JP7373428B2 - Thin film transistors, oxide semiconductor thin films, and sputtering targets - Google Patents

Description

The present invention relates to a thin film transistor (TFT), an oxide semiconductor thin film for the thin film transistor, and a sputtering target for forming the oxide semiconductor thin film.

Oxide semiconductors have higher mobility and larger optical band gaps than general-purpose amorphous silicon (a-Si), and can be formed at low temperatures. Therefore, it is expected to be applied to next-generation displays that require large size, high resolution, and high-speed drive, as well as resin substrates with low heat resistance. For example, as the oxide semiconductor, an amorphous oxide semiconductor (In-Ga-Zn-O, hereinafter sometimes referred to as "IGZO") made of indium, gallium, zinc, and oxygen is widely used.

When using an oxide semiconductor as a semiconductor layer of a thin film transistor, the thin film transistor is required to have excellent static characteristics. Specifically, (1) the on-current, that is, the maximum drain current when applying a positive voltage to the gate electrode and the drain electrode, is high, and (2) the off-current, that is, the maximum drain current when applying a negative voltage to the gate electrode and the drain voltage, is high. The drain current when a positive voltage is applied is low, (3) the S value (subthreshold swing), that is, the gate voltage required to increase the drain current by one order of magnitude, is low, and (4) the threshold voltage, that is, The voltage at which the drain current begins to flow when a positive voltage is applied to the drain electrode and either positive or negative voltage is applied to the gate voltage is stable without changing over time, and (5) mobility is high, etc. is required.

Further, a TFT using an oxide semiconductor is required to have a small amount of change in threshold voltage before and after applying optical stress such as light irradiation, that is, to have excellent optical stress resistance. For example, when we continue to irradiate the blue band where light absorption begins, charges are trapped at the interface between the gate insulating film of the TFT and the oxide semiconductor thin film, and changes in the charge inside the oxide semiconductor thin film cause the threshold voltage to can change (shift) significantly to the negative side. It has been pointed out that this changes the static characteristics of the TFT. In addition, light leaking from the liquid crystal cell is irradiated onto the TFT when driving the liquid crystal panel or turning on a pixel by applying a negative bias to the gate electrode, but this light puts stress on the TFT, causing image unevenness. This may cause deterioration of TFT characteristics. When a TFT is actually used, if its static characteristics change due to light irradiation stress, the reliability of the display device itself will deteriorate.

Similarly, in an organic EL display, light leaking from a light emitting layer may be irradiated onto a semiconductor layer, causing a problem that values such as threshold voltage may vary.

In this way, a shift in the threshold voltage in particular causes a decrease in the reliability of the display device itself, such as a liquid crystal display or an organic EL display including a TFT, so there is a strong desire to improve the resistance to optical stress.

In addition, in recent years, in order to further improve the characteristics of oxide semiconductors, an amorphous oxide semiconductor containing indium, gallium, and tin (In-Ga-Sn-O, hereinafter sometimes referred to as "IGTO") has been used instead of IGZO. ) has been developed.

For example, Patent Document 1 discloses an oxide sintered body in which the atomic ratio of each element of indium, gallium, and tin is controlled within a predetermined range. Patent Document 1 states that an oxide semiconductor film suitable for a patterning process in manufacturing a semiconductor element can be formed.

Furthermore, Patent Document 2 discloses that oxide sintering is performed to control the atomic ratio of each element of indium, gallium, zinc, and tin within a predetermined range, and to achieve a density of 5.5 g/cm ³ to 6.8 g/cm ³ . The body is disclosed. Patent Document 2 states that an oxide thin film with high resistance to hydrofluoric acid (an inorganic acid wet etching solution) can be obtained, and a thin film transistor with good transistor characteristics can be manufactured.

Japanese Patent Application Publication No. 2017-165646 JP2017-206430A

However, in Patent Document 1 and Patent Document 2, light stress resistance is not considered at all. Therefore, the thin film transistors using the oxide semiconductor thin films described in Patent Documents 1 and 2 sometimes have insufficient optical stress resistance.

The present invention has been made in view of these circumstances, and provides a thin film transistor with excellent static properties and optical stress resistance, an oxide semiconductor thin film for thin film transistors, and a sputtering target for forming the oxide semiconductor thin film. The purpose is to provide the following.

