Oxide semiconductor film and thin-film transistor

Abstract

The purpose of the present disclosure is to provide a crystalline oxide semiconductor film having high carrier mobility.　An oxide semiconductor film according to one aspect of the present disclosure is disposed on a substrate, said oxide semiconductor film comprising In as an element and having a grain boundary line density of not more than 27 / μm, wherein the grain boundary line density is a value found by EBSD analysis in which a boundary having an orientation difference of 2° or greater is considered a grain boundary.

Description

Oxide semiconductor films and thin-film transistors

[0001] This disclosure relates to oxide semiconductor films and thin-film transistors.

[0002] In recent years, flat panel displays such as liquid crystal displays (LCDs) and organic electroluminescent displays (OLEDs) have been widely used in smartphones, tablets, laptop computers, monitors, and televisions. Thin-film transistors (TFTs) are used to drive each pixel in flat panel displays.

[0003] Conventionally, amorphous oxide semiconductor films have been widely used as oxide semiconductor films in thin-film transistors. However, in oxide semiconductor films with an amorphous structure, carriers are formed by electrons generated by oxygen vacancies, which tends to result in high carrier density. Therefore, when amorphous oxide semiconductor films are used in thin-film transistors with small channel lengths and widths, the electrical properties may not be stable.

[0004] From this perspective, crystalline oxide semiconductor films, which tend to have lower carrier densities compared to amorphous oxide semiconductor films, are attracting attention today (see Patent Documents 1-3). However, crystalline oxide semiconductor films have the challenge of being difficult to achieve high-speed carrier mobility.

[0005] International Publication No. 2017 / 017966, International Publication No. 2018 / 143073, Japanese Patent Publication No. 2012-253315

[0006] Patent Document 1 describes a crystalline oxide semiconductor film mainly composed of indium oxide, formed by deposition and heating without introducing impurities such as water, and containing surface crystal grains having a single crystal orientation. Patent Document 1 also describes including a trivalent metal element other than indium in a concentration of more than 8 atomic percent and up to 17 atomic percent relative to the total metal content.

[0007] Patent Document 2 describes a crystalline oxide semiconductor film mainly composed of indium oxide, containing surface crystal grains having a single crystal orientation, and having a band gap of 3.90 eV or more. Patent Document 2 describes forming a protective film on an oxide semiconductor film without performing heat treatment in an oxidizing atmosphere on the oxide semiconductor film. Patent Document 2 describes that by not performing heat treatment before forming the protective film, the carrier density of the oxide semiconductor film can be increased compared to when heat treatment is performed, and the band gap can be made 3.90 eV or more. Furthermore, Patent Document 2 describes that it is preferable to include trivalent metal elements other than indium in amounts of more than 7 atomic percent and 15 atomic percent or less relative to the total metal content.

[0008] Patent Document 3 describes a crystalline oxide layer laminated on an insulating layer, wherein the carrier density of the oxide layer is 10 １８ / cm ３ The following describes a laminated structure in which the average crystal grain size is 1 μm or more, and the crystals of the oxide layer are arranged columnarly on the surface of the insulating layer. Patent Document 3 describes that by forming an amorphous oxide thin film on an insulating layer and then heating and crystallizing it, the direction of crystal arrangement and crystal grain size can be made homogeneous. Patent Document 3 describes that the average crystal grain size in the oxide layer is determined by performing azimuthal angle mapping of EBSD and analyzing the region surrounded by components with an azimuthal difference of 15° or more as crystal grains. Furthermore, Patent Document 3 describes that indium oxide, Ga-doped indium oxide, Al-doped indium oxide, Zn-doped indium oxide, and Sn-doped indium oxide are preferred as materials constituting the oxide layer.

[0009] Patent documents 1 and 2 describe how carrier density can be stabilized by including surface crystal particles having a single crystal orientation. Furthermore, patent document 3 describes how field-effect mobility can be improved and TFTs with good S values ​​can be reproducibly formed by controlling the crystal arrangement direction and particle size.

