JP2000256842A - Ito sputtering target, and production of ito sintered compact and transparent conductive film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ITO sputtering target capable of forming an ITO thin film of extremely low resistivity even if the formation of the ITO thin film is carried out by the ordinary sputtering method. SOLUTION: The thin film with low resistivity can be obtained by using, as a sputtering target, an ITO sintered compact which has a composition consisting essentially of indium, tin, and oxygen and also has >=7.08 g/cm3 sintered density of sintered compact and >=62% oxygen content by an atomic ratio of O/(In+Sn+O). Such a sintered compact can be produced by performing sintering under a gaseous-mixture atmosphere of ozone and oxygen.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing an ITO sintered body, an ITO sputtering target for forming a transparent conductive film made of the sintered body, and a method for manufacturing a transparent conductive film using the sputtering target.

[0002]

2. Description of the Related Art ITO (Indium) is a transparent conductive film.
Tin Oxide) thin films are manufactured by chemical film forming methods such as spray pyrolysis and CVD, electron beam evaporation, and the like.
It can be roughly classified into physical film forming methods such as a sputtering method. Among them, the sputtering method is used in various fields because it is a film forming method that can easily increase the area and obtain a high-performance film.

When an ITO thin film is produced by a sputtering method, an alloy target composed of indium metal and tin (IT target) or a composite oxide sintered body target composed of indium oxide and tin oxide (ITO target) is used as a sputtering target. Is used. Among them, the method using an ITO target has a smaller change with time in the resistance value and transmittance of the obtained film than the method using an IT target, and the film forming conditions can be easily controlled. Has become mainstream. Among them, a magnet is arranged behind a sputtering target, and a magnetic field is present on the target surface to converge the plasma.
The C magnetron sputtering method is often used in mass production equipment because it has a feature that the film formation speed is high.

[0004] ITO thin films have features such as high conductivity and high transmittance, and can be easily processed in a fine manner. Therefore, they are used in a wide range of fields such as display electrodes for flat panel displays, window materials for solar cells, and antistatic films. Used throughout. For this reason, researches on sputtering targets for providing ITO thin films and ITO sintered bodies as a raw material thereof have been actively conducted. For example, Japanese Patent Application Laid-Open No. 9-255426 discloses a method for obtaining a high-density and uniform sintered body. A method of sintering a molded body obtained by casting in an ozone-containing atmosphere is disclosed.

On the other hand, in particular, in the field of flat panel displays such as liquid crystal display devices, the size and definition have been advanced in recent years, and the demand for lower resistance of ITO thin films as display electrodes has been increasing. .

Means for reducing the resistance of the thin film include, for example, a method of increasing the thickness of the thin film and a method of reducing the resistivity of the thin film. However, the method (1) is not preferable because the etching characteristics deteriorate as the film thickness increases, and it becomes difficult to form a fine pattern. Therefore, it is necessary to lower the resistivity of the thin film.
It is said to be 100 μΩ · cm.

A low-voltage sputtering method is known as an approach for lowering the resistivity of an ITO thin film from a film forming method. In this method, it is considered that the resistivity of the film is reduced because the damage to the film due to high energy neutral particles or negative ions of oxygen can be reduced by lowering the impedance at the time of discharge. As a means for reducing the discharge voltage, there is a method of increasing the magnetic field of the magnet.

When this method is adopted, the experiments performed by the present inventors have shown that the magnetic field intensity of the horizontal component immediately above the target is 400 μm.
By changing from Gauss to 1000 Gauss, DC600
The discharge voltage when W was applied decreased from about 360 V to 260 V. As a result, the resistivity of the obtained thin film was 300 μm.
Ω · cm to 150 μΩ · cm. But,
Even if this technology is adopted, the demand for a low resistivity thin film on the market is not satisfied.

Attempts have been made to reduce the resistivity by methods other than the sputtering method, and an ion plating method using arc discharge, a vacuum evaporation method with a gradient in the substrate temperature, and the like have been reported. However, it is difficult to form a uniform film on a large area such as a liquid crystal display by any of the methods, and it has not been established as a mass production technique.

