Composite thin film

Abstract

An object of the present disclosure is to provide an oxide semiconductor thin film whose properties are less likely to be degraded by heat treatment.SOLUTION: The metallic elements include In, Al, and an inevitable impurity, and the number of Al atoms is 5atm% or more and 15atm% or less with respect to the total number of In and Al atoms.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This disclosure relates to oxide semiconductor thin films, composite thin films, and sputtering targets. [Background technology]

[0002] Flat display panels (FDPs) utilize semiconductors with a three-dimensional stacked structure. In these three-dimensional stacked semiconductors, thin-film transistors are formed on a metal wiring layer with an oxide semiconductor as the channel layer. Known oxide semiconductors include In-Ga-Zn-O (IGZO) oxide semiconductors containing indium, gallium, zinc, and oxygen, as well as In-Zn-O (IZO) containing indium, zinc, and oxygen, and In-Ga-O (IGO) containing indium, gallium, and oxygen (see, for example, Japanese Patent Publication No. 2017-59838 and Japanese Patent Publication No. 2014-98211).

[0003] While these oxide semiconductors exhibit high mobility, they also tend to have high carrier densities. Therefore, to ensure stable operation as a thin-film transistor (TFT), the total number of carriers passing through the channel must be kept low by reducing the film thickness, etc. For this reason, oxide semiconductors used as channel layers in thin-film transistors used in FDPs are required to be thinned to about 8 nm. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-59838 [Patent Document 2] Japanese Patent Publication No. 2014-98211 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] When heat is applied to oxide semiconductors, a decrease in carrier mobility or an increase in carrier density may occur, potentially degrading the desired properties.

[0006] Looking at the heat treatment of semiconductor processes when forming three-dimensional stacked semiconductor structures, one heat treatment applied to the metal wiring is called sintering, which is used to ensure ohmic contact between the metal wiring and silicon. The temperature for this is typically between 350°C and 500°C.

[0007] Furthermore, in recent semiconductor processes, high-dielectric-constant (high-k) materials such as HfOx are increasingly being used for gate insulating films. This is because, by having a higher dielectric constant than conventional gate insulating films such as SiOx, the actual insulating film thickness can be reduced in accordance with LSI scaling while increasing the electrical insulating film thickness, thereby reducing transistor current leakage. For example, when HfOx, a well-known high-k film, is formed by the ALD method, crystallization can occur at temperatures above 450°C. When HfOx crystallizes, current leakage is more likely to occur along grain boundaries. Therefore, the heat treatment temperature is limited to below 450°C.

[0008] Based on the above, it is desirable that oxide semiconductors do not degrade in properties at temperatures of 350°C to 500°C, which is the heat treatment temperature for metal wiring, or below 450°C, which is the heat treatment temperature when using a high-k film.

[0009] This disclosure is made based on the circumstances described above and aims to provide oxide semiconductor thin films and composite thin films that are less susceptible to degradation of properties by heat treatment, as well as sputtering targets for forming these oxide semiconductor thin films. [Means for solving the problem]

[0010] An oxide semiconductor thin film according to one aspect of this disclosure is an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm% or more and 15 atm% or less relative to the total number of In and Al atoms.

[0011] Another aspect of the present disclosure is an oxide semiconductor thin film comprising a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is greater than 15 atm% and less than 100 atm% relative to the total number of In and Al atoms.

[0012] A composite thin film according to one aspect of the present disclosure is a composite thin film composed of two oxide semiconductor thin films, an upper layer and a lower layer, wherein the upper layer is InO or an oxide semiconductor thin film according to the above aspect, and the lower layer is an oxide semiconductor thin film according to another aspect.

[0013] A sputtering target according to one aspect of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm% or more and 15 atm% or less relative to the total number of In and Al atoms.

