JP2019024058A - Oxide semiconductor thin film and method for manufacturing thin-film transistor - Google Patents

Abstract

An object of the present invention is to provide a method for manufacturing an oxide semiconductor thin film in which the carrier concentration is reduced while maintaining a high carrier mobility different from that of a conventional crystalline oxide semiconductor thin film. An oxide sintered body containing indium and gallium as oxides in an atmosphere having a water pressure in the system of 2.0 × 10 −3 Pa to 5.0 × 10 −1 Pa. Using a target, a film forming step of forming an oxide thin film on the surface of the substrate by sputtering, and an atmosphere in which the relative humidity measured at a temperature of 22 ° C. in the system is 5% or more and the oxygen concentration is 30% or more A heat treatment step of heat-treating the oxide thin film to form a crystalline oxide semiconductor thin film containing indium and gallium as oxides and further containing hydrogen. It is a manufacturing method. [Selection] Figure 1

Description

  The present invention relates to a method for producing a crystalline oxide semiconductor thin film, and more particularly, by containing indium and gallium as oxides and further containing hydrogen, the carrier is maintained while maintaining high carrier mobility. The present invention relates to a method for manufacturing a crystalline oxide semiconductor thin film with reduced concentration.

  A thin film transistor (Thin Film Transistor, TFT) is one type of field effect transistor (hereinafter referred to as FET). A TFT is a three-terminal element having a gate terminal, a source terminal, and a drain terminal as a basic configuration, and a semiconductor thin film formed on the surface of a substrate is used as a channel layer in which electrons or holes move as carriers. The active element has a function of switching a current between a source terminal and a drain terminal by applying a voltage to a gate terminal to control a current flowing in a channel layer.

  The TFT is the most widely used electronic device at present, and a typical application is a TFT for driving a liquid crystal. Many liquid crystal driving TFTs use an n-type channel layer in which electrons move as carriers. Currently, the most widely used n-type channel layer is a low-temperature polysilicon thin film or an amorphous silicon thin film.

  In recent years, however, liquid crystal drive TFTs are required to be driven at high speed as the definition of liquid crystals increases. The driving speed of the TFT depends on the electron mobility of the channel layer. In order to realize high-speed driving, it is necessary to use a semiconductor thin film whose electron mobility is higher than that of amorphous silicon for the channel layer. Low-temperature polysilicon has sufficiently high electron mobility, but when formed on a large glass substrate, in-plane uniformity is low and yield is low, or there are many processes compared to amorphous silicon, and capital investment is required. For this reason, there are problems such as high costs.

With respect to such a situation, Patent Document 1 discloses a transparent amorphous oxide thin film formed by vapor phase film formation and composed of elements of In, Ga, Zn, and O, and the oxide The composition of the thin film is InGaO ₃ (ZnO) _m (m is a natural number of less than 6) when crystallized, and carrier mobility (also referred to as carrier electron mobility) is added without adding impurity ions. A transparent semi-insulating amorphous oxide thin film characterized by being semi-insulating and having a carrier concentration (also referred to as carrier electron concentration) of 10 ¹⁶ cm ⁻³ or less, exceeding ¹ cm ² V ⁻¹ sec ⁻¹ ; A thin film transistor characterized by using the transparent semi-insulating amorphous oxide thin film as a channel layer has been proposed.

However, the transparent amorphous oxide formed by vapor phase deposition method of either sputtering method or pulse laser deposition method proposed in Patent Document 1 and composed of elements of In, Ga, Zn and O Since the thin film (a-IGZO film) stays in the carrier mobility in the range of approximately 1 cm ² V ⁻¹ sec ^{−1 to} 10 cm ² V ⁻¹ sec ⁻¹ , the carrier mobility when formed as a TFT channel layer. It was pointed out that there was a shortage.

In order to solve the shortage of carrier mobility, other materials have been studied. For example, in Patent Document 2, gallium is dissolved in indium oxide, the atomic ratio Ga / (Ga + In) is 0.001 or more and 0.12 or less, and the content ratio of indium and gallium with respect to all metal atoms is 80. There has been proposed a thin film transistor characterized by using an oxide semiconductor thin film having an atomic percent or more and an In ₂ O ₃ bixbyite structure. Compared with Patent Document 1, Patent Document 2 increases carrier mobility by increasing the indium content and suppresses increase in carrier concentration by crystallizing into a bixbite structure of In ₂ O ₃ . When applied to the TFT channel layer, the problem remains that the crystal grain boundary causes variations in TFT characteristics. Further, in Patent Document 2, there are some examples in which the carrier concentration exceeds 2.0 × 10 ¹⁸ cm ^−3, and it is left as a problem that the oxide semiconductor thin film applied to the TFT channel layer is slightly high. It was.

In Patent Document 3, in order to solve the high carrier concentration of Patent Document 2, DC sputtering is performed using a sputtering target at a moisture pressure in the system of 3.0 × 10 ⁻⁴ Pa to 5.0 × 10 ⁻² Pa. There has been proposed a method of manufacturing an oxide semiconductor thin film by forming a film formation body and crystallizing the film formation body. Patent Document 4 states that the content of hydrogen element contained in the oxide semiconductor film is 0.1 atomic% or more and 5 atomic% or less with respect to all elements forming the oxide semiconductor thin film. A characteristic thin film transistor has been proposed. However, since both are inventions related to an oxide semiconductor thin film of a crystalline film, knowledge about the influence of hydrogen or the like on an oxide semiconductor thin film other than a crystal has not been obtained. In addition, there still remains a problem of crystal grain boundaries that cause important in-plane variations in TFT characteristics.

