum (E c ) of the insulating film 104 , the oxide semiconductor films 108 _ 1 , 108 _ 2 , and 108 _ 3 , and the insulating film 110 .
  • a silicon oxide film is used as each of the insulating films 104 and 110 , an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108 _ 1 , an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 1082 , and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 1083 .
  • a silicon oxide film is used as each of the insulating films 104 and 110 , an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 108 _ 2 , and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108 _ 3 .
  • a silicon oxide film is used as each of the insulating films 104 and 110 , an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108 _ 1 , and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 108 _ 2 .
  • the conduction band minimum gradually varies between the oxide semiconductor films 108 _ 1 , 108 _ 2 , and 108 _ 3 .
  • the conduction band minimum gradually varies between the oxide semiconductor films 108 _ 2 and 108 _ 3 .
  • the conduction band minimum gradually varies between the oxide semiconductor films 108 _ 1 and 108 _ 2 . In other words, the conduction band minimum is continuously changed or continuously connected.
  • the oxide semiconductor film 108 _ 2 serves as a well, and a channel region is formed in the oxide semiconductor film 108 _ 2 in the transistor with the stacked-layer structure.
  • the oxide semiconductor film 108 _ 2 can be distanced away from defect states.
  • the defect states might be more distant from the vacuum level than the conduction band minimum (E c ) of the oxide semiconductor film 108 _ 2 functioning as a channel region, so that electrons are likely to be accumulated in the defect states.
  • the defect states When the electrons are accumulated in the defect states, the electrons become negative fixed electric charge, so that the threshold voltage of the transistor is shifted in the positive direction. Therefore, it is preferable that the defect states be closer to the vacuum level than the conduction band minimum (E c ) of the oxide semiconductor film 108 _ 2 .
  • Such a structure inhibits accumulation of electrons in the defect states. As a result, the on-state current and the field-effect mobility of the transistor can be increased.
  • the conduction band minimum of each of the oxide semiconductor films 108 _ 1 and 1083 is closer to the vacuum level than that of the oxide semiconductor film 108 _ 2 .
  • a typical difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of each of the oxide semiconductor films 108 _ 1 and 1083 is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. That is, the electron affinity of the oxide semiconductor film 108 _ 2 is higher than those of the oxide semiconductor films 108 _ 1 and 108 _ 3 .
  • the difference between the electron affinity of each of the oxide semiconductor films 108 _ 1 and 108 _ 3 and the electron affinity of the oxide semiconductor film 108 _ 2 is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less.
  • the oxide semiconductor film 108 _ 2 serves as a main path of current.
  • the oxide semiconductor film 1082 serves as a channel region
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 serve as oxide insulating films.
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 each include one or more metal elements constituting the oxide semiconductor film 108 _ 2 in which a channel region is formed.
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 can be referred to as oxide insulating films for such properties and/or functions.
  • a material used for the oxide semiconductor films 108 _ 1 and 108 _ 3 has a smaller electron affinity (a difference between the vacuum level and the conduction band minimum) than the oxide semiconductor film 108 _ 2 and is selected such that a difference (band offset) exists between the conduction band minimum of each of the oxide semiconductor films 108 _ 1 and 1083 and that of the oxide semiconductor film 108 _ 2 .
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 using a material whose conduction band minimum is closer to the vacuum level than that of the oxide semiconductor film 1082 .
  • a difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of each of the oxide semiconductor films 108 _ 1 and 108 _ 3 is preferably greater than or equal to 0.2 eV, further preferably greater than or equal to 0.5 eV.
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 not have a spinel crystal structure. This is because if the oxide semiconductor films 108 _ 1 and 108 _ 3 have a spinel crystal structure, constituent elements of the conductive films 120 a and 120 b might be diffused into the oxide semiconductor film 108 _ 2 at the interface between the spinel crystal structure and another region.
  • each of the oxide semiconductor films 108 _ 1 and 108 _ 3 is preferably a CAAC-OS film described later, in which case a higher blocking property against constituent elements of the conductive films 120 a and 120 b , for example, copper elements, can be obtained.
  • an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2, is used as each of the oxide semiconductor films 108 _ 1 and 1083 is described in this embodiment, one embodiment of the present invention is not limited thereto.
  • an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, 1:1:1.2, 1:3:4, 1:3:6, 1:4:5, 1:5:6, or 1:10:1 may be used as each of the oxide semiconductor films 108 _ 1 and 108 _ 3 .
  • oxide semiconductor films formed using a metal oxide target whose atomic ratio of Ga to Zn is 10:1 may be used as the oxide semiconductor films 108 _ 1 and 108 _ 3 .
  • oxide semiconductor films formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1 is used as the oxide semiconductor film 1082 and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of Ga to Zn is 10:1 is used as each of the oxide semiconductor films 108 _ 1 and 108 _ 3 because the difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of the oxide semiconductor film 108 _ 1 or 108 _ 3 can be 0.6 eV or more.
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108 _ 1 and 108 _ 3 might be 1: ⁇ 1: ⁇ 2 (0 ⁇ 1 ⁇ 2, 0 ⁇ 2 ⁇ 2).
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:4, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108 _ 1 and 108 _ 3 might be 1: ⁇ 3: ⁇ 4 (1 ⁇ 3 ⁇ 5, 2 ⁇ 4 ⁇ 6).
  • the oxide semiconductor films 108 _ 1 and 108 _ 3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:6, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108 _ 1 and 108 _ 3 might be 1: ⁇ 5: ⁇ 6 (1 ⁇ 5 ⁇ 5, 4 ⁇ 6 ⁇ 8).
  • FIG. 32A is a top view of a transistor 300 A.
  • FIG. 32B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 32A .
  • FIG. 32C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 32A .
  • some components of the transistor 300 A e.g., an insulating film functioning as a gate insulating film
  • the direction of dashed-dotted line X 1 -X 2 may be referred to as a channel length direction
  • the direction of dashed-dotted line Y 1 -Y 2 may be referred to as a channel width direction.
  • some components are not illustrated in some cases in top views of transistors described below.
  • the transistor 300 A illustrated in FIGS. 32A to 32C includes a conductive film 304 over a substrate 302 , an insulating film 306 over the substrate 302 and the conductive film 304 , an insulating film 307 over the insulating film 306 , an oxide semiconductor film 308 over the insulating film 307 , a conductive film 312 a over the oxide semiconductor film 308 , and a conductive film 312 b over the oxide semiconductor film 308 .
  • an insulating film 314 , an insulating film 316 , and an insulating film 318 are provided.
  • the insulating films 306 and 307 function as the gate insulating films of the transistor 300 A, and the insulating films 314 , 316 , and 318 function as protective insulating films of the transistor 300 A.
  • the conductive film 304 functions as a gate electrode
  • the conductive film 312 a functions as a source electrode
  • the conductive film 312 b functions as a drain electrode.
  • the insulating films 306 and 307 may be referred to as a first insulating film
  • the insulating films 314 and 316 may be referred to as a second insulating film
  • the insulating film 318 may be referred to as a third insulating film.
  • the transistor 300 A illustrated in FIGS. 32A to 32C is a channel-etched transistor.
  • the oxide semiconductor film of one embodiment of the present invention is suitable for a channel-etched transistor.
  • FIG. 33A is a top view of a transistor 300 B.
  • FIG. 33B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 33A .
  • FIG. 33C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 33A .
  • the transistor 300 B illustrated in FIGS. 33A to 33C includes the conductive film 304 over the substrate 302 , the insulating film 306 over the substrate 302 and the conductive film 304 , the insulating film 307 over the insulating film 306 , the oxide semiconductor film 308 over the insulating film 307 , the insulating film 314 over the oxide semiconductor film 308 , the insulating film 316 over the insulating film 314 , the conductive film 312 a electrically connected to the oxide semiconductor film 308 through an opening 341 a provided in the insulating films 314 and 316 , and the conductive film 312 b electrically connected to the oxide semiconductor film 308 through an opening 341 b provided in the insulating films 314 and 316 .
  • the insulating film 318 is provided.
  • the insulating films 306 and 307 each function as a gate insulating film of the transistor 300 B
  • the insulating films 314 and 316 each function as a protective insulating film of the oxide semiconductor film 308
  • the insulating film 318 functions as a protective insulating film of the transistor 300 B.
  • the conductive film 304 functions as a gate electrode
  • the conductive film 312 a functions as a source electrode
  • the conductive film 312 b functions as a drain electrode.
  • the transistor 300 A illustrated in FIGS. 32A to 32C has a channel-etched structure, whereas the transistor 300 B in FIGS. 33A to 33C has a channel-protective structure.
  • the oxide semiconductor film of one embodiment of the present invention is suitable for a channel-protective transistor as well.
  • FIG. 34A is a top view of a transistor 300 C.
  • FIG. 34B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 34A .
  • FIG. 34C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 34A .
  • the transistor 300 C illustrated in FIGS. 34A to 34C is different from the transistor 300 B in FIGS. 33A to 33C in the shapes of the insulating films 314 and 316 .
  • the insulating films 314 and 316 of the transistor 300 C have island shapes and are provided over a channel region of the oxide semiconductor film 308 .
  • Other components are similar to those of the transistor 300 B.
  • FIG. 35A is a top view of a transistor 300 D.
  • FIG. 35B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 35A .
  • FIG. 35C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 35A .
  • the transistor 300 D illustrated in FIGS. 35A to 35C includes the conductive film 304 over the substrate 302 , the insulating film 306 over the substrate 302 and the conductive film 304 , the insulating film 307 over the insulating film 306 , the oxide semiconductor film 308 over the insulating film 307 , the conductive film 312 a over the oxide semiconductor film 308 , the conductive film 312 b over the oxide semiconductor film 308 , the insulating film 314 over the oxide semiconductor film 308 and the conductive films 312 a and 312 b , the insulating film 316 over the insulating film 314 , the insulating film 318 over the insulating film 316 , and conductive films 320 a and 320 b over the insulating film 318 .
  • the insulating films 306 and 307 function as first gate insulating films of the transistor 300 D, and the insulating films 314 , 316 , and 318 function as second gate insulating films of the transistor 300 D.
  • the conductive film 304 functions as a first gate electrode
  • the conductive film 320 a functions as a second gate electrode
  • the conductive film 320 b functions as a pixel electrode used for a display device.
  • the conductive film 312 a functions as a source electrode
  • the conductive film 312 b functions as a drain electrode.
  • the conductive film 320 a is connected to the conductive film 304 in an opening 342 b and an opening 342 c provided in the insulating films 306 , 307 , 314 , 316 , and 318 .
  • the same potential is applied to the conductive film 320 a and the conductive film 304 .
  • the structure of the transistor 300 D is not limited to that described above, in which the openings 342 b and 342 c are provided so that the conductive film 320 a is connected to the conductive film 304 .
  • a structure in which only one of the openings 342 b and 342 c is provided so that the conductive film 320 a is connected to the conductive film 304 , or a structure in which the conductive film 320 a is not connected to the conductive film 304 without providing the openings 342 b and 342 c may be employed. Note that in the case where the conductive film 320 a is not connected to the conductive film 304 , it is possible to apply different potentials to the conductive film 320 a and the conductive film 304 .
  • the conductive film 320 b is connected to the conductive film 312 b through an opening 342 a provided in the insulating films 314 , 316 , and 318 .
  • transistor 300 D has the S-channel structure described above.
  • the oxide semiconductor film 308 included in the transistor 300 A in FIGS. 32A to 32C may have a stacked-layer structure.
  • FIGS. 36A and 36B and FIGS. 37A and 37B illustrate examples of such a case.
  • FIGS. 36A and 36B are cross-sectional views of a transistor 300 E and FIGS. 37A and 37B are cross-sectional views of a transistor 300 F.
  • the top views of the transistors 300 E and 300 F are similar to that of the transistor 300 A illustrated in FIG. 32A .
  • the oxide semiconductor film 308 of the transistor 300 E illustrated in FIGS. 36A and 36B includes an oxide semiconductor film 308 _ 1 , an oxide semiconductor film 3082 , and an oxide semiconductor film 3083 .
  • the oxide semiconductor film 308 of the transistor 300 F illustrated in FIGS. 37A and 37B includes the oxide semiconductor film 3082 and the oxide semiconductor film 3083 .
  • the conductive film 304 , the insulating film 306 , the insulating film 307 , the oxide semiconductor film 308 , the oxide semiconductor film 308 _ 1 , the oxide semiconductor film 3082 , the oxide semiconductor film 3083 , the conductive films 312 a and 312 b , the insulating film 314 , the insulating film 316 , the insulating film 318 , and the conductive films 320 a and 320 b can be formed using the materials of the conductive film 106 , the insulating film 116 , the oxide semiconductor film 108 , the oxide semiconductor film 108 _ 1 , the oxide semiconductor film 1082 , the oxide semiconductor film 108 _ 3 , the conductive films 120 a and 120 b , the insulating film 104 , the insulating film 118 , the insulating film 116 , and the conductive film 112 , respectively, described above.
  • FIG. 38A is a top view of a transistor 300 G.
  • FIG. 38B is a cross-sectional view taken along dashed-dotted line X 1 -X 2 in FIG. 38A .
  • FIG. 38C is a cross-sectional view taken along dashed-dotted line Y 1 -Y 2 in FIG. 38A .
  • the transistor 300 G illustrated in FIGS. 38A to 38C includes the conductive film 304 over the substrate 302 , the insulating film 306 over the substrate 302 and the conductive film 304 , the insulating film 307 over the insulating film 306 , the oxide semiconductor film 308 over the insulating film 307 , the conductive film 312 a over the oxide semiconductor film 308 , the conductive film 312 b over the oxide semiconductor film 308 , the insulating film 314 over the oxide semiconductor film 308 and the conductive films 312 a and 312 b , the insulating film 316 over the insulating film 314 , the conductive film 320 a over the insulating film 316 , and the conductive film 320 b over the insulating film 316 .
  • the insulating films 306 and 307 have an opening 351 .
  • a conductive film 312 c which is electrically connected to the conductive film 304 through the opening 351 , is formed over the insulating films 306 and 307 .
  • the insulating films 314 and 316 have an opening 352 a that reaches the conductive film 312 b and an opening 352 b that reaches the conductive film 312 c.
  • the oxide semiconductor film 308 includes the oxide semiconductor film 3082 on the conductive film 304 side and the oxide semiconductor film 3083 over the oxide semiconductor film 3082 .
  • the insulating film 318 is provided over the transistor 300 G.
  • the insulating film 318 is formed to cover the insulating film 316 , the conductive film 320 a , and the conductive film 320 b.
  • the insulating films 306 and 307 function as first gate insulating films of the transistor 300 G
  • the insulating films 314 and 316 function as second gate insulating films of the transistor 300 G
  • the insulating film 318 functions as a protective insulating film of the transistor 300 G.
  • the conductive film 304 functions as a first gate electrode
  • the conductive film 320 a functions as a second gate electrode
  • the conductive film 320 b functions as a pixel electrode used for a display device.
  • the conductive film 312 a functions as a source electrode
  • the conductive film 312 b functions as a drain electrode
  • the conductive film 312 c functions as a connection electrode.
  • transistor 300 G has the S-channel structure described above.
  • the structures of the transistors 300 A to 300 G can be freely combined with each other.
  • Examples of a display device that includes the semiconductor device described in the above embodiments are described below with reference to FIGS. 39 to 44 , FIGS. 45A to 45D , and FIG. 46 .
  • FIG. 39 is a top view illustrating an example of a display device.
  • a display device 700 in FIG. 39 includes a pixel portion 702 provided over a first substrate 701 , a source driver circuit portion 704 and a gate driver circuit portion 706 that are provided over the first substrate 701 , a sealant 712 provided to surround the pixel portion 702 , the source driver circuit portion 704 , and the gate driver circuit portion 706 , and a second substrate 705 provided to face the first substrate 701 .
  • the first substrate 701 and the second substrate 705 are sealed with the sealant 712 .
  • the pixel portion 702 , the source driver circuit portion 704 , and the gate driver circuit portion 706 are enclosed by the first substrate 701 , the sealant 712 , and the second substrate 705 .
  • a display element is provided between the first substrate 701 and the second substrate 705 .
  • a flexible printed circuit (FPC) terminal portion 708 that is electrically connected to the pixel portion 702 , the source driver circuit portion 704 , and the gate driver circuit portion 706 is provided in a region different from the region that is over the first substrate 701 and surrounded by the sealant 712 .
  • an FPC 716 is connected to the FPC terminal portion 708 , and a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702 , the source driver circuit portion 704 , and the gate driver circuit portion 706 .
  • a signal line 710 is connected to the pixel portion 702 , the source driver circuit portion 704 , the gate driver circuit portion 706 , and the FPC terminal portion 708 . Through the signal line 710 , a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702 , the source driver circuit portion 704 , the gate driver circuit portion 706 , and the FPC terminal portion 708 .
  • a plurality of gate driver circuit portions 706 may be provided in the display device 700 .
  • the structure of the display device 700 is not limited to the example shown here, in which the source driver circuit portion 704 and the gate driver circuit portion 706 as well as the pixel portion 702 are formed over the first substrate 701 .
  • only the gate driver circuit portion 706 may be formed over the first substrate 701 , or only the source driver circuit portion 704 may be formed over the first substrate 701 .
  • a substrate over which a source driver circuit, a gate driver circuit, or the like is formed e.g., a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film
  • a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film
  • the pixel portion 702 , the source driver circuit portion 704 , and the gate driver circuit portion 706 included in the display device 700 include a plurality of transistors.
  • the display device 700 can include a variety of elements.
  • electroluminescent (EL) element e.g., an EL element containing organic and inorganic materials, an organic EL element, an inorganic EL element, or an LED
  • a light-emitting transistor element a transistor that emits light depending on current
  • GLV grating light valve
  • DMD digital micromirror device
  • DMS digital micro shutter
  • MIMOD interferometric modulator display
  • An example of a display device including an EL element is an EL display.
  • Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display).
  • An example of a display device including a liquid crystal element is a liquid crystal display (a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display).
  • An example of a display device including an electronic ink display or an electrophoretic element is electronic paper.
  • some or all of pixel electrodes may function as reflective electrodes.
  • some or all of pixel electrodes may contain aluminum, silver, or the like.
  • a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.
  • a progressive system, an interlace system, or the like can be employed.
  • color elements controlled in pixels at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively).
  • R, G, and B correspond to red, green, and blue, respectively.
  • four pixels of an R pixel, a G pixel, a B pixel, and a W (white) pixel may be used.
  • a color element may be composed of two colors of R, G, and B as in PenTile layout. The two colors may differ depending on the color elements.
  • one or more colors of yellow, cyan, magenta, and the like may be added to RGB.
  • the size of a display region may differ between dots of color elements.
  • One embodiment of the disclosed invention is not limited to a color display device; the disclosed invention can also be applied to a monochrome display device.
  • a coloring layer may be used to obtain a full-color display device in which white light (W) is used for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp).
  • a backlight e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp.
  • a red (R) coloring layer, a green (G) coloring layer, a blue (B) coloring layer, and a yellow (Y) coloring layer can be combined as appropriate.
  • RGB red
  • G green
  • B blue
  • Y yellow
  • the coloring layer high color reproducibility can be obtained as compared with the case without the coloring layer.
  • white light in the region without the coloring layer may be directly utilized for display.
  • a decrease in the luminance of a bright image due to the coloring layer can be suppressed, and power consumption can be reduced by approximately 20% to 30% in some cases.
  • elements may emit light in their respective colors R, G, B, Y, and W.
  • power consumption may be further reduced as compared with the case of using a coloring layer.
  • any of the following systems may be used: the above-described color filter system in which part of white light is converted into red light, green light, and blue light through color filters; a three-color system in which red light, green light, and blue light are used; and a color conversion system or a quantum dot system in which part of blue light is converted into red light or green light.
  • FIG. 40 and FIG. 41 are each a cross-sectional view taken along dashed-dotted line Q-R in FIG. 39 and illustrate the structure including a liquid crystal element as a display element.
  • FIG. 42 is a cross-sectional view taken along dashed-dotted line Q-R in FIG. 39 and illustrates the structure including an EL element as a display element.
  • FIG. 40 Portions common to FIG. 40 , FIG. 41 , and FIG. 42 are described first, and then, different portions are described.
  • the display device 700 in FIG. 40 , FIG. 41 , and FIG. 42 includes a lead wiring portion 711 , the pixel portion 702 , the source driver circuit portion 704 , and the FPC terminal portion 708 .
  • the lead wiring portion 711 includes the signal line 710 .
  • the pixel portion 702 includes a transistor 750 and a capacitor 790 .
  • the source driver circuit portion 704 includes a transistor 752 .
  • the transistor 750 and the transistor 752 each have a structure similar to that of the transistor 100 A described above. Note that the transistor 750 and the transistor 752 may each have the structure of any of the other transistors described in the above embodiments.
  • the transistor used in this embodiment includes an oxide semiconductor film that is highly purified and in which formation of oxygen vacancies is inhibited.
  • the transistor can have a low off-state current. Accordingly, an electrical signal such as an image signal can be held for a long time, and a long writing interval can be set in an on state. Accordingly, the frequency of refresh operation can be reduced, which suppresses power consumption.
  • the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high-speed operation.
  • a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, no additional semiconductor device formed using a silicon wafer or the like is needed as a driver circuit; therefore, the number of components of the semiconductor device can be reduced.
  • the transistor capable of high-speed operation in the pixel portion a high-quality image can be provided.
  • the capacitor 790 includes a lower electrode and an upper electrode.
  • the lower electrode is formed through a step of processing a conductive film to be a conductive film functioning as a first gate electrode of the transistor 750 .
  • the upper electrode is formed through a step of processing a conductive film to be a conductive film functioning as source and drain electrodes of the transistor 750 or a second gate electrode of the transistor 750 .
  • an insulating film formed through a step of forming an insulating film to be an insulating film functioning as a first gate insulating film of the transistor 750 and insulating films formed through a step of forming insulating films to be insulating films functioning as protective insulating films over the transistor 750 are provided. That is, the capacitor 790 has a stacked-layer structure in which an insulating film functioning as a dielectric film is positioned between the pair of electrodes.
  • a planarization insulating film 770 is provided over the transistor 750 , the transistor 752 , and the capacitor 790 .
  • FIG. 40 , FIG. 41 , and FIG. 42 each illustrate an example in which the transistor 750 included in the pixel portion 702 and the transistor 752 included in the source driver circuit portion 704 have the same structure
  • the pixel portion 702 and the source driver circuit portion 704 may include different transistors. Specifically, a structure in which a top-gate transistor is used in the pixel portion 702 and a bottom-gate transistor is used in the source driver circuit portion 704 , or a structure in which a bottom-gate transistor is used in the pixel portion 702 and a top-gate transistor is used in the source driver circuit portion 704 may be employed. Note that the term “source driver circuit portion 704 ” can be replaced by the term “gate driver circuit portion.”
  • the signal line 710 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752 .
  • the signal line 710 is formed using a material containing a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.
  • the FPC terminal portion 708 includes a connection electrode 760 , an anisotropic conductive film 780 , and the FPC 716 .
  • the connection electrode 760 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752 .
  • the connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780 .
  • glass substrates can be used as the first substrate 701 and the second substrate 705 .
  • flexible substrates may also be used.
  • An example of the flexible substrate is a plastic substrate.
  • a structure 778 is provided between the first substrate 701 and the second substrate 705 .
  • the structure 778 is a columnar spacer obtained by selective etching of an insulating film and is provided to control the distance (cell gap) between the first substrate 701 and the second substrate 705 .
  • a spherical spacer may also be used as the structure 778 .
  • a light-blocking film 738 functioning as a black matrix, a coloring film 736 functioning as a color filter, and an insulating film 734 in contact with the light-blocking film 738 and the coloring film 736 are provided on the second substrate 705 side.
  • the display device 700 in FIG. 40 includes a liquid crystal element 775 .
  • the liquid crystal element 775 includes a conductive film 772 , a conductive film 774 , and a liquid crystal layer 776 .
  • the conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode.
  • the display device 700 in FIG. 40 can display an image in such a manner that transmission or non-transmission of light is controlled by the alignment state in the liquid crystal layer 776 that is changed depending on the voltage applied between the conductive film 772 and the conductive film 774 .
  • the conductive film 772 is electrically connected to the conductive film functioning as the source electrode or the drain electrode of the transistor 750 .
  • the conductive film 772 is formed over the planarization insulating film 770 and functions as a pixel electrode, that is, one electrode of the display element.
  • a conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film 772 .
  • a material containing an element selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light.
  • a material containing aluminum or silver is preferably used for the conductive film that reflects visible light.
  • the display device 700 is a reflective liquid crystal display device. In the case where a conductive film that transmits visible light is used as the conductive film 772 , the display device 700 is a transmissive liquid crystal display device.
  • the method for driving the liquid crystal element can be changed by changing the structure over the conductive film 772 , an example of this case is illustrated in FIG. 41 .
  • the display device 700 illustrated in FIG. 41 is an example of employing a horizontal electric field mode (e.g., an FFS mode) as a driving mode of the liquid crystal element.
  • a horizontal electric field mode e.g., an FFS mode
  • an insulating film 773 is provided over the conductive film 772
  • the conductive film 774 is provided over the insulating film 773 .
  • the conductive film 774 functions as a common electrode, and an electric field generated between the conductive film 772 and the conductive film 774 through the insulating film 773 can control the alignment state in the liquid crystal layer 776 .
  • the conductive film 772 and/or the conductive film 774 may be provided with an alignment film on a side in contact with the liquid crystal layer 776 .
  • an optical member (optical substrate) or the like such as a polarizing member, a retardation member, or an anti-reflection member, may be provided as appropriate.
  • circular polarization may be obtained by using a polarizing substrate and a retardation substrate.
  • a backlight, a sidelight, or the like may be used as a light source.
  • thermotropic liquid crystal In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. These liquid crystal materials exhibit a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.
  • a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used.
  • the blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase when the temperature of a cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range.
  • the liquid crystal composition containing a liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy, which eliminates the need for an alignment process.
  • An alignment film does not need to be provided, and thus, rubbing treatment is not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented, and defects and damage of a liquid crystal display device in the manufacturing process can be reduced. Moreover, the liquid crystal material that exhibits a blue phase has small viewing angle dependence.
  • a liquid crystal element In the case where a liquid crystal element is used as a display element, a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, or the like can be used.
  • TN twisted nematic
  • IPS in-plane switching
  • FFS fringe field switching
  • ASM axially symmetric aligned micro-cell
  • OBC optical compensated birefringence
  • FLC ferroelectric liquid crystal
  • AFLC anti-ferroelectric liquid crystal
  • a normally black liquid crystal display device such as a vertical alignment (VA) mode transmissive liquid crystal display device may also be used.
  • VA vertical alignment
  • a vertical alignment mode for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, and an ASV mode, or the like can be employed.
  • MVA multi-domain vertical alignment
  • PVA patterned vertical alignment
  • ASV ASV mode
  • the display device 700 illustrated in FIG. 42 includes a light-emitting element 782 .
  • the light-emitting element 782 includes a conductive film 772 , an EL layer 786 , and a conductive film 788 .
  • the display device 700 illustrated in FIG. 42 can display an image by utilizing light emission from the EL layer 786 of the light-emitting element 782 .
  • the EL layer 786 contains an organic compound or an inorganic compound such as a quantum dot.
  • Examples of materials that can be used for an organic compound include a fluorescent material and a phosphorescent material.
  • Examples of materials that can be used for a quantum dot include a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, and a core quantum dot material.
  • a material containing elements belonging to Groups 12 and 16, elements belonging to Groups 13 and 15, or elements belonging to Groups 14 and 16, may be used.
  • a quantum dot material containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.
  • an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.
  • the above-described organic compound and the inorganic compound can be deposited by a method such as an evaporation method (including a vacuum evaporation method), a droplet discharge method (also referred to as an ink-jet method), a coating method, or a gravure printing method.
  • a method such as an evaporation method (including a vacuum evaporation method), a droplet discharge method (also referred to as an ink-jet method), a coating method, or a gravure printing method.
  • a low molecular material, a middle molecular material (including an oligomer and a dendrimer), or a high molecular material may be included in the EL layer 786 .
  • FIGS. 45A to 45D are cross-sectional views illustrating the method for forming the EL layer 786 .
  • the conductive film 772 is formed over the planarization insulating film 770 , and an insulating film 730 is formed to cover part of the conductive film 772 (see FIG. 45A ).
  • a droplet 784 is discharged to an exposed portion of the conductive film 772 , which is an opening of the insulating film 730 , from a droplet discharge apparatus 783 , so that a layer 785 containing a composition is formed.
  • the droplet 784 is a composition containing a solvent and is attached to the conductive film 772 (see FIG. 45B ).
  • step of discharging the droplet 784 may be performed under reduced pressure.
  • the solvent is removed from the layer 785 containing the composition, and the resulting layer is solidified to form the EL layer 786 (see FIG. 45C ).
  • the solvent may be removed by drying or heating.
  • the conductive film 788 is formed over the EL layer 786 ; thus, the light-emitting element 782 is formed (see FIG. 45D ).
  • the composition can be selectively discharged; accordingly, waste of material can be reduced. Furthermore, a lithography process or the like for shaping is not needed, and thus, the process can be simplified and cost reduction can be achieved.
  • the droplet discharge method described above is a general term for a means including a nozzle equipped with a composition discharge opening or a means to discharge droplets such as a head having one or a plurality of nozzles.
  • FIG. 46 is a conceptual diagram illustrating a droplet discharge apparatus 1400 .
  • the droplet discharge apparatus 1400 includes a droplet discharge means 1403 .
  • the droplet discharge means 1403 is equipped with a head 1405 and a head 1412 .
  • the heads 1405 and 1412 are connected to a control means 1407 , and this control means 1407 is controlled by a computer 1410 ; thus, a preprogrammed pattern can be drawn.
  • the drawing may be conducted at a timing, for example, based on a marker 1411 formed over a substrate 1402 .
  • the reference point may be determined on the basis of an outer edge of the substrate 1402 .
  • the marker 1411 is detected by an imaging means 1404 and converted into a digital signal by an image processing means 1409 . Then, the digital signal is recognized by the computer 1410 , and then, a control signal is generated and transmitted to the control means 1407 .
  • An image sensor or the like using a charge coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) can be used as the imaging means 1404 .
  • CCD charge coupled device
  • CMOS complementary metal-oxide-semiconductor
  • information about a pattern to be formed over the substrate 1402 is stored in a storage medium 1408 , and a control signal is transmitted to the control means 1407 based on the information, so that each of the heads 1405 and 1412 of the droplet discharge means 1403 can be individually controlled.
  • the heads 1405 and 1412 are supplied with a material to be discharged from material supply sources 1413 and 1414 through pipes, respectively.
  • a space as indicated by a dotted line 1406 to be filled with a liquid material and a nozzle which is a discharge outlet are provided inside the head 1405 .
  • an inside structure of the head 1412 is similar to that of the head 1405 .
  • the nozzle sizes of the heads 1405 and 1412 are different from each other, different materials with different widths can be discharged simultaneously.
  • Each head can discharge and draw a plurality of light emitting materials. In the case of drawing over a large area, the same material can be simultaneously discharged to be drawn from a plurality of nozzles in order to improve throughput.
  • the heads 1405 and 1412 can freely scan the substrate in directions indicated by arrows X, Y, and Z in FIG. 46 , and a region in which a pattern is drawn can be freely set. Thus, a plurality of the same patterns can be drawn over one substrate.