[0014] A sputtering target according to another aspect of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is greater than 15 atm% and less than 100 atm% relative to the total number of In and Al atoms. [Effects of the Invention]

[0015] The oxide semiconductor thin films and composite thin films of this disclosure are less susceptible to degradation of properties by heat treatment. Furthermore, the sputtering target of this disclosure can form oxide semiconductor thin films that are less susceptible to degradation of properties by heat treatment. [Brief explanation of the drawing]

[0016] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a composite thin film according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a graph showing the temperature dependence of the hole mobility of a single film in an example. [Figure 3] FIG. 3 is a graph showing the temperature dependence of the carrier density of a single film in an example. [Figure 4] FIG. 10 is a graph showing the temperature dependence of the resistivity of a single film in an example. [Figure 5] FIG. 13 is a graph showing the temperature dependence of the hole mobility of a composite thin film having InO as the upper layer in an example. [Figure 6] FIG. 16 is a graph showing the temperature dependence of the carrier density of a composite thin film having InO as the upper layer in an example. [Figure 7] FIG. 19 is a graph showing the temperature dependence of the resistivity of a composite thin film having InO as the upper layer in an example. [Figure 8] FIG. 22 is a graph showing the temperature dependence of the hole mobility of a composite thin film having IAO as the upper layer in an example. [Figure 9] FIG. 25 is a graph showing the temperature dependence of the carrier density of a composite thin film having IAO as the upper layer in an example. [Figure 10] FIG. 28 is a graph showing the temperature dependence of the resistivity of a composite thin film having IAO as the upper layer in an example. [Figure 11] FIG. 31 is a graph showing the switching characteristics of the No. 1 TFT in an example. [Figure 12] FIG. 34 is a graph showing the switching characteristics of the No. 2 TFT in an example. [Figure 13] FIG. 37 is a graph showing the switching characteristics of the No. 1 TFT in an example. [Figure 14] FIG. 40 is a graph showing the switching characteristics of the No. 2 TFT in an example.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] [Description of Embodiments in this Disclosure] The Discloser has discovered that, while heat treatment generally reduces carrier mobility in oxide semiconductor thin films, in oxide semiconductor thin films where the metal elements are In and Al, keeping the number of Al atoms within a specific range actually increases carrier mobility, and the carrier density shows a similar trend. Based on these findings, the Discloser has completed the oxide semiconductor thin film of this Disclosure.

[0018] (1) That is, an oxide semiconductor thin film according to one aspect of the present disclosure is an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al and unavoidable impurities, and the number of Al atoms is 5 atm% or more and 15 atm% or less relative to the total number of In and Al atoms.

[0019] In an oxide semiconductor thin film containing metal elements consisting of In, Al, and unavoidable impurities, if the number of Al atoms is within the above range, then, for example, heat treatment in the temperature range of 350°C to 500°C increases carrier mobility, and the carrier density shows a similar trend. In other words, the properties of this oxide semiconductor thin film do not easily deteriorate due to heat treatment.

[0020] (2) In the above (1), it is preferable that the oxide semiconductor thin film is crystallized. The crystallization of the oxide semiconductor thin film improves the stability and reliability of its electrical properties.

[0021] (3) In (1) or (2) above, it is preferable that the thickness of the oxide semiconductor thin film be 15 nm or less. The oxide semiconductor thin film can be suitably used as a thin film with a thickness of 15 nm or less.

[0022] When an oxide semiconductor thin film containing In as a metallic element is in contact with a thin film containing AlO, oxygen tends to move from the AlO-containing thin film to the In-containing oxide semiconductor thin film, which can easily lead to a decrease in carrier mobility. The Discloser has discovered that in an oxide semiconductor thin film where the metallic elements are In and Al, the region with a large number of Al atoms has the effect of suppressing this oxygen movement, and has completed the oxide semiconductor thin film of this disclosure.

[0023] (4) That is, an oxide semiconductor thin film according to another aspect of the present disclosure is an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al and unavoidable impurities, and the number of Al atoms is greater than 15 atm% and less than 100 atm% relative to the total number of In and Al atoms.

[0024] In an oxide semiconductor thin film containing metal elements consisting of In, Al, and unavoidable impurities, if the number of Al atoms is kept within the above range, the movement of oxygen to other In-containing oxide semiconductor thin films in contact with this oxide semiconductor thin film can be suppressed. Therefore, by using this oxide semiconductor thin film as a lower layer for an In-containing oxide semiconductor thin film, the deterioration of the properties of other In-containing oxide semiconductor thin films due to heat treatment can be suppressed.

[0025] (5) In (4) above, it is preferable that the oxide semiconductor thin film be amorphous. By making the oxide semiconductor thin film amorphous in this way, it is easier to make the oxide semiconductor thin film highly resistive.