Aspect 1 of the present invention is
Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. This is a thin film transistor including an oxide semiconductor thin film in which the amount of change in light transmittance at a wavelength of 450 nm before and after is 5% or less in absolute value.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3)

Aspect 2 of the present invention is
The thin film transistor according to aspect 1 includes an oxide semiconductor thin film in which the atomic ratio satisfies the following formulas (4) to (6) and the amount of change in the light transmittance is 2% or less in absolute value.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6)

Aspect 3 of the present invention is
The thin film transistor according to aspect 1 includes an oxide semiconductor thin film in which the atomic ratio satisfies the following formulas (7) to (9) and the amount of change in the light transmittance is 1% or less in absolute value.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

Aspect 4 of the present invention is
Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. The oxide semiconductor thin film for a thin film transistor according to Aspect 1, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 5% or less in absolute value.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3)

Aspect 5 of the present invention is
Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (4) to (6), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. The oxide semiconductor thin film for a thin film transistor according to Aspect 2, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 2% or less in absolute value.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6)

Aspect 6 of the present invention is
Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (7) to (9), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. The oxide semiconductor thin film for a thin film transistor according to Aspect 3, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 1% or less in absolute value.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

Aspect 7 of the present invention is
The oxide according to aspect 1 or aspect 4, which is composed of In, Ga, Sn, and O, and wherein the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3). A sputtering target used for forming semiconductor thin films.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3)

Aspect 8 of the present invention is
The oxide according to aspect 2 or aspect 5, which is composed of In, Ga, Sn, and O, and wherein the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (4) to (6). A sputtering target used for forming semiconductor thin films.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6)

Aspect 9 of the present invention is
The oxide according to aspect 3 or aspect 6, which is composed of In, Ga, Sn, and O, and wherein the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (7) to (9). A sputtering target used for forming semiconductor thin films.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

According to the present invention, it is possible to provide a thin film transistor with excellent static properties and optical stress resistance, an oxide semiconductor thin film for the thin film transistor, and a sputtering target for forming the oxide semiconductor thin film.

FIG. 1 is a schematic cross-sectional view of a thin film transistor according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a thin film transistor according to an embodiment of the present invention.

The present inventors have conducted studies to realize TFTs with excellent optical stress resistance in addition to static characteristics when using oxides containing In, Ga, and Sn as metal elements in the semiconductor layer of thin film transistors. I've been piling it up. As a result, we determined the atomic ratio of each metal element in the In-Ga-Sn-O-based oxide semiconductor thin film and the absolute change in light transmittance at a wavelength of 450 nm when heat treated at 350°C for 1 hour in the air. The present invention was completed based on the discovery that the intended purpose could be achieved by appropriately controlling the values.

That is, the thin film transistor according to the embodiment of the present invention is composed of In, Ga, Sn, and O, and the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3). It is characterized in that it includes an oxide semiconductor thin film whose absolute value of light transmittance at a wavelength of 450 nm changes by 5% or less before and after heat treatment at 350° C. for 1 hour in an air atmosphere.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3)

An oxide semiconductor whose composition satisfies all of the above formulas (1) to (3), and whose absolute value of change in light transmittance at a wavelength of 450 nm is 5% or less before and after heat treatment at 350° C. for 1 hour in the above air atmosphere. By using a thin film, both the static characteristics and optical stress resistance of the formed thin film transistor can be improved.

Further, the thin film transistor according to the embodiment of the present invention includes an oxide semiconductor thin film in which the above atomic ratio satisfies the following formulas (4) to (6) and the above change in light transmittance is 2% or less in absolute value. It is preferable to include.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6)

Further, the thin film transistor according to the embodiment of the present invention includes an oxide semiconductor thin film in which the above atomic ratio satisfies the following formulas (7) to (9) and the above change in light transmittance is 1% or less in absolute value. It is even more preferable to include.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

In the formulas (1) to (9) of this specification, the above In refers to the content of In relative to the total of all metal elements excluding oxygen, ie, In, Ga, and Sn. Similarly, the above Ga and the above Sn in formulas (1) to (9) respectively mean the respective contents of Ga and Sn relative to the total of all metal elements excluding oxygen (O), that is, In, Ga, and Sn. In the following, the above "In/(In+Ga+Sn)" will be referred to as the "In atomic ratio", the above "Ga/(In+Ga+Sn)" will be referred to as the "Ga atomic ratio", and the above "Sn/(In+Ga+Sn)" will be referred to as the "Sn atomic ratio". There is a thing.

Hereinafter, thin film transistors according to embodiments of the present invention will be described in detail.

[1. Oxide semiconductor thin film]
First, an oxide semiconductor thin film included in a thin film transistor according to an embodiment of the present invention will be described. As described above, the oxide semiconductor thin film according to the embodiment of the present invention is made of an amorphous oxide composed of In, Ga, Sn, and O, and satisfies the above formulas (1) to (3).

Among the metal elements constituting the oxide, In is thought to contribute to improving electrical conductivity, Ga to reducing oxygen vacancies and controlling carrier density, and Sn to improving wet etching resistance. Furthermore, each of In, Ga, and Sn also contributes to light transmittance.

