JP2010084177A - Zinc oxide-based sintered target and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a zinc oxide-based sintered target which can stably form a thin film with low resistance, and to provide a method for producing the same. <P>SOLUTION: The zinc oxide-based sintered target contains 0.2 to 3 mass% aluminum, sintered density is ≥5.5 g/cm<SP>3</SP>, and also, ≥8 atomic% of the aluminum is present in a solid-soluted form. The target can be produced by regulating zinc oxide powder with the average grain size of ≤1 μm and the composite oxide powder of zinc and aluminum with the average grain size of ≤1.5 μm in such a manner that the aluminum is comprised by 0.2 to 3 mass% so as to be mixed, thereafter press-molding the mixture, and then performing sintering at 1,200 to 1,500°C in a nitrogen gas flow with an oxygen concentration of ≤1 ppm so as to obtain a sintered compact in which sintered density is ≥5.5 g/cm<SP>3</SP>, and ≥8 atomic% of the aluminum is present in a solid-soluted form. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a zinc oxide based sintered target for forming a transparent conductive film by a sputtering method and a method for producing the same.

A transparent conductive film is required for flat panel displays such as liquid crystal displays, solar cells, touch panels, and the like. For example, in a liquid crystal display, a transparent conductive film is formed before and after the liquid crystal. By applying an electric field from the transparent conductive film, the gradient of the liquid crystal is controlled, and the amount of light transmitted from the backlight is adjusted. Display. For this reason, the transparent conductive film for liquid crystal displays is required to have an average transmittance of 90% or more in the visible light region and a resistivity on the order of 10 ⁻⁴ Ωcm. Currently, indium oxide to which tin is added (hereinafter referred to as ITO) is generally used as a transparent conductive film that satisfies such conditions.

  In the field of flat panel displays, ITO has a high average transmittance in the visible light region and can form a low-resistance thin film. However, since indium, which is a main component, is expensive, a zinc oxide (hereinafter referred to as ZnO) transparent conductive film that is low in cost and has no resource problems has recently attracted attention as an alternative to ITO. In addition, in thin-film Si-based and CIS-based solar cells that can also absorb near-infrared light, a ZnO-based transparent conductive film that can increase the transmittance in the near-infrared light region is used.

One of the methods for forming this ZnO-based conductive film is a sputtering method using a target containing Al, Ga, Ti, Ge, Mg or the like as an additive element imparting low resistance characteristics, and various studies have been made. Yes.
A target used in the sputtering method is usually manufactured by a sintering method, and the target itself is required to have low resistance. As a method of ensuring this low resistance, a method of sintering at a relatively high temperature, or a method of sintering in an inert gas or vacuum is effective (see Patent Document 1).
Further, as a means for preventing the generation of vacancies in the process of manufacturing the target, a composite oxide powder of zinc (hereinafter referred to as Zn) and aluminum (hereinafter referred to as Al) is formed in advance, and this is mixed with the ZnO powder. There is also proposed a method of forming and then sintering (see Patent Document 2).
JP-A-2-14959 JP 2008-63214 A

As described above, the additive element to ZnO plays an extremely important role in the practical application of the conductive thin film. However, despite various studies, there is a target that can exert its effect to the maximum. The current state of research is not complete.
An object of the present invention is to provide a ZnO-based sintered target capable of maximizing the effects of additive elements and stably forming a low-resistance thin film, and a method for producing the same.

  In order to obtain an excellent thin film, it is effective for the present inventors to achieve a form in which the presence state of Al in the target structure is dissolved in the structure as much as possible when adding Al to ZnO. The inventors have found that this can be realized and have reached the present invention.

That is, the present invention is a ZnO-based target containing 0.2 to 3% by mass of Al, in which the sintered density is 5.5 g / cm ³ or more, and 8 atomic% or more of Al is in a solid solution form. It is an existing ZnO-based sintered target.

The target of the present invention is prepared by mixing ZnO powder having an average particle diameter of 1 μm or less and Zn-Al composite oxide powder having an average particle diameter of 1.5 μm or less so as to contain 0.2 to 3 mass% of Al. After that, press molding is performed, and then sintering is performed at a temperature of 1200 ° C. to 1500 ° C. in a nitrogen stream having an oxygen concentration of 1 ppm or less. The sintered density is 5.5 g / cm ³ or more, and 8 atomic% or more of Al. It can manufacture by obtaining the sintered compact which was made to exist in the form which dissolved in solid solution.

  According to the present invention, a high-quality ZnO-based conductive film can be provided by a sputtering method, which is very important industrially.

