JP2011109032A - Thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thin film transistor using an oxide semiconductor favorable in electric characteristics. <P>SOLUTION: The thin film transistor has a structure including a gate electrode 20 formed on a substrate 10, a gate insulating film 30 on the gate electrode 20, an oxide semiconductor film 40 on the gate electrode 20 and the gate insulating film 30, and a metal film 70 on the oxide semiconductor film 40, wherein the oxide semiconductor film 40 has a region 50 (a high metal concentration region 50) higher in metal concentration than other regions of the oxide semiconductor film 40 on an interface with the metal film 70. A metal contained in the oxide semiconductor film may be present as a crystal grain or a microcrystal in the high metal concentration region 50. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The technical field relates to a thin film transistor using an oxide semiconductor.

  In recent years, an oxide semiconductor has attracted attention as a new semiconductor material that combines high mobility obtained from polysilicon and uniform element characteristics obtained from amorphous silicon. For example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like can be given as metal oxides exhibiting semiconductor characteristics.

  Patent Documents 1 and 2 propose a thin film transistor using a metal oxide exhibiting semiconductor characteristics in a channel formation region.

JP 2007-123861 A JP 2007-96055 A

  It is an object to provide a thin film transistor using an oxide semiconductor with favorable electrical characteristics.

  One embodiment of the present invention includes a gate electrode formed over a substrate, a gate insulating film over the gate electrode, an oxide semiconductor film over the gate electrode and the gate insulating film, a metal film over the oxide semiconductor film, The oxide semiconductor film is a thin film transistor including a region (high metal concentration region) having a metal concentration higher than that of another region of the oxide semiconductor film at an interface with the metal film.

  In the high metal concentration region, the metal contained in the oxide semiconductor film may exist as crystal grains or microcrystals.

  One embodiment of the present invention includes a gate electrode formed over a substrate, a gate insulating film over the gate electrode, an oxide semiconductor film containing indium, gallium, and zinc over the gate electrode and the gate insulating film, and an oxide semiconductor. The oxide semiconductor film is a thin film transistor including a region having a higher indium concentration than other regions of the oxide semiconductor film at an interface with the titanium film.

  Indium may exist as crystal grains or microcrystals in a region where the concentration of indium is higher than that of other regions of the oxide semiconductor film.

  A thin film transistor using an oxide semiconductor with favorable electrical characteristics can be provided.

Cross-sectional schematic diagram of thin film transistor using oxide semiconductor Energy band diagram between source electrode and drain electrode in the thin film transistor shown in FIG. Diagram showing the crystal structure of metal and oxygen in IGZO Diagram showing the structural model of metal atoms and oxygen atoms in the vicinity of the interface between the tungsten film and the oxide semiconductor film Diagram showing the structural model of metal atoms and oxygen atoms in the vicinity of the interface between the molybdenum film and the oxide semiconductor film Diagram showing the structural model of metal atoms and oxygen atoms in the vicinity of the interface between the titanium film and the oxide semiconductor film (A) Graph showing CV characteristics of sample 1 (B) Graph showing relationship between Vg of sample 1 and (1 / C) ² (A) Graph showing CV characteristics of sample 2 (B) Graph showing relationship between Vg of sample 2 and (1 / C) ² FIG. 5 is a diagram illustrating an example of an electronic device to which the thin film transistor illustrated in FIG. 1 is applied.

  Hereinafter, embodiments of the disclosed invention will be described with reference to the drawings. However, the present invention is not limited to the following description, and it is easily understood by those skilled in the art that the modes and details can be variously changed without departing from the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below.

(Embodiment 1)
FIG. 1A is a schematic cross-sectional view of a thin film transistor using an oxide semiconductor. In this thin film transistor, the substrate 50, the gate electrode 20, the gate insulating film 30, the oxide semiconductor film 40, and the region 50 having a higher metal concentration than the other regions of the oxide semiconductor film 40 (hereinafter referred to as “high metal concentration region 50”). , A metal film 70 and an insulating film 80.

  The thin film transistor illustrated in FIG. 1A is a bottom-gate type with a channel etch structure. However, the structure of the thin film transistor is not limited to this, and an arbitrary top gate structure, bottom gate structure, or the like can be used.

