US20150008428A1 - Semiconductor device and method for manufacturing semiconductor device - Google Patents

Semiconductor device and method for manufacturing semiconductor device Download PDF

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US20150008428A1
US20150008428A1 US14/313,154 US201414313154A US2015008428A1 US 20150008428 A1 US20150008428 A1 US 20150008428A1 US 201414313154 A US201414313154 A US 201414313154A US 2015008428 A1 US2015008428 A1 US 2015008428A1
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insulating layer
layer
oxide semiconductor
semiconductor layer
oxide
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Yoshitaka Yamamoto
Tetsuhiro Tanaka
Toshihiko Takeuchi
Yasumasa Yamane
Takayuki Inoue
Shunpei Yamazaki
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Semiconductor Energy Laboratory Co Ltd
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Semiconductor Energy Laboratory Co Ltd
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Assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD. reassignment SEMICONDUCTOR ENERGY LABORATORY CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: TAKEUCHI, TOSHIHIKO, TANAKA, TETSUHIRO, YAMANE, YASUMASA, YAMAMOTO, YOSHITAKA, YAMAZAKI, SHUNPEI, INOUE, TAKAYUKI
Publication of US20150008428A1 publication Critical patent/US20150008428A1/en
Priority to US15/946,149 priority Critical patent/US11404585B2/en
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    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/673—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes
    • H10D30/6733—Multi-gate TFTs
    • H01L29/7869—
    • H01L29/66969—
    • H01L29/792—
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/673—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/673—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes
    • H10D30/6733—Multi-gate TFTs
    • H10D30/6734—Multi-gate TFTs having gate electrodes arranged on both top and bottom sides of the channel, e.g. dual-gate TFTs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/6737—Thin-film transistors [TFT] characterised by the electrodes characterised by the electrode materials
    • H10D30/6739—Conductor-insulator-semiconductor electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/674—Thin-film transistors [TFT] characterised by the active materials
    • H10D30/6755—Oxide semiconductors, e.g. zinc oxide, copper aluminium oxide or cadmium stannate
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6757—Thin-film transistors [TFT] characterised by the structure of the channel, e.g. transverse or longitudinal shape or doping profile
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D64/00—Electrodes of devices having potential barriers
    • H10D64/01—Manufacture or treatment
    • H10D64/031—Manufacture or treatment of data-storage electrodes
    • H10D64/035—Manufacture or treatment of data-storage electrodes comprising conductor-insulator-conductor-insulator-semiconductor structures
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D99/00—Subject matter not provided for in other groups of this subclass
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10B—ELECTRONIC MEMORY DEVICES
    • H10B41/00—Electrically erasable-and-programmable ROM [EEPROM] devices comprising floating gates
    • H10B41/70—Electrically erasable-and-programmable ROM [EEPROM] devices comprising floating gates the floating gate being an electrode shared by two or more components
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/673—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes
    • H10D30/6736—Thin-film transistors [TFT] characterised by the electrodes characterised by the shapes, relative sizes or dispositions of the gate electrodes characterised by the shape of gate insulators
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D64/00—Electrodes of devices having potential barriers
    • H10D64/01—Manufacture or treatment
    • H10D64/031—Manufacture or treatment of data-storage electrodes
    • H10D64/037—Manufacture or treatment of data-storage electrodes comprising charge-trapping insulators

Definitions

  • the present invention relates to a semiconductor device and a device including the semiconductor device.
  • a “semiconductor device” refers to a device that can function by utilizing semiconductor characteristics; an electro-optical device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
  • a technique in which a transistor is formed using a semiconductor has attracted attention.
  • the transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device).
  • IC integrated circuit
  • display device display device
  • a silicon-based semiconductor material is widely known as a material for a semiconductor applicable to a transistor.
  • an oxide semiconductor has been attracting attention.
  • Patent Document 1 a transistor including an amorphous oxide semiconductor layer containing indium (In), gallium (Ga), and zinc (Zn) is disclosed in Patent Document 1.
  • Patent Documents 2 and 3 Techniques for improving carrier mobility by stacking oxide semiconductor layers are disclosed in Patent Documents 2 and 3.
  • a transistor including an oxide semiconductor layer has an extremely small leakage current when the transistor is off.
  • a low-power-consumption CPU utilizing the small leakage current characteristic of a transistor including an oxide semiconductor layer is disclosed (see Patent Document 4).
  • Miniaturization of transistors has been progressing with an increase in integration of circuits.
  • the miniaturization of a transistor may cause deterioration of electrical characteristics, such as on-state current, off-state current, threshold, and an S value (subthreshold swing), of the transistor (see Patent Document 5).
  • electrical characteristics such as on-state current, off-state current, threshold, and an S value (subthreshold swing) of the transistor (see Patent Document 5).
  • decreasing the channel length increases the on-state current, but at the same time increases the off-state current, a variation in threshold, and the S value.
  • the on-state current is decreased.
  • An object of one embodiment of the present invention is to provide a semiconductor device in which the threshold voltage is adjusted to an appropriate value.
  • An object is to provide a semiconductor device in which deterioration of electrical characteristics which becomes noticeable when the semiconductor device is miniaturized can be suppressed.
  • Another object is to provide a highly integrated semiconductor device.
  • Another object is to provide a semiconductor device in which deterioration of on-state current characteristics is reduced.
  • Another object is to provide a semiconductor device with low power consumption.
  • Another object is to provide a semiconductor device with high reliability.
  • Another object is to provide a semiconductor device which can retain data even when power supply is stopped.
  • Another object is to provide a semiconductor device with favorable characteristics.
  • One embodiment of the present invention is a semiconductor device including a first semiconductor, an electrode which is electrically connected to the first semiconductor, a first gate electrode and a second gate electrode between which the first semiconductor is sandwiched, an electron trap layer between the first gate electrode and the first semiconductor, and a gate insulating layer between the second gate electrode and the first semiconductor.
  • Another embodiment of the present invention is a manufacturing method of the semiconductor device, in which electrons are trapped in the electron trap layer by performing heat treatment at higher than or equal to 125° C. and lower than or equal to 450° C. and at the same time, keeping the potential of the first gate electrode higher than the potential of the electrode for one second or longer.
  • a second semiconductor and a third semiconductor between which the first semiconductor is sandwiched may be further included.
  • the second semiconductor may be placed between the first semiconductor and the first gate electrode.
  • the third semiconductor may be placed between the first semiconductor and the gate insulating layer.
  • the first gate electrode preferably faces the top surface and the side surface of the first semiconductor.
  • the electron trap layer contains any one of hafnium oxide, aluminum oxide, tantalum oxide, and aluminum silicate.
  • a semiconductor device whose threshold is adjusted to an appropriate value can be provided. Furthermore, a semiconductor device can be provided in which lowering of electrical characteristics, which becomes noticeable when the semiconductor device is miniaturized, can be suppressed. A highly integrated semiconductor device can be provided. A semiconductor device with low power consumption can be provided. A highly reliable semiconductor device can be provided. A semiconductor device in which data is retained even when power supply is stopped can be provided.
  • FIGS. 1A to 1C illustrate examples of a semiconductor device of an embodiment
  • FIG. 2 illustrates an example of a semiconductor device of an embodiment
  • FIGS. 3A to 3D illustrate band diagram examples of a semiconductor device of an embodiment
  • FIG. 4A schematically shows characteristics of a semiconductor device of an embodiment and FIG. 4B illustrates an example of a circuit in which the semiconductor device is used;
  • FIGS. 5A and 5B illustrate examples of a memory cell of an embodiment
  • FIGS. 6A to 6C illustrate a manufacturing process of a semiconductor device
  • FIGS. 7A to 7C are a top view and cross-sectional views of a transistor
  • FIGS. 8A and 8B are schematic band diagrams of stacked semiconductor layers
  • FIGS. 9A to 9C are a top view and cross-sectional views of a transistor
  • FIGS. 10A to 10D illustrate a method for manufacturing a transistor
  • FIGS. 11A to 11C are a top view and cross-sectional views of a transistor
  • FIGS. 12A to 12C are a top view and cross-sectional views of a transistor
  • FIGS. 13A and 13B are cross-sectional views of a transistor
  • FIGS. 14A to 14D illustrate circuits that use a semiconductor device of one embodiment of the present invention
  • FIG. 15 is a circuit diagram of a semiconductor device of an embodiment
  • FIG. 16 is a block diagram of a semiconductor device of an embodiment
  • FIG. 17 is a circuit diagram of a memory device of an embodiment
  • FIGS. 18A to 18F illustrate examples of an electronic device
  • FIG. 19 show measurement results of electrical characteristics of transistors manufactured in Reference Example.
  • FIG. 1A shows a semiconductor device including a semiconductor layer 101 , an electron trap layer 102 , a gate electrode 103 , a gate insulating layer 104 , and a gate electrode 105 .
  • a stacked body of a first insulating layer 102 a and a second insulating layer 102 b as shown in FIG. 1B a stacked body of the first insulating layer 102 a , the second insulating layer 102 b , and a third insulating layer 102 c as shown in FIG. 1C , or a stacked body of four or more layers may be used.
  • a conductive layer 102 d which is electrically insulated may be included in an insulator 102 e .
  • the insulator 102 e may be composed of a plurality of insulating layers.
  • FIG. 3A is an example of a band diagram between points A and B in the semiconductor device illustrated in FIG. 1B .
  • Ec and Ev denote the conduction band minimum and the valence band maximum, respectively.
  • the potential of the gate electrode 103 is equal to the potential of a source electrode or drain electrode (not illustrated).
  • the band gap of the first insulating layer 102 a is larger than that of the second insulating layer 102 b , and the electron affinity of the first insulating layer 102 a is smaller than that of the second insulating layer 102 b ; however, the present invention is not limited to this example.
  • Electron trap states 106 exist at the interface between the first insulating layer 102 a and the second insulating layer 102 b or inside the second insulating layer 102 b .
  • FIG. 3B shows the state where the potential of the gate electrode 103 is higher than the potential of the source or drain electrode.
  • the process for increasing the potential of the gate electrode 103 to a potential higher than the potential of the source or drain electrode under constant conditions is referred to as a threshold adjust process.
  • the potential of the gate electrode 103 may be higher than the potential of the source or drain electrode by 1 V or more. After this process, the potential of the gate electrode 103 may be lower than the highest potential applied to the gate electrode 105 . In the process, the difference between the potential of the gate electrode 103 and the potential of the source or drain electrode is typically less than 4 V.
  • the potential of the gate electrode 105 is preferably equal to the potential of the source or drain electrode.
  • electrons 107 that exist in the semiconductor layer 101 move toward the gate electrode 103 having a higher potential. Some of the electrons 107 moving from the semiconductor layer 101 toward the gate electrode 103 are trapped by the electron trap states 106 .
  • the first is a process by the tunnel effect.
  • the tunnel effect (Fowler-Nordheim tunnel effect) can be obtained by applying an appropriate voltage to the gate electrode 103 .
  • a tunnel current increases with the square of the electric field between the gate electrode 103 and the semiconductor layer 101 .
  • the second is the process that the electrons 107 hop from trap states to trap states in the band gap such as defect states in the first insulating layer 102 a to reach the second insulating layer 102 b .
  • This is a conduction mechanism called Poole-Frenkel conduction, in which as the absolute temperature is higher and trap states are shallower, the electric conductivity is higher.
  • the third is the process that the electrons 107 go over the barrier of the first insulating layer 102 a by thermal excitation.
  • the distribution of electrons existing in the semiconductor layer 101 follows the Fermi-Dirac distribution, in general, the proportion of electrons having high energy is larger as the temperature is higher. Assuming that the density of electrons having energy 3 eV higher than the Fermi level at 300 K (27° C.) is 1, for example, the density is 6 ⁇ 10 16 at 450 K (177° C.), 1.5 ⁇ 10 25 at 600 K (327° C.), and 1.6 ⁇ 10 30 at 750 K (477° C.).
