Oxide semiconductor thin film, composite thin film, and sputtering target

Abstract

An oxide semiconductor thin film according to one embodiment of the present disclosure contains metal elements. The metal elements comprises In, Al, and inevitable impurities. The total number of Al atoms is 5 to 15 atm% with respect to the total number of In atoms plus Al atoms.

Description

Oxide semiconductor thin films, composite thin films and sputtering targets

[0001] The present disclosure relates to oxide semiconductor thin films, composite thin films, and sputtering targets.

[0002] Flat display panels (FDPs) use semiconductors with a three-dimensional stacked structure. In these semiconductors, thin-film transistors are formed in a metal wiring layer, with an oxide semiconductor as the channel layer. Examples of oxide semiconductors include In—Ga—Zn—O (IGZO) oxide semiconductors containing indium, gallium, zinc, and oxygen, as well as In—Zn—O (IZO) and In—Ga—O (IGO) containing indium, gallium, and oxygen (see, for example, JP 2017-59838 A and JP 2014-98211 A).

[0003] While these oxide semiconductors exhibit high mobility, they also tend to have high carrier densities, so in order to ensure stable operation as thin film transistors (TFTs), the total number of carriers passing through the channel is 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.

[0004] JP 2017-59838 A JP 2014-98211 A

[0005] When heat is applied to an oxide semiconductor, the carrier mobility decreases, the carrier density increases, and so on, which may result in deterioration of desired characteristics.

[0006] When we look at the heat treatments involved in the semiconductor process for forming three-dimensional stacked semiconductors, one example is the heat treatment applied to metal wiring, called sintering, which is used to ensure ohmic contact between the metal wiring and silicon. This temperature is generally between 350°C and 500°C.

[0007] Furthermore, recent semiconductor processes are increasingly using high-dielectric-constant (high-k) materials such as HfOx for gate insulating films. 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 the electrical insulating film is increased, thereby reducing transistor current leakage. For example, when HfOx, a well-known high-k film, is formed using the ALD method, it may crystallize at temperatures above 450°C. When HfOx crystallizes, it is prone to current leakage along the grain boundaries. For this reason, the heat treatment temperature is limited to below 450°C.

[0008] In view of the above, it is desirable for the oxide semiconductor to have characteristics that do not deteriorate at least at temperatures of 350°C to 500°C, which are the heat treatment temperatures required for metal wiring, or at temperatures below 450°C, which are the heat treatment temperatures required for high-k films.

[0009] The present disclosure has been made in light of the above-mentioned circumstances, and aims to provide an oxide semiconductor thin film and a composite thin film whose characteristics are less likely to be deteriorated by heat treatment, and a sputtering target for forming the oxide semiconductor thin film.

[0010] An oxide semiconductor thin film according to one embodiment of the present disclosure is an oxide semiconductor thin film containing metal elements, the metal elements being composed of In, Al, and inevitable impurities, and the number of Al atoms being 5 atm % or more and 15 atm % or less with respect to the total number of In and Al atoms.

[0011] An oxide semiconductor thin film according to another embodiment of the present disclosure is an oxide semiconductor thin film containing a metal element, the metal element being composed of In, Al, and unavoidable impurities, and the number of Al atoms being more than 15 atomic % and less than 100 atomic % with respect to the total number of In and Al atoms.

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

[0013] A sputtering target according to one embodiment of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, and contains a metal element, the metal element being composed 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 embodiment of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, and includes a metal element, the metal element being composed of In, Al, and unavoidable impurities, and the number of Al atoms is more than 15 atomic % and less than 100 atomic % relative to the total number of In and Al atoms.

[0015] The oxide semiconductor thin film and composite thin film of the present disclosure are resistant to deterioration in properties due to heat treatment. Furthermore, the sputtering target of the present disclosure can form an oxide semiconductor thin film whose properties are resistant to deterioration due to heat treatment.

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

[0017] [Description of Embodiments of the Present Disclosure] The present inventors have found that, while heat treatment of an oxide semiconductor thin film generally reduces carrier mobility, when the number of Al atoms is within a specific range in an oxide semiconductor thin film containing In and Al as metal elements, carrier mobility actually increases and the carrier density also tends to be comparable, and have completed the oxide semiconductor thin film of the present disclosure.