[0010] Meanwhile, the inventors diligently studied how to achieve high-speed carrier mobility in crystalline oxide semiconductor films. To improve carrier mobility in crystalline oxide semiconductor films, it is important to reduce grain boundaries that act as barriers to electron movement and to create crystals with low electron scattering factors. The inventors found that, considering that electrons move along an electric field in a semiconductor, it is important to reduce the density of boundaries (including unclosed grain boundaries) contained in a unit area than the size of the crystal grain (the area of ​​the crystal surrounded by closed grain boundaries), and that it is important to define boundaries with an orientation difference of 2° or more as grain boundaries using EBSD analysis. Patent document 3 describes performing EBSD azimuth mapping and analyzing the region surrounded by components with an orientation difference of 15° or more as crystal grains. However, the crystal grains described in cited document 3 do not represent the density of boundaries contained in a unit area. Furthermore, according to the inventors' research results, it was found that even if the orientation difference is less than 15°, boundaries with an orientation difference of 2° or more have an adverse effect on carrier mobility.

[0011] This disclosure is made in light of these circumstances and aims to provide a crystalline oxide semiconductor film with high carrier mobility.

[0012] An oxide semiconductor film according to one aspect of this disclosure is an oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, and the grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries.

[0013] An oxide semiconductor film according to one aspect of this disclosure can increase carrier mobility.

[0014] Figure 1 is a schematic cross-sectional view showing a thin-film transistor according to one embodiment of the present disclosure. Figure 2 is a graph showing the relationship between grain boundary line density and carrier mobility analyzed by EBSD analysis. Figure 3 is a graph showing the measurement results of the static characteristics (Id-Vg characteristics) of thin-film transistor No. 1. Figure 4 is an SEM backscattered electron image of oxide semiconductor films No. 1, No. 3, No. 4, and No. 6. Figure 5 is an SEM backscattered electron image of oxide semiconductor films No. 1, No. 7, and No. 8. Figure 6 is an example of the grain boundary line density measurement results by EBSD analysis in No. 1. Figure 7 is an example of the grain boundary line density measurement results using the SEM reflection image in No. 1 (left: backscattered electron image (original image), right: image analysis result).

[0015] [Description of Embodiments of the Disclosure] First, embodiments of the Disclosure will be listed and described.

[0016] (1) An oxide semiconductor film according to one aspect of the present disclosure is an oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, and the grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries.

[0017] According to the inventors' findings, the carrier mobility in a crystalline oxide semiconductor film correlates with the grain boundary line density, which is determined by EBSD (Electron BackScatter Diffraction Pattern) analysis using boundaries with an orientation difference of 2° or more as grain boundaries. The oxide semiconductor film in question contains In as an element, has a grain boundary line density of 27 / μm or less, and since the above grain boundary line density is a value determined by EBSD analysis using boundaries with an orientation difference of 2° or more as grain boundaries, the carrier mobility can be increased.

[0018] (2) In the above (1), the oxide semiconductor film has a carrier surface density of 6 × 10 １２ cm －２ The following configuration is preferable. This configuration allows for stable driving of thin-film transistors.

[0019] (3) In (1) or (2) above, the oxide semiconductor film is preferably of average thickness of 10 nm or more and 45 nm or less. With this configuration, the grain boundary line density can be easily controlled within a desired range. Furthermore, with this configuration, it is easy to reduce the carrier surface density of the oxide semiconductor film.

[0020] (4) A thin-film transistor according to another aspect of the present disclosure comprises an oxide semiconductor film as described in any of (1) to (3) above.

[0021] Because the thin-film transistor is equipped with the oxide semiconductor film, it has high carrier mobility.

[0022] In this disclosure, "grain boundary line density" means a value measured by the following methods (1) or (2): (1) Perform EBSD analysis, remove unmeasurable points and measurement points with low data reliability, derive adjacent analysis points with an orientation difference of 2° or more from the obtained measurement points, and sum the lengths of the edges shared by the analysis points (here, "edge" includes not only closed boundaries (boundaries that can be traced around) but also open boundaries (boundaries that cannot be traced around)). Then calculate Σ (length of edges shared by analysis points with an orientation difference of 2° or more) / Σ (analysis points), and the obtained value is taken as the grain boundary line density. (2) Set observation conditions so that contrast due to differences in crystal orientation (channeling contrast) can be obtained in the SEM (Scanning Electron Microscope) reflection image, and then acquire the SEM reflection image. The obtained image is color-coded in black and white based on Otsu's binarization method. The number of pixels corresponding to the white edge and black edge of the painted boundary is measured, and the total number of these pixels is divided by 2 to obtain the boundary pixel count. The length density is obtained by dividing the obtained boundary pixel count by the total number of pixels in the image. This length density correlates with the grain boundary line density obtained in (1), and can be converted back to the grain boundary line density in (1) by linear approximation (see Figures 6 and 7). "Average thickness" means the average value of the thickness of any five points.