The substrate 10 is suitably a glass substrate. When the temperature of the subsequent heat treatment is high, a glass substrate having a strain point of 730 ° C. or higher is preferably used. In view of heat resistance, a glass substrate containing more barium oxide (BaO) than boric acid (B ₂ O ₃ ) is preferable.

  In addition to the glass substrate, a substrate made of an insulator such as a ceramic substrate, a quartz glass substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 10. In addition, crystallized glass or the like can be used as the substrate 10.

  Further, an insulating film serving as a base film may be provided between the substrate 10 and the gate electrode 20. The base film has a function of preventing diffusion of impurity elements from the substrate 10. Note that the base film can be formed using one or more films selected from silicon nitride, silicon oxide, silicon nitride oxide, and silicon oxynitride.

  As the gate electrode 20, a metal conductive film can be used. As a material of the metal conductive film, an element selected from aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo) or tungsten (W), or these An alloy containing any of these elements as a component can be used. For example, a three-layer structure of titanium film-aluminum film-titanium film or a three-layer structure of molybdenum film-aluminum film-molybdenum film can be used. Note that the metal conductive film is not limited to a three-layer structure, and a single layer, a two-layer structure, or a stacked structure of four or more layers may be used.

Examples of the oxide semiconductor film 40 include an In—Sn—Ga—Zn—O film that is a quaternary metal oxide, an In—Ga—Zn—O film that is a ternary metal oxide, and In—Sn—Zn. -O film, In-Al-Zn-O film, Sn-Ga-Zn-O film, Al-Ga-Zn-O film, Sn-Al-Zn-O system, and In, which is a binary metal oxide -Zn-O film, Sn-Zn-O film, Al-Zn-O film, Zn-Mg-O film, Sn-Mg-O film, In-Mg-O film, In-O film, Sn-O film A film, a Zn—O film, or the like can be used. Each of the oxide semiconductor films may include silicon oxide (SiO ₂ ).

As the oxide semiconductor film 40, an oxide semiconductor film having a structure represented by InMO ₃ (ZnO) _m (m> 0) can also be used. Here, M represents one or more metal elements selected from gallium (Ga), aluminum (Al), manganese (Mn), and cobalt (Co). Examples corresponding to M include gallium alone, gallium and aluminum, gallium and manganese, gallium and cobalt, and the like.

Note that among oxide semiconductor films having a structure represented by InMO ₃ (ZnO) _m (m> 0), an oxide semiconductor having a structure containing gallium (Ga) as M is converted into an In—Ga—Zn—O-based oxide. Also referred to as a physical semiconductor.

  The oxide semiconductor film 40 intentionally excludes impurities such as hydrogen, moisture, hydroxyl groups, or hydroxides (also referred to as hydrogen compounds) that are considered to be a cause of the donor, and then decreases at the same time in the step of removing these impurities. By supplying such oxygen, it is highly purified and electrically i-type (intrinsic). This is for suppressing variation in electrical characteristics of the thin film transistor.

The smaller the hydrogen in the oxide semiconductor film 40 is, the closer the oxide semiconductor film 40 is to i-type. Therefore, hydrogen contained in the oxide semiconductor film 40 is 5 × 10 ¹⁹ / cm ³ or less, preferably 5 × 10 ¹⁸ / cm ³ or less, more preferably 5 × 10 ¹⁷ / cm ³ or less, or 5 × 10 ^16. It is better to be less than / cm ³ . The hydrogen concentration can be measured by secondary ion mass spectrometry (SIMS).

By removing hydrogen contained in the oxide semiconductor film 40 as much as possible, the carrier density in the oxide semiconductor film 40 is less than 5 × 10 ¹⁴ / cm ³ , preferably 5 × 10 ¹² / cm ³ or less, more preferably 5 × 10 ¹⁰ / cm ³ or less. The carrier density can be measured by CV (capacity and voltage) measurement.

  The oxide semiconductor is a wide gap semiconductor. For example, the band gap of silicon is 1.12 eV, whereas the band gap of an In—Ga—Zn—O-based oxide semiconductor is 3.15 eV.

  An oxide semiconductor that is a wide gap semiconductor has a low minority carrier density and is less likely to induce minority carriers. Therefore, in the thin film transistor using the oxide semiconductor film 40, it can be said that a tunnel current hardly occurs, and that an off current hardly flows. Therefore, an off current per channel width of 1 μm of the thin film transistor using the oxide semiconductor film 40 can be 100 aA / μm or less, preferably 10 aA / μm or less, more preferably 1 aA / μm or less.

