JP2018104772A - Method of manufacturing oxide semiconductor thin film and method of manufacturing thin film transistor - Google Patents

Abstract

A method for producing a crystalline oxide semiconductor thin film which exhibits high carrier mobility and a crystal grain size of less than 1 μm is provided. The method of manufacturing an oxide semiconductor thin film according to the present invention has an oxygen partial pressure of 1.0 × 10 −3 Pa or more and 3.0 × 10 −2 Pa or less with respect to a total pressure of a rare gas and oxygen, A target including an oxide having a moisture pressure with respect to a total gas pressure of 2.0 × 10 −3 Pa to 5.0 × 10 −1 Pa and a Ga / (In + Ga) atomic ratio of 0.15 to 0.30. After forming a non-crystalline first oxide thin film by sputtering, the oxygen partial pressure is more than 3.0 × 10 −2 Pa and not more than 2.0 × 10 −1 Pa, and the water pressure is 2.0 × 10 −3 Pa. It is not crystalline on the first oxide thin film by a sputtering method using a target containing an oxide having a Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less at a ratio of 5.0 × 10 −1 Pa or less. A second oxide thin film is formed and heat-treated. [Selection] Figure 1

Description

  The present invention relates to a method for manufacturing an oxide semiconductor thin film and a method for manufacturing a thin film transistor. More specifically, a crystalline oxide semiconductor thin film containing indium, gallium, and hydrogen and exhibiting high carrier mobility, and an oxide semiconductor thin film in which coarsening of crystal grain size is suppressed, and a surface using the same The present invention relates to a method for manufacturing a thin film transistor excellent in internal uniformity.

  A thin film transistor (hereinafter referred to as “TFT”) is one type of a 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 a substrate is used as a channel layer in which electrons or holes move as carriers. This is an active element having a function of switching a current between a source terminal and a drain terminal by applying a voltage to a terminal to control a current flowing in a channel layer.

  The TFT is an electronic device that is currently most practically used, and a typical application is a TFT for driving a liquid crystal. Many of the liquid crystal driving TFTs use an n-type channel layer in which electrons move as carriers. At present, 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 a high speed as the definition of liquid crystals increases. The driving speed of the TFT depends on the electron mobility of the channel layer. Therefore, 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 a sufficiently high electron mobility, but when formed on a large glass substrate, the in-plane uniformity is low and the yield is low, or the manufacturing process is larger than amorphous silicon and requires capital investment. For these reasons, 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 composition is InGaO ₃ (ZnO) _m (m is a natural number of less than 6) when crystallized, and the carrier mobility (also referred to as carrier electron mobility) is 1 cm without adding impurity ions. A transparent semi-insulating amorphous oxide thin film characterized by having a semi-insulating property that exceeds ² V ⁻¹ sec ⁻¹ and has a carrier concentration (also referred to as carrier electron concentration) of 10 ¹⁶ cm ⁻³ or less, and A thin film transistor characterized in that this transparent semi-insulating amorphous oxide thin film is used 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 It has been pointed out that a thin film (a-IGZO film) has a carrier mobility in the range of approximately ^{1 to} 10 cm ² V ⁻¹ sec ⁻¹ , so that the carrier mobility is insufficient when formed as a TFT channel layer. It was.

In order to solve the shortage of carrier mobility, materials other than a-IGZO have been studied. For example, in Patent Document 2, gallium is dissolved in indium oxide, the atomic ratio Ga / (Ga + In) is 0.001 to 0.12, and the content ratio of indium and gallium with respect to all metal atoms is 80 atoms. %, And a thin film transistor characterized by using an oxide thin film having an In ₂ O ₃ bixbyite structure has been proposed.

In Patent Document 2, the indium content is higher than that in Patent Document 1, and the carrier mobility is increased by crystallizing the In ₂ O ₃ bixbite structure. However, the carrier concentration is 2.0 × 10 ^18. There are some examples exceeding cm ^−3, and the problem remains that the oxide semiconductor thin film applied to the TFT channel layer is high.

In Patent Document 3, a laminated structure composed of an oxide layer and an insulating layer serving as a channel layer of a thin film transistor, and a material constituting the oxide layer is indium oxide doped with Ga substantially having a bixbite structure. The oxide layer has a carrier concentration of 10 ¹⁸ / cm ³ or less, an average crystal grain size of 1 μm or more, and the oxide layer crystals are arranged in a columnar shape on the surface of the insulating layer. Has been proposed. In particular, it is disclosed that an oxide thin film (oxide layer) grows in a columnar shape from the substrate side in a heat treatment process by introducing water during thin film deposition and suppressing the formation of microcrystals. An oxide thin film in which crystals are arranged has few oxygen vacancies, so that the carrier concentration can be controlled to 10 ¹⁸ / cm ³ or less. Further, it has been reported that the average crystal grain size of such an oxide thin film is 1 μm or more, preferably 1 to 27 μm.

Certainly, if it is a columnar crystal oxide thin film with few defects, the carrier concentration can be 10 ¹⁸ / cm ³ or less, but the average crystal grain size should be 1 μm or more from the viewpoint of variation in the TFT substrate surface. Is rather undesirable. The shape of the oxide semiconductor layer of a mass-produced TFT is several to several tens of μm in length and several to several tens of μm in width. Therefore, when the average crystal grain size is 1 μm or more, the semiconductor characteristics are deteriorated. There may be only a few crystal grain boundaries to be formed per side of the semiconductor layer. The semiconductor characteristics are easily influenced by the number of crystal grain boundaries, and for example, it becomes difficult to form a large number of TFTs uniformly in a large-area glass substrate surface.