JP 2010-219538 A WO2010 / 032422 JP 2011-222557 A WO2010 / 047077

A. Takagi, K .; Nomura, H .; Ohta, H .; Yanagi, T .; Kamiya, M .; Hirano, and H.M. Hosono, Thin Solid Films 486, 38 (2005)

  An object of the present invention is to provide a method for producing an oxide semiconductor thin film in which carrier concentration is reduced while maintaining high carrier mobility by further containing hydrogen in a crystalline oxide semiconductor thin film. is there. In particular, a manufacturing method for obtaining a crystalline oxide semiconductor thin film containing indium and gallium as oxides, and further containing hydrogen, comprising the crystalline oxide semiconductor thin films of Patent Documents 2 to 4 Another object of the present invention is to provide a suitable manufacturing method capable of manufacturing a crystalline oxide semiconductor thin film different from the above.

As a result of intensive studies to solve the above-described problems, the present inventors have incorporated a suitable amount of hydrogen into a crystalline oxide semiconductor thin film, whereby carrier mobility is 10 cm ² V ⁻¹ sec ^−1. While maintaining the above, it was confirmed that a sufficiently low carrier concentration was obtained as a semiconductor, and a manufacturing method for obtaining the oxide semiconductor thin film was newly found.

The first of the present invention is an oxide containing indium and gallium as oxides in an atmosphere having a moisture pressure in the system of 2.0 × 10 ⁻³ Pa to 5.0 × 10 ⁻¹ Pa. A film forming step of forming an oxide thin film on the surface of the substrate by a sputtering method using a target made of a sintered body, a relative humidity measured at 22 ° C. in the system is 5% or more, and an oxygen concentration is 30%. A heat treatment step of heat-treating the oxide thin film so as to become a crystalline oxide semiconductor thin film containing indium and gallium as oxides and further containing hydrogen in the above atmosphere. Is a method for producing a high quality oxide semiconductor thin film.

  2nd of this invention is a manufacturing method of an oxide semiconductor thin film which makes the said relative humidity 10% or more in the said heat processing process in 1st invention.

  3rd of this invention is a manufacturing method of an oxide semiconductor thin film which makes the said oxygen concentration in the said heat processing process 100% in 1st or 2nd invention.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the gallium content in the oxide sintered body of the target in the film forming step is 0.15 in terms of Ga / (In + Ga) atomic ratio. The manufacturing method of the oxide semiconductor thin film is set to 0.30 or less.

  According to a fifth aspect of the present invention, in the target used in the film formation step according to any one of the first to third aspects, the gallium content in the oxide sintered body is expressed by a Ga / (In + Ga) atomic ratio. The manufacturing method of an oxide semiconductor thin film is 0.17 or more and 0.30 or less.

  6th of this invention is a manufacturing of the thin-film transistor provided with the said oxide semiconductor thin film as a channel layer including the process of manufacturing a crystalline oxide semiconductor thin film with the manufacturing method in any one of 1-5 Is the method.

  The oxide semiconductor thin film manufactured by the manufacturing method of the present invention contains hydrogen uniformly in the depth direction of the crystalline oxide semiconductor thin film containing hydrogen, and has a high carrier mobility of the oxide semiconductor thin film. The carrier concentration can be reduced while maintaining the above. Accordingly, since a thin film transistor (TFT) to which an oxide semiconductor thin film is applied as a channel layer operates stably, the method for producing a crystalline oxide semiconductor thin film of the present invention is extremely useful industrially.

It is a figure showing the X-ray-diffraction measurement result in the X-ray-diffraction measurement of the oxide semiconductor thin film of Example 1 which is one Embodiment of this invention. It is a figure showing the change of the hydrogen concentration of the film depth direction by the secondary ion mass spectrometry of the oxide semiconductor thin film of Example 13 which is one Embodiment of this invention.

  Below, the manufacturing method of the crystalline oxide semiconductor thin film of this invention and the manufacturing method of a thin-film transistor (TFT) using the same are demonstrated in detail. The present invention is not limited to the following description, and can be implemented with appropriate modifications as long as the object of the present invention is achieved.

1. Oxide Semiconductor Thin Film (1) Metal Composition The oxide semiconductor thin film obtained by the production method of the present invention is a crystalline oxide semiconductor thin film containing indium and gallium as oxides and further containing hydrogen. . The term “crystalline” generally refers to a state in which a clear diffraction peak corresponding to a plane index based on a crystal structure is observed in an X-ray diffraction measurement result in an X-ray diffraction measurement, which is composed of a crystal structure. In general, the arrangement of constituent atoms is amorphous, which indicates a solid state that does not have long-range regularity such as a crystal structure, generally a crystal component having a small crystal grain size (1 nm to 100 nm or less), and an amorphous It differs from the microcrystal in the state which forms the mixed phase with a quality component.

  Note that, in the crystalline oxide semiconductor thin film, for example, in the X-ray diffraction measurement result in the X-ray diffraction measurement, not only a clear diffraction peak corresponding to the plane index based on the crystal structure is seen, but also the TEM- In the electron beam diffraction diagram of EDX measurement, it can be identified from the fact that diffraction spots corresponding to the plane index based on the crystal structure are formed. On the other hand, the amorphous oxide semiconductor thin film does not show a clear diffraction peak corresponding to the plane index based on the crystal structure in the X-ray diffraction measurement result in the X-ray diffraction measurement, and has a cross-sectional structure. In the TEM-EDX measurement electron diffraction pattern, a halo or a halo in which some spots remain is formed. The microcrystalline oxide semiconductor thin film has, for example, no clear diffraction peak in the X-ray diffraction measurement result in the X-ray diffraction measurement, and in the electron diffraction pattern of the TEM-EDX measurement of the cross-sectional structure, A diffraction pattern consisting of a combination of rings is formed.