  Further, in the thin film transistor using the oxide semiconductor film 40 which is a wide gap semiconductor, collision ionization and avalanche breakdown hardly occur. Therefore, it can be said that the thin film transistor using the oxide semiconductor film 40 has resistance to hot carrier deterioration. The main cause of hot carrier deterioration is that carriers are increased by avalanche breakdown and carriers accelerated at a high speed are injected into the gate insulating film.

  The metal film 70 is used as a source electrode or a drain electrode. As the metal film 70, a metal material such as aluminum (Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo) or tungsten (W), or these metal materials An alloy material containing as a component can be used. Further, the metal film 70 is formed of chromium (Cr), tantalum (Ta), titanium (Ti), molybdenum (Mo) or tungsten (W) on one or both of metal films such as aluminum (Al) and copper (Cu). Alternatively, a structure in which a refractory metal film such as the above is laminated may be used. Note that an aluminum film such as silicon (Si), titanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium (Cr), neodymium (Nd), scandium (Sc), or yttrium (Y). By using an aluminum material to which an element for preventing generation of hillocks and whiskers is added, a metal film 70 having excellent heat resistance can be obtained.

  FIG. 1B is a schematic cross-sectional view in which the region 100 in FIG.

  As shown in FIG. 1B, the metal contained in the oxide semiconductor film 40 may exist as crystal grains or microcrystals in the high metal concentration region 50.

  FIG. 2 is an energy band diagram (schematic diagram) between a source electrode and a drain electrode in the thin film transistor having the configuration shown in FIG. This figure corresponds to the case where the potential difference between the source electrode and the drain electrode is zero.

In this energy band diagram, since the metal is degenerated, the conduction band and the Fermi level coincide. The high metal concentration region 50 is treated as a metal. Further, by removing impurities as much as possible, the oxide semiconductor film 40 is highly purified and electrically i-type (intrinsic). As a result, the Fermi level (E _f ) can be approximately the same as the intrinsic Fermi level (E _i ).

  From this energy band diagram, it can be seen that there is no barrier at the interface between the oxide semiconductor film 40 and the high metal concentration region 50 and a good contact is obtained.

(Embodiment 2)
A manufacturing process of the thin film transistor having the structure shown in FIG. 1 will be described.

  First, after a conductive film is formed over the substrate 10 having an insulating surface, the gate electrode 20 is formed by a first photolithography process.

  The resist mask used for the first photolithography process may be formed by an inkjet method. When the resist mask is formed by an ink-jet method, a manufacturing cost can be reduced because a photomask is not used.

  Next, a gate insulating film 30 is formed on the gate electrode 20.

  The gate insulating film 30 is formed by a method such as a plasma CVD method or a sputtering method. As the gate insulating film 30, a film such as silicon oxide, silicon nitride, silicon oxynitride, silicon nitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide, or hafnium oxide is preferable.

  The gate insulating film 30 in contact with the oxide semiconductor film 40 is desirably a dense film having high insulation resistance. Therefore, it is particularly suitable to form the gate insulating film 30 by a high-density plasma CVD method using μ waves (2.45 GHz).

  The interface characteristics between the gate insulating film 30 which is a dense film having high insulation resistance obtained in this way and the oxide semiconductor film 40 which is made to be nearly i-type by removing impurities as much as possible are good.

If the interface characteristics between the oxide semiconductor film 40 and the gate insulating film 30 are poor, in the gate bias / thermal stress test (BT test: 85 ° C., 2 × 10 ⁶ V / cm, 12 hours), As a result, the bond between the impurity and the main component of the oxide semiconductor is cut, and the generated unpaired bond causes a threshold voltage drift.

The gate insulating film 30 may have a stacked structure of a nitride insulating film and an oxide insulating film. For example, after a silicon nitride film (SiN _y (y> 0)) having a thickness of 50 nm to 200 nm is formed as the first gate insulating film by a sputtering method, the second gate insulating film is formed on the first gate insulating film. By forming a silicon oxide film (SiO _x (x> 0)) having a thickness of 5 nm to 300 nm, the gate insulating film 30 having a stacked structure can be obtained. The thickness of the gate insulating film 30 may be set as appropriate depending on the characteristics required for the thin film transistor, and may be about 350 nm to 400 nm.