Further, in Patent Document 4, in order to solve the high carrier concentration of Patent Document 2, the sputtering target is formed by DC sputtering under a condition where the water pressure in the system is 3 × 10 ^{−4 to} 5 × 10 ⁻² Pa. A method for forming an oxide semiconductor in which a film body is formed and the film body is crystallized has been proposed.

The film forming method described in Patent Document 4 is almost the same as the film forming method disclosed in Patent Document 3. In particular, in Patent Document 4, the only example of indium oxide doped with Ga, that is, Example 14 related to In ₂ O ₃ : Ga ₂ O ₃ = 95: 5, and Ga / (In + Ga) = 0.058 in Patent Document 3. In Example 9, the composition of the sputtering target as well as the film forming method is almost the same. Therefore, in Patent Document 4, as in Patent Document 3, TFTs cannot be manufactured uniformly on a large-area substrate with coarse crystal grains having an average crystal grain size of 1 μm or more.

JP 2010-219538 A WO2010 / 032422 JP 2013-16866 A JP 2011-222557 A

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 a crystalline oxide semiconductor thin film that exhibits high carrier mobility and a crystal grain size of less than 1 μm.

  The inventors of the present invention formed an oxide semiconductor thin film by a two-stage sputtering method in a predetermined atmosphere using a predetermined target, and contained indium, gallium and hydrogen, and an average crystal grain size of less than 1 μm The inventors have found that a crystalline oxide semiconductor thin film can be obtained, and have completed the present invention. Specifically, the present invention provides the following.

(1) In the first invention of the present invention, the oxygen partial pressure relative to the total pressure of the rare gas and oxygen is 1.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻² Pa or less, and all gases in the system In an atmosphere where the water pressure relative to the total pressure is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, indium and gallium are contained, and the gallium content is Ga / (In + Ga) atomic ratio. A first film forming step of forming a non-crystalline first oxide thin film by a sputtering method using a target containing an oxide of 0.15 or more and 0.30 or less, and a total pressure of a rare gas and oxygen The oxygen partial pressure is more than 3.0 × 10 ⁻² Pa and 2.0 × 10 ⁻¹ Pa or less, and the water pressure with respect to the total pressure of all gases in the system is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10. ^{-1 Pa} under the following atmosphere containing indium and gallium, the A second non-crystalline material is formed on the first oxide thin film by a sputtering method using a target containing an oxide having a Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less. An oxide semiconductor thin film manufacturing method including a second film forming step of forming an oxide thin film to form a laminated thin film, and a heat treatment step of heat-treating the laminated thin film.

  (2) The second invention of the present invention is the method for manufacturing an oxide semiconductor thin film according to the first invention, wherein the heat treatment step is performed in an oxygen-containing atmosphere.

(3) In the third invention of the present invention, the partial pressure of oxygen with respect to the total pressure of the rare gas and oxygen is 1.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻² Pa or less. In an atmosphere where the water pressure relative to the total pressure is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, indium and gallium are contained, and the gallium content is Ga / (In + Ga) atomic ratio. A first film forming step of forming a first oxide thin film by a sputtering method using a target including an oxide of 0.15 or more and 0.30 or less, and an oxygen partial pressure with respect to a total pressure of a rare gas and oxygen is More than 3.0 × 10 ⁻² Pa 2.0 × 10 ⁻¹ Pa or less, moisture pressure with respect to the total pressure of all gases in the system is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻¹ Pa or less. Containing indium and gallium under the atmosphere of A second oxide thin film is formed on the first oxide thin film by a sputtering method using a target containing an oxide having a Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less. A method of manufacturing a thin film transistor including a second film forming step for forming a laminated thin film and a heat treatment step for heat-treating the laminated thin film.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the crystalline oxide semiconductor thin film which shows high carrier mobility and whose crystal grain diameter is less than 1 micrometer can be provided. A TFT in which such an oxide semiconductor thin film is applied to a semiconductor layer operates uniformly and stably on a large-area substrate.

2 is a transmission electron microscope (TEM) image of the surface of the oxide semiconductor thin film of Example 1. FIG. 2 is a TEM image of a cross section of the oxide semiconductor thin film of Example 1. FIG.

  Hereinafter, a specific embodiment of the present invention (hereinafter referred to as “the present embodiment”) will be described in detail. In addition, this invention is not limited to the following embodiment, It can implement by adding a change suitably within the range of the objective of this invention.

1. Oxide Semiconductor Thin Film (1) Metal Composition Although the oxide semiconductor thin film according to this embodiment is not particularly limited, for example, it is crystalline and contains indium, gallium, and hydrogen, and the gallium content is Ga / ( In + Ga) atomic ratio is 0.15 or more and 0.30 or less.

Although the gallium content of the oxide semiconductor thin film of the present invention is not particularly limited, for example, the Ga / (In + Ga) atomic ratio is 0.15 or more and 0.30 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 of the present invention. This effect can be sufficiently obtained when the gallium content is 0.15 or more in terms of the Ga / (In + Ga) atomic ratio. In addition, a crystalline oxide semiconductor thin film having a crystal grain size of less than 1 μm can be obtained. On the other hand, by being 0.30 or less, the oxide semiconductor thin film can exhibit carrier mobility of 10 cm ² V ⁻¹ sec ⁻¹ or more.

  The gallium content is not particularly limited, but the Ga / (In + Ga) atomic ratio is preferably 0.17 or more, more preferably more than 0.20. Moreover, as gallium content, it is preferable that it is 0.25 or less, and it is more preferable that it is 0.23 or less.

  The oxide semiconductor thin film can contain a specific positive trivalent element as an element other than indium and gallium. Specific positive trivalent elements include boron, aluminum, scandium, and yttrium. When these elements are contained in the oxide semiconductor thin film, the carrier concentration of the oxide semiconductor thin film can be reduced. Note that it hardly contributes to improvement of carrier mobility.