[0026] (6) In (4) or (5) above, the surface resistivity is 10 7 It is desirable that the surface resistivity be greater than or equal to Ω. By keeping the surface resistivity above the above lower limit, the increase in the number of unwanted carriers can be suppressed.

[0027] (7) A composite thin film according to one aspect of the present disclosure is a composite thin film comprising two oxide semiconductor thin films, an upper layer and a lower layer, wherein the upper layer is an oxide semiconductor thin film of InO or any of (1) to (3) above, and the lower layer is an oxide semiconductor thin film of any of (4) to (6) above.

[0028] This composite thin film can suppress the movement of oxygen from the lower layer to the upper layer. Therefore, it can suppress the deterioration of the properties of the upper layer's In-containing oxide semiconductor thin film due to heat treatment.

[0029] (8) A sputtering target according to one aspect of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm% or more and 15 atm% or less relative to the total number of In and Al atoms.

[0030] Since the number of Al atoms relative to the total number of In and Al atoms in the sputtering target is within the above range, it is possible to form an oxide semiconductor thin film in which degradation of properties due to heat treatment is suppressed.

[0031] (9) A sputtering target according to another aspect of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is greater than 15 atm% and less than 100 atm% relative to the total number of In and Al atoms.

[0032] Since the number of Al atoms relative to the total number of In and Al atoms in the sputtering target is within the above range, it is possible to form an oxide semiconductor thin film in which the movement of oxygen to other In-containing oxide semiconductor thin films in contact with it is suppressed.

[0033] Here, "carrier mobility" refers to the carrier mobility obtained by Hall effect measurement. "Surface resistivity" refers to the value of the electrical resistance of the film measured using the four-terminal method.

[0034] [Details of the embodiments of this disclosure] Hereinafter, an oxide semiconductor thin film, a composite thin film, and a sputtering target according to one embodiment of this disclosure will be described.

[0035] [Composite Thin Film] The composite thin film 1 shown in Figure 1 is composed of two oxide semiconductor thin films: an upper layer (first oxide semiconductor thin film 11) and a lower layer (second oxide semiconductor thin film 12).

[0036] [First oxide semiconductor thin film] The first oxide semiconductor thin film 11 is itself an oxide semiconductor thin film according to one aspect of the present disclosure and contains a metal element. The metal element consists of In, Al, and unavoidable impurities. That is, the first oxide semiconductor thin film 11 substantially does not contain any metal elements other than In and Al.

[0037] With respect to the total number of In and Al atoms mentioned above, the lower limit for the number of Al atoms is 5 atm%, with 6 atm% being more preferable. On the other hand, the upper limit for the number of Al atoms is 15 atm%, with 12 atm% being more preferable. By setting the number of Al atoms above the lower limit, the decrease in carrier mobility due to heat treatment can be suppressed. Conversely, by setting the number of Al atoms below the upper limit, the increase in the resistivity of the first oxide semiconductor thin film 11 can be suppressed.

[0038] The planar shape and size of the first oxide semiconductor thin film 11 are not particularly limited. They are appropriately determined according to the thin-film transistor formed on the first oxide semiconductor thin film 11. For example, the planar shape of the first oxide semiconductor thin film 11 can be the same as that of the gate electrode from the viewpoint of controllability of the channel length and channel width of the thin-film transistor, and the planar size of the first oxide semiconductor thin film 11 can be set to a size that ensures the channel length and channel width of the thin-film transistor.

[0039] The first oxide semiconductor thin film 11 is preferably crystalline. The crystalline state of the first oxide semiconductor thin film 11 improves the stability and reliability of its electrical properties. The Discloser is aware that the first oxide semiconductor thin film 11 readily crystallizes with heat treatment in a temperature range of at least 350°C to 500°C. In other words, the first oxide semiconductor thin film 11 can be crystallized in the process of forming a three-dimensional stacked semiconductor without any special treatment.

[0040] As the upper limit of the film thickness of the first oxide semiconductor thin film 11, 15 nm is preferable, and 10 nm is more preferable. The first oxide semiconductor thin film 11 can be preferably used as a thin film having a film thickness not exceeding the above upper limit. The lower limit of the film thickness of the first oxide semiconductor thin film 11 is not particularly limited, but from the viewpoint of the stability and reliability of the electrical characteristics of the first oxide semiconductor thin film 11, 2 nm is preferable.