  • the movement of the electrons 107 toward the gate electrode 103 by going over the barrier of the first insulating layer 102 a occurs by the above three processes or the combination of these processes.
  • the second and the third processes indicate that current increases exponentially as the temperature is higher.
  • the Fowler-Nordheim tunnel effect is more likely to occur as the density of electrons in a thin part (a high-energy portion) of a barrier layer of the first insulating layer 102 a is higher; thus, a higher temperature is better.
  • the potential of the gate electrode 103 is kept higher than that of the source or drain electrode at a high temperature (a temperature higher than the operating temperature or the storage temperature of the semiconductor device, or higher than or equal to 125° C. and lower than or equal to 450° C., for example higher than or equal to 150° C. and lower than or equal to 300° C.) for one second or longer, for example, one minute or longer.
  • a high temperature a temperature higher than the operating temperature or the storage temperature of the semiconductor device, or higher than or equal to 125° C. and lower than or equal to 450° C., for example higher than or equal to 150° C. and lower than or equal to 300° C.
  • process temperature The temperature of the process for trapping electrons is referred to as process temperature below.
  • the amount of electrons trapped by the electron trap states 106 can be controlled by the potential of the gate electrode 103 .
  • the electric field of the gate electrode 103 is blocked and a channel formed in the semiconductor layer 101 disappears.
  • the total number of electrons trapped by the electron trap states 106 increases linearly at first, and then, the rate of increase gradually decreases and converges at a certain value.
  • the convergence value depends on the potential of the gate electrode 103 . As the potential is higher, the number of trapped electrons is more likely to be large; however, it never exceeds the total number of electron trap states 106 .
  • each thickness of the first and second insulating layers 102 a and 102 b is preferably set at a thickness at which the tunnel effect is not a problem.
  • the physical thickness is preferably more than 1 nm.
  • the channel length is more than or equal to four times, typically more than or equal to ten times as large as the equivalent silicon oxide thickness of the first and second insulating layers 102 a and 102 b . Note that when a so-called High-K material is used, the equivalent silicon oxide thickness is less than the physical thickness.
  • the thickness of the first insulating layer 102 a is preferably more than or equal to 10 nm and less than or equal to 20 nm, and the equivalent silicon oxide thickness of the second insulating layer 102 b is more than or equal to 1 nm and less than or equal to 25 nm.
  • Another method is to set the operating temperature or the storage temperature of the semiconductor device at a temperature that is lower enough than the process temperature. For example, the probability that electrons go over a 3 eV-barrier when the temperature is 120° C. is less than a one hundred-thousandth that when the temperature is 300° C. In this way, although electrons that easily go over a barrier to be trapped by the electron trap states 106 during process at 300° C., the electrons are difficult to go over the barrier during storage 120° C. and are kept trapped by the electron trap states 106 for a long time.
  • the effective mass of a hole is extremely large or is substantially localized in the semiconductor layer 101 .
  • the injection of holes from the semiconductor layer 101 to the first and second insulating layers 102 a and 102 b does not occur and consequently a phenomenon in which electrons trapped by the electron trap states 106 bond to holes and disappear does not occur.
  • a material showing Poole-Frenkel conduction may be used for the second insulating layer 102 b .
  • the Poole-Frenkel conduction is, as described above, electron hopping conduction between defect states and the like in a material.
  • a material including a large number of defect states or including deep defect states has low electric conductivity and consequently can hold electrons trapped by the electron trap states 106 for a long time.
  • Circuit design or material selection may be made so that no voltage at which electrons trapped in the first and second insulating layers 102 a and 102 b are released is applied.
  • a material whose effective mass of holes is extremely large or is substantially localized such as an In—Ga—Zn-based oxide semiconductor
  • a channel is formed when the potential of the gate electrode 103 is higher than that of the source or drain electrode; however, when the potential of the gate electrode 103 is lower than that of the source or drain electrode, the material shows characteristics similar to an insulator.
  • the electric field between the gate electrode 103 and the semiconductor layer 101 is extremely small and consequently the Fowler-Nordheim tunnel effect or electron conduction according to the Poole-Frenkel conduction is significantly decreased.
  • the electron trap layer 102 is formed using three insulating layers as illustrated in FIG. 1C , that the electron affinity of the third insulating layer 102 c is smaller than that of the second insulating layer 102 b , and that the bandgap of the third insulating layer 102 c is larger than that of the second insulating layer 102 b.
  • the third insulating layer 102 c As a material of the third insulating layer 102 c , the same material as or a material similar to that of the first insulating layer 102 a can be used. Alternatively, a material whose constituent elements are the same as those of the second insulating layer 102 b but the number of electron trap states is small enough may be used. The number (density) of electron trap states depends on the formation method.
  • FIGS. 3C and 3D illustrate the examples.
  • the potential of the gate electrode 103 is equal to that of the source or drain electrode.
  • FIG. 3D shows the state where the potential of the gate electrode 103 is higher than the potential of the source or drain electrode. Electrons that exist in the semiconductor layer 101 move toward the gate electrode 103 having a higher potential. Some of the electrons moving from the semiconductor layer 101 toward the gate electrode 103 are trapped in the conductive layer 102 d . In other word, in the semiconductor device illustrated in FIG. 2 , the conductive layer 102 d functions as the electron trap states 106 in the semiconductor device in FIG. 1B .
  • each of the first to third insulating layers 102 a to 102 c may be composed of a plurality of insulating layers.
  • a plurality of insulating layers containing the same constituting elements and formed by different formation methods may be used.
  • the first and second insulating layers 102 a and 102 b are formed using insulating layers composed of the same constituting elements (e.g. hafnium oxide)
  • the first insulating layer 102 a may be formed by a chemical vapor deposition (CVD) method or an atomic layer deposition (ALD) method and the second insulating layer 102 b may be formed by sputtering.
  • CVD chemical vapor deposition
  • ALD atomic layer deposition
  • Examples of the CVD method that can be used here are a thermal CVD method, a photo CVD method, a plasma CVD (PECVD) method, a metal organic CVD (MOCVD) method, or a low pressure CVD (LPCVD) method.
  • PECVD plasma CVD
  • MOCVD metal organic CVD
  • LPCVD low pressure CVD
  • insulating films may be formed by different CVD methods.
  • an insulating layer formed by sputtering includes more defects and stronger electron trapping characteristics than an insulating layer formed by CVD or ALD. From this reason, the second insulating layer 102 b may be formed by sputtering and the third insulating layer 102 c may be formed by CVD or ALD when the second and third insulating layers 102 b and 102 c contain the same constituent elements.
  • one insulating layer may be formed by sputtering and another insulating layer may be formed by CVD or ALD.
  • the threshold of a semiconductor device is increased by the trap of electrons in the electron trap layer 102 .
  • a source-drain current (cut-off current, Icut) when the potential of the gate electrodes 103 and 105 is equal to the potential of the source electrode can be significantly decreased.
  • the Icut density (a current value per micrometer of a channel width) of an In—Ga—Zn-based oxide whose bandgap is 3.2 eV is 1 zA/ ⁇ m (1 ⁇ 10 ⁇ 21 A/ ⁇ m) or less, typically 1 yA/ ⁇ m (1 ⁇ 10 ⁇ 24 A/ ⁇ m) or less.
  • FIG. 4A schematically shows dependence of current per micrometer of channel width (Id) between source and drain electrodes on the potential of the gate electrode 105 (Vg) at room temperature, before and after electron trap in the electron trap layer 102 .
  • each potential of the source electrode and the gate electrode 103 is 0 V and the potential of the drain electrode is +1 V.
  • current smaller than 1 fA cannot be measured directly, it can be estimated from a value measured by another method, the subthreshold value, and the like. Note that Example is referred to for the measurement method.
  • FIG. 4B illustrates a circuit in which charge stored in a capacitor 111 is controlled by a transistor 110 . Leakage current between electrodes of the capacitor 111 is ignored.
  • the capacitance of the capacitor 111 is 1 fF
  • the potential of the capacitor 111 on the transistor 110 side is +1 V
  • the potential of Vd is 0 V.
  • the curve 108 in FIG. 4A denotes the Id-Vg characteristics of the transistor 110 and the channel width is 0.1 ⁇ m, in which case the Icut is approximately 1 ⁇ 10 ⁇ 15 A (approximately 1 fA) and the resistivity of the transistor 110 at this time is approximately 1 ⁇ 10 15 ⁇ . Accordingly, the time constant of a circuit composed of the transistor 110 and the capacitor 111 is approximately one second. This means that most of the charge stored in the capacitor 111 is lost in approximately one second.
  • charge can be held for 10 years in a simple circuit composed of a transistor and a capacitor without applying such a large voltage.
  • This can be applied to various kinds of memory devices, such as a memory cell shown in FIGS. 5A and 5B .
  • the memory cell illustrated in FIG. 5A includes a transistor 121 , a transistor 122 , and a capacitor 123 .
  • the transistor 121 includes the electron trap layer 102 as illustrated in FIG. 1A .
  • the above-described process for increasing the threshold is performed to lower Icut. Note that in the drawing, the transistor with the changed threshold due to electrons in the electron trap layer 102 is represented by a symbol that is different from the symbol for a normal transistor.
  • Memory cells in FIG. 5A are formed in a matrix. For example, to the memory cell in the n-th row and m-th column, a read word line Pn, the write word line Qn, the threshold correction line Tn, the bit line Rm, and the source line Sm are connected. Note that all the threshold correction wirings may be arranged to be connected to each other to have the same potential.
  • the threshold correction can be performed as follows. First, potentials of all read word lines, all write word lines, all source lines, and all bit lines are set at 0 V. Then, a wafer or chip over which the memory cells are formed is set at an appropriate temperature and the potentials of all the threshold correction lines are set at an appropriate value (e.g., +3 V), and these conditions are held for an appropriate period. In this way, the threshold becomes an appropriate value.
  • an appropriate value e.g., +3 V
  • the memory cell may have a structure including a transistor 124 and a capacitor 125 as illustrated in FIG. 5B .
  • the word line Qn, the threshold correction line Tn, the bit line Rm, and the source line Sn are connected.
  • the method for adjusting the threshold can be similar to that in the case of FIG. 5A .
  • the threshold is adjusted to an appropriate value by making the electron trap layer trap electrons as described above, it is preferable to avoid further addition of electrons to the electron trap layer in normal use after that.
  • further addition of electrons means a further increase of threshold, resulting in circuit deterioration.
  • the electron trap layer When the electron trap layer is in the vicinity of a semiconductor layer, and a wiring or an electrode whose potential is higher than that of the semiconductor layer faces the semiconductor layer with the electron trap layer sandwiched therebetween, electrons might be trapped in the electron trap layer in normal use.
  • the potential of the threshold control wiring Tn is preferably set lower than or equal to the lowest potential of the bit line Rm in the memory cell shown in FIG. 5A or 5 B.
  • steps illustrated in FIGS. 6A to 6C can be performed.
  • initial characteristics are measured to select a conforming item (see FIG. 6A ).
  • items without malfunctions that cannot be recovered due to a break in a wire or the like are regarded as conforming items.
  • the threshold has not been corrected to an appropriate value and thus charge in the capacitor cannot be held for a long time; however, this is not the criteria of selection.
  • the difference between the potential of the gate electrode 103 and the potential of the one with the lower potential of the source electrode and the drain electrode is more than or equal to 1 V and less than 4V and, in addition, less than or equal to the difference between the potential of the gate electrode 105 and the potential of the one with the lower potential of the source electrode and the drain electrode after shipment of this memory cell.
  • One of the criteria for conforming items is the threshold increased as planned.