  The oxide semiconductor thin film preferably does not substantially contain a positive trivalent element other than the specific positive trivalent element described above. Examples of positive trivalent elements other than specific positive trivalent elements include lanthanum, praseodymium, dyspronium, holmium, erbium, ytterbium, and lutetium. These elements not only do not contribute to the reduction of the carrier concentration of the oxide semiconductor thin film, but also may reduce the carrier mobility. Note that “substantially free” means that it is unavoidable to inevitably contain impurities as described later.

  The oxide semiconductor thin film preferably does not substantially contain a positive tetravalent or higher element. Examples of the positive tetravalent element or more include titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, silicon, germanium, tin, lead, antimony, bismuth, and cerium. When such an element is contained in the oxide semiconductor thin film, it acts as a scattering factor, so that carrier mobility may be reduced.

  It is preferable that the oxide semiconductor thin film does not substantially contain a positive divalent element or less. Examples of elements having a positive divalent value or less include lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, and zinc. While such an element contributes to the reduction of the carrier concentration to the oxide semiconductor, it may act as a scattering factor to reduce the carrier movement, and the carrier mobility may be lowered as a whole. .

  The composition of the oxide semiconductor thin film is not particularly limited. For example, it is preferably made of indium, gallium, hydrogen and oxygen, or made of indium, gallium, hydrogen, oxygen and unavoidable impurities.

(2) Inevitable impurities The oxide semiconductor thin film can contain inevitable impurities. The content of inevitable impurities is not particularly limited, but for example, it is preferably 500 ppm or less, more preferably 300 ppm or less, and further preferably 100 ppm or less. When the amount of impurities is large, problems such as an increase in carrier concentration and a decrease in carrier mobility may occur.

(3) Hydrogen concentration The concentration of hydrogen contained in the oxide semiconductor thin film is determined by secondary ion mass spectrometry (SIMS), Rutherford Backscattering Spectroscopy (RBS, Rutherford Backscattering Spectrometry), or hydrogen forward scattering. It can be measured by an analysis method (HFS: Hydrogen Forward Scattering) or the like. As the hydrogen concentration, for example, a value obtained by SIMS analysis is preferably 1 × 10 ²⁰ to 1 × 10 ²² atoms / cm ³ , and preferably 3 × 10 ^{20 to} 5 × 10 ²¹ atoms / cm ^3. More preferably, it is 5 * 10 < ²⁰ > -1 * 10 < ²¹ > atoms / cm < ³ >. It is considered that hydrogen exists in the vicinity of oxygen in the oxide semiconductor thin film and contributes to a reduction in carrier concentration. When the hydrogen concentration is 1 × 10 ²⁰ atoms / cm ³ or more, the carrier concentration can be sufficiently reduced to 2 × 10 ¹⁸ cm ⁻³ or less. On the other hand, when the hydrogen concentration is 1 × 10 ²² atoms / cm ³ or less, it is possible to prevent excessive hydrogen from acting as a scattering factor, to prevent a decrease in carrier mobility, and 10 cm ² V ⁻¹ sec ^{−. One} or more can be maintained.

(4) Film quality and film structure The oxide semiconductor thin film is crystalline. The crystal structure of the oxide semiconductor thin film is preferably a bixbite structure. The crystal structure of the oxide semiconductor thin film can be confirmed by a diffraction peak in X-ray diffraction measurement, a thin film structure confirmation in electron microscope observation, a diffraction spot (electron diffraction pattern) corresponding to a plane index, or the like.

  The crystal grain size of the oxide semiconductor thin film is not particularly limited, but is, for example, less than 1 μm. In general, the channel length of an oxide semiconductor layer of a TFT that is mass-produced is about several μm to several tens of μm. Therefore, by setting the crystal grain size to less than 1 μm, about 10 or more crystal grain boundaries can exist stably with respect to the channel length. Thereby, it becomes possible to average the influence of the carrier scattering by the crystal grain boundary on the mobility reduction, and the TFT can be manufactured even on a large substrate. On the other hand, the average crystal grain size in the oxide semiconductor thin film (oxide layer) after the heat crystallization of Patent Document 3 is 1 to 27 μm. In Patent Document 3, the reason why crystal grains grow in this way is that, in a film in which water is introduced during thin film deposition to suppress the formation of microcrystals, crystals grow in a columnar shape from the substrate side in the heat treatment step, and the heated crystals It is described that the crystal grain size becomes larger than that of a thin film not introduced with water because crystal growth is not inhibited by microcrystals in the film in the crystallization step. Therefore, in the technique of Patent Document 3, an indium oxide thin film doped with Ga having an atomic ratio of Ga / (Ga + In) of 0.01 or more and 0.09 or less is deposited by introducing water, and then crystallized by heat treatment. The crystal grain size inevitably becomes 1 μm or more. Although details will be described later, in the present invention, the gallium content is further increased as compared with the prior art, and the oxygen partial pressure during film formation is further increased, so that even if an appropriate amount of water is introduced, indium, gallium, The crystal grain size of the oxide semiconductor thin film containing oxygen and hydrogen can be suppressed to less than 1 μm.

  The “crystal grain size” is an average grain size obtained by measuring the grain size by an intersection method from an image obtained by observation with a transmission electron microscope (TEM), and includes 50 to 100 crystal grains. Average value.