  • a step of discharging the composition may be performed under reduced pressure.
  • a substrate may be heated when the composition is discharged.
  • either drying or baking or both of them are performed. Both the drying and baking are heat treatments but different in purpose, temperature, and time period.
  • the steps of drying and baking are performed under normal pressure or under reduced pressure by laser irradiation, rapid thermal annealing, heating using a heating furnace, or the like. Note that there is no particular limitation on the timing of the heat treatment and the number of times of the heat treatment.
  • the temperature for performing each of the steps of drying and baking in a favorable manner depends on the materials of the substrate and the properties of the composition.
  • the EL layer 786 can be formed with the droplet discharge apparatus.
  • the display device 700 shown in FIG. 42 is described again.
  • the insulating film 730 is provided over the planarization insulating film 770 and the conductive film 772 .
  • the insulating film 730 covers part of the conductive film 772 .
  • the light-emitting element 782 has a top-emission structure.
  • the conductive film 788 has a light-transmitting property and transmits light emitted from the EL layer 786 .
  • the top-emission structure is described as an example in this embodiment, the structure is not limited thereto.
  • a bottom-emission structure in which light is emitted to the conductive film 772 side or a dual-emission structure in which light is emitted to both the conductive film 772 side and the conductive film 788 side may also be employed.
  • the coloring film 736 is provided to overlap with the light-emitting element 782 , and the light-blocking film 738 is provided in the lead wiring portion 711 and the source driver circuit portion 704 to overlap with the insulating film 730 .
  • the coloring film 736 and the light-blocking film 738 are covered with the insulating film 734 .
  • a space between the light-emitting element 782 and the insulating film 734 is filled with a sealing film 732 .
  • the structure of the display device 700 is not limited to the example in FIG. 42 , in which the coloring film 736 is provided.
  • a structure without the coloring film 736 may also be employed in the case where the EL layer 786 is formed by separate coloring.
  • An input/output device may be provided in the display device 700 illustrated in FIG. 41 and FIG. 42 .
  • a touch panel or the like can be given.
  • FIG. 43 illustrates a structure in which the display device 700 illustrated in FIG. 41 includes a touch panel 791 .
  • FIG. 44 illustrates a structure in which the display device 700 illustrated in FIG. 42 includes the touch panel 791 .
  • FIG. 43 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 41
  • FIG. 44 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 42 .
  • the touch panel 791 illustrated in FIG. 43 and FIG. 44 is what is called an in-cell touch panel provided between the second substrate 705 and the coloring film 736 .
  • the touch panel 791 is formed on the second substrate 705 side before the coloring film 736 is formed.
  • the touch panel 791 includes the light-blocking film 738 , an insulating film 792 , an electrode 793 , an electrode 794 , an insulating film 795 , an electrode 796 , and an insulating film 797 .
  • a change in the capacitance between the electrodes 793 and 794 can be detected when an object such as a finger or a stylus approaches, for example.
  • a portion in which the electrode 793 intersects with the electrode 794 is illustrated in the upper portion of the transistor 750 illustrated in FIG. 43 and FIG. 44 .
  • the electrode 796 is electrically connected to the two electrodes 793 between which the electrode 794 is sandwiched through openings provided in the insulating film 795 .
  • a structure in which a region where the electrode 796 is provided is provided in the pixel portion 702 is illustrated in FIG. 43 and FIG. 44 as an example; however, one embodiment of the present invention is not limited thereto.
  • the region where the electrode 796 is provided may be provided in the source driver circuit portion 704 .
  • the electrode 793 and the electrode 794 are provided in a region overlapping with the light-blocking film 738 . As illustrated in FIG. 43 , it is preferable that the electrode 793 not overlap with the light-emitting element 782 . As illustrated in FIG. 44 , it is preferable that the electrode 793 not overlap with the liquid crystal element 775 . In other words, the electrode 793 has an opening in a region overlapping with the light-emitting element 782 and the liquid crystal element 775 . That is, the electrode 793 has a mesh shape. With such a structure, the electrode 793 does not block light emitted from the light-emitting element 782 , or alternatively the electrode 793 does not block light transmitted through the liquid crystal element 775 . Thus, since luminance is hardly reduced even when the touch panel 791 is provided, a display device with high visibility and low power consumption can be obtained. Note that the electrode 794 can have a structure similar to that of the electrode 793 .
  • the electrode 793 and the electrode 794 do not overlap with the light-emitting element 782 , a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794 . Furthermore, since the electrode 793 and the electrode 794 do not overlap with the liquid crystal element 775 , a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794 .
  • a conductive nanowire may be used for the electrodes 793 , 794 , and 796 .
  • the nanowire may have a mean diameter of greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, further preferably greater than or equal to 5 nm and less than or equal to 25 nm.
  • a carbon nanotube or a metal nanowire such as an Ag nanowire, a Cu nanowire, or an Al nanowire may be used.
  • the transmittance of visible light can be greater than or equal to 89% and the sheet resistance can be greater than or equal to 40 ⁇ /sq. and less than or equal to 100 ⁇ /sq.
  • FIG. 43 and FIG. 44 Although the structure of the in-cell touch panel is illustrated in FIG. 43 and FIG. 44 , one embodiment of the present invention is not limited thereto.
  • a touch panel formed over the display device 700 what is called an on-cell touch panel, or a touch panel attached to the display device 700 , what is called an out-cell touch panel may be used.
  • the display device of one embodiment of the present invention can be combined with various types of touch panels.
  • FIGS. 47A to 47C a display device including a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 47A to 47C .
  • a display device illustrated in FIG. 47A includes a region including pixels of display elements (hereinafter referred to as a pixel portion 502 ), a circuit portion that is provided outside the pixel portion 502 and includes a circuit for driving the pixels (hereinafter, the circuit portion is referred to as a driver circuit portion 504 ), circuits having a function of protecting elements (hereinafter, the circuits are referred to as protection circuits 506 ), and a terminal portion 507 . Note that the protection circuits 506 are not necessarily provided.
  • Part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed. Thus, the number of components and the number of terminals can be reduced.
  • the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).
  • the pixel portion 502 includes a plurality of circuits for driving display elements arranged in X (X is a natural number of 2 or more) rows and Y (Y is a natural number of 2 or more) columns (hereinafter, the circuits are referred to as pixel circuits 501 ).
  • the driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a ) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b ).
  • the gate driver 504 a includes a shift register or the like.
  • the gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal.
  • the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal.
  • the gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter referred to as scan lines GL_ 1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_ 1 to GL_X separately.
  • the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the gate driver 504 a.
  • the source driver 504 b includes a shift register or the like.
  • the source driver 504 b receives a signal (image signal) from which a data signal is generated, as well as a signal for driving the shift register, through the terminal portion 507 .
  • the source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 from the image signal.
  • the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like.
  • the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter referred to as data lines DL_ 1 to DL_Y).
  • the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the source driver 504 b.
  • the source driver 504 b includes a plurality of analog switches, for example.
  • the source driver 504 b can output, as data signals, time-divided image signals obtained by sequentially turning on the plurality of analog switches.
  • the source driver 504 b may include a shift register or the like.
  • a pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively.
  • Writing and holding of the data signal in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a .
  • a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.
  • the protection circuit 506 in FIG. 47A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501 .
  • the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501 .
  • the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507 .
  • the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507 .
  • the terminal portion 507 refers to a portion having terminals for inputting power, control signals, and image signals from external circuits to the display device.
  • the protection circuit 506 electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is supplied to the wiring connected to the protection circuit.
  • the protection circuits 506 provided for the pixel portion 502 and the driver circuit portion 504 can improve the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like.
  • ESD electrostatic discharge
  • the configuration of the protection circuits 506 is not limited thereto; for example, the protection circuit 506 can be connected to the gate driver 504 a or the source driver 504 b . Alternatively, the protection circuit 506 can be connected to the terminal portion 507 .
  • the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b .
  • the gate driver 504 a may be formed, and a separately prepared substrate over which a source driver circuit is formed (e.g., a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.
  • Each of the plurality of pixel circuits 501 in FIG. 47A can have the configuration illustrated in FIG. 47B , for example.
  • the pixel circuit 501 in FIG. 47B includes a liquid crystal element 570 , a transistor 550 , and a capacitor 560 .
  • the transistor 550 the transistor described in the above embodiment can be used.
  • the potential of one of a pair of electrodes of the liquid crystal element 570 is set as appropriate in accordance with the specifications of the pixel circuit 501 .
  • the alignment state of the liquid crystal element 570 depends on data written thereto.
  • a common potential may be supplied to the one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501 .
  • the potential supplied to the one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 may differ between rows.
  • Examples of a method for driving the display device including the liquid crystal element 570 include a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, and a transverse bend alignment (TBA) mode.
  • ECB electrically controlled birefringence
  • PDLC polymer-dispersed liquid crystal
  • PNLC polymer network liquid crystal
  • guest-host mode an electrically controlled birefringence (ECB) mode
  • EMB electrically controlled birefringence
  • PDLC polymer-dispersed liquid crystal
  • PNLC polymer network liquid crystal
  • guest-host mode an electrically controlled birefringence (ECB) mode
  • PDLC polymer-dispersed liquid crystal
  • PNLC polymer network liquid crystal
  • one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the data line DL_n, and the other of the source electrode and the drain electrode of the transistor 550 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570 .
  • a gate electrode of the transistor 550 is electrically connected to the scan line GL_m.
  • the transistor 550 is configured to be turned on or off to control whether a data signal is written.
  • One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other of the pair of electrodes of the capacitor 560 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570 .
  • the potential of the potential supply line VL is set as appropriate in accordance with the specifications of the pixel circuit 501 .
  • the capacitor 560 functions as a storage capacitor for storing written data.
  • the gate driver 504 a in FIG. 47A sequentially selects the pixel circuits 501 row by row to turn on the transistors 550 , and data signals are written.
  • the transistor 550 When the transistor 550 is turned off, the pixel circuit 501 to which the data has been written is brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.
  • each of the plurality of pixel circuits 501 in FIG. 47A can have the configuration illustrated in FIG. 47C , for example.
  • the pixel circuit 501 in FIG. 47C includes transistors 552 and 554 , a capacitor 562 , and a light-emitting element 572 .
  • the transistor described in the above embodiment can be used as the transistor 552 and/or the transistor 554 .
  • One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring through which a data signal is supplied (hereinafter referred to as a data line DL_n).
  • a gate electrode of the transistor 552 is electrically connected to a wiring through which a gate signal is supplied (hereinafter referred to as a scan line GL_m).
  • the transistor 552 is configured to be turned on or off to control whether a data signal is written.
  • One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other of the pair of electrodes of the capacitor 562 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552 .
  • the capacitor 562 functions as a storage capacitor for storing written data.
  • One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a.
  • a gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552 .
  • One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other of the anode and the cathode of the light-emitting element 572 is electrically connected to the other of the source electrode and the drain electrode of the transistor 554 .
  • an organic electroluminescent element also referred to as an organic EL element
  • the light-emitting element 572 is not limited thereto and may be an inorganic EL element including an inorganic material.
  • a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential V SS is supplied to the other of the potential supply line VL_a and the potential supply line VL_b.
  • the gate driver 504 a in FIG. 47A sequentially selects the pixel circuits 501 row by row to turn on the transistors 552 , and data signals are written.
  • the pixel circuit 501 to which the data has been written is brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal.
  • the light-emitting element 572 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.
  • FIG. 48 a display module and electronic devices, each of which includes a semiconductor device of one embodiment of the present invention, are described with reference to FIG. 48 , FIGS. 49A to 49E , FIGS. 50A to 50G , and FIGS. 51A and 51B .
  • a touch panel 7004 connected to an FPC 7003 a display panel 7006 connected to an FPC 7005 , a backlight 7007 , a frame 7009 , a printed-circuit board 7010 , and a battery 7011 are provided between an upper cover 7001 and a lower cover 7002 .
  • the semiconductor device of one embodiment of the present invention can be used for the display panel 7006 , for example.
  • the shapes and sizes of the upper cover 7001 and the lower cover 7002 can be changed as appropriate in accordance with the sizes of the touch panel 7004 and the display panel 7006 .
  • the touch panel 7004 can be a resistive touch panel or a capacitive touch panel and overlap with the display panel 7006 .
  • a counter substrate (sealing substrate) of the display panel 7006 can have a touch panel function.
  • a photosensor may be provided in each pixel of the display panel 7006 to form an optical touch panel.
  • the backlight 7007 includes a light source 7008 .
  • One embodiment of the present invention is not limited to the structure in FIG. 48 , in which the light source 7008 is provided over the backlight 7007 .
  • a structure in which the light source 7008 is provided at an end portion of the backlight 7007 and a light diffusion plate is further provided may be employed.
  • the backlight 7007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.
  • the frame 7009 protects the display panel 7006 and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed-circuit board 7010 .
  • the frame 7009 may also function as a radiator plate.
  • the printed-circuit board 7010 includes a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal.
  • a power source for supplying power to the power supply circuit an external commercial power source or the separate battery 7011 may be used.
  • the battery 7011 can be omitted in the case where a commercial power source is used.
  • the display module 7000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.
  • FIGS. 49A to 49E illustrate examples of electronic devices.
  • FIG. 49A is an external view of a camera 8000 to which a finder 8100 is attached.
  • the camera 8000 includes a housing 8001 , a display portion 8002 , an operation button 8003 , a shutter button 8004 , and the like. Furthermore, an attachable lens 8006 is attached to the camera 8000 .
  • the lens 8006 of the camera 8000 here is detachable from the housing 8001 for replacement, the lens 8006 may be included in the housing 8001 .
  • Images can be taken with the camera 8000 at the press of the shutter button 8004 .
  • images can be taken at the touch of the display portion 8002 that serves as a touch panel.
  • the housing 8001 of the camera 8000 includes a mount including an electrode, so that the finder 8100 , a stroboscope, or the like can be connected to the housing 8001 .
  • the finder 8100 includes a housing 8101 , a display portion 8102 , a button 8103 , and the like.
  • the housing 8101 includes a mount for engagement with the mount of the camera 8000 so that the finder 8100 can be connected to the camera 8000 .
  • the mount includes an electrode, and an image or the like received from the camera 8000 through the electrode can be displayed on the display portion 8102 .
  • the button 8103 serves as a power button.
  • the display portion 8102 can be turned on and off with the button 8103 .
  • a display device of one embodiment of the present invention can be used in the display portion 8002 of the camera 8000 and the display portion 8102 of the finder 8100 .
  • the housing 8001 of the camera 8000 may include a finder having a display device.
  • FIG. 49B is an external view of a head-mounted display 8200 .
  • the head-mounted display 8200 includes a mounting portion 8201 , a lens 8202 , a main body 8203 , a display portion 8204 , a cable 8205 , and the like.
  • the mounting portion 8201 includes a battery 8206 .
  • the main body 8203 includes a wireless receiver or the like to receive video data, such as image data, and display it on the display portion 8204 .
  • video data such as image data
  • the movement of the eyeball and the eyelid of a user is captured by a camera in the main body 8203 and then coordinates of the points the user looks at are calculated using the captured data to utilize the eye of the user as an input means.
  • the mounting portion 8201 may include a plurality of electrodes so as to be in contact with the user.
  • the main body 8203 may be configured to sense current flowing through the electrodes with the movement of the user's eyeball to recognize the direction of his or her eyes.
  • the main body 8203 may be configured to sense current flowing through the electrodes to monitor the user's pulse.
  • the mounting portion 8201 may include sensors, such as a temperature sensor, a pressure sensor, or an acceleration sensor so that the user's biological information can be displayed on the display portion 8204 .
  • the main body 8203 may be configured to sense the movement of the user's head or the like to move an image displayed on the display portion 8204 in synchronization with the movement of the user's head or the like.
  • the display device of one embodiment of the present invention can be used in the display portion 8204 .
  • FIGS. 49C to 49E are external views of a head-mounted display 8300 .
  • the head-mounted display 8300 includes a housing 8301 , a display portion 8302 , an object for fixing, such as a band, 8304 , and a pair of lenses 8305 .
  • a user can see display on the display portion 8302 through the lenses 8305 . It is favorable that the display portion 8302 be curved. When the display portion 8302 is curved, a user can feel high realistic sensation of images.
  • the structure described in this embodiment as an example has one display portion 8302 , the number of the display portions 8302 provided is not limited to one. For example, two display portions 8302 may be provided, in which case one display portion is provided for one corresponding user's eye, so that three-dimensional display using parallax or the like is possible.
  • the display device of one embodiment of the present invention can be used in the display portion 8302 .
  • the display device including the semiconductor device of one embodiment of the present invention has an extremely high resolution; thus, even when an image is magnified using the lenses 8305 as illustrated in FIG. 49E , the user does not perceive pixels, and thus a more realistic image can be displayed.
  • FIGS. 50A to 50G illustrate examples of electronic devices that are different from those illustrated in FIGS. 49A to 49E .
  • Electronic devices illustrated in FIGS. 50A to 50G include a housing 9000 , a display portion 9001 , a speaker 9003 , an operation key 9005 (including a power switch or an operation switch), a connection terminal 9006 , a sensor 9007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008 , and the like.
  • a sensor 9007 a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray
  • a microphone 9008
  • the electronic devices in FIGS. 50A to 50G have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a memory medium and displaying it on the display portion.
  • the electronic devices 50A to 50G are not limited thereto, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 50A to 50G , the electronic devices may each have a plurality of display portions. Furthermore, the electronic devices may each be provided with a camera and the like to have a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.
  • a memory medium an external memory medium or a memory medium incorporated in the camera
  • FIGS. 50A to 50G are described in detail below.
  • FIG. 50A is a perspective view illustrating a television device 9100 .
  • the television device 9100 can include the display portion 9001 having a large screen size of, for example, 50 inches or more, or 100 inches or more.
  • FIG. 50B is a perspective view of a portable information terminal 9101 .
  • the portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system.
  • the portable information terminal 9101 can be used as a smartphone.
  • the portable information terminal 9101 may include a speaker, a connection terminal, a sensor, or the like.
  • the portable information terminal 9101 can display text and image information on its plurality of surfaces.
  • three operation buttons 9050 also referred to as operation icons or simply as icons
  • information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001 .
  • Examples of the information 9051 include display indicating reception of an e-mail, a social networking service (SNS) message, or a telephone call, the title and sender of an e-mail or an SNS message, date, time, remaining battery, and reception strength of an antenna.
  • the operation buttons 9050 or the like may be displayed in place of the information 9051 .
  • FIG. 50C is a perspective view of a portable information terminal 9102 .
  • the portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001 .
  • information 9052 , information 9053 , and information 9054 are displayed on different surfaces.
  • a user of the portable information terminal 9102 can see the display (here, the information 9053 ) on the portable information terminal 9102 put in a breast pocket of his/her clothes.
  • a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102 .
  • the user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.
  • FIG. 50D is a perspective view of a watch-type portable information terminal 9200 .
  • the portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game.
  • the display surface of the display portion 9001 is curved, and display can be performed on the curved display surface.
  • the portable information terminal 9200 can employ near field communication conformable to a communication standard. For example, hands-free calling can be achieved by mutual communication between the portable information terminal 9200 and a headset capable of wireless communication.
  • the portable information terminal 9200 includes the connection terminal 9006 and can perform direct data communication with another information terminal via a connector. Charging through the connection terminal 9006 is also possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006 .
  • FIGS. 50E, 50F, and 50G are perspective views of a foldable portable information terminal 9201 that is opened, that is shifted from the opened state to the folded state or from the folded state to the opened state, and that is folded, respectively.
  • the portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable.
  • the display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined by hinges 9055 . By being folded at the hinges 9055 between the two adjacent housings 9000 , the portable information terminal 9201 can be reversibly changed in shape from the opened state to the folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature greater than or equal to 1 mm and less than or equal to 150 mm.
  • FIGS. 51A and 51B are perspective views of a display device including a plurality of display panels. The plurality of display panels are wound in the perspective view in FIG. 51A and are unwound in the perspective view in FIG. 51B .
  • a display device 9500 illustrated in FIGS. 51A and 51B includes a plurality of display panels 9501 , a hinge 9511 , and a bearing 9512 .
  • the plurality of display panels 9501 each include a display region 9502 and a light-transmitting region 9503 .
  • Each of the plurality of display panels 9501 is flexible. Two adjacent display panels 9501 are provided so as to partly overlap with each other. For example, the light-transmitting regions 9503 of the two adjacent display panels 9501 can overlap with each other. A display device having a large screen can be obtained with the plurality of display panels 9501 .
  • the display device is highly versatile because the display panels 9501 can be wound depending on its use.
  • the display regions 9502 of the adjacent display panels 9501 are separated from each other in FIGS. 51A and 51B , without limitation to this structure, the display regions 9502 of the adjacent display panels 9501 may overlap with each other without any space so that a continuous display region 9502 is obtained, for example.
  • the electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information.
  • the semiconductor device of one embodiment of the present invention can also be used for an electronic device that does not have a display portion.
  • the IGZO film was formed in an atmosphere including an argon gas at 200 sccm, where the pressure was controlled to 0.6 Pa, the substrate temperature was room temperature, and an alternating-current power of 2.5 kW was applied.
  • the formed IGZO film of the sample was observed by HAADF-STEM and was subjected to measurement using EDX.
  • EDX atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.
  • the HAADF-STEM image was taken and the EDX measurement was performed under conditions where the acceleration voltage was 200 kV, and irradiation with an electron beam with a diameter of approximately 0.1 nm ⁇ was performed.
  • an energy dispersive X-ray spectrometer JED- 2300 T was used as an elemental analysis apparatus.
  • a Si drift detector was used to detect X-rays emitted from the sample.
  • an EDX spectrum of a point is obtained in such a manner that electron beam irradiation is performed on the point in an analysis target region of a sample, and the energy of characteristic X-rays of the sample generated by the irradiation and its frequency are measured.
  • peaks of an EDX spectrum of the point were attributed to electron transition to the L shell in an In atom, electron transition to the K shell in a Ga atom, and electron transition to the K shell in a Zn atom and the K shell in an O atom, and the proportions of the atoms in the point were calculated.
  • An EDX mapping image indicating distributions of the proportions of the atoms can be obtained through this process in an analysis target region of the sample.
  • FIGS. 52A to 52E and FIGS. 53A to 53E show the HAADF-STEM images and EDX mapping images of the IGZO film of the sample.
  • FIGS. 52A to 52E show the HAADF-STEM images and EDX mapping images of a plane of the IGZO film
  • FIGS. 53A to 53E show those of a cross section of the IGZO film.
  • FIG. 52A and FIG. 53A are the HAADF-STEM images of the sample.
  • FIG. 52B and FIG. 53B are EDX mapping images of O atoms
  • FIG. 52C and FIG. 53C are EDX mapping images of Zn atoms
  • FIG. 52D and FIG. 53D are EDX mapping images of Ga atoms
  • FIG. 52E and FIG. 53E are EDX mapping images of In atoms. Note that the HAADF-STEM images and EDX mapping images in FIGS. 52A to 52E and FIGS. 53A to 53E were taken at a magnification of 7,200,000 times.
  • FIGS. 52B to 52E and FIGS. 53B to 53E show relative distribution of brightness indicating that the atoms have distributions in the IGZO film.
  • attention is focused on Outlined Square 1 A and Outlined Square 1 B in FIGS. 52B to 52E and Outlined Square 2 A and Outlined Square 2 B in FIGS. 53B to 53E .
  • a relatively bright region occupies a large area in each of Outlined Squares 1 A and 2 A
  • a relatively dark region occupies a large area in each of Outlined Squares 1 B and 2 B. That is, the number of In atoms is relatively large in the regions indicated by Outlined Squares 1 A and 2 A and the number of In atoms is relatively small in the regions indicated by Outlined Squares 1 B and 2 B.
  • the relatively bright regions correspond to Regions A 1 described in the above embodiment
  • the relatively dark regions correspond to Regions B 1 described in the above embodiment.
  • a relatively dark region occupies a large area in each of Outlined Squares 1 A and 2 A
  • a relatively bright region occupies a large area in each of Outlined Squares 1 B and 2 B. That is, the number of Ga atoms is relatively small in the regions indicated by Outlined Squares 1 A and 2 A and the number of Ga atoms is relatively large in the regions indicated by Outlined Squares 1 B and 2 B.
  • the number of Ga atoms tends to be relatively small in the region including a relatively large number of In atoms
  • the number of Ga atoms tends to be relatively large in the region including a relatively small number of In atoms. Accordingly, in FIG. 52D and FIG. 53D , the relatively bright regions roughly correspond to Regions B 1 described in the above embodiment, and the relatively dark regions roughly correspond to Regions A 1 described in the above embodiment.
  • a relatively bright region occupies a large area in each of Outlined Square 1 B and Outlined Square 2 B, while Outlined Square 1 A and Outlined Square 2 A each include a bright region whose area is not as large as that of the relatively bright region in Outlined Square 1 B and Outlined Square 2 B.
  • the number of Zn atoms is relatively large in the regions indicated by Outlined Square 1 B and Outlined Square 2 B, and the number of Zn atoms in the regions indicated by Outlined Square 1 A and Outlined Square 2 A is not as large as that in the regions indicated by Outlined Square 1 B and Outlined Square 2 B.
  • Region A 1 of the IGZO film includes a large number of In atoms and a large number of O atoms and also includes a smaller number of Zn atoms than Region B 1 . It is thus suggested that Region A 1 has a high content of, for example, indium, indium oxide, and an In—Zn oxide. Accordingly, Region A 1 serves as a region having higher conductivity than Region B 1 , so that Region A 1 contributes to increases in field-effect mobility and on-state current of a transistor.
  • Region A 1 e.g., Outlined Square 1 A and Outlined Square 2 A
  • Region A 1 Observation of the particulate portions shows that they have a diameter of greater than or equal to 0.5 nm and less than or equal to 1.5 nm.
  • Regions A 1 appear to be formed by the plurality of particulate portions that are connected to each other. In this manner, Regions A 1 extend in a cloud-like manner.
  • the particulate portions included in Regions A 1 correspond to the clusters in Regions A 1 described in the above embodiment.
  • Region B 1 of the IGZO film includes a large number of Ga atoms, a large number of Zn atoms, and a large number of 0 atoms and also includes a smaller number of In atoms than Region A 1 . It is thus suggested that Region B 1 has a high content of, for example, an In-Ga—Zn oxide. Accordingly, Region B 1 serves as a region having a higher semiconductor property than Region A 1 , so that Region B 1 contributes to the switching characteristics of a transistor.
  • Regions B 1 e.g., Outlined Square 1 B and Outlined Square 2 B
  • Regions B 1 also appear to be formed by the plurality of particulate portions that are connected to each other. In this manner, Regions B 1 also extend in a cloud-like manner.
  • the particulate portions included in Regions B 1 correspond to the clusters in Regions B 1 described in the above embodiment.
  • the IGZO film of the sample fabricated in this example is a composite oxide semiconductor where In-rich Regions A 1 and In-poor Regions B 1 are formed.
  • Region A 1 contributes to the on-state current and field-effect mobility of a transistor
  • Region B 1 contributes to the switching characteristics of a transistor. Therefore, with the use of the composite oxide semiconductor, a transistor with electrical characteristics in which the on-state current and the mobility are high and the S value is small can be manufactured.
  • composite oxide semiconductors were deposited and their crystallinities were examined by XRD.
  • samples including oxide semiconductor films were fabricated with different oxygen flow rate percentages and substrate temperatures during the deposition.
  • Each sample was fabricated by forming an oxide semiconductor film over a 600 mm ⁇ 720 mm glass substrate.
  • 18 kinds of samples were fabricated in total using three substrate temperatures and six oxygen flow rate percentages.
  • the substrate temperature during the deposition was room temperature, 130° C., or 170° C.
  • the oxygen flow rate percentage was 0%, 10%, 30%, 50%, 70%, or 100%.
  • the proportions of an oxygen gas and an argon gas were varied such that the sum of the oxygen gas flow rate and the argon gas flow rate was 200 sccm.
  • the XRD analysis was conducted by a powder method (also referred to as a ⁇ -2 ⁇ method) which is a kind of an out-of-plane method.
  • a powder method also referred to as a ⁇ -2 ⁇ method
  • X-ray diffraction intensity is measured while an incident angle of an X-ray is changed and the angle of a detector facing an X-ray source is equal to the incident angle.
  • a grazing-incidence XRD (GIXRD) method also referred to as a thin film method or a Seemann-Bohlin method
  • the GIXRD method is a kind of an out-of-plane method for measuring X-ray diffraction intensity in which X-ray is incident at an angle approximately 0.40° from a film surface with use of a variable-angle detector.
  • FIG. 54 shows the results of the XRD analysis conducted on the samples.
  • the horizontal axis represents the diffraction angle 219 and the vertical axis represents diffraction intensity (any unit).
  • Each of the graphs in FIG. 54 shows three diffraction profiles obtained from the respective measurement points. In each graph, the upper and lower profiles were obtained from the center of the substrate (A) and the periphery of the substrate (C), respectively, and the middle profile was obtained from a point between A and C.
  • the samples from which the peak of the diffraction intensity was observed include a crystal part where the c-axes are aligned in the thickness direction (hereinafter also referred to as crystal part having orientation).
  • the crystallinity of an oxide semiconductor film to be deposited can be controlled with the oxygen flow rate percentage and substrate temperature during the deposition. It was also found that the crystallinity can be high by setting the substrate temperature high even when the oxygen flow rate percentage is 0%. Moreover, it was found that the crystallinity can be high by setting the oxygen flow rate percentage high even when the substrate temperature is room temperature.
  • Sample S 1 is a sample in which a transistor with a channel length L of 3 ⁇ m and a channel width W of 50 ⁇ m is formed.
  • a 10-nm-thick titanium film and a 100-nm-thick copper film were formed over a glass substrate with a sputtering apparatus.
  • the conductive film was processed by a photolithography method.
  • a stack including four insulating films was formed over the substrate and the conductive film.
  • the insulating films were formed in succession in a vacuum with a plasma-enhanced chemical deposition (PECVD) apparatus.
  • PECVD plasma-enhanced chemical deposition
  • As the insulating films a 50-nm-thick silicon nitride film, a 300-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film, and a 50-nm-thick silicon oxynitride film were used and stacked in this order.
  • oxide semiconductor film was formed over the insulating films and was processed into an island shape, whereby a semiconductor layer was formed.
  • a 40-nm-thick oxide semiconductor film was formed as the oxide semiconductor film 108 .
  • the oxide semiconductor film is the above-described composite oxide semiconductor or C/IGZO.
  • the oxygen flow rate percentage in deposition of the oxide semiconductor film was 0%.
  • an insulating film was formed over the insulating films and the semiconductor layer.
  • a 150-nm-thick silicon oxynitride film was formed with a PECVD apparatus.
  • the heat treatment was performed at 350° C. for one hour in a mixed gas atmosphere of nitrogen and oxygen.
  • An opening was formed in a desired region of the insulating film.
  • the opening was formed by a dry etching method.
  • a conductive film was formed over the insulating film and in the opening and the conductive film was processed into an island shape.
  • the insulating film in contact with the bottom surface of the conductive film was processed in succession after the formation of the conductive film, whereby the insulating film was formed.
  • the conductive film As the conductive film, a 10-nm-thick oxide semiconductor film, a 50-nm-thick titanium nitride film, and a 100-nm-thick copper film were formed in this order.