  • chips with a threshold abnormality are regarded as nonconforming items, and these chips may again be subjected to electron injection. Conforming items are shipped after dicing, resin sealing, and packaging.
  • the degree of the threshold increase depends on the density of electrons trapped by the electron trap layer 102 .
  • the threshold is increased by Q/C, where Q is the surface density of trapped electrons and C is the dielectric constant of the first insulating layer 102 a.
  • the potential of the gate electrode 103 determines the value at which the number of trapped electrons converges. Accordingly, the degree of the threshold increase can be adjusted by the potential of the gate electrode 103 .
  • the potential of the gate electrode 103 is set higher than the potentials of the source electrode and the drain electrode by 1.5 V and the temperature is set at 150° C. to 250° C., typically 200° C. ⁇ 20° C. is considered.
  • first threshold, Vth1 the threshold of the semiconductor device before electrons are trapped in the electron trap layer 102
  • Vth1 the threshold of the semiconductor device before electrons are trapped in the electron trap layer 102
  • Vth1 the threshold of the semiconductor device before electrons are trapped in the electron trap layer 102
  • the number of trapped electrons in the electron trap layer 102 increases, and the channel disappears.
  • trap of electrons in the electron trap layer 102 stops.
  • the threshold voltage becomes +1.5 V. It can also be said that the threshold voltage is increased by 0.4 V by electrons trapped in the electron trap layer 102 .
  • the threshold that has been changed by electrons trapped in the electron trap layer 102 is referred to as a second threshold (Vth2).
  • the thresholds of a plurality of semiconductor devices which are initially largely different from each other can converge at values within an appropriate range. For example, if three semiconductor devices with the first threshold voltages of +1.2 V, +1.1 V, and +0.9 V are subjected to the process under above-described conditions, trap of electrons does not make the threshold voltage to become significantly higher than +1.5 V in each semiconductor device; the second threshold voltage of each semiconductor device can become approximately +1.5 V. In this case, the number of trapped electrons in the electron trap layer 102 (e.g., the surface density of electrons) varies among the three semiconductor devices.
  • the number of electrons trapped in the electron trap layer 102 also depends on the length of time for the threshold adjust process and thus a desired threshold can be obtained by adjusting time for the threshold adjust process.
  • any of a variety of materials can be used for the gate electrode 103 .
  • a conductive layer of Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used.
  • the gate electrode 103 may have a stacked-layer structure of any of these materials.
  • a conductive layer containing nitrogen may be used for the gate electrode 103 .
  • a titanium nitride layer over which a tungsten layer is stacked, a tungsten nitride layer over which a tungsten layer is stacked, a tantalum nitride layer over which a tungsten layer is stacked, or the like can be used as the gate electrode 103 .
  • the work function of the gate electrode 103 that faces the semiconductor layer 101 is one factor determining the threshold of the semiconductor device; in general, as the work function of a material is smaller, the threshold becomes lower.
  • the threshold can be adjusted by adjusting the number of trapped electrons in the electron trap layer 102 ; accordingly, the range of choices for the material of the gate electrode 103 can be widened.
  • any of a variety of materials can be used for the semiconductor layer 101 .
  • materials can be used for the semiconductor layer 101 .
  • silicon, germanium, and silicon germanium any of a variety of oxide semiconductors described later can be used.
  • any of a variety of materials can be used for the first insulating layer 102 a .
  • an insulating layer containing one or more kinds selected from magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide can be used.
  • the second insulating layer 102 b can be an insulating layer containing one or more kinds selected from hafnium oxide, aluminum oxide, tantalum oxide, aluminum silicate, and the like, for example.
  • the third insulating layer 102 c can be an insulating layer containing one or more kinds selected from magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide, for example.
  • the conductive layer 102 d can be formed using any kind of materials.
  • a conductive layer of Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, Pt, Pd, or the like can be used.
  • the conductive layer 102 d may have a stacked-layer structure of any of these materials.
  • a conductive layer containing nitrogen may be used as the conductive layer 102 d.
  • a metal of the platinum group such as platinum or palladium: a nitride such as indium nitride, zinc nitride, In—Zn-based oxynitride, In—Ga-based oxynitride, or In—Ga—Zn-based oxynitride; or the like may be used.
  • any of a variety of materials can be used for the insulator 102 e .
  • silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, or tantalum oxide can be used.
  • the semiconductor device in which a necessary number of electrons are trapped in the electron trap layer 102 is the same as a normal MOS semiconductor device.
  • the potential of the gate electrode 103 be always set at the lowest potential in the circuit.
  • timing of the process for trapping electrons in the electron trap layer 102 is not limited to that described above and may be any of the following timings before leaving the factory, for example: after formation of wiring metal connected to the source electrode or the drain electrode of the semiconductor device, after backplane process (wafer process), after wafer dicing, and after packaging. In either case, it is preferable that the semiconductor device be not exposed to temperatures of 125° C. or higher for one hour or more after the process for trapping electrons.
  • a semiconductor device which is one embodiment of the present invention is described with reference to drawings.
  • a transistor in which a threshold control gate electrode exists between a substrate and a semiconductor layer is described below, a transistor in which a semiconductor layer exists between a threshold control gate electrode and a substrate may be used.
  • FIGS. 7A to 7C are a top view and cross-sectional views illustrating a transistor of one embodiment of the present invention.
  • FIG. 7A is the top view
  • FIG. 7B illustrates a cross section taken along the dashed-dotted line A-B in FIG. 7A
  • FIG. 7C illustrates a cross section taken along the dashed-dotted line C-D in FIG. 7A .
  • the direction of the dashed-dotted line A-B and the direction of the dashed-dotted line C-D may be referred to as a channel length direction and a channel width direction, respectively.
  • the transistor 450 in FIGS. 7A to 7C includes a gate electrode 401 embedded in a substrate 400 ; a base insulating layer 402 including a projection and a depression over the substrate 400 and the gate electrode 401 ; an oxide semiconductor layer 404 a and an oxide semiconductor layer 404 b over the projection of the base insulating layer 402 ; a source electrode 406 a and a drain electrode 406 b over the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 b ; an oxide semiconductor layer 404 c in contact with the depression of the base insulating layer 402 , a side surface of the projection (depression) of the base insulating layer 402 , a side surface of the oxide semiconductor layer 404 a , a side surface and a top surface of the oxide semiconductor layer 404 b , the source electrode 406 a , and the drain electrode 406 b ; a gate insulating layer 408 over the oxide semiconductor layer 404 c ; a gate electrode 410 provided over and
  • the base insulating layer 402 includes a first insulating layer 402 a , a second insulating layer 402 b , and a third insulating layer 402 c and functions as the electron trap layer described in Embodiment 1.
  • the oxide semiconductor layers 404 a , 404 b , and 404 c are collectively referred to as a multilayer semiconductor layer 404 .
  • the second insulating layer 402 b can be formed thick.
  • the second insulating layer 402 b can be formed approximately four times as thick as the second insulating layer 402 b using silicon oxide with a relative dielectric constant of 3.9.
  • the increase in the thickness of the second insulating layer 402 b is preferable in terms of preventing the leakage of trapped electrons.
  • each thickness of the first insulating layer 402 a and the third insulating layer 402 c is more than or equal to 1 nm and less than or equal to 30 nm, preferably more than or equal to 10 nm and less than or equal to 20 nm.
  • the thickness of the second insulating layer 402 b is more than or equal to 1 nm and less than or equal to 100 nm, preferably more than or equal to 5 nm and less than or equal to 20 nm.
  • the channel length refers to the distance between a source (a source region, source electrode) and a drain (drain region, drain electrode) in a region where a semiconductor layer overlaps with a gate electrode in the top view. That is, the channel length in FIG. 7A is the distance between the source electrode 406 a and the drain electrode 406 b in the region where the oxide semiconductor layer 404 b overlaps with the gate electrode 410 .
  • the channel width refers to the width of a source or a drain in a region where a semiconductor layer overlaps with a gate electrode. That is, the channel width in FIG. 7A is the width of the source electrode 406 a or the drain electrode 406 b in the region where the semiconductor layer 404 b overlaps with the gate electrode 410 .
  • the base insulating layer 402 When the base insulating layer 402 functions as an electron trap layer, electrons can be trapped in electron trap states existing at the interface between the third insulating layer 402 c and the second insulating layer 402 b or inside the second insulating layer 402 as described in Embodiment 1.
  • the number of electrons trapped in the electron trap states can be adjusted by the potential of the gate electrode 401 .
  • the gate electrode 410 electrically covers the oxide semiconductor layer 404 b , increasing the on-state current.
  • This transistor structure is referred to as a surrounded channel (s-channel) structure.
  • a current flows through an entire region of the oxide semiconductor layer 404 b (bulk). Since a current flows through the oxide semiconductor layer 404 b , an adverse effect of interface scattering is unlikely to occur, leading to a high on-state current. Note that as the oxide semiconductor layer 404 b is thicker, the on-state current can be increased.
  • the electrode, the semiconductor layer, or the like has a rounded upper end portion (curved surface) in some cases.
  • the coverage with the gate insulating layer 408 , the gate electrode 410 , and the oxide insulating layer 412 , which are to be formed over the oxide semiconductor layer 404 b can be improved.
  • electric field concentration that might occur at end portions of the source electrode 406 a and the drain electrode 406 b can be reduced, which can suppress deterioration of the transistor.
  • the channel length of the transistor is less than or equal to 100 nm, preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, and still further preferably less than or equal to 20 nm
  • the channel width of the transistor is less than or equal to 100 nm, preferably less than or equal to 40 nm, further preferably less than or equal to 30 nm, and still further preferably less than or equal to 20 nm. Even with such a small channel width, a transistor of one embodiment of the present invention can increase the on-state current by having the s-channel structure.
  • the substrate 400 is not limited to a simple supporting substrate, and may be a substrate where another device such as a transistor is formed. In that case, at least one of the gate electrode 410 , the source electrode 406 a , and the drain electrode 406 b of the transistor 450 may be electrically connected to the above device.
  • the base insulating layer 402 can have a function of supplying oxygen to the multilayer semiconductor layer 404 as well as a function of preventing diffusion of impurities from the substrate 400 .
  • the base insulating layer 402 also has a function as an interlayer insulating layer.
  • the base insulating layer 402 is preferably subjected to planarization treatment such as chemical mechanical polishing (CMP) treatment so as to have a flat surface.
  • CMP chemical mechanical polishing
  • the multilayer semiconductor layer 404 in the channel formation region of the transistor 450 has a structure in which the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c are stacked in this order from the substrate 400 side.
  • the oxide semiconductor layer 404 b is surrounded by the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c .
  • the gate electrode 410 electrically covers the oxide semiconductor layer 404 b.
  • the oxide semiconductor layer 404 b for example, an oxide semiconductor whose electron affinity (an energy difference between a vacuum level and the conduction band minimum) is higher than those of the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c is used.
  • the electron affinity can be obtained by subtracting an energy difference between the conduction band minimum and the valence band maximum (what is called an energy gap) from an energy difference between the vacuum level and the valence band maximum (what is called an ionization potential).
  • the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c each contain one or more kinds of metal elements forming the oxide semiconductor layer 404 b .
  • the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c are preferably formed using an oxide semiconductor whose conduction band minimum is closer to a vacuum level than that of the oxide semiconductor layer 404 b by 0.05 eV or more, 0.07 eV or more, 0.1 eV or more, or 0.15 eV or more and 2 eV or less, 1 eV or less, 0.5 eV or less, or 0.4 eV or less.
  • the oxide semiconductor layer 404 c is formed between the oxide semiconductor layer 404 b and the gate insulating layer 408 , whereby a structure in which the channel of the transistor is provided in a region that is not in contact with the gate insulating layer 408 is obtained.