Therefore, there is a need in the market for a technique for obtaining an ITO thin film having a low resistivity by a technique which can be used as a mass production technique.

In addition, particles adhering to the substrate, apart from the characteristics of the thin film, have been regarded as a problem. In particular, the yellow powder that accumulates on the non-erosion portion of the target surface as the cumulative sputtering time of the target increases, peels off from the target surface when the amount of deposition increases, and floats in a vacuum to adhere to the substrate surface. This yellow powder changes its direction by colliding with gas particles (argon, oxygen, etc.) in the vacuum chamber before the target material beaten out from the erosion part reaches the opposing substrate, and is deposited on the target surface. It is believed that.

When this yellow powder adheres to the substrate, it degrades the display quality of a liquid crystal display device or the like, causing a defect.
Significantly lowers product yield. Therefore, it is desired to reduce the amount of yellow powder deposited on the target surface.

[0013]

The problem to be solved by the present invention is that the IT
IT by ordinary sputtering method using O target
An object of the present invention is to provide an ITO sputtering target capable of obtaining a lower resistivity than before even when an O thin film is manufactured, and a method for manufacturing a transparent conductive film using the ITO target.

It is another object of the present invention to provide an ITO sputtering target capable of reducing the amount of yellow powder deposited on a non-erosion portion as the cumulative use time of the ITO target increases.

[0015]

The present inventors have conducted intensive studies on the relationship between the density and oxygen content of the ITO target and the resistivity of the obtained thin film. 1) When the density of the ITO target increases, the resistance of the thin film decreases. 2) that the resistivity of the thin film decreases as the amount of oxygen in the target increases.

Based on this finding, further investigations have been carried out, and the sintered body has a sintered density of 7.08 g / cm ³ or more and an oxygen content of 62% or more in an atomic ratio of O / (In + Sn + O). By using an ITO sputtering target consisting of a body to produce a thin film by ordinary sputtering,
It has been found that a thin film having an extremely low resistivity of 00 μΩ · cm can be obtained. Also, with the high density as described above,
By using a sputtering target made of an ITO sintered body having a high oxygen content, it was also found that the amount of yellow powder deposited on the non-erosion portion can be reduced with an increase in the cumulative use time of the target, and the present invention was completed. Was.

That is, the present invention substantially comprises indium, tin and oxygen, has a sintered density of 7.08 g / cm ³ or more, and has an oxygen content of O / (In).
+ Sn + O) An ITO sputtering target made of an ITO sintered body having an atomic ratio of 62% or more and a method of manufacturing a transparent conductive film for forming a transparent conductive film on a substrate, wherein the above-mentioned ITO sputtering target is used. The present invention relates to a method for producing a transparent conductive film.

According to the present invention, the cause of the decrease in the resistivity of the obtained thin film and the cause of the decrease in the amount of yellow powder generated with the increase in the accumulated use time have not yet been clarified. However, when a target having a high oxygen content is used, the oxygen partial pressure in the sputtering gas used at the time of sputtering is reduced, and of the oxygen provided to the thin film, oxygen supplied from the target and O ₂ as the sputtering gas are used. It is thought that the change in the ratio to the amount of oxygen provided in the form has an effect, but such an inference has no effect on the present invention.

Hereinafter, the present invention will be described in detail.

The sintered density according to the present invention is 7.08 g / c.
m ³ or more, and the oxygen content is 62% or more, preferably 64% or more in atomic ratio of O / (In + Sn + O).
A sputtering target using a TO sintered body can be manufactured, for example, by the following method.