  Preferably, as a pretreatment for forming the gate insulating film 30, impurities such as hydrogen and moisture adsorbed on the substrate 10 are preliminarily heated in the preheating chamber of the sputtering apparatus by preheating the substrate 10 on which the gate electrode 20 is formed. Desorption and evacuation are recommended. This is for preventing the gate insulating film 30 and the oxide semiconductor film 40 formed thereafter from containing impurities such as hydrogen and moisture as much as possible. Further, the substrate 10 on which the gate insulating film 30 is formed may be preheated.

  The preheating temperature is suitably 100 ° C. or higher and 400 ° C. or lower. If it is 150 degreeC or more and 300 degrees C or less, it is still more suitable. In addition, a cryopump is appropriate as the exhaust means in the preheating chamber.

  Next, the oxide semiconductor film 40 is formed over the gate insulating film 30. An appropriate thickness of the oxide semiconductor film 40 is 2 nm to 200 nm.

  The oxide semiconductor film 40 is formed by a sputtering method. The sputtering method is performed in a rare gas (typically argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen.

As a target used for forming the oxide semiconductor film 40 by a sputtering method, a metal oxide target containing zinc oxide as a main component can be used. The composition ratios were In ₂ O ₃ : Ga ₂ O ₃ : ZnO = 1: 1: 1 [mol%], In: Ga: Zn = 1: 1: 0.5 [atom%], and In: Ga. : Zn = 1: 1: 1 [atom%] or In: Ga: Zn = 1: 1: 2 [atom%]), an oxide semiconductor containing indium (In), gallium (Ga), and zinc (Zn) A target can also be used. In addition, the filling rate of the oxide semiconductor target is appropriately 90% to 100%. If it is 95% or more and 99.9% or less, it is more preferable. This is because a denser oxide semiconductor film can be formed as an oxide semiconductor target with a higher filling rate is used.

  Prior to the formation of the oxide semiconductor film 40, the substrate 10 is held in a processing chamber in a reduced pressure state, and the substrate 10 is heated to room temperature or a temperature lower than 400 ° C. Then, a voltage is applied between the substrate 10 and the target while introducing a sputtering gas from which hydrogen and moisture have been removed while removing residual moisture in the processing chamber, whereby the oxide semiconductor film 40 is formed on the substrate 10. Is deposited.

It is appropriate to use an adsorption-type vacuum pump as an evacuation unit for removing moisture remaining in the processing chamber. Examples include a cryopump, an ion pump, and a titanium sublimation pump. Moreover, what added the cold trap to the turbo pump can also be used as an exhaust means. Impurities contained in the oxide semiconductor film 40 formed in the treatment chamber by exhausting a compound containing hydrogen atoms (more preferably a compound containing carbon atoms) such as water (H ₂ O) from the treatment chamber. The concentration of can be reduced. Further, by performing sputtering film formation while removing moisture remaining in the processing chamber with a cryopump, the temperature of the substrate 10 in forming the oxide semiconductor film 40 can be set to room temperature to less than 400 ° C. .

  Note that dust attached to the surface of the gate insulating film 30 is preferably removed by reverse sputtering before the oxide semiconductor film 40 is formed by a sputtering method. Reverse sputtering is a method of cleaning the substrate surface with reactive plasma generated by applying a voltage to the substrate side using an RF power source without applying a voltage to the target side. Note that reverse sputtering is performed in an argon atmosphere. Further, nitrogen, helium, oxygen, or the like may be used instead of argon.

  After the oxide semiconductor film 40 is formed, the oxide semiconductor film 40 is dehydrated or dehydrogenated. The temperature of the heat treatment for dehydration or dehydrogenation is suitably 400 ° C. or higher and 750 ° C. or lower, and particularly preferably 425 ° C. or higher. Note that the heat treatment time may be 1 hour or less if the temperature of the heat treatment is 425 ° C. or more, but if the temperature is 425 ° C. or less, the heat treatment time should be longer than 1 hour.