  • the oxide semiconductor layer 404 a contains one or more metal elements contained in the oxide semiconductor layer 404 b , an interface state is unlikely to be formed at the interface between the oxide semiconductor layer 404 b and the oxide semiconductor layer 404 a , compared with the interface between the oxide semiconductor layer 404 b and the base insulating layer 402 on the assumption that the oxide semiconductor layer 404 b is in contact with the base insulating layer 402 .
  • the interface state sometimes forms a channel, leading to a change in the threshold of the transistor.
  • a variation in the electrical characteristics of the transistor, such as threshold can be reduced. Further, the reliability of the transistor can be improved.
  • the oxide semiconductor layer 404 c contains one or more metal elements contained in the oxide semiconductor layer 404 b , scattering of carriers is unlikely to occur at the interface between the oxide semiconductor layer 404 b and the oxide semiconductor layer 404 c , compared with the interface between the oxide semiconductor layer 404 b and the gate insulating layer 408 on the assumption that the oxide semiconductor layer 404 b is in contact with the gate insulating layer 408 .
  • the field-effect mobility of the transistor can be increased.
  • oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c for example, a material containing Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf with a higher atomic ratio than that used for the oxide semiconductor layer 404 b can be used.
  • an atomic ratio of any of the above metal elements in the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c is 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as much as that in the oxide semiconductor layer 404 b .
  • Any of the above metal elements is strongly bonded to oxygen and thus has a function of suppressing generation of an oxygen vacancy in the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c . That is, an oxygen vacancy is less likely to be generated in the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c than in the oxide semiconductor layer 404 b.
  • each of the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c is an In-M-Zn oxide containing at least indium, zinc, and M (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf), and when the oxide semiconductor layer 404 a has an atomic ratio of In to M and Zn which is x 1 :y 1 :z 1 , the oxide semiconductor layer 404 b has an atomic ratio of In to M and Zn which is x 2 :y 2 :z 2 , and the oxide semiconductor layer 404 c has an atomic ratio of In to M and Zn which is x 3 :y 3 :z 3 , y 1 /x 1 and y 3 /x 3 is each preferably larger than y 2 /x 2 .
  • M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or H
  • Y 1 /x 1 and y 3 /x 3 is each 1.5 times or more, preferably 2 times or more, further preferably 3 times or more as large as y 2 /x 2 .
  • the transistor can have stable electrical characteristics.
  • y 2 is 3 times or more as large as x 2 , the field-effect mobility of the transistor is reduced; accordingly, y 2 is preferably less than 3 times x 2 .
  • the proportion of In and the proportion of M in the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c when summation of In and M is assumed to be 100 atomic % are preferably less than 50 atomic % and greater than or equal to 50 atomic %, respectively, and further preferably less than 25 atomic % and greater than or equal to 75 atomic %, respectively.
  • the proportion of In and the proportion of M in the oxide semiconductor layer 404 b when summation of In and M is assumed to be 100 atomic % are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively.
  • the thicknesses of the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c are each greater than or equal to 3 nm and less than or equal to 100 nm, preferably greater than or equal to 3 nm and less than or equal to 50 nm.
  • the thickness of the oxide semiconductor layer 404 b is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm.
  • the oxide semiconductor layer 404 b is preferably thicker than the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c.
  • an oxide semiconductor containing indium, zinc, and gallium can be used, for example.
  • the oxide semiconductor layer 404 b preferably contains indium because carrier mobility can be increased.
  • substantially intrinsic refers to the state where an oxide semiconductor layer has a carrier density lower than 1 ⁇ 10 17 /cm 3 , preferably lower than 1 ⁇ 10 15 /cm 3 , further preferably lower than 1 ⁇ 10 13 /cm 3 .
  • hydrogen, nitrogen, carbon, silicon, and a metal element other than main components of the oxide semiconductor layer are impurities.
  • hydrogen and nitrogen form donor levels to increase the carrier density.
  • silicon in the oxide semiconductor layer forms an impurity level.
  • the impurity level might become a trap, so that the electrical characteristics of the transistor might deteriorate. Accordingly, in the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c and at interfaces between these layers, the impurity concentration is preferably reduced.
  • the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 1 ⁇ 10 19 atoms/cm 3 , further preferably lower than 5 ⁇ 10 18 atoms/cm 3 , still further preferably lower than 1 ⁇ 10 18 atoms/cm 3 .
  • the concentration of hydrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than or equal to 2 ⁇ 10 20 atoms/cm 3 , further preferably lower than or equal to 5 ⁇ 10 19 atoms/cm 3 , still further preferably lower than or equal to 1 ⁇ 10 19 atoms/cm 3 , yet still further preferably lower than or equal to 5 ⁇ 10 18 atoms/cm 3 .
  • the concentration of nitrogen at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer is preferably lower than 5 ⁇ 10 19 atoms/cm 3 , further preferably lower than or equal to 5 ⁇ 10 18 atoms/cm 3 , still further preferably lower than or equal to 1 ⁇ 10 18 atoms/cm 3 , yet still further preferably lower than or equal to 5 ⁇ 10 17 atoms/cm 3 .
  • the oxide semiconductor layer includes crystals
  • high concentration of silicon or carbon might reduce the crystallinity of the oxide semiconductor layer.
  • the concentration of silicon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1 ⁇ 10 19 atoms/cm 3 , preferably lower than 5 ⁇ 10 18 atoms/cm 3 , further preferably lower than 1 ⁇ 10 18 atoms/cm 3 .
  • the concentration of carbon at a certain depth of the oxide semiconductor layer or in a region of the oxide semiconductor layer may be lower than 1 ⁇ 10 19 atoms/cm 3 , preferably lower than 5 ⁇ 10 18 atoms/cm 3 , further preferably lower than 1 ⁇ 10 18 atoms/cm 3 , for example.
  • a transistor in which the above-described highly purified oxide semiconductor layer is used for a channel formation region has an extremely low off-state current.
  • the voltage between a source and a drain is set at about 0.1 V, 5 V, or 10 V, for example, the off-state current standardized on the channel width of the transistor can be as low as several yoctoamperes per micrometer to several zeptoamperes per micrometer.
  • the gate insulating layer of the transistor an insulating layer containing silicon is used in many cases; thus, it is preferable that, as in the transistor of one embodiment of the present invention, a region of the multilayer semiconductor layer, which serves as a channel, be not in contact with the gate insulating layer for the above-described reason. In the case where a channel is formed at the interface between the gate insulating layer and the multilayer semiconductor layer, scattering of carriers occurs at the interface, whereby the field-effect mobility of the transistor may be reduced. Also from the view of the above, it is preferable that the region of the multilayer semiconductor layer, which serves as a channel, be separated from the gate insulating layer.
  • the multilayer semiconductor layer 404 having a stacked structure of the oxide semiconductor layers 404 a , 404 b , and 404 c , a channel can be formed in the oxide semiconductor layer 404 b ; thus, the transistor can have high field-effect mobility and stable electrical characteristics.
  • the band structure of the multilayer semiconductor layer 404 is described.
  • a stacked film corresponding to the multilayer semiconductor layer 404 is formed.
  • In the stacked film In—Ga—Zn oxide with an energy gap of 3.5 eV is used for layers corresponding to the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c , and In—Ga—Zn oxide with an energy gap of 3.15 eV is used for a layer corresponding to the oxide semiconductor layer 404 b.
  • each of the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c was 10 nm.
  • the energy gap was measured with the use of a spectroscopic ellipsometer (UT-300 manufactured by HORIBA Jobin Yvon). Further, the energy difference between the vacuum level and the valence band maximum was measured using an ultraviolet photoelectron spectroscopy (UPS) device (VersaProbe, ULVAC-PHI. Inc.).
  • UPS ultraviolet photoelectron spectroscopy
  • FIG. 8A is part of a schematic band structure showing an energy difference (electron affinity) between the vacuum level and the conduction band minimum of each layer, which is calculated by subtracting the energy gap from the energy difference between the vacuum level and the valence band maximum.
  • FIG. 8A is a band diagram showing the case where silicon oxide layers are provided in contact with the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c .
  • Evac represents energy of the vacuum level
  • EcI1 and EcI2 represent the conduction band minimum of the silicon oxide layer
  • EcS1 represents the conduction band minimum of the oxide semiconductor layer 404 a
  • EcS2 represents the conduction band minimum of the oxide semiconductor layer 404 b
  • EcS3 represents the conduction band minimum of the oxide semiconductor layer 404 c.
  • the conduction band minimum continuously varies among the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c .
  • This can be understood also from the fact that the constituent elements are common among the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c and oxygen is easily diffused among the oxide semiconductor layers 404 a to 404 c . Accordingly, the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c have a continuous physical property although they have different compositions in a stack.
  • the multilayer semiconductor layer 404 in which layers containing the same main components are stacked is formed to have not only a simple stacked-layer structure of the layers but also a continuous energy band (here, in particular, a well structure having a U shape in which the conduction band minimum continuously varies among the layers).
  • the stacked-layer structure is formed such that there exist no impurities that form a defect level such as a trap center or a recombination center at each interface. If impurities exist between the stacked layers in the multilayer semiconductor layer, the continuity of the energy band is lost and carriers at the interface disappear by a trap or recombination.
  • FIG. 8A shows the case where EcS1 and EcS3 are equal to each other; however, EcS1 and EcS3 may be different from each other.
  • part of the band structure in the case where EcS1 is higher than EcS3 is shown in FIG. 8B .
  • EcS1 when EcS1 is equal to EcS3, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, 1:3:4, 1:6:4, or 1:9:6 can be used for the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c and an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 or 3:1:2 can be used for the oxide semiconductor layer 404 b .
  • EcS1 when EcS1 is higher than EcS3, an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:6:4 or 1:9:6 can be used for the oxide semiconductor layer 404 a
  • an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 or 3:1:2 can be used for the oxide semiconductor layer 404 b
  • an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:2, 1:3:3, or 1:3:4 can be used for the oxide semiconductor layer 404 c , for example.
  • the oxide semiconductor layer 404 b of the multilayer semiconductor layer 404 serves as a well, so that a channel is formed in the oxide semiconductor layer 404 b in a transistor including the multilayer semiconductor layer 404 .
  • the multilayer semiconductor layer 404 can also be referred to as a U-shaped well.
  • a channel formed to have such a structure can also be referred to as a buried channel.
  • trap levels due to impurities or defects might be formed in the vicinity of the interface between the oxide semiconductor layers 404 a and 404 c and an insulating layer such as a silicon oxide layer.
  • the oxide semiconductor layer 404 b can be distanced away from the trap levels owing to existence of the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c .
  • an electron in the oxide semiconductor layer 404 b might reach the trap levels by passing over the oxide semiconductor layer 404 a or the oxide semiconductor layer 404 c .
  • the threshold of the transistor shifts in the positive direction.
  • each of the energy differences is preferably greater than or equal to 0.1 eV, further preferably greater than or equal to 0.15 eV.
  • the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c preferably include crystal parts.
  • the transistor can have stable electrical characteristics.
  • the oxide semiconductor layer 404 c contain less In than the oxide semiconductor layer 404 b so that diffusion of In to the gate insulating layer is prevented.
  • a conductive material that can be bonded to oxygen is preferably used.
  • Al, Cr, Cu, Ta, Ti, Mo, or W can be used.
  • Ti which is easily bonded to oxygen
  • W with a high melting point, which allows subsequent process temperatures to be relatively high.
  • the conductive material that can be bonded to oxygen includes, in its category, a material to which oxygen is easily diffused.