  • the titanium nitride film and the copper film were each formed using a sputtering apparatus.
  • plasma treatment was performed from above the semiconductor layer, the insulating film, and the conductive film.
  • the plasma treatment was performed with a PECVD apparatus at a substrate temperature of 220° C. in a mixed gas atmosphere containing an argon gas and a nitrogen gas.
  • an insulating film was formed over the semiconductor layer, the insulating film, and the conductive film.
  • the insulating film was formed by stacking a 100-nm-thick silicon nitride film and a 300-nm-thick silicon oxynitride film with a PECVD apparatus.
  • a mask was formed over the formed insulating film and an opening was formed in the insulating film with use of the mask.
  • a conductive film was formed to fill the opening and was processed into an island shape, whereby the conductive films serving as a source electrode and a drain electrode were formed.
  • the conductive films a 10-nm-thick titanium film and a 100-nm-thick copper film were formed with a sputtering apparatus.
  • an insulating film was formed over the insulating film and the conductive film.
  • a 1.5- ⁇ m-thick acrylic photosensitive resin was used for the insulating film.
  • I d -V g characteristics of the fabricated transistor of Sample S 1 were measured.
  • a voltage applied to the conductive film serving as a first gate electrode hereinafter the voltage is also referred to as gate voltage (V g )
  • a voltage applied to the conductive film serving as a second gate electrode hereinafter the voltage is also referred to as back gate voltage (V bg )
  • V g gate voltage
  • V bg back gate voltage
  • a voltage applied to the conductive film serving as the source electrode (hereinafter the voltage is also referred to as source voltage (V s )) was 0 V (comm) and voltages applied to the conductive film serving as the drain electrode (hereinafter the voltages are also referred to as drain voltage (V d )) were 0.1 V and 20 V.
  • FIG. 55 shows the results of I d -V g characteristics of Sample S 1 .
  • the first vertical axis represents I d [A]
  • the second vertical axis represents field-effect mobility ( ⁇ FE ) [cm 2 /Vs]
  • the horizontal axis represents V g [V]. Note that the field-effect mobility was measured when V d was 20 V.
  • FIG. 55 shows the field-effect mobility in the range where V g is lower than or equal to 7.5 V as the field-effect mobility estimated from such I d -V g characteristics.
  • the transistor which is a semiconductor device of one embodiment of the present invention, has excellent electrical characteristics as shown in FIG. 55 .
  • Table 1 lists the transistor characteristics that are shown in FIG. 55 .
  • the field-effect mobility of the transistor that is a semiconductor device of one embodiment of the present invention exceeds 100 cm 2 /Vs.
  • This field-effect mobility is equivalent to that of a transistor including low-temperature polysilicon and means extraordinary characteristics for a transistor using an oxide semiconductor.
  • a transistor including the composite oxide semiconductor or C/IGZO in its semiconductor layer can have both high carrier mobility and excellent switching characteristics.
  • Sample S 2 is a sample in which a transistor with a channel length L of 2 ⁇ m and a channel width W of 3 ⁇ m is formed.
  • a 10-nm-thick titanium film and a 100-nm-thick copper film were formed over a glass substrate with a sputtering apparatus.
  • the conductive film was processed by a photolithography method.
  • a stack including four insulating films was formed over the substrate and the conductive film.
  • the insulating films were formed in succession in a vacuum with a plasma-enhanced chemical deposition (PECVD) apparatus.
  • PECVD plasma-enhanced chemical deposition
  • As the insulating films a 50-nm-thick silicon nitride film, a 100-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film, and a 50-nm-thick silicon oxynitride film were used and stacked in this order.
  • oxide semiconductor film was formed over the insulating films and was processed into an island shape, whereby a semiconductor layer was formed.
  • a 40-nm-thick oxide semiconductor film was formed as the oxide semiconductor film 108 .
  • the oxide semiconductor film is the above-described composite oxide semiconductor or C/IGZO.
  • the oxygen flow rate percentage in deposition of the oxide semiconductor film was 10%.
  • an insulating film was formed over the insulating films and the semiconductor layer.
  • a 150-nm-thick silicon oxynitride film was formed with a PECVD apparatus.
  • the heat treatment was performed at 350° C. for one hour in a mixed gas atmosphere of nitrogen and oxygen.
  • An opening was formed in a desired region of the insulating film.
  • the opening was formed by a dry etching method.
  • a conductive film was formed over the insulating film and in the opening and the conductive film was processed into an island shape.
  • the insulating film in contact with the bottom surface of the conductive film was processed in succession after the formation of the conductive film, whereby the insulating film was formed.
  • the conductive film As the conductive film, a 10-nm-thick first oxide semiconductor film and a 90-nm-thick second oxide semiconductor film were formed in this order.
  • plasma treatment was performed from above the semiconductor layer, the insulating film, and the conductive film.
  • the plasma treatment was performed with a PECVD apparatus at a substrate temperature of 220° C. in a mixed gas atmosphere containing an argon gas and a nitrogen gas.
  • an insulating film was formed over the semiconductor layer, the insulating film, and the conductive film.
  • the insulating film was formed by stacking a 100-nm-thick silicon nitride film and a 300-nm-thick silicon oxynitride film with a PECVD apparatus.
  • a mask was formed over the formed insulating film and an opening was formed in the insulating film with use of the mask.
  • a conductive film was formed to fill the opening and was processed into an island shape, whereby the conductive films serving as a source electrode and a drain electrode were formed.
  • the conductive films a 50-nm-thick titanium film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film were formed in this order with a sputtering apparatus.
  • an insulating film was formed over the insulating film and the conductive film.
  • a 1.5- ⁇ m-thick acrylic photosensitive resin was used for the insulating film.
  • I d -V g characteristics of the fabricated transistor of Sample S 2 were measured.
  • the measurement conditions of I d -V g characteristics of the transistor were the same as those in Example 3.
  • FIG. 56 shows the results of I d -V g characteristics of Sample S 2 .
  • the transistor which is a semiconductor device of one embodiment of the present invention, has excellent electrical characteristics as shown in FIG. 56 .
  • Table 2 lists the transistor characteristics that are shown in FIG. 56 .
  • a transistor including the composite oxide semiconductor or C/IGZO in its semiconductor layer can have both high carrier mobility and excellent switching characteristics.
  • FIG. 57 shows a cross-sectional STEM image of the transistor of Sample S 2 .
  • Sample S 2 fabricated in this example was confirmed to have a favorable cross-sectional shape.
  • the channel length L which was designed to be 2 ⁇ m, was found to be 1.78 ⁇ m in the completed transistor.

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Abstract

The field-effect mobility and reliability of a transistor including an oxide semiconductor film are improved. A semiconductor layer of a transistor is formed using a composite oxide semiconductor in which a first region and a second region are mixed. The first region includes a plurality of first clusters containing one or more of indium, zinc, and oxygen as a main component. The second region includes a plurality of second clusters containing one or more of indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen. The first region includes a portion in which the plurality of first clusters are connected to each other. The second region includes a portion in which the plurality of second clusters are connected to each other.

Description

    BACKGROUND OF THE INVENTION 1. Field of the Invention
  • One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device.
  • Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a memory device, a driving method thereof, or a manufacturing method thereof.
  • In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each one embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic device may each include a semiconductor device.
    • 2. Description of the Related Art
  • Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as a field-effect transistor (FET) or a thin film transistor (TFT)). Such transistors are used in a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.
  • In addition, a semiconductor device achieving high field-effect mobility (simply referred to as mobility or μFE in some cases) with such a structure that a plurality of oxide semiconductor layers are stacked, the oxide semiconductor layer serving as a channel in the plurality of oxide semiconductor layers contains indium and gallium, and the proportion of indium is higher than the proportion of gallium (see Patent Document 1).
    • REFERENCE Patent Document
    • [Patent Document 1] Japanese Published Patent Application No. 2014-007399
    SUMMARY OF THE INVENTION
  • The field-effect mobility of a transistor that uses an oxide semiconductor film as a channel region is preferably as high as possible. However, when the field-effect mobility is increased, the transistor has a problem with its characteristics, that is, the transistor tends to be normally on. Note that “normally on” means a state where a channel exists without application of a voltage to a gate electrode and current flows through the transistor.
  • Furthermore, in a transistor that uses an oxide semiconductor film in a channel region, oxygen vacancies which are formed in the oxide semiconductor film adversely affect the transistor characteristics. For example, oxygen vacancies formed in the oxide semiconductor film are bonded with hydrogen to serve as carrier supply sources. The carrier supply sources generated in the oxide semiconductor film cause a change in the electrical characteristics, typically, shift in the threshold voltage, of the transistor including the oxide semiconductor film.
  • When the amount of oxygen vacancies in the oxide semiconductor film is too large, for example, the threshold voltage of the transistor is shifted in the negative direction, and the transistor has normally-on characteristics. Thus, especially in the channel region of the oxide semiconductor film, the amount of oxygen vacancies is preferably small or the amount with which the normally-on characteristics are not exhibited.
  • In view of the foregoing problems, an object of one embodiment of the present invention is to improve field-effect mobility and reliability of a transistor including an oxide semiconductor film. Another object of one embodiment of the present invention is to prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and to improve the reliability of the transistor. Another object of one embodiment of the present invention is to provide a semiconductor device with reduced power consumption. Another object of one embodiment of the present invention is to provide a novel semiconductor device. Another object of one embodiment of the present invention is to provide a novel display device.
  • Note that the description of the above objects does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all of these objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.
  • One embodiment of the present invention is a composite oxide semiconductor in which a first region and a second region are mixed. The first region includes a plurality of first clusters containing one or more of indium (In), zinc (Zn), and oxygen (O) as a main component. The second region includes a plurality of second clusters containing one or more of indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen as a main component. The first region includes a portion in which the plurality of first clusters are connected to each other, and the second region includes a portion in which the plurality of second clusters are connected to each other.
  • In the above embodiment, the first region preferably exists being surrounded by the second region.
  • In the above embodiments, the first clusters preferably have higher conductivity than the second clusters, and the second clusters preferably have a higher semiconductor property than the first clusters.
  • Furthermore, the composite oxide semiconductor preferably includes a portion in which each of the first clusters has a size of greater than or equal to 0.5 nm and less than or equal to 1.5 nm.
  • In the above embodiments, it is preferable that an atomic ratio of In to an element M and Zn be In:M:Zn=4:2:3 or in a neighborhood thereof, and the proportion of the element M be higher than or equal to 1.5 and lower than or equal to 2.5 and the proportion of Zn be higher than or equal to 2 and lower than or equal to 4 when the proportion of In is 4.
  • Alternatively, in the above embodiments, it is preferable that an atomic ratio of In to an element M and Zn be In:M:Zn=5:1:6 or in a neighborhood thereof, and the proportion of the element M be higher than or equal to 0.5 and lower than or equal to 1.5 and the proportion of Zn be higher than or equal to 5 and lower than or equal to 7 when the proportion of In is 5.
  • One embodiment of the present invention is a semiconductor device including a semiconductor layer, a gate, and a gate insulating layer. The semiconductor layer includes the composite oxide semiconductor described in any of the above embodiments. At this time, it is preferable that a maximum value of field-effect mobility be greater than or equal to 100 cm2/Vs and less than or equal to 200 cm2/Vs in a range in which a gate voltage is higher than 0 V and lower than or equal to 10 V and a drain voltage is higher than 0 V and lower than or equal to 20 V.
  • Another embodiment of the present invention is a method for manufacturing a composite oxide semiconductor, which includes a first step of placing a substrate in a deposition chamber; a second step of introducing one or both of an argon gas and an oxygen gas into the deposition chamber; a third step of applying voltage to a target containing indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen; and a fourth step of depositing the composite oxide semiconductor from the target onto the substrate. The fourth step includes a first step in which the element M and the zinc are preferentially sputtered from the target; and a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target.
  • Another embodiment of the present invention is a method for manufacturing a composite oxide semiconductor, which includes a first step of placing a substrate in a deposition chamber; a second step of introducing, into the deposition chamber, a deposition gas containing an argon gas and not containing an oxygen gas; a third step of applying voltage to a target containing indium, an element M (M represents Al, Ga, Y, or Sn), zinc, and oxygen; and a fourth step of depositing the composite oxide semiconductor from the target onto the substrate. The fourth step includes a first step in which the element M and the zinc are preferentially sputtered from the target; and a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target.
  • In the above manufacturing methods, it is preferable that the substrate not be heated intentionally.
  • Another embodiment of the present invention is a display device which includes the semiconductor device of any one of the above-described embodiments, and a display element. Another embodiment of the present invention is a display module which includes the display device and a touch sensor. Another embodiment of the present invention is an electronic device which includes the semiconductor device of any one of the above-described embodiments, the above-described display device, or the above-described display module; and an operation key or a battery.
  • One embodiment of the present invention can improve field-effect mobility and reliability of a transistor including an oxide semiconductor film. One embodiment of the present invention can prevent a change in electrical characteristics of a transistor including an oxide semiconductor film and improve the reliability of the transistor. One embodiment of the present invention can provide a semiconductor device with reduced power consumption. One embodiment of the present invention can provide a novel semiconductor device. One embodiment of the present invention can provide a novel display device.
  • Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not necessarily have all the effects listed above. Other effects can be derived from the description of the specification, the drawings, the claims, and the like.
    • BRIEF DESCRIPTION OF THE DRAWINGS
  • FIGS. 1A and 1B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIG. 2 illustrates atomic ratios of an oxide semiconductor.
  • FIGS. 3A and 3B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 4A and 4B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 5A and 5B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 6A and 6B are a schematic top view and a schematic cross-sectional view illustrating a composite oxide semiconductor.
  • FIGS. 7A and 7B illustrate a sputtering apparatus.
  • FIG. 8 is a process flow chart illustrating a method for manufacturing a composite oxide semiconductor.
  • FIGS. 9A to 9C illustrate a cross section of the vicinity of a target.
  • FIG. 10 illustrates an analysis method.
  • FIG. 11 shows SEM observation and EDX mapping images.
  • FIG. 12 shows SEM observation and EDX mapping images.
  • FIGS. 13A and 13B show compositions of a sample.
  • FIG. 14 shows compositions of a sample.
  • FIG. 15 shows SEM observation and EDX mapping images.
  • FIG. 16 shows SEM observation and EDX mapping images.
  • FIG. 17 shows SEM observation and EDX mapping images.
  • FIG. 18 shows SEM observation and EDX mapping images.
  • FIG. 19 shows SEM observation and EDX mapping images.
  • FIG. 20 shows SEM observation and EDX mapping images.
  • FIGS. 21A to 21C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIG. 22 is a cross-sectional view illustrating a semiconductor device.
  • FIGS. 23A and 23B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 24A and 24B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 25A and 25B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 26A and 26B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 27A and 27B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 28A and 28B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 29A and 29B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 30A and 30B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 31A to 31C each illustrate a band structure.
  • FIGS. 32A to 32C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 33A to 33C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 34A to 34C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 35A to 35C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIGS. 36A and 36B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 37A and 37B are cross-sectional views illustrating a semiconductor device.
  • FIGS. 38A to 38C are a top view and cross-sectional views illustrating a semiconductor device.
  • FIG. 39 is a top view illustrating one mode of a display device.
  • FIG. 40 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 41 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 42 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 43 is a cross-sectional view illustrating one mode of a display device.
  • FIG. 44 is a cross-sectional view illustrating one mode of a display device.
  • FIGS. 45A to 45D are cross-sectional views illustrating a method for forming an EL layer.
  • FIG. 46 is a conceptual diagram illustrating a droplet discharge apparatus.
  • FIGS. 47A to 47C are a block diagram and circuit diagrams illustrating a display device.
  • FIG. 48 illustrates a display module.
  • FIGS. 49A to 49E illustrate electronic devices.
  • FIGS. 50A to 50G illustrate electronic devices.
  • FIGS. 51A and 51B are perspective views illustrating a display device.
  • FIGS. 52A to 52E are a HAADF-STEM image and EDX mapping images of a plane.
  • FIGS. 53A to 53E are a HAADF-STEM image and EDX mapping images of a cross section.
  • FIG. 54 shows XRD analysis results.
  • FIG. 55 shows Id-Vg characteristics of a transistor.
  • FIG. 56 shows Id-Vg characteristics of a transistor.
  • FIG. 57 is a cross-sectional STEM image of a transistor.
    •  DETAILED DESCRIPTION OF THE INVENTION
  • Hereinafter, embodiments will be described with reference to drawings.
  • However, the embodiments can be implemented in many different modes, and it will be readily appreciated by those skilled in the art that modes and details thereof can be changed in various ways without departing from the spirit and scope of the present invention. Thus, the present invention should not be interpreted as being limited to the following description of the embodiments.
  • In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, embodiments of the present invention are not limited to such a scale. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to shapes or values shown in the drawings.
  • Note that in this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not limit the components numerically.
  • Note that in this specification, terms for describing arrangement, such as “over”, “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. Further, the positional relation between components is changed as appropriate in accordance with a direction in which the components are described. Thus, the positional relation is not limited to that described with a term used in this specification and can be explained with another term as appropriate depending on the situation.
  • In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, or a drain electrode) and a source (a source terminal, a source region, or a source electrode), and current can flow between the source and the drain through the channel region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.
  • Further, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.
  • Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” include a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.
  • In this specification and the like, the term “parallel” means that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also covers the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” means that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly also covers the case where the angle is greater than or equal to 85° and less than or equal to 95°.
  • In this specification and the like, the terms “film” and “layer” can be interchanged with each other. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.
  • Unless otherwise specified, the off-state current in this specification and the like refers to a drain current of a transistor in an off state (also referred to as non-conduction state and cutoff state). Unless otherwise specified, the off state of an n-channel transistor means that a voltage (Vgs) between its gate and source is lower than the threshold voltage (Vth), and the off state of a p-channel transistor means that the gate-source voltage Vgs is higher than the threshold voltage Vth. For example, the off-state current of an n-channel transistor sometimes refers to a drain current that flows when the gate-source voltage Vgs is lower than the threshold voltage Vth.
  • The off-state current of a transistor depends on Vgs in some cases. Thus, “the off-state current of a transistor is lower than or equal to I” may mean “there is Vgs with which the off-state current of the transistor becomes lower than or equal to I”. Furthermore, “the off-state current of a transistor” means “the off-state current in an off state at predetermined Vgs”, “the off-state current in an off state at Vgs in a predetermined range”, “the off-state current in an off state at Vgs with which sufficiently reduced off-state current is obtained”, or the like.
  • As an example, the assumption is made of an n-channel transistor where the threshold voltage Vth is 0.5 V and the drain current is 1×10−9 A at Vgs of 0.5 V, 1×10−13 A at Vgs of 0.1 V, 1×10−19 A at Vgs of −0.5 V, and 1×10−22 A at Vgs of −0.8 V. The drain current of the transistor is 1×10−19 A or lower at Vgs of −0.5 V or at Vgs in the range of −0.8 V to −0.5 V; therefore, it can be said that the off-state current of the transistor is 1×10−19 A or lower. Since there is Vgs at which the drain current of the transistor is 1×10−22 A or lower, it may be said that the off-state current of the transistor is 1×10−22 A or lower.
  • In this specification and the like, the off-state current of a transistor with a channel width W is sometimes represented by a current value in relation to the channel width W or by a current value per given channel width (e.g., 1 μm). In the latter case, the off-state current may be expressed in the unit with the dimension of current per length (e.g., A/μm).
  • The off-state current of a transistor depends on temperature in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at room temperature, 60° C., 85° C., 95° C., or 125° C. Alternatively, the off-state current may be an off-state current at a temperature at which the required reliability of a semiconductor device or the like including the transistor is ensured or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.). The description “an off-state current of a transistor is lower than or equal to I” may refer to a situation where there is Vgs at which the off-state current of a transistor is lower than or equal to I at room temperature, 60° C., 85° C., 95° C., 125° C., a temperature at which the required reliability of a semiconductor device or the like including the transistor is ensured, or a temperature at which the semiconductor device or the like including the transistor is used (e.g., temperature in the range of 5° C. to 35° C.).
  • The off-state current of a transistor depends on voltage Vds between its drain and source in some cases. Unless otherwise specified, the off-state current in this specification may be an off-state current at Vds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V. Alternatively, the off-state current may be an off-state current at Vds at which the required reliability of a semiconductor device or the like including the transistor is ensured or Vds at which the semiconductor device or the like including the transistor is used. The description “an off-state current of a transistor is lower than or equal to I” may refer to a situation where there is Vgs at which the off-state current of a transistor is lower than or equal to I at Vds of 0.1 V, 0.8 V, 1 V, 1.2 V, 1.8 V, 2.5 V, 3 V, 3.3 V, 10 V, 12 V, 16 V, or 20 V, Vds at which the required reliability of a semiconductor device or the like including the transistor is ensured, or Vds at which the semiconductor device or the like including the transistor is used.
  • In the above description of off-state current, a drain may be replaced with a source. That is, the off-state current sometimes refers to current that flows through a source of a transistor in an off state.
  • In this specification and the like, the term “leakage current” sometimes expresses the same meaning as off-state current. In this specification and the like, the off-state current sometimes refers to current that flows between a source and a drain when a transistor is off, for example.
  • In this specification and the like, the threshold voltage of a transistor refers to a gate voltage (Vg) at which a channel is formed in the transistor. Specifically, in a graph where the horizontal axis represents the gate voltage (Vg) and the vertical axis represents the square root of drain current (Id), the threshold voltage of a transistor may refer to a gate voltage (Vg) at the intersection of the square root of drain current (Id) of 0 (Id=0 A) and an extrapolated straight line that is tangent with the highest inclination to a plotted curve (Vg-√Id characteristics). Alternatively, the threshold voltage of a transistor may refer to a gate voltage (Vg) at which the value of Id[A]×L [μm]/W [μm] is 1×10−9 [A] where L is channel length and W is channel width.
  • In this specification and the like, a “semiconductor” can have characteristics of an “insulator” when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called an “insulator” in some cases. Similarly, an “insulator” in this specification and the like can be called a “semiconductor” in some cases. An “insulator” in this specification and the like can be called a “semi-insulator” in some cases.
  • In this specification and the like, a “semiconductor” can have characteristics of a “conductor” when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification and the like can be called a “conductor” in some cases. Similarly, a “conductor” in this specification and the like can be called a “semiconductor” in some cases.
  • In this specification and the like, an impurity in a semiconductor refers to an element that is not a main component of the semiconductor. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. If a semiconductor contains an impurity, the density of states (DOS) may be formed therein, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. In the case where the semiconductor includes an oxide semiconductor, examples of the impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specific examples include hydrogen (also included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. When the semiconductor is an oxide semiconductor, oxygen vacancies may be formed by entry of impurities such as hydrogen, for example. Furthermore, in the case where the semiconductor includes silicon, examples of the impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements except hydrogen, Group 2 elements, Group 13 elements, and Group 15 elements.
    • Embodiment 1
  • In this embodiment, an oxide semiconductor film that includes a composite oxide semiconductor of one embodiment of the present invention and a semiconductor device that includes this oxide semiconductor film are described.
    • <1-1. Oxide Semiconductor Film>
  • Indium is preferably contained in an oxide semiconductor film. In particular, indium and zinc are preferably contained. In addition, aluminum, gallium, yttrium, tin, or the like is preferably contained. Furthermore, one or more elements selected from boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained.
  • Here, the case where an oxide semiconductor film contains indium, an element M, and zinc is considered. The element M is aluminum, gallium, yttrium, tin, or the like. Other elements that can be used as the element M include boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, and magnesium. Note that two or more of the above elements can be used in combination as the element M. The terms of the atomic ratio of indium, the element M, and zinc contained in the oxide semiconductor film are denoted by [In], [M], and [Zn], respectively.
    • <1-2. Structure of Oxide Semiconductor Film>
  • FIGS. 1A and 1B are schematic views of an oxide semiconductor film that includes a composite oxide semiconductor of one embodiment of the present invention.
  • FIG. 1A is a schematic view of a top surface of an oxide semiconductor film (a-b plane direction), and FIG. 1B is a schematic view of a cross section of the oxide semiconductor film (c-axis direction) formed over a substrate Sub.
  • FIGS. 1A and 1B illustrate an example in which the oxide semiconductor film is formed over the substrate; however, one embodiment of the present invention is not limited to this example and an insulating film such as a base film or an interlayer film or another semiconductor film such as an oxide semiconductor film may be formed between the substrate and the oxide semiconductor film.
  • The oxide semiconductor film of one embodiment of the present invention is a composite oxide semiconductor having a structure in which Region A1 and Region B1 are mixed as shown in FIGS. 1A and 1B. Therefore, in the following description, the oxide semiconductor film is referred to as a composite oxide semiconductor in some cases.
  • Region A1 shown in FIGS. 1A and 1B is high in In with [In]:[M]:[Zn]=x:y:z (x>0, y≥0, z≥0). In contrast, Region B1 is low in In with [In]:[M]:[Zn]=a:b:c (a>0, b>0, c>0).
  • Note that in this specification, when the atomic ratio of In to the element M in Region A1 is greater than the atomic ratio of In to the element M in Region B1, Region A1 has higher In concentration than Region B1. Therefore, in this specification, Region A1 is also referred to as an In-rich region, and Region B1 is also referred to as an In-poor region.
  • For example, the In concentration in Region A1 is 1.1 or more times, preferably 2 to 10 times that in Region B1. Region A1 is an oxide containing at least In and does not necessarily contain the element M and Zn.
  • The atomic ratio of elements included in the composite oxide semiconductor of one embodiment of the present invention will be described here.
  • A phase diagram in FIG. 2 can be used to show the atomic ratio of elements in the case where Region A1 in the composite oxide semiconductor contains In, the element M, and Zn. The atomic ratio of In to the element M and Zn is denoted by x:y:z. This atomic ratio can be shown as coordinates (x:y:z) in FIG. 2. Note that the proportion of oxygen atoms is not illustrated in FIG. 2.
  • In FIG. 2, dashed lines correspond to a line representing the atomic ratio of [In]:[M]:[Zn]=(1+a):(1-α):1 (−1≤α≤1), a line representing the atomic ratio of [In]:[M]:[Zn]=(1+α):(1-α):2, a line representing the atomic ratio of [In]:[M]:[Zn]=(1+α):(1-α):3, a line representing the atomic ratio of [In]:[M]:[Zn]=(1+α):(1-α):4, and a line representing the atomic ratio of [In]:[M]:[Zn]=(1+α):(1−α):5.
  • Dashed-dotted lines correspond to a line representing the atomic ratio of [In]:[M]:[Zn]=1:1:β (β≥0), a line representing the atomic ratio of [In]:[M]:[Zn]=1:2:β, a line representing the atomic ratio of [In]:[M]:[Zn]=1:3:β, a line representing the atomic ratio of [In]:[M]:[Zn]=1:4:β, a line representing the atomic ratio of [In]:[M]: [Zn]=1:7:β, a line representing the atomic ratio of [In]:[M]:[Zn]=2:1:β, and a line representing the atomic ratio of [In]:[M]:[Zn]=5:1:β.
  • An oxide semiconductor having the atomic ratio of [In]:[M]:[Zn]=0:2:1 or a neighborhood thereof in FIG. 2 tends to have a spinel crystal structure.
  • Region A2 in FIG. 2 represents an example of a preferred range of atomic ratios of indium to the element M and zinc contained in Region A1. Note that Region A2 includes atomic ratios on a line representing the atomic ratio of [In]:[M]:[Zn]=(1+γ):0:(1−γ) (−1≤γ≤1).
  • Region B2 in FIG. 2 represents an example of a preferred range of atomic ratios of indium to the element M and zinc contained in Region B1. Note that Region B2 includes atomic ratios from [In]:[M]:[Zn]=4:2:3 to [In]:[M]:[Zn]=4:2:4.1 and a neighborhood thereof. The neighborhood includes an atomic ratio of [In]:[M]:[Zn]=5:3:4. Region B2 includes an atomic ratio of [In]:[M]:[Zn]=5:1:6 and a neighborhood thereof.
  • Region A2 with high In concentrations provides higher conductivity than Region B2 and has a function of increasing carrier mobility of the oxide semiconductor film (or field-effect mobility of a transistor). Therefore, the on-state current and carrier mobility of a transistor using an oxide semiconductor film including Region A1 can be increased.
  • In contrast, Region B2 with low In concentrations provides lower conductivity than Region A2 and has a function of decreasing leakage current of the oxide semiconductor film or a transistor. Therefore, the off-state current of a transistor using an oxide semiconductor film including Region B1 can be decreased.
  • In the oxide semiconductor film of one embodiment of the present invention, Region A1 and Region B1 form a composite. That is, carrier movement occurs easily in Region A1, whereas carrier movement does not occur easily in Region B1. Therefore, an oxide semiconductor of one embodiment of the present invention can be used as a material with high carrier mobility, excellent switching characteristics, and favorable semiconductor characteristics.
  • In other words, Region A1 has a lower semiconductor property and higher conductivity than Region B1. Conversely, Region B1 has a higher semiconductor property and lower conductivity than Region A1. Here, a high semiconductor property means a wide band gap, favorable switching characteristics, being close to an i-type semiconductor, or the like.
  • For example, a plurality of Regions A1 are present in particulate form (in cluster form) in the a-b plane direction and the c-axis direction as shown in FIGS. 1A and 1B. Note that clusters may be distributed unevenly and irregularly. A plurality of clusters overlap with each other or are connected to each other in some cases. For example, in some cases, shapes each including a cluster overlapping with another cluster are connected to each other, so that Region A1 is observed to extend in a cloud-like manner.
  • In other words, the clusters included in Region A1 have a lower semiconductor property and higher conductivity than the clusters included in Region B1. Conversely, the clusters included in Region B1 have a higher semiconductor property and lower conductivity than the clusters included in Region A1.
  • In other words, in the composite oxide semiconductor of one embodiment of the present invention, a first region with a high In concentration and a second region with a low In concentration are connected in a cloud-like manner. Alternatively, in the composite oxide semiconductor of one embodiment of the present invention, the first region where In is distributed at a high concentration and the second region where In is not distributed at a high concentration are connected in a cloud-like manner.
  • As shown in FIGS. 1A and 1B, Regions A1 are connected to each other in the a-b plane direction, so that Regions A1 can serve as a current path. Accordingly, the oxide semiconductor film can have increased conductivity and a transistor using this oxide semiconductor film can have increased field-effect mobility.
  • In other words, Regions B1 shown in FIGS. 1A and 1B are scattered in Regions A1. Therefore, Region B1 can exist in a state of being sandwiched three-dimensionally by Regions A1. In other words, Region B1 can exist in a state of being surrounded by Region A1. That is, Region B1 is enclosed by Region A1.
  • From another perspective, Region B1 includes a cluster (also referred to as a second cluster) that is different from a cluster (also referred to as a first cluster) included in Region A1. In this structure, Region B1 includes a plurality of second clusters and has a portion where the plurality of second clusters are connected to each other. In other words, the first cluster included in Region A1 includes a portion where the first cluster and another first cluster are connected to each other in a cloud-like manner, and the second cluster included in Region B1 includes a portion where the second cluster and another second cluster are connected to each other in a cloud-like manner.