For example, the substrate 10 over which the oxide semiconductor film 40 is formed is introduced into an electric furnace which is one of heat treatment apparatuses, and heat treatment is performed in a nitrogen atmosphere. Thereafter, high purity oxygen gas, high purity dinitrogen monoxide (N ₂ O) gas or ultra-dry air (with a dew point of −40 ° C. or lower, preferably −60 ° C. or lower, and 4 pairs of nitrogen and oxygen in the same furnace) The gas mixed at a ratio of 1) is introduced for cooling. It is desirable that oxygen gas or N ₂ O gas does not contain water, hydrogen, or the like. Further, the purity of oxygen gas or N ₂ O gas is 6N (99.9999%) or more, preferably 7N (99.9999999%) or more (that is, the impurity concentration in oxygen gas or N ₂ O gas is 1 ppm or less, It is preferable to set it to 0.1 ppm or less.

  Note that the heat treatment apparatus is not limited to an electric furnace, and for example, a rapid thermal annealing (RTA) apparatus such as a GRTA (Gas Rapid Thermal Anneal) apparatus or an LRTA (Lamp Rapid Thermal Anneal) apparatus can be used.

  Further, the heat treatment for dehydration or dehydrogenation of the oxide semiconductor film 40 can be performed on the oxide semiconductor film 40 before and after being processed into an island shape.

  Through the above steps, the entire oxide semiconductor film 40 is brought into an oxygen-excess state, whereby the resistance of the entire oxide semiconductor film 40 is increased, that is, it is made i-type.

  Next, a metal film 70 is formed over the gate insulating film 30 and the oxide semiconductor film 40. The metal film 70 may be formed by a sputtering method, a vacuum evaporation method, or the like. Further, the metal film 70 may have a single layer structure or a laminated structure of two or more layers.

  Thereafter, a resist mask is formed over the metal film 70 by a third photolithography process, and selective etching is performed to form a source electrode and a drain electrode, and then the resist mask is removed.

  The channel length L of the thin film transistor is determined by the interval width between the lower end portion of the source electrode adjacent to the lower end portion of the drain electrode on the oxide semiconductor film 40. That is, it can be said that the channel length L of the thin film transistor is determined by the degree of exposure at the time of forming the resist mask in the third photolithography process. Ultraviolet light, KrF laser light, and ArF laser light can be used for light exposure for forming the resist mask in the third photolithography process. When the channel length L is less than 25 nm, exposure may be performed using extreme ultraviolet (Extreme Ultraviolet) having a very short wavelength of several nanometers to several tens of nanometers. This is because the exposure with extreme ultraviolet rays has a high resolution and a large depth of focus. Therefore, the channel length L of the thin film transistor can be 10 nm or more and 1000 nm or less depending on the type of light used for exposure.

  Note that when the metal film 70 is etched, the material of the metal film 70, the material of the oxide semiconductor film 40, and etching conditions must be adjusted as appropriate so that the oxide semiconductor film 40 is not removed.

  As an example, in the case where a titanium film is used as the metal film 70 and an In—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductor film 40, a hydrogen peroxide solution (ammonia, water, and hydrogen peroxide) is used as an etchant. It is preferable to use a mixed solution of hydrogen oxide water.

  Note that in the third photolithography step, only part of the oxide semiconductor film 40 is etched, whereby the oxide semiconductor film 40 having a groove (a depressed portion) can be formed. Further, the resist mask for forming the source electrode and the drain electrode may be formed by an inkjet method. When the resist mask is formed by an ink-jet method, a manufacturing cost can be reduced because a photomask is not used.

The surface of the exposed oxide semiconductor film 40 is formed by plasma treatment using a gas such as dinitrogen monoxide (N ₂ O), nitrogen (N ₂ ), or argon (Ar) after forming the source electrode and the drain electrode. You may remove the adsorbed water etc. which adhered to. In the plasma treatment, a mixed gas of oxygen and argon can be used.

  In the case where plasma treatment is performed, the insulating film 80 that is in contact with part of the oxide semiconductor film 40 is formed without being exposed to the air as it is. In the thin film transistor illustrated in FIG. 1, the oxide semiconductor film 40 is formed so that the oxide semiconductor film 40 and the insulating film 80 are in contact with each other in a region that does not overlap with the metal film 70.

  As an example of the insulating film 80, the substrate 10 on which the oxide semiconductor film 40 and the metal film 70 are formed is heated to a temperature of room temperature to less than 100 ° C., and then a sputtering gas containing high-purity oxygen from which hydrogen and moisture are removed. And a silicon oxide film having defects formed by using a silicon semiconductor target.