The lower limit of the gallium content in the oxide semiconductor thin film obtained by the production method of the present invention is preferably 0.15 or more, and more preferably 0.17 or more, as a Ga / (In + Ga) atomic ratio. More preferably, it is more preferably more than 0.20. As an upper limit, it is preferable that it is 0.30 or less, it is more preferable that it is 0.25 or less, and it is still more preferable that it is 0.23 or less. Gallium has a strong bonding force with oxygen and has an effect of reducing the amount of oxygen vacancies in the crystalline oxide semiconductor thin film obtained by the manufacturing method of the present invention. When the gallium content is 0.15 or more in terms of Ga / (In + Ga) atomic ratio, this effect can be sufficiently obtained. On the other hand, in the case of 0.30 or less, carrier mobility of 10 cm ² V ⁻¹ sec ⁻¹ or more that is sufficiently high as an oxide semiconductor thin film can be obtained.

  The crystalline oxide semiconductor thin film obtained by the production method of the present invention may contain a specific positive trivalent element among elements other than indium and gallium. Specific positive trivalent elements include boron, aluminum, scandium, and yttrium. When these elements are contained in the crystalline oxide semiconductor thin film obtained by the production method of the present invention, it contributes to a reduction in carrier concentration, but hardly contributes to an improvement in carrier mobility. It is preferable that the crystalline oxide semiconductor thin film obtained by the production method of the present invention does not substantially contain a positive trivalent element other than the above. That is, it is preferable that lanthanum, praseodymium, dyspronium, holmium, erbium, ytterbium, and lutetium are not included. This is because the carrier mobility is lowered without contributing to the reduction of the carrier concentration. In the present specification, the phrase “substantially not containing an element” means that a metal element or a metalloid element is not intentionally added.

  The crystalline oxide semiconductor thin film obtained by the production method of the present invention preferably does not substantially contain a positive tetravalent or higher element. Examples of elements having a positive tetravalent or higher include titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, silicon, germanium, tin, lead, antimony, bismuth, and cerium. When these elements are contained in the oxide semiconductor thin film obtained by the manufacturing method of the present invention, it acts as a scattering factor, so that the carrier mobility of the crystalline oxide semiconductor thin film is lowered.

  It is preferable that the crystalline oxide semiconductor thin film obtained by the production method of the present invention does not substantially contain a positive divalent element or less. Elements that are less than positive divalent include lithium, sodium, potassium, rubidium, cesium, magnesium, calcium oxide, strontium, barium, and zinc. When these elements are contained in the oxide semiconductor thin film obtained by the production method of the present invention, although it contributes somewhat to the reduction of the carrier concentration, it acts as a scattering factor, so the carrier mobility is lowered more than the effect. End up.

(2) Inevitable impurities The total amount of inevitable impurities contained in the oxide semiconductor thin film obtained by the production method of the present invention is preferably 500 ppm or less, more preferably 300 ppm or less, and further preferably 100 ppm or less. preferable. In the present specification, the inevitable impurities are impurities that are inevitably mixed in the manufacturing process of each raw material even though they are not intentionally added. When the amount of impurities is large, problems such as an increase in carrier concentration or a decrease in carrier mobility may occur.

(3) Hydrogen content, film depth direction distribution and bonding state The hydrogen content contained in the crystalline oxide semiconductor thin film obtained by the production method of the present invention is determined by secondary ion mass spectrometry (SIMS, Measured by Secondary Ion Mass Spectroscopy, Rutherford Backscattering Spectroscopy (RBS), Hydrogen Forward Scattering (HFS), Hydrogen Forward Scattering, etc. For example, the hydrogen content measured by secondary ion mass spectrometry is preferably 1.0 × 10 ²⁰ atoms / cm ³ or more and 1.0 × 10 ²² atoms / cm ³ or less, and 3.0 × It is more preferably 10 ²⁰ atoms / cm ³ or more and 5.0 × 10 ²¹ atoms / cm ³ or less, and 5.0 × 10 ²⁰ atoms / cm ³ or more and 1.0 × 10 ²¹ atoms / cm ³ or less. Is more preferable.

Hydrogen is considered to be present in the vicinity of oxygen in the crystalline oxide semiconductor thin film and contribute to the reduction of the carrier concentration of the oxide semiconductor thin film. When the hydrogen content in the oxide semiconductor thin film is 1.0 × 10 ²⁰ atoms / cm ³ or more, the carrier concentration of the oxide semiconductor thin film is reduced to 2.0 × 10 ¹⁸ cm ⁻³ or less. Therefore, it is preferable. On the other hand, when the content of hydrogen in the oxide semiconductor thin film is 1.0 × 10 ²² atoms / cm ³ or more, excess hydrogen acts as a scattering factor, and the carrier mobility of the oxide semiconductor thin film is 10 cm. ^Since it is possible to reduce the possibility of lowering to less than ² V ⁻¹ sec ^−1, it is preferable.

  In the crystalline oxide semiconductor thin film obtained by the production method of the present invention, it is preferable that the distribution of hydrogen contained in the film depth direction is as uniform as possible. “Uniform” means that the ratio of the average hydrogen concentration in the vicinity of the thin film surface to the average hydrogen concentration in the vicinity of the substrate is in the range of 0.50 to 1.20. It is even better if this ratio is in the range of 0.80 to 1.10.

  The average hydrogen concentration in the vicinity of the surface of the thin film in this specification is a boundary between the surface of the oxide semiconductor thin film and the boundary data that is not affected by the surface, starting from boundary data up to 10 nm in the positive direction of the film depth by SIMS. It means the average value of hydrogen concentration at 5 or more random points. In the present specification, the average hydrogen concentration in the vicinity of the substrate refers to the boundary data that is not affected by the substrate in the vicinity of the interface between the substrate and the oxide semiconductor thin film, and is from 10 nm in the negative direction of the film depth by SIMS. Mean the average value of 5 or more random hydrogen concentrations. Note that the positive direction of the film depth by SIMS is the direction from the film surface to the substrate, and the negative direction indicates the opposite direction.