[0041] As the lower limit of the carrier density of the first oxide semiconductor thin film 11, 1×10 12 cm -3 is preferable, 1×10 13 cm -3 is more preferable, and 1×10 14 cm -3 is even more preferable. On the other hand, as the upper limit of the carrier density of the first oxide semiconductor thin film 11, 1×10 20 cm -3 is preferable, 1×10 19 cm -3 is more preferable, and 1×10 18 cm -3 is even more preferable. If the carrier density of the first oxide semiconductor thin film 11 is less than the above lower limit, when a thin film transistor is formed on the first oxide semiconductor thin film 11, there is a risk that the drain current is insufficient. Conversely, if the carrier density of the first oxide semiconductor thin film 11 exceeds the above upper limit, it becomes difficult to completely deplete the inside of the first oxide semiconductor thin film 11, so the threshold voltage shifts to the negative side, and the above thin film transistor may not function as a switching element.

[0042] As the lower limit of the hole mobility of the first oxide semiconductor thin film 11, 5 cm 2 / Vs is preferable, and 10 cm 2 / Vs is more preferable. If the hole mobility of the first oxide semiconductor thin film 11 is less than the above lower limit, the switching characteristics of the thin film transistor when a thin film transistor is formed on the first oxide semiconductor thin film 11 may deteriorate. On the other hand, the upper limit of the hole mobility of the first oxide semiconductor thin film 11 is not particularly limited, but usually the hole mobility of the first oxide semiconductor thin film 11 is 100 cm2 It is less than or equal to / Vs. "Hall mobility" refers to carrier mobility obtained by measuring the Hall effect.

[0043] The first oxide semiconductor thin film 11 can be laminated as a first oxide semiconductor layer on the surface of a predetermined location (in this case, the second oxide semiconductor thin film 12) by sputtering using, for example, a known sputtering apparatus. By using the sputtering method, a first oxide semiconductor layer with excellent in-plane uniformity of its composition and film thickness can be easily formed.

[0044] (First sputtering target) A first sputtering target used in the sputtering method is itself another embodiment of the present disclosure. That is, the first sputtering target is a sputtering target used for forming a first oxide semiconductor thin film 11, and contains a metal element, wherein the metal element consists of In, Al, and unavoidable impurities. An example of the first sputtering target is an oxide target containing In and Al (IAO target), specifically a target obtained by adding Al2O3 to In2O3.

[0045] With respect to the total number of In and Al atoms in the first sputtering target, the lower limit for the number of Al atoms is 5 atm%, with 6 atm% being more preferable. On the other hand, the upper limit for the number of Al atoms is 15 atm%, with 12 atm% being more preferable. Since the number of Al atoms in the first sputtering target is within the above range relative to the total number of In and Al atoms, it is possible to form a first oxide semiconductor thin film 11 in which deterioration of properties due to heat treatment is suppressed.

[0046] It is preferable that the first sputtering target has the same composition as the desired first oxide semiconductor thin film 11. By making the composition of the first sputtering target the same as the desired first oxide semiconductor thin film 11, compositional deviations in the formed first oxide semiconductor thin film 11 can be suppressed, making it easier to obtain a first oxide semiconductor thin film 11 having the desired composition.

[0047] The first sputtering target can be manufactured, for example, by a powder sintering method.

[0048] The first sputtering target for stacking the first oxide semiconductor thin film 11 is not limited to the target containing In and Al as described above, but may be a plurality of targets with different compositions. In this case, the plurality of targets are configured to contain In and Al as a whole. Each target may also contain both In and Al. The plurality of targets may also be oxide targets containing one or more of In and Al. The plurality of targets can also be manufactured, for example, by powder sintering. When using the plurality of targets, the co-sputtering method, in which the plurality of targets are discharged simultaneously, can be used as the sputtering method.

[0049] The conditions for stacking the first oxide semiconductor thin film 11 by sputtering are not particularly limited, but for example, the substrate temperature can be 20°C to 50°C, the deposition power can be 200W to 300W, the pressure can be 0.1Pa to 0.3Pa, and the carrier gas can be Ar. In addition, it is preferable to include oxygen in the atmosphere as an oxygen source. The oxygen content in the atmosphere can be 3% to 60% by volume.