[0018] (1) That is, an oxide semiconductor thin film according to one embodiment of the present disclosure is an oxide semiconductor thin film containing metal elements, the metal elements being composed of In, Al, and unavoidable impurities, and the number of Al atoms is 5 atm % or more and 15 atm % or less with respect 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, the carrier mobility increases and the carrier density tends to be the same when heat treatment is performed at a temperature range of, for example, 350° C. to 500° C. In other words, the characteristics of the oxide semiconductor thin film are less likely to deteriorate due to heat treatment.

[0020] (2) In the above (1), the oxide semiconductor thin film may be crystallized, which improves the stability and reliability of electrical characteristics.

[0021] (3) In the above (1) or (2), the oxide semiconductor thin film may have a thickness of 15 nm or less. The oxide semiconductor thin film can be suitably used as a thin film having a thickness of 15 nm or less.

[0022] When an oxide semiconductor thin film containing In as a metal element is in contact with a thin film containing AlO, oxygen tends to migrate from the thin film containing AlO to the oxide semiconductor thin film containing In, which tends to reduce carrier mobility. The present inventors have found that in an oxide semiconductor thin film containing In and Al as metal elements, a region with a large range of Al atomic numbers has the effect of inhibiting this oxygen migration, and have completed the oxide semiconductor thin film of the present disclosure.

[0023] (4) That is, an oxide semiconductor thin film according to another embodiment of the present disclosure is an oxide semiconductor thin film containing a metal element, the metal element being composed of In, Al, and unavoidable impurities, and the number of Al atoms is more than 15 atomic % and less than 100 atomic % with respect 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 within the above range, it is possible to prevent oxygen from migrating to another oxide semiconductor thin film containing In that is in contact with the oxide semiconductor thin film. Therefore, by using the oxide semiconductor thin film as an underlying layer of an oxide semiconductor thin film containing In, it is possible to prevent deterioration of the characteristics of the other oxide semiconductor thin films containing In due to heat treatment.

[0025] (5) In the above (4), the oxide semiconductor thin film may be amorphous. By making the oxide semiconductor thin film amorphous in this way, the oxide semiconductor thin film is likely to have a high resistance.

[0026] (6) In the above (4) or (5), the surface resistivity is 10 ７ By setting the surface resistivity to the above lower limit or more, an increase in the number of unnecessary carriers can be suppressed.

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

[0028] The composite thin film can prevent oxygen from moving from the lower layer to the upper layer, thereby preventing the deterioration of the characteristics of the upper layer of the oxide semiconductor thin film containing In due to heat treatment.

[0029] (8) A sputtering target according to one embodiment of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, and includes a metal element, the metal element being composed 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] In the sputtering target, the number of Al atoms relative to the total number of In and Al atoms is within the above range, and therefore an oxide semiconductor thin film can be formed in which deterioration of characteristics due to heat treatment is suppressed.

[0031] (9) A sputtering target according to another embodiment of the present disclosure is a sputtering target used for forming an oxide semiconductor thin film, the sputtering target including a metal element, the metal element including In, Al, and unavoidable impurities, and a ratio of the number of Al atoms to the total number of In and Al atoms is more than 15 atomic % and less than 100 atomic %.

[0032] In the sputtering target, the ratio of the number of Al atoms to the total number of In and Al atoms is within the above range, and therefore an oxide semiconductor thin film can be formed in which oxygen migration to other oxide semiconductor thin films containing In that are in contact with the oxide semiconductor thin film is suppressed.

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

[0034] [Details of Embodiments of the Present Disclosure] Hereinafter, an oxide semiconductor thin film, a composite thin film, and a sputtering target according to an embodiment of the present disclosure will be described.

[0035] [Composite Thin Film] The composite thin film 1 shown in FIG. 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 embodiment of the present disclosure and contains metal elements. The metal elements are composed of In, Al, and unavoidable impurities. That is, the first oxide semiconductor thin film 11 contains substantially no metal elements other than In and Al.

[0037] The lower limit of the number of Al atoms relative to the total number of In and Al atoms is 5 atm%, more preferably 6 atm%. On the other hand, the upper limit of the number of Al atoms is 15 atm%, more preferably 12 atm%. By setting the number of Al atoms to the lower limit or more, it is possible to prevent a decrease in carrier mobility due to heat treatment. Conversely, by setting the number of Al atoms to the upper limit or less, it is possible to prevent an increase in the resistivity of the first oxide semiconductor thin film 11.