Here, boundary data that is not influenced by the surface in the vicinity of the surface of the oxide semiconductor thin film is apparent by analyzing the SIMS measurement results. For example, in the SIMS measurement result of FIG. 2, the boundary data that is not affected by the surface in the vicinity of the surface of the oxide semiconductor thin film is the average hydrogen concentration of 1.0 × 10 ^{21 to} 1.0 × 10 ²² atoms / and the range of film depth 0~3.9nm varying greatly in the range of cm ³ greater, film depth 3.9nm greater range boundary of which is constant at approximately 1.0 × ¹⁰ 21 atoms / cm ³ It has become. On the other hand, in the vicinity of the interface between the substrate and the oxide semiconductor thin film, the same applies to the boundary data that is not affected by the substrate, and the average hydrogen concentration is in the range of 1.0 × 10 ^{21 to} 3.7 × 10 ²¹ atoms / cm ³ . This is data of 149 nm, which is a boundary between the range of the film depth of 149 nm or more, which greatly changes in, and the range of the film depth of less than 149 nm where the average hydrogen concentration is approximately constant. From these boundary data, the average hydrogen concentration in the vicinity of the substrate or the average hydrogen concentration in the vicinity of the thin film surface can be obtained, respectively.

Hydrogen contained in the crystalline oxide semiconductor thin film obtained by the manufacturing method of the present invention exists as OH ⁻ in which a hydrogen atom or a hydrogen ion and an oxygen ion are combined in an indium oxide phase having a bixbyite structure. Most are things. OH ⁻ is present at a specific lattice position or interstitial position in the crystalline oxide semiconductor thin film obtained by the production method of the present invention. In particular, OH ⁻ can be confirmed by time-of-flight SIMS measurement (TOF-SIMS, Time of Flight-Secondary Ion Mass Spectroscopy). On the other hand, it is not preferable that hydrogen forms a different phase other than the bixbite structure with indium and / or gallium.

(4) Film quality The oxide semiconductor thin film obtained by the production method of the present invention is a crystalline oxide semiconductor thin film. In general, a crystal film made of a crystal shows a clear diffraction peak corresponding to a plane index based on the crystal structure in X-ray diffraction measurement, but is clear in an amorphous film made of amorphous and a microcrystal film made of microcrystal. Does not show diffraction peaks. Even in the case of a microcrystalline film, only a bulge level that cannot be clearly recognized as a diffraction peak can be confirmed in the diffraction pattern at the diffraction angle at which the peak of the crystal film appears. In addition, when a TEM image of the cross-sectional structure of each thin film observed with a transmission electron microscope (hereinafter sometimes referred to as TEM) is compared, a crystal grain boundary is confirmed in the crystalline film. Clear crystal grain boundaries are not confirmed not only in the film but also in the microcrystalline film. In the case of a crystalline film, a diffraction spot corresponding to the plane index is confirmed in the electron diffraction image. However, in the case of an amorphous film and a microcrystalline film, a halo, a halo with a slight remaining spot, or a spot is observed. Only a diffraction pattern consisting of a ring combination is confirmed.

(5) Film thickness The film thickness of the crystalline oxide semiconductor thin film obtained by the production method of the present invention is preferably 10 nm or more as the lower limit, more preferably 30 nm or more, and even more preferably 50 nm or more. preferable. On the other hand, the upper limit is not particularly limited. For example, when applied as a channel layer of a thin film transistor (TFT) of a device that requires flexibility, it is preferably 1000 nm or less, more preferably 500 nm or less, and 300 nm or less. If so, it is even more preferable. If the thickness exceeds 1000 nm, characteristics required as a channel layer of a thin film transistor (TFT) may not be maintained when the device is bent. In general, it can be said that a thickness of 30 nm or more and 300 nm or less is suitable in consideration of the throughput in the manufacturing process and the small variation in performance.

(6) Carrier concentration / carrier mobility The oxide semiconductor thin film obtained by the production method of the present invention has a carrier concentration of 2.0 × 10 ¹⁸ cm ⁻³ or less, more preferably a carrier concentration of 1.0 × 10 ¹⁸ cm ^−3. Hereinafter, it is particularly preferably 8.0 × 10 ¹⁷ cm ⁻³ or less, and more preferably 5.0 × 10 ¹⁷ cm ⁻³ or less. As represented by the amorphous oxide semiconductor thin film made of indium, gallium, and zinc described in Non-Patent Document 1, an amorphous oxide semiconductor thin film containing a large amount of indium has a carrier concentration of 4.0. Since it is in a degenerate state at × 10 ¹⁸ cm ⁻³ or more, a thin film transistor (TFT) in which this is applied to the channel layer does not show normally-off. Therefore, the crystalline oxide semiconductor thin film according to the present invention is convenient because the carrier concentration is controlled in a range in which the above-described thin film transistor (TFT) shows normally-off. Further, the carrier mobility is 10 cm ² V ⁻¹ sec ⁻¹ or more, more preferably the carrier mobility is 15 cm ² V ⁻¹ sec ⁻¹ or more, and 20 cm ² V ⁻¹ sec ⁻¹ or more is further shown. preferable.

2. Manufacturing method of oxide semiconductor thin film The manufacturing method of an oxide semiconductor thin film according to the present invention includes an oxide baking containing indium and gallium as oxides in an atmosphere in which the moisture pressure in the system is a predetermined pressure. A film forming process for forming an oxide thin film on the surface of a substrate by a sputtering method using a target composed of a combination, and an oxide thin film formed on the surface of the substrate containing indium and gallium as oxides And a heat treatment step of heat-treating to form a crystalline oxide semiconductor thin film containing hydrogen.

  Hereinafter, a preferred embodiment of the method for producing an oxide semiconductor thin film of the present invention will be described.

2-1. Film Forming Process The film forming process in the manufacturing method of the present invention uses a target made of an oxide sintered body containing indium and gallium as oxides in an atmosphere having a moisture pressure within a predetermined range, and the surface of the substrate. In this step, an oxide thin film is formed by sputtering. Note that the term “oxide thin film” is used as a thin film that is a precursor of an oxide semiconductor thin film to distinguish it from the “oxide semiconductor thin film” and does not have properties as a semiconductor. Does not mean.