  • the conductive material that can be bonded to oxygen When the conductive material that can be bonded to oxygen is in contact with a multilayer semiconductor layer, a phenomenon occurs in which oxygen in the multilayer semiconductor layer is diffused to the conductive material that can be bonded to oxygen. The phenomenon noticeably occurs when the temperature is high. Since the fabricating process of the transistor involves some heat treatment steps, the above phenomenon causes generation of oxygen vacancies in the vicinity of a region which is in the multilayer semiconductor layer and is in contact with the source electrode or the drain electrode. The oxygen vacancies bond to hydrogen that is slightly contained in the layer, whereby the region is changed to an n-type region. Thus, the n-type region can serve as a source or a drain of the transistor.
  • a transistor with an extremely short channel length In the case of forming a transistor with an extremely short channel length, an n-type region which is formed by the generation of oxygen vacancies might extend in the channel length direction of the transistor, causing a short circuit. In that case, the electrical characteristics of the transistor change; for example, the threshold voltage shifts to cause a state in which on and off states of the transistor cannot be controlled with the gate voltage (conduction state). Accordingly, when a transistor with an extremely short channel length is formed, it is not always preferable that a conductive material that can be bonded to oxygen be used for a source electrode and a drain electrode.
  • a conductive material which is less likely to be bonded to oxygen than the above material is preferably used for the source electrode 406 a and the drain electrode 406 b .
  • the conductive material which is not easily bonded to oxygen for example, a material containing tantalum nitride, titanium nitride, or ruthenium or the like can be used. Note that in the case where the conductive material is in contact with the oxide semiconductor layer 404 b , the conductive materials may be stacked with the above-described conductive material which is easily bonded to oxygen.
  • the first insulating layer 402 a , the third insulating layer 402 c , and the gate insulating layer 408 can be formed using an insulating layer containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, and tantalum oxide.
  • the second insulating layer 402 b can be formed using an insulating layer containing one or more of hafnium oxide, aluminum oxide, aluminum silicate, and the like.
  • each thickness of the first insulating layer 402 a and the third insulating layer 402 c is more than or equal to 1 nm and less than or equal to 30 nm, preferably more than or equal to 10 nm and less than or equal to 20 nm.
  • the thickness of the second insulating layer 402 b is more than or equal to 1 nm and less than or equal to 20 nm, preferably more than or equal to 5 nm and less than or equal to 10 nm.
  • a conductive layer formed using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or the like can be used.
  • the gate electrodes 401 and 410 may be a stack including any of the above materials.
  • a conductive layer containing nitrogen may be used for the gate electrodes 401 and 410 .
  • the gate electrodes 401 and 410 can be a stack in which a tungsten layer is formed over a titanium nitride layer, a stack in which a tungsten layer is formed over a tungsten nitride layer, or a stack in which a tungsten layer is formed over a tantalum nitride layer.
  • the oxide insulating layer 412 may be formed over the gate insulating layer 408 and the gate electrode 410 .
  • the oxide insulating layer 412 can be formed using an insulating layer containing one or more of magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, and tantalum oxide.
  • the oxide insulating layer 412 may be a stack including any of the above materials.
  • the oxide insulating layer 412 preferably contains excess oxygen.
  • An oxide insulating layer containing excess oxygen refers to an oxide insulating layer from which oxygen can be released by heat treatment or the like.
  • the oxide insulating layer containing excess oxygen is preferably a layer in which the amount of released oxygen when converted into oxygen atoms is 1.0 ⁇ 10 19 atoms/cm 3 or more in thermal desorption spectroscopy analysis. Oxygen released from the oxide insulating layer can be diffused to the channel formation region in the multilayer semiconductor layer 404 through the gate insulating layer 408 , so that oxygen vacancies formed in the channel formation region can be filled with the oxygen. In this manner, stable electrical characteristics of the transistor can be achieved.
  • the oxide semiconductor layer 404 c is formed so as to cover a region where a channel is formed in the oxide semiconductor layer 404 b , and a channel formation layer and the gate insulating layer are not in contact with each other. Accordingly, scattering of carriers at the interface between a channel formation layer and the gate insulating layer can be reduced and the on-state current of the transistor can be increased.
  • the field-effect mobility might be reduced because of a decrease in the number of carriers contained in the oxide semiconductor layer.
  • a gate electric field is applied to the oxide semiconductor layer in the side surface direction in addition to the perpendicular direction. That is, the gate electric field is applied to the whole of the oxide semiconductor layer, whereby current flows in the bulk of the oxide semiconductor layer. Consequently, a change in the electrical characteristics can be suppressed owing to the highly purified intrinsic oxide semiconductor layer and the field-effect mobility of the transistor can be increased.
  • the oxide semiconductor layer 404 b is formed over the oxide semiconductor layer 404 a , so that an interface state is less likely to be formed.
  • impurities do not enter the oxide semiconductor layer 404 b from above and below because the oxide semiconductor layer 404 b is an intermediate layer in a three-layer structure.
  • on-state current of the transistor is increased as described above, and in addition, threshold voltage can be stabilized and an S value can be reduced.
  • Icut can be reduced and power consumption can be reduced.
  • the threshold of the transistor becomes stable; thus, long-term reliability of the semiconductor device can be improved.
  • FIGS. 9A to 9C are a top view and cross-sectional views which illustrate the transistor 470 .
  • FIG. 9A is the top view.
  • FIG. 9B illustrates a cross section taken along the dashed-dotted line A-B in FIG. 9A .
  • FIG. 9C illustrates a cross section taken along the dashed-dotted line C-D in FIG. 9A . Note that for simplification of the drawing, some components are not illustrated in the top view in FIG. 9A .
  • the base insulating layer 402 is not etched because overetching of a conductive layer to be the source electrode 406 a and the drain electrode 406 b does not occur when the source electrode 406 a and the drain electrode 406 b are formed.
  • the etching rate of the base insulating layer 402 is preferably set (sufficiently) lower than the etching rate of the conductive layer.
  • one embodiment of the present invention may have a structure in which only the oxide semiconductor layer 404 b is provided without the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 c and is electrically covered with the gate electrode.
  • a method for forming the transistor 450 which is described in Embodiment 2 with reference to FIGS. 7A to 7C , is described with reference to FIGS. 10A to 10D and FIGS. 11A to 11C .
  • a plurality of linear grooves is formed on the substrate 400 , a conductive layer is deposited using Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as a main component, and is planarized and etched (see FIG. 10A ).
  • the conductive layer can be formed by sputtering, CVD, or the like.
  • a glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used.
  • a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon-on-insulator (SOI) substrate, or the like may be used. Any of these substrates further provided with a semiconductor element thereover may be used.
  • the base insulating layer 402 composed of the first to third insulating layers 402 a to 402 c is formed (see FIG. 10B ).
  • Oxygen may be added to the base insulating layer 402 by an ion implantation method, an ion doping method, a plasma immersion ion implantation method, or the like. Adding oxygen enables the base insulating layer 402 to supply oxygen much easily to the multilayer semiconductor layer 404 .
  • the oxide semiconductor layers 404 a and 404 b are formed over the base insulating layer 402 by sputtering, CVD (including MOCVD, ALD, or PECVD), vacuum evaporation, or pulse laser deposition (PLD) (see FIG. 10C ).
  • CVD including MOCVD, ALD, or PECVD
  • PVD pulse laser deposition
  • the base insulating layer 402 can be slightly over-etched.
  • the gate electrode 410 to be formed later can cover the oxide semiconductor layer 404 c easily.
  • a layer to be a hard mask e.g. a tungsten layer
  • a resist mask are provided over the oxide semiconductor layer 404 b , and the layer to be a hard mask is etched to form a hard mask.
  • the resist mask is removed and then the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 b are etched using the hard mask as a mask. Then, the resist mask is removed.
  • the hard mask is gradually reduced as the etching progresses; as a result, the edges of the hard mask is rounded to have a curved surface. Accordingly, the edges of the oxide semiconductor layer 404 b is rounded to have a curved surface.
  • This structure improves the coverage with the oxide semiconductor layer 404 c , the gate insulating layer 408 , the gate electrode 410 , and the oxide insulating layer 412 , which are to be formed over the oxide semiconductor layer 404 b , and can prevent shape defects such as disconnection.
  • electric field concentration which might occur at end portions of the source electrode 406 a and the drain electrode 406 b can be reduced, which can reduce deterioration of the transistor.
  • the layers need to be formed successively without exposure to the air with the use of a multi-chamber deposition apparatus (e.g., a sputtering apparatus) including a load lock chamber.
  • a multi-chamber deposition apparatus e.g., a sputtering apparatus
  • each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (to about 5 ⁇ 10 ⁇ 7 Pa to 1 ⁇ 10 ⁇ 4 Pa) by an adsorption vacuum pump such as a cryopump and that the chamber be able to heat a substrate to 100° C. or higher, preferably 500° C. or higher so that water and the like acting as impurities of the oxide semiconductor can be removed as much as possible.
  • a combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber.
  • 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 is used, whereby entry of moisture or the like into the oxide semiconductor layer can be prevented as much as possible.
  • the materials described in Embodiment 2 can be used for the oxide semiconductor layer 404 a , the oxide semiconductor layer 404 b , and the oxide semiconductor layer 404 c that is to be formed in a later step.
  • an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4 or 1:3:2 can be used for the oxide semiconductor layer 404 a
  • an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:1:1 can be used for the oxide semiconductor layer 404 b
  • an In—Ga—Zn oxide whose atomic ratio of In to Ga and Zn is 1:3:4 or 1:3:2 can be used for the oxide semiconductor layer 404 c.
  • An oxide semiconductor that can be used for each of the oxide semiconductor layers 404 a , 404 b , and 404 c preferably contains at least indium (In) or zinc (Zn). Both In and Zn are preferably contained. Furthermore, in order to reduce variations in electrical characteristics of the transistors including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn.
  • gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), and the like are used.
  • lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given.
  • La lanthanum
  • Ce cerium
  • Pr praseodymium
  • Nd neodymium
  • Sm samarium
  • Eu europium
  • Gd gadolinium
  • Tb terbium
  • Dy dysprosium
  • Ho holmium
  • Er erbium
  • Tm
  • In—Ga—Zn oxide means an oxide containing In, Ga, and Zn as its main components.
  • the In—Ga—Zn oxide may contain another metal element in addition to In, Ga, and Zn.
  • a layer containing the In—Ga—Zn oxide is also referred to as an IGZO layer.
  • a material represented by InMO 3 (ZnO) m (m>0 is satisfied, and m is not an integer) may be used.
  • M represents one or more metal elements selected from Ga, Fe, Mn, or Co.
  • a material represented by In 2 SnO 5 (ZnO) (n>0, n is an integer) may be used.
  • materials are selected so that the oxide semiconductor layers 404 a and 404 c each have an electron affinity lower than that of the oxide semiconductor layer 404 b.
  • the oxide semiconductor layer is preferably formed by a sputtering method.
  • a sputtering method an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used.
  • a DC sputtering method is preferably used because dust generated in the film formation can be reduced and the film thickness can be uniform.
  • a material whose atomic ratio of In to Ga and Zn is any of 1:1:1, 2:2:1, 3:1:2, 1:3:2, 1:3:4, 1:4:3, 1:5:4, 1:6:6, 2:1:3 1:6:4, 1:9:6, 1:1:4, and 1:1:2 is used so that the oxide semiconductor layers 404 a and 404 c each have an electron affinity lower than that of the oxide semiconductor layer 404 b.
  • the indium content in the oxide semiconductor layer 404 b is preferably higher than those in the oxide semiconductor layers 404 a and 404 c .
  • the s orbital of heavy metal mainly contributes to carrier transfer, and when the proportion of In in the oxide semiconductor is increased, overlap of the s orbitals is likely to be increased. Therefore, an oxide having a composition in which the proportion of In is higher than that of Ga has higher mobility than an oxide having a composition in which the proportion of In is equal to or lower than that of Ga.