  • Note that the proportion of scattered Regions A1 can be adjusted by changing the formation conditions or composition of the composite oxide semiconductor. For example, it is possible to form a composite oxide semiconductor with a low proportion of Regions A1 or a composite oxide semiconductor with a high proportion of Regions A1. In a composite oxide semiconductor of one embodiment of the present invention, the proportion of Regions A1 is not always lower than that of Regions B1. In a composite oxide semiconductor with an extremely high proportion of Regions A1, depending on the observation range, Region B1 is sometimes formed in Region A1. The size of the particulate region of Region A1 can be appropriately adjusted by changing, for example, the formation conditions or composition of the composite oxide semiconductor.
  • FIGS. 3A and 3B show a composite oxide semiconductor that has a lower proportion of Regions A1 and a higher proportion of Regions B1 than the composite oxide semiconductor shown in FIGS. 1A and 1B.
  • FIGS. 4A and 4B show a composite oxide semiconductor that has a lower proportion of Regions A1 and a higher proportion of Regions B1 than the composite oxide semiconductor shown in FIGS. 3A and 3B. Because of the low proportion of Regions A1, the first clusters scattered without overlapping with each other may be included as shown in FIGS. 4A and 4B.
  • Note that by changing the formation conditions or composition of the composite oxide semiconductor, it is possible to form a composite oxide semiconductor that has a higher proportion of Regions A1 and a lower proportion of Regions B1 than the composite oxide semiconductor shown in FIGS. 1A and 1B.
  • Here, when all of Regions A1 are connected in the a-b plane direction, the switching characteristics of a transistor deteriorate in some cases. For example, the off-state current of the transistor might increase. It is thus preferable that as shown in FIGS. 3A and 3B and FIGS. 4A and 4B, Regions A1 be scattered in Regions B1. Therefore, Region A1 can exist in a state of being sandwiched three-dimensionally by Regions B1. In other words, Region A1 can exist in a state of being surrounded by Region B1. That is, Region A1 is enclosed by Region B1. As a result, the switching characteristics of the transistor can be improved. Specifically, the off-state current can be reduced.
  • In some cases, the boundary between Region A1 and Region B1 is not clearly observed. The sizes of Region A1 and Region B1 can be measured with energy dispersive X-ray spectroscopy (EDX) mapping images obtained by EDX. For example, the diameter of a cluster in Region A1 is greater than or equal to 0.1 nm and less than or equal to 2.5 nm in the EDX mapping image of a cross-sectional photograph or a plan-view photograph in some cases. Note that the diameter of the cluster is preferably greater than or equal to 0.5 nm and less than or equal to 1.5 nm.
  • As described above, an oxide semiconductor of one embodiment of the present invention is a composite oxide semiconductor in which Region A1 and Region B1 are mixed and have different functions that are complementary. For example, when an oxide semiconductor of one embodiment of the present invention is an In—Ga—Zn oxide (hereinafter referred to as IGZO), in which Ga is used as the element M, the oxide semiconductor of one embodiment of the present invention can be called complementary IGZO (abbreviation: C/IGZO).
  • In contrast, when Region A1 and Region B1 are stacked in a layered manner, for example, interaction does not take place or is unlikely to take place between Region A1 and Region B1, so that the function of Region A1 and that of Region B1 are independently performed in some cases. In that case, even when the carrier mobility is increased owing to Regions A1, the off-state current of the transistor might be increased. Therefore, in the case where the above-described composite oxide semiconductor or C/IGZO is used, a function of achieving high carrier mobility and a function of achieving excellent switching characteristics can be obtained at the same time. This is an advantageous effect obtained by using a composite oxide semiconductor of one embodiment of the present invention.
  • Note that in the case where the oxide semiconductor is deposited with a sputtering apparatus, a film having an atomic ratio deviated from the atomic ratio of the target is formed. Especially for zinc, [Zn] in the atomic ratio of a deposited film is smaller than that in the atomic ratio of the target in some cases depending on the substrate temperature during deposition.
  • The characteristics of the composite oxide semiconductor of one embodiment of the present invention are not uniquely determined by the atomic ratio. Therefore, the illustrated regions represent preferred atomic ratios of Region A1 and Region B1 of the composite oxide semiconductor; a boundary therebetween is not clear.
  • Here, Region B1 may have crystallinity. Region B1 preferably includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) described later. The CAAC-OS has c-axis alignment, its nanocrystals are connected in the a-b plane direction, and its crystal structure has distortion. Note that the distortion in the CAAC-OS is a portion where the direction of a lattice arrangement changes between a region with a regular lattice arrangement and another region with a regular lattice arrangement.
  • In FIG. 5A, a plurality of nanocrystals that are included in Region B1 are schematically shown by dashed lines. The shape of the nanocrystal is basically hexagon. However, the shape is not always a regular hexagon and is a non-regular hexagon in some cases. At the distorted portion, a polygonal nanocrystal such as a pentagonal nanocrystal or a heptagonal nanocrystal is included in some cases.
  • Note that a clear grain boundary cannot be observed even in the vicinity of the distorted portion in the CAAC-OS. That is, a lattice arrangement is distorted so that formation of a grain boundary is inhibited. This is probably because the CAAC-OS can tolerate distortion owing to a low density of arrangement of oxygen atoms in an a-b plane direction, the interatomic bond distance changed by substitution of a metal element, and the like.
  • Furthermore, FIG. 5B schematically shows that nanocrystals have c-axis alignment and the c-axes are aligned in a direction substantially perpendicular to a surface over which the CAAC-OS film is formed (also referred to as a formation surface) or the top surface of the CAAC-OS film. The CAAC-OS has a layered crystal structure (also referred to as a layered structure) having c-axis alignment and includes a layer containing indium and oxygen (hereinafter referred to as an In layer) and a layer containing the element M, zinc, and oxygen (hereinafter referred to as an (M,Zn) layer) that are stacked.
  • Note that indium and the element M are replaced with each other in some cases. Therefore, when some of the elements M in the (M,Zn) layer are replaced with indium, the layer can also be referred to as an (In,M,Zn) layer. In that case, the In layer and the (In,M,Zn) layer are stacked in the layered structure.
  • FIGS. 6A and 6B show an example in which the proportion of Regions A1 is low and the proportion of Regions B1 is high as compared to those in FIGS. 5A and 5B.
  • Oxide semiconductors have various structures and various properties. An oxide semiconductor of one embodiment of the present invention may be a composite oxide semiconductor including two or more of the amorphous oxide semiconductor, the polycrystalline oxide semiconductor, an a-like OS described later, an nc-OS described later, and a CAAC-OS described later. Region A1 and Region B1 may have different crystallinities.
  • For example, Region A1 is preferably non-single-crystal. Note that in the case where Region A1 has crystallinity, when Region A1 is formed of indium, Region A1 tends to have a tetragonal crystal structure. Furthermore, when Region A1 is formed of indium oxide ([In]:[M]:[Zn]=x:0:0 (x>0)), Region A1 tends to have a bixbyite crystal structure. Furthermore, when Region A1 is formed of an In—Zn oxide ([In]:[M]:[Zn]=x:0:z (x>0, z>0)), Region A1 tends to have a layered crystal structure.
  • For example, Region B1 is preferably non-single-crystal. Region B1 preferably includes a CAAC-OS. Note that Region B1 does not necessarily include only a CAAC-OS and may also include a region of a polycrystalline oxide semiconductor, an nc-OS, or the like.
  • The CAAC-OS is an oxide semiconductor with high crystallinity. In contrast, in the CAAC-OS, a reduction in electron mobility due to a grain boundary is less likely to occur because a clear grain boundary cannot be observed. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has few impurities and defects (e.g., oxygen vacancies). Thus, with the CAAC-OS, a composite oxide semiconductor is physically stable; thus, a composite oxide semiconductor which is resistant to heat and has high reliability can be provided.
    • <1-3. Transistor Including Oxide Semiconductor Film>
  • Next, the case where the above-described oxide semiconductor film is used for a transistor will be described.
  • With the use of the composite oxide semiconductor in a transistor, the transistor can have high carrier mobility and high switching characteristics. In addition, the transistor can have high reliability.
  • An oxide semiconductor film with low carrier density is preferably used for the transistor. For example, an oxide semiconductor film whose carrier density is lower than 8×1011/cm3, preferably lower than 1×1011/cm3, or further preferably lower than 1×1010/cm3, and greater than or equal to 1×10−9/cm3 is used as the oxide semiconductor film.
  • In order to reduce the carrier density of the oxide semiconductor film, the impurity concentration in the oxide semiconductor film is reduced so that the density of defect states can be reduced. In this specification and the like, a state with a low impurity concentration and a low density of defect states is referred to as a highly purified intrinsic or substantially highly purified intrinsic state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has few carrier generation sources, and thus can have a low carrier density. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has a low density of trap states in some cases.
  • Charges trapped by the trap states in the oxide semiconductor film take a long time to be released and may behave like fixed charges. Thus, the transistor whose channel region is formed in the oxide semiconductor having a high density of trap states has unstable electrical characteristics in some cases.
  • To obtain stable electrical characteristics of the transistor, it is effective to reduce the concentration of impurities in the oxide semiconductor film. In order to reduce the concentration of impurities in the oxide semiconductor film, the concentration of impurities in a film that is adjacent to the oxide semiconductor film is preferably reduced. As examples of the impurities, hydrogen, nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, and the like are given.
  • Here, the influence of impurities in the oxide semiconductor film will be described.
  • When silicon or carbon that is one of Group 14 elements is contained in the oxide semiconductor film, defect states are formed. Thus, the concentration of silicon or carbon in the oxide semiconductor and around an interface with the oxide semiconductor (measured by secondary ion mass spectrometry (SIMS)) is set lower than or equal to 2×1018 atoms/cm3, and preferably lower than or equal to 2×1017 atoms/cm3.
  • When the oxide semiconductor film contains alkali metal or alkaline earth metal, defect states are formed and carriers are generated, in some cases. Thus, a transistor including an oxide semiconductor film which contains alkali metal or alkaline earth metal is likely to be normally on. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film. Specifically, the concentration of alkali metal or alkaline earth metal in the oxide semiconductor film measured by SIMS is set lower than or equal to 1×1018 atoms/cm3, and preferably lower than or equal to 2×1016 atoms/cm3.
  • When the oxide semiconductor film contains nitrogen, the oxide semiconductor film easily becomes n-type by generation of electrons serving as carriers and an increase of carrier density. Thus, a transistor whose semiconductor includes an oxide semiconductor that contains nitrogen is likely to be normally-on. For this reason, nitrogen in the oxide semiconductor is preferably reduced as much as possible; the nitrogen concentration measured by SIMS is set, for example, lower than 5×1019 atoms/cm3, preferably lower than or equal to 5×1018 atoms/cm3, further preferably lower than or equal to 1×1018 atoms/cm3, and still further preferably lower than or equal to 5×1017 atoms/cm3.
  • Hydrogen contained in an oxide semiconductor film reacts with oxygen bonded to a metal atom to be water, and thus causes an oxygen vacancy (Vo), in some cases. Due to entry of hydrogen into the oxygen vacancy (Vo), an electron serving as a carrier is generated in some cases. Furthermore, in some cases, bonding of part of hydrogen to oxygen bonded to a metal atom causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor which contains hydrogen is likely to be normally on. Accordingly, it is preferable that hydrogen in the oxide semiconductor be reduced as much as possible. Specifically, the hydrogen concentration measured by SIMS in the oxide semiconductor is lower than 1×1020 atoms/cm3, preferably lower than 1×1019 atoms/cm3, further preferably lower than 5×1018 atoms/cm3, and still further preferably lower than 1×1018 atoms/cm3.
  • The oxygen vacancies (Vo) in the oxide semiconductor film can be reduced by introduction of oxygen into the oxide semiconductor film. That is, the oxygen vacancies (Vo) in the oxide semiconductor film disappear when the oxygen vacancies (Vo) are filled with oxygen. Accordingly, diffusion of oxygen in the oxide semiconductor film can reduce the oxygen vacancies (Vo) in a transistor and improve the reliability of the transistor.
  • As a method for introducing oxygen into the oxide semiconductor film, for example, an oxide in which oxygen content is higher than that in the stoichiometric composition is provided in contact with the oxide semiconductor film. That is, in the oxide, a region including oxygen in excess of that in the stoichiometric composition (hereinafter also referred to as an excess oxygen region) is preferably formed. In particular, in the case of using an oxide semiconductor film in a transistor, an oxide including an excess oxygen region is provided in a base film, an interlayer film, or the like in the vicinity of the transistor, whereby oxygen vacancies in the transistor are reduced, and the reliability can be improved.
  • When an oxide semiconductor film with sufficiently reduced impurity concentration is used for a channel formation region in a transistor, the transistor can have stable electrical characteristics.
  • One embodiment of the present invention is a semiconductor device including a transistor. The transistor includes a first gate electrode, a first insulating film over the first gate electrode, an oxide semiconductor film over the first insulating film, a second insulating film over the oxide semiconductor film, a second gate electrode over the second insulating film, and a third insulating film over the oxide semiconductor film and the second gate electrode. The oxide semiconductor film includes a channel region overlapping with the first or second gate electrode, a source region in contact with the third insulating film, and a drain region in contact with the third insulating film. The first gate electrode and the second gate electrode are electrically connected to each other.
  • The above transistor includes, in terms of its electrical characteristics, a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 10 cm2/Vs and smaller than 200 cm2/Vs, a second region where the threshold voltage is higher than or equal to −1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1×10−12 A/cm2. Furthermore, μFE(max)/μFE(Vg=2V) is larger than or equal to 1 and smaller than 10 where μFE(max) represents the maximum value of the field-effect mobility of the transistor and μFE(Vg=2V) represents the value of the field-effect mobility of the transistor at a gate voltage of 2 V.
  • Preferably, the above transistor includes, in terms of its electrical characteristics, a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 60 cm2/Vs and smaller than 200 cm2/Vs, a second region where the threshold voltage is higher than or equal to −1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1×10−12 A/cm2. Furthermore, μFE(max)/μFE(Vg=2V) is larger than or equal to 1 and smaller than 2 where μFE(max) represents the maximum value of the field-effect mobility of the transistor and μFE(Vg=2V) represents the value of the field-effect mobility of the transistor at a gate voltage of 2 V.
  • In other words, the semiconductor device of one embodiment of the present invention is a transistor which includes an oxide semiconductor film in its channel region and whose field-effect mobility, threshold voltage, off-state current, and S value are extremely favorable. Such a semiconductor device can be favorably used as a transistor in a pixel of a liquid crystal display or an organic EL display or a transistor in a driver circuit of a liquid crystal display or an organic EL display, for example.
    • <1-4. Method for Manufacturing Composite Oxide Semiconductor>
  • An example of a method for manufacturing the composite oxide semiconductor shown in FIGS. 1A and 1B and other drawings is described. A composite oxide semiconductor of one embodiment of the present invention can be formed with a sputtering apparatus.
    • [Sputtering Apparatus]
  • FIG. 7A is a cross-sectional view of a deposition chamber 2501 of the sputtering apparatus. FIG. 7B is a plan view of a magnet unit 2530 a and a magnet unit 2530 b of the sputtering apparatus.
  • The deposition chamber 2501 illustrated in FIG. 7A includes a target holder 2520 a, a target holder 2520 b, a backing plate 2510 a, a backing plate 2510 b, a target 2502 a, a target 2502 b, a member 2542, and a substrate holder 2570. Note that the target 2502 a is placed over the backing plate 2510 a. The backing plate 2510 a is placed over the target holder 2520 a. The magnet unit 2530 a is placed under the target 2502 a with the backing plate 2510 a therebetween. The target 2502 b is placed over the backing plate 2510 b. The backing plate 2510 b is placed over the target holder 2520 b. The magnet unit 2530 b is placed under the target 2502 b with the backing plate 2510 b therebetween.
  • As illustrated in FIGS. 7A and 7B, the magnet unit 2530 a includes a magnet 2530N1, a magnet 2530N2, a magnet 2530S, and a magnet holder 2532. The magnet 2530N1, the magnet 2530N2, and the magnet 2530S are placed over the magnet holder 2532 in the magnet unit 2530 a. The magnet 2530N1, the magnet 2530N2, and the magnet 2530S are spaced. Note that the magnet unit 2530 b has a structure similar to that of the magnet unit 2530 a. When the substrate 2560 is transferred into the deposition chamber 2501, the substrate 2560 is placed in contact with the substrate holder 2570.
  • The target 2502 a, the backing plate 2510 a, and the target holder 2520 a are separated from the target 2502 b, the backing plate 2510 b, and the target holder 2520 b by the member 2542. Note that the member 2542 is preferably an insulator. The member 2542 may be a conductor or a semiconductor. The member 2542 may be a conductor or a semiconductor whose surface is covered with an insulator.
  • The target holder 2520 a and the backing plate 2510 a are fixed to each other with a screw (e.g., a bolt) and have the same potential. The target holder 2520 a has a function of supporting the target 2502 a with the backing plate 2510 a positioned therebetween. The target holder 2520 b and the backing plate 2510 b are fixed to each other with a screw (e.g., a bolt) and have the same potential. The target holder 2520 b has a function of supporting the target 2502 b with the backing plate 2510 b positioned therebetween.
  • The backing plate 2510 a has a function of fixing the target 2502 a. The backing plate 2510 b has a function of fixing the target 2502 b.
  • Magnetic lines of force 2580 a and 2580 b formed by the magnet unit 2530 a are illustrated in FIG. 7A.
  • As illustrated in FIG. 7B, the magnet unit 2530 a has a structure in which the magnet 2530N1 having a rectangular or substantially rectangular shape, the magnet 2530N2 having a rectangular or substantially rectangular shape, and the magnet 2530S having a rectangular or substantially rectangular shape are fixed to the magnet holder 2532. The magnet unit 2530 a can be oscillated horizontally as shown by an arrow in FIG. 7B. For example, the magnet unit 2530 a may be oscillated with a beat of greater than or equal to 0.1 Hz and less than or equal to 1 kHz.
  • The magnetic field over the target 2502 a changes in accordance with oscillation of the magnet unit 2530 a. The region with an intense magnetic field is a high-density plasma region; thus, sputtering of the target 2502 a easily occurs in the vicinity of the region. The same applies to the magnet unit 2530 b.
    • <1-5. Manufacturing Flow of Composite Oxide Semiconductor>
  • FIG. 8 is a process flow chart showing a method for manufacturing a composite oxide semiconductor.
  • The composite oxide semiconductor shown in FIGS. 1A and 1B is manufactured through at least first to fourth processes shown in FIG. 8.
    • [First Process: Process of Placing Substrate in Deposition Chamber]
  • The first process includes a step of placing a substrate in a deposition chamber (see Step S101 in FIG. 8).
  • In the first process, for example, the substrate 2560 is placed on the substrate holder 2570 of the deposition chamber 2501 shown in FIGS. 7A and 7B.
  • The temperature of the substrate 2560 in deposition influences the electrical properties of a composite oxide semiconductor. The higher the substrate temperature is, the higher the crystallinity and reliability of the composite oxide semiconductor can be. In contrast, the lower the substrate temperature is, the lower the crystallinity of the composite oxide semiconductor can be and the higher the carrier mobility thereof can be. In particular, the lower the substrate temperature in deposition is, the more the field-effect mobility at a low gate voltage (e.g., higher than 0 V and lower than or equal to 2 V) is notably increased in a transistor including the composite oxide semiconductor.
  • The temperature of the substrate 2560 can be set higher than or equal to room temperature (25° C.) and lower than or equal to 200° C., preferably higher than or equal to room temperature and lower than or equal to 170° C., further preferably higher than or equal to room temperature and lower than or equal to 130° C. The substrate temperature in the above range is suitable for the case of using a large glass substrate (e.g., a glass substrate having a size of the 8th generation or the 10th generation described later). In particular, when the substrate temperature in deposition of a composite oxide semiconductor is room temperature, i.e., the substrate is not heated intentionally, the substrate can be favorably prevented from bending or warping.
  • The substrate 2560 may be cooled with a cooling mechanism or the like provided for the substrate holder 2570.
  • In the case where the temperature of the substrate 2560 is set higher than or equal to 100° C. and lower than or equal to 130° C., water in the composite oxide semiconductor can be removed. Water as an impurity is removed in this manner, whereby the field-effect mobility and the reliability can be improved at the same time.
  • Furthermore, in the case where the temperature of the substrate 2560 is set higher than or equal to 100° C. and lower than or equal to 130° C., the sputtering apparatus can be prevented from warping due to overheat. Accordingly, semiconductor devices can be manufactured with higher productivity. The productivity is thus stabilized, so that a large-scale production apparatus is easy to employ. Thus, a large display device including a large substrate can be easily manufactured.
  • When the temperature of the substrate 2560 is high, water in the composite oxide semiconductor can be more effectively removed and moreover, the composite oxide semiconductor can have increased crystallinity. For example, the temperature of the substrate 2560 is set to higher than or equal to 80° C. and lower than or equal to 200° C., preferably higher than or equal to 100° C. and lower than or equal to 170° C., whereby a composite oxide semiconductor with high crystallinity can be formed.
  • [Second Process: Process of Introducing Gas into Deposition Chamber]
  • The second process includes a step of introducing a gas into the deposition chamber (see Step S201 in FIG. 8).
  • In the second process, for example, a gas is introduced into the deposition chamber 2501 in FIGS. 7A and 7B. One or both of an argon gas and an oxygen gas are introduced as the gas. Note that instead of an argon gas, an inert gas such as helium, xenon, or krypton can be used.
  • The proportion of oxygen in the whole deposition gas in forming a composite oxide semiconductor using an oxygen gas is referred to as an oxygen flow rate percentage in some cases.
  • The higher the oxygen flow rate percentage is, the higher the crystallinity and reliability of the composite oxide semiconductor can be. In contrast, the lower the oxygen flow rate percentage is, the lower the crystallinity of the composite oxide semiconductor can be and the higher the carrier mobility thereof can be. In particular, the lower the oxygen flow rate percentage is, the more the field-effect mobility at a low gate voltage (e.g., higher than 0 V and lower than or equal to 2 V) is notably increased in a transistor including the composite oxide semiconductor.
  • The oxygen flow rate percentage can be appropriately set in the range from 0% to 100% inclusive so that favorable characteristics of the composite oxide semiconductor suitable to the uses can be obtained.
  • For example, in the case where the composite oxide semiconductor is used for a semiconductor layer of a transistor having high field-effect mobility, the oxygen flow rate percentage is set to higher than 0% and lower than or equal to 30%, preferably higher than or equal to 5% and lower than or equal to 30%, further preferably higher than or equal to 7% and lower than or equal to 15% in deposition of the composite oxide semiconductor. Alternatively, the composite oxide semiconductor may be formed without using an oxygen gas, in which case the oxygen flow rate percentage is 0%.
  • To make both the field-effect mobility and reliability of a transistor relatively high, the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than 30% and lower than 70%, preferably higher than 30% and lower than or equal to 50%. Alternatively, the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than or equal to 10% and lower than or equal to 50%, preferably higher than or equal to 30% and lower than or equal to 50%.
  • To make the reliability of a transistor high, the oxygen flow rate percentage in deposition of the composite oxide semiconductor is set to higher than or equal to 70% and lower than or equal to 100%.
  • When the substrate temperature and the oxygen flow rate percentage in deposition are controlled in this manner, a composite oxide semiconductor that provides desired electrical characteristics can be deposited. For example, a reduction (an increase) of substrate temperature and a reduction (an increase) of oxygen flow rate percentage contribute to the field-effect mobility to the same degree, in some cases. Therefore, even when the substrate temperature cannot be increased sufficiently with the constraints of an apparatus, for example, a transistor having field-effect mobility substantially the same as that when the substrate temperature is increased can be obtained by increasing the oxygen flow rate percentage.
  • Note that even when a composite oxide semiconductor whose carrier mobility is increased by control of the substrate temperature and oxygen flow rate percentage in deposition is used, a transistor with high reliability can be obtained by reducing oxygen vacancies and impurities by a method described later.
  • In addition, increasing the purity of the gas is necessary. For example, as an oxygen gas or an argon gas used as the gas, a gas which is highly purified to have a dew point of −40° C. or lower, preferably −80° C. or lower, further preferably −100° C. or lower, still further preferably −120° C. or lower is used, whereby entry of moisture or the like into the composite oxide semiconductor can be minimized.
  • The deposition chamber 2501 is preferably evacuated to high vacuum (about 5×10−7 Pa to 1×10−4 Pa) with an entrapment vacuum evacuation pump such as a cryopump so that water or the like, which is an impurity for the composite oxide semiconductor, is removed as much as possible. In particular, the partial pressure of gas molecules corresponding to H2O (gas molecules corresponding to m/z=18) in the deposition chamber 2501 in the standby mode of the sputtering apparatus is preferably lower than or equal to 1×10−4 Pa, further preferably lower than or equal to 5×10−5 Pa.
    • [Third Process: Process of Applying Voltage to Target]
  • The third process includes a step of applying voltage to a target (see Step S301 in FIG. 8).
  • In the third process, for example, voltage is applied to the target holder 2520 a and the target holder 2520 b in FIGS. 7A and 7B. As an example, a potential applied to a terminal V1 connected to the target holder 2520 a is lower than a potential applied to a terminal V2 connected to the substrate holder 2570. A potential applied to a terminal V4 connected to the target holder 2520 b is lower than the potential applied to the terminal V2 connected to the substrate holder 2570. The potential applied to the terminal V2 connected to the substrate holder 2570 is a ground potential. A potential applied to a terminal V3 connected to the magnet holder 2532 is a ground potential.
  • Note that the potentials applied to the terminals V1, V2, V3, and V4 are not limited to the above potentials. Not all the target holder 2520, the substrate holder 2570, and the magnet holder 2532 are necessarily supplied with potentials. For example, the substrate holder 2570 may be electrically floating. Note that it is assumed that a power source capable of controlling a potential applied to the terminal V1 is electrically connected to the terminal V1. As the power source, a DC power source, an AC power source, or an RF power source may be used.
  • As the target 2502 a and the target 2502 b, a target including indium, the element M (M is Al, Ga, Y, or Sn), zinc, and oxygen is preferably used. For example, an In-Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 [atomic ratio]) or an In-Ga—Zn metal oxide target (In:Ga:Zn=5:1:7 [atomic ratio]) can be used as the target 2502 a and the target 2502 b. In the following description, the case of using an In-Ga—Zn metal oxide target (In:Ga:Zn=4:2:4.1 [atomic ratio]) is described.
    • [Fourth Process: Process of Depositing Composite Oxide Semiconductor on Substrate]
  • The fourth process includes a step in which sputtered particles are ejected from the target and a composite oxide semiconductor is deposited on the substrate (see Step S401 in FIG. 8).
  • In the fourth process, for example, in the deposition chamber 2501 illustrated in FIGS. 7A and 7B, an argon gas or an oxygen gas is ionized to be separated into cations and electrons, and plasma is created. Then, the cations in the plasma are accelerated toward the targets 2502 a and 2502 b by the potentials applied to the target holders 2520 a and 2520 b. Sputtered particles are generated when the cations collide with the In-Ga—Zn metal oxide target, and the sputtered particles are deposited on the substrate 2560.
  • Note that in an In-Ga—Zn metal oxide target with an atomic ratio of In:Ga:Zn=4:2:4.1 or 5:1:7 that is used as the targets 2502 a and 2502 b, a plurality of crystal grains with different compositions are included in some cases. In most cases, for example, the diameters of the plurality of crystal grains are each 10 μm or less. In the case where, for example, crystal grains with a high proportion of In are included in the In-Ga—Zn metal oxide target, the proportion of Region A1 described above is increased in some cases.
    • <1-6. Deposition Model>
  • In the fourth process, a deposition model shown in FIGS. 9A to 9C can be presumed.
  • FIGS. 9A to 9C are schematic cross-sectional views of the vicinity of the target 2502 a shown in FIGS. 7A and 7B. Note that FIG. 9A shows the state of the target before use, FIG. 9B shows the state of the target before deposition, and FIG. 9C shows the state of the target during the deposition. Note that in FIGS. 9A to 9C, the target 2502 a, plasma 2190, a cation 2192, sputtered particles 2504 a and 2506 a, and the like are shown.
  • In FIG. 9A, a surface of the target 2502 a is relatively flat and its composition (e.g., the composition ratio between In, Ga, and Zn) is uniform. In contrast, in FIG. 9B, unevenness is formed and compositional segregation occurs on the surface of the target 2502 a by sputtering performed in advance or the like. The unevenness and the segregation can occur because of plasma (e.g., Ar plasma) generated in the sputtering performed in advance. Note that FIG. 9B illustrates a segregation region 2504 and a segregation region 2506. Here, the segregation region 2504 is a region containing a large amount of Ga and a large amount of Zn (a Ga,Zn-rich region), and the segregation region 2506 is a region containing a large amount of In (an In-rich region). The segregation region 2504, which contains a large amount of Ga, is formed because the melting point of Ga lower than that of In allows part of Ga to be melted by heat applied to the target 2502 a during the plasma treatment and aggregate.
    • [First Step]
  • In FIG. 9C, an argon gas or an oxygen gas is ionized and separated into the cations 2192 and electrons (not illustrated), and the plasma 2190 is generated. After that, the cations 2192 in the plasma 2190 are accelerated toward the target 2500 a (here, an In-Ga—Zn oxide target). The cations 2192 collide with the In-Ga—Zn oxide target, whereby the sputtered particles 2504 a and 2506 a are generated and ejected from the In-Ga—Zn oxide target. Note that since the sputtered particles 2504 a are ejected from the segregation region 2504, they form a Ga,Zn-rich cluster in some cases. Since the sputtered particles 2506 a are ejected from the segregation region 2506, they form an In-rich cluster in some cases.
  • When an In-Ga—Zn oxide target is used, presumably, the sputtered particles 2504 a are preferentially sputtered first from the segregation region 2504. This is because Ga and Zn, which have lower relative atomic masses than In, are preferentially ejected from the In-Ga—Zn oxide target by collision of the cation 2192 with the In-Ga—Zn oxide target. The sputtered particles 2504 a that are ejected are deposited over the substrate, thereby forming Region B1 illustrated in FIGS. 1A and 1B and other drawings.
    • [Second Step]
  • Next, as illustrated in FIG. 9C, the sputtered particles 2506 a are sputtered from the segregation region 2506. The sputtered particles 2506 a collide with Region B1 that has been formed over the substrate, thereby forming Region A1 illustrated in FIGS. 1A and 1B and other drawings.
  • As illustrated in FIG. 9C, the target 2502 a is subjected to sputtering throughout the deposition; thus, generation of the segregation region 2504 and disappearance of the segregation region 2504 occur intermittently.
  • The deposition model including the first step and the second step is repeated, whereby the composite oxide semiconductor that is one embodiment of the present invention and shown in FIGS. 1A and 1B and other drawings can be obtained.
  • That is, the sputtered particles (2506 a and 2504 a) are respectively ejected from the In-rich segregation region 2506 and the Ga,Zn-rich segregation region 2504 to be deposited over the substrate. The In-rich regions are connected to each other in a cloud-like manner over the substrate, so that a composite oxide semiconductor of one embodiment of the present invention as illustrated in FIGS. 1A and 1B can be formed. In a film of the composite oxide semiconductor, the In-rich regions are connected to each other in a cloud-like manner. Thanks to this, a transistor using the composite oxide semiconductor has a high on-state current (Ion) and high field-effect mobility (μFE).