(1) Sputtering method Preferred sputtering methods include DC sputtering, AC sputtering with a frequency of 1 MHz or less, and pulse sputtering. Of these, the DC sputtering method is particularly preferable from the industrial viewpoint. Although RF sputtering can be applied, since it is non-directional, it is not necessary to select it because it is difficult to establish conditions for uniform film formation on a large glass substrate.

(2) Moisture pressure In the production method of the present invention, in the film forming step of forming an oxide thin film by a sputtering method, the moisture pressure in the system is 2.0 × 10 −3 Pa or more and 5.0 × 10 −1 Pa or less. Control by atmosphere. It is preferably 2.0 × 10 −2 Pa to 2.0 × 10 −1 Pa, and more preferably controlled in an atmosphere of 5.1 × 10 −2 Pa to 1.0 × 10 −1 Pa. The water in the system is preferably introduced as water vapor in the sputtering apparatus chamber. When the water pressure in the system is less than 2.0 × 10 −3 Pa, since the amount of hydrogen or hydroxyl as a component of water taken into the oxide thin film is small, the effect of reducing the carrier concentration of the oxide semiconductor thin film is sufficiently obtained. I can't. On the other hand, when it exceeds 5.0 × 10 −1 Pa, the carrier concentration of the oxide semiconductor thin film increases and the carrier mobility of the oxide semiconductor thin film decreases. This is probably because hydrogen or a hydroxyl group behaves as a donor or as a scattering factor. In addition, for hydrogen addition to the oxide semiconductor thin film, it is possible to substitute the control of the hydrogen partial pressure in the system instead of the control of the water pressure in the system in this film forming process. Since the manufacturing process is required and the cost may be increased for ensuring safety, control by moisture pressure is preferable.

(3) Other gas conditions In the film forming step, the kind of gas constituting the atmosphere gas for film formation by sputtering is preferably a rare gas, oxygen, and water vapor, and more preferably the rare gas is argon. More preferably, the water vapor is introduced into the sputtering apparatus chamber as water vapor. The total pressure of these atmospheric gases is preferably controlled in the range of 0.1 Pa to 3.0 Pa, more preferably in the range of 0.2 Pa to 0.8 Pa, and 0.3 Pa to 0.7 Pa. If it is a range, it is still more preferable.

Of the atmospheric gases in the system, it is preferable to control not only the moisture pressure in the system but also the oxygen partial pressure in the system. The range of oxygen partial pressure in the system is preferably 9.0 × 10 ⁻³ Pa to 3.0 × 10 ⁻¹ Pa, more preferably 1.0 × 10 ⁻² Pa to 2.0 × 10 ⁻¹ Pa. Preferably, 2.5 × 10 ⁻² Pa or more and 9.0 × 10 ⁻² Pa or less is more preferable. By setting the oxygen partial pressure to 9.0 × 10 ⁻³ Pa, the carrier concentration of the oxide semiconductor thin film can be sufficiently reduced. In addition, variation in carrier concentration in the surface of the oxide semiconductor thin film can be reduced. By setting the oxygen partial pressure in the system to 3.0 × 10 ⁻¹ Pa or less, the ratio of the rare gas, particularly argon, in the atmospheric gas is relatively increased, so that the film formation rate is increased and industrial practical use is achieved. Increases nature.

In order to optimize the carrier concentration and carrier mobility of the oxide semiconductor thin film obtained by the production method of the present invention, it is particularly preferable to combine the oxygen partial pressure in the system and the water pressure in the system well. When the oxygen partial pressure in the system is too low, it may be difficult to reduce the carrier concentration of the oxide semiconductor thin film even if the water pressure in the system is controlled. That is, the oxygen partial pressure in the system is 1.0 × 10 ⁻² Pa to 3.0 × 10 ⁻¹ Pa and the water pressure in the system is 5.0 × 10 ⁻² Pa to 2.0 × 10 ^{−. More} preferably, the oxygen partial pressure in the system is 5.0 × 10 ⁻² Pa or more and 2.0 × 10 ⁻¹ Pa or less, and the water pressure in the system is 5.1 ×. It is even more preferable to control the pressure in the range of 10 ⁻² Pa to 7.5 × 10 ⁻¹ Pa.

(4) Substrate In this film forming step, the substrate used for film formation is an inorganic material such as alkali glass, non-alkali glass, or quartz glass, or a transparent heat-resistant resin, such as a plate, sheet, or film. Can be used. Further, the substrate may be a substrate made of a base material in which an inorganic material such as silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, or hafnium oxide or an organic material having heat resistance is further formed on the above substrate. .

(5) Substrate temperature In this film formation step, the substrate temperature for film formation by sputtering is preferably room temperature or higher and 300 ° C. or lower, more preferably substrate temperature of 100 ° C. or higher and 300 ° C. or lower. If the substrate temperature is less than 100 ° C. and the oxygen partial pressure in the system is 2.4 × 10 ⁻² Pa or more, excessive oxygen may be taken into the film. Excess oxygen causes a decrease in the carrier concentration of the oxide semiconductor thin film, or causes a large variation in the carrier concentration in the surface of the oxide semiconductor thin film. On the other hand, when the substrate temperature exceeds 300 ° C., the oxide thin film immediately after film formation may be crystallized, which is not preferable. The oxide thin film formed on the substrate in the film formation step is preferably an amorphous or microcrystalline thin film. As will be described later, in the etching process after the film forming process, the microfabrication into a predetermined channel layer shape is facilitated particularly by wet etching.