  • a transistor having high mobility can be achieved.
  • a term “parallel” indicates 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 includes the case where the angle is greater than or equal to ⁇ 5° and less than or equal to 5°.
  • the term “perpendicular” indicates that the angle formed between two straight lines is greater than or equal to 80° and less than or equal to 100°, and accordingly includes the case where the angle is greater than or equal to 85° and less than or equal to 95°.
  • trigonal and rhombohedral crystal systems are included in a hexagonal crystal system.
  • An oxide semiconductor layer is classified roughly into a non-single-crystal oxide semiconductor layer and a single crystal oxide semiconductor layer.
  • the non-single-crystal oxide semiconductor layer includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS) layer, a polycrystalline oxide semiconductor layer, a microcrystalline oxide semiconductor layer, an amorphous oxide semiconductor layer, and the like.
  • CAAC-OS c-axis aligned crystalline oxide semiconductor
  • the CAAC-OS layer is an oxide semiconductor layer including a plurality of crystal parts. Most of the crystal parts each fit inside a cube whose one side is less than 100 nm. Thus, the CAAC-OS layer may include a crystal part that fits inside a cube whose one side is less than 10 nm, less than 5 nm, or less than 3 nm.
  • TEM transmission electron microscope
  • metal atoms are arranged in a layered manner in the crystal parts.
  • Each metal atom layer has a morphology reflected by a surface over which the CAAC-OS layer is formed (hereinafter, a surface over which the CAAC-OS layer is formed is referred to as a formation surface) or a top surface of the CAAC-OS layer, and is arranged in parallel to the formation surface or the top surface of the CAAC-OS layer.
  • metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity in arrangement of metal atoms between different crystal parts.
  • a CAAC-OS layer is subjected to structural analysis with an X-ray diffraction (XRD) apparatus.
  • XRD X-ray diffraction
  • each metal atom layer arranged in a layered manner observed in the cross-sectional TEM image corresponds to a plane parallel to the a-b plane of the crystal.
  • the crystal part is formed concurrently with deposition of the CAAC-OS layer or is formed through crystallization treatment such as heat treatment.
  • the c-axis of the crystal is oriented in a direction parallel to a normal vector of a formation surface or a normal vector of a top surface.
  • the c-axis might not be necessarily parallel to a normal vector of a formation surface or a normal vector of a top surface of the CAAC-OS layer.
  • the degree of crystallinity in the CAAC-OS layer is not necessarily uniform. For example, if crystal growth leading to the CAAC-OS layer occurs from the vicinity of the top surface of the layer, the degree of the crystallinity in the vicinity of the top surface may be higher than that in the vicinity of the formation surface. Moreover, when an impurity is added to the CAAC-OS layer, the crystallinity in a region to which the impurity is added is changed, and the degree of crystallinity in the CAAC-OS layer varies depending on regions.
  • a peak of 2 ⁇ may be observed at around 36°, in addition to the peak of 2 ⁇ at around 31°.
  • the peak of 2 ⁇ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS layer. It is preferable that a peak of 2 ⁇ appears at around 31° and a peak of 2 ⁇ do not appear at around 36°.
  • the CAAC-OS layer is an oxide semiconductor layer having low impurity concentration.
  • the impurity is an element other than the main components of the oxide semiconductor layer, such as hydrogen, carbon, silicon, or a transition metal element.
  • an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor layer, such as silicon disturbs the atomic arrangement of the oxide semiconductor layer by depriving the oxide semiconductor layer of oxygen and causes a decrease in crystallinity.
  • a heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor layer and causes a decrease in crystallinity if contained in the oxide semiconductor layer.
  • the impurity contained in the oxide semiconductor layer might serve as a carrier trap or a carrier generation source.
  • the CAAC-OS layer is an oxide semiconductor layer having a low density of defect states. Oxygen vacancies in the oxide semiconductor layer may serve as carrier traps or carrier generation sources when hydrogen is captured therein.
  • the state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) 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 layer has few carrier generation sources, and thus can have a low carrier density.
  • a transistor including the oxide semiconductor layer rarely has negative threshold voltage (is rarely normally on).
  • the highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor layer has few carrier traps. Accordingly, the transistor including the oxide semiconductor layer has little variation in electrical characteristics and high reliability. Electric charge trapped by the carrier traps in the oxide semiconductor layer takes a long time to be released, and thus may behave like fixed electric charge. Accordingly, the transistor which includes the oxide semiconductor layer having high impurity concentration and a high density of defect states can have unstable electrical characteristics.
  • the crystal part size in the microcrystalline oxide semiconductor is more than or equal to 1 nm and less than or equal to 100 nm, or more than or equal to 1 nm and less than or equal to 10 nm.
  • a microcrystal with a size more than or equal to 1 nm and less than or equal to 10 nm, or a size more than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as nanocrystal (nc).
  • An oxide semiconductor layer including nanocrystal is referred to as an nc-OS (nanocrystalline oxide semiconductor) layer. In a TEM image of the nc-OS layer, for example, a boundary between crystal parts is not clearly detected in some cases.
  • a microscopic region for example, a region with a size more than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size more than or equal to 1 nm and less than or equal to 3 nm
  • a microscopic region has a periodic atomic order.
  • the orientation of the whole layer is not observed. Accordingly, the nc-OS layer sometimes cannot be distinguished from an amorphous oxide semiconductor layer depending on an analysis method.
  • nc-OS layer when the nc-OS layer is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Furthermore, a halo pattern is shown in an electron diffraction pattern (also referred to as a selected-area electron diffraction pattern) of the nc-OS layer obtained by using an electron beam having a probe diameter (e.g., more than or equal to 50 nm) larger than the diameter of a crystal part.
  • a probe diameter e.g., more than or equal to 50 nm
  • spots are shown in a nanobeam electron diffraction pattern of the nc-OS layer obtained by using an electron beam having a probe diameter (e.g., more than or equal to 1 nm and smaller than or equal to 30 nm) close to, or smaller than or equal to a diameter of a crystal part.
  • a probe diameter e.g., more than or equal to 1 nm and smaller than or equal to 30 nm
  • regions with high luminance in a circular (ring) pattern may be shown, and a plurality of spots may be shown in the ring-like region.
  • the nc-OS layer is an oxide semiconductor layer that has high regularity as compared with an amorphous oxide semiconductor layer. For this reason, the nc-OS layer has a lower density of defect states than an amorphous oxide semiconductor layer. However, there is no regularity of crystal orientation between different crystal parts in the nc-OS layer; hence, the nc-OS layer has a higher density of defect states than the CAAC-OS layer.
  • an oxide semiconductor layer may be a stacked layer including two or more layers of an amorphous oxide semiconductor layer, a microcrystalline oxide semiconductor layer, and a CAAC-OS layer, for example.
  • the CAAC-OS layer can be deposited by a sputtering method using a polycrystalline oxide semiconductor sputtering target.
  • a crystal region included in the sputtering target is sometimes separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) flakes off from the sputtering target.
  • the flat-plate-like sputtered particle or pellet-like sputtered particle is electrically charged and thus reaches the substrate while maintaining its crystal state, without being aggregation in plasma, forming a CAAC-OS layer.
  • First heat treatment may be performed after the oxide semiconductor layer 404 b is formed.
  • the first heat treatment may be performed at a temperature higher than or equal to 250° C. and lower than or equal to 650° C., typically higher than or equal to 300° C. and lower than or equal to 500° C., in an inert gas atmosphere, an atmosphere containing an oxidizing gas at 10 ppm or more, or a reduced pressure state.
  • the first heat treatment may be performed in such a manner that heat treatment is performed in an inert gas atmosphere, and then another heat treatment is performed in an atmosphere containing an oxidizing gas at 10 ppm or more, in order to compensate desorbed oxygen.
  • the crystallinity of the oxide semiconductor layer 404 b can be improved, and in addition, impurities such as hydrogen and water can be removed from the base insulating layer 402 and the oxide semiconductor layer 404 a .
  • the first heat treatment may be performed before etching for formation of the oxide semiconductor layer 404 b.
  • a first conductive layer to be the source electrode 406 a and the drain electrode 406 b is formed over the oxide semiconductor layers 404 a and 404 b .
  • Al, Cr, Cu, Ta, Ti, Mo, W, or an alloy material containing any of these as a main component can be used.
  • a 100-nm-thick titanium layer is formed by a sputtering method or the like.
  • a tungsten layer is formed by a CVD method or the like.
  • the first conductive layer is etched so as to be divided over the oxide semiconductor layer 404 b to form the source electrode 406 a and the drain electrode 406 b (see FIG. 10D ).
  • the oxide semiconductor layer 403 c is formed over the oxide semiconductor layer 404 b , the source electrode 406 a , and the drain electrode 406 b.
  • second heat treatment may be performed after the oxide semiconductor layer 403 c is formed.
  • the second heat treatment can be performed in a condition similar to that of the first heat treatment.
  • the second heat treatment can remove impurities such as hydrogen and water from the oxide semiconductor layer 403 c .
  • impurities such as hydrogen and water can be further removed from the oxide semiconductor layer 404 a and 404 b.
  • an insulating layer 407 to be the gate insulating layer 408 is formed over the oxide semiconductor layer 403 c (see FIG. 11A ).
  • the insulating layer 407 can be formed by sputtering. CVD (including MOCVD, ALD, or PECVD), vacuum evaporation, PLD.
  • a second conductive layer 409 to be the gate electrode 410 is formed over the insulating layer 407 (see FIG. 11B ).
  • Al, Ti, Cr, Co, Ni, Cu, Y, Zr, Mo, Ru, Ag, Ta, W, or an alloy material containing any of these as its main component can be used.
  • the second conductive layer 409 can be formed by a sputtering method, a CVD method, or the like.
  • a stack including a conductive layer containing any of the above materials and a conductive layer containing nitrogen, or a conductive layer containing nitrogen may be used for the second conductive layer 409 .
  • the second conductive layer 409 is selectively etched using a resist mask to form the gate electrode 410 (see FIG. 11C ). Note that as shown in FIG. 7C , the oxide semiconductor layer 404 b is electrically surrounded by the gate electrode 410 .
  • the insulating layer 407 is selectively etched using the resist mask or the gate electrode 410 as a mask to form the gate insulating layer 408 .
  • the oxide semiconductor layer 403 c is etched using the resist mask or the gate electrode 410 as a mask to form the oxide semiconductor layer 404 c.
  • the top edge of the oxide semiconductor 404 c is aligned with the bottom edge of the gate insulating layer 408 .
  • the top edge of the gate insulating layer 408 is aligned with the bottom edge of the gate electrode 410 .
  • the gate insulating layer 408 and the oxide semiconductor layer 404 c are formed using the gate electrode 410 as a mask, the gate insulating layer 408 and the oxide semiconductor layer 404 c may be formed before the second conductive layer 409 is formed.
  • the oxide insulating layer 412 is formed over the source electrode 406 a , the drain electrode 406 b , and the gate electrode 410 (see FIGS. 7B and 7C ).
  • a material and a method for the oxide insulating layer 412 can be similar to those for the first insulating layer 402 a .
  • the oxide insulating layer 412 may be formed using aluminum oxide, magnesium oxide, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon oxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide, or an oxide insulating layer containing nitrogen.
  • the oxide insulating layer 412 can be formed by sputtering, CVD (including MOCVD, ALD, or PECVD), vacuum evaporation, PLD.
  • CVD including MOCVD, ALD, or PECVD
  • PLD vacuum evaporation
  • the oxide insulating layer 412 preferably contains excessive oxygen so as to be able to supply oxygen to the multilayer semiconductor layer 404 .
  • third heat treatment may be performed.
  • the third heat treatment can be performed under a condition similar to that of the first heat treatment.