  • In this manner, for a transistor having a high on-state current (Ion) and high field-effect mobility (μFE), In is of importance and other metals (e.g., Ga) are not always necessary.
  • Note that described above as an example is a model in which a composite oxide semiconductor of one embodiment of the present invention is formed with the use of an argon gas. In that case, the composite oxide semiconductor might contain many oxygen vacancies. When the composite oxide semiconductor contains many oxygen vacancies, shallow defect states (also referred to as sDOS) are formed in the composite oxide semiconductor in some cases. When sDOS is formed in the composite oxide semiconductor, the sDOS serves as a carrier trap, resulting in a reduction in on-state current and field-effect mobility.
  • Therefore, when a composite oxide semiconductor is formed with the use of an argon gas, it is preferable to supply oxygen into the composite oxide semiconductor after formation thereof so that oxygen vacancies in the composite oxide semiconductor are compensated and sDOS is reduced.
  • Such oxygen supply can be conducted by, for example, performing heat treatment in an atmosphere that contains oxygen after the formation of the composite oxide semiconductor, or performing plasma treatment in an atmosphere that contains oxygen. Alternatively, either an insulating film that is in contact with the composite oxide semiconductor of one embodiment of the present invention or an insulating film in the vicinity of the composite oxide semiconductor is made to contain excess oxygen. Details of the mode in which such an insulating film contains excess oxygen are described in Embodiment 2.
  • Note that the formation method is not limited to a sputtering method; a pulsed laser deposition (PLD) method, a plasma-enhanced chemical vapor deposition (PECVD) method, a thermal chemical vapor deposition (CVD) method, an atomic layer deposition (ALD) method, a vacuum evaporation method, or the like may be used. As an example of a thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method can be given.
    • <1-7. Examination of Deposition Model>
  • Next, to examine the above-described deposition model, the shape of the surface of the sputtering target and distributions of the compositions over the surface were examined. Here, a change in the sputtering target before and after the sputtering was examined.
  • FIG. 10 illustrates fabrication of a sample and an analysis method.
  • For the sample, a part that is cut from a metal oxide target was used. Here, four kinds of targets were used: a metal oxide target in which the atomic ratio between the metal elements is In:Ga:Zn=4:2:3, a metal oxide target in which the atomic ratio between the metal elements is In:Ga:Zn=1:1:1, a metal oxide target in which the atomic ratio between the metal elements is In:Ga:Zn=5:1:6, and a metal oxide target in which the atomic ratio between the metal elements is In:Ga:Zn=5:1:8.
  • Next, a surface of the sample was polished. Then, the polished surface was observed with a scanning electron microscope (SEM) and analyzed by energy dispersive X-ray spectroscopy (SEM-EDX) with regard to the composition. The SEM observation and EDX measurement were performed with EX-370 produced by HORIBA, Ltd. at an acceleration voltage of 15 kV.
  • Next, sputtering was performed on the sample surface. The sputtering was performed for one hour using an argon gas as a deposition gas under a pressure of 0.4 Pa and with a DC power of 200 W.
  • The surface subjected to the sputtering was observed with a SEM and analyzed by SEM-EDX with regard to the composition in the above-described manner.
  • In the analysis by EDX, an EDX spectrum of a point is obtained in such a manner that electron beam irradiation is performed on the point in an analysis target region of a sample, and the energy of characteristic X-rays of the sample generated by the irradiation and its frequency are measured. Here, peaks of an EDX spectrum of the point were attributed to electron transition to the L shell in an In atom, electron transition to the L shell in a Ga atom, and electron transition to the L shell in a Zn atom and the K shell in an O atom, and the proportions of the atoms in the point were calculated. An EDX mapping image indicating distributions of the proportions of the atoms can be obtained through the process in an analysis target region of a sample.
  • First, SEM images and EDX mapping images of the metal oxide target with a ratio of In:Ga:Zn=4:2:3 are shown in FIG. 11 and FIG. 12. FIG. 11 shows the SEM image and EDX mapping images of the sample surface before the sputtering and FIG. 12 shows the SEM image and EDX mapping images of the sample surface after the sputtering. In each of FIG. 11 and FIG. 12, the EDX mapping images of 0 atoms, Zn atoms, Ga atoms, and In atoms existing in the range of the SEM image are shown.
  • As can be seen from FIG. 11, although voids (holes) are observed in the SEM image, the sample surface is relatively flat. In addition, a plurality of grains are observed in the SEM image and it is shown that the sample is polycrystalline. Although the EDX mapping image shows Zn-rich regions in some parts, it was found that the distribution of each element is substantially uniform.
  • Meanwhile, as shown in FIG. 12, it was found that the sputtering formed unevenness on the sample surface. Specifically, a particulate precipitate with a diameter of approximately greater than or equal to 0.1 μm and less than or equal to 5 μm was observed on the sample surface. Note that in the EDX mapping images in FIG. 12, there are points the EDX spectra of which were not obtained owing to the influence of the unevenness on the surface.
  • The EDX mapping images in FIG. 12 show that the 0 atoms, the Zn atoms, and the Ga atoms have distribution reflecting the shape of the sample surface and their proportions vary widely from part to part. Moreover, it is shown that the proportions of the Ga atoms and the Zn atoms tend to be high on the surface of the precipitate. In contrast, the distribution of the In atoms does not reflect the shape of the sample surface and the In atoms are more uniformly distributed than other atoms.
  • FIGS. 13A and 13B show pie graphs showing the ratios between In, Ga, and
  • Zn in some positions. In FIG. 14, the results in FIGS. 13A and 13B are shown in the form of column graphs. As shown in FIGS. 13A and 13B and FIG. 14, the composition ratio is substantially uniform before the sputtering and is substantially the same as that of the target; however, after the sputtering, the composition ratio varies from position to position. Specifically, it was found that the precipitate observed after the sputtering included a region where the amount of In is small and Ga and Zn were segregated.
  • SEM images and EDX mapping images of the metal oxide target with a ratio of In:Ga:Zn=1:1:1 are shown in FIG. 15 and FIG. 16. SEM images and EDX mapping images of the metal oxide target with a ratio of In:Ga:Zn=5:1:6 are shown in FIG. 17 and FIG. 18. SEM images and EDX mapping images of the metal oxide target with a ratio of In:Ga:Zn=5:1:8 are shown in FIG. 19 and FIG. 20.
  • As described above, in any of the cases of the metal oxide targets with different composition ratios, a precipitate having a low proportion of In and high proportions of Ga and Zn exists on the surface after the sputtering. It was also shown that In is distributed more uniformly than O, Ga, and Zn.
  • The above results show that a segregation region that contains a large amount of Ga and a large amount of Zn is formed on the surface of the metal oxide target by the sputtering. It is also found that In is uniformly distributed. This indicates that the above deposition model is quite appropriate.
    • <1-8. Classification of Oxide Semiconductor>
  • Next, classification of oxide semiconductors is described.
  • Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of the non-single-crystal oxide semiconductor include a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a nanocrystalline oxide semiconductor (nc-OS), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.
  • From another perspective, oxide semiconductors are classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of the crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and an nc-OS.
  • An amorphous structure is generally thought to be isotropic and have no non-uniform structure, to be metastable and have no fixed atomic arrangement, to have a flexible bond angle, and to have a short-range order but have no long-range order, for example.
  • In other words, a stable oxide semiconductor cannot be regarded as a completely amorphous oxide semiconductor. Moreover, an oxide semiconductor that is not isotropic (e.g., an oxide semiconductor that has a periodic structure in a microscopic region) cannot be regarded as a completely amorphous oxide semiconductor. In contrast, an a-like OS, which is not isotropic, has an unstable structure that contains a void. Because of its instability, an a-like OS has physical properties similar to those of an amorphous oxide semiconductor.
    • [CAAC-OS]
  • First, a CAAC-OS is described.
  • A CAAC-OS is one of oxide semiconductors and has a plurality of c-axis aligned crystal parts (also referred to as pellets).
  • The CAAC-OS is an oxide semiconductor with high crystallinity. Entry of impurities, formation of defects, or the like might decrease the crystallinity of an oxide semiconductor. This means that the CAAC-OS has few impurities and defects (e.g., oxygen vacancies).
  • Note that an impurity means an element other than the main components of an oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon or the like) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.
  • [nc-OS]
  • Next, an nc-OS is described.
  • Analysis of an nc-OS by XRD is described. When the structure of an nc-OS is analyzed by an out-of-plane method, a peak indicating orientation does not appear. That is, a crystal of an nc-OS does not have orientation.
  • The nc-OS is an oxide semiconductor that has higher regularity than an amorphous oxide semiconductor. Thus, the nc-OS has a lower density of defect states than the a-like OS and the amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS in some cases.
    • [A-Like OS]
  • An a-like OS has a structure between the structure of an nc-OS and the structure of an amorphous oxide semiconductor.
  • The a-like OS contains a void or a low-density region. The a-like OS has an unstable structure because it contains a void.
  • The a-like OS has a lower density than the nc-OS and the CAAC-OS because it contains a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. It is difficult to deposit an oxide semiconductor having a density lower than 78% of the density of the single crystal oxide semiconductor.
  • For example, in the case of an oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of single crystal InGaZnO4 with a rhombohedral crystal structure is 6.357 g/cm3. Accordingly, in the case of the oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm3 and lower than 5.9 g/cm3, for example. In the case of the oxide semiconductor whose atomic ratio of In to Ga and Zn is 1:1:1, the density of the nc-OS and the density of the CAAC-OS are each higher than or equal to 5.9 g/cm3 and lower than 6.3 g/cm3, for example.
  • In the case where an oxide semiconductor having a certain composition does not exist in a single crystal state, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate a density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition may be calculated using a weighted average with respect to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.
  • As described above, oxide semiconductors have various structures and various properties. In the oxide semiconductor film of one embodiment of the present invention, two or more of an amorphous oxide semiconductor, an a-like OS, an nc-OS, and a CAAC-OS may be mixed.
  • Note that Region A1 described above is preferably non-single-crystal. Region B1 is preferably non-single-crystal. Region A1 and Region B1 may have different crystallinities.
  • At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.
    • Embodiment 2
  • In this embodiment, a transistor in a mode different from that of the transistor in Embodiment 1 is described with reference to FIGS. 21A to 21C, FIG. 22, FIGS. 23A and 23B, FIGS. 24A and 24B, FIGS. 25A and 25B, FIGS. 26A and 26B, FIGS. 27A and 27B, FIGS. 28A and 28B, FIGS. 29A and 29B, FIGS. 30A and 30B, FIGS. 31A to 31C, FIGS. 32A to 32C, FIGS. 33A to 33C, FIGS. 34A to 34C, FIGS. 35A to 35C, FIGS. 36A and 36B, FIGS. 37A and 37B, and FIGS. 38A to 38C.
    • <2-1. Structure Example of Transistor>
  • Structures of a transistor of one embodiment of the present invention are described.
    • [Structure Example 1 of Transistor]
  • FIG. 21A is a top view of a transistor 100A. FIG. 21B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 21A. FIG. 21C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 21A. For clarity, some components such as an insulating film 110 are not illustrated in FIG. 21A. As in FIG. 21A, some components are not illustrated in some cases in top views of transistors described below. In addition, the direction of dashed-dotted line X1-X2 may be referred to as the channel length (L) direction, and the direction of dashed-dotted line Y1-Y2 may be referred to as the channel width (W) direction.
  • The transistor 100A illustrated in FIGS. 21A to 21C includes a conductive film 106 over a substrate 102, an insulating film 104 over the conductive film 106, an oxide semiconductor film 108 over the insulating film 104, the insulating film 110 over the oxide semiconductor film 108, a conductive film 112 over the insulating film 110, and an insulating film 116 over the insulating film 104, the oxide semiconductor film 108, and the conductive film 112. The oxide semiconductor film 108 includes a channel region 108 i overlapping with the conductive film 112, a source region 108 s in contact with the insulating film 116, and a drain region 108 d in contact with the insulating film 116.
  • The insulating film 116 contains nitrogen or hydrogen. The insulating film 116 is in contact with the source region 108 s and the drain region 108 d, so that nitrogen or hydrogen that is contained in the insulating film 116 is added to the source region 108 s and the drain region 108 d. The source region 108 s and the drain region 108 d each have a high carrier density when nitrogen or hydrogen is added thereto.
  • The transistor 100A may further include an insulating film 118 over the insulating film 116, a conductive film 120 a electrically connected to the source region 108 s through an opening 141 a provided in the insulating films 116 and 118, and a conductive film 120 b electrically connected to the drain region 108 d through an opening 141 b provided in the insulating films 116 and 118. In addition, an insulating film 122 may be provided over the insulating film 118, the conductive film 120 a, and the conductive film 120 b. Although the structure where the insulating film 122 is provided is shown in FIGS. 21B and 21C, one embodiment of the present invention is not limited thereto, and the insulating film 122 is not necessarily provided.
  • In this specification and the like, the insulating film 104 may be referred to as a first insulating film, the insulating film 110 may be referred to as a second insulating film, the insulating film 116 may be referred to as a third insulating film, the insulating film 118 may be referred to as a fourth insulating film, and the insulating film 122 may be referred to as a fifth insulating film. The insulating film 104 functions as a first gate insulating film and the insulating film 110 functions as a second gate insulating film. The insulating films 116 and 118 function as a protective insulating film and the insulating film 122 functions as a planarization film.
  • The insulating film 110 includes an excess oxygen region. Since the insulating film 110 includes the excess oxygen region, excess oxygen can be supplied to the channel region 108 i included in the oxide semiconductor film 108. As a result, oxygen vacancies that might be formed in the channel region 108 i can be filled with excess oxygen, which can provide a highly reliable semiconductor device.
  • To supply excess oxygen to the oxide semiconductor film 108, excess oxygen may be supplied to the insulating film 104 that is formed under the oxide semiconductor film 108. In that case, excess oxygen contained in the insulating film 104 might also be supplied to the source region 108 s and the drain region 108 d included in the oxide semiconductor film 108. When excess oxygen is supplied to the source region 108 s and the drain region 108 d, the resistance of the source region 108 s and the drain region 108 d might be increased.
  • In contrast, in the structure in which the insulating film 110 formed over the oxide semiconductor film 108 contains excess oxygen, excess oxygen can be selectively supplied only to the channel region 108 i. Alternatively, the carrier density of the source and drain regions 108 s and 108 d can be selectively increased after excess oxygen is supplied to the channel region 108 i and the source and drain regions 108 s and 108 d, in which case an increase in the resistance of the source and drain regions 108 s and 108 d can be prevented.
  • Furthermore, each of the source region 108 s and the drain region 108 d included in the oxide semiconductor film 108 preferably contains an element that forms an oxygen vacancy or an element that is bonded to an oxygen vacancy. Typical examples of the element that forms an oxygen vacancy or the element that is bonded to an oxygen vacancy include hydrogen, boron, carbon, nitrogen, fluorine, phosphorus, sulfur, chlorine, titanium, and a rare gas. Typical examples of the rare gas element include helium, neon, argon, krypton, and xenon. In the case where one or more of the elements that form an oxygen vacancy are contained in the insulating film 116, the element that forms an oxygen vacancy is diffused from the insulating film 116 to the source region 108 s and the drain region 108 d. In addition or alternatively, the element that forms an oxygen vacancy is added to the source region 108 s and the drain region 108 d by impurity addition treatment.
  • An impurity element added to the oxide semiconductor film cuts a bond between a metal element and oxygen in the oxide semiconductor film, so that an oxygen vacancy is formed. Alternatively, when an impurity element is added to the oxide semiconductor film, oxygen bonded to a metal element in the oxide semiconductor film is bonded to the impurity element and detached from the metal element, so that an oxygen vacancy is formed. As a result, the oxide semiconductor film has a higher carrier density, and thus, the conductivity thereof becomes higher.
  • FIG. 22 is an enlarged view showing the oxide semiconductor film 108 in FIG. 21B and its vicinity. As illustrated in FIG. 22, oxygen (O) released from the insulating film 110 because of heat treatment or the like is supplied to the channel region 108 i, so that oxygen vacancies are reduced. As a result, the channel region 108 i can be of an i-type. Meanwhile, hydrogen (H) is supplied to the source region 108 s and the drain region 108 d and this hydrogen and an oxygen vacancy are bonded. Thus, the source region 108 s and the drain region 108 d can be of an n-type. Examples of the above hydrogen include hydrogen contained in a deposition gas used for forming the insulating film 116 and hydrogen released from the insulating film 116 because of heat treatment or the like.
  • The conductive film 106 illustrated in FIGS. 21B and 21C functions as a first gate electrode and the conductive film 112 functions as a second gate electrode. The conductive film 120 a functions as a source electrode and the conductive film 120 b functions as a drain electrode.
  • As illustrated in FIG. 21C, an opening 143 is provided in the insulating films 104 and 110. The conductive film 106 is electrically connected to the conductive film 112 through the opening 143. Thus, the same potential is applied to the conductive film 106 and the conductive film 112. Note that different potentials may be applied to the conductive film 106 and the conductive film 112 without providing the opening 143. Alternatively, the conductive film 106 may be used as a light-shielding film without providing the opening 143. For example, when the conductive film 106 is formed using a light-shielding material, light irradiation of the channel region 108 i from the bottom can be reduced.
  • As illustrated in FIGS. 21B and 21C, the oxide semiconductor film 108 faces the conductive film 106 functioning as the first gate electrode and the conductive film 112 functioning as the second gate electrode and is positioned between the two conductive films functioning as the gate electrodes.
  • Furthermore, the length of the conductive film 112 in the channel width direction is larger than the length of the oxide semiconductor film 108 in the channel width direction. In the channel width direction, the whole oxide semiconductor film 108 is covered with the conductive film 112 with the insulating film 110 placed therebetween. Since the conductive film 112 is connected to the conductive film 106 through the opening 143 provided in the insulating films 104 and 110, a side surface of the oxide semiconductor film 108 in the channel width direction faces the conductive film 112 with the insulating film 110 placed therebetween.
  • In other words, in the channel width direction of the transistor 100A, the conductive films 106 and 112 are connected to each other through the opening 143 provided in the insulating films 104 and 110, and the conductive films 106 and 112 surround the oxide semiconductor film 108 with the insulating films 104 and 110 positioned therebetween.
  • Such a structure enables the oxide semiconductor film 108 included in the transistor 100A to be electrically surrounded by electric fields of the conductive film 106 functioning as the first gate electrode and the conductive film 112 functioning as the second gate electrode. A device structure of a transistor, like that of the transistor 100A, in which electric fields of a first gate electrode and a second gate electrode electrically surround the oxide semiconductor film 108 in which a channel region is formed can be referred to as a surrounded channel (S-channel) structure. Note that the transistor 100A can also be referred to as a dual-gate transistor from the number of its gate electrodes.
  • Since the transistor 100A has the S-channel structure, an electric field for inducing a channel can be effectively applied to the oxide semiconductor film 108 by the conductive film 106 or the conductive film 112; thus, the current drive capability of the transistor 100A can be improved and high on-state current characteristics can be obtained. Owing to the high on-state current, it is possible to reduce the size of the transistor 100A. Furthermore, since the transistor 100A has a structure in which the oxide semiconductor film 108 is surrounded by the conductive film 106 and the conductive film 112, the mechanical strength of the transistor 100A can be increased.
  • When seen in the channel width direction of the transistor 100A, an opening different from the opening 143 may be formed on the side of the oxide semiconductor film 108 on which the opening 143 is not formed.
  • The transistor 100A may be called a top-gate self-aligned (TGSA) FET from the position of the conductive film 112 relative to the oxide semiconductor film 108 or the formation method of the conductive film 112. Note that the semiconductor device of one embodiment of the present invention is not limited to the above example and may be a bottom-gate top-contact (BGTC) FET.
    • <2-2. Components of Transistor>
  • Next, details of the components of the transistor in FIGS. 21A to 21C will be described.
    • [Substrate]
  • The substrate 102 can be formed using a material having heat resistance high enough to withstand heat treatment in the manufacturing process.
  • Specifically, non-alkali glass, soda-lime glass, alkali glass, crystal glass, quartz, sapphire, or the like can be used. Alternatively, an inorganic insulating film may be used. Examples of the inorganic insulating film include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and an aluminum oxide film.
  • The non-alkali glass may have a thickness of greater than or equal to 0.2 mm and less than or equal to 0.7 mm, for example. The non-alkali glass may be polished to obtain the above thickness.
  • As the non-alkali glass, a large-area glass substrate having any of the following sizes can be used: the 6th generation (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the 8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm), and the 10th generation (2950 mm×3400 mm). Thus, a large-sized display device can be manufactured.
  • Alternatively, as the substrate 102, a single-crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon or silicon carbide, a compound semiconductor substrate made of silicon germanium or the like, an SOI substrate, or the like may be used.
  • For the substrate 102, an inorganic material such as a metal may be used. Examples of the inorganic material such as a metal include stainless steel and aluminum.
  • Alternatively, for the substrate 102, an organic material such as a resin, a resin film, or plastic may be used. Examples of the resin film include polyester, polyolefin, polyamide (e.g., nylon or aramid), polyimide, polycarbonate, polyurethane, an acrylic resin, an epoxy resin, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyether sulfone (PES), and a resin having a siloxane bond.
  • For the substrate 102, a composite material of an inorganic material and an organic material may be used. Examples of the composite material include a resin film to which a metal plate or a thin glass plate is bonded, a resin film into which a fibrous or particulate metal or a fibrous or particulate glass is dispersed, and an inorganic material into which a fibrous or particulate resin is dispersed.
  • The substrate 102 can at least support films or layers formed thereover or thereunder and may be one or more of an insulating film, a semiconductor film, and a conductive film.
    • [First Insulating Film]
  • The insulating film 104 can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like as appropriate. For example, the insulating film 104 can be formed to have a single-layer structure or a stacked-layer structure of an oxide insulating film and/or a nitride insulating film. To improve the properties of the interface with the oxide semiconductor film 108, at least a region of the insulating film 104 which is in contact with the oxide semiconductor film 108 is preferably formed using an oxide insulating film. When the insulating film 104 is formed using an oxide insulating film from which oxygen is released by heating, oxygen contained in the insulating film 104 can be moved to the oxide semiconductor film 108 by heat treatment.
  • The thickness of the insulating film 104 can be greater than or equal to 50 nm, greater than or equal to 100 nm and less than or equal to 3000 nm, or greater than or equal to 200 nm and less than or equal to 1000 nm. By increasing the thickness of the insulating film 104, the amount of oxygen released from the insulating film 104 can be increased, and interface states at the interface between the insulating film 104 and the oxide semiconductor film 108 and oxygen vacancies included in the channel region 108 i of the oxide semiconductor film 108 can be reduced.
  • For example, the insulating film 104 can be formed to have a single-layer structure or stacked-layer structure of a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, a silicon nitride film, an aluminum oxide film, a hafnium oxide film, a gallium oxide film, a Ga—Zn oxide film, or the like. In this embodiment, the insulating film 104 has a stacked-layer structure of a silicon nitride film and a silicon oxynitride film. With the insulating film 104 having such a layered structure including a silicon nitride film as a lower layer and a silicon oxynitride film as an upper layer, oxygen can be efficiently introduced into the oxide semiconductor film 108.
    • [Oxide Semiconductor Film]
  • It is favorable that the above-described composite oxide semiconductor or C/IGZO is used for the oxide semiconductor film 108.
    • [Second Insulating Film]
  • The insulating film 110 has a function of supplying oxygen to the oxide semiconductor film 108, particularly to the channel region 108 i. The insulating film 110 can be formed to have a single-layer structure or a stacked-layer structure of an oxide insulating film and/or a nitride insulating film, for example. To improve the interface properties with the oxide semiconductor film 108, a region which is in the insulating film 110 and in contact with the oxide semiconductor film 108 is preferably formed using at least an oxide insulating film. For example, a silicon oxide film, a silicon oxynitride film, a silicon nitride oxide film, or a silicon nitride film can be used as the insulating film 110.
  • The thickness of the insulating film 110 can be greater than or equal to 5 nm and less than or equal to 400 nm, greater than or equal to 5 nm and less than or equal to 300 nm, or greater than or equal to 10 nm and less than or equal to 250 nm.
  • It is preferable that the insulating film 110 have few defects and typically have as few signals observed by electron spin resonance (ESR) spectroscopy as possible. Examples of the signals include a signal due to an E center observed at a g-factor of 2.001. Note that the E center is due to the dangling bond of silicon. As the insulating film 110, a silicon oxide film or a silicon oxynitride film whose spin density of a signal due to the E center is lower than or equal to 3×1017 spins/cm3, preferably lower than or equal to 5×1016 spins/cm3 may be used.
  • In addition to the above-described signal, a signal due to nitrogen dioxide (NO2) might be observed in the insulating film 110. The signal is divided into three signals according to the N nuclear spin; a first signal, a second signal, and a third signal. The first signal is observed at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039. The second signal is observed at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003. The third signal is observed at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966.
  • It is suitable to use an insulating film whose spin density of a signal due to nitrogen dioxide (NO2) is higher than or equal to 1×1017 spins/cm3 and lower than 1×1018 spins/cm3 as the insulating film 110, for example.
  • Note that nitrogen oxide (NOx) such as nitrogen dioxide (NO2) forms a state in the insulating film 110. The state is positioned in the energy gap of the oxide semiconductor film 108. Thus, when nitrogen oxide (NOx) is diffused to the interface between the insulating film 110 and the oxide semiconductor film 108, an electron might be trapped by the state on the insulating film 110 side. As a result, the trapped electron remains in the vicinity of the interface between the insulating film 110 and the oxide semiconductor film 108, leading to a positive shift of the threshold voltage of the transistor. Accordingly, the use of a film with a low nitrogen oxide content as the insulating film 110 can reduce a shift of the threshold voltage of the transistor.
  • As an insulating film that releases a small amount of nitrogen oxide (NOx), for example, a silicon oxynitride film can be used. The silicon oxynitride film releases more ammonia than nitrogen oxide (NOx) in thermal desorption spectroscopy (TDS); the typical released amount of ammonia is greater than or equal to 1×1018/cm3 and less than or equal to 5×1019/cm3. Note that the released amount of ammonia is the total amount of ammonia released by heat treatment in a range of 50° C. to 650° C. or 50° C. to 550° C. in TDS.
  • Since nitrogen oxide (NOx) reacts with ammonia and oxygen in heat treatment, the use of an insulating film that releases a large amount of ammonia reduces nitrogen oxide (NOx).
  • Note that in the case where the insulating film 110 is analyzed by SIMS, nitrogen concentration in the film is preferably lower than or equal to 6×1020 atoms/cm3.
  • The insulating film 110 may be formed using a high-k material such as hafnium silicate (HfSiOx), hafnium silicate containing nitrogen (HfSixOyNz), hafnium aluminate containing nitrogen (HfAlxOyNz), or hafnium oxide. The use of such a high-k material enables a reduction in gate leakage current of a transistor.
    • [Third Insulating Film]
  • The insulating film 116 contains nitrogen or hydrogen. The insulating film 116 may contain fluorine. As the insulating film 116, for example, a nitride insulating film can be used. The nitride insulating film can be formed using silicon nitride, silicon nitride oxide, silicon oxynitride, silicon nitride fluoride, silicon fluoronitride, or the like. The hydrogen concentration in the insulating film 116 is preferably higher than or equal to 1×1022 atoms/cm3. Furthermore, the insulating film 116 is in contact with the source region 108 s and the drain region 108 d of the oxide semiconductor film 108. Thus, the concentration of an impurity (nitrogen or hydrogen) in the source region 108 s and the drain region 108 d in contact with the insulating film 116 is increased, leading to an increase in the carrier density of the source region 108 s and the drain region 108 d.
    • [Fourth Insulating Film]
  • As the insulating film 118, an oxide insulating film can be used. Alternatively, a layered film of an oxide insulating film and a nitride insulating film can be used as the insulating film 118. The insulating film 118 can be formed using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, aluminum oxide, hafnium oxide, gallium oxide, or a Ga—Zn oxide.
  • Furthermore, the insulating film 118 preferably functions as a barrier film against hydrogen, water, and the like from the outside.
  • The thickness of the insulating film 118 can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.
    • [Fifth Insulating Film]
  • The insulating film 122 has an insulating property and is formed using an inorganic material or an organic material. Examples of the inorganic material include silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, and aluminum nitride. Examples of the organic material include photosensitive resin materials such as an acrylic resin and a polyimide resin.
    • [Conductive Film]
  • The conductive films 106, 112, 120 a, and 120 b can be formed by a sputtering method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, a thermal CVD method, or the like. As each of the conductive films 106, 112, 120 a, and 120 b, a metal film having conductivity, a conductive film having a function of reflecting visible light, or a conductive film having a function of transmitting visible light may be used.
  • A material containing a metal element selected from aluminum, gold, platinum, silver, copper, chromium, tantalum, titanium, molybdenum, tungsten, nickel, iron, cobalt, palladium, and manganese can be used for the metal film having conductivity. Alternatively, an alloy containing any of the above metal elements may be used.
  • For the metal film having conductivity, specifically a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a titanium nitride film, a two-layer structure in which a copper film is stacked over a tantalum nitride film, a three-layer structure in which a titanium film, a copper film, and a titanium film are stacked in this order, or the like may be used. In particular, a conductive film containing a copper element is preferably used because the resistance can be reduced. As an example of the conductive film containing a copper element, an alloy film containing copper and manganese is given. The alloy film is favorable because it can be processed by a wet etching method.
  • Note that a tantalum nitride film is preferably used for the conductive films 106, 112, 120 a, and 120 b. A tantalum nitride film has conductivity and a high barrier property against copper or hydrogen. Because a tantalum nitride film releases little hydrogen from itself, it can be favorably used as a metal film in contact with the oxide semiconductor film 108 or a metal film in the vicinity of the oxide semiconductor film 108.
  • As the metal film having conductivity, a conductive macromolecule or a conductive polymer may be used.
  • For the conductive film having a function of reflecting visible light, a material containing a metal element selected from gold, silver, copper, and palladium can be used. In particular, a conductive film containing a silver element is preferably used because reflectance of visible light can be improved.
  • For the conductive film having a function of transmitting visible light, a material containing an element selected from indium, tin, zinc, gallium, and silicon can be used. Specifically, an In oxide, a Zn oxide, an In—Sn oxide (also referred to as ITO), an In—Sn—Si oxide (also referred to as ITSO), an In—Zn oxide, an In-Ga—Zn oxide, or the like can be used.
  • As the conductive film having a function of transmitting visible light, a film containing graphene or graphite may be used. The film containing graphene can be formed in the following manner: a film containing graphene oxide is formed and is reduced. As a reducing method, a method with application of heat, a method using a reducing agent, or the like can be employed.
  • The conductive films 112, 120 a, and 120 b can be formed by electroless plating. As a material that can be deposited by electroless plating, for example, one or more elements selected from Cu, Ni, Al, Au, Sn, Co, Ag, and Pd can be used. It is further favorable to use Cu or Ag because the resistance of the conductive film can be reduced.
  • When the conductive film is formed by electroless plating, a diffusion prevention film may be formed under the conductive film to prevent component elements of the conductive film from diffusing outward. A seed layer that can make the conductive film grow may be formed between the diffusion prevention film and the conductive film. The diffusion prevention film can be formed by sputtering, for example. For the diffusion prevention film, a tantalum nitride film or a titanium nitride film can be used, for example. The seed layer can be formed by electroless plating. For the seed layer, a material similar to the material for the conductive film that can be formed by electroless plating can be used.