[0023] [Details of Embodiments of the Disclosure] Embodiments of the Disclosure will be described in detail below with reference to the drawings as appropriate. Note that for the numerical values ​​described herein, it is possible to adopt only one of the upper and lower limits, or to combine the upper and lower limits as desired. Also, Figure 1 is schematic and may not correspond to the actual dimensions, shape, etc.

[0024] <Oxide Semiconductor Film> An oxide semiconductor film according to one aspect of this disclosure is disposed on a substrate. More specifically, the oxide semiconductor film is disposed on a substrate in a thin-film transistor. The oxide semiconductor film is polycrystalline. The oxide semiconductor film contains indium (In) as an element. The oxide semiconductor film has a grain boundary line density of 27 / μm or less, and the above grain boundary line density is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. Note that "disposed on a substrate" includes not only configurations in which the film is directly disposed on the substrate, but also configurations in which it is disposed on the substrate via other layers (configurations in which it is indirectly disposed on the substrate).

[0025] To improve carrier mobility in crystalline oxide semiconductor films, it is desirable to suppress electron scattering within the oxide semiconductor film. Specifically, it is desirable to suppress scattering within crystal grains, such as phonon scattering, scattering due to defects such as oxygen vacancies (defect scattering), and scattering due to impurities such as dopants (impurity scattering), as well as scattering at crystal grain boundaries.

[0026] In this regard, Patent Documents 1 and 2 state that it is preferable to include 7 atomic percent or more of Al (aluminum), Ga (gallium), etc., in order to reduce scattering at grain boundaries. However, it is anticipated that adding large amounts of other elements in this way will make impurity scattering more likely. Furthermore, Patent Document 3 describes a technique for determining the average grain size using EBSD analysis, where boundaries with an orientation difference of 15° or more are considered grain boundaries. However, it is difficult to sufficiently increase carrier mobility using this technique.

[0027] Taking the above-mentioned disadvantages into consideration, the inventors diligently studied how to increase the proportion of In in the composition while simultaneously improving the carrier mobility.

[0028] When the oxide semiconductor film contains In as an element and the grain boundary line density is 27 / μm or less, and the grain boundary line density is a value obtained by determining a boundary having an orientation difference of 2° or more as a grain boundary by EBSD analysis, the carrier mobility can be increased. More specifically, even if the area of the crystal grains is the same, if the shape of the crystal grains is closer to a circle, the length of the grain boundary is shorter, and the more the shape deviates from a circle, the longer the grain boundary length becomes. Further, since electrons related to conduction are pulled by an electric field, it is presumed that when there are boundaries (finite line segments) with different orientations, they cannot bypass the boundaries. From such a viewpoint, it is considered that the carrier mobility can be increased by setting the grain boundary line density obtained by determining a boundary having an orientation difference of 2° or more as a grain boundary by EBSD analysis to 27 / μm or less. Further, since the oxide semiconductor film is a polycrystal, it is easy to keep the carrier density low.

[0029] (In) In is an element that contributes to the improvement of conductivity (electrical conductivity). The greater the content of In, the more the conductivity of the oxide semiconductor film is improved and the carrier mobility is improved. The lower limit of the content of In in all metal elements may be 90 atomic%, may be 92 atomic%, or may be 95 atomic% from the viewpoint of sufficiently increasing the carrier mobility. Also, the upper limit of the content of In in all metal elements may be 100 atomic% or may be 98 atomic%.

[0030] In the oxide semiconductor film, the elements other than In may be O (oxygen) and unavoidable impurities. According to this configuration, the acceleration of the carrier mobility based on In can be utilized to the limit. ２ O ３ The acceleration of the carrier mobility based on In can be utilized to the limit.