(5) Film thickness Although it does not specifically limit as a film thickness of an oxide semiconductor thin film, For example, it is preferable that it is 10 nm or more, it is more preferable that it is 30 nm or more, and it is further more preferable that it is 50 nm or more. On the other hand, the film thickness is not particularly limited. For example, when applied as a TFT channel layer of a device requiring flexibility, it is preferably 1000 nm or less, more preferably 500 nm or less, and 300 nm or less. More preferably. When the thickness is 1000 nm or less, characteristics necessary as a channel layer of a thin film transistor (TFT) can be maintained even when the device is bent. In consideration of the throughput in the manufacturing process and the small variation in performance, the overall thickness is preferably 30 to 300 nm.

(6) Carrier concentration / carrier mobility The carrier concentration is not particularly limited, but is preferably 2.0 × 10 ¹⁸ cm ⁻³ or less, for example, 1.0 × 10 ¹⁸ cm ⁻³ or less. Is more preferably 8.0 × 10 ¹⁷ cm ⁻³ or less, and particularly preferably 8.0 × 10 ¹⁶ cm ⁻³ or less. When the carrier concentration is 2.0 × 10 ¹⁸ cm ⁻³ or less, a TFT formed using the oxide semiconductor thin film exhibits normally-off. As represented by the amorphous oxide semiconductor thin film composed of indium, gallium, and zinc described in Non-Patent Document 1, the amorphous oxide semiconductor thin film containing a large amount of indium has a carrier concentration of 4 × 10 ^18. Since it is in a degenerated state at cm ⁻³ or more, there is a possibility that a TFT in which this is applied to the channel layer does not show normally-off.

The carrier mobility, but are not limited to, for example, preferably 10cm ² V ^-1 sec ^-1 or more, and more preferably 15cm ² V ^-1 sec ^-1 or more.

2. Manufacturing method of oxide semiconductor thin film The manufacturing method of the oxide semiconductor thin film is such that the oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is 1.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻² Pa or less. In an atmosphere where the water pressure with respect to the total pressure of all gases is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, indium and gallium are contained, and the gallium content is Ga / (In + Ga). A first film formation step of forming a non-crystalline first oxide thin film by a sputtering method using a target including an oxide having an atomic ratio of 0.15 to 0.30; 4. The oxygen partial pressure with respect to the total pressure is more than 3.0 × 10 ⁻² Pa and 2.0 × 10 ⁻¹ Pa or less, and the water pressure with respect to the total pressure of all the gases in the system is 2.0 × 10 ⁻³ Pa or more. 0 × ¹⁰ -1 Pa under the following atmosphere, indium On the first oxide thin film by a sputtering method using a target containing an oxide containing gallium and an gallium content of 0.15 or more and 0.30 or less in a Ga / (In + Ga) atomic ratio. It includes a second film forming step of forming a non-crystalline second oxide thin film to form a laminated thin film, and a heat treatment step of heat-treating the laminated thin film.

  As described above, the film is formed by the two-stage sputtering method of the first film formation process and the second film formation process, and the oxygen partial pressure in the first film formation process is changed to the oxygen content in the second film formation process. By making it lower than the pressure, an oxide semiconductor thin film having an average crystal grain size of less than 1 μm can be obtained.

2-1. 1st film-forming process The 1st film-forming process has an oxygen partial pressure of 1.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻² Pa or less relative to the total pressure of rare gas and oxygen, In an atmosphere where the water pressure relative to the total pressure is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, indium and gallium are contained, and the gallium content is Ga / (In + Ga) atomic ratio. In this step, a non-crystalline first oxide thin film is formed by a sputtering method using a target including an oxide of 0.15 or more and 0.30 or less.

(1) Sputtering method The sputtering method is not particularly limited, and for example, a direct current sputtering method, alternating current sputtering with a frequency of 1 MHz or less, and pulse sputtering can be used. Among these, DC sputtering is preferable from an industrial viewpoint. Although RF sputtering can be applied, since it is non-directional, it may be difficult to establish conditions for uniform film formation on a large substrate.

(2) Sputtering gas conditions Although it does not specifically limit as a gas seed | species which comprises the atmospheric gas of sputtering film-forming, For example, it is preferable that it is an atmosphere containing a noble gas, oxygen, and water. The rare gas is preferably argon. Moreover, it is preferable that water is introduced into the sputtering apparatus chamber as water vapor.

  The total pressure of these atmospheric gases is not particularly limited, but is preferably in the range of 0.1 Pa or more and 3.0 Pa or less, more preferably 0.2 Pa or more and 0.8 Pa or less. More preferably, it is in the range of 3 Pa or more and 0.7 Pa or less.

(3) Oxygen partial pressure In the first film forming step, the oxygen partial pressure with respect to the total pressure of the rare gas and oxygen in the atmospheric gas in the system for sputtering is 1.0 × 10 ⁻³ Pa or more and 3.0 ×. Control to 10 ⁻² Pa or less. Moreover, as oxygen partial pressure, it is preferable to control to 5.0 * 10 < ^-3 > Pa or more and 2.0 * 10 ^<-2 > Pa or less, for example. Oxygen partial pressure is 1.0 × 10 ⁻³ Pa or more, so that the oxide semiconductor thin film has a carrier concentration after heat treatment of 2.0 × 10 ¹⁸ cm ⁻³ or less and an average crystal grain size of less than 1 μm. Can be obtained. On the other hand, when the oxygen partial pressure is 3.0 × 10 ⁻² Pa or less, the channel region of the semiconductor layer in the vicinity of the gate insulating film of the bottom-gate TFT has an extremely high oxygen residue after the heat treatment. The resistance can be prevented, and the problem that the TFT does not normally turn on can be prevented.