  • Note that an oxide semiconductor typified by an In-Ga—Zn oxide may be used for the conductive film 112. The oxide semiconductor can have a high carrier density when nitrogen or hydrogen is supplied from the insulating film 116. In other words, the oxide semiconductor functions as an oxide conductor (OC). Accordingly, the oxide semiconductor can be used for a gate electrode.
  • The conductive film 112 can have, for example, a single-layer structure of an oxide conductor (OC), a single-layer structure of a metal film, or a stacked-layer structure of an oxide conductor (OC) and a metal film.
  • Note that it is favorable that the conductive film 112 has a single-layer structure of a light-shielding metal film or a stacked-layer structure of an oxide conductor (OC) and a light-shielding metal film because the channel region 108 i formed under the conductive film 112 can be shielded from light. In the case where the conductive film 112 has a stacked-layer structure of an oxide semiconductor or an oxide conductor (OC) and a light-shielding metal film, formation of a metal film (e.g., a titanium film or a tungsten film) over the oxide semiconductor or the oxide conductor (OC) produces any of the following effects: the resistance of the oxide semiconductor or the oxide conductor (OC) is reduced by the diffusion of the constituent element of the metal film to the oxide semiconductor or oxide conductor (OC) side, the resistance is reduced by damage (e.g., sputtering damage) during the deposition of the metal film, and the resistance is reduced when oxygen vacancies are formed by the diffusion of oxygen in the oxide semiconductor or the oxide conductor (OC) to the metal film.
  • The thickness of the conductive films 106, 112, 120 a, and 120 b can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.
    • <2-3. Structure Example 2 of Transistor>
  • FIGS. 23A and 23B are cross-sectional views of a transistor 100B. FIGS. 24A and 24B are cross-sectional views of a transistor 100C. FIGS. 25A and 25B are cross-sectional views of a transistor 100D. The top views of the transistors 100B, 100C, and 100D are not illustrated because they are similar to the top view of the transistor 100A in FIG. 21A.
  • The transistor 100B illustrated in FIGS. 23A and 23B is different from the transistor 100A in the layered structure of the conductive film 112, the shape of the conductive film 112, and the shape of the insulating film 110.
  • The conductive film 112 in the transistor 100B includes a conductive film 112_1 over the insulating film 110 and the conductive film 112_2 over the conductive film 112_1. For example, an oxide conductive film is used as the conductive film 112_1, so that excess oxygen can be added to the insulating film 110. The oxide conductive film can be formed by a sputtering method in an atmosphere containing an oxygen gas. As the oxide conductive film, an oxide film containing indium and tin, an oxide film containing tungsten and indium, an oxide film containing tungsten, indium, and zinc, an oxide film containing titanium and indium, an oxide film containing titanium, indium, and tin, an oxide film containing indium and zinc, an oxide film containing silicon, indium, and tin, or an oxide film containing indium, gallium, and zinc can be used, for example.
  • As illustrated in FIG. 23B, the conductive film 112_2 is connected to the conductive film 106 through the opening 143. By forming the opening 143 after a conductive film to be the conductive film 112_1 is formed, the shape illustrated in FIG. 23B can be obtained. In the case where an oxide conductive film is used as the conductive film 112_1, the structure in which the conductive film 112_2 is connected to the conductive film 106 can decrease the contact resistance between the conductive film 112 and the conductive film 106.
  • The conductive film 112 and the insulating film 110 in the transistor 100B have a tapered shape. More specifically, the lower end portion of the conductive film 112 is located outward from the upper end portion of the conductive film 112. The lower end portion of the insulating film 110 is located outward from the upper end portion of the insulating film 110. In addition, the lower end portion of the conductive film 112 is formed in substantially the same position as that of the upper end portion of the insulating film 110.
  • It is favorable that the conductive film 112 and the insulating film 110 of the transistor 100B are formed to have tapered shapes because the coverage with the insulating film 116 can be high as compared with the case of the transistor 100A in which the conductive film 112 and the insulating film 110 have rectangular shapes.
  • The other components of the transistor 100B are similar to those of the transistor 100A described above and have similar effects.
  • The transistor 100C illustrated in FIGS. 24A and 24B is different from the transistor 100A in the layered structure of the conductive film 112, the shape of the conductive film 112, and the shape of the insulating film 110.
  • The conductive film 112 in the transistor 100C includes the conductive film 112_1 over the insulating film 110 and the conductive film 112_2 over the conductive film 112_1. A lower end portion of the conductive film 112_1 is located outward from an upper end portion of the conductive film 112_2. For example, the conductive film 112_1, the conductive film 112_2, and the insulating film 110 are processed with one mask, the conductive film 112_2 is processed by a wet etching method, and the conductive film 112_1 and the insulating film 110 are processed by a dry etching method, whereby the above-described structure can be obtained.
  • With the structure of the transistor 100C, regions 108 f are formed in the oxide semiconductor film 108 in some cases. The regions 108 f are formed between the channel region 108 i and the source region 108 s and between the channel region 108 i and the drain region 108 d.
  • The regions 108 f function as high-resistance regions or low-resistance regions. The high-resistance regions have the same level of resistance as the channel region 108 i and do not overlap with the conductive film 112 functioning as a gate electrode. In the case where the regions 108 f are high-resistance regions, the regions 108 f function as offset regions. To suppress a decrease in the on-state current of the transistor 100C, the regions 108 f functioning as offset regions may each have a length of 1 μm or less in the channel length (L) direction.
  • The low-resistance regions have a resistance that is lower than that of the channel region 108 i and higher than that of the source region 108 s and the drain region 108 d. In the case where the regions 108 f are low-resistance regions, the regions 108 f function as lightly doped drain (LDD) regions. The regions 108 f functioning as LDD regions can relieve an electric field in the drain region, thereby reducing a change in the threshold voltage of the transistor due to the electric field in the drain region.
  • Note that in the case where the regions 108 f serve as LDD regions, for example, the regions 108 f are formed by supplying one or more of nitrogen, hydrogen, and fluorine from the insulating film 116 to the regions 108 f or by adding an impurity element from above the conductive film 112_1 using the insulating film 110 and the conductive film 112_1 as a mask so that the impurity element is added to the oxide semiconductor film 108 through the conductive film 112_1 and the insulating film 110.
  • As illustrated in FIG. 24B, the conductive film 112_2 is connected to the conductive film 106 through the opening 143.
  • The other components of the transistor 100C are similar to those of the transistor 100A described above and have similar effects.
  • The transistor 100D illustrated in FIGS. 25A and 25B is different from the transistor 100A in the layered structure of the conductive film 112, the shape of the conductive film 112, and the shape of the insulating film 110.
  • The conductive film 112 in the transistor 100D includes the conductive film 112_1 over the insulating film 110 and the conductive film 112_2 over the conductive film 112_1. A lower end portion of the conductive film 112_1 is located outward from a lower end portion of the conductive film 112_2. Furthermore, a lower end portion of the insulating film 110 is located outward from the lower end portion of the conductive film 112_1. For example, the conductive film 112_1, the conductive film 112_2, and the insulating film 110 are processed with one mask, the conductive film 112_2 and the conductive film 112_1 are processed by a wet etching method, and the insulating film 110 is processed by a dry etching method, whereby the above-described structure can be obtained.
  • As in the transistor 100C, the regions 108 f are formed in the oxide semiconductor film 108 in the transistor 100D, in some cases. The regions 108 f are formed between the channel region 108 i and the source region 108 s and between the channel region 108 i and the drain region 108 d.
  • As illustrated in FIG. 25B, the conductive film 112_2 is connected to the conductive film 106 through the opening 143.
  • The other components of the transistor 100D are similar to those of the transistor 100A described above and have similar effects.
    • <2-4. Structure Example 3 of Transistor>
  • FIGS. 26A and 26B are cross-sectional views of a transistor 100E. FIGS. 27A and 27B are cross-sectional views of a transistor 100F. FIGS. 28A and 28B are cross-sectional views of a transistor 100G. FIGS. 29A and 29B are cross-sectional views of a transistor 100H. FIGS. 30A and 30B are cross-sectional views of a transistor 100J. The top views of the transistors 100E, 100F, 100G, 100H, and 100J are not illustrated because they are similar to the top view of the transistor 100A in FIG. 21A.
  • The transistors 100E, 100F, 100G, 100H, and 100J are different from the above-described the transistor 100A in the structure of the oxide semiconductor film 108. The other components are similar to those of the transistor 100A and have similar effects.
  • The oxide semiconductor film 108 of the transistor 100E illustrated in FIGS. 26A and 26B includes an oxide semiconductor film 108_1 over the insulating film 104, an oxide semiconductor film 108_2 over the oxide semiconductor film 108_1, and an oxide semiconductor film 1083 over the oxide semiconductor film 1082. The channel region 108 i, the source region 108 s, and the drain region 108 d each have a three-layer structure of the oxide semiconductor film 108_1, the oxide semiconductor film 1082, and the oxide semiconductor film 108_3.
  • The oxide semiconductor film 108 of the transistor 100F illustrated in FIGS. 27A and 27B includes the oxide semiconductor film 108_2 over the insulating film 104, and the oxide semiconductor film 1083 over the oxide semiconductor film 108_2. The channel region 108 i, the source region 108 s, and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108_2 and the oxide semiconductor film 108_3.
  • The oxide semiconductor film 108 of the transistor 100G illustrated in FIGS. 28A and 28B includes the oxide semiconductor film 108_1 over the insulating film 104, and the oxide semiconductor film 1082 over the oxide semiconductor film 108_1. The channel region 108 i, the source region 108 s, and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108_1 and the oxide semiconductor film 108_2.
  • The oxide semiconductor film 108 of the transistor 100H illustrated in FIGS. 29A and 29B includes the oxide semiconductor film 108_1 over the insulating film 104, the oxide semiconductor film 1082 over the oxide semiconductor film 1081, and the oxide semiconductor film 1083 over the oxide semiconductor film 1082. The channel region 108 i has a three-layer structure of the oxide semiconductor film 108_1, the oxide semiconductor film 1082, and the oxide semiconductor film 1083. The source region 108 s and the drain region 108 d each have a two-layer structure of the oxide semiconductor film 108_1 and the oxide semiconductor film 1082. Note that in the cross section of the transistor 100H in the channel width (W) direction, the oxide semiconductor film 108_3 covers side surfaces of the oxide semiconductor film 108_1 and the oxide semiconductor film 1082.
  • The oxide semiconductor film 108 of the transistor 100J illustrated in FIGS. 30A and 30B includes the oxide semiconductor film 108_2 over the insulating film 104, and the oxide semiconductor film 1083 over the oxide semiconductor film 108_2. The channel region 108 i has a two-layer structure of the oxide semiconductor film 108_2 and the oxide semiconductor film 108_3. The source region 108 s and the drain region 108 d each have a single-layer structure of the oxide semiconductor film 108_2. Note that in the cross section of the transistor 100J in the channel width (W) direction, the oxide semiconductor film 108_3 covers side surfaces of the oxide semiconductor film 108_2.
  • A side surface of the channel region 108 i in the channel width (W) direction or a region in the vicinity of the side surface is easily damaged by processing, resulting in a defect (e.g., oxygen vacancy), or easily contaminated by an impurity attached thereto. Therefore, even when the channel region 108 i is substantially intrinsic, stress such as an electric field applied thereto activates the side surface of the channel region 108 i in the channel width (W) direction or the region in the vicinity of the side surface and turns it into a low-resistance (n-type) region easily. Moreover, if the side surface of the channel region 108 i in the channel width (W) direction or the region in the vicinity of the side surface is an n-type region, a parasitic channel may be formed because the n-type region serves as a carrier path.
  • Thus, in the transistor 100H and the transistor 100J, the channel region 108 i has a stacked-layer structure and side surfaces of the channel region 108 i in the channel width (W) direction are covered with one layer of the stacked layers. With such a structure, defects on or in the vicinity of the side surfaces of the channel region 108 i can be suppressed or adhesion of an impurity to the side surfaces of the channel region 108 i or to regions in the vicinity of the side surfaces can be reduced.
    • [Band Structure]
  • Here, a band structure of the insulating film 104, the oxide semiconductor films 108_1, 108_2, and 108_3, and the insulating film 110, a band structure of the insulating film 104, the oxide semiconductor films 108_2 and 108_3, and the insulating film 110, and a band structure of the insulating film 104, the oxide semiconductor films 108_1 and 108_2, and the insulating film 110 will be described with reference to FIGS. 31A to 31C. Note that FIGS. 31A to 31C are each a band structure of the channel region 108 i.
  • FIG. 31A shows an example of a band structure in the thickness direction of a stack including the insulating film 104, the oxide semiconductor films 108_1, 108_2, and 108_3, and the insulating film 110. FIG. 31B shows an example of a band structure in the thickness direction of a stack including the insulating film 104, the oxide semiconductor films 108_2 and 108_3, and the insulating film 110. FIG. 31C shows an example of a band structure in the thickness direction of a stack including the insulating film 104, the oxide semiconductor films 108_1 and 108_2, and the insulating film 110. For easy understanding, the band structures show the conduction band minimum (Ec) of the insulating film 104, the oxide semiconductor films 108_1, 108_2, and 108_3, and the insulating film 110.
  • In the band structure of FIG. 31A, a silicon oxide film is used as each of the insulating films 104 and 110, an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108_1, an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 1082, and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 1083.
  • In the band structure of FIG. 31B, a silicon oxide film is used as each of the insulating films 104 and 110, an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 108_2, and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108_3.
  • In the band structure of FIG. 31C, a silicon oxide film is used as each of the insulating films 104 and 110, an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2 is used as the oxide semiconductor film 108_1, and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 4:2:4.1 is used as the oxide semiconductor film 108_2.
  • As illustrated in FIG. 31A, the conduction band minimum gradually varies between the oxide semiconductor films 108_1, 108_2, and 108_3. As illustrated in FIG. 31B, the conduction band minimum gradually varies between the oxide semiconductor films 108_2 and 108_3. As illustrated in FIG. 31C, the conduction band minimum gradually varies between the oxide semiconductor films 108_1 and 108_2. In other words, the conduction band minimum is continuously changed or continuously connected. To obtain such a band structure, there exists no impurity, which forms a defect state such as a trap center or a recombination center, at the interface between the oxide semiconductor films 108_1 and 1082 or the interface between the oxide semiconductor films 1082 and 1083.
  • To form a continuous junction between the oxide semiconductor films 108_1, 108_2, and 108_3, it is necessary to form the films successively without exposure to the air with a multi-chamber deposition apparatus (sputtering apparatus) provided with a load lock chamber.
  • With the band structure of FIG. 31A, FIG. 31B, or FIG. 31C, the oxide semiconductor film 108_2 serves as a well, and a channel region is formed in the oxide semiconductor film 108_2 in the transistor with the stacked-layer structure.
  • By providing the oxide semiconductor films 108_1 and 108_3, the oxide semiconductor film 108_2 can be distanced away from defect states.
  • In addition, the defect states might be more distant from the vacuum level than the conduction band minimum (Ec) of the oxide semiconductor film 108_2 functioning as a channel region, so that electrons are likely to be accumulated in the defect states. When the electrons are accumulated in the defect states, the electrons become negative fixed electric charge, so that the threshold voltage of the transistor is shifted in the positive direction. Therefore, it is preferable that the defect states be closer to the vacuum level than the conduction band minimum (Ec) of the oxide semiconductor film 108_2. Such a structure inhibits accumulation of electrons in the defect states. As a result, the on-state current and the field-effect mobility of the transistor can be increased.
  • The conduction band minimum of each of the oxide semiconductor films 108_1 and 1083 is closer to the vacuum level than that of the oxide semiconductor film 108_2. A typical difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of each of the oxide semiconductor films 108_1 and 1083 is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less. That is, the electron affinity of the oxide semiconductor film 108_2 is higher than those of the oxide semiconductor films 108_1 and 108_3. The difference between the electron affinity of each of the oxide semiconductor films 108_1 and 108_3 and the electron affinity of the oxide semiconductor film 108_2 is 0.15 eV or more or 0.5 eV or more and 2 eV or less or 1 eV or less.
  • In such a structure, the oxide semiconductor film 108_2 serves as a main path of current. In other words, the oxide semiconductor film 1082 serves as a channel region, and the oxide semiconductor films 108_1 and 108_3 serve as oxide insulating films. It is preferable that the oxide semiconductor films 108_1 and 108_3 each include one or more metal elements constituting the oxide semiconductor film 108_2 in which a channel region is formed. With such a structure, interface scattering hardly occurs at the interface between the oxide semiconductor film 108_1 and the oxide semiconductor film 1082 or at the interface between the oxide semiconductor film 108_2 and the oxide semiconductor film 108_3. Thus, the transistor can have high field-effect mobility because the movement of carriers is not hindered at the interface.
  • To prevent each of the oxide semiconductor films 108_1 and 108_3 from functioning as part of a channel region, a material having sufficiently low conductivity is used for the oxide semiconductor films 108_1 and 108_3. Thus, the oxide semiconductor films 108_1 and 108_3 can be referred to as oxide insulating films for such properties and/or functions. A material used for the oxide semiconductor films 108_1 and 108_3 has a smaller electron affinity (a difference between the vacuum level and the conduction band minimum) than the oxide semiconductor film 108_2 and is selected such that a difference (band offset) exists between the conduction band minimum of each of the oxide semiconductor films 108_1 and 1083 and that of the oxide semiconductor film 108_2. Furthermore, to inhibit generation of a difference in threshold voltage due to the value of the drain voltage, it is preferable to form the oxide semiconductor films 108_1 and 108_3 using a material whose conduction band minimum is closer to the vacuum level than that of the oxide semiconductor film 1082. For example, a difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of each of the oxide semiconductor films 108_1 and 108_3 is preferably greater than or equal to 0.2 eV, further preferably greater than or equal to 0.5 eV.
  • It is preferable that the oxide semiconductor films 108_1 and 108_3 not have a spinel crystal structure. This is because if the oxide semiconductor films 108_1 and 108_3 have a spinel crystal structure, constituent elements of the conductive films 120 a and 120 b might be diffused into the oxide semiconductor film 108_2 at the interface between the spinel crystal structure and another region. Note that each of the oxide semiconductor films 108_1 and 108_3 is preferably a CAAC-OS film described later, in which case a higher blocking property against constituent elements of the conductive films 120 a and 120 b, for example, copper elements, can be obtained.
  • Although the example where an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:2, is used as each of the oxide semiconductor films 108_1 and 1083 is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, 1:1:1.2, 1:3:4, 1:3:6, 1:4:5, 1:5:6, or 1:10:1 may be used as each of the oxide semiconductor films 108_1 and 108_3. Alternatively, oxide semiconductor films formed using a metal oxide target whose atomic ratio of Ga to Zn is 10:1 may be used as the oxide semiconductor films 108_1 and 108_3. In that case, it is favorable that an oxide semiconductor film formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1 is used as the oxide semiconductor film 1082 and an oxide semiconductor film formed using a metal oxide target whose atomic ratio of Ga to Zn is 10:1 is used as each of the oxide semiconductor films 108_1 and 108_3 because the difference between the conduction band minimum of the oxide semiconductor film 1082 and the conduction band minimum of the oxide semiconductor film 108_1 or 108_3 can be 0.6 eV or more.
  • When the oxide semiconductor films 108_1 and 108_3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:1:1, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108_1 and 108_3 might be 1:β1:β2 (0<β1≤2, 0<β2≤2). When the oxide semiconductor films 108_1 and 108_3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:4, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108_1 and 108_3 might be 1:β3:β4 (1≤β3≤5, 2≤β4≤6). When the oxide semiconductor films 108_1 and 108_3 are formed using a metal oxide target whose atomic ratio of In to Ga and Zn is 1:3:6, the atomic ratio of In to Ga and Zn in the oxide semiconductor films 108_1 and 108_3 might be 1:β5:β6 (1≤β5≤5, 4≤β6≤8).
    • <2-5. Structure Example 4 of Transistor>
  • FIG. 32A is a top view of a transistor 300A. FIG. 32B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 32A. FIG. 32C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 32A. Note that in FIG. 32A, some components of the transistor 300A (e.g., an insulating film functioning as a gate insulating film) are not illustrated to avoid complexity. The direction of dashed-dotted line X1-X2 may be referred to as a channel length direction, and the direction of dashed-dotted line Y1-Y2 may be referred to as a channel width direction. As in FIG. 32A, some components are not illustrated in some cases in top views of transistors described below.
  • The transistor 300A illustrated in FIGS. 32A to 32C includes a conductive film 304 over a substrate 302, an insulating film 306 over the substrate 302 and the conductive film 304, an insulating film 307 over the insulating film 306, an oxide semiconductor film 308 over the insulating film 307, a conductive film 312 a over the oxide semiconductor film 308, and a conductive film 312 b over the oxide semiconductor film 308. Over the transistor 300A, specifically, over the conductive films 312 a and 312 b and the oxide semiconductor film 308, an insulating film 314, an insulating film 316, and an insulating film 318 are provided.
  • In the transistor 300A, the insulating films 306 and 307 function as the gate insulating films of the transistor 300A, and the insulating films 314, 316, and 318 function as protective insulating films of the transistor 300A. Furthermore, in the transistor 300A, the conductive film 304 functions as a gate electrode, the conductive film 312 a functions as a source electrode, and the conductive film 312 b functions as a drain electrode.
  • In this specification and the like, the insulating films 306 and 307 may be referred to as a first insulating film, the insulating films 314 and 316 may be referred to as a second insulating film, and the insulating film 318 may be referred to as a third insulating film.
  • The transistor 300A illustrated in FIGS. 32A to 32C is a channel-etched transistor. The oxide semiconductor film of one embodiment of the present invention is suitable for a channel-etched transistor.
    • <2-6. Structure Example 5 of Transistor>
  • FIG. 33A is a top view of a transistor 300B. FIG. 33B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 33A. FIG. 33C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 33A.
  • The transistor 300B illustrated in FIGS. 33A to 33C includes the conductive film 304 over the substrate 302, the insulating film 306 over the substrate 302 and the conductive film 304, the insulating film 307 over the insulating film 306, the oxide semiconductor film 308 over the insulating film 307, the insulating film 314 over the oxide semiconductor film 308, the insulating film 316 over the insulating film 314, the conductive film 312 a electrically connected to the oxide semiconductor film 308 through an opening 341 a provided in the insulating films 314 and 316, and the conductive film 312 b electrically connected to the oxide semiconductor film 308 through an opening 341 b provided in the insulating films 314 and 316. Over the transistor 300B, specifically, over the conductive films 312 a and 312 b and the insulating film 316, the insulating film 318 is provided.
  • In the transistor 300B, the insulating films 306 and 307 each function as a gate insulating film of the transistor 300B, the insulating films 314 and 316 each function as a protective insulating film of the oxide semiconductor film 308, and the insulating film 318 functions as a protective insulating film of the transistor 300B. Moreover, in the transistor 300B, the conductive film 304 functions as a gate electrode, the conductive film 312 a functions as a source electrode, and the conductive film 312 b functions as a drain electrode.
  • The transistor 300A illustrated in FIGS. 32A to 32C has a channel-etched structure, whereas the transistor 300B in FIGS. 33A to 33C has a channel-protective structure. The oxide semiconductor film of one embodiment of the present invention is suitable for a channel-protective transistor as well.
    • <2-7. Structure Example 6 of Transistor>
  • FIG. 34A is a top view of a transistor 300C. FIG. 34B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 34A. FIG. 34C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 34A.
  • The transistor 300C illustrated in FIGS. 34A to 34C is different from the transistor 300B in FIGS. 33A to 33C in the shapes of the insulating films 314 and 316. Specifically, the insulating films 314 and 316 of the transistor 300C have island shapes and are provided over a channel region of the oxide semiconductor film 308. Other components are similar to those of the transistor 300B.
    • <2-8. Structure Example 7 of Transistor>
  • FIG. 35A is a top view of a transistor 300D. FIG. 35B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 35A. FIG. 35C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 35A.
  • The transistor 300D illustrated in FIGS. 35A to 35C includes the conductive film 304 over the substrate 302, the insulating film 306 over the substrate 302 and the conductive film 304, the insulating film 307 over the insulating film 306, the oxide semiconductor film 308 over the insulating film 307, the conductive film 312 a over the oxide semiconductor film 308, the conductive film 312 b over the oxide semiconductor film 308, the insulating film 314 over the oxide semiconductor film 308 and the conductive films 312 a and 312 b, the insulating film 316 over the insulating film 314, the insulating film 318 over the insulating film 316, and conductive films 320 a and 320 b over the insulating film 318.
  • In the transistor 300D, the insulating films 306 and 307 function as first gate insulating films of the transistor 300D, and the insulating films 314, 316, and 318 function as second gate insulating films of the transistor 300D. Furthermore, in the transistor 300D, the conductive film 304 functions as a first gate electrode, the conductive film 320 a functions as a second gate electrode, and the conductive film 320 b functions as a pixel electrode used for a display device. The conductive film 312 a functions as a source electrode, and the conductive film 312 b functions as a drain electrode.
  • As illustrated in FIG. 35C, the conductive film 320 a is connected to the conductive film 304 in an opening 342 b and an opening 342 c provided in the insulating films 306, 307, 314, 316, and 318. Thus, the same potential is applied to the conductive film 320 a and the conductive film 304.
  • The structure of the transistor 300D is not limited to that described above, in which the openings 342 b and 342 c are provided so that the conductive film 320 a is connected to the conductive film 304. For example, a structure in which only one of the openings 342 b and 342 c is provided so that the conductive film 320 a is connected to the conductive film 304, or a structure in which the conductive film 320 a is not connected to the conductive film 304 without providing the openings 342 b and 342 c may be employed. Note that in the case where the conductive film 320 a is not connected to the conductive film 304, it is possible to apply different potentials to the conductive film 320 a and the conductive film 304.
  • The conductive film 320 b is connected to the conductive film 312 b through an opening 342 a provided in the insulating films 314, 316, and 318.
  • Note that the transistor 300D has the S-channel structure described above.
    • <2-9. Structure Example 8 of Transistor>
  • The oxide semiconductor film 308 included in the transistor 300A in FIGS. 32A to 32C may have a stacked-layer structure. FIGS. 36A and 36B and FIGS. 37A and 37B illustrate examples of such a case.
  • FIGS. 36A and 36B are cross-sectional views of a transistor 300E and FIGS. 37A and 37B are cross-sectional views of a transistor 300F. The top views of the transistors 300E and 300F are similar to that of the transistor 300A illustrated in FIG. 32A.
  • The oxide semiconductor film 308 of the transistor 300E illustrated in FIGS. 36A and 36B includes an oxide semiconductor film 308_1, an oxide semiconductor film 3082, and an oxide semiconductor film 3083. The oxide semiconductor film 308 of the transistor 300F illustrated in FIGS. 37A and 37B includes the oxide semiconductor film 3082 and the oxide semiconductor film 3083.
  • Note that the conductive film 304, the insulating film 306, the insulating film 307, the oxide semiconductor film 308, the oxide semiconductor film 308_1, the oxide semiconductor film 3082, the oxide semiconductor film 3083, the conductive films 312 a and 312 b, the insulating film 314, the insulating film 316, the insulating film 318, and the conductive films 320 a and 320 b can be formed using the materials of the conductive film 106, the insulating film 116, the oxide semiconductor film 108, the oxide semiconductor film 108_1, the oxide semiconductor film 1082, the oxide semiconductor film 108_3, the conductive films 120 a and 120 b, the insulating film 104, the insulating film 118, the insulating film 116, and the conductive film 112, respectively, described above.
    • <2-10. Structure Example 9 of Transistor>
  • FIG. 38A is a top view of a transistor 300G. FIG. 38B is a cross-sectional view taken along dashed-dotted line X1-X2 in FIG. 38A. FIG. 38C is a cross-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 38A.
  • The transistor 300G illustrated in FIGS. 38A to 38C includes the conductive film 304 over the substrate 302, the insulating film 306 over the substrate 302 and the conductive film 304, the insulating film 307 over the insulating film 306, the oxide semiconductor film 308 over the insulating film 307, the conductive film 312 a over the oxide semiconductor film 308, the conductive film 312 b over the oxide semiconductor film 308, the insulating film 314 over the oxide semiconductor film 308 and the conductive films 312 a and 312 b, the insulating film 316 over the insulating film 314, the conductive film 320 a over the insulating film 316, and the conductive film 320 b over the insulating film 316.
  • The insulating films 306 and 307 have an opening 351. A conductive film 312 c, which is electrically connected to the conductive film 304 through the opening 351, is formed over the insulating films 306 and 307. The insulating films 314 and 316 have an opening 352 a that reaches the conductive film 312 b and an opening 352 b that reaches the conductive film 312 c.
  • The oxide semiconductor film 308 includes the oxide semiconductor film 3082 on the conductive film 304 side and the oxide semiconductor film 3083 over the oxide semiconductor film 3082.
  • The insulating film 318 is provided over the transistor 300G. The insulating film 318 is formed to cover the insulating film 316, the conductive film 320 a, and the conductive film 320 b.
  • In the transistor 300G, the insulating films 306 and 307 function as first gate insulating films of the transistor 300G, and the insulating films 314 and 316 function as second gate insulating films of the transistor 300G, and the insulating film 318 functions as a protective insulating film of the transistor 300G. Furthermore, in the transistor 300G, the conductive film 304 functions as a first gate electrode, the conductive film 320 a functions as a second gate electrode, and the conductive film 320 b functions as a pixel electrode used for a display device. Moreover, in the transistor 300G, the conductive film 312 a functions as a source electrode, the conductive film 312 b functions as a drain electrode, and the conductive film 312 c functions as a connection electrode.
  • Note that the transistor 300G has the S-channel structure described above.
  • The structures of the transistors 300A to 300G can be freely combined with each other.
  • At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.
    • Embodiment 3
  • In this embodiment, examples of a display device that includes the semiconductor device described in the above embodiments are described below with reference to FIGS. 39 to 44, FIGS. 45A to 45D, and FIG. 46.
  • FIG. 39 is a top view illustrating an example of a display device. A display device 700 in FIG. 39 includes a pixel portion 702 provided over a first substrate 701, a source driver circuit portion 704 and a gate driver circuit portion 706 that are provided over the first substrate 701, a sealant 712 provided to surround the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706, and a second substrate 705 provided to face the first substrate 701. The first substrate 701 and the second substrate 705 are sealed with the sealant 712. That is, the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 are enclosed by the first substrate 701, the sealant 712, and the second substrate 705. Although not illustrated in FIG. 39, a display element is provided between the first substrate 701 and the second substrate 705.
  • In the display device 700, a flexible printed circuit (FPC) terminal portion 708 that is electrically connected to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 is provided in a region different from the region that is over the first substrate 701 and surrounded by the sealant 712. Furthermore, an FPC 716 is connected to the FPC terminal portion 708, and a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706. Furthermore, a signal line 710 is connected to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708. Through the signal line 710, a variety of signals and the like are supplied from the FPC 716 to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708.
  • A plurality of gate driver circuit portions 706 may be provided in the display device 700. The structure of the display device 700 is not limited to the example shown here, in which the source driver circuit portion 704 and the gate driver circuit portion 706 as well as the pixel portion 702 are formed over the first substrate 701. For example, only the gate driver circuit portion 706 may be formed over the first substrate 701, or only the source driver circuit portion 704 may be formed over the first substrate 701. In this case, a substrate over which a source driver circuit, a gate driver circuit, or the like is formed (e.g., a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film) may be formed on the first substrate 701. Note that there is no particular limitation on the method for connecting the separately prepared driver circuit board, and a chip on glass (COG) method, a wire bonding method, or the like can be used.