[0031] (Unavoidable impurities) The unavoidable impurities may be contained due to raw materials, materials, manufacturing equipment, etc. Examples of such unavoidable impurities include Al, Pb, Si, Fe, Ni, Ti, Mg, Cr, Zr, W, Ta, and rare earths. The content of unavoidable impurities in the oxide semiconductor film is preferably 1 mass% or less for each element, and more preferably 500 mass ppm or less.

[0032] (Other Elements) On the other hand, the oxide semiconductor film can also actively contain elements other than In (hereinafter also referred to as "other elements") within a certain range. Examples of the above other elements include B (boron), Fe (iron), Al, and Ga. Further, examples of the above other elements can also include Zn, Mg, La, Nb, Hf, Gd, Dy, Ce, Nd, Ti, V, Mo, Mn, Co, Ni, Si, Bi, Ge, Zr, Ta, W, Ag, and Ru. The content of each of these elements in all the metal elements can be set within a range where the content of In is not insufficient, and the upper limit thereof may be, for example, 7 atomic %, 5 atomic %, or 4.5 atomic %.

[0033] As described above, the oxide semiconductor film is polycrystalline. By being polycrystalline, the oxide semiconductor film can suppress the carrier density to a small value. More specifically, indium oxide is said to easily increase carriers due to oxygen deficiency, hydrogen, etc. If the carrier density increases too much, the oxide semiconductor film behaves like a conductor, and the transistor cannot enter the OFF state. In this regard, by crystallizing indium oxide, it is considered that defects are less likely to occur in the film. As a result, it is considered that an excessive increase in the carrier density can be suppressed, and the switching of the transistor can be stably achieved.

[0034] As the upper limit of the carrier surface density of the oxide semiconductor film, from the viewpoint of enabling it to function well as a switching element in a thin-film transistor, 1×10 １３ cm －２ is preferable, and 6×10 １２ cm －２ is more preferable. In order to drive a thin-film transistor, it is necessary to deplete the channel region during OFF. To deplete it, it is necessary to push out the carriers contained in the channel region by an electric field, so it is presumed that the absolute amount of carriers (carrier surface density) has an influence. Therefore, by setting the carrier surface density below the above upper limit, the thin-film transistor can be stably driven. On the other hand, as the lower limit of the carrier surface density, for example, 1×10 １１ cm－２ This can be done. Note that carrier surface density [cm －２ ] is the carrier density [cm²] －３ This refers to the value obtained by integrating ] in the thickness direction. In this disclosure, the carrier surface density can be determined by carrier density × average thickness.

[0035] As described above, the oxide semiconductor film has a high In content. Therefore, although the oxide semiconductor film is polycrystalline, it tends to have a high carrier surface density. On the other hand, by controlling the average thickness of the oxide semiconductor film, the carrier mobility and carrier surface density can be controlled more reliably.

[0036] Furthermore, in order to improve carrier mobility in the oxide semiconductor film, it is desirable to have a low grain boundary line density, which is determined by EBSD analysis using boundaries with an orientation difference of 2° or more as grain boundaries. In this regard, if the average thickness of the oxide semiconductor film is large, crystallization occurs during film formation, and even if heat treatment is applied at around 400°C or below afterward, the crystallization from the time of film formation is maintained, making it difficult for crystal grains to grow and preventing a decrease in grain boundary line density. As a result, a large number of grain boundaries that hinder electron movement are generated, making it difficult to increase carrier mobility. Moreover, if crystallization occurs during film formation, patterning with chemicals becomes difficult, making the manufacture of thin-film transistors itself difficult.

[0037] In contrast, by reducing the average thickness of the oxide semiconductor film, it becomes easier to maintain an amorphous state immediately after deposition. Therefore, it becomes easier to grow crystals in the in-plane direction during subsequent heat treatment crystallization.

[0038] The upper limit of the average thickness of the oxide semiconductor film is preferably 45 nm, more preferably 40 nm, even more preferably 35 nm, and may also be 30 nm or 25 nm, from the viewpoint of suppressing the carrier plane density and easily increasing the grain boundary line density. On the other hand, the lower limit of the average thickness may be, for example, 10 nm or 15 nm.