(4) Moisture pressure In the film forming step, the moisture pressure with respect to the total pressure of all the gases in the system among the atmosphere gases in the system to be sputtered is 2.0 × 10 ⁻³ Pa or more and 5.0 × 10 ⁻ Control to ¹ Pa or less. Moreover, as a water pressure, it is preferable to control to 5.0 * 10 ^<-2 > Pa or more and 2.0 * 10 ^{< -1} > Pa or less, for example. When the water pressure is 2 × 10 ⁻³ Pa or more, the amount of hydrogen or hydroxyl which is a component of water taken into the oxide semiconductor thin film can be increased, and a sufficient carrier concentration reduction effect can be obtained. it can. On the other hand, when the water pressure is 5.0 × 10 ⁻¹ Pa or less, an increase in carrier concentration and a decrease in carrier mobility can be suppressed. As the water pressure increases, hydrogen or a hydroxyl group may behave as a donor or a scattering factor.

  The method for introducing water into the system is not particularly limited. For example, it is preferably introduced as water vapor in the sputtering apparatus chamber.

  Note that hydrogen can be added to the oxide semiconductor thin film by substituting control of the hydrogen partial pressure in the system in the above process. However, it is assumed that other film forming conditions, particularly the optimum oxygen partial pressure, change. In addition, the cost is increased to ensure safety, such as the need for an explosion-proof manufacturing process. Therefore, control by moisture pressure is preferable.

  Note that, as described above, in addition to optimizing the carrier concentration and carrier mobility of the oxide semiconductor thin film, in order to control the crystal grain size to be less than 1 μm, both the oxygen partial pressure and the water pressure are within the above ranges. Need to control. For example, if the oxygen partial pressure is too low, the carrier concentration may not be reduced even if the moisture pressure is increased, and the crystal grain size may exceed 1 μm.

(5) Substrate The substrate used for film formation is not particularly limited. For example, inorganic materials such as alkali glass, alkali-free glass, and quartz glass, or polycarbonate, polyarylate, polyethersulfone, polyethernitrile, polyethylene terephthalate, Organic materials such as polyvinylphenol can be used. In addition, a substrate in which an inorganic material such as silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, tantalum oxide, or hafnium or an organic material such as PMA or a fluorine-based polymer is further formed over such a material is used. Can do.

  Although it does not specifically limit as a shape of a board | substrate, The thing of shapes, such as a board, a sheet | seat, or a film, can be used.

(6) Substrate temperature Although it does not specifically limit as substrate temperature of sputtering film-forming, For example, it is preferable that it is room temperature-300 degreeC, and it is more preferable that it is 100-300 degreeC. However, if the substrate temperature is less than 100 ° C. and the oxygen partial pressure is 2.4 × 10 ⁻² or more, excessive oxygen may be taken into the film. Excessive oxygen may cause a decrease in carrier concentration of the oxide semiconductor thin film or cause in-plane variation. In order to form a large number of TFTs more uniformly in the surface of a large-area glass substrate, in addition to making the average crystal grain size less than 1 μm, the substrate can be used from the viewpoint of uniformity of oxygen and hydrogen concentrations in the film. More preferably, the temperature is 200 ° C. or higher. Therefore, as described above, the preferable gallium content of the oxide semiconductor thin film is more than 0.20 and not more than 0.23 in terms of the Ga / (In + Ga) atomic ratio.

(7) T-S distance The distance between the target and the substrate (T-S distance) is not particularly limited. For example, it is preferably 10 mm or more, more preferably 20 mm or more, and 30 mm or more. More preferably. On the other hand, the T-S distance is preferably 150 mm or less, more preferably 110 mm or less, and even more preferably 80 mm or less. When the distance between T and S is 10 mm or more, the thin film to be formed can be prevented from being damaged by plasma. When the T-S distance is 150 mm or less, it is possible to prevent the film formation rate from being lowered, and industrial practicality can be improved.

(8) Target The target contains indium and gallium, and includes an oxide having a gallium content of 0.15 or more and 0.30 or less in a Ga / (In + Ga) atomic ratio. The target is not particularly limited. For example, it is preferable to use an oxide sintered target, and an oxide sintered target composed of indium, gallium, and oxygen, or an oxide composed of indium, gallium, oxygen, and inevitable impurities. It is more preferable to use a sintered compact target. Furthermore, for example, an oxide sintered body target to which at least one of the positive trivalent elements boron, aluminum, scandium, and yttrium is added may be used.

Further, the oxide sintered body target is not particularly limited, but for example, it is preferable to contain at least an In ₂ O ₃ phase having a bixbite structure, and β-Ga as a generated phase other than the In ₂ O ₃ phase. GaInO _{3-phase 2} O ₃ -type structure, or _beta-Ga 2 _{O 3} -type structure GaInO ₃ phase and the (Ga, _{an in)} is more preferably constructed by 2 _{O 3} phase.

  The gallium content in the target is not particularly limited, but is preferably 0.17 or more and more preferably more than 0.20 in terms of the Ga / (In + Ga) atomic ratio. Moreover, as gallium content, it is preferable that it is 0.25 or less, and it is more preferable that it is 0.23 or less.

Although it does not specifically limit as a density of an oxide sintered compact target, For example, it is preferable that it is 6.3 g / cm < ³ > or more. When the density is 6.3 g / cm ³ or more, generation of nodules during mass production can be prevented.

  When film formation is performed by direct current sputtering film formation, good conductivity is required, so the oxide sintered compact target is preferably sintered in an oxygen-containing atmosphere, and more preferably in an oxygen atmosphere. preferable.

(9) Film thickness Although it does not specifically limit as a film thickness of the oxide thin film formed in a 1st film-forming process, For example, it is preferable that it is 10 nm or more, and it is more preferable that it is 20 nm or more. On the other hand, the film thickness is preferably 1000 nm or less, for example. When the film thickness is 1000 nm or less, when the film thickness is 10 nm or more and 1000 nm or less, the average crystal grain size of the oxide semiconductor thin film after the heat treatment can be made finer.