[0050] Furthermore, the method for stacking the first oxide semiconductor thin film 11 is not limited to sputtering; chemical film deposition methods such as coating may also be used.

[0051] (Advantages of first oxide semiconductor thin films) In a first oxide semiconductor thin film 11 containing metallic elements consisting of In, Al, and unavoidable impurities, if the number of Al atoms is between 5 atm% and 15 atm%, then, for example, when heat treatment is performed in a temperature range of 350°C to 500°C, the carrier mobility increases, and the carrier density shows a similar trend. In other words, the properties of the first oxide semiconductor thin film 11 do not easily deteriorate due to heat treatment.

[0052] [Doxy-2 semiconductor thin film] The second oxide semiconductor thin film 12 is itself an oxide semiconductor thin film according to another aspect of the present disclosure and contains a metallic element. The metallic element consists of In, Al, and unavoidable impurities. That is, the second oxide semiconductor thin film 12 substantially does not contain any metallic elements other than In and Al.

[0053] The number of Al atoms is greater than 15 atm% of the total number of In and Al atoms mentioned above, and more preferably greater than 20 atm%. By exceeding the above upper limit for the number of Al atoms, the amount of InO relatively decreases, and the resistive component increases, resulting in a high-resistance thin film. If a thin film with low electrical resistance is used for the lower layer, the oxide semiconductor thin film 12, electrons will flow to both the upper layer where the thin-film transistor is formed and the lower layer, the oxide semiconductor thin film 12. If electrons flow to the lower layer, the total number of carriers in the upper and lower layers combined will increase, making it difficult for the thin-film transistor to operate. For this reason, the oxide semiconductor thin film 12 needs to be a high-resistance film. On the other hand, there is no particular upper limit for the number of Al atoms, but the number of Al atoms is less than 100 atm%, and more preferably less than 85 atm%.

[0054] The planar shape and size of the second oxide semiconductor thin film 12 are not particularly limited, but can be the same as those of the first oxide semiconductor thin film 11.

[0055] The oxide semiconductor thin film 12 is preferably amorphous. By making the oxide semiconductor thin film 12 amorphous, it is easier to increase its resistance. The oxide semiconductor thin film 12 is more likely to be amorphous when the number of Al atoms exceeds 20 atm% relative to the total number of In and Al atoms.

[0056] Furthermore, by making the lower layer amorphous, the interface with the upper layer is modulated, changing the nucleation state which is the starting point for crystallization of the upper layer, making it easier to obtain an increase in the grain size and orientation of the upper layer. This promotes the crystallization of the upper layer, the first oxide semiconductor thin film 11, and improves the stability and reliability of the electrical properties of the first oxide semiconductor thin film 11.

[0057] The upper limit of the thickness of the oxide semiconductor thin film 12 is preferably 10 nm, and more preferably 8 nm. Current flows along the plane of the oxide semiconductor thin film 12. By keeping the thickness of the oxide semiconductor thin film 12 below the above upper limit, the resistance in the plane direction can be increased. The lower limit of the thickness of the oxide semiconductor thin film 12 is not particularly limited, but from the viewpoint of the stability and reliability of the electrical properties of the oxide semiconductor thin film 12, 2 nm is preferred.

[0058] The lower limit of surface resistivity is 10 7 Ω is preferred, 10 8 Ω is more preferable. By setting the surface resistivity above the above lower limit, the increase in the number of unwanted carriers in the lower layer can be suppressed as described above. The upper limit of the surface resistivity is not particularly limited, but the surface resistivity of the second oxide semiconductor thin film 12 is usually 10 10 It is less than or equal to Ω.

[0059] The oxide dioxide semiconductor thin film 12 can be laminated as an oxide dioxide semiconductor layer on the surface of a predetermined location (in this case, for example, a metal wiring) by sputtering using, for example, a known sputtering apparatus. By using the sputtering method, an oxide dioxide semiconductor layer with excellent in-plane uniformity of its composition and film thickness can be easily formed.