  • excess oxygen is easily released from the base insulating layer 402 , the gate insulating layer 408 , and the oxide insulating layer 412 , so that oxygen vacancies in the multilayer semiconductor layer 404 can be reduced.
  • fourth heat treatment is performed.
  • the potential of the gate electrode 401 is kept higher than that of the source or drain electrode at a high temperature higher than or equal to 125° C. and lower than or equal to 450° C., for example higher than or equal to 150° C. and lower than or equal to 300° C. for one second or longer, for example, one minute or longer.
  • the needed number of electrons moves from the multilayer semiconductor layer 404 toward the gate electrode 401 and some of them are trapped by the electron trap states 106 existing inside the second insulating layer 402 b or at the interface with another insulating layer.
  • the degree of the threshold increase can be controlled.
  • the transistor 450 illustrated in FIGS. 7A to 7C can be fabricated.
  • FIGS. 12A to 12C are a top view and cross-sectional views which illustrate a transistor of one embodiment of the present invention.
  • FIG. 12A is the top view.
  • FIG. 12B illustrates a cross section taken along the dashed-dotted line A-B in FIG. 12A .
  • FIG. 12C illustrates a cross section taken along the dashed-dotted line C-D in FIG. 12A . Note that for simplification of the drawing, some components in the top view in FIG. 12A are not illustrated.
  • the direction of the dashed-dotted line A-B is referred to as a channel length direction
  • the direction of the dashed-dotted line C-D is referred to as a channel width direction.
  • a transistor 550 illustrated in FIGS. 12A to 12C includes the base insulating layer 402 over the substrate 400 ; the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 b over the base insulating layer 402 ; the source electrode 406 a and the drain electrode 406 b over the oxide semiconductor layer 404 a and the oxide semiconductor layer 404 b ; the oxide semiconductor layer 404 c in contact with the oxide semiconductor layer 404 b , the source electrode 406 a , and the drain electrode 406 b ; the gate insulating layer 408 over the oxide semiconductor layer 404 c ; the gate electrode 410 over the gate insulating layer 408 ; and the oxide insulating layer 412 over the source electrode 406 a , the drain electrode 406 b , and the gate electrode 410 .
  • the base insulating layer 402 includes the first insulating layer 402 a , the second insulating layer 402 b , and the third insulating layer 402 c and functions as the electron trap layer described in Embodiment 1.
  • the oxide semiconductor layers 404 a , 404 b , and 404 c are collectively referred to as multilayer semiconductor layer 404 .
  • the transistor 450 in Embodiment 2 is different from the transistor 550 in this embodiment in that each of the channel length and the channel width is more than or equal to twice, typically ten times as large as the thickness of the multilayer semiconductor layer 404 .
  • a channel length refers to a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor layer and a gate electrode overlap with each other in a top view. Accordingly, in FIG. 12A , a channel length is a distance between the source electrode 406 a and the drain electrode 406 b in a region where the oxide semiconductor layer 404 b and the gate electrode 410 overlap with each other.
  • a channel width refers to a length of a portion where a source and a drain face each other in parallel and where a semiconductor layer and a gate electrode overlap with each other. Accordingly, in FIG. 12A , a channel width is a length of a portion where the source electrode 406 a and the drain electrode 406 b face each other and where the oxide semiconductor layer 404 b and the gate electrode 410 overlap with each other.
  • FIG. 13A is a cross-sectional view of the transistor 560 .
  • the difference between the transistors 550 and 560 is that the gate electrode 401 of the transistor 560 overlaps the source electrode 406 a but does not overlap the drain electrode 406 b .
  • parasitic capacitance generated between the gate electrode 401 and the drain electrode 406 b can be reduced.
  • the gate electrode 401 may be placed to overlap the drain electrode 406 b but not the source electrode 406 a.
  • FIG. 13B is a cross-sectional view of the transistor 570 .
  • the difference between the transistors 560 and 570 is that the gate electrode 401 of the transistor 570 overlaps neither the source electrode 406 a nor the drain electrode 406 b .
  • parasitic capacitance generated between the gate electrode 401 and the source and drain electrodes 406 a and 406 b can be reduced. Because Icut can be reduced as long as the threshold is high in at least part of a channel, such a structure can be used.
  • this embodiment has a structure in which the oxide semiconductor layer 404 b is sandwiched between the oxide semiconductor layers 404 a and 404 c , without limitation to this structure, a structure including only the oxide semiconductor layer 404 b and not including the oxide semiconductor layers 404 a and 404 c is possible. Alternatively, a structure including any one or two of the oxide semiconductor layers 404 a , 404 b , and 404 c is possible.
  • FIGS. 14A and 14B are circuit diagrams of a semiconductor device and FIGS. 14C and 14D are cross-sectional views of a semiconductor device.
  • FIGS. 14C and 14D each illustrate a cross-sectional view of the transistor 450 in a channel length direction on the left and a cross-sectional view of the transistor 450 in a channel width direction on the right.
  • “OS” is written beside a transistor in order to clearly demonstrate that the transistor includes an oxide semiconductor.
  • the semiconductor devices illustrated in FIGS. 14C and 14D each include a transistor 2200 containing a first semiconductor material in a lower portion and a transistor containing a second semiconductor material in an upper portion.
  • a transistor 2200 containing a first semiconductor material in a lower portion and a transistor containing a second semiconductor material in an upper portion.
  • the transistor 450 described in Embodiment 2 as an example is used as the transistor containing the second semiconductor material.
  • the first semiconductor material and the second semiconductor material are preferably materials having different band gaps.
  • the first semiconductor material may be a semiconductor material (e.g., silicon, germanium, silicon germanium, silicon carbide, or gallium arsenic) other than an oxide semiconductor
  • the second semiconductor material may be the oxide semiconductor described in Embodiment 2.
  • a transistor using a material other than an oxide semiconductor, such as single crystal silicon, can operate at high speed easily.
  • a transistor including an oxide semiconductor has the low off-state current.
  • the transistor 2200 is a p-channel transistor here, it is needless to say that an n-channel transistor can be used to form a circuit having a different configuration.
  • the specific structure of the semiconductor device such as the material used for the semiconductor device and the structure of the semiconductor device, is not necessarily limited to those described here except for the use of the transistor described in Embodiment 2, which is formed using an oxide semiconductor.
  • FIGS. 14A , 14 C, and 14 D each illustrate a configuration example of what is called a CMOS circuit, in which a p-channel transistor and an n-channel transistor are connected in series and gates of the transistors are connected.
  • the circuit can operate at high speed because the transistor of one embodiment of the present invention including an oxide semiconductor has high on-state current.
  • the transistor 450 is provided over the transistor 2200 with an insulating layer 2201 positioned therebetween.
  • Wirings 2202 are provided between the transistor 2200 and the transistor 450 .
  • wirings and electrodes provided in the upper portion and the lower portion are electrically connected to each other through a plurality of plugs 2203 embedded in insulating layers.
  • an insulating layer 2204 covering the transistor 450 , a wiring 2205 over the insulating layer 2204 , and a wiring 2206 formed by processing a conductive layer that is also used for a pair of electrodes of the transistor are provided.
  • the area occupied by the circuit can be reduced and a plurality of circuits can be arranged with higher density.
  • one of the source and the drain of the transistor 450 is electrically connected to one of a source and a drain of the transistor 2200 via the wiring 2202 and the plug 2203 .
  • the gate of the transistor 450 is electrically connected to a gate of the transistor 2200 via the wiring 2205 , the wiring 2206 , the plug 2203 , the wiring 2202 , and the like.
  • an opening portion in which the plug 2203 is embedded is provided in a gate insulating layer of the transistor 450 , and the gate of the transistor 450 is in contact with the plug 2203 in the opening portion.
  • a connection between the electrodes of the transistor 450 and the transistor 2200 is changed from that in the configuration illustrated in FIG. 14C or FIG. 14D , a variety of circuits can be formed.
  • a circuit having a configuration in which a source and a drain of a transistor are connected to those of another transistor as illustrated in FIG. 14B can operate as what is called an analog switch.
  • a semiconductor device having an image sensor function for reading data of an object can be fabricated with the use of the transistor described in any of the above embodiments.
  • FIG. 15 illustrates an example of an equivalent circuit of a semiconductor device having an image sensor function.
  • One electrode of a photodiode 602 is electrically connected to a photodiode reset signal line 658 , and the other electrode of the photodiode 602 is electrically connected to one gate of a transistor 640 .
  • One of a source and a drain of the transistor 640 is electrically connected to a photo sensor reference signal line 672 , and the other of the source and the drain thereof is electrically connected to one of a source and a drain of a transistor 656 .
  • One gate of the transistor 656 is electrically connected to a gate signal line 659 , and the other of the source and the drain thereof is electrically connected to a photo sensor output signal line 671 .
  • the other gate (backgate) of the transistor 640 and the other gate (backgate) of the transistor 656 are connected to a ground line 673 .
  • the photodiode 602 for example, a pin photodiode in which a semiconductor layer having p-type conductivity, a high-resistance semiconductor layer (semiconductor layer having i-type conductivity), and a semiconductor layer having n-type conductivity are stacked can be used.
  • a light source such as a backlight can be used at the time of reading data of an object.
  • the transistor in which a channel is formed in an oxide semiconductor which is described in any of the above embodiments, can be used.
  • “OS” is written beside the transistor 640 and the transistor 656 so that the transistors 640 and 656 can be identified as transistors including an oxide semiconductor.
  • each of the transistor 640 and the transistor 656 be one of the transistors described in the above embodiments, in which the oxide semiconductor layer is electrically covered with the gate electrode.
  • the oxide semiconductor layer has round end portions and a curved surface in the transistor, coverage with a film formed over the oxide semiconductor layer can be improved.
  • electric field concentration which might occur at end portions of the source electrode and the drain electrode can be reduced, which can suppress deterioration of the transistor. Therefore, variation in the electric characteristics of the transistor 640 and the transistor 656 is suppressed, and the transistor 640 and the transistor 656 are electrically stable.
  • the semiconductor device having an image sensor function which is illustrated in FIG. 15 , can be highly reliable.
  • FIG. 16 is a block diagram illustrating a configuration example of a CPU at least partly including the transistor shown in Embodiment 2.
  • the CPU illustrated in FIG. 16 includes an arithmetic logic unit (ALU) 1191 , an ALU controller 1192 , an instruction decoder 1193 , an interrupt controller 1194 , a timing controller 1195 , a register 1196 , a register controller 1197 , a bus interface 1198 (BUS I/F), a rewritable ROM 1199 , and an ROM interface (ROM I/F) 1189 over a substrate 1190 .
  • a semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190 .
  • the ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Needless to say, the CPU in FIG.
  • the CPU may have the following configuration: a structure including the CPU illustrated in FIG. 16 or an arithmetic circuit is considered as one core; a plurality of the cores is included; and the cores operate in parallel.
  • the number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example.
  • An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192 , the interrupt controller 1194 , the register controller 1197 , and the timing controller 1195 .
  • the ALU controller 1192 , the interrupt controller 1194 , the register controller 1197 , and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191 . While the CPU is executing a program, the interrupt controller 1194 processes an interrupt request from an external input/output device or a peripheral circuit depending on its priority or a mask state. The register controller 1197 generates an address of the register 1196 , and reads/writes data from/to the register 1196 depending on the state of the CPU.
  • the timing controller 1195 generates signals for controlling operation timings of the ALU 1191 , the ALU controller 1192 , the instruction decoder 1193 , the interrupt controller 1194 , and the register controller 1197 .
  • the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK 2 on the basis of a reference clock signal CLK 1 , and supplies the internal clock signal CLK 2 to the above circuits.
  • a memory cell is provided in the register 1196 .
  • the memory cell of the register 1196 or a cache memory any of the transistors described in the above embodiments can be used.