2-2. Second film forming step The oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is more than 3.0 × 10 ⁻² Pa and not more than 2.0 × 10 ⁻¹ Pa, and the water pressure with respect to the total pressure of all the gases in the system is 2 In an atmosphere of 0.0 × 10 ⁻³ Pa to 5.0 × 10 ⁻¹ Pa, indium and gallium are contained, and the content of gallium is 0.15 or more and 0.30 in terms of Ga / (In + Ga) atomic ratio. This is a step of forming a laminated thin film by depositing a non-crystalline second oxide thin film on the first oxide thin film by sputtering using a target containing an oxide as described below.

(1) Oxygen partial pressure In the second film forming step, the oxygen partial pressure with respect to the total pressure of the rare gas and oxygen in the atmosphere gas in the system for sputtering is set to 3.0 × 10 ⁻² Pa over 2.0 ×. Control to 10 ⁻¹ Pa or less. Moreover, as oxygen partial pressure, it is preferable to control to 3.5 * 10 ^<-2 > Pa or more and 1.0 * 10 ^{< -1} > Pa or less, for example. When the oxygen partial pressure is 3.0 × 10 ⁻² Pa or more, an oxide semiconductor thin film having an average crystal grain size after heat treatment of less than 1 μm can be obtained. On the other hand, when the oxygen partial pressure is 4.0 × 10 ⁻¹ Pa or less, by increasing the ratio of rare gas in the atmospheric gas and increasing the film formation rate, industrial practicality can be achieved. Can be increased.

(2) Film thickness Although it does not specifically limit as a film thickness of the oxide thin film formed in a 2nd film-forming process, For example, it is preferable that it is 20 nm or more. On the other hand, the film thickness is preferably 1000 nm or less, for example. When the film thickness is 1000 nm or less, the average crystal grain size of the oxide semiconductor thin film after the heat treatment can be made finer when the film thickness is 20 nm or more and 1000 nm or less.

(3) Other conditions In the second film formation step, film formation conditions other than the oxygen partial pressure (sputtering method, sputtering gas conditions, moisture pressure, substrate, substrate temperature, T-S distance, target, etc.) Is the same.

2-3. Heat treatment step An oxide thin film obtained by sputtering film formation in a non-equilibrium process is made of amorphous or microcrystal. Therefore, the obtained oxide thin film is crystallized by heat treatment. Defects can be greatly reduced by such crystallization, so that an oxide semiconductor thin film with high mobility controlled to an appropriate carrier concentration can be obtained.

(1) Heat treatment method Heat treatment or laser treatment can be used as a heat treatment method for stabilizing the structure. The heat treatment method is not particularly limited, and specifically, it is preferable to use a rapid thermal annealing method (RTA: Rapid Thermal Annealing) using infrared heating or a heat treatment method (LA: Lamp Annealing) using lamp heating. . As the laser treatment, treatment with an excimer laser or a YAG laser using a wavelength that can be absorbed by the oxide semiconductor is preferably used. Considering application to a large glass substrate, heat treatment such as RTA is more preferable.

(2) Heat treatment conditions The heat treatment temperature is not particularly limited as long as the substrate is within a range in which the substrate is not deformed or damaged. For example, the heat treatment temperature is preferably 100 ° C or higher and 500 ° C or lower, and preferably 100 ° C or higher and 450 ° C or lower. More preferred. Moreover, when using a film substrate, it is preferable that it is 100 to 300 degreeC, and it is more preferable that it is 100 to 200 degreeC. Especially, when using versatile PET film, it is preferable that it is 100 to 150 degreeC. When the heat treatment temperature is 100 ° C. or higher, the structure of the thin film after film formation can be sufficiently stabilized. Moreover, it can prevent that the board | substrate which can be used will be restrict | limited extremely because heat processing temperature is 500 degrees C or less.

  The rate of temperature increase up to the heat treatment temperature is not particularly limited, but is preferably 10 ° C./min or more, more preferably 50 ° C./min or more, for example, from the viewpoint of increasing the throughput in the manufacturing process. More preferably, it is 100 ° C./min or more.

  Although it does not specifically limit as heat processing time, For example, it is preferable that they are 1 minute or more and 120 minutes or less, and it is more preferable that they are 5 minutes or more and 60 minutes or less. The heat treatment atmosphere is not particularly limited, but for example, an oxygen-containing atmosphere is preferable. As the oxygen-containing atmosphere, for example, an atmosphere containing oxygen, ozone, water vapor, or nitrogen oxide is preferable. “Heat treatment time” refers to the time during which the heat treatment temperature is maintained.

  In addition, by combining the heat treatment temperature, the heat treatment time, the temperature raising time, and the atmosphere within the preferable ranges described above, a specific effect based on each configuration can be obtained.

(3) Etching conditions The amorphous or microcrystalline thin film after film formation and before heat treatment can be subjected to fine processing necessary for applications such as TFT by wet etching or dry etching. 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 is not particularly limited as long as it is a weak acid. For example, it is preferable to use a weak acid mainly composed of PAN or oxalic acid. More specifically, ITO-06N manufactured by Kanto Chemical Co., Ltd. can be used as the weak acid. Depending on the configuration of the TFT, dry etching can be selected.

3. Thin Film Transistor and Manufacturing Method Thereof A thin film transistor according to this embodiment includes the above-described oxide semiconductor thin film as a channel layer. In the oxide semiconductor thin film described above, the channel concentration can be reduced while the channel layer maintains high carrier mobility, so that the TFT operates stably.