[0060] Here, we will explain the role of the oxide-second semiconductor thin film 12. Let's assume that AlO is used as the lower layer, which is an insulating film. In AlO, oxygen movement (movement from Al2O3 to In2O3) is likely to occur between it and InO during the heat treatment process. In the composite thin film 1, oxygen moves from the lower layer, which has a large proportion of AlO, to the upper oxide-first semiconductor thin film 11. In this case, the amount of oxygen in the oxide-first semiconductor thin film 11 (InO side) increases, causing the InO crystal grains to become smaller and the carrier mobility to decrease. Also, the increase in oxygen reduces the oxygen vacancies in the carrier source, which tends to lower the carrier density. As a result, the properties of the oxide-first semiconductor thin film 11 deteriorate due to heat treatment. In contrast, by using the oxide-second semiconductor thin film 12 containing In2O3 and Al2O3, the movement of oxygen to the oxide-first semiconductor thin film 11 can be suppressed. Therefore, the deterioration of the properties of the oxide-first semiconductor thin film 11 due to heat treatment can be suppressed.

[0061] (Second sputtering target) A second sputtering target used in the sputtering method is itself another embodiment of the present disclosure. That is, the second sputtering target is a sputtering target used for forming a second oxide semiconductor thin film 12, and contains a metal element, wherein the metal element consists of In, Al, and unavoidable impurities. An example of such a second sputtering target is an oxide target containing In and Al (IAO target), specifically a target obtained by adding Al to In2O3.

[0062] In the second sputtering target, the number of Al atoms is more than 15 atm%, and more preferably more than 20 atm%, relative to the total number of In and Al atoms. On the other hand, the number of Al atoms is less than 100 atm%, and more preferably less than 85 atm%. By keeping the number of Al atoms in the second oxide semiconductor thin film 12 within the above range, the movement of oxygen to the first oxide semiconductor thin film 11, which is an oxide semiconductor thin film containing In and in contact with the second oxide semiconductor thin film 12, can be suppressed. Therefore, by using the second oxide semiconductor thin film 12 as the lower layer of the first oxide semiconductor thin film 11, the deterioration of the properties of the upper first oxide semiconductor thin film 11 due to heat treatment can be suppressed.

[0063] The second sputtering target can be configured in the same way as the first sputtering target described above, except that the number of Al atoms is different from the total number of In and Al atoms, for the purpose of depositing the second oxide semiconductor thin film 12. Therefore, a detailed explanation is omitted.

[0064] (Advantages of oxide semiconductor thin films) In a second oxide semiconductor thin film 12 containing metal elements consisting of In, Al, and unavoidable impurities, if the number of Al atoms is set to more than 15 atm% and less than 100 atm%, the movement of oxygen to a first oxide semiconductor thin film 11, which is another oxide semiconductor thin film containing In and in contact with the second oxide semiconductor thin film 12, can be suppressed. Therefore, by using the second oxide semiconductor thin film 12 as the lower layer of the first oxide semiconductor thin film 11, the deterioration of the properties of the upper first oxide semiconductor thin film 11 due to heat treatment can be suppressed.

[0065] (Advantages of composite thin films) The composite thin film 1 can suppress the movement of oxygen from the lower layer to the upper layer. Therefore, it can suppress the deterioration of the properties of the upper layer, which contains In and is a first oxide semiconductor thin film 11, due to heat treatment.

[0066] [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.

[0067] In the above embodiment, a case was described in which a first oxide semiconductor thin film is used as a composite thin film with a second oxide semiconductor thin film. However, the first oxide semiconductor thin film can also be used on its own. Even when used on its own, the first oxide semiconductor thin film is less susceptible to degradation of properties due to heat treatment.

[0068] In the above embodiment, the upper layer of the second oxide semiconductor thin film is described as a first oxide semiconductor thin film, i.e., an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm% to 15 atm% relative to the total number of In and Al atoms. However, the first oxide semiconductor thin film can also be InO. The second oxide semiconductor thin film of this disclosure is more likely to exhibit its effects when used as a lower layer of an oxide semiconductor thin film containing In. [Examples]

[0069] The present invention will be described in detail below based on the examples provided, but the present invention should not be interpreted as being limited based on the description of these examples.

[0070] <Single membrane> A glass substrate (Corning's "EagleXG," 4 inches in diameter, 0.7 mm thick) was prepared, and an oxide semiconductor thin film (IAO) containing metal elements consisting of In, Al, and unavoidable impurities, or an oxide semiconductor thin film (InO) containing metal elements consisting of In and unavoidable impurities, was formed on its surface by sputtering to a thickness of 8 nm.