(6) Distance between TS In the film formation step, the distance between the target and the substrate (the distance between TS) in film formation by sputtering is preferably 150 mm or less, more preferably 110 mm or less, and particularly preferably 80 mm or less. It is. When the T-S distance exceeds 150 mm, the film formation rate is remarkably reduced, and industrial practicality may be poor. Although the film formation rate can be increased by shortening the T-S distance, it is excellent in industrial practicality. On the other hand, since the formed oxide thin film may be damaged by plasma, it is 10 mm or more. Is preferably 20 mm or more, and particularly preferably 30 mm or more.

(7) Target In the film formation step, a target made of an oxide sintered body containing indium and gallium as oxides is used for film formation by sputtering. In particular, it is preferable to use a target made of an oxide sintered body containing indium and gallium as oxides, but further, an oxide firing in which one or more of the positive trivalent elements boron, aluminum, scandium, and yttrium are added. You may use the target which consists of a combination. The target composed of an oxide sintered body containing indium and gallium as an oxide preferably contains at least a bixbite-type In ₂ O ₃ phase, and β− as a generated phase other than the In ₂ O ₃ phase. GaInO ₃ phase of Ga ₂ O ₃ type structure, or _beta-Ga 2 _{O 3} -type structure GaInO ₃ phase and the (Ga, _{an in)} it is particularly preferably constructed by 2 _{O 3} phase. The density of the target made of an oxide sintered body having such a structure is preferably 6.3 g / cm ³ or more. If the density is less than 6.3 g / cm ³ , it may cause nodules during mass production. In addition, since it is mainly used in direct current sputtering film formation and requires good conductivity, it is preferable that a target made of an oxide sintered body is sintered in an oxygen-containing atmosphere, particularly in an oxygen atmosphere. It is preferable to be sintered.

  In the oxide sintered body containing indium and gallium as oxides or a target thereof, the content of gallium is Ga / (In + Ga) atomic ratio, and the lower limit is preferably 0.15 or more. More preferably, it is 17 or more, and further more preferably more than 0.20. The upper limit is preferably 0.30 or less, more preferably 0.25 or less, and still more preferably 0.23 or less. The gallium content in the oxide sintered body or its target is the same as the gallium content in the oxide semiconductor thin film obtained by the production method of the present invention.

2-2. Heat treatment step The heat treatment step is a step of heat-treating the oxide thin film formed on the surface of the substrate so as to become a crystalline oxide semiconductor thin film. Defects are excessively introduced into the oxide thin film obtained by film formation by sputtering in a non-equilibrium process. The introduction of excessive defects causes disturbance of the thin film structure such as ion (atom) and lattice arrangement, which results in an increase in carrier concentration and a decrease in carrier mobility. Therefore, by heat-treating this oxide thin film, excessive defects in the oxide thin film can be reduced, the disordered oxide thin film structure can be recovered, and the carrier concentration and carrier mobility can be stabilized. . Further, the oxide thin film is formed into a crystalline oxide semiconductor thin film by heat treatment. By performing the heat treatment, a crystalline oxide semiconductor thin film having high carrier mobility controlled to an appropriate carrier concentration can be obtained.

(1) Heat treatment method As a method of heat treatment so as to form a crystalline oxide semiconductor thin film, an infrared heating method, a lamp heat treatment method, or a laser treatment method can be used in addition to a general so-called resistance heating method. Specific heat treatment methods include rapid thermal annealing (RTA) using infrared heating, heat treatment using lamp heating (LA), and the like. Examples of the laser treatment method include a treatment method using an excimer laser or a YAG laser using a wavelength that can be absorbed by the oxide semiconductor. Considering application to a large glass substrate, heat treatment such as RTA is preferable.

(2) Heat treatment conditions The heat treatment temperature in the heat treatment step can be appropriately selected as long as it is equal to or higher than the temperature at which the amorphous or microcrystalline oxide thin film is crystallized and the substrate is not deformed or damaged. . Specifically, although it varies depending on the gallium content in the oxide thin film, it is preferably 100 ° C. or higher and lower than 500 ° C., more preferably 100 ° C. or higher and 450 ° C. or lower.

  The rate of temperature rise to the heat treatment temperature in the heat treatment step is not particularly limited, but is preferably 10 ° C./min or more, more preferably 50 ° C./min or more, and particularly preferably 100 ° C./min or more. By increasing the rate of temperature increase, it becomes possible to perform the heat treatment while limiting the target temperature as much as possible. There is also an advantage that throughput in the process can be increased. The heat treatment time is preferably 1 minute to 120 minutes, and more preferably 5 minutes to 60 minutes, which is maintained at the heat treatment temperature.

  In the heat treatment step, the heat treatment atmosphere is an oxygen-containing atmosphere with an oxygen concentration of 30% or more. In addition, an atmosphere with an oxygen concentration of 100% (pure oxygen) is preferable. These oxidizing atmospheres preferably contain moisture. The moisture in the oxidizing atmosphere has a relative humidity at a temperature of 22 ° C. of 5% or more, preferably 10% or more, and more preferably 30% or more in consideration of the stability of manufacturing conditions. The oxygen concentration is a volume ratio, and the moisture volume ratio is not included.

(3) Etching conditions In the manufacturing method of the present invention, after an oxide thin film is formed in the film forming step, a thin film transistor (TFT) or the like is formed by wet etching or dry etching in an amorphous or microcrystalline state before heat treatment. The fine processing required for the application can be applied. Usually, a substrate temperature is appropriately selected from a temperature lower than the crystallization temperature, for example, a range from room temperature to 300 ° C., and once an oxide thin film is formed, fine processing by wet etching can be performed. The etchant can be generally used as long as it is a weak acid, but is preferably a weak acid mainly composed of PAN or oxalic acid. For example, ITO-06N manufactured by Kanto Chemical Co., Ltd. can be used. Note that dry etching may be selected depending on the structure of the thin film transistor (TFT).

3. Thin Film Transistor (TFT) and Manufacturing Method Thereof According to the crystalline oxide semiconductor thin film obtained by the manufacturing method of the present invention, the carrier concentration can be reduced while maintaining high carrier mobility. Furthermore, a stable operation can be realized by forming a thin film transistor (TFT) using the oxide semiconductor thin film as a channel layer.