  The structure of the thin film transistor is not particularly limited. For example, a thin film transistor including a source electrode, a drain electrode, a gate electrode, a channel layer, and a gate insulating film can be used.

  The thin film transistor can be manufactured by combining a conventionally known method for manufacturing a thin film transistor and a method for manufacturing an oxide semiconductor thin film. For example, a gate insulating film is formed on the surface of the gate electrode, and the oxide semiconductor thin film is formed on the surface of the gate insulating film by the above-described manufacturing method of the oxide semiconductor thin film, heat-treated, etched, and patterned. Examples include a method of forming a (channel layer) and then forming a patterned source electrode and drain electrode on the surface of the oxide semiconductor thin film (channel layer).

As a method of forming a gate insulating film on the surface of the gate electrode, for example, a method of forming a SiO ₂ film (gate insulating film) on the surface of a conductive Si substrate (gate electrode) by thermal oxidation, a metal film, ITO conductive oxide thin film and the like, conductive nitride or the like on the surface of the (gate electrode), a nitride film of the SiO ₂ film or the like is added an oxide thin film and further nitrogen such as SiO ₂ film by a CVD method, a sputtering method, or the like A method of forming (gate insulating film) can be given.

  As a method for forming the source electrode and the drain electrode on the surface of the oxide semiconductor thin film (channel layer), for example, a metal thin film such as Mo, Al, Ta, Ti, Au, Pt or the like is formed by DC magnetron sputtering. Metal alloy thin films, conductive oxide nitride thin films of these metals, various conductive polymer materials, or methods for depositing ITO or the like for transparent TFTs can be used.

  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 fabricated 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) capable of measuring gas pressure up to 1.3 Pa. . An oxide sintered compact target made of indium and gallium was used as the target. The target gallium content was 0.20 in terms of the Ga / (In + Ga) atomic ratio. This target can contain a trace amount of inevitable impurities. In actual film formation, an oxide thin film was formed after 10 minutes of pre-sputtering. Details of the film forming conditions are shown below.

[Sputtering conditions for the first film forming step]
Oxygen partial pressure: 1.2 × 10 ⁻² Pa
Water pressure: 6.5 × 10 ⁻² Pa
Film thickness 10nm

[Sputtering conditions for the second film formation step]
Oxygen partial pressure: 6.0 × 10 ⁻² Pa
Water pressure: 6.5 × 10 ⁻² Pa
Film thickness 40nm

[Sputtering conditions common to the first film-forming process and the second film-forming process]
Substrate temperature: 200 ° C
Ultimate vacuum: less than 3 × 10 ⁻⁵ Pa Target-substrate (TS) distance: 60 mm
Sputtering gas total pressure: 0.6Pa
Input power: Direct current (DC) 300W
Total film thickness: 50 nm

Subsequently, an oxide semiconductor thin film was obtained by subjecting the oxide thin film after film formation to heat treatment under the following conditions using a RTA (Rapid Thermal Annealing) apparatus.
[Heat treatment conditions]
Heat treatment temperature: 340 ° C
Atmosphere: Air Temperature rising rate: 500 ° C / min

<Characteristic evaluation of oxide semiconductor thin film>
The composition of the oxide thin film was confirmed to be the same composition as the sintered compact target 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 mobility were determined by a Hall effect measuring device (manufactured by Toyo Technica). The crystal structure of the oxide semiconductor thin film was confirmed by X-ray diffraction measurement (manufactured by Philips). The average crystal grain size was measured with a transmission electron microscope (HF-2200 manufactured by Hitachi High-Technologies Corporation). Table 1 shows the results.
(Examples 2-6, Comparative Example 1)
Except for changing the film thicknesses of the first oxide thin film and the second oxide thin film, the oxygen partial pressure and the water pressure of the first film forming process and the second film forming structure as shown in Table 1, An oxide semiconductor thin film was prepared and evaluated in the same manner as in Example 1. Table 1 shows these results.

In Examples 1 to 6, the oxygen partial pressure in the first film formation step is 1.0 × 10 ⁻³ Pa or more and 3.0 × 10 ⁻² Pa or less, and the moisture pressure is 2.0 × 10 ⁻³ Pa or more. The oxygen partial pressure in the second film forming step is controlled to 5.0 × 10 ⁻¹ Pa or less, the oxygen partial pressure is more than 3.0 × 10 ⁻² Pa and 2.0 × 10 ⁻¹ Pa or less, and the water pressure is 2.0. An oxide semiconductor thin film containing indium, gallium, and hydrogen was obtained by performing film formation in two stages while controlling the pressure from × 10 ⁻³ Pa to 5.0 × 10 ⁻¹ Pa. Such an oxide semiconductor film was crystallized by heat treatment to be a single layer. Moreover, such an oxide semiconductor thin film had a bixbite structure, and the average crystal grain size of the crystal grains was less than 1 μm.

  On the other hand, in Comparative Example 1, the average crystal grain size of the oxide semiconductor thin film was 2.7 μm by performing the film formation in one stage.

(Examples 7-12, Comparative Examples 3-4)
The gallium content of the target used for sputtering film formation in the first film formation process and the second film formation process is changed to 0.15 in terms of the Ga / (In + Ga) atomic ratio, and accordingly, the first film formation process and the second film formation process are performed. An oxide semiconductor thin film was produced and evaluated in the same manner as in Example 2 except that the substrate temperature in the film formation step and the heat treatment temperature in the heat treatment step were changed as shown in Table 1. Table 1 shows these results.

  In Examples 7 to 11, the oxide semiconductor thin film had a bixbite structure, and the average crystal grain size of the crystal grains was less than 1 μm.