[0039] As described above, in this disclosure, the grain boundary line density of the oxide semiconductor film is a value obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries. Because the oxide semiconductor film has a high In content, the nucleus density increases during crystallization, and grain boundary scattering tends to occur easily. In this regard, the oxide semiconductor film can form crystals with a low scattering factor if the grain boundary line density obtained by EBSD analysis, where boundaries with an orientation difference of 2° or more are defined as grain boundaries, is 27 / μm or less. Furthermore, according to the inventors' findings, boundaries with an orientation difference of less than 2° do not substantially affect carrier mobility. As an upper limit for the grain boundary line density, from the viewpoint of reducing grain boundaries that act as barriers to electron transfer, 26 / μm or less is preferred, and 22 / μm or less is more preferred. On the other hand, the lower limit for the grain boundary line density is not particularly limited, but for example, it may be 2 / μm, preferably 2.5 / μm, or more preferably 3 / μm.

[0040] The crystalline form of the oxide semiconductor film may be faceted or radial (dendrite-like).

[0041] <Method for Manufacturing an Oxide Semiconductor Film> The oxide semiconductor film is deposited, for example, by a sputtering method using a sputtering target. The sputtering target may also be formed using an oxide sintered body. After deposition, the oxide semiconductor film is patterned by photolithography or the like. The oxide semiconductor film is then heat-treated before or after patterning to form a polycrystalline structure. This heat treatment may be an annealing treatment performed after patterning to improve the film quality. The temperature of the heat treatment is not particularly limited, as long as it is a temperature at which almost the entire film crystallizes, but for example, it may be between 300°C and 450°C. The heat treatment may be performed, for example, in an atmospheric environment.

[0042] <Oxide Sintered Body> An oxide sintered body according to one aspect of the present disclosure contains In as an element. More specifically, the oxide sintered body has the same chemical composition as the oxide semiconductor film. The oxide sintered body may be formed, for example, through a step of weighing raw material powder (weighing step S1), a step of drying and granulating the raw material powder weighed in weighing step S1 (drying and granulation step S2), a step of molding the granulated powder obtained in drying and granulation step S2 (molding step S3), and a step of sintering the molded powder obtained in molding step S3 (sintering step S4).

[0043] <Sputtering Target> A sputtering target according to one aspect of this disclosure contains In as an element. More specifically, the sputtering target has the same chemical composition as the oxide semiconductor film. The shape of the target in the sputtering target can be appropriately set according to the shape and structure of the sputtering apparatus. For example, the shape of the target can be a rectangular plate, a circular plate, a cylindrical plate, etc.

[0044] In the sputtering method, the carrier gas is, for example, a mixed gas of argon and oxygen (Ar / O). ２ A mixed gas can be used. As the carrier gas, for example, a mixed gas containing hydrogen gas in addition to argon gas and oxygen gas (Ar / O ２ / H ２ A mixed gas can also be used, but this complicates the apparatus configuration, so a more common Ar / O2 mixture is preferred. ２ It is preferable to use a mixed gas. The stage temperature during film formation can be, for example, within the range of room temperature or higher and 200°C or lower.

[0045] <Thin-Film Transistor> An example of a thin-film transistor comprising the oxide semiconductor film is shown in Figure 1. Figure 1 shows a bottom-gate type thin-film transistor 10, and more specifically, a bottom-gate type thin-film transistor with an etch-stop structure. However, the thin-film transistor in this disclosure is not limited to the structure shown in Figure 1. For example, in the case of a bottom-gate type, the thin-film transistor may have a back-channel etch structure. The thin-film transistor may also be, for example, a top-gate type, and in terms of the number of gates, it may be a double-gate type. In the case of a top-gate type, the thin-film transistor may have, for example, a coplanar structure.

[0046] Because the thin-film transistor 10 is equipped with the oxide semiconductor film, it has high carrier mobility. Furthermore, because the thin-film transistor 10 is equipped with the oxide semiconductor film, it is easy to keep the carrier density low.

[0047] The thin-film transistor 10 has a structure in which a substrate 1, a gate electrode 2, a gate insulating film 3, the oxide semiconductor film 4, a protective layer 5 (ESL protective film), source and drain electrodes 6 (source electrode 6a and drain electrode 6b), and an insulating film 7 (passivation insulating film) are stacked in this order. The thin-film transistor 10 also includes a conductive film 8 which is disposed on the insulating film 7 and connected to the drain electrode 6b via a contact hole 7a provided in the insulating film 7. In the thin-film transistor 10, the oxide semiconductor film 4 is indirectly disposed on the substrate 1 via the gate electrode 2 and the gate insulating film 3.