The reason for defining each range of the In atomic ratio, Ga atomic ratio, and Sn atomic ratio expressed by the above formulas (1) to (9) will be explained below.

- Regarding the In atomic ratio The In atomic ratio shown in the above formulas (1), (4), and (7) will be explained first. As the In atomic ratio increases, that is, as the amount of In in the metal element increases, the conductivity of the oxide semiconductor thin film improves, and the field effect mobility increases. In order to effectively exhibit this effect, the In atomic ratio needs to be 0.34 or more. The above In atomic ratio is preferably 0.36 or more, more preferably 0.38 or more. However, if the In atomic ratio is too large, the carrier density will increase too much and the threshold voltage will decrease. Furthermore, if the In atomic ratio is too large, a large amount of In oxide will be included in the oxide semiconductor thin film. The In oxide is likely to cause oxygen vacancies, and the oxygen vacancies may promote light absorption and reduce a given light transmittance. Therefore, the In atomic ratio is set to 0.43 or less. The In atomic ratio is preferably 0.42 or less, more preferably 0.41 or less.

- Regarding the Ga atomic ratio Next, the Ga atomic ratio shown in the above formulas (2), (5), and (8) will be explained. As the Ga atomic ratio increases, the electrical stability of the oxide semiconductor thin film improves, and the effect of suppressing excessive generation of carriers is exhibited. Further, Ga oxide is less likely to cause oxygen vacancies, and further has the effect of compensating for oxygen vacancies contained in the above-mentioned In oxides and the like. Therefore, the larger the Ga atomic ratio, the higher the light transmittance, and as a result, the effect of suppressing fluctuations in the light transmittance is also exhibited. In order to exhibit the above effect more effectively, the Ga atomic ratio needs to be 0.30 or more. The Ga atomic ratio is preferably 0.32 or more, more preferably 0.34 or more. However, if the Ga atomic ratio is too large, the conductivity of the oxide semiconductor thin film decreases and the field effect mobility tends to decrease. Therefore, the Ga atomic ratio is set to 0.39 or less. The Ga atomic ratio is preferably 0.38 or less, more preferably 0.37 or less.

- Regarding the Sn atomic ratio The Sn atomic ratio shown in the above formulas (3), (6), and (9) will be explained. As the Sn atomic ratio increases, the resistance of the oxide semiconductor thin film to acid-based etching solutions improves. In order to exhibit this effect more effectively, the Sn atomic ratio needs to be 0.21 or more. The Sn atomic ratio is preferably 0.22 or more, more preferably 0.23 or more. On the other hand, if the Sn atomic ratio is too large, the field effect mobility of the oxide semiconductor thin film decreases, and the resistance to acid-based etching solutions increases more than necessary, making it difficult to process the oxide semiconductor thin film itself. Further, Sn can bond with hydrogen or the like, promote light absorption through the bond, and reduce a predetermined light transmittance. Therefore, the Sn atomic ratio is set to 0.30 or less. The Sn atomic ratio is preferably 0.28 or less, more preferably 0.26 or less.

In one embodiment of the present invention, the oxide semiconductor thin film may contain unavoidable impurities. Unavoidable impurities may be introduced due to circumstances such as raw materials, materials, or manufacturing equipment. Examples of unavoidable impurities include Zn, Al, Pb, Si, Fe, Ni, Ti, Mg, Cr, and Zr. The content of unavoidable impurities is preferably 1% by mass or less, more preferably 500 mass ppm or less, based on the mass of the oxide semiconductor thin film.

Next, the reason why the amount of change in light transmittance at a wavelength of 450 nm was specified before and after heat treatment at 350° C. for 1 hour in an air atmosphere will be explained.

The present inventors conducted studies focusing on oxide semiconductor thin films in order to obtain TFTs that are reliably superior in optical stress resistance. The present inventors have hitherto believed that the characteristics of the oxide semiconductor thin film can be improved by performing pre-annealing immediately after forming the oxide semiconductor thin film that satisfies the above-described component composition. However, it has been found that even when the pre-annealed oxide semiconductor thin film is used in a TFT, there are variations in optical stress resistance. Therefore, the study focused on oxide semiconductor thin films that have poor optical stress resistance even after the above-mentioned pre-annealing.

First, the above-mentioned optical stress resistance is considered to be caused by defects caused by oxygen vacancies or bonds with hydrogen in the oxide semiconductor thin film. The degree of these defects can be evaluated by the light transmittance at a wavelength of 450 nm (hereinafter simply referred to as "light transmittance"), and the more defects there are, the lower the light transmittance is. By performing the above pre-annealing, the amount of defects in each oxide semiconductor thin film is reduced and exhibits high light transmittance.