  • the register controller 1197 selects operation of holding data in the register 1196 in accordance with an instruction from the ALU 1191 . That is, the register controller 1197 selects whether data is held by a flip-flop or by a capacitor in the memory cell included in the register 1196 . When data holding by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196 . When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.
  • FIG. 17 is an example of a circuit diagram of a storage element that can be used as the register 1196 .
  • a memory element 700 includes a circuit 701 in which stored data is volatile when power supply is stopped, a circuit 702 in which stored data is nonvolatile when power supply is stopped, a switch 703 , a switch 704 , a logic element 706 , a capacitor 707 , and a circuit 720 having a selecting function.
  • the circuit 702 includes a capacitor 708 , a transistor 709 , and a transistor 710 .
  • the memory element 700 may further include another element such as a diode, a resistor, or an inductor, as needed.
  • the transistor described in the above embodiments can be used.
  • a ground potential (GND) is input to both gates (a first gate and a second gate) of the transistor 709 .
  • the gate of the transistor 709 is grounded through a load such as a resistor.
  • the transistor 709 has an extremely low Icut because electrons are trapped in the electron trap layer and thereby the threshold is increased; thus, charge stored in the capacitor 708 can be held for a long period.
  • the switch 703 is a transistor 713 having one conductivity type (e.g., an n-channel transistor) and the switch 704 is a transistor 714 having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor).
  • a first terminal of the switch 703 corresponds to one of a source and a drain of the transistor 713
  • a second terminal of the switch 703 corresponds to the other of the source and the drain of the transistor 713
  • conduction or non-conduction between the first terminal and the second terminal of the switch 703 i.e., the on/off state of the transistor 713
  • a control signal RD input to a gate of the transistor 713 .
  • a first terminal of the switch 704 corresponds to one of a source and a drain of the transistor 714
  • a second terminal of the switch 704 corresponds to the other of the source and the drain of the transistor 714
  • conduction or non-conduction between the first terminal and the second terminal of the switch 704 i.e., the on/off state of the transistor 714 .
  • One of a source and a drain of the transistor 709 is electrically connected to one of a pair of electrodes of the capacitor 708 and a gate of the transistor 710 .
  • the connection portion is referred to as a node M 2 .
  • One of a source and a drain of the transistor 710 is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch 703 (the one of the source and the drain of the transistor 713 ).
  • the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ) is electrically connected to the first terminal of the switch 704 (the one of the source and the drain of the transistor 714 ).
  • the second terminal of the switch 704 (the other of the source and the drain of the transistor 714 ) is electrically connected to a line which can supply a power supply potential VDD is supplied.
  • the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ), the first terminal of the switch 704 (the one of the source and the drain of the transistor 714 ), an input terminal of the logic element 706 , and one of a pair of electrodes of the capacitor 707 are electrically connected to each other.
  • the connection portion is referred to as a node M 1 .
  • the other of the pair of electrodes of the capacitor 707 can be supplied with a constant potential.
  • the other of the pair of electrodes of the capacitor 707 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD).
  • the other of the pair of electrodes of the capacitor 707 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line).
  • the other of the pair of electrodes of the capacitor 708 can be supplied with a constant potential.
  • the other of the pair of electrodes of the capacitor 707 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD).
  • the other of the pair of electrodes of the capacitor 708 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line).
  • the capacitor 707 and the capacitor 708 are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized.
  • a control signal WE is input to the first gate (first gate electrode) of the transistor 709 .
  • the potential of the second gate (second gate electrode) of the transistor 709 is kept at GND.
  • the control signal RD which is different from the control signal WE.
  • a signal corresponding to data held in the circuit 701 is input to the other of the source and the drain of the transistor 709 .
  • FIG. 17 illustrates an example in which a signal output from the circuit 701 is input to the other of the source and the drain of the transistor 709 .
  • the logic value of a signal output from the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ) is inverted by the logic element 706 , and the inverted signal is input to the circuit 701 through the circuit 720 .
  • a signal output from the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ) is input to the circuit 701 through the logic element 706 and the circuit 720 ; however, this embodiment is not limited thereto.
  • the signal output from the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ) may be input to the circuit 701 without its logic value being inverted.
  • the signal output from the second terminal of the switch 703 (the other of the source and the drain of the transistor 713 ) can be input to the node.
  • the transistor described in Embodiment 2 can be used.
  • the control signal WE and the lowest potential in the circuit e.g., GND
  • the control signal WE and the lowest potential in the circuit may be input to the first gate and the second gate, respectively.
  • the transistors included in the memory element 700 except for the transistor 709 can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate 1190 .
  • the transistor can be a transistor in which a channel is formed in a silicon layer or a silicon substrate.
  • a transistor in which a channel is formed in an oxide semiconductor layer can be used for all the transistors used for the memory element 700 .
  • a transistor in which a channel is formed in an oxide semiconductor layer can be included besides the transistor 709 , and a transistor in which a channel is formed in a layer or the substrate 1190 including a semiconductor other than an oxide semiconductor can be used for the rest of the transistors.
  • circuit 701 in FIG. 17 for example, a flip-flop circuit can be used.
  • logic element 706 for example, an inverter, a clocked inverter, or the like can be used.
  • data stored in the circuit 701 can be held by the capacitor 708 which is provided in the circuit 702 .
  • the off-state current of a transistor in which a channel is formed in an oxide semiconductor layer is extremely low.
  • the off-state current of a transistor whose channel is formed in an oxide semiconductor layer is much lower than that of a transistor whose channel is formed in crystalline silicon.
  • the memory element Since the switch 703 and the switch 704 are provided, the memory element performs the above pre-charge operation; thus, the time required for the circuit 701 to hold original data again after the supply of the power supply voltage is restarted can be shortened.
  • a signal held by the capacitor 708 is input to the gate of the transistor 710 . Therefore, after supply of the power supply voltage to the memory element 700 is restarted, the signal held by the capacitor 708 can be converted into the one corresponding to the state (the on state or the off state) of the transistor 710 to be read from the circuit 702 . Consequently, an original signal can be accurately read even when a potential corresponding to the signal held by the capacitor 708 fluctuates to some degree.
  • the memory element 700 By applying the above-described memory element 700 to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Further, shortly after the supply of the power supply voltage is restarted, the memory element can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor. Accordingly, power consumption can be suppressed.
  • the storage element 700 can also be used in a digital signal processor (DSP), a custom LSI, an LSI such as a programmable logic device (PLD), and a radio frequency identification (RF-ID).
  • DSP digital signal processor
  • PLD programmable logic device
  • RFID radio frequency identification
  • the semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images), or the like.
  • Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data appliances, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines. Specific examples of these electronic devices are illustrated in FIGS. 18A to 18F .
  • FIG. 18A illustrates a portable game machine including a housing 501 , a housing 502 , a display portion 503 , a display portion 504 , a microphone 505 , a speaker 506 , an operation key 507 , a stylus 508 , and the like.
  • the portable game machine in FIG. 18A has the two display portions 503 and 504 , the number of display portions included in a portable game machine is not limited to this.
  • FIG. 18B illustrates a portable data terminal including a first housing 511 , a second housing 512 , a first display portion 513 , a second display portion 514 , a joint 515 , an operation key 516 , and the like.
  • the first display portion 513 is provided in the first housing 511
  • the second display portion 514 is provided in the second housing 512 .
  • the first housing 511 and the second housing 512 are connected to each other with the joint 515 , and the angle between the first housing 511 and the second housing 512 can be changed with the joint 515 .
  • An image on the first display portion 513 may be switched depending on the angle between the first housing 511 and the second housing 512 at the joint 515 .
  • a display device with a position input function may be used as at least one of the first display portion 513 and the second display portion 514 .
  • the position input function can be added by providing a touch panel in a display device.
  • the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel area of a display device.
  • FIG. 18C illustrates a laptop personal computer, which includes a housing 521 , a display portion 522 , a keyboard 523 , a pointing device 524 , and the like.
  • FIG. 18D illustrates the electric refrigerator-freezer including a housing 531 , a door for a refrigerator 532 , a door for a freezer 533 , and the like.
  • FIG. 18E illustrates a video camera, which includes a first housing 541 , a second housing 542 , a display portion 543 , operation keys 544 , a lens 545 , a joint 546 , and the like.
  • the operation keys 544 and the lens 545 are provided for the first housing 541
  • the display portion 543 is provided for the second housing 542 .
  • the first housing 541 and the second housing 542 are connected to each other with the joint 546 , and the angle between the first housing 541 and the second housing 542 can be changed with the joint 546 .
  • Images displayed on the display portion 543 may be switched in accordance with the angle at the joint 546 between the first housing 541 and the second housing 542 .
  • FIG. 18F illustrates a passenger car including a car body 551 , wheels 552 , a dashboard 553 , lights 554 , and the like.
  • Example 2 a transistor having as low Icut as 1 yA/ ⁇ m as described in Embodiment 1 was fabricated, and the off-state current was measured. The results will be described below.
  • the structure of the transistor of reference example is the same as that of the transistor 450 shown in FIGS. 7A to 7C , FIG. 9A to 9C , and FIG. 10A to 10D . Note that a gate insulating layer of the transistor of reference example does not function as an electron trap layer.
  • a silicon oxynitride (SiON) layer to be a base insulating layer was formed to a thickness of 300 nm over a silicon substrate.
  • a surface of the silicon oxide layer was subjected to polishing treatment, and a 20-nm-thick first oxide semiconductor layer and a 15-nm-thick oxide semiconductor layer were stacked.
  • the heat treatment was performed under a nitrogen atmosphere at 450° C. for one hour, and then performed under an oxygen atmosphere at 450° C. for one hour.
  • ICP inductively coupled plasma
  • a tungsten layer to be a source electrode and a drain electrode was formed to a thickness of 100 nm over the first oxide semiconductor layer and the second oxide semiconductor layer.
  • the layer was formed by sputtering using a tungsten target under the following conditions: argon (80 sccm) atmosphere: pressure, 0.8 Pa; power supply (power supply output), 1.0 kW; distance between the silicon substrate and the target, 60 mm; and substrate temperature, 230° C.
  • first etching was performed thereon.
  • first etching was performed thereon.
  • first etching was performed thereon.
  • first etching was performed thereon.
  • second etching was performed thereon.
  • third etching was performed.
  • CF 4 :Cl 2 :O 2 45 sccm:45 sccm:55 sccm
  • power supply 3000 W
  • bias power 110 W
  • pressure 0.67 Pa
  • a third oxide semiconductor layer was formed to a thickness of 5 nm over the second oxide semiconductor layer, the source electrode, and the drain electrode.
  • a 10-nm-thick titanium nitride layer and a 10-nm-thick tungsten layer were formed by a sputtering method.
  • the stack of the 10-nm-thick titanium nitride layer and the 10-nm-thick tungsten layer was etched by an ICP etching method.
  • first etching and second etching were performed.
  • a gate electrode was formed.
  • a 20-nm-thick aluminum oxide layer was formed over the gate electrode by a sputtering method, and a 150-nm-thick silicon oxynitride film was formed thereover by a CVD method.
  • the reference example transistor was formed.
  • the channel length of the transistor was 50 nm and the channel width thereof was 40 nm.
  • the off-state current of the fabricated transistor was calculated. Because a current smaller than 1 fA cannot be measured directly, 250,000 transistors of reference example were connected in parallel to manufacture a transistor whose channel width was substantially 10 mm (40 nm ⁇ 250,000), and the Icut density was calculated.
  • FIG. 19 shows Id-Vg characteristics when the drain potential and the source potential of the transistor whose channel width is 10 mm are 1 V and 0 V, respectively.
  • the off-state current of 10 ⁇ 13 A or lower i.e. the off-state current density of 10 ⁇ 17 A/ ⁇ m or lower was obtained.

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