[0048] (Substrate) The substrate 1 is not particularly limited, but examples include transparent substrates such as glass substrates and silicone resin substrates. The glass used for the glass substrate is not particularly limited, but examples include alkali-free glass, high-strain point glass, and soda-lime glass. In addition, a metal substrate such as a stainless steel thin film and a resin substrate such as a polyethylene terephthalate (PET) film can also be used as the substrate 1.

[0049] (Gate Socket) The gate socket 2 is conductive. The thin film constituting the gate socket 2 is not particularly limited, but examples include an Al alloy or an Al alloy with thin films or alloy films of Mo, Cu, Ti laminated on the surface, or a Cu alloy with thin films or alloy films of Mo, Ti laminated on the surface.

[0050] (Gate insulating film) The gate insulating film 3 is laminated on the substrate 1 so as to cover the gate electrode 2. The thin film constituting the gate insulating film 3 is not particularly limited, but a silicon oxide film (SiO ２ Film), silicon nitride film (SiN film), silicon oxynitride film, Al ２ O ３ Ya Y ２ O ３ Examples include metal oxide films such as those mentioned above. Furthermore, the gate insulating film 3 may be a single-layer structure of these thin films, or it may be a multilayer structure in which two or more thin films are stacked.

[0051] (Oxide semiconductor film) The structure of the oxide semiconductor film 4 is as described above.

[0052] (Protective layer) The protective layer 5 prevents damage to the oxide semiconductor film 4 when the source and drain electrodes 6 are formed by etching. The thin film constituting the protective layer 5 is not particularly limited, but an example is a silicon oxide film.

[0053] (Source and Drain Electrodes) The source and drain electrodes 6 cover a portion of the gate insulating film 3 and the protective layer 5, and are electrically connected to the oxide semiconductor film 4 at both ends of the channel of the oxide semiconductor film 4. A drain current of the thin-film transistor 10 flows between the source electrode 6a and the drain electrode 6b, depending on the voltage between the gate electrode 2 and the source electrode 6a, and the voltage between the source electrode 6a and the drain electrode 6b. The thin films constituting the source electrode 6a and the drain electrode 6b are not particularly limited as long as they are conductive, and for example, a thin film similar to that of the gate electrode 2 can be used.

[0054] (Insulating Film) The insulating film 7 covers the gate electrode 2, the gate insulating film 3, the oxide semiconductor film 4, the protective film 5, and the source and drain electrodes 6, preventing the degradation of the characteristics of the thin-film transistor 10. The thin film constituting the insulating film 7 is not particularly limited, but a silicon nitride film can be used because its sheet resistance can be relatively easily controlled by its hydrogen content. The insulating film 7 may also be a two-layer structure, for example, a silicon oxide film and a silicon nitride film.

[0055] The insulating film 7 has a contact hole 7a that extends to the drain electrode 6b. The contact hole 7a penetrates the insulating film 7 in the thickness direction.

[0056] (Conductive film) The conductive film 8 is filled into the contact hole 7a. The conductive film 8 constitutes wiring for acquiring drain current. The conductive film 8 is not particularly limited, and a thin film similar to that of the gate electrode 2 can be used. Among these, a transparent conductive film suitable for display applications is preferred. Examples of such transparent conductive films include ITO film, ZnO film, and IZO film.

[0057] [Other Embodiments] The above embodiments do not limit the configuration of the present invention. Therefore, the above embodiments allow for the omission, substitution, or addition of components of each part of the above embodiments based on the description herein and common technical knowledge, and all such omissions, substitutions, or additions should be interpreted as falling within the scope of the present invention.

[0058] The present disclosure will be described in detail below based on examples, but the present disclosure should not be construed as being limited based on the description of these examples.