However, when an oxide semiconductor thin film having poor optical stress resistance was checked, it was found that the light transmittance before the above-mentioned pre-annealing was low. In other words, the effect of pre-annealing on improving the light transmittance was large. This oxide semiconductor thin film exhibited high light transmittance as some of the defects were eliminated by pre-annealing, but since it originally had a large amount of defects, it is thought that the defects remaining even after pre-annealing resulted in poor optical stress resistance. . In other words, the present inventors have found that an oxide semiconductor thin film that contributes to excellent optical stress resistance has a small amount of change in light transmittance before and after pre-annealing.

The present inventors conducted extensive studies based on the above considerations, and found that if the absolute value of the change in light transmittance at a wavelength of 450 nm before and after heat treatment at 350°C for 1 hour in the air is 5% or less, optical stress It has been found that resistance can be improved. In addition, if the amount of change in the light transmittance is 5% or less in absolute value, it is possible to suppress the influence on TFT characteristics due to thermal processes during TFT manufacturing and display panel manufacturing, and to maintain a stable yield of TFTs. It is also expected that it will be easier to manufacture. The amount of change in the light transmittance is preferably 2% or less in absolute value, more preferably 1% or less in absolute value. Note that "heat treatment at 350° C. for 1 hour in an air atmosphere" simulates pre-annealing conditions after forming an oxide semiconductor thin film. Further, in order to obtain desired TFT static characteristics, the light transmittance of the oxide semiconductor thin film after pre-annealing is preferably 80% or more, more preferably 85% or more.

The oxide semiconductor thin film used in the present invention has been described above.

[1-1. Method for manufacturing oxide semiconductor thin film]
The above-mentioned oxide semiconductor thin film can also be formed by a film forming method such as a coating method in addition to a sputtering method. Preferably, the film is formed by a sputtering method using a sputtering target. Hereinafter, the sputtering target may be simply referred to as a "target". According to the sputtering method, a thin film with excellent in-plane uniformity of components and film thickness can be easily formed.

When forming a film using a sputtering method using a target, in order to interpolate the oxygen released from the thin film during sputtering film formation and increase the density of the oxide semiconductor thin film as much as possible, the gas pressure and oxygen partial pressure during film formation must be adjusted. It is preferable to appropriately control the power input to the sputtering target, the substrate temperature, the T-S distance, which is the distance between the sputtering target and the substrate, etc.

Specifically, for example, it is preferable to form a film under the following sputtering conditions.

The preferred gas pressure during film formation is approximately 1 to 10 mTorr. It is considered that when the gas pressure is lowered to such an extent that the sputtering discharge is stabilized, scattering of sputtered atoms is eliminated and a dense, ie, high-density film can be formed.

The amount of oxygen added may be appropriately controlled depending on the sputtering apparatus, the composition of the target, the thin film transistor manufacturing process, etc. so that the oxide semiconductor thin film behaves as a semiconductor. In addition, in order to suppress the amount of change in light transmittance before and after heat treatment to 5% or less in absolute value, the addition flow rate ratio (100 x O ₂ / (Ar + O ₂ )) is preferably 1% or more, more preferably 4% or more. It is recommended to do so.

It is recommended that the substrate temperature during film formation be controlled within the range of approximately room temperature to 200°C.

Furthermore, the amount of defects in the oxide semiconductor thin film is also affected by the conditions of heat treatment (pre-annealing) after film formation. Therefore, it is preferable to appropriately control the pre-annealing conditions. As for the pre-annealing conditions, it is recommended to carry out the pre-annealing under an atmospheric atmosphere (atmospheric pressure) at approximately 250 to 400° C. for 10 minutes to 3 hours. A specific example of the above-mentioned heat treatment is a pre-annealing treatment described below, that is, a heat treatment performed after the formation of the oxide semiconductor thin film and before the formation of the source/drain electrodes.

The preferred thickness of the oxide semiconductor thin film can be 10 nm or more, more preferably 20 nm or more, and can be 200 nm or less, further 100 nm or less, further 90 nm or less, and further still 80 nm or less.

[2. Sputtering target]
As a target used in the sputtering method, it is preferable to use a sputtering target that contains the above-mentioned elements and has the same composition as the desired oxide semiconductor thin film. Thereby, an oxide semiconductor thin film having a desired component composition can be formed with little compositional deviation. Specifically, a sputtering target is made of an oxide containing In, Ga, and Sn as metal elements, and the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3). Recommended to use.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3)

Preferably, it is recommended to use a sputtering target in which the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (4) to (6).
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6)

More preferably, it is recommended to use a sputtering target in which the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (7) to (9).
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

In one embodiment of the invention, the sputtering target may contain unavoidable impurities. Unavoidable impurities may be introduced due to circumstances such as raw materials, materials, or manufacturing equipment. Examples of unavoidable impurities include Zn, Al, Pb, Si, Fe, Ni, Ti, Mg, Cr, and Zr. The content of unavoidable impurities is preferably 1% by mass or less, more preferably 500 mass ppm or less, based on the mass of the sputtering target.