First, tin oxide powder having a maximum particle diameter of 1 μm or less and a median diameter (particle diameter of powder corresponding to 50% of the cumulative particle size distribution) of 0.4 μm or less is mixed with indium oxide powder at a desired ratio. Into a ball mill pot and dry or wet mixed to produce a mixed powder.
By using such a powder, an effect of increasing the density of the sintered body can be obtained. In the present invention, the content of tin oxide in the mixed powder is 5 to 1 as SnO ₂ / (In ₂ O ₃ + SnO ₂ ).
Preferably it is 5% by weight. By doing this,
The atomic ratio of Sn / (In + Sn) according to the present invention is 4.5.
~ 14% of a mixed powder is obtained, and the amount of tin oxide in a target obtained by molding and sintering this powder is Sn / (In +
The atomic ratio of Sn) is 4.5 to 14%. By doing so, the resistivity of the thin film obtained when the film is formed by the sputtering method decreases.

The tap density of the mixed powder of the tin oxide powder and the indium oxide powder is preferably adjusted to 1.8 g / cm ³ or more. Tap density 1.8 g / cm ³
In order to obtain the above mixed powder, for example, it is obtained by mixing the mixed powder with the above mixing means for 10 hours or more. If the tap density is less than 1.8 g / cm ³ , a compact having sufficient strength cannot be obtained when the mixed powder is compacted and subjected to the sintering step, and as a result, cracks are formed in the sintered compact during sintering. May occur.

The tap density referred to here is a physical property value of the powder generally used in the powder handling industry. For example, a powder for measuring the tap density is put into a measuring cylinder, and then a scalpel containing the powder is measured. It is a value calculated from the volume and weight of the powder obtained by tapping the cylinder until the bulk of the powder no longer changes.

The powder thus obtained is molded by a molding method such as a pressing method or a casting method to produce an ITO molded body. When a molded body is manufactured by press molding, after filling the mixed powder into a mold having a predetermined size, the molded body is pressed by a press at a pressure of 100 to 300 kg / cm ² to obtain a molded body. At this time, a binder such as PVA may be added as needed. On the other hand, when a molded body is manufactured by the casting method, the mixed powder is mixed with water, a binder and a dispersant to form a slurry, and the thus obtained 5
A slurry having a viscosity of 0 to 5000 centipoise is poured into a casting mold to produce a molded article.

Next, the compact thus obtained is subjected to a consolidation treatment by a cold isostatic press (CIP) as required. At this time, the pressure of CIP is ² ton / cm ² or more, preferably ² to 5 ton / c in order to obtain a sufficient consolidation effect.
m ² is desirable. When the molding is performed by the casting method, a drying treatment and a debinding treatment are performed at a temperature of 300 to 500 ° C. for about 5 to 20 hours in order to remove moisture and organic substances such as a binder remaining in the molded article after the CIP. Is preferably applied. Even when molding is performed by a press method, when a binder is used at the time of molding, it is preferable to perform the same binder removal treatment.

Next, the compact thus obtained is sintered. The sintering temperature can be appropriately selected, but is desirably 1450 to 1650 ° C. in order to obtain a sufficient density increasing effect and to suppress evaporation of tin oxide. Also, the sintering time is desirably 5 hours or more, preferably 5 to 30 hours, in order to obtain a sufficient density increasing effect.

The sintering atmosphere is such that the oxygen content in the sintered body is O
For example, an oxygen gas atmosphere containing ozone is used so that the atomic ratio of / (In + Sn + O) becomes 62% or more.
At this time, the ozone concentration in the oxygen gas (O ₃ / (O ₂ +
O ₃ ), the volume ratio) is preferably 0.1 to 20%. The flow rate of the ozone-containing oxygen gas is an oxygen linear velocity of 2.0 cm.
/ Min or more, particularly preferably 2.5 cm / min. By controlling the flow rate of the ozone-containing oxygen gas in this manner, an ITO thin film having an extremely low resistivity can be obtained, and the amount of yellow powder adhering to the non-erosion portion of the target surface can be reduced. Become. The pressure is, 30~100mmH ₂ O is preferably a gauge pressure.

In order to prevent the dissociation of oxygen from the compact and the sintered body, it is preferable to introduce a mixed gas of oxygen and ozone before the furnace temperature reaches 400 ° C. at the time of heating. Further, it is preferable to keep the mixed gas flowing until the temperature in the furnace reaches 400 ° C. when the temperature is lowered. In this case, the atmosphere before the introduction of the mixed gas and / or the atmosphere after the introduction of the mixed gas is stopped is preferably an air or oxygen atmosphere.