[0059] [Fabrication of Oxide Semiconductor Films] A crystalline oxide semiconductor film with an average thickness of 15 nm made of InO film (No. 1), a crystalline oxide semiconductor film with an average thickness of 40 nm made of InO film (No. 2), a crystalline oxide semiconductor film with an average thickness of 100 nm made of InO film (No. 3), a crystalline oxide semiconductor film with an average thickness of 100 nm made of InO film with 8.5 atomic% of Al added (No. 4), a crystalline oxide semiconductor film with an average thickness of 15 nm made of InO film (No. 5), and a film with 10 atomic% of Ga added. A crystalline oxide semiconductor film with an average thickness of 100 nm consisting of an InO film (No. 6), a crystalline oxide semiconductor film with an average thickness of 23 nm consisting of an InO film with 1 atomic% of B added (No. 7), and an IBO / IGBO multilayer film (No. 8) consisting of an InO film with 1 atomic% of B added and stacked on an IGBO film with an average thickness of 10 nm were arranged in the bottom-gate thin-film transistor shown in Figure 1 by DC magnetron sputtering. The quality of the obtained oxide semiconductor films is shown in Table 1. The content [atomic %] of each element in Table 1 was calculated from the composition of the target used for simultaneous discharge during film deposition and the deposition rate of each target. The composition of the target was determined by XRF (X-ray fluorescence analysis). The configuration of each part and the manufacturing conditions of the oxide semiconductor films are as follows. Crystalline oxide semiconductor films No. 3, 4, and 6 crystallized immediately after deposition. The reason for this is presumed to be that the large average thickness resulted in a longer film deposition time, and the rise in substrate temperature during deposition caused crystallization. On the other hand, all crystalline oxide semiconductor films other than those mentioned above were in an amorphous state before heat treatment 1. (Sputtering conditions) Atmosphere gas: Ar / O ２ Mixed gas (oxygen partial pressure ratio 4%) Film deposition pressure: 1 mTorr Stage temperature: Room temperature (no heating) (Heat treatment conditions) Heat treatment 1: Nos. 1 to 4 are heated in air with added water vapor by bubbling into water (350°C), Nos. 5 to 8 are annealed in air (350°C) Heat treatment 2: 250°C, 30 minutes (after device fabrication) (common to Nos. 1 to 8)

[0060] <Grain Boundary Line Density> The crystal morphology of the oxide semiconductor films No. 1 to No. 8 was as follows: No. 1, No. 5, and No. 7 were faceted, while No. 2, No. 3, and No. 8 were dendrite. For No. 4 and No. 6, it was difficult to distinguish due to the fineness of the crystal grains, but it is presumed to be faceted, as a certain degree of grain expansion is necessary for dendrite formation. For No. 1, 7, and 8, the grain boundary line density was determined by EBSD analysis, and for the others, by SEM reflection images, with boundaries having an orientation difference of 2° or more being considered as grain boundaries. More specifically, for No. 1, 7, and 8, EBSD analysis was performed, and after excluding measurement points that could not be measured and measurement points with low data reliability, adjacent analysis points with an orientation difference of 2° or more were derived from the obtained measurement points, and the lengths of the sides shared by the analysis points were summed. Here, "edge" includes not only closed boundaries but also open boundaries. The formula Σ(length of edges shared by analysis points with an orientation difference of 2° or more) / Σ(analysis points) was calculated, and the resulting value was defined as the grain boundary line density. For samples No. 2 to No. 6, observation conditions were set to obtain contrast due to differences in crystal orientation (channeling contrast) in the SEM reflection image, and then the SEM reflection image was acquired. The obtained image was then color-coded in black and white based on Otsu's binarization method. The number of pixels corresponding to the white and black edges of the colored boundaries was measured, and the total number of these pixels was divided by 2 to determine the boundary pixel count. The length density was then calculated by dividing the obtained boundary pixel count by the total number of pixels in the image. Finally, with the SEM length density on the x-axis and the EBSD grain boundary line density on the y-axis, the equation y = 2.3433x + 0.4071 was used, and the coefficient of determination R was set to R. ２ The grain boundary line density was calculated using a linear approximation with a value of 0.8632. The results are shown in Table 1. Figure 2 shows the relationship between grain boundary line density and carrier mobility for No. 1 to No. 8.

[0061] <Carrier Density and Carrier Surface Density> For oxide semiconductor films No. 1 to No. 8, the carrier density [cm³] －３ ] and carrier surface density [cm －２ The carrier surface density was calculated by multiplying the carrier density by the average thickness. The results are shown in Table 1.