Alternatively, a film may be formed using a combinatorial sputtering method in which targets having different compositions are discharged simultaneously. For example, oxide targets of each of the elements In, Ga, and Sn, such as In ₂ O ₃ , Ga ₂ O ₃ , and SnO ₂ , or oxide targets of a mixture containing at least two of the above elements can also be used. Another method is to use one or more pure metal targets or alloy targets containing an element such as In, and to form a film while supplying oxygen as an atmospheric gas. For example, using an alloy target in which In, Ga, and Sn, excluding O, satisfy the above (1) to (3), (4) to (6), or (7) to (9), oxygen is used as the atmospheric gas. An example of this is to form a film while supplying it.

The target can be manufactured, for example, by a powder sintering method.

[3. Thin film transistor]
Next, a thin film transistor including the above oxide semiconductor thin film as a semiconductor layer of the thin film transistor will be described. The thin film transistor only needs to have at least a gate electrode, a gate insulating film, a source/drain electrode, the above-mentioned oxide semiconductor thin film, and a protective film on a substrate, and its structure is not particularly limited as long as it is a commonly used one. .

[3-1. Manufacturing method of thin film transistor]
Hereinafter, a preferred embodiment of a method for manufacturing a thin film transistor according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic cross-sectional view of a thin film transistor according to an embodiment of the present invention. Note that FIG. 1 and the manufacturing method described below show an example of a preferred embodiment of the present invention, and are not intended to be limited thereto. For example, although FIG. 1 shows a thin film transistor with a bottom gate structure, the thin film transistor is not limited thereto, and may be a top gate thin film transistor in which a gate insulating film and a gate electrode are sequentially provided on an oxide semiconductor thin film. Further, as in FIG. 2 and an embodiment described later, a thin film transistor may be provided with an ESL (etch-stop layer) protective film on an oxide semiconductor thin film.

In FIG. 1, a gate electrode 2 and a gate insulating film 3 are formed on a substrate 1, and an oxide semiconductor thin film 4 is formed thereon. Source/drain electrodes 5 are formed on the oxide semiconductor thin film 4 and are electrically connected to the oxide semiconductor thin film 4. Furthermore, a protective film 6 is formed, and a transparent conductive film (pad) 8 is electrically connected to the drain electrode 5 via a contact hole 7 .

The method of forming the gate electrode 2 and the gate insulating film 3 on the substrate 1 is not particularly limited, and a commonly used method can be adopted. Further, the types of the gate electrode 2 and the gate insulating film 3 are not particularly limited, and commonly used ones can be used. For example, as the gate electrode 2, an Al-based metal such as pure Al or an Al alloy, or a Cu-based metal such as pure Cu or a Cu alloy, which has a low electrical resistivity; a high-melting point metal such as Mo, Cr, or Ti, which has high heat resistance, or a metal such as these Alloys; can be preferably used. Furthermore, typical examples of the gate insulating film include a silicon oxide film, a silicon nitride film, a silicon oxynitride film, and the like. As the gate insulating film, oxides such as Al ₂ O ₃ and Y ₂ O ₃ or a stack of these can also be used.

Next, an oxide semiconductor thin film 4 is formed as a semiconductor layer. The oxide semiconductor thin film 4 is preferably formed by a DC sputtering method or an RF sputtering method using a sputtering target having the same composition as the oxide semiconductor thin film 4. Further, in order to obtain an oxide semiconductor thin film in which a decrease in light transmittance due to pre-annealing described below is suppressed, it is preferable to form the oxide semiconductor thin film under the above-mentioned conditions. Alternatively, the film may be formed by a combinatorial sputtering method using a plurality of types of sputtering targets.

After forming the oxide semiconductor thin film, patterning is performed by wet etching. As described above, after the oxide semiconductor thin film is formed and before the source/drain electrodes 5 are formed, pre-annealing is preferably performed to improve the film quality of the oxide semiconductor thin film 4. Pre-annealing may be performed before or after patterning the oxide semiconductor thin film. Thereby, the on-current and field effect mobility of the transistor characteristics can be increased, and as a result, the transistor performance can be improved.