Further, the flow rate of ozone-containing oxygen (L / min) flowing into the sintering furnace and the charged weight (kg) of the compact are expressed by the formula: charged weight (kg) / ozone-containing oxygen flow rate (L / min).
Is preferably determined to be 1.0 or less,
More preferably 0.5 or less, even more preferably 0.3
It is as follows.

As described above, for example, by controlling the flow rate and the flow rate of the ozone-containing oxygen gas, the oxygen content in the sintered body can be made to be 62% or more in O / (In + Sn + O) atomic ratio. .

Next, the obtained sintered body is ground into a desired shape and, if necessary, is bonded to a backing plate made of oxygen-free copper or the like using indium solder or the like, thereby providing the present invention. An ITO sputtering target is manufactured.

The obtained target is set in a sputtering apparatus, and sputtering is performed by applying an dc or rf electric field using an inert gas such as argon and, if necessary, an oxygen gas as a sputtering gas. A conductive film is obtained.

The sputtering conditions include a substrate temperature,
There are parameters to be controlled, such as the sputtering gas pressure, the oxygen partial pressure in the sputtering gas pressure, and the strength of the applied electric field. The set values of these parameters for obtaining the lowest resistivity vary depending on the type of substrate used and the equipment used. For example, substrate temperature: 200 ° C., sputtering gas (Ar + O ₂ ) pressure: 5 mTorr, oxygen concentration:
A value such as 0.1% and DC power: 600 W can be exemplified.

The ITO thin film thus obtained is composed of 1
It has an extremely low resistivity of 00 μΩ · cm.

[0035]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited thereto.

EXAMPLE 1 900 g of indium oxide powder and tin oxide powder 10 having a maximum particle diameter of 1 μm or less and a median diameter (particle diameter of powder corresponding to 50% of the cumulative distribution of particle diameter) of 0.4 μm or less
0 g was placed in a polyethylene pot and mixed for 72 hours by a dry ball mill to produce a mixed powder. When the tap density of the mixed powder was measured, it was 2.0 g / cm ³ .

This mixed powder was placed in a mold, and 300 kg /
It was pressed at a pressure of cm ² to obtain a molded body. This compact was subjected to a densification treatment by CIP at a pressure of 3 ton / cm ² . Next, this compact was placed in a pure oxygen atmosphere sintering furnace and sintered under the following conditions.

(Sintering Conditions) Sintering temperature: 1500 ° C., heating rate: 25 ° C./Hr, sintering time: 6 hours, gas introduced into the sintering furnace: oxygen + 2% ozone, introduced gas pressure: 30 mmH ₂ O (gauge pressure), introduced gas linear velocity: 2.6 cm / min, charged weight / gas flow rate: 0.2.
4 kg · min / L, gas introduction start temperature (when temperature rises): 4
00 ° C., gas introduction stop temperature (during cooling): 400 ° C. The density of the obtained sintered body was 7.12 g / cm ^{3 as} measured by Archimedes' method. The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. 6
It contained 2.3 atomic% oxygen.

[0039]

[Table 1]

This sintered body was 4 inches × by wet processing.
A 7 inch, 6 mm thick sintered body was processed and bonded to an oxygen-free copper backing plate using indium solder to obtain a target.

The thin film was evaluated by sputtering this target under the following sputtering conditions.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.05%, film thickness: 30
00 °, substrate temperature: 200 ° C.

The resistivity of the obtained film is 104 μΩ · cm
The transmittance at 550 nm was 87.0%.
The transmittance was measured as a transmittance including a glass substrate using air as a reference. Cor on the glass substrate
# 7059 manufactured by Ning Co. was used.

Example 2 In the same manner as in Example 1, a mixed powder of indium oxide and tin oxide having a tap density of 2.0 g / cm ³ was obtained. This mixed powder was placed in a mold, pressed and CIPed under the same conditions as in Example 1 to obtain a molded body. Next, this compact was placed in a pure oxygen atmosphere sintering furnace and sintered under the following conditions.