  The thin film transistor is not particularly limited as long as it includes a crystalline oxide semiconductor thin film obtained by the manufacturing method of the present invention as a channel layer. For example, the thin film transistor includes a source electrode, a drain electrode, a gate electrode, a channel layer, and a gate insulation. A membrane may be provided.

  A thin film transistor can be manufactured by combining a conventionally well-known method and the manufacturing method of the oxide semiconductor thin film of this invention. For example, a gate insulating film is formed on the surface of the gate electrode, and an oxide semiconductor thin film is formed as a channel layer on the surface of the gate insulating film. Specifically, after forming an oxide thin film on the surface of the gate insulating film according to the method for manufacturing an oxide semiconductor thin film of the present invention, a heat treatment is performed to obtain a crystalline oxide semiconductor thin film, which is then etched. Thus, a patterned oxide semiconductor thin film (channel layer) is formed. A thin film transistor can be manufactured by forming a patterned source electrode and drain electrode on the surface of the oxide semiconductor thin film (channel layer).

The method of forming the gate insulating film on the surface of the gate electrode is, for example, a method of forming a SiO ₂ film (gate insulating film) on the surface of the Si substrate (gate electrode) by thermal oxidation or the like, or a surface of the ITO film (gate electrode) In addition, a method of forming a SiO ₂ film (gate insulating film) by high-frequency magnetron sputtering can be used.

  The source electrode and the drain electrode are formed on the surface of the oxide semiconductor thin film (channel layer) by a direct current magnetron sputtering method, such as a metal thin film such as Mo, Al, Ta, Ti, Au, Pt or an alloy of these metals. Examples include thin film, conductive oxide or nitride thin films of these metals, various conductive polymer materials, and methods for forming ITO or the like for transparent TFTs.

  As a method for forming the patterned source electrode and drain electrode on the surface of the oxide semiconductor thin film (channel layer), for example, an etching method using a photolithography technique, a lift-off method, or the like can be used.

  Hereinafter, the present invention will be described in more detail using examples, but the present invention is not limited to these examples.

Example 1
An oxide semiconductor thin film was manufactured and evaluated by the process described below.

<Production of oxide semiconductor thin film>
Film formation by direct current sputtering was performed using a load-lock magnetron sputtering apparatus (manufactured by ULVAC) equipped with a direct current power source, a 6-inch cathode, and a quadrupole mass spectrometer (manufactured by INFICON). As a target, a target composed of an oxide sintered body containing indium and gallium as oxides was used. The target gallium content is the content shown in Table 1 (Ga / (In + Ga) atomic ratio). In actual film formation, after pre-sputtering for 10 minutes, the substrate was transported directly above the sputtering target, that is, to a stationary facing position, to form an oxide thin film having a thickness of 50 nm. Details of the film forming conditions are shown below.

[Film formation conditions]
Substrate temperature: 200 ° C
Ultimate vacuum: less than 3.0 × 10 ⁻⁵ Pa Target-substrate (TS) distance: 60 mm
Sputtering gas total pressure: 0.6Pa
Oxygen partial pressure: 1.2 × 10 ⁻² Pa
Moisture pressure: 6.5 × 10 ⁻³² Pa
Input power: Direct current (DC) 300W

  Subsequently, the oxide oxide thin film after film formation is subjected to heat treatment using an RTA (Rapid Thermal Annealing) apparatus so as to become a crystalline oxide semiconductor thin film under the following conditions. Got.

[Heat treatment conditions]
Heat treatment temperature: 350 ° C
Atmosphere: Oxygen (excluding moisture)
Relative humidity: 50%
Temperature increase rate: 500 ° C / min

<Characteristic evaluation of oxide semiconductor thin film>
The composition of the oxide thin film and the oxide semiconductor thin film was examined by ICP emission spectroscopy. The film thickness of the oxide semiconductor thin film was measured with a surface roughness meter (manufactured by Tencor). The carrier concentration and carrier mobility of the oxide semiconductor thin film were determined by a Hall effect measuring device (manufactured by Toyo Technica). The film quality of the oxide thin film before the heat treatment step and the oxide semiconductor thin film after the heat treatment step were confirmed by X-ray diffraction measurement (manufactured by Philips). Tables 1 and 2 show the results. Note that it is confirmed that the gallium content in the oxide thin film before the heat treatment step and the gallium content in the oxide semiconductor thin film after the heat treatment step are the same in terms of Ga / (In + Ga) atomic ratio. It was.

  Among Examples and Comparative Examples, representative oxide semiconductor thin films were measured by SIMS (secondary ion mass spectrometry, manufactured by ULVAC-PHI).

(Examples 2 to 12, Comparative Examples 1 to 5)
The target, sputtering conditions, and heat treatment conditions were the same as in Example 1 except that the targets and conditions were made of an oxide sintered body containing indium and gallium having the compositions shown in Table 1 as oxides. An oxide semiconductor thin film was prepared and evaluated. Tables 1 and 2 summarize the results.

From Examples 1 to 12, a crystalline oxide semiconductor thin film containing indium and gallium as oxides and further containing hydrogen obtained by the production method of the present invention, wherein gallium is a Ga / (In + Ga) atom. The oxide semiconductor thin film having a number ratio of 0.15 or more and 0.30 or less has an oxygen partial pressure in the system of 9.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻¹ Pa or less and moisture in the system. The film was formed by sputtering in an atmosphere in which the pressure was controlled in the range of 2.0 × 10 ⁻³ Pa to 5.0 × 10 ⁻¹ Pa, and the relative humidity measured at a temperature of 22 ° C. in the system was 5 % Or higher and an oxygen concentration of 30% or higher, whereby the carrier concentration is less than 1.0 × 10 ¹⁸ cm ⁻³ and the carrier mobility is 10 cm ² V ⁻¹ sec ⁻¹ or higher. I understand that.