Next, source/drain electrodes 5 are formed. The type of source/drain electrodes 5 is not particularly limited, and commonly used ones can be used. For example, similarly to the gate electrode 2 described above, a high melting point metal such as Mo or an alloy containing the metal; an Al-based metal; a Cu-based metal; etc. can be used.

As a method for forming the source/drain electrodes 5, for example, a metal thin film is formed by magnetron sputtering, patterned by photolithography, and wet etched to form the source/drain electrodes 5. For the wet etching, an inorganic acid etching solution or a hydrogen peroxide etching solution can be used.

After forming the source/drain electrodes 5 and before forming the protective film 6, heat treatment (200° C. to 300° C.) or N ₂ O plasma treatment may be performed as necessary to recover damage to the oxide surface. .

Next, a protective film (passivation insulating film) 6 is formed on the oxide semiconductor thin film 4 by a CVD (Chemical Vapor Deposition) method. As the protective film 6, for example, a laminated film including a silicon nitride film as an upper layer and a silicon oxide film as a lower layer can be used. The material is not limited to these, and a silicon oxynitride film, as well as oxides such as Al ₂ O ₃ and Y ₂ O ₃ , or a stack of these may also be used. Specifically, as the protective film 6, a laminated film having a total thickness of 250 nm is formed by laminating a SiOx film with a thickness of 100 nm and an SiNx film with a thickness of 150 nm. A mixed gas of SiH ₄ , N ₂ and N ₂ O is used to form the SiO ₂ film, and a mixed gas of SiH ₄ , N ₂ and NH ₃ is used to form the SiNx film. Examples of the conditions include a film formation power density of 0.32 W/cm ² , a film formation temperature of 200° C., and a gas pressure during film formation of 133 Pa.

Next, the transparent conductive film (pad) 8 is electrically connected to the drain electrode 5 via the contact hole 7 based on a conventional method. The type of transparent conductive film is not particularly limited, and commonly used ones can be used. For example, an ITO film having a thickness of 80 nm may be formed using a DC sputtering method under the conditions of carrier gas: a mixed gas of argon and oxygen gas, film forming power: 200 W, and gas pressure: 5 mTorr. By applying a probe such as a semiconductor testing device (tester) to the pad 8, electrical measurements of the thin film transistor can be performed.

Hereinafter, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples, and can be practiced with modifications within the scope that fits the spirit of the preceding and following examples. All of them are included within the technical scope of the present invention.

That is, in Experimental Method 1 below, an oxide thin film assuming an oxide semiconductor thin film was formed on glass, the thin film was heat-treated under the following conditions, and the amount of change in transmittance before and after the heat treatment was evaluated. In addition, in Experimental Method 2 below, a thin film transistor was fabricated using an oxide semiconductor thin film having the same composition as the oxide thin film of Experimental Method 1 as a semiconductor layer, and the static characteristics and optical stress resistance were evaluated.

(Experimental method 1: Spectroscopic measurement)
An oxide thin film was formed by sputtering on a glass substrate ("EagleXG" manufactured by Corning Inc.) having a diameter of 4 inches and a thickness of 0.7 mm. The sputtering conditions were as follows.

<Sputtering conditions>
・Sputtering equipment: “CS-200” manufactured by ULVAC
・Substrate temperature: 25℃ (room temperature)
・Thickness of oxide thin film: 300nm
・Carrier gas: Ar
・Film forming power density: 2.55W/ ^cm2
・Gas pressure: 1mTorr
・Amount of oxygen added: O ₂ / (Ar + O ₂ ) = 4% (volume ratio)

In addition, the atomic ratio of each element of In, Ga, and Sn of a sputtering target was adjusted suitably so that it might be the same as the target atomic ratio of each element of an oxide thin film.

The transmittance of the formed oxide thin film at a wavelength of 450 nm (hereinafter, this transmittance will be referred to as "transmittance before heat treatment") was measured. The transmittance was measured using an ultraviolet-visible near-infrared spectrophotometer "V-570" manufactured by JASCO Corporation. The measurement used S-polarized light in transmission mode asynchronous mode. The measurement was performed with the incident angle set at 5 degrees and the detector angle set at 0 degrees. Subsequently, heat treatment was performed at 350° C. for 1 hour in an atmospheric atmosphere and under atmospheric pressure. This heat treatment simulates the post-forming pre-annealing of the oxide semiconductor thin film in Experimental Method 2. Subsequently, the transmittance of the heat-treated oxide semiconductor thin film at a wavelength of 450 nm (hereinafter, this transmittance will be referred to as "transmittance after heat treatment") was measured. The measurement method was the same as the transmittance measurement performed before heat treatment. The absolute value of the value obtained by subtracting the transmittance before heat treatment from the transmittance after heat treatment was taken as the absolute value of the amount of change in transmittance before and after heat treatment (hereinafter sometimes simply referred to as "amount of change in transmittance"). Table 1 shows the transmittance and the amount of change in transmittance after heat treatment for each sample. Samples with a transmittance change of 5% or less were accepted. In Table 1, numerical values outside the range of the embodiments of the present invention are underlined.