  In Example 12, different targets were used in the first film forming step and the second oxide thin film. Specifically, the Ga / (In + Ga) atom number ratio in the first film forming step was set to 0.17, whereas the Ga / (In + Ga) atom number ratio in the second film forming step was set to 0.20. . The average crystal grain size of the crystal grains of the obtained oxide semiconductor thin film was less than 1 μm.

  On the other hand, in Comparative Example 2, the film was formed in one stage, and the gallium content of the target used for the sputtering film formation was 0.08 in terms of the Ga / (In + Ga) atomic ratio. In addition, Example 8 described in patent document 3 was referred for the film-forming conditions. The obtained oxide semiconductor thin film had a bixbite structure, but the average crystal grain size of the crystal grains was 1 μm or more.

  In Comparative Example 4, the gallium content of the target used for sputtering film formation in the first film formation step and the second film formation step was 0.08 in terms of the Ga / (In + Ga) atomic ratio. Although the obtained oxide semiconductor thin film had a bixbite structure, the average grain size of the crystal grains was 1 μm or more as in Comparative Example 2. That is, when the gallium content of the target used for sputtering deposition of the first oxide thin film is 0.08 in terms of the Ga / (In + Ga) atomic ratio, the average crystal grain size of the oxide semiconductor thin film is 1 μm or more. I found out that

  In Comparative Example 3, the gallium content of the target used for sputtering film formation in the first film formation step and the second film formation step was 0.35 in terms of the Ga / (In + Ga) atomic ratio. The obtained oxide semiconductor thin film had a bixbite structure, and the average crystal grain size of the crystal grains was less than 1 μm, but because of the high resistance, the Hall effect measurement could not be performed.

  1 is a TEM image of the oxide semiconductor thin film of Example 1. FIG. In Example 1, it is clear that the average crystal grain size is controlled to be less than 1 μm.

  2A and 2B are TEM images of a cross section of the oxide semiconductor thin film of Example 1. FIG. In FIG. 2A, a portion having a strong contrast (portion close to black) is one crystal grain. In FIG. 2B, it was confirmed that there were no grain boundaries at the top and bottom of the film. In Example 1, the oxide thin film having a two-layer structure was obtained by performing sputtering in two stages, but it was found that an oxide semiconductor thin film having a single layer structure was obtained after the heat treatment.

<Production of thin film transistor and evaluation of operating characteristics>
Next, a thin film transistor was manufactured and evaluated by the process described below.

(Example 13)
A thin film transistor (TFT) 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. On the SiO ₂ gate insulating film, an oxide thin film (Ga / (In + Ga) atomic ratio = 0.20) similar to that in Example 1 was stacked.

  The oxide thin film thus laminated was patterned 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, 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, thereby forming a source electrode and a drain made of an Au / Ti laminated film. An electrode was formed. Patterning was performed by a lift-off method, and a source electrode and a drain electrode were formed so as to have a channel length of 20 μm and a channel width of 500 μm.

  Finally, the laminated oxide thin film was heat-treated under the same conditions as in Example 1 to crystallize and form a single layer. Thereby, a thin film transistor including a crystalline oxide semiconductor thin film as a channel layer was obtained.

The operating characteristics of the thin film transistor were evaluated using a semiconductor parameter analyzer (manufactured by Agilent). As a result, operation characteristics as a thin film transistor were confirmed. The thin film transistor of Example 13 was confirmed to have a favorable value with a field effect mobility of 48 cm ² V ⁻¹ sec ⁻¹ , an on / off ratio of 6 × 10 ⁸ , and an S value of 0.29.

Claims (3)

The oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is 1.0 × 10 ⁻³ Pa to 3.0 × 10 ⁻² Pa and the water pressure with respect to the total pressure of all the gases in the system is 2.0 × 10 ⁻ Oxidation containing indium and gallium in an atmosphere of ³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, and the gallium content is Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less A first film forming step of forming a non-crystalline first oxide thin film by sputtering using a target including an object,
The oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is more than 3.0 × 10 ⁻² Pa and not more than 2.0 × 10 ⁻¹ Pa, and the water pressure with respect to the total pressure of all the gases in the system is 2.0 × 10 ⁻ Oxidation containing indium and gallium in an atmosphere of ³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, and the gallium content is Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less A second film forming step of forming a laminated thin film by forming a non-crystalline second oxide thin film on the first oxide thin film by a sputtering method using a target including an object;
A method of manufacturing an oxide semiconductor thin film, comprising: a heat treatment step of heat-treating the laminated thin film. The method for manufacturing an oxide semiconductor thin film according to claim 1, wherein the heat treatment step is performed in an oxygen-containing atmosphere. The oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is 1.0 × 10 ⁻³ Pa to 3.0 × 10 ⁻² Pa and the water pressure with respect to the total pressure of all the gases in the system is 2.0 × 10 ⁻ Oxidation containing indium and gallium in an atmosphere of ³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, and the gallium content is Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less A first film forming step of forming a first oxide thin film by sputtering using a target including an object,
The oxygen partial pressure with respect to the total pressure of the rare gas and oxygen is more than 3.0 × 10 ⁻² Pa and not more than 2.0 × 10 ⁻¹ Pa, and the water pressure with respect to the total pressure of all the gases in the system is 2.0 × 10 ⁻ Oxidation containing indium and gallium in an atmosphere of ³ Pa or more and 5.0 × 10 ⁻¹ Pa or less, and the gallium content is Ga / (In + Ga) atomic ratio of 0.15 or more and 0.30 or less A second film forming step of forming a laminated thin film by forming a second oxide thin film on the first oxide thin film by a sputtering method using a target including an object;
A method of manufacturing a thin film transistor, comprising: a heat treatment step of heat-treating the laminated thin film.