[0071] For the IAO sputtering target, we used In2O3 with Al added in the range of 3 atm% to 22 atm%. For the InO sputtering target, we used In2O3. The deposition conditions were a substrate temperature of 25°C (room temperature), deposition power of 250W, pressure of 1 mTorr (=0.133 Pa), and carrier gas of Ar. The oxygen content of the atmosphere was set to 4 volume%. The flow rates were 24 sccm for Ar and 1 sccm for O2.

[0072] Under the conditions described above, in addition to the InO thin film, six thin films of IAO were deposited by controlling the amount of Al added, with Al atoms at concentrations of 3 atm%, 6 atm%, 8.5 atm%, 15 atm%, and 22 atm% relative to the total number of In and Al atoms.

[0073] For each of the six thin films mentioned above, four thin films were prepared under a total of four conditions: one without heat treatment, and one treated with heat at 350°C, 400°C, and 500°C for one hour in an air atmosphere.

[0074] The hole mobility, carrier density, and resistivity of the obtained thin films were measured. The results are shown in Figures 2, 3, and 4, respectively.

[0075] In InO without added Al, it can be seen that the hole mobility decreases and the carrier density also tends to decrease when the heat treatment temperature rises from 400°C to 500°C.

[0076] In the range of Al addition (~4%), hole mobility decreases when heat treatment temperature is increased from 350°C to 500°C, while carrier density tends to remain similar. When a small amount of Al is added, the hole mobility without heat treatment is lower than that of InO, whereas when heat-treated at 350°C, the hole mobility becomes higher than that of InO (Al 3%). This is thought to be due to factors such as a slight increase in grain size, a reduction in lattice constant, and suppression of impurity scattering due to strengthened oxygen bonds. On the other hand, at heat treatment temperatures of 400°C to 500°C, the hole mobility decreases for both InO and IAO (Al 3%). This is thought to be due to increased impurity scattering caused by activation such as hydrogen desorption at grain boundaries.

[0077] In the Al addition range of 5-15%, hole mobility increases from 400°C to 500°C during heat treatment, while carrier density remains similar. This is thought to be due to an increase in the crystallization rate of IAO due to high-temperature heat treatment. Additionally, since the heat treatment is performed in an atmospheric environment, an increase in the composition ratio of O atoms to Al atoms is considered a contributing factor. In this case, the relative number of OH bonds also increases due to heat treatment in air, which is thought to make grain boundaries less likely to be activated.

[0078] As the amount of Al added increases further, the relative amount of InO decreases, and the resistive component increases. The resistance value increases with the amount of Al added. All of these IAOs are amorphous films.

[0079] Based on these results, it is determined that IAO in which the amount of Al atoms is between 5 atm% and 15 atm% relative to the total number of In and Al atoms is less susceptible to degradation of properties due to heat treatment.

[0080] <Composite Thin Film 1> A composite thin film consisting of two oxide semiconductor thin films, an upper and a lower layer, was deposited on the same glass substrate as the single film using the following procedure.

[0081] First, five sputtering targets were used: InO+Zn+Ru, InO+B+Ru, Ga2O3, Al2O3, and In2O3 with 22 atm% Al added. Thin films were then formed on the surface of each of the five glass substrates by sputtering to a thickness of 5 nm. All of the thin films were amorphous.

[0082] Next, using InO as the sputtering target, a thin upper layer was formed on the surface of the lower layer deposited on each of the five glass substrates by sputtering, resulting in a film thickness of 8 nm.

[0083] The deposition conditions for the upper and lower layers were the same as for the single-layer film. Furthermore, after heat treatment at various temperatures (including no heat treatment), the hole mobility, carrier density, and resistivity of the resulting thin films were measured. The results are shown in Figures 5, 6, and 7, respectively.

[0084] These results indicate that in a composite thin film using IAO in the lower layer, where the number of Al atoms is 22 atm% relative to the total number of In and Al atoms, carrier mobility is high and carrier density is kept low.

[0085] <Composite Thin Film 2> A composite thin film was obtained in the same manner as composite thin film 1, except that the upper layer was IAO with an Al atomic content of 6 atm%, and the lower layer was IAO or IGZO with an Al atomic content of 22 atm%, with the upper layer being IAO.