[0038] The shape and size in a planar view of the first oxide semiconductor thin film 11 are not particularly limited and are determined appropriately depending on the thin film transistor to be formed on the first oxide semiconductor thin film 11. For example, the shape in a planar view of the first oxide semiconductor thin film 11 may be the same shape as that of a gate electrode from the viewpoint of controllability of the channel length and channel width of the thin film transistor, and the size in a planar view of the first oxide semiconductor thin film 11 may be 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 crystallized. The crystallization of the first oxide semiconductor thin film 11 improves the stability and reliability of electrical characteristics. The present inventors have found that the first oxide semiconductor thin film 11 is easily crystallized by heat treatment at 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 structure semiconductor without any special treatment.

[0040] The upper limit of the film thickness of the first oxide semiconductor thin film 11 is preferably 15 nm, more preferably 10 nm. The first oxide semiconductor thin film 11 can be suitably used as a thin film having a film thickness equal to or less than the upper limit. The lower limit of the film thickness of the first oxide semiconductor thin film 11 is not particularly limited, but is preferably 2 nm from the viewpoint of the stability and reliability of the electrical properties of the first oxide semiconductor thin film 11.

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

[0042] The lower limit of the hole mobility of the first oxide semiconductor thin film 11 is 5 cm ２ / Vs is preferred, and 10 cm ２ If the hole mobility of the first oxide semiconductor thin film 11 is less than the above lower limit, the switching characteristics of a thin film transistor formed on the first oxide semiconductor thin film 11 may be degraded. On the other hand, the upper limit of the hole mobility of the first oxide semiconductor thin film 11 is not particularly limited, but the hole mobility of the first oxide semiconductor thin film 11 is usually 100 cm ２ / Vs or less. "Hall mobility" refers to carrier mobility obtained by Hall effect measurement.

[0043] The first oxide semiconductor thin film 11 can be deposited 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 device. By using the sputtering method, a first oxide semiconductor layer excellent in in-plane uniformity of its components and film thickness can be easily formed.

[0044] (First Sputtering Target) The 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 to form the first oxide semiconductor thin film 11, and contains metal elements, and the metal elements are composed of In, Al, and inevitable impurities. The first sputtering target is an oxide target containing In and Al (IAO target), specifically, In ２ O ３Al ２ O ３ Examples of targets include those doped with

[0045] The lower limit of the number of Al atoms relative to the total number of In and Al atoms in the first sputtering target is 5 atm %, more preferably 6 atm %, while the upper limit of the number of Al atoms is 15 atm %, more preferably 12 atm %. Since the first sputtering target has the number of Al atoms relative to the total number of In and Al atoms within the above range, it is possible to form the first oxide semiconductor thin film 11 whose characteristics are prevented from deteriorating due to heat treatment.

[0046] The first sputtering target preferably 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 in this way, deviation in the composition of the formed first oxide semiconductor thin film 11 can be suppressed, and therefore, it is easy to obtain the first oxide semiconductor thin film 11 having the desired composition.

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

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

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

[0050] The method for depositing the first oxide semiconductor thin film 11 is not limited to sputtering, and a chemical film formation method such as coating may also be used.

[0051] (Advantages of the first oxide semiconductor thin film) In the first oxide semiconductor thin film 11 containing metal elements consisting of In, Al, and unavoidable impurities, if the number of Al atoms is 5 atm % or more and 15 atm % or less, the carrier mobility increases and the carrier density tends to be the same when heat treatment is performed at a temperature range of, for example, 350° C. to 500° C. In other words, the characteristics of the first oxide semiconductor thin film 11 are less likely to deteriorate due to heat treatment.

[0052] [Second Oxide Semiconductor Thin Film] The second oxide semiconductor thin film 12 is itself an oxide semiconductor thin film according to another embodiment of the present disclosure and contains metal elements. The metal elements are composed of In, Al, and unavoidable impurities. That is, the second oxide semiconductor thin film 12 contains substantially no metal elements other than In and Al.