  The insulating film 80 is suitably formed while removing residual moisture in the processing chamber. This is for preventing hydrogen, a hydroxyl group, or moisture from being contained in the oxide semiconductor film 40 and the insulating film 80.

It is appropriate to use an adsorption-type vacuum pump as an evacuation unit for removing moisture remaining in the processing chamber. Examples include a cryopump, an ion pump, and a titanium sublimation pump. Moreover, what added the cold trap to the turbo pump can also be used as an exhaust means. By exhausting hydrogen atoms, compounds containing hydrogen atoms such as water (H ₂ O), or the like from the treatment chamber, the concentration of impurities contained in the insulating film 80 formed in the treatment chamber can be reduced.

  Note that as the insulating film 80, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, or the like can be used in addition to the silicon oxide film.

  After the insulating film 80 is formed, heat treatment may be performed at 100 ° C. to 400 ° C., preferably 150 ° C. to less than 350 ° C. in an inert gas atmosphere or a nitrogen gas atmosphere. When heat treatment is performed, impurities such as hydrogen, moisture, a hydroxyl group, or hydride contained in the oxide semiconductor film 40 diffuse into the insulating film 80 including defects. As a result, impurities contained in the oxide semiconductor film 40 can be further reduced.

  Further, a high metal concentration region 50 is formed at the interface between the oxide semiconductor film 40 and the metal film 70 by the heat treatment.

  Note that the high metal concentration region 50 may be formed over the oxide semiconductor film 40 by a sputtering method or the like before the metal film 70 is formed.

  Through the above steps, the thin film transistor having the structure illustrated in FIG. 1 can be formed.

(Embodiment 3)
The result of having verified by the computational science about the phenomenon in which the high metal concentration area | region 50 is formed in the interface of the oxide semiconductor film 40 and the metal film 70 of the thin-film transistor of the structure shown in FIG.

  In the following calculation, the case where the oxide semiconductor film 40 is a film formed of an In—Ga—Zn—O-based oxide semiconductor is considered. Further, consider the case where the metal film 70 is any one of a tungsten (W) film, a molybdenum (Mo) film, and a titanium (Ti) film.

[Phenomenon in which the high metal concentration region 50 is formed]
First, energy (defect formation energy E _def ) necessary for each of the oxides of indium, gallium, and zinc constituting the In—Ga—Zn—O-based oxide semiconductor to form an oxygen-deficient state is calculated. To do.

The defect formation energy E _def is defined by the following equation (1).

However, E (A _m O _n-1 ) is the energy of oxide A _m O _n-1 having oxygen vacancies, E (O) is half of the energy of oxygen molecules, and E (A _m O _n ) is oxygen deficient. it is the energy of a oxide _a m _{O n.} A is any one of indium alone, gallium alone, zinc alone, indium, gallium and zinc.

Further, the relationship between the defect concentration n and the defect formation energy E _def is approximately expressed by the following equation (2).

Here, N is the number of oxygen positions in a state where no defect is formed, k _B is the Boltzmann constant, and T is the absolute temperature.

From the equation (2), it can be seen that as the deficiency formation energy E _def increases, the oxygen deficiency concentration n, that is, the amount of oxygen deficiency decreases.

CASTEP which is a program of the density functional method is used for calculation of the defect formation energy E _def . A plane wave basis pseudopotential method is used as the density functional method, and GGAPBE is used as the functional. The cut-off energy is 500 eV. k points, for _{IGZO 3 × 3 × 1, In} 2 O 3 using a grid of 4 × 4 × 1 for the 2 × 3 × 2, ZnO for _{2 × 2 × 2, Ga 2} O 3 is about.

The crystal structure is such that for IGZO crystals, the energy of Ga and Zn is minimized with respect to the structure of 84 atoms doubled respectively on the a-axis and b-axis for the structure of symmetry R-3 (international number: 148). The structure arranged in is used. For In ₂ O ₃ , an 80-atom binary structure is used, for Ga ₂ O ₃ , an 80-atom β-Gallia structure is used, and for ZnO, an 80-atom wurtzite structure is used.

Table 1 is a table showing the values of the defect formation energy E _{def in} the case where A is indium alone, gallium alone, zinc alone, indium, gallium and zinc in formula (1).