  • The pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 included in the display device 700 include a plurality of transistors.
  • The display device 700 can include a variety of elements. As examples of the elements, electroluminescent (EL) element (e.g., an EL element containing organic and inorganic materials, an organic EL element, an inorganic EL element, or an LED), a light-emitting transistor element (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, an electronic ink display, an electrophoretic element, an electrowetting element, a plasma display panel (PDP), micro electro mechanical systems (MEMS) display (e.g., a grating light valve (GLV), a digital micromirror device (DMD), a digital micro shutter (DMS) element, or an interferometric modulator display (IMOD) element), a piezoelectric ceramic display, and the like can be given.
  • An example of a display device including an EL element is an EL display. Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). An example of a display device including a liquid crystal element is a liquid crystal display (a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including an electronic ink display or an electrophoretic element is electronic paper. In a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes may function as reflective electrodes. For example, some or all of pixel electrodes may contain aluminum, silver, or the like. In this case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.
  • As a display system of the display device 700, a progressive system, an interlace system, or the like can be employed. Furthermore, color elements controlled in pixels at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of an R pixel, a G pixel, a B pixel, and a W (white) pixel may be used. Alternatively, a color element may be composed of two colors of R, G, and B as in PenTile layout. The two colors may differ depending on the color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Note that the size of a display region may differ between dots of color elements. One embodiment of the disclosed invention is not limited to a color display device; the disclosed invention can also be applied to a monochrome display device.
  • A coloring layer (also referred to as a color filter) may be used to obtain a full-color display device in which white light (W) is used for a backlight (e.g., an organic EL element, an inorganic EL element, an LED, or a fluorescent lamp). For example, a red (R) coloring layer, a green (G) coloring layer, a blue (B) coloring layer, and a yellow (Y) coloring layer can be combined as appropriate. With the use of the coloring layer, high color reproducibility can be obtained as compared with the case without the coloring layer. Here, by providing a region with a coloring layer and a region without a coloring layer, white light in the region without the coloring layer may be directly utilized for display. By partly providing the region without a coloring layer, a decrease in the luminance of a bright image due to the coloring layer can be suppressed, and power consumption can be reduced by approximately 20% to 30% in some cases. In the case where full-color display is performed using a self-luminous element such as an organic EL element or an inorganic EL element, elements may emit light in their respective colors R, G, B, Y, and W. By using a self-luminous element, power consumption may be further reduced as compared with the case of using a coloring layer.
  • As a coloring system, any of the following systems may be used: the above-described color filter system in which part of white light is converted into red light, green light, and blue light through color filters; a three-color system in which red light, green light, and blue light are used; and a color conversion system or a quantum dot system in which part of blue light is converted into red light or green light.
  • In this embodiment, a structure including a liquid crystal element as a display element and a structure including an EL element as a display element are described with reference to FIGS. 40 to 42. FIG. 40 and FIG. 41 are each a cross-sectional view taken along dashed-dotted line Q-R in FIG. 39 and illustrate the structure including a liquid crystal element as a display element. FIG. 42 is a cross-sectional view taken along dashed-dotted line Q-R in FIG. 39 and illustrates the structure including an EL element as a display element.
  • Portions common to FIG. 40, FIG. 41, and FIG. 42 are described first, and then, different portions are described.
    • <3-1. Portions Common to Display Devices>
  • The display device 700 in FIG. 40, FIG. 41, and FIG. 42 includes a lead wiring portion 711, the pixel portion 702, the source driver circuit portion 704, and the FPC terminal portion 708. The lead wiring portion 711 includes the signal line 710. The pixel portion 702 includes a transistor 750 and a capacitor 790. The source driver circuit portion 704 includes a transistor 752.
  • The transistor 750 and the transistor 752 each have a structure similar to that of the transistor 100A described above. Note that the transistor 750 and the transistor 752 may each have the structure of any of the other transistors described in the above embodiments.
  • The transistor used in this embodiment includes an oxide semiconductor film that is highly purified and in which formation of oxygen vacancies is inhibited. The transistor can have a low off-state current. Accordingly, an electrical signal such as an image signal can be held for a long time, and a long writing interval can be set in an on state. Accordingly, the frequency of refresh operation can be reduced, which suppresses power consumption.
  • In addition, the transistor used in this embodiment can have relatively high field-effect mobility and thus is capable of high-speed operation. For example, in a liquid crystal display device that includes such a transistor capable of high-speed operation, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, no additional semiconductor device formed using a silicon wafer or the like is needed as a driver circuit; therefore, the number of components of the semiconductor device can be reduced. In addition, by using the transistor capable of high-speed operation in the pixel portion, a high-quality image can be provided.
  • The capacitor 790 includes a lower electrode and an upper electrode. The lower electrode is formed through a step of processing a conductive film to be a conductive film functioning as a first gate electrode of the transistor 750. The upper electrode is formed through a step of processing a conductive film to be a conductive film functioning as source and drain electrodes of the transistor 750 or a second gate electrode of the transistor 750. Between the lower electrode and the upper electrode, an insulating film formed through a step of forming an insulating film to be an insulating film functioning as a first gate insulating film of the transistor 750 and insulating films formed through a step of forming insulating films to be insulating films functioning as protective insulating films over the transistor 750 are provided. That is, the capacitor 790 has a stacked-layer structure in which an insulating film functioning as a dielectric film is positioned between the pair of electrodes.
  • In FIG. 40, FIG. 41, and FIG. 42, a planarization insulating film 770 is provided over the transistor 750, the transistor 752, and the capacitor 790.
  • Although FIG. 40, FIG. 41, and FIG. 42 each illustrate an example in which the transistor 750 included in the pixel portion 702 and the transistor 752 included in the source driver circuit portion 704 have the same structure, one embodiment of the present invention is not limited thereto. For example, the pixel portion 702 and the source driver circuit portion 704 may include different transistors. Specifically, a structure in which a top-gate transistor is used in the pixel portion 702 and a bottom-gate transistor is used in the source driver circuit portion 704, or a structure in which a bottom-gate transistor is used in the pixel portion 702 and a top-gate transistor is used in the source driver circuit portion 704 may be employed. Note that the term “source driver circuit portion 704” can be replaced by the term “gate driver circuit portion.”
  • The signal line 710 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752. In the case where the signal line 710 is formed using a material containing a copper element, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.
  • The FPC terminal portion 708 includes a connection electrode 760, an anisotropic conductive film 780, and the FPC 716. Note that the connection electrode 760 is formed through the same process as the conductive films functioning as source electrodes and drain electrodes of the transistors 750 and 752. The connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780.
  • For example, glass substrates can be used as the first substrate 701 and the second substrate 705. As the first substrate 701 and the second substrate 705, flexible substrates may also be used. An example of the flexible substrate is a plastic substrate.
  • A structure 778 is provided between the first substrate 701 and the second substrate 705. The structure 778 is a columnar spacer obtained by selective etching of an insulating film and is provided to control the distance (cell gap) between the first substrate 701 and the second substrate 705. Alternatively, a spherical spacer may also be used as the structure 778.
  • A light-blocking film 738 functioning as a black matrix, a coloring film 736 functioning as a color filter, and an insulating film 734 in contact with the light-blocking film 738 and the coloring film 736 are provided on the second substrate 705 side.
    • <3-2. Structure Example of Display Device Including Liquid Crystal Element>
  • The display device 700 in FIG. 40 includes a liquid crystal element 775. The liquid crystal element 775 includes a conductive film 772, a conductive film 774, and a liquid crystal layer 776. The conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode. The display device 700 in FIG. 40 can display an image in such a manner that transmission or non-transmission of light is controlled by the alignment state in the liquid crystal layer 776 that is changed depending on the voltage applied between the conductive film 772 and the conductive film 774.
  • The conductive film 772 is electrically connected to the conductive film functioning as the source electrode or the drain electrode of the transistor 750. The conductive film 772 is formed over the planarization insulating film 770 and functions as a pixel electrode, that is, one electrode of the display element.
  • A conductive film that transmits visible light or a conductive film that reflects visible light can be used as the conductive film 772. For example, a material containing an element selected from indium (In), zinc (Zn), and tin (Sn) is preferably used for the conductive film that transmits visible light. For example, a material containing aluminum or silver is preferably used for the conductive film that reflects visible light.
  • In the case where a conductive film that reflects visible light is used as the conductive film 772, the display device 700 is a reflective liquid crystal display device. In the case where a conductive film that transmits visible light is used as the conductive film 772, the display device 700 is a transmissive liquid crystal display device.
  • The method for driving the liquid crystal element can be changed by changing the structure over the conductive film 772, an example of this case is illustrated in FIG. 41. The display device 700 illustrated in FIG. 41 is an example of employing a horizontal electric field mode (e.g., an FFS mode) as a driving mode of the liquid crystal element. In the structure illustrated in FIG. 41, an insulating film 773 is provided over the conductive film 772, and the conductive film 774 is provided over the insulating film 773. In such a structure, the conductive film 774 functions as a common electrode, and an electric field generated between the conductive film 772 and the conductive film 774 through the insulating film 773 can control the alignment state in the liquid crystal layer 776.
  • Although not illustrated in FIG. 40 and FIG. 41, the conductive film 772 and/or the conductive film 774 may be provided with an alignment film on a side in contact with the liquid crystal layer 776. Although not illustrated in FIG. 40 and FIG. 41, an optical member (optical substrate) or the like, such as a polarizing member, a retardation member, or an anti-reflection member, may be provided as appropriate. For example, circular polarization may be obtained by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.
  • In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. These liquid crystal materials exhibit a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.
  • In the case where a horizontal electric field mode is employed, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. The blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase when the temperature of a cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition containing a liquid crystal exhibiting a blue phase and a chiral material has a short response time and optical isotropy, which eliminates the need for an alignment process. An alignment film does not need to be provided, and thus, rubbing treatment is not necessary; accordingly, electrostatic discharge damage caused by the rubbing treatment can be prevented, and defects and damage of a liquid crystal display device in the manufacturing process can be reduced. Moreover, the liquid crystal material that exhibits a blue phase has small viewing angle dependence.
  • In the case where a liquid crystal element is used as a display element, a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, or the like can be used.
  • Furthermore, a normally black liquid crystal display device such as a vertical alignment (VA) mode transmissive liquid crystal display device may also be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, and an ASV mode, or the like can be employed.
    • <3-3. Display Device Including Light-Emitting Element>
  • The display device 700 illustrated in FIG. 42 includes a light-emitting element 782. The light-emitting element 782 includes a conductive film 772, an EL layer 786, and a conductive film 788. The display device 700 illustrated in FIG. 42 can display an image by utilizing light emission from the EL layer 786 of the light-emitting element 782. Note that the EL layer 786 contains an organic compound or an inorganic compound such as a quantum dot.
  • Examples of materials that can be used for an organic compound include a fluorescent material and a phosphorescent material. Examples of materials that can be used for a quantum dot include a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, and a core quantum dot material. A material containing elements belonging to Groups 12 and 16, elements belonging to Groups 13 and 15, or elements belonging to Groups 14 and 16, may be used. Alternatively, a quantum dot material containing an element such as cadmium (Cd), selenium (Se), zinc (Zn), sulfur (S), phosphorus (P), indium (In), tellurium (Te), lead (Pb), gallium (Ga), arsenic (As), or aluminum (Al) may be used.
  • The above-described organic compound and the inorganic compound can be deposited by a method such as an evaporation method (including a vacuum evaporation method), a droplet discharge method (also referred to as an ink-jet method), a coating method, or a gravure printing method. A low molecular material, a middle molecular material (including an oligomer and a dendrimer), or a high molecular material may be included in the EL layer 786.
  • Here, a method for forming the EL layer 786 by a droplet discharge method is described with reference to FIGS. 45A to 45D. FIGS. 45A to 45D are cross-sectional views illustrating the method for forming the EL layer 786.
  • First, the conductive film 772 is formed over the planarization insulating film 770, and an insulating film 730 is formed to cover part of the conductive film 772 (see FIG. 45A).
  • Then, a droplet 784 is discharged to an exposed portion of the conductive film 772, which is an opening of the insulating film 730, from a droplet discharge apparatus 783, so that a layer 785 containing a composition is formed. The droplet 784 is a composition containing a solvent and is attached to the conductive film 772 (see FIG. 45B).
  • Note that the step of discharging the droplet 784 may be performed under reduced pressure.
  • Next, the solvent is removed from the layer 785 containing the composition, and the resulting layer is solidified to form the EL layer 786 (see FIG. 45C).
  • The solvent may be removed by drying or heating.
  • Next, the conductive film 788 is formed over the EL layer 786; thus, the light-emitting element 782 is formed (see FIG. 45D).
  • When the EL layer 786 is formed by a droplet discharge method as described above, the composition can be selectively discharged; accordingly, waste of material can be reduced. Furthermore, a lithography process or the like for shaping is not needed, and thus, the process can be simplified and cost reduction can be achieved.
  • The droplet discharge method described above is a general term for a means including a nozzle equipped with a composition discharge opening or a means to discharge droplets such as a head having one or a plurality of nozzles.
  • Next, a droplet discharge apparatus used for the droplet discharge method is described with reference to FIG. 46. FIG. 46 is a conceptual diagram illustrating a droplet discharge apparatus 1400.
  • The droplet discharge apparatus 1400 includes a droplet discharge means 1403. In addition, the droplet discharge means 1403 is equipped with a head 1405 and a head 1412.
  • The heads 1405 and 1412 are connected to a control means 1407, and this control means 1407 is controlled by a computer 1410; thus, a preprogrammed pattern can be drawn.
  • The drawing may be conducted at a timing, for example, based on a marker 1411 formed over a substrate 1402. Alternatively, the reference point may be determined on the basis of an outer edge of the substrate 1402. Here, the marker 1411 is detected by an imaging means 1404 and converted into a digital signal by an image processing means 1409. Then, the digital signal is recognized by the computer 1410, and then, a control signal is generated and transmitted to the control means 1407.
  • An image sensor or the like using a charge coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS) can be used as the imaging means 1404. Note that information about a pattern to be formed over the substrate 1402 is stored in a storage medium 1408, and a control signal is transmitted to the control means 1407 based on the information, so that each of the heads 1405 and 1412 of the droplet discharge means 1403 can be individually controlled. The heads 1405 and 1412 are supplied with a material to be discharged from material supply sources 1413 and 1414 through pipes, respectively.
  • Inside the head 1405, a space as indicated by a dotted line 1406 to be filled with a liquid material and a nozzle which is a discharge outlet are provided. Although it is not shown, an inside structure of the head 1412 is similar to that of the head 1405. When the nozzle sizes of the heads 1405 and 1412 are different from each other, different materials with different widths can be discharged simultaneously. Each head can discharge and draw a plurality of light emitting materials. In the case of drawing over a large area, the same material can be simultaneously discharged to be drawn from a plurality of nozzles in order to improve throughput. When a large substrate is used, the heads 1405 and 1412 can freely scan the substrate in directions indicated by arrows X, Y, and Z in FIG. 46, and a region in which a pattern is drawn can be freely set. Thus, a plurality of the same patterns can be drawn over one substrate.
  • Furthermore, a step of discharging the composition may be performed under reduced pressure. A substrate may be heated when the composition is discharged. After discharging the composition, either drying or baking or both of them are performed. Both the drying and baking are heat treatments but different in purpose, temperature, and time period. The steps of drying and baking are performed under normal pressure or under reduced pressure by laser irradiation, rapid thermal annealing, heating using a heating furnace, or the like. Note that there is no particular limitation on the timing of the heat treatment and the number of times of the heat treatment. The temperature for performing each of the steps of drying and baking in a favorable manner depends on the materials of the substrate and the properties of the composition.
  • In the above-described manner, the EL layer 786 can be formed with the droplet discharge apparatus.
  • The display device 700 shown in FIG. 42 is described again.
  • In the display device 700 in FIG. 42, the insulating film 730 is provided over the planarization insulating film 770 and the conductive film 772. The insulating film 730 covers part of the conductive film 772. Note that the light-emitting element 782 has a top-emission structure. Thus, the conductive film 788 has a light-transmitting property and transmits light emitted from the EL layer 786. Although the top-emission structure is described as an example in this embodiment, the structure is not limited thereto. For example, a bottom-emission structure in which light is emitted to the conductive film 772 side or a dual-emission structure in which light is emitted to both the conductive film 772 side and the conductive film 788 side may also be employed.
  • The coloring film 736 is provided to overlap with the light-emitting element 782, and the light-blocking film 738 is provided in the lead wiring portion 711 and the source driver circuit portion 704 to overlap with the insulating film 730. The coloring film 736 and the light-blocking film 738 are covered with the insulating film 734. A space between the light-emitting element 782 and the insulating film 734 is filled with a sealing film 732. The structure of the display device 700 is not limited to the example in FIG. 42, in which the coloring film 736 is provided. For example, a structure without the coloring film 736 may also be employed in the case where the EL layer 786 is formed by separate coloring.
  • <3-4. Structure Example of Display Device Provided with Input/Output Device>
  • An input/output device may be provided in the display device 700 illustrated in FIG. 41 and FIG. 42. As an example of the input/output device, a touch panel or the like can be given.
  • FIG. 43 illustrates a structure in which the display device 700 illustrated in FIG. 41 includes a touch panel 791. FIG. 44 illustrates a structure in which the display device 700 illustrated in FIG. 42 includes the touch panel 791.
  • FIG. 43 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 41, and FIG. 44 is a cross-sectional view of the structure in which the touch panel 791 is provided in the display device 700 illustrated in FIG. 42.
  • First, the touch panel 791 illustrated in FIG. 43 and FIG. 44 is described below.
  • The touch panel 791 illustrated in FIG. 43 and FIG. 44 is what is called an in-cell touch panel provided between the second substrate 705 and the coloring film 736. The touch panel 791 is formed on the second substrate 705 side before the coloring film 736 is formed.
  • Note that the touch panel 791 includes the light-blocking film 738, an insulating film 792, an electrode 793, an electrode 794, an insulating film 795, an electrode 796, and an insulating film 797. A change in the capacitance between the electrodes 793 and 794 can be detected when an object such as a finger or a stylus approaches, for example.
  • A portion in which the electrode 793 intersects with the electrode 794 is illustrated in the upper portion of the transistor 750 illustrated in FIG. 43 and FIG. 44. The electrode 796 is electrically connected to the two electrodes 793 between which the electrode 794 is sandwiched through openings provided in the insulating film 795. Note that a structure in which a region where the electrode 796 is provided is provided in the pixel portion 702 is illustrated in FIG. 43 and FIG. 44 as an example; however, one embodiment of the present invention is not limited thereto. For example, the region where the electrode 796 is provided may be provided in the source driver circuit portion 704.
  • The electrode 793 and the electrode 794 are provided in a region overlapping with the light-blocking film 738. As illustrated in FIG. 43, it is preferable that the electrode 793 not overlap with the light-emitting element 782. As illustrated in FIG. 44, it is preferable that the electrode 793 not overlap with the liquid crystal element 775. In other words, the electrode 793 has an opening in a region overlapping with the light-emitting element 782 and the liquid crystal element 775. That is, the electrode 793 has a mesh shape. With such a structure, the electrode 793 does not block light emitted from the light-emitting element 782, or alternatively the electrode 793 does not block light transmitted through the liquid crystal element 775. Thus, since luminance is hardly reduced even when the touch panel 791 is provided, a display device with high visibility and low power consumption can be obtained. Note that the electrode 794 can have a structure similar to that of the electrode 793.
  • Since the electrode 793 and the electrode 794 do not overlap with the light-emitting element 782, a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794. Furthermore, since the electrode 793 and the electrode 794 do not overlap with the liquid crystal element 775, a metal material having low transmittance with respect to visible light can be used for the electrode 793 and the electrode 794.
  • Thus, as compared with the case of using an oxide material whose transmittance of visible light is high, resistance of the electrodes 793 and 794 can be reduced, whereby sensitivity of the sensor of the touch panel can be increased.
  • For example, a conductive nanowire may be used for the electrodes 793, 794, and 796. The nanowire may have a mean diameter of greater than or equal to 1 nm and less than or equal to 100 nm, preferably greater than or equal to 5 nm and less than or equal to 50 nm, further preferably greater than or equal to 5 nm and less than or equal to 25 nm. As the nanowire, a carbon nanotube or a metal nanowire such as an Ag nanowire, a Cu nanowire, or an Al nanowire may be used. For example, in the case where an Ag nanowire is used for any one of or all of electrodes 793, 794, and 796, the transmittance of visible light can be greater than or equal to 89% and the sheet resistance can be greater than or equal to 40 Ω/sq. and less than or equal to 100 Ω/sq.
  • Although the structure of the in-cell touch panel is illustrated in FIG. 43 and FIG. 44, one embodiment of the present invention is not limited thereto. For example, a touch panel formed over the display device 700, what is called an on-cell touch panel, or a touch panel attached to the display device 700, what is called an out-cell touch panel may be used.
  • In this manner, the display device of one embodiment of the present invention can be combined with various types of touch panels.
  • At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.
    • Embodiment 4
  • In this embodiment, a display device including a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 47A to 47C.
    • <4. Circuit Configuration of Display Device>
  • A display device illustrated in FIG. 47A includes a region including pixels of display elements (hereinafter referred to as a pixel portion 502), a circuit portion that is provided outside the pixel portion 502 and includes a circuit for driving the pixels (hereinafter, the circuit portion is referred to as a driver circuit portion 504), circuits having a function of protecting elements (hereinafter, the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.
  • Part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed. Thus, the number of components and the number of terminals can be reduced. When part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).
  • The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X (X is a natural number of 2 or more) rows and Y (Y is a natural number of 2 or more) columns (hereinafter, the circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).
  • The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter referred to as scan lines GL_1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the gate driver 504 a.
  • The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (image signal) from which a data signal is generated, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 from the image signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter referred to as data lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, another signal can be supplied from the source driver 504 b.
  • The source driver 504 b includes a plurality of analog switches, for example. The source driver 504 b can output, as data signals, time-divided image signals obtained by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.
  • A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of data lines DL supplied with data signals, respectively. Writing and holding of the data signal in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of X or less, and n is a natural number of Y or less), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the data line DL_n in accordance with the potential of the scan line GL_m.
  • The protection circuit 506 in FIG. 47A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the data line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 refers to a portion having terminals for inputting power, control signals, and image signals from external circuits to the display device.
  • The protection circuit 506 electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is supplied to the wiring connected to the protection circuit.
  • As illustrated in FIG. 47A, the protection circuits 506 provided for the pixel portion 502 and the driver circuit portion 504 can improve the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like. Note that the configuration of the protection circuits 506 is not limited thereto; for example, the protection circuit 506 can be connected to the gate driver 504 a or the source driver 504 b. Alternatively, the protection circuit 506 can be connected to the terminal portion 507.
  • One embodiment of the present invention is not limited to the example in FIG. 47A, in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b. For example, only the gate driver 504 a may be formed, and a separately prepared substrate over which a source driver circuit is formed (e.g., a driver circuit board formed using a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.
  • Each of the plurality of pixel circuits 501 in FIG. 47A can have the configuration illustrated in FIG. 47B, for example.
  • The pixel circuit 501 in FIG. 47B includes a liquid crystal element 570, a transistor 550, and a capacitor 560. As the transistor 550, the transistor described in the above embodiment can be used.
  • The potential of one of a pair of electrodes of the liquid crystal element 570 is set as appropriate in accordance with the specifications of the pixel circuit 501. The alignment state of the liquid crystal element 570 depends on data written thereto. A common potential may be supplied to the one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501. The potential supplied to the one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 may differ between rows.
  • Examples of a method for driving the display device including the liquid crystal element 570 include a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an anti-ferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, and a transverse bend alignment (TBA) mode. Other examples of the method for driving the display device include an electrically controlled birefringence (ECB) mode, a polymer-dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Without being limited thereto, various liquid crystal elements and driving methods can be used.
  • In the pixel circuit 501 in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the data line DL_n, and the other of the source electrode and the drain electrode of the transistor 550 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. A gate electrode of the transistor 550 is electrically connected to the scan line GL_m. The transistor 550 is configured to be turned on or off to control whether a data signal is written.
  • One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other of the pair of electrodes of the capacitor 560 is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. The potential of the potential supply line VL is set as appropriate in accordance with the specifications of the pixel circuit 501. The capacitor 560 functions as a storage capacitor for storing written data.
  • For example, in the display device including the pixel circuits 501 in FIG. 47B, the gate driver 504 a in FIG. 47A sequentially selects the pixel circuits 501 row by row to turn on the transistors 550, and data signals are written.
  • When the transistor 550 is turned off, the pixel circuit 501 to which the data has been written is brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.
  • Alternatively, each of the plurality of pixel circuits 501 in FIG. 47A can have the configuration illustrated in FIG. 47C, for example.
  • The pixel circuit 501 in FIG. 47C includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. The transistor described in the above embodiment can be used as the transistor 552 and/or the transistor 554.
  • One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring through which a data signal is supplied (hereinafter referred to as a data line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring through which a gate signal is supplied (hereinafter referred to as a scan line GL_m).
  • The transistor 552 is configured to be turned on or off to control whether a data signal is written.
  • One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring through which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other of the pair of electrodes of the capacitor 562 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.
  • The capacitor 562 functions as a storage capacitor for storing written data.
  • One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a. A gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.
  • One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other of the anode and the cathode of the light-emitting element 572 is electrically connected to the other of the source electrode and the drain electrode of the transistor 554.
  • As the light-emitting element 572, an organic electroluminescent element (also referred to as an organic EL element) can be used, for example. Note that the light-emitting element 572 is not limited thereto and may be an inorganic EL element including an inorganic material.
  • A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other of the potential supply line VL_a and the potential supply line VL_b.
  • In the display device including the pixel circuits 501 in FIG. 47C, the gate driver 504 a in FIG. 47A sequentially selects the pixel circuits 501 row by row to turn on the transistors 552, and data signals are written.
  • When the transistor 552 is turned off, the pixel circuit 501 to which the data has been written is brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.
  • At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.
    • Embodiment 5
  • In this embodiment, a display module and electronic devices, each of which includes a semiconductor device of one embodiment of the present invention, are described with reference to FIG. 48, FIGS. 49A to 49E, FIGS. 50A to 50G, and FIGS. 51A and 51B.
    • <5-1. Display Module>
  • In a display module 7000 illustrated in FIG. 48, a touch panel 7004 connected to an FPC 7003, a display panel 7006 connected to an FPC 7005, a backlight 7007, a frame 7009, a printed-circuit board 7010, and a battery 7011 are provided between an upper cover 7001 and a lower cover 7002.
  • The semiconductor device of one embodiment of the present invention can be used for the display panel 7006, for example.
  • The shapes and sizes of the upper cover 7001 and the lower cover 7002 can be changed as appropriate in accordance with the sizes of the touch panel 7004 and the display panel 7006.
  • The touch panel 7004 can be a resistive touch panel or a capacitive touch panel and overlap with the display panel 7006. Alternatively, a counter substrate (sealing substrate) of the display panel 7006 can have a touch panel function. Alternatively, a photosensor may be provided in each pixel of the display panel 7006 to form an optical touch panel.
  • The backlight 7007 includes a light source 7008. One embodiment of the present invention is not limited to the structure in FIG. 48, in which the light source 7008 is provided over the backlight 7007. For example, a structure in which the light source 7008 is provided at an end portion of the backlight 7007 and a light diffusion plate is further provided may be employed. Note that the backlight 7007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.
  • The frame 7009 protects the display panel 7006 and functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed-circuit board 7010. The frame 7009 may also function as a radiator plate.
  • The printed-circuit board 7010 includes a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the separate battery 7011 may be used. The battery 7011 can be omitted in the case where a commercial power source is used.
  • The display module 7000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.
    • <5-2. Electronic Device 1>
  • Next, FIGS. 49A to 49E illustrate examples of electronic devices.
  • FIG. 49A is an external view of a camera 8000 to which a finder 8100 is attached.
  • The camera 8000 includes a housing 8001, a display portion 8002, an operation button 8003, a shutter button 8004, and the like. Furthermore, an attachable lens 8006 is attached to the camera 8000.
  • Although the lens 8006 of the camera 8000 here is detachable from the housing 8001 for replacement, the lens 8006 may be included in the housing 8001.
  • Images can be taken with the camera 8000 at the press of the shutter button 8004. In addition, images can be taken at the touch of the display portion 8002 that serves as a touch panel.
  • The housing 8001 of the camera 8000 includes a mount including an electrode, so that the finder 8100, a stroboscope, or the like can be connected to the housing 8001.
  • The finder 8100 includes a housing 8101, a display portion 8102, a button 8103, and the like.
  • The housing 8101 includes a mount for engagement with the mount of the camera 8000 so that the finder 8100 can be connected to the camera 8000. The mount includes an electrode, and an image or the like received from the camera 8000 through the electrode can be displayed on the display portion 8102.
  • The button 8103 serves as a power button. The display portion 8102 can be turned on and off with the button 8103.
  • A display device of one embodiment of the present invention can be used in the display portion 8002 of the camera 8000 and the display portion 8102 of the finder 8100.
  • Although the camera 8000 and the finder 8100 are separate and detachable electronic devices in FIG. 49A, the housing 8001 of the camera 8000 may include a finder having a display device.
  • FIG. 49B is an external view of a head-mounted display 8200.
  • The head-mounted display 8200 includes a mounting portion 8201, a lens 8202, a main body 8203, a display portion 8204, a cable 8205, and the like. The mounting portion 8201 includes a battery 8206.
  • Power is supplied from the battery 8206 to the main body 8203 through the cable 8205. The main body 8203 includes a wireless receiver or the like to receive video data, such as image data, and display it on the display portion 8204. The movement of the eyeball and the eyelid of a user is captured by a camera in the main body 8203 and then coordinates of the points the user looks at are calculated using the captured data to utilize the eye of the user as an input means.
  • The mounting portion 8201 may include a plurality of electrodes so as to be in contact with the user. The main body 8203 may be configured to sense current flowing through the electrodes with the movement of the user's eyeball to recognize the direction of his or her eyes. The main body 8203 may be configured to sense current flowing through the electrodes to monitor the user's pulse. The mounting portion 8201 may include sensors, such as a temperature sensor, a pressure sensor, or an acceleration sensor so that the user's biological information can be displayed on the display portion 8204. The main body 8203 may be configured to sense the movement of the user's head or the like to move an image displayed on the display portion 8204 in synchronization with the movement of the user's head or the like.
  • The display device of one embodiment of the present invention can be used in the display portion 8204.
  • FIGS. 49C to 49E are external views of a head-mounted display 8300. The head-mounted display 8300 includes a housing 8301, a display portion 8302, an object for fixing, such as a band, 8304, and a pair of lenses 8305.
  • A user can see display on the display portion 8302 through the lenses 8305. It is favorable that the display portion 8302 be curved. When the display portion 8302 is curved, a user can feel high realistic sensation of images. Although the structure described in this embodiment as an example has one display portion 8302, the number of the display portions 8302 provided is not limited to one. For example, two display portions 8302 may be provided, in which case one display portion is provided for one corresponding user's eye, so that three-dimensional display using parallax or the like is possible.