Further, the atomic ratio of each metal element in the formed oxide thin film was measured by ICP (Inductively Coupled Plasma) emission spectroscopy. The measurement was performed by separately preparing a sample in which only an oxide semiconductor thin film was formed on a glass substrate by the sputtering method in the same manner as described above. The measurement was performed using "CIROS Mark II" manufactured by Rigaku Corporation. The measurement results for each sample are shown in Table 1.

(Experimental method 2: Fabrication of thin film transistor)
Next, experimental method 2 will be explained with reference to FIG. FIG. 2 is a schematic cross-sectional view of a thin film transistor manufactured by Experimental Method 2. First, a glass substrate 1 ("Eagle A film was formed. The film forming conditions were a substrate temperature of 25° C. (room temperature), a film forming power density of 3.8 W/cm ² , a pressure of 0.266 Pa, and a carrier gas of Ar. After forming the Mo thin film, a gate electrode 2 was formed by patterning.

Next, as a gate insulating film, a silicon oxide film having an average thickness of 250 nm was formed to cover the gate electrode 2 by CVD. A mixed gas of N ₂ O and SiH ₄ was used as the raw material gas. The film forming conditions were a substrate temperature of 320° C., a film forming power density of 0.96 W/cm ² , and a pressure of 133 Pa.

Next, an oxide semiconductor thin film containing In, Ga, and Sn as metal elements and having an average thickness of 40 nm was formed as an oxide semiconductor thin film on the front surface side of the glass substrate 1 by a sputtering method. Various conditions for the sputtering method were performed in the same manner as in the formation of the oxide thin film in Experimental Method 1, to obtain an oxide semiconductor thin film. Therefore, the atomic ratios of In, Ga, and Sn in the oxide semiconductor thin film are the same as the atomic ratios in the corresponding oxide thin film of Experimental Method 1.

The obtained oxide semiconductor thin film was patterned by photolithography and wet etching to form an oxide semiconductor thin film 4. Note that "ITO-07N" manufactured by Kanto Kagaku Co., Ltd. was used as the wet etchant.

Here, pre-annealing treatment was performed to improve the film quality of the oxide semiconductor thin film 4. The conditions for the pre-annealing treatment were 60 minutes at 350° C. in an air atmosphere (atmospheric pressure).

Next, a silicon oxide film was formed on the surface side of the glass substrate 1 by CVD to have an average thickness of 100 nm. A mixed gas of N ₂ O and SiH ₄ was used as the raw material gas. The film forming conditions were a substrate temperature of 230° C., a film forming power density of 0.32 W/cm ² , and a pressure of 133 Pa. After forming the silicon oxide film, an ESL protective film 9 was formed by patterning.

Next, a Mo thin film was formed on the surface side of the glass substrate 1 to have an average thickness of 200 nm. The film forming conditions were a substrate temperature of 25° C. (room temperature), a film forming power density of 3.8 W/cm ² , a pressure of 0.266 Pa, and a carrier gas of Ar. After forming the Mo thin film, a source electrode and a drain electrode 5 were formed by patterning.

Next, a passivation insulating film 6 having a two-layer structure of a silicon oxide film (average thickness: 100 nm) and a silicon nitride film (average thickness: 150 nm) was formed on the surface side of the glass substrate 1 by the CVD method. As the source gas, a mixed gas of N ₂ O and SiH ₄ was used to form the silicon oxide film, and a mixed gas of NH ₃ and SiH ₄ was used to form the silicon nitride film. The film forming conditions were a substrate temperature of 150° C., a film forming power density of 0.32 W/cm ² , and a pressure of 133 Pa.

Next, a contact hole was formed by photolithography and dry etching, and a pad 8 for electrical connection to the drain electrode was provided.

Next, a post-annealing process was performed. Note that the conditions for the post-annealing treatment were 30 minutes at 250° C. in an N ₂ atmosphere at atmospheric pressure.

A thin film transistor was obtained as described above. Note that the channel length of this thin film transistor was 20 μm, and the channel width was 200 μm.

For each of the obtained TFTs, field effect mobility (μFE), threshold voltage (Vth), S value, resistance to optical stress due to light irradiation and negative bias (NBTIS, Bias Thermal Illumination Stress) were evaluated as transistor characteristics. . Various transistor characteristics were measured using a Keithley 4200SCS semiconductor parameter analyzer.