The defect formation energy E _def of IGZO (Model 1) is a value for oxygen adjacent to three indium and one zinc (see FIG. 3A) in the IGZO crystal when A is indium, gallium, and zinc. is there.

The defect formation energy E _def of IGZO (Model 2) is a value for oxygen adjacent to three indium and one gallium in the IGZO crystal when A is indium, gallium, and zinc (see FIG. 3B). is there.

The defect formation energy E _def of IGZO (Model 3) is a value for oxygen adjacent to two zincs and two galliums in the IGZO crystal when A is indium, gallium, and zinc (see FIG. 3C). is there.

The larger the value of deficiency formation energy E _def , the higher the energy required to form the oxygen deficiency state. That is, the larger the value of the defect formation energy E _def, the stronger the bond with oxygen. In other words, from Table 1, it can be said that indium having the smallest value of the defect formation energy E _def has the weakest bond with oxygen.

  The oxygen deficiency state in the In—Ga—Zn—O-based oxide semiconductor is considered to occur because the metal film 70 used as the source electrode or the drain electrode extracts oxygen from the oxide semiconductor film 40. A part of the oxide semiconductor film 40 thus in an oxygen deficient state becomes a high metal concentration region 50. Depending on the presence or absence of the high metal concentration region 50, the carrier density of the oxide semiconductor film 40 differs by at least two orders of magnitude. This is because oxygen is extracted from the oxide semiconductor film 40 so that the oxide semiconductor film 40 becomes n. In addition, n-ization means that the electron which is a majority carrier increases.

[Carrier density in oxide semiconductor film 40]
Next, an element is actually fabricated and evaluated for the extraction of oxygen from the oxide semiconductor film 40 by the metal film 70. Specifically, an oxide semiconductor in which a metal film having an oxygen extracting effect is stacked on an oxide semiconductor film and in a case where a metal film having no oxygen extracting effect is stacked on an oxide semiconductor film The carrier density in the film 40 is calculated and the results are compared.

  The carrier density in the oxide semiconductor film can be obtained by fabricating a MOS capacitor using the oxide semiconductor film and evaluating the CV measurement result (CV characteristics) of the MOS capacitor.

The carrier density is measured by the following procedures (1) to (3). (1) A CV characteristic in which the relationship between the gate voltage Vg of the MOS capacitor and the capacitance C is plotted is acquired. (2) A graph representing the relationship between the gate voltage Vg and (1 / C) ² is acquired from the C-V characteristic, and a differential value of (1 / C) ² in the weak inversion region is obtained in the graph. (3) the resulting derivative value is substituted into the following expression for the carrier density N _d (3).

Here, e is the elementary charge, ε ₀ is the dielectric constant of vacuum, and ε is the dielectric constant of the oxide semiconductor.

  As a sample for measurement, a MOS capacitor using a metal film having an oxygen extraction effect (hereinafter referred to as “sample 1”) and a MOS capacitor using a metal film having no oxygen extraction effect (hereinafter referred to as “sample”). Prepared as “Sample 2”). Note that a titanium film was applied as a metal film having an oxygen extracting effect. Further, a film having a titanium nitride film on the surface (on the oxide semiconductor film side) of the titanium film was applied as a metal film having no oxygen extraction effect.

The details of the sample are as follows.
Sample 1:
A titanium film with a thickness of 400 nm is formed over a glass substrate, and an oxide semiconductor film with a thickness of 2 μm using an In—Ga—Zn—O-based oxide semiconductor (a-IGZO) is formed over the titanium film. Then, a silicon oxynitride film with a thickness of 300 nm is formed over the oxide semiconductor film, and a silver film with a thickness of 300 nm is formed over the silicon oxynitride film.
Sample 2:
A titanium film having a titanium film with a thickness of 300 nm on a glass substrate, a titanium nitride film with a thickness of 100 nm on the titanium film, and an In—Ga—Zn—O-based oxide on the titanium nitride film It has an oxide semiconductor film with a thickness of 2 μm using a semiconductor (a-IGZO), a silicon oxynitride film with a thickness of 300 nm on the oxide semiconductor film, and a silver with a thickness of 300 nm on the silicon oxynitride film Has a membrane.