(Sintering Conditions) Sintering temperature: 1500 ° C., heating rate: 25 ° C./Hr, sintering time: 6 hours, gas introduced into the sintering furnace: oxygen + 10% ozone, gas introduced pressure: 80 mmH ₂ O (gauge pressure), introduced gas linear velocity: 3.0 cm / min, charged weight / gas flow rate: 0.
3 kg · min / L, gas introduction start temperature (at elevated temperature): 4
00 ° C, gas introduction stop temperature (during cooling): 100 ° C.

When the density of the obtained sintered body was measured by the Archimedes method, it was 7.13 g / cm ³ . The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. It contained 64.1 atomic percent oxygen.

The sintered body was processed by a wet processing method to 4 inches ×
A 7 inch, 6 mm thick sintered body was processed and bonded to an oxygen-free copper backing plate using indium solder to obtain a target.

The thin film was evaluated by sputtering this target under the following sputtering conditions.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.02%, film thickness: 30
00 °, substrate temperature: 200 ° C.

The resistivity of the obtained film is 100 μΩ · cm
The transmittance at 550 nm was 86.9%.

Comparative Example 1 In the same manner as in Example 1, a mixed powder of indium oxide and tin oxide having a tap density of 2.0 g / cm ³ was obtained. This mixed powder was placed in a mold, pressed and CIPed under the same conditions as in Example 1 to obtain a molded body. Next, this compact was placed in an air atmosphere sintering furnace and sintered under the following conditions.

(Sintering Conditions) Sintering temperature: 1500 ° C., heating rate: 25 ° C./Hr, sintering time: 6 hours, gas introduced into the sintering furnace: oxygen, gas introduced pressure: 50 mmH ₂ O (gauge Pressure), linear velocity of introduced gas: 2.
5 cm / min, charge weight / gas flow rate: 0.5 kg · mi
n / L, gas introduction start temperature: 400 ° C., gas introduction stop temperature: 100 ° C. When the density of the obtained sintered body was measured by the Archimedes method, it was 7.13 g / cm ³ . The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. 6
It contained 0.3 atomic% oxygen.

The sintered body was 4 inches × by wet processing.
A 7 inch, 6 mm thick sintered body was processed and bonded to an oxygen-free copper backing plate using indium solder to obtain a target.

The thin film was evaluated by sputtering this target under the following sputtering conditions.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.2%, film thickness: 300
0Å.

The resistivity of the obtained film was 150 μΩ · cm.
The transmittance at 550 nm was 86.9%.

Comparative Example 2 In the same manner as in Example 1, a mixed powder of indium oxide and tin oxide having a tap density of 2.0 g / cm ³ was obtained. This mixed powder was placed in a mold, pressed and CIPed under the same conditions as in Example 1 to obtain a molded body. Next, this compact was placed in a pure oxygen atmosphere sintering furnace and sintered under the following conditions.

(Sintering Conditions) Sintering temperature: 1500 ° C., heating rate: 25 ° C./Hr, sintering time: 6 hours, gas introduced into the sintering furnace: oxygen + 2% ozone, gas introduced pressure: 50 mmH ₂ O (gauge pressure), introduced gas linear velocity: 1.2 cm / min, charged weight / gas flow rate: 1.
1 kg · min / L, gas introduction start temperature: 400 ° C., gas introduction stop temperature: 100 ° C. When the density of the obtained sintered body was measured by the Archimedes method, it was 7.10 g / cm ³ . The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. 6
It contained 0.5 atomic% oxygen.

The sintered body was 4 inches × by wet processing.
A 7 inch, 6 mm thick sintered body was processed and bonded to an oxygen-free copper backing plate using indium solder to obtain a target.

The thin film was evaluated by sputtering this target under the following sputtering conditions.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.2%, film thickness: 300
0Å.

The resistivity of the obtained film is 145 μΩ · cm
The transmittance at 550 nm was 87.0%.

Example 3 An ITO sintered body was manufactured in the same manner as in Example 1.