[0053] The number of Al atoms is more than 15 atm%, more preferably more than 20 atm%, relative to the total number of In and Al atoms. Setting the number of Al atoms above the lower limit relatively reduces the amount of InO, increasing the resistance component, resulting in a high-resistance thin film. If a thin film with low electrical resistance is used for the lower second oxide semiconductor thin film 12, electrons will flow to both the upper layer where the thin film transistor is formed and the lower second 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. Therefore, the second oxide semiconductor thin film 12 must be a high-resistance film. While there is no particular upper limit on the number of Al atoms, the number of Al atoms is less than 100 atm%, more preferably less than 85 atm%.

[0054] The shape and size in a plan view 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 second oxide semiconductor thin film 12 is preferably amorphous. By making the second oxide semiconductor thin film 12 amorphous in this way, the resistance of the second oxide semiconductor thin film 12 is likely to be high. Note that the second oxide semiconductor thin film 12 is likely to be amorphous when the number of Al atoms exceeds 20 atm % with respect 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 state of nucleation that serves as the starting point for crystallization of the upper layer, making it easier to enlarge and orient the crystal grain size of the upper layer, thereby accelerating the crystallization of the upper layer, the first oxide semiconductor thin film 11, and improving the stability and reliability of the electrical properties of the first oxide semiconductor thin film 11.

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

[0058] The lower limit of the surface resistivity is 10 ７ Ω is preferred, 10 ８ Ω is more preferable. By setting the surface resistivity to the above lower limit or more, it is possible to prevent an increase in the number of unnecessary carriers in the lower layer, 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 set to 10 １０ It is less than Ω.

[0059] The second oxide semiconductor thin film 12 can be deposited as a second oxide 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 device. By using the sputtering method, a second oxide semiconductor layer with excellent in-plane uniformity in its components and film thickness can be easily formed.

[0060] Here, the role of the second oxide semiconductor thin film 12 will be described. It is assumed that an insulating film of AlO is used as the lower layer. AlO is a material that allows oxygen to migrate between InO (Al ２ O ３ From In ２ O ３ In the composite thin film 1, oxygen migrates from the lower layer, which has a higher proportion of AlO, to the upper first oxide semiconductor thin film 11. In this case, the oxygen in the first oxide semiconductor thin film 11 (InO side) increases, which reduces the size of the InO crystal grains and decreases the carrier mobility. In addition, the increase in oxygen reduces oxygen vacancies in the carrier generation source, which tends to decrease the carrier density. This causes the characteristics of the first oxide semiconductor thin film 11 to deteriorate due to heat treatment. In contrast, ２ O ３ and Al ２ O ３ By using the second oxide semiconductor thin film 12 containing the above, it is possible to prevent oxygen from moving to the first oxide semiconductor thin film 11. Therefore, it is possible to prevent the deterioration of the characteristics of the first oxide semiconductor thin film 11 due to heat treatment.

[0061] (Second Sputtering Target) The 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 to form the second oxide semiconductor thin film 12, and contains metal elements, and the metal elements are composed of In, Al, and inevitable impurities. The second sputtering target is an oxide target containing In and Al (IAO target), specifically, In ２ O ３ Examples of such targets include those containing Al.

[0062] The number of Al atoms is more than 15 atm%, more preferably more than 20 atm%, relative to the total number of In and Al atoms in the second sputtering target. On the other hand, the number of Al atoms is less than 100 atm%, more preferably less than 85 atm%. When the number of Al atoms in the second oxide semiconductor thin film 12 is within the above range, oxygen migration to the first oxide semiconductor thin film 11, which is another oxide semiconductor thin film containing In and is in contact with the second oxide semiconductor thin film 12, can be suppressed. Therefore, by using the second oxide semiconductor thin film 12 as a lower layer of the first oxide semiconductor thin film 11, deterioration of the characteristics 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 manner as the first sputtering target described above, except that the number of Al atoms relative to the total number of In and Al atoms is different for the purpose of forming the second oxide semiconductor thin film 12, and therefore detailed description thereof will be omitted.

[0064] (Advantages of the second oxide semiconductor thin film) When the number of Al atoms is more than 15 atm % and less than 100 atm % in the second oxide semiconductor thin film 12 containing metal elements such as In, Al, and unavoidable impurities, it is possible to prevent oxygen from migrating to the first oxide semiconductor thin film 11, which is another oxide semiconductor thin film containing In and is in contact with the second oxide semiconductor thin film 12. Therefore, by using the second oxide semiconductor thin film 12 as a lower layer of the first oxide semiconductor thin film 11, it is possible to prevent deterioration in the characteristics of the upper first oxide semiconductor thin film 11 due to heat treatment.

