JP2007280756A - Zinc oxide based transparent conductive film and liquid crystal display as well as zinc oxide based sputtering target using same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a transparent conductive film which is low in resistivity, which is superior in heat resistance, and which has zinc oxide superior in chemical resistance as the base material. <P>SOLUTION: This is the transparent conductive film in which gallium is added to aluminum added zinc oxide (ZAO), and which is low in resistivity, superior in heat resistance and superior in chemical resistance, and this can be obtained by making aluminum content more than 2% and less than 6% by an atomic ratio of Al/(Zn+Al+Ga), and by making gallium content more than 0.1% and less than 1.0% by the atomic ratio of Ga/(Zn+Al+Ga). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a transparent conductive film used for flat panel displays, solar cells and the like.

  An ITO (Indium Tin Oxide) thin film is widely used as a transparent electrode such as a flat panel display centering on a liquid crystal display and a solar cell because it has a low resistivity and a high transmittance for visible light. However, in recent years, interest in a transparent conductive film (ITO substitute material) that does not use indium is increasing due to a rise in the price of indium as a raw material and resource problems. As ITO substitute materials, materials based on zinc oxide and tin oxide are known. In particular, a film in which aluminum oxide is added to zinc oxide (hereinafter abbreviated as ZAO) has been reported to be 190 μΩcm, which is comparable to ITO (see Non-Patent Document 1, for example).

  Examples of a method for forming a film using zinc oxide as a base material include an rf magnetron sputtering method, a dc magnetron sputtering method, a pulse laser deposition method, an ion plating method, and an evaporation method. The value of 190 μΩcm is obtained by the rf magnetron sputtering method. However, the flat panel display manufacturing process requires large-area uniform film formation and high-speed film formation, and the dc magnetron sputtering method is employed. Therefore, when considering the correspondence to the existing manufacturing process, it is necessary to form a film showing practical characteristics by this dc magnetron sputtering method.

  When forming a film using zinc oxide as a base material by the dc magnetron sputtering method, there is a problem that the discharge stability is inferior to that of ITO. As means for solving such problems, a method of increasing the density of a sintered body using zinc oxide as a base material and reducing the bulk resistivity (for example, see Patent Document 1), or a method of adding a plurality of different elements (See, for example, Patent Documents 2 to 3).

Japanese Patent Laid-Open No. H02-149459 JP H11-236219 JP H11-256320 A T. T. et al. Minami, H .; Nanto and S.M. Takata, JpnJ. Appl. Phys. 23, 280-282 (1984).

As described above, various problems were solved, and it was considered that a material using zinc oxide as a base material could be used as an alternative material for ITO. However, when it is actually applied to a liquid crystal display, it must be used with a thin film thickness of 200 nm or less. When various thin film characteristics were examined with such a thin film thickness, the following problems were clarified.
1) The resistivity of the thin film increases rapidly as the film thickness decreases.
2) Resistivity increases due to high temperature (eg, 250 ° C.) treatment in the atmosphere.
3) Dissolve in an alkaline solution (developer, stripper used in the photolithography process)

  As described above, a thin film based on zinc oxide having a low resistivity, excellent heat resistance, and good chemical resistance in a film thickness region thinner than 200 nm has not yet been reported.

  The subject of this invention is providing the thin film which uses zinc oxide as a base material which solves all the said problems.

  In order to solve the above-mentioned problems, the present inventors have intensively studied an additive added to aluminum-added zinc oxide (hereinafter referred to as ZAO), which is well known for exhibiting a low resistivity.

  As a result, gallium is added to ZAO, and the addition amount of aluminum is more than 2% by atomic ratio of Al / (Zn + Al + Ga) and less than 6%, and gallium is more than 0.1% by atomic ratio of Ga / (Zn + Al + Ga). It has been found that the above problem can be solved by setting the content to less than 1.0%.