  • The display device of one embodiment of the present invention can be used in the display portion 8302. The display device including the semiconductor device of one embodiment of the present invention has an extremely high resolution; thus, even when an image is magnified using the lenses 8305 as illustrated in FIG. 49E, the user does not perceive pixels, and thus a more realistic image can be displayed.
    • <5-3. Electronic Device 2>
  • Next, FIGS. 50A to 50G illustrate examples of electronic devices that are different from those illustrated in FIGS. 49A to 49E.
  • Electronic devices illustrated in FIGS. 50A to 50G include a housing 9000, a display portion 9001, a speaker 9003, an operation key 9005 (including a power switch or an operation switch), a connection terminal 9006, a sensor 9007 (a sensor having a function of measuring force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 9008, and the like.
  • The electronic devices in FIGS. 50A to 50G have a variety of functions such as a function of displaying a variety of information (e.g., a still image, a moving image, and a text image) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling processing with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, and a function of reading out a program or data stored in a memory medium and displaying it on the display portion. Note that functions of the electronic devices in FIGS. 50A to 50G are not limited thereto, and the electronic devices can have a variety of functions. Although not illustrated in FIGS. 50A to 50G, the electronic devices may each have a plurality of display portions. Furthermore, the electronic devices may each be provided with a camera and the like to have a function of taking a still image, a function of taking a moving image, a function of storing the taken image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying the taken image on the display portion, or the like.
  • The electronic devices in FIGS. 50A to 50G are described in detail below.
  • FIG. 50A is a perspective view illustrating a television device 9100. The television device 9100 can include the display portion 9001 having a large screen size of, for example, 50 inches or more, or 100 inches or more.
  • FIG. 50B is a perspective view of a portable information terminal 9101. The portable information terminal 9101 functions as, for example, one or more of a telephone set, a notebook, and an information browsing system. Specifically, the portable information terminal 9101 can be used as a smartphone. Note that the portable information terminal 9101 may include a speaker, a connection terminal, a sensor, or the like. The portable information terminal 9101 can display text and image information on its plurality of surfaces. For example, three operation buttons 9050 (also referred to as operation icons or simply as icons) can be displayed on one surface of the display portion 9001. Furthermore, information 9051 indicated by dashed rectangles can be displayed on another surface of the display portion 9001. Examples of the information 9051 include display indicating reception of an e-mail, a social networking service (SNS) message, or a telephone call, the title and sender of an e-mail or an SNS message, date, time, remaining battery, and reception strength of an antenna. Alternatively, the operation buttons 9050 or the like may be displayed in place of the information 9051.
  • FIG. 50C is a perspective view of a portable information terminal 9102. The portable information terminal 9102 has a function of displaying information on three or more surfaces of the display portion 9001. Here, information 9052, information 9053, and information 9054 are displayed on different surfaces. For example, a user of the portable information terminal 9102 can see the display (here, the information 9053) on the portable information terminal 9102 put in a breast pocket of his/her clothes. Specifically, a caller's phone number, name, or the like of an incoming call is displayed in a position that can be seen from above the portable information terminal 9102. The user can see the display without taking out the portable information terminal 9102 from the pocket and decide whether to answer the call.
  • FIG. 50D is a perspective view of a watch-type portable information terminal 9200. The portable information terminal 9200 is capable of executing a variety of applications such as mobile phone calls, e-mailing, reading and editing texts, music reproduction, Internet communication, and a computer game. The display surface of the display portion 9001 is curved, and display can be performed on the curved display surface. The portable information terminal 9200 can employ near field communication conformable to a communication standard. For example, hands-free calling can be achieved by mutual communication between the portable information terminal 9200 and a headset capable of wireless communication. Moreover, the portable information terminal 9200 includes the connection terminal 9006 and can perform direct data communication with another information terminal via a connector. Charging through the connection terminal 9006 is also possible. Note that the charging operation may be performed by wireless power feeding without using the connection terminal 9006.
  • FIGS. 50E, 50F, and 50G are perspective views of a foldable portable information terminal 9201 that is opened, that is shifted from the opened state to the folded state or from the folded state to the opened state, and that is folded, respectively. The portable information terminal 9201 is highly portable when folded. When the portable information terminal 9201 is opened, a seamless large display region is highly browsable. The display portion 9001 of the portable information terminal 9201 is supported by three housings 9000 joined by hinges 9055. By being folded at the hinges 9055 between the two adjacent housings 9000, the portable information terminal 9201 can be reversibly changed in shape from the opened state to the folded state. For example, the portable information terminal 9201 can be bent with a radius of curvature greater than or equal to 1 mm and less than or equal to 150 mm.
  • Next, an example of an electronic device that is different from the electronic devices illustrated in FIGS. 49A to 49E and FIGS. 50A to 50G is illustrated in FIGS. 51A and 51B. FIGS. 51A and 51B are perspective views of a display device including a plurality of display panels. The plurality of display panels are wound in the perspective view in FIG. 51A and are unwound in the perspective view in FIG. 51B.
  • A display device 9500 illustrated in FIGS. 51A and 51B includes a plurality of display panels 9501, a hinge 9511, and a bearing 9512. The plurality of display panels 9501 each include a display region 9502 and a light-transmitting region 9503.
  • Each of the plurality of display panels 9501 is flexible. Two adjacent display panels 9501 are provided so as to partly overlap with each other. For example, the light-transmitting regions 9503 of the two adjacent display panels 9501 can overlap with each other. A display device having a large screen can be obtained with the plurality of display panels 9501. The display device is highly versatile because the display panels 9501 can be wound depending on its use.
  • Although the display regions 9502 of the adjacent display panels 9501 are separated from each other in FIGS. 51A and 51B, without limitation to this structure, the display regions 9502 of the adjacent display panels 9501 may overlap with each other without any space so that a continuous display region 9502 is obtained, for example.
  • The electronic devices described in this embodiment are characterized by having a display portion for displaying some sort of information. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic device that does not have a display portion.
  • At least part of this embodiment can be implemented in combination with any of the other embodiments described in this specification as appropriate.
    • Example 1
  • In this example, results of observation and elemental analysis of In-Ga—Zn oxide films (hereinafter referred to as IGZO films) formed by the method described in the above embodiment will be described.
  • An IGZO film of a sample of this example was formed over a glass substrate with the intended thickness set to 100 nm by a sputtering method using an In-Ga—Zn oxide target (with an atomic ratio of In:Ga:Zn=5:1:7). The IGZO film was formed in an atmosphere including an argon gas at 200 sccm, where the pressure was controlled to 0.6 Pa, the substrate temperature was room temperature, and an alternating-current power of 2.5 kW was applied.
  • The formed IGZO film of the sample was observed by HAADF-STEM and was subjected to measurement using EDX. With the use of an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd., the HAADF-STEM image was taken and the EDX measurement was performed under conditions where the acceleration voltage was 200 kV, and irradiation with an electron beam with a diameter of approximately 0.1 nmϕ was performed.
  • In the EDX measurement, an energy dispersive X-ray spectrometer JED-2300T was used as an elemental analysis apparatus. A Si drift detector was used to detect X-rays emitted from the sample.
  • In the EDX measurement, an EDX spectrum of a point is obtained in such a manner that electron beam irradiation is performed on the point in an analysis target region of a sample, and the energy of characteristic X-rays of the sample generated by the irradiation and its frequency are measured. In this example, peaks of an EDX spectrum of the point were attributed to electron transition to the L shell in an In atom, electron transition to the K shell in a Ga atom, and electron transition to the K shell in a Zn atom and the K shell in an O atom, and the proportions of the atoms in the point were calculated. An EDX mapping image indicating distributions of the proportions of the atoms can be obtained through this process in an analysis target region of the sample.
  • FIGS. 52A to 52E and FIGS. 53A to 53E show the HAADF-STEM images and EDX mapping images of the IGZO film of the sample. FIGS. 52A to 52E show the HAADF-STEM images and EDX mapping images of a plane of the IGZO film, and FIGS. 53A to 53E show those of a cross section of the IGZO film. FIG. 52A and FIG. 53A are the HAADF-STEM images of the sample. FIG. 52B and FIG. 53B are EDX mapping images of O atoms, FIG. 52C and FIG. 53C are EDX mapping images of Zn atoms, FIG. 52D and FIG. 53D are EDX mapping images of Ga atoms, and FIG. 52E and FIG. 53E are EDX mapping images of In atoms. Note that the HAADF-STEM images and EDX mapping images in FIGS. 52A to 52E and FIGS. 53A to 53E were taken at a magnification of 7,200,000 times.
  • Bars above the EDX mapping images shown in FIGS. 52B to 52E and FIGS. 53B to 53E indicate the proportions [atomic %] of atoms in some points of the IGZO film.
  • The EDX mapping images in FIGS. 52B to 52E and FIGS. 53B to 53E show relative distribution of brightness indicating that the atoms have distributions in the IGZO film. Here, attention is focused on Outlined Square 1A and Outlined Square 1B in FIGS. 52B to 52E and Outlined Square 2A and Outlined Square 2B in FIGS. 53B to 53E.
  • In FIG. 52E and FIG. 53E, a relatively bright region occupies a large area in each of Outlined Squares 1A and 2A, while a relatively dark region occupies a large area in each of Outlined Squares 1B and 2B. That is, the number of In atoms is relatively large in the regions indicated by Outlined Squares 1A and 2A and the number of In atoms is relatively small in the regions indicated by Outlined Squares 1B and 2B. In FIG. 52E and FIG. 53E, the relatively bright regions correspond to Regions A1 described in the above embodiment, and the relatively dark regions correspond to Regions B1 described in the above embodiment.
  • In contrast to FIG. 52E and FIG. 53E, in FIG. 52D and FIG. 53D, a relatively dark region occupies a large area in each of Outlined Squares 1A and 2A, while a relatively bright region occupies a large area in each of Outlined Squares 1B and 2B. That is, the number of Ga atoms is relatively small in the regions indicated by Outlined Squares 1A and 2A and the number of Ga atoms is relatively large in the regions indicated by Outlined Squares 1B and 2B. In this manner, the number of Ga atoms tends to be relatively small in the region including a relatively large number of In atoms, and the number of Ga atoms tends to be relatively large in the region including a relatively small number of In atoms. Accordingly, in FIG. 52D and FIG. 53D, the relatively bright regions roughly correspond to Regions B1 described in the above embodiment, and the relatively dark regions roughly correspond to Regions A1 described in the above embodiment.
  • In FIG. 52C and FIG. 53C, a relatively bright region occupies a large area in each of Outlined Square 1B and Outlined Square 2B, while Outlined Square 1A and Outlined Square 2A each include a bright region whose area is not as large as that of the relatively bright region in Outlined Square 1B and Outlined Square 2B. In other words, the number of Zn atoms is relatively large in the regions indicated by Outlined Square 1B and Outlined Square 2B, and the number of Zn atoms in the regions indicated by Outlined Square 1A and Outlined Square 2A is not as large as that in the regions indicated by Outlined Square 1B and Outlined Square 2B.
  • As for FIG. 52B and FIG. 53B, similarly, the number of oxygen atoms is relatively large in the regions indicated by Outlined Square 1A, Outlined Square 1B, Outlined Square 2A, and Outlined Square 2B.
  • As described above, Region A1 of the IGZO film includes a large number of In atoms and a large number of O atoms and also includes a smaller number of Zn atoms than Region B1. It is thus suggested that Region A1 has a high content of, for example, indium, indium oxide, and an In—Zn oxide. Accordingly, Region A1 serves as a region having higher conductivity than Region B1, so that Region A1 contributes to increases in field-effect mobility and on-state current of a transistor.
  • Here, when the regions that correspond to Regions A1 (e.g., Outlined Square 1A and Outlined Square 2A) in FIG. 52E and FIG. 53E are examined, a plurality of particulate portions can be seen in Region A1. Observation of the particulate portions shows that they have a diameter of greater than or equal to 0.5 nm and less than or equal to 1.5 nm. Regions A1 appear to be formed by the plurality of particulate portions that are connected to each other. In this manner, Regions A1 extend in a cloud-like manner. The particulate portions included in Regions A1 correspond to the clusters in Regions A1 described in the above embodiment.
  • Furthermore, Region B1 of the IGZO film includes a large number of Ga atoms, a large number of Zn atoms, and a large number of 0 atoms and also includes a smaller number of In atoms than Region A1. It is thus suggested that Region B1 has a high content of, for example, an In-Ga—Zn oxide. Accordingly, Region B1 serves as a region having a higher semiconductor property than Region A1, so that Region B1 contributes to the switching characteristics of a transistor.
  • Here, when the regions that correspond to Regions B1 (e.g., Outlined Square 1B and Outlined Square 2B) in FIG. 52D and FIG. 53D are examined, a plurality of particulate portions can be seen in Region B1. Regions B1 also appear to be formed by the plurality of particulate portions that are connected to each other. In this manner, Regions B1 also extend in a cloud-like manner. The particulate portions included in Regions B1 correspond to the clusters in Regions B1 described in the above embodiment.
  • As described above, the IGZO film of the sample fabricated in this example is a composite oxide semiconductor where In-rich Regions A1 and In-poor Regions B1 are formed. Region A1 contributes to the on-state current and field-effect mobility of a transistor, and Region B1 contributes to the switching characteristics of a transistor. Therefore, with the use of the composite oxide semiconductor, a transistor with electrical characteristics in which the on-state current and the mobility are high and the S value is small can be manufactured.
  • At least part of this example can be implemented in combination with any of the embodiments and the other examples described in this specification as appropriate.
    • Example 2
  • In this example, composite oxide semiconductors were deposited and their crystallinities were examined by XRD.
    • [Fabrication of Sample]
  • In this example, samples including oxide semiconductor films were fabricated with different oxygen flow rate percentages and substrate temperatures during the deposition.
  • Each sample was fabricated by forming an oxide semiconductor film over a 600 mm×720 mm glass substrate.
  • Note that the oxide semiconductor film was formed under the following conditions: a deposition gas(es) with a flow rate of 200 sccm was/were introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=5:1:7).
  • Here, 18 kinds of samples were fabricated in total using three substrate temperatures and six oxygen flow rate percentages. The substrate temperature during the deposition was room temperature, 130° C., or 170° C. The oxygen flow rate percentage was 0%, 10%, 30%, 50%, 70%, or 100%. The proportions of an oxygen gas and an argon gas were varied such that the sum of the oxygen gas flow rate and the argon gas flow rate was 200 sccm.
    • [XRD Analysis Result]
  • The XRD analysis was conducted by a powder method (also referred to as a θ-2θ method) which is a kind of an out-of-plane method. Note that in a θ-2θ method, X-ray diffraction intensity is measured while an incident angle of an X-ray is changed and the angle of a detector facing an X-ray source is equal to the incident angle. Note that a grazing-incidence XRD (GIXRD) method (also referred to as a thin film method or a Seemann-Bohlin method) may be used. The GIXRD method is a kind of an out-of-plane method for measuring X-ray diffraction intensity in which X-ray is incident at an angle approximately 0.40° from a film surface with use of a variable-angle detector.
  • FIG. 54 shows the results of the XRD analysis conducted on the samples. In FIG. 54, the horizontal axis represents the diffraction angle 219 and the vertical axis represents diffraction intensity (any unit). Each of the graphs in FIG. 54 shows three diffraction profiles obtained from the respective measurement points. In each graph, the upper and lower profiles were obtained from the center of the substrate (A) and the periphery of the substrate (C), respectively, and the middle profile was obtained from a point between A and C.
  • The diffraction angle (around at 2θ=31°) at which the peak of the diffraction intensity was observed corresponds to a diffraction angle on the (009) plane of the structure model of single crystal InGaZnO4. This indicates that the samples from which the peak of the diffraction intensity was observed include a crystal part where the c-axes are aligned in the thickness direction (hereinafter also referred to as crystal part having orientation).
  • As seen in FIG. 54, no clear peak is observed from the film deposited at room temperature and with an oxygen flow rate percentage of 0%. This suggests that the proportion of crystal parts having orientation in the film is extremely low.
  • Meanwhile, clear peaks are observed from the films that were deposited at room temperature using the deposition gas containing oxygen. The peak intensities of the films deposited at room temperature and with an oxygen flow rate percentage of higher than or equal to 70% tend to be low.
  • Furthermore, clear peaks are observed from the films that were deposited with an oxygen flow rate percentage of 0% at high substrate temperatures. At substrate temperatures of 130° C. and 170° C., clearer peaks are observed in the case of using the deposition gas containing oxygen. The peak intensities of the films deposited with high oxygen flow rate percentages (higher than or equal to 70%) tend to be low, as in the films deposited at room temperature.
  • The above-described results show that the crystallinity of an oxide semiconductor film to be deposited can be controlled with the oxygen flow rate percentage and substrate temperature during the deposition. It was also found that the crystallinity can be high by setting the substrate temperature high even when the oxygen flow rate percentage is 0%. Moreover, it was found that the crystallinity can be high by setting the oxygen flow rate percentage high even when the substrate temperature is room temperature.
  • At least part of this example can be implemented in combination with any of the embodiments and the other examples described in this specification as appropriate.
    • Example 3
  • In this example, a transistor which corresponds to the transistor 100A described in Embodiment 2 was formed and the electrical characteristics of the transistor were evaluated. In this example, Sample S1 described below was fabricated. Sample S1 is a sample in which a transistor with a channel length L of 3 μm and a channel width W of 50 μm is formed.
    • [Fabrication Method of Sample S1]
  • First, a 10-nm-thick titanium film and a 100-nm-thick copper film were formed over a glass substrate with a sputtering apparatus. Next, the conductive film was processed by a photolithography method.
  • A stack including four insulating films was formed over the substrate and the conductive film. The insulating films were formed in succession in a vacuum with a plasma-enhanced chemical deposition (PECVD) apparatus. As the insulating films, a 50-nm-thick silicon nitride film, a 300-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film, and a 50-nm-thick silicon oxynitride film were used and stacked in this order.
  • Next, an oxide semiconductor film was formed over the insulating films and was processed into an island shape, whereby a semiconductor layer was formed. A 40-nm-thick oxide semiconductor film was formed as the oxide semiconductor film 108. The oxide semiconductor film is the above-described composite oxide semiconductor or C/IGZO.
  • The oxide semiconductor film was formed under the following conditions: the substrate temperature was room temperature (25° C.); an argon gas with a flow rate of 200 sccm was introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=5:1:7). In this example, the oxygen flow rate percentage in deposition of the oxide semiconductor film was 0%.
  • Next, an insulating film was formed over the insulating films and the semiconductor layer. As the insulating film, a 150-nm-thick silicon oxynitride film was formed with a PECVD apparatus.
  • Next, heat treatment was performed. The heat treatment was performed at 350° C. for one hour in a mixed gas atmosphere of nitrogen and oxygen.
  • An opening was formed in a desired region of the insulating film. The opening was formed by a dry etching method.
  • Then, a conductive film was formed over the insulating film and in the opening and the conductive film was processed into an island shape. In addition, the insulating film in contact with the bottom surface of the conductive film was processed in succession after the formation of the conductive film, whereby the insulating film was formed.
  • As the conductive film, a 10-nm-thick oxide semiconductor film, a 50-nm-thick titanium nitride film, and a 100-nm-thick copper film were formed in this order. The oxide semiconductor film was formed under the following conditions: the substrate temperature was 170° C.; an oxygen gas with a flow rate of 200 sccm was introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1). The titanium nitride film and the copper film were each formed using a sputtering apparatus.
  • Next, plasma treatment was performed from above the semiconductor layer, the insulating film, and the conductive film. The plasma treatment was performed with a PECVD apparatus at a substrate temperature of 220° C. in a mixed gas atmosphere containing an argon gas and a nitrogen gas.
  • Next, an insulating film was formed over the semiconductor layer, the insulating film, and the conductive film. The insulating film was formed by stacking a 100-nm-thick silicon nitride film and a 300-nm-thick silicon oxynitride film with a PECVD apparatus.
  • Then, a mask was formed over the formed insulating film and an opening was formed in the insulating film with use of the mask.
  • A conductive film was formed to fill the opening and was processed into an island shape, whereby the conductive films serving as a source electrode and a drain electrode were formed. For the conductive films, a 10-nm-thick titanium film and a 100-nm-thick copper film were formed with a sputtering apparatus.
  • After that, an insulating film was formed over the insulating film and the conductive film. A 1.5-μm-thick acrylic photosensitive resin was used for the insulating film.
  • In the above manner, Sample S1 was fabricated.
  • [Id-Vg Characteristics of Transistor]
  • Next, Id-Vg characteristics of the fabricated transistor of Sample S1 were measured. In measuring the Id-Vg characteristics of the transistor, a voltage applied to the conductive film serving as a first gate electrode (hereinafter the voltage is also referred to as gate voltage (Vg)) and a voltage applied to the conductive film serving as a second gate electrode (hereinafter the voltage is also referred to as back gate voltage (Vbg)) were changed from −10 V to +10 V in increments of 0.25 V. A voltage applied to the conductive film serving as the source electrode (hereinafter the voltage is also referred to as source voltage (Vs)) was 0 V (comm) and voltages applied to the conductive film serving as the drain electrode (hereinafter the voltages are also referred to as drain voltage (Vd)) were 0.1 V and 20 V.
  • FIG. 55 shows the results of Id-Vg characteristics of Sample S1. In FIG. 55, the first vertical axis represents Id [A], the second vertical axis represents field-effect mobility (μFE) [cm2/Vs], and the horizontal axis represents Vg [V]. Note that the field-effect mobility was measured when Vd was 20 V.
  • Note that the results in FIG. 55 were obtained with the upper limit of Id in the measurement set to 1 mA. In FIG. 55, when Vd is 20 V, Id exceeds this upper limit at a Vg of 7.5 V. For this reason, FIG. 55 shows the field-effect mobility in the range where Vg is lower than or equal to 7.5 V as the field-effect mobility estimated from such Id-Vg characteristics.
  • The transistor, which is a semiconductor device of one embodiment of the present invention, has excellent electrical characteristics as shown in FIG. 55. Here, Table 1 lists the transistor characteristics that are shown in FIG. 55.
  • TABLE 1
    μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/
    [cm2V−1s−1] [V] [V/decade] [A/cm2] [cm2V−1s−1] μFE(@Vg = 2 V)
    103 −0.1 0.12 <1 × 10−12 70 1.47
  • As described above, the field-effect mobility of the transistor that is a semiconductor device of one embodiment of the present invention exceeds 100 cm2/Vs. This field-effect mobility is equivalent to that of a transistor including low-temperature polysilicon and means extraordinary characteristics for a transistor using an oxide semiconductor.
  • As shown in Table 1, Sample S1 includes a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 60 cm2/Vs and smaller than 150 cm2/Vs, a second region where the threshold voltage is higher than or equal to −1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1×10−12 A/cm2, and μFE(max)/μFE(Vg=2V) is larger than or equal to 1 and smaller than 2 where μFE(max) represents the maximum value of the field-effect mobility of the transistor and μFE(Vg=2V) represents the value of the field-effect mobility of the transistor at a gate voltage of 2 V.
  • The above-described characteristics of the transistor can be obtained with the use of the composite oxide semiconductor or C/IGZO described above. A transistor including the composite oxide semiconductor or C/IGZO in its semiconductor layer can have both high carrier mobility and excellent switching characteristics.
  • At least part of this example can be implemented in combination with any of the embodiments and the other examples described in this specification as appropriate.
    • Example 4
  • In this example, a transistor which corresponds to the transistor 100A described in Embodiment 2 was formed and the electrical characteristics and the cross-sectional shape of the transistor were evaluated. In this example, Sample S2 described below was fabricated. Sample S2 is a sample in which a transistor with a channel length L of 2 μm and a channel width W of 3 μm is formed.
    • [Fabrication Method of Sample S2]
  • First, a 10-nm-thick titanium film and a 100-nm-thick copper film were formed over a glass substrate with a sputtering apparatus. Next, the conductive film was processed by a photolithography method.
  • A stack including four insulating films was formed over the substrate and the conductive film. The insulating films were formed in succession in a vacuum with a plasma-enhanced chemical deposition (PECVD) apparatus. As the insulating films, a 50-nm-thick silicon nitride film, a 100-nm-thick silicon nitride film, a 50-nm-thick silicon nitride film, and a 50-nm-thick silicon oxynitride film were used and stacked in this order.
  • Next, an oxide semiconductor film was formed over the insulating films and was processed into an island shape, whereby a semiconductor layer was formed. A 40-nm-thick oxide semiconductor film was formed as the oxide semiconductor film 108. The oxide semiconductor film is the above-described composite oxide semiconductor or C/IGZO.
  • The oxide semiconductor film was formed under the following conditions: the substrate temperature was 70° C.; an argon gas with a flow rate of 180 sccm and an oxygen gas with a flow rate of 20 sccm were introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1). In this example, the oxygen flow rate percentage in deposition of the oxide semiconductor film was 10%.
  • Next, an insulating film was formed over the insulating films and the semiconductor layer. As the insulating film, a 150-nm-thick silicon oxynitride film was formed with a PECVD apparatus.
  • Next, heat treatment was performed. The heat treatment was performed at 350° C. for one hour in a mixed gas atmosphere of nitrogen and oxygen.
  • An opening was formed in a desired region of the insulating film. The opening was formed by a dry etching method.
  • Then, a conductive film was formed over the insulating film and in the opening and the conductive film was processed into an island shape. In addition, the insulating film in contact with the bottom surface of the conductive film was processed in succession after the formation of the conductive film, whereby the insulating film was formed.
  • As the conductive film, a 10-nm-thick first oxide semiconductor film and a 90-nm-thick second oxide semiconductor film were formed in this order. The first oxide semiconductor film was formed under the following conditions: the substrate temperature was 170° C.; an oxygen gas with a flow rate of 200 sccm was introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1). The second oxide semiconductor film was formed under the following conditions: the substrate temperature was 170° C.; an argon gas with a flow rate of 180 sccm and an oxygen gas with a flow rate of 20 sccm were introduced into a chamber of the sputtering apparatus; the pressure was set to 0.6 Pa; and an AC power of 2.5 kW was applied to a metal oxide target containing indium, gallium, and zinc (with an atomic ratio of In:Ga:Zn=4:2:4.1).
  • Next, plasma treatment was performed from above the semiconductor layer, the insulating film, and the conductive film. The plasma treatment was performed with a PECVD apparatus at a substrate temperature of 220° C. in a mixed gas atmosphere containing an argon gas and a nitrogen gas.
  • Next, an insulating film was formed over the semiconductor layer, the insulating film, and the conductive film. The insulating film was formed by stacking a 100-nm-thick silicon nitride film and a 300-nm-thick silicon oxynitride film with a PECVD apparatus.
  • Then, a mask was formed over the formed insulating film and an opening was formed in the insulating film with use of the mask.
  • A conductive film was formed to fill the opening and was processed into an island shape, whereby the conductive films serving as a source electrode and a drain electrode were formed. For the conductive films, a 50-nm-thick titanium film, a 400-nm-thick aluminum film, and a 100-nm-thick titanium film were formed in this order with a sputtering apparatus.
  • After that, an insulating film was formed over the insulating film and the conductive film. A 1.5-μm-thick acrylic photosensitive resin was used for the insulating film.
  • In the above manner, Sample S2 was fabricated.
  • [Id-Vg Characteristics of Transistor]
  • Next, Id-Vg characteristics of the fabricated transistor of Sample S2 were measured. The measurement conditions of Id-Vg characteristics of the transistor were the same as those in Example 3.
  • FIG. 56 shows the results of Id-Vg characteristics of Sample S2.
  • The transistor, which is a semiconductor device of one embodiment of the present invention, has excellent electrical characteristics as shown in FIG. 56. Here, Table 2 lists the transistor characteristics that are shown in FIG. 56.
  • TABLE 2
    μFE(max) Vth S Ioff μFE(@Vg = 2 V) μFE(max)/
    [cm2V−1s−1] [V] [V/decade] [A/cm2] [cm2V−1s−1] μFE(@Vg = 2 V)
    65.5 0.23 0.09 <1 × 10−12 58 1.13
  • As shown in Table 2, Sample S2 includes a first region where the maximum value of the field-effect mobility of the transistor at a gate voltage of higher than 0 V and lower than or equal to 10 V is larger than or equal to 60 cm2/Vs and smaller than 150 cm2/Vs, a second region where the threshold voltage is higher than or equal to −1 V and lower than or equal to 1 V, a third region where the S value is smaller than 0.3 V/decade, and a fourth region where the off-state current is lower than 1×10−12 A/cm2, and μFE(max)/μFE(Vg=2V) is larger than or equal to 1 and smaller than 2 where μFE(max) represents the maximum value of the field-effect mobility of the transistor and μFE(Vg=2V) represents the value of the field-effect mobility of the transistor at a gate voltage of 2 V.
  • The above-described characteristics of the transistor can be obtained with the use of the composite oxide semiconductor or C/IGZO described above. A transistor including the composite oxide semiconductor or C/IGZO in its semiconductor layer can have both high carrier mobility and excellent switching characteristics.
    • [Cross-Sectional Shape of Transistor]
  • Next, the cross-sectional shape of the fabricated transistor of Sample S2 was evaluated. Cross-sectional STEM observation was conducted to evaluate the cross-sectional shape of the transistor. FIG. 57 shows a cross-sectional STEM image of the transistor of Sample S2.
  • As shown in FIG. 57, Sample S2 fabricated in this example was confirmed to have a favorable cross-sectional shape. The channel length L, which was designed to be 2 μm, was found to be 1.78 μm in the completed transistor.
  • At least part of this example can be implemented in combination with any of the embodiments and the other examples described in this specification as appropriate.
  • This application is based on Japanese Patent Application serial no. 2016-074493 filed with Japan Patent Office on Apr. 1, 2016 and Japanese Patent Application serial no. 2016-075853 filed with Japan Patent Office on Apr. 5, 2016, the entire contents of which are hereby incorporated by reference.

Claims (4)

What is claimed is:
1. A method for manufacturing a composite oxide semiconductor, comprising:
a first step of placing a substrate in a deposition chamber;
a second step of introducing one or both of an argon gas and an oxygen gas into the deposition chamber;
a third step of applying voltage to a target comprising indium, an element M, zinc, and oxygen; and
a fourth step of depositing the composite oxide semiconductor from the target onto the substrate,
wherein the fourth step comprises:
a first step in which the element M and the zinc are preferentially sputtered from the target; and
a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target, and
wherein the M represents Al, Ga, Y, or Sn.
2. The method for manufacturing the composite oxide semiconductor, according to claim 1,
wherein the substrate is not heated intentionally.
3. A method for manufacturing a composite oxide semiconductor, comprising:
a first step of placing a substrate in a deposition chamber;
a second step of introducing, into the deposition chamber, a deposition gas comprising an argon gas and not comprising an oxygen gas;
a third step of applying voltage to a target comprising indium, an element M, zinc, and oxygen; and
a fourth step of depositing the composite oxide semiconductor from the target onto the substrate,
wherein the fourth step comprises:
a first step in which the element M and the zinc are preferentially sputtered from the target; and
a second step in which the indium has a cluster-like shape and then the indium with the cluster-like shape is sputtered from the target, and
wherein the M represents Al, Ga, Y, or Sn.
4. The method for manufacturing the composite oxide semiconductor, according to claim 3,
wherein the substrate is not heated intentionally.
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