[0065] (Advantages of the Composite Thin Film) The composite thin film 1 can prevent oxygen from moving from the lower layer to the upper layer, thereby preventing the deterioration of the characteristics of the upper layer, the first oxide semiconductor thin film 11 containing In, due to heat treatment.

[0066] [Other Embodiments] The above-described embodiments do not limit the configuration of the present invention. Therefore, the above-described embodiments may include omissions, substitutions, or additions of components based on the description in this specification and common general technical knowledge, and all of these should be construed as falling within the scope of the present invention.

[0067] In the above embodiment, the first oxide semiconductor thin film is used as a composite thin film with the second oxide semiconductor thin film, but the first oxide semiconductor thin film can also be used alone. Even when used alone, the characteristics of the first oxide semiconductor thin film are less likely to deteriorate due to heat treatment.

[0068] In the above embodiment, the upper layer of the second oxide semiconductor thin film is the first oxide semiconductor thin film, i.e., an oxide semiconductor thin film containing a metal element, the metal element being composed of In, Al, and unavoidable impurities, and the number of Al atoms being 5 atm % or more and 15 atm % or less relative to the total number of In and Al atoms, but the first oxide semiconductor thin film may also be InO. The second oxide semiconductor thin film of the present disclosure is more likely to exhibit its effects when used as a lower layer of an oxide semiconductor thin film containing In.

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

[0070] <Single Film> A glass substrate (Corning Incorporated's "Eagle XG," diameter 4 inches, thickness 0.7 mm) was prepared, and an oxide semiconductor thin film containing metal elements such as In, Al, and unavoidable impurities (IAO) or an oxide semiconductor thin film containing metal elements such as In and unavoidable impurities (InO) was formed on the surface of the glass substrate by sputtering to a thickness of 8 nm.

[0071] The sputtering target of IAO contains In ２ O ３ The InO sputtering target was made of InO with Al added in the range of 3 atm% to 22 atm%. ２ O ３ The deposition conditions were a substrate temperature of 25°C (room temperature), deposition power of 250W, pressure of 1mTorr (=0.133Pa), and carrier gas of Ar. The oxygen content of the atmosphere was 4% by volume. The flow rates were Ar at 24 sccm and O ２ The flow rate was set to 1 sccm.

[0072] Under the above conditions, in addition to the InO thin film, by controlling the amount of Al added, a total of six IAO thin films were formed, each with an Al atomic ratio 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, four thin films were prepared under four conditions: one without heat treatment, and one heat-treated in air at temperatures of 350°C, 400°C, and 500°C for one hour.

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

[0075] It can be seen that in InO to which Al is not added, the Hall mobility decreases when the heat treatment temperature is increased from 400° C. to 500° C., and the carrier density also tends to decrease.

[0076] It can be seen that when the Al content is in the range of up to 4%, the Hall mobility decreases when the heat treatment temperature is changed from 350°C to 500°C, while the carrier density tends to be the same. When a small amount of Al is added, the Hall mobility without heat treatment is smaller than that of InO, but when heat treated at a temperature of 350°C, the Hall mobility is 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, when the heat treatment temperature is changed from 400°C to 500°C, the Hall mobility decreases for both InO and IAO (Al 3%). This is thought to be due to increased impurity scattering caused by activation due to hydrogen desorption from the grain boundaries, etc.

[0077] It can be seen that when the Al content is in the range of 5 to 15%, the Hall mobility increases when the heat treatment temperature is changed from 400°C to 500°C, while the carrier density tends to be the same. This is thought to be because the crystallization rate of IAO increases due to the high-temperature heat treatment. In addition, because the heat treatment is performed in an air atmosphere, the composition ratio of O atoms to Al atoms increases, which is thought to be a factor. In this case, because the heat treatment is performed in air, the number of OH bonds also increases relatively, which is thought to make it difficult to activate the grain boundaries.

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

[0079] From the above results, it is concluded that IAO in which 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 is less likely to have its characteristics deteriorated by heat treatment.