  That is, the present invention is composed of zinc, aluminum, gallium, and oxygen, aluminum is contained in an Al / (Zn + Al + Ga) atomic ratio of more than 2% and less than 6%, and gallium is Ga / (Zn + Al + Ga) atomic ratio. Thus, a transparent conductive film characterized in that it is contained in a proportion of more than 0.1% and less than 1.0% is provided.

  The aluminum content is more than 2% and less than 6% in the above atomic ratio. This is because the resistivity of the thin film is increased outside this range, but the aluminum content is more preferably 2.4% or more and 5.3% or less in the above atomic ratio.

  The addition amount of gallium is more than 0.1% and less than 1.0% in the above atomic ratio. If it is 0.1% or less, it is difficult to obtain excellent low resistivity, heat resistance, and chemical resistance according to the present invention, and if it is 1.0% or more, the resistivity of the obtained thin film becomes high. The addition amount of gallium is more preferably 0.2% or more and 0.8% or less in the above atomic ratio.

  The transparent conductive film of the present invention is a transparent conductive film made of zinc, aluminum, gallium, and oxygen, but is contained in the raw materials used for film formation or the film formation process as long as the effects of the present invention are not impaired. It may contain inevitable impurities mixed in. In order to sufficiently obtain the excellent effects of the present invention such as low resistivity, high heat resistance, and high chemical resistance, it is preferable that zinc, aluminum, gallium, and oxygen are 99.9 at% or more of the whole.

  Next, the effect of adding gallium will be described. The present inventor examined the heat resistance deterioration factor of the ZAO film. As a result, it was found that a large depression exists in the vicinity of the grain boundary of the ZAO film surface, and that the resistivity of the ZAO film is increased by adsorbing oxygen or moisture to this portion. The inventors have found that the addition of a small amount of gallium to the ZAO film reduces the number and depth of the depressions, and prevents heat or moisture from being absorbed by preventing adsorption of oxygen or moisture.

  Further, it is known that the dissolution of the film by the chemical solution also occurs from the grain boundary, and it has been found that the chemical resistance can be improved by reducing the depression.

  The above effect can be obtained even if the amount of gallium added is increased to 1% or more, but the resistivity of the thin film increases, making it impossible to adapt to a liquid crystal display or the like.

  The transparent conductive film of the present invention can be produced by sputtering using a target made of a sintered body containing zinc oxide, aluminum oxide and gallium oxide. Alternatively, a metal target including zinc, aluminum, and gallium can be used and a reactive sputtering method can be used in an atmosphere containing oxygen.

  In the zinc oxide sputtering target of the present invention, for example, a zinc oxide powder, an aluminum oxide powder and a gallium oxide powder are mixed so as to have a target composition, molded by a press or the like, and then fired to obtain a sintered body. It is obtained by shaping and polishing, if necessary, and then bonding to a backing plate. Specifically, the content of aluminum in the target is more than 2% and less than 6% by atomic ratio of Al / (Zn + Al + Ga), and the amount of gallium in the target is 0 by atomic ratio of Ga / (Zn + Al + Ga). More than 1% and less than 1.0%. The aluminum content in the target is more preferably 2.4% or more and 5.3% or less in the above atomic ratio, and the gallium content is 0.2% or more and 0.00% in the above atomic ratio. More preferably, it is 8% or less.

  The transparent conductive film of the present invention is produced, for example, as follows. The zinc oxide-based sputtering target is placed in a sputtering apparatus and evacuated. In order to obtain good crystals, the substrate temperature is preferably 180 ° C. or higher. More preferably, it is 180 degreeC-240 degreeC. When the substrate temperature is low, the crystallinity of the resulting film is not improved, and the effects of the present invention are difficult to obtain. Moreover, when the substrate temperature is high, the load on the apparatus increases, which is not preferable.

  As the sputtering gas, an inert gas such as Ar is used. If necessary, an oxidizing gas or a reducing gas may be introduced.

  As the sputtering method, a DC sputtering method, an RF sputtering method, an AC sputtering method, or a method combining these can be used.