[0062] <Evaluation of Static Characteristics> For thin-film transistors No. 1, 3, 4, and 6, the drain current (Id)-gate voltage (Vg) characteristics (Id-Vg characteristics) were measured using a prober and a semiconductor parameter analyzer (Keitley "4200SCS") under the following conditions. The measurement results of the static characteristics of No. 1 (n=3) are shown in Figure 3. Note that the oxide semiconductor films other than No. 1 had crystallized immediately after deposition, making it impossible to pattern them for TFT formation, and therefore their static characteristics were not evaluated. Gate voltage: -30V to 30V (step 0.25V) Source voltage: 0V Drain voltage: 10V Stage temperature: Room temperature (23℃)

[0063] <Field effect mobility> Field effect mobility μ ＦＥ [cm ２ [Vs] is defined as the gate voltage Vg [V], threshold voltage Vth [V], drain current Id [A], channel length L [m], channel width W [m], and gate dielectric capacitance C. ｏｘ As [F], in the saturation region of the above static characteristics (Vg > Vd - Vth), the μ shown in the following equation (1) is ＦＥ The calculation was performed using the following method. The results are shown in Table 1.

[0064] <TFT saturation mobility> From the above formula (1), the TFT saturation mobility [cm ２ The [Vs] was calculated. The TFT saturation mobility was calculated only for No. 1. The results are shown in Table 1.

[0065] <Threshold Voltage> The threshold voltage [V] is the voltage when the transistor's drain current is 10 －９ The gate voltage at which A occurs was calculated from the static characteristics described above. The threshold voltage was calculated only for No. 1. The calculation results are shown in Table 1.

[0066] <S-value> The S-value [V / decade] was calculated by determining the minimum change in gate voltage required to increase the drain current by one order of magnitude based on the static characteristics described above. The S-value was calculated only for No. 1. The calculation results are shown in Table 1.

[0067]

[0068] As shown in Figure 3 and Table 1, the oxide semiconductor film No. 1 has a grain boundary line density of 10.2 / μm, determined by EBSD analysis using boundaries with an orientation difference of 2° or more as grain boundaries, thus enabling a high field-effect mobility. Furthermore, the oxide semiconductor film No. 1 has a carrier surface density of 3.3 × 10⁻¹⁶. １２ cm －２ Therefore, the TFT saturation mobility is large, resulting in excellent switching characteristics.

[0069] [Relationship between film thickness and crystallization] Figures 4 and 5 show the relationship between film thickness (average thickness) and crystallization in oxide semiconductor films No. 1, No. 3, No. 4, and No. 6 through No. 8. Figures 4 and 5 are SEM backscattered electron images of the oxide semiconductor film surface. As shown in Figure 4, oxide semiconductor films No. 3, No. 4, and No. 6 crystallize immediately after deposition as the film thickness increases, making it difficult for crystal grains to grow even if heat treatment is applied afterward. Therefore, controlling the film thickness is considered important from the viewpoint of improving carrier mobility.

[0070] [Relationship between the addition of other elements and carrier density] As shown in Figure 5, when other elements are added to In, the crystal grains become larger compared to No. 1, suggesting that crystallization immediately after film formation can be suppressed.

[0071] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to these examples. It is clear to those skilled in the art that various modifications or alterations can be conceived within the scope of the claims, and these will naturally also fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any way without departing from the spirit of the invention.

[0072] This application is based on a Japanese patent application (Patent Application No. 2025-003090) filed on January 8, 2025, the contents of which are incorporated herein by reference.

[0073] 1. Substrate 2. Gate electrode 3. Gate insulating film 4. Oxide semiconductor film 5. Protective layer 6. Source / drain electrodes 6a. Source electrode 6b. Drain electrode 7. Insulating film 7a. Contact hole 8. Conductive film 10. Thin-film transistor

Claims

1. An oxide semiconductor film disposed on a substrate, containing In as an element, having a grain boundary line density of 27 / μm or less, wherein the grain boundary line density is determined by EBSD analysis, with boundaries having an orientation difference of 2° or more being defined as grain boundaries.

2. Carrier area density is 6 × 10 １２ cm －２ The oxide semiconductor film according to claim 1, wherein the following applies:

3. The oxide semiconductor film according to claim 1, wherein the average thickness is 10 nm or more and 45 nm or less.

4. A thin-film transistor comprising an oxide semiconductor film according to any one of claims 1 to 3.

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