[Evaluation of static characteristics (μFE, Vth, S value)]
Static characteristics were evaluated under the following measurement conditions.
・Drain voltage: 10V
・Gate voltage: -30V to 30V (measurement interval: 0.25V)
・Substrate temperature: room temperature

The measurement results of the static properties of each sample are shown in Table 1. In addition, the criteria for determining each static characteristic were as follows.
(μFE)
・5 cm ² /Vs or more: high mobility and pass ・Less than 5 cm ² /Vs: low mobility and fail (Vth)
・0V or more: Pass ・Less than 0V: Fail (S value)
・1.0V/dec or less: Pass ・More than 1.0V/dec: Fail

[Evaluation of light stress tolerance]
Next, using the above TFT, NBTIS was evaluated as photo stress resistance in the following manner. Photo stress resistance was evaluated by performing a stress application test in which light was irradiated while applying a negative bias to the gate electrode. The stress application conditions are as follows.
・Gate voltage: -20V
・Source/drain voltage: 10V
・Substrate temperature: 60℃
・Light stress conditions:
Stress application time: 2 hours Light intensity: 25000NIT
Light source: white LED

The difference ΔVth (V) between threshold voltages (Vth) before and after stress application was measured. The measurement results for each sample are shown in Table 1 as NBTIS. The criteria for determining ΔVth were as follows.
・6.0V or less: Excellent resistance to light stress, passed ・Over 6.0V: Poor resistance to light stress, failed

Consider the results in Table 1. Sample No. Examples 1 to 3 are examples using an oxide semiconductor thin film that satisfies the requirements specified in the embodiments of the present invention. Sample No. Samples 1 to 3 were excellent in both static properties (μFE, Vth, S value) and light stress resistance (NBTIS). On the other hand, sample No. 4 is a comparative example using an oxide semiconductor thin film that does not satisfy the requirements specified in the embodiments of the present invention. Sample No. Although Sample No. 4 had excellent static characteristics, the amount of change in transmittance was large, so it was inferior in light stress resistance.

1 Substrate 2 Gate electrode 3 Gate insulating film 4 Oxide semiconductor thin film 5 Source/drain electrode 6 Protective film 7 Contact hole 8 Transparent conductive film (pad)
9 ESL protective film

Claims (9)

Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. A thin film transistor including an oxide semiconductor thin film in which the amount of change in light transmittance at a wavelength of 450 nm before and after is 5% or less in absolute value.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3) The thin film transistor according to claim 1, comprising an oxide semiconductor thin film in which the atomic ratio satisfies the following formulas (4) to (6) and the amount of change in the light transmittance is 2% or less in absolute value.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6) The thin film transistor according to claim 1, comprising an oxide semiconductor thin film in which the atomic ratio satisfies the following formulas (7) to (9) and the amount of change in the light transmittance is 1% or less in absolute value.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9) Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. The oxide semiconductor thin film for a thin film transistor according to claim 1, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 5% or less in absolute value.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3) Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (4) to (6), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. 3. The oxide semiconductor thin film for a thin film transistor according to claim 2, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 2% or less in absolute value.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6) Consisting of In, Ga, Sn, and O, the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (7) to (9), and is heat-treated at 350 ° C. for 1 hour in an air atmosphere. 4. The oxide semiconductor thin film for a thin film transistor according to claim 3, wherein the amount of change in light transmittance at a wavelength of 450 nm before and after is 1% or less in absolute value.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9) The metal element according to claim 1 or 4, which is composed of In, Ga, Sn, and O, and the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (1) to (3). A sputtering target used to form oxide semiconductor thin films.
0.34≦In/(In+Ga+Sn)≦0.43 (1)
0.30≦Ga/(In+Ga+Sn)≦0.39 (2)
0.21≦Sn/(In+Ga+Sn)≦0.30 (3) The metal element according to claim 2 or 5, which is composed of In, Ga, Sn, and O, and the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (4) to (6). A sputtering target used to form oxide semiconductor thin films.
0.36≦In/(In+Ga+Sn)≦0.42 (4)
0.32≦Ga/(In+Ga+Sn)≦0.38 (5)
0.22≦Sn/(In+Ga+Sn)≦0.28 (6) The metal element according to claim 3 or 6, which is composed of In, Ga, Sn, and O, and the atomic ratio of each metal element to the total of In, Ga, and Sn satisfies the following formulas (7) to (9). A sputtering target used to form oxide semiconductor thin films.
0.38≦In/(In+Ga+Sn)≦0.41 (7)
0.34≦Ga/(In+Ga+Sn)≦0.37 (8)
0.23≦Sn/(In+Ga+Sn)≦0.26 (9)

Images