In particular, as can be seen from Examples 1 to 5, the crystalline oxide semiconductor thin film containing indium and gallium as oxides and further containing hydrogen obtained by the production method of the present invention is subjected to heat treatment. By controlling the relative humidity of the atmosphere to 5% or more, a carrier concentration of less than 1.0 × 10 ¹⁸ cm ⁻³ can be realized.

On the other hand, in Comparative Example 2, since the relative humidity of the heat treatment atmosphere is 0%, hydrogen is desorbed from the oxide semiconductor thin film by the heat treatment, and as a result, the carrier concentration of the oxide semiconductor thin film is 1.0 ×. It has become 10 ¹⁸ cm ⁻³ or more.

Further, in Comparative Examples 1 and 3, since the oxygen concentration is less than 30% in the atmosphere in the heat treatment step, even when the relative humidity is controlled, the oxygen deficiency of the oxide semiconductor thin film does not disappear during the heat treatment. The carrier concentration of the oxide semiconductor thin film is 1.0 × 10 ¹⁸ cm ⁻³ or more.

<X-ray diffraction measurement and TEM-EDX measurement of cross-sectional structure>
In FIG. 1, the X-ray-diffraction measurement result of the oxide semiconductor thin film of Example 1 is shown. A clear diffraction peak of the In ₂ O ₃ bixbite structure was observed, confirming that the oxide semiconductor thin film of Example 1 was a crystalline oxide semiconductor thin film.

<Measurement of hydrogen concentration distribution in the film depth direction by SIMS>
As Example 13, the hydrogen concentration distribution in the film depth direction was measured by SIMS before and after the heat treatment for an oxide semiconductor thin film manufactured under the same film formation conditions and heat treatment conditions as in Example 1 and having a thickness of about 150 nm. did. Note that by increasing the film thickness to 150 nm, the difference in hydrogen concentration distribution in the film depth direction can be easily compared.

  FIG. 2 shows the SIMS measurement results. In the oxide semiconductor thin film of Example 13 heat-treated in an oxygen atmosphere with a relative humidity of 50%, it was confirmed that there was no change in the hydrogen concentration distribution in the film depth direction before and after the heat treatment. Therefore, it was found that an oxide semiconductor having a uniform hydrogen concentration in the film depth direction can be obtained even after heat treatment by the manufacturing method of the present invention.

<Production of thin film transistor and evaluation of operating characteristics>
(Example 14)
Next, a thin film transistor (TFT) was fabricated and evaluated. That is, the thin film transistor was manufactured using a 475 μm thick, 20 mm square conductive p-type Si substrate on which a 100 nm thick SiO ₂ film was formed by thermal oxidation. Here, the SiO ₂ film functions as a gate insulating film, and the conductive p-type Si substrate functions as a gate electrode.

Specifically, the oxide thin film (Ga / (In + Ga) atomic ratio = 0.20) of Example 1 was formed on the SiO ₂ film gate insulating film. The sputtering conditions were the same as in Example 1.

  Subsequently, patterning was performed on the formed oxide thin film by a photolithography method using a resist (OFPR # 800 manufactured by Tokyo Ohka Kogyo Co., Ltd.) and an etchant (ITO-06N manufactured by Kanto Chemical).

Next, the oxide thin film was heat-treated under the same conditions as in Example 1 to obtain a crystalline oxide semiconductor thin film. Thus, a channel layer made of a crystalline oxide semiconductor thin film was formed on the SiO ₂ film gate insulating film.

  Then, a 10 nm thick Ti film and a 50 nm thick Au film are formed in this order on the surface of the channel layer by direct current magnetron sputtering, so that a source electrode and a drain electrode made of an Au / Ti laminated film are formed. Was deposited. Patterning was performed by a lift-off method to form a thin film transistor by forming a source electrode and a drain electrode so as to have a channel length of 20 μm and a channel width of 500 μm.

The operating characteristics of the thin film transistor were evaluated using a semiconductor parameter analyzer (manufactured by Agilent). As a result, the operating characteristics as a thin film transistor could be confirmed. In addition, the obtained thin film transistor has a field effect mobility of 54.2 cm ² V ⁻¹ sec ⁻¹ , an on / off ratio of 2 × 10 ⁸ , and an S value of 0.27, each showing a good value. Was confirmed.

Claims (6)

A target made of an oxide sintered body containing indium and gallium as oxides in an atmosphere having a moisture pressure of 2.0 × 10 ⁻³ Pa to 5.0 × 10 ⁻¹ Pa in the system. A film forming step of forming an oxide thin film on the surface of the substrate by a sputtering method;
The oxide thin film contains indium and gallium as oxides, and further contains hydrogen in an atmosphere having a relative humidity of 5% or more and an oxygen concentration of 30% or more measured at a temperature of 22 ° C. in the system. A method for producing a crystalline oxide semiconductor thin film, comprising: a heat treatment step of performing a heat treatment so as to form a crystalline oxide semiconductor thin film.   The method for manufacturing an oxide semiconductor thin film according to claim 1, wherein the relative humidity is 10% or more in the heat treatment step.   The method for manufacturing an oxide semiconductor thin film according to claim 1 or 2, wherein the oxygen concentration is set to 100% in the heat treatment step.   The content of the gallium in the oxide sintered body is 0.15 or more and 0.30 or less in a Ga / (In + Ga) atomic ratio in the target used in the film forming step. The manufacturing method of the oxide semiconductor thin film in any one.   The content of the gallium in the oxide sintered body is 0.17 or more and 0.30 or less in a Ga / (In + Ga) atomic ratio in the target used in the film forming step. The manufacturing method of the oxide semiconductor thin film in any one.   A method for producing a thin film transistor comprising the oxide semiconductor thin film as a channel layer, comprising a step of producing a crystalline oxide semiconductor thin film by the production method according to claim 1.

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