Note that in Samples 1 and 2, the oxide semiconductor film is an oxide semiconductor target containing indium (In), gallium (Ga), and zinc (Zn) (In: Ga: Zn = 1: 1: 0.5 [ atom%]). The atmosphere for forming the oxide semiconductor film was a mixed atmosphere of argon (Ar) and oxygen (O ₂ ) (Ar: O ₂ = 30 (sccm): 15 (sccm)).

FIG. 7A shows the CV characteristics of Sample 1. FIG. FIG. 7B shows the relationship between Vg of Sample 1 and (1 / C) ² . When the differential value of (1 / C) ² in the weak inversion region in FIG. 7B is substituted into Expression (3), a carrier density of 1.8 × 10 ¹² / cm ³ in the oxide semiconductor film is obtained.

FIG. 8A shows the CV characteristics of Sample 2. FIG. 8B shows the relationship between Vg of Sample 2 and (1 / C) ² . When the differential value of (1 / C) ² in the weak inversion region in FIG. 8B is substituted into Expression (3), a carrier density of 6.0 × 10 ¹⁰ / cm ³ in the oxide semiconductor film is obtained.

  From the above results, in the MOS capacitor using the metal film having the effect of extracting oxygen (sample 1) and the MOS capacitor using the metal film having no effect of extracting oxygen (sample 2), It can be seen that the carrier density of each is at least two orders of magnitude different. This suggests that oxygen is extracted from the oxide semiconductor film by the metal film and oxygen vacancies in the oxide semiconductor film are increased, so that the oxide semiconductor film in the vicinity of the metal film is n-type.

(Embodiment 4)
The thin film transistor having the structure illustrated in FIG. 1 can be applied to a variety of electronic devices (including game machines). Examples of the electronic device include a television device (also referred to as a television or a television receiver), a monitor for a computer, a digital camera, a digital video camera, a digital photo frame, a mobile phone (also referred to as a mobile phone or a mobile phone device). ), Portable game machines, portable information terminals, sound reproduction devices, large game machines such as pachinko machines, solar cells, and the like.

  FIG. 9A illustrates an example of a mobile phone to which the thin film transistor having the structure illustrated in FIG. 1 is applied. This mobile phone includes a display unit 121 incorporated in a housing 120.

  This mobile phone can input information by touching the display unit 121 with a finger or the like. In addition, operations such as making a call or typing a mail can be performed by touching the display unit 121 with a finger or the like.

  For example, by arranging a plurality of thin film transistors having the structure shown in FIG. 1 as switching elements of pixels in the display portion 121, the performance of this mobile phone can be improved.

  FIG. 9B illustrates an example of a television device to which the thin film transistor having the structure illustrated in FIG. 1 is applied. In this television apparatus, a display portion 131 is incorporated in a housing 130.

  For example, by arranging a plurality of thin film transistors having the structure shown in FIG. 1 as switching elements of pixels in the display portion 131, the performance of the television device can be improved.

  As described above, the thin film transistor having the structure illustrated in FIG. 1 can be provided on display panels of various electronic devices, whereby the performance of the electronic device can be improved.

DESCRIPTION OF SYMBOLS 10 Substrate 20 Gate electrode 30 Gate insulating film 40 Oxide semiconductor film 50 High metal concentration region 70 Metal film

Claims (5)

A gate electrode formed on the substrate;
A gate insulating film on the gate electrode;
An oxide semiconductor film containing indium, gallium and zinc on the gate electrode and the gate insulating film;
A titanium film on the oxide semiconductor film,
The thin film transistor, wherein the oxide semiconductor film has a region having a higher indium concentration than other regions of the oxide semiconductor film at an interface with the titanium film. In claim 1,
A thin film transistor, wherein indium is present as a crystal grain or a microcrystal in a region where the concentration of indium is higher than that of another region of the oxide semiconductor film. In claim 1 or claim 2,
A thin film transistor, wherein a hydrogen concentration in the oxide semiconductor film is less than 5 × 10 ¹⁶ / cm ³ . In any one of Claims 1 thru | or 3,
A thin film transistor, wherein a carrier density in the oxide semiconductor film is 5 × 10 ¹⁰ / cm ³ or less. In any one of Claims 1 thru | or 4,
The region having a higher indium concentration than the other region of the oxide semiconductor film is a region formed by performing heat treatment after forming the titanium film over the oxide semiconductor. .