[0086] After heat treatment at various temperatures (including no heat treatment), similar to that performed on single films, the hole mobility, carrier density, and resistivity of the resulting thin films were measured. The results are shown in Figures 8, 9, and 10, respectively.

[0087] These results show that even when the upper layer is IAO with 6 atm% Al atoms, the composite thin film using IAO with 22 atm% Al atoms relative to the total number of In and Al atoms in the lower layer maintains high carrier mobility and low carrier density.

[0088] <tft> Among bottom-gate type transistors, a TFT (Thin Film Transistor) with an etch-stop (ESL) structure was formed.

[0089] The cross-sectional structure of the TFT consists of, from bottom to top: a Si substrate, a Mo gate electrode (average thickness 30 nm), an HfO gate insulating film (average thickness 10 nm), an oxide semiconductor thin film (average thickness 8 nm), an ESL-SiO insulating film (average thickness 30 nm), Mo source and drain electrodes (average thickness 100 nm), and a passivation SiO insulating film (average thickness 100 nm).

[0090] The HfO gate insulating film was formed using the ALD (atomic layer volume) method. The SiO films, which are insulating films for the ESL and passivation, were formed using the CVD method. The oxide semiconductor thin film was formed using the sputtering method.

[0091] Four TFTs (No. 1 to No. 4) of different types of oxide semiconductor thin films were formed. The composition and average film thickness of each oxide semiconductor thin film are shown below. Note that in the multilayer structures No. 2 and No. 3, the left side is the lower layer. No. 4 is an oxide semiconductor with an Al content of 5 atm% relative to the total number of In and Al atoms. (No.1) In2O3 (8nm) (No.2) Al2O3 / In2O3 (5nm / 8nm) (No.3)Al2O3 / In2O3 / Al2O3(5nm / 8nm / 3nm) (No.4) In2O3 + Al2O3 (15nm)

[0092] After forming the oxide semiconductor thin films described above, samples No. 1 to No. 3 were heat-treated at 350°C for 1 hour in an air atmosphere, while sample No. 4 was heat-treated at 250°C for 1 hour in an air atmosphere. The microstructure of the semiconductor thin films (or each layer in the case of multilayer structures) of No. 1 to No. 4 was crystalline.

[0093] The switching characteristics of TFTs No. 1 to No. 4 were measured. The results are shown in Figures 11 to 14. For each TFT, the source-drain current (Id) was measured when the source-drain voltage (Vd) was fixed at 0.2V and the gate voltage (Vg) was swept from -2V to 6V.

[0094] TFT No. 4 switches (see Figure 14), but TFT No. 1 does not switch due to its high carrier density (see Figure 11). Furthermore, TFTs No. 2 and No. 3, which contain Al2O3 in the lower layer, tend to switch (see Figures 12 and 13). This is thought to be because the oxygen in Al2O3 reduces oxygen vacancies, lowering the carrier density.

[0095] However, it is possible that the crystal grain size will decrease due to the supply of oxygen. By placing the highly resistive In2O3+Al2O3 as the lower layer, it becomes possible to control the movement of oxygen, and by increasing the size of the In2O3 crystal grains in the upper layer, it is thought that it can contribute to reducing carrier density and improving mobility. [Industrial applicability]

[0096] The oxide semiconductor thin films and composite thin films of this disclosure are less susceptible to degradation of properties by heat treatment. Furthermore, the sputtering target of this disclosure can form oxide semiconductor thin films that are less susceptible to degradation of properties by heat treatment. [Explanation of Symbols]

[0097] 1. Composite thin film 11. First oxide semiconductor thin film 12. Dioxide semiconductor thin film< / tft>

Claims

1. A composite thin film composed of two layers of oxide semiconductor thin films, an upper layer and a lower layer, The upper layer is an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm% or more and 15 atm% or less relative to the total number of In and Al atoms. The lower layer is an oxide semiconductor thin film containing a metal element, wherein the metal element consists of In, Al, and unavoidable impurities, and the number of Al atoms is greater than 15 atm% and less than 100 atm% relative to the total number of In and Al atoms. A composite thin film in which the thickness of the upper layer is 10 nm or less.

2. The composite thin film according to claim 1, wherein the upper layer is crystallized.

3. The composite thin film according to claim 1 or claim 2, wherein the lower layer is amorphous.

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