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

[0081] First, as a sputtering target, InO+Zn+Ru, InO+B+Ru, Ga ２ O ３ , Al ２ O ３ and In ２ O ３% of Al was added to the SiO2 film, and a thin film having a thickness of 5 nm was formed on the surface of each of five glass substrates by sputtering.

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

[0083] The deposition conditions for the upper and lower layers were the same as for the single film. After heat treatment at various temperatures (including without heat treatment), the resulting thin films were measured for Hall mobility, carrier density, and resistivity. The results are shown in Figures 5, 6, and 7, respectively.

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

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

[0086] After heat treatment at various temperatures (including without heat treatment) in the same manner as for the single film, the resulting thin film was measured for Hall mobility, carrier density, and resistivity. The results are shown in Figures 8, 9, and 10, respectively.

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

[0088] <TFT> A bottom gate type TFT (thin film transistor) having an etch stop (ESL) structure was formed.

[0089] The cross-sectional structure of the TFT is, from the bottom up, a Si substrate, a Mo gate electrode (average thickness 30 nm), a 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 film serving as the gate insulating film was formed by atomic layer deposition (ALD). The SiO film serving as the insulating film for ESL and passivation was formed by CVD. The oxide semiconductor thin film was formed by sputtering.

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

[0092] After forming the oxide semiconductor thin films, Nos. 1 to 3 were heat-treated in an air atmosphere at 350° C. for 1 hour, and No. 4 was heat-treated in an air atmosphere at 250° C. for 1 hour. The structure of the semiconductor thin films Nos. 1 to 4 (or each layer in the case of a multilayer structure) was all crystalline.

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

[0094] The TFT No. 4 switches (see FIG. 14), but the TFT No. 1 does not switch due to its high carrier density (see FIG. 11). ２ O ３ In the TFTs No. 2 and No. 3 containing Al in the lower layer, a tendency to switch is observed (see Figures 12 and 13). ２ O ３ This is thought to be because the oxygen in the film reduces oxygen vacancies, resulting in a decrease in carrier density.

[0095] However, it is thought that the supply of oxygen reduces the size of the crystal grains. ２ O ３ +Al ２ O ３ By placing it in the lower layer, it is possible to control the movement of oxygen, and the upper layer, In ２ O ３ It is believed that increasing the size of the crystal grains can contribute to reducing the carrier density and improving the mobility.

[0096] The oxide semiconductor thin film and composite thin film of the present disclosure are resistant to deterioration in properties due to heat treatment. Furthermore, the sputtering target of the present disclosure can form an oxide semiconductor thin film whose properties are resistant to deterioration due to heat treatment.

[0097] 1 Composite thin film 11 First oxide semiconductor thin film 12 Second oxide semiconductor thin film

Claims

1. An oxide semiconductor thin film containing a metal element, wherein the metal element is composed of In, Al and unavoidable impurities, and the number of Al atoms is 5 atomic % or more and 15 atomic % or less relative to the total number of In and Al atoms.

2. The oxide semiconductor thin film according to claim 1, which is crystallized.

3. The oxide semiconductor thin film according to claim 1 or 2, which has a film thickness of 15 nm or less.

4. An oxide semiconductor thin film containing a metal element, wherein the metal element is composed of In, Al and unavoidable impurities, and the number of Al atoms is more than 15 atomic % and less than 100 atomic % relative to the total number of In and Al atoms.

5. The oxide semiconductor thin film according to claim 4, which is amorphous.

6. Surface resistivity is 10 ７ 6. The oxide semiconductor thin film according to claim 4, wherein the resistance is Ω or more.

7. A composite thin film comprising two oxide semiconductor thin films, an upper layer and a lower layer, wherein the upper layer is InO or the oxide semiconductor thin film described in claim 1, and the lower layer is the oxide semiconductor thin film described in claim 4.

8. A sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, the metal element consisting of In, Al and inevitable impurities, and the number of Al atoms relative to the total number of In and Al atoms is 5 atm % or more and 15 atm % or less.

9. A sputtering target used for forming an oxide semiconductor thin film, comprising a metal element, the metal element consisting of In, Al and unavoidable impurities, and the number of Al atoms relative to the total number of In and Al atoms is more than 15 atomic % and less than 100 atomic %.

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