  According to the present invention, it is possible to provide a transparent conductive film based on zinc oxide, which is suitable for liquid crystal displays, and has a low resistivity, excellent heat resistance, and good chemical resistance. In particular, when there is a step of heating after forming a transparent electrode such as a liquid crystal display manufacturing step, it has an excellent feature that the resistivity of the transparent electrode can be prevented from increasing in the step after the thin film formation. .

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

(Examples 1-7 and Comparative Examples 1-8)
A predetermined amount of zinc oxide powder having an average particle diameter of 1 μm, aluminum oxide powder having an average particle diameter of 0.2 μm, and gallium oxide powder having an average particle diameter of 0.5 μm are placed in a pot and mixed by a dry ball mill for 48 hours. A mixed powder was prepared. This mixed powder was put into a mold and pressed at a pressure of 300 kg / cm ² to obtain a molded body. This compact was subjected to densification treatment with CIP at a pressure of 3 ton / cm ² . Next, the compact was sintered under the following conditions.

(Sintering conditions)
Sintering temperature: 1500 ° C
Temperature increase rate: 50 ° C / hr
Holding time: 5 hours Sintering atmosphere: In nitrogen atmosphere The obtained sintered body was processed into 4 inches φ × 6 mmt and bonded to an oxygen-free copper backing plate using indium solder.

  Using this target, transparent conductive films having different contents of Al and Ga were produced by the DC magnetron sputtering method under the following conditions.

(Sputtering film formation conditions)
Apparatus: dc magnetron sputtering apparatus Magnetic field intensity: 1000 Gauss (directly above the target, horizontal component)
Substrate temperature: 200 ° C
Ultimate vacuum: 5 × 10 ⁻⁵ Pa
Sputtering gas: Ar
Sputtering gas pressure: 0.5 Pa
DC power: 300W
Film thickness: 100nm
Substrate used: alkali-free glass (Corning # 1737 glass)
The resistivity of the obtained thin film was measured using a Hall effect measuring device. The results are shown in Table 1. It is possible to obtain a film having a low resistivity of 1000 μΩcm or less in a range of aluminum exceeding 2% and less than 6% and gallium being Ga / (Zn + Al + Ga) exceeding 0.1% and less than 1.0%. did it.

  Table 1 summarizes the resistivity of the obtained thin film after high-temperature treatment (maintained in the atmosphere at 250 ° C. for 30 minutes). The film having the composition of the present invention showed a low resistivity of 1000 μΩcm or less even after the treatment. On the other hand, the samples of Comparative Examples 2, 4, and 6 that exhibited a low resistivity before the treatment increased the resistivity to 1000 μΩcm or more by this high temperature treatment.

  Further, the alkali solubility of the obtained thin film (the amount of decrease in film thickness when immersed in a 2% KOH aqueous solution at 25 ° C. for 4 minutes) was examined. The film thickness dissolved by this treatment is summarized in Table 1. In the film having the composition of the present invention, the dissolution film thickness by alkali treatment is 20 nm or less, and as is clear from comparison with Comparative Examples 2, 4, and 6 (dissolution film thickness 38 to 40 nm) not containing gallium, Alkali resistance is also significantly improved.

Claims (5)

It is composed of zinc, aluminum, gallium, and oxygen, and aluminum is contained in an Al / (Zn + Al + Ga) atomic ratio of more than 2% and less than 6%, and gallium is Ga / (Zn + Al + Ga) atomic ratio of 0.1%. And a transparent conductive film characterized by being contained in a proportion of less than 1.0%. The transparent conductive film according to claim 1, wherein the thickness of the film is less than 200 nm. A liquid crystal display comprising the transparent conductive film according to claim 1. A device comprising the transparent conductive film according to claim 1. It is composed of zinc, aluminum, gallium, and oxygen, and aluminum is contained in an Al / (Zn + Al + Ga) atomic ratio of more than 2% and less than 6%, and gallium is Ga / (Zn + Al + Ga) atomic ratio of 0.1%. 2. The zinc oxide-based sputtering target for forming a thin film according to claim 1, wherein a zinc oxide-based sintered body contained in a proportion of more than 1.0% and less than 1.0% is used as a target material.