When the density of the obtained sintered body was measured by the Archimedes method, it was 7.12 g / cm ³ . The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. It contained 62.3 atomic% oxygen.

This sintered body was processed into a sintered body having a diameter of 4 inches and a thickness of 5 mm by a wet processing method, and was bonded to a backing plate made of oxygen-free copper using indium solder to obtain a target.

The target was continuously discharged under the following sputtering conditions, and the amount of yellow powder generated was examined.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.1%, substrate temperature:
200 ° C. Here, the oxygen gas concentration was set to a value at which the resistivity of the obtained thin film decreased most.

FIG. 1 shows a photograph of the external appearance of the target three hours after the start of discharge, and FIG. 2 shows a photograph of the target after a lapse of 5.5 hours. Only a slight yellow powder is generated at the center of the target.

Comparative Example 3 An ITO sintered body was manufactured in the same manner as in Comparative Example 1.

When the density of the obtained sintered body was measured by the Archimedes method, it was 7.13 g / cm ³ . The composition of this sintered body was analyzed using EPMA. Table 1 shows the results. It contained 60.3 atom% oxygen.

This sintered body was processed into a sintered body having a diameter of 4 inches and a thickness of 5 mm by a wet processing method, and was bonded to a backing plate made of oxygen-free copper using indium solder to obtain a target.

The target was continuously discharged under the following sputtering conditions, and the amount of yellow powder generated was examined.

(Sputtering conditions) DC power: 200 W, gas pressure: 5.0 mTorr, sputtering gas: Ar + oxygen, oxygen gas concentration (O ₂ / Ar) in the sputtering gas: 0.4%, substrate temperature:
200 ° C. Here, the oxygen gas concentration was set to a value at which the resistivity of the obtained thin film decreased most.

FIG. 3 shows a photograph of the appearance of the target three hours after the start of the discharge, and FIG. 4 shows a photograph of the target after a lapse of 5.5 hours. A large amount of yellow powder adheres to the non-erosion part (target center part).

[0075]

According to the present invention, an ITO thin film having an extremely low resistivity can be obtained by producing a thin film by a sputtering method using a sputtering target made of an ITO sintered body. Further, it is possible to reduce the amount of yellow powder that adheres to the non-erosion portion on the target surface.

[Brief description of the drawings]

FIG. 1 shows a discharge start 3 of a target obtained in Example 3.
It is a figure which shows the surface after time.

FIG. 2 is a diagram showing the surface of the target obtained in Example 3 5.5 hours after the start of discharge.

FIG. 3 shows a discharge start 3 of the target obtained in Comparative Example 2.
It is a figure which shows the surface after time.

FIG. 4 is a diagram showing the surface of a target obtained in Comparative Example 2 5.5 hours after the start of discharge.

Claims (6)

[Claims] 1. A sintered body substantially consisting of indium, tin and oxygen, having a sintered density of 7.08 g / cm ³ or more and an oxygen content of 62% by atomic ratio of O / (In + Sn + O). An ITO sputtering target made of the above ITO sintered body. 2. The method according to claim 1, wherein the tin content in the sintered body is Sn / (In).
2. The ITO sputtering target according to claim 1, wherein the atomic ratio of + Sn) is 4.5 to 14%. 3. A method for producing an ITO sintered body obtained by sintering a molded body substantially composed of indium, tin and oxygen, while flowing an ozone-containing oxygen gas at a linear velocity of 2.0 cm / min or more. A method for producing an ITO sintered body, comprising sintering a molded body. 4. The method for producing an ITO sintered body according to claim 3, wherein the ozone content in the ozone-containing oxygen gas is 0.1 to 20%. 5. A method for producing a transparent conductive film on a substrate using a sputtering target substantially consisting of indium, tin and oxygen, wherein the sputtering target according to claim 1 is used. A method for producing a transparent conductive film. 6. The production of a transparent conductive film according to claim 5, wherein a sputtering target having a tin content of 4.5 to 14% in an atomic ratio of Sn / (In + Sn) in the sputtering target is used. Method.