An important feature of the present invention lies in achieving a form in which the presence state of Al in the target structure is dissolved in the structure as much as possible. This will be described in detail below.
First, the present invention is directed to a ZnO-based target containing 0.2 to 3% by mass of Al. In the sputtering method, the composition of the target is substantially the same as the composition of the thin film, and therefore the Al amount imparting basic conductivity is related to the electrical characteristics required for the thin film.
When the amount of Al is less than 0.2% by mass, the thin film is not sufficiently conductive, and when the amount of Al exceeds 3% by mass, the near-infrared transmittance is low and the conductivity starts to increase. Therefore, it was set as the said range in this invention.

In the target of this invention, after satisfy | filling said basic composition, it is necessary to make 8 atomic% or more of Al exist in a solid solution form.
The solid solution state does not take a crystal structure such as Al ₂ O ₃ or ZnAl ₂ O ₄ but means that Al atoms are in a solid solution form in substitutional form in ZnO crystal or the like. That is, the resistivity of the target itself can be reduced by adopting this form.
If the resistivity of the target itself can be reduced, the film can be formed at a low voltage. As a result, the voltage for accelerating oxygen ions toward the substrate is lowered, the collision energy of oxygen ions is reduced, damage to the thin film is reduced, and the resistivity of the thin film can be lowered.
Further, by increasing the solid solution Al in the target, the distribution of Al in the target can be made uniform. When sputtering is performed using a target having a uniform Al distribution, the Al distribution in the thin film can also be made uniform. In the thin film, all Al is dissolved in ZnO, but when the Al distribution is non-uniform in the thin film, the influence of ionic impurity scattering becomes strong and the resistivity increases at a location where the Al density is locally high. As a result, there is a problem that the resistivity of the entire thin film is increased.
In the present invention, it is possible to efficiently produce a low-resistance thin film by forming a solid solution of 8 atomic% or more of Al.

Nuclear magnetic resonance (NMR) can be used for analysis of the amount of solute Al in the target. In the results of NMR, chemical shifts 0ppm hexacoordinate near ZnAl ₂ O ₄ and ZnAl ₂ O ₄ tetracoordinate around 60ppm is detected.
On the other hand, according to the study of the present inventors, a peak is confirmed at about 180 ppm in the Al-added ZnO target. No Al compound having a peak in the vicinity of 180 ppm has been reported, and this peak can be quantified as a solid solution Al peak.
Moreover, if the sintered density as a target is low, it will break during machining of the target. Or, various problems such as generation of foreign matters occurred during film formation, so in the present invention, it is defined as 5.5 g / cm ³ or more. By making it higher than this density, both the productivity of the target and the productivity of the thin film can be improved, such as easy machining.

In the production method of the present invention, it is important to manage raw materials and sintering conditions in order to dissolve Al in a large amount.
In the production method of the present invention, the average particle size of ZnO powder as a raw material was defined as 1 μm or less, and the average particle size of the complex oxide of Zn and Al was defined as 1.5 μm or less. When the average particle diameter of the ZnO powder or the composite oxide is larger than the specified value, segregation of the composite oxide containing Al becomes remarkable in the state before sintering. In such a state, the time for diffusing Al during sintering becomes longer. That is, the average particle size of the Zn powder was set to 1 μm and the particle size of the composite oxide powder was set to 1.5 μm or less so that the processing time for increasing the amount of solute Al was not increased and the production efficiency was not lowered. These powders are adjusted so as to have the basic composition of the target described above, mixed, and press-molded.

Sintering needs to be performed at 1200 to 1500 ° C. in a nitrogen stream while supplying nitrogen gas with an oxygen concentration of 1 ppm or less. Sintering in a nitrogen atmosphere in which the oxygen concentration is limited as much as possible has the effect of significantly promoting the solid solution of Al.
When sintering is performed at 1200 ° C. or higher in a nitrogen atmosphere, the sintering proceeds and the density can be increased while removing oxygen from the molded body. Further, if the temperature exceeds 1500 ° C., the mass decrease during sintering becomes significant, causing problems in production yield and maintenance of the sintering furnace. Therefore, it is necessary to perform sintering at a temperature of 1500 ° C. or lower.
By the said manufacturing method, the sintered compact made to exist in the form which the sintered density of 5.5 g / cm < ³ > or more and 8 atomic% or more of Al was made into a solid solution can be obtained.

The following examples further illustrate the present invention. However, the present invention is not limited to these examples.
First, ZnO powder having an average particle diameter of 0.3 μm and aluminum oxide (Al ₂ O ₃ ) powder having an average particle diameter of 0.3 μm were prepared.
The ZnO powder and Al ₂ O ₃ powder were mixed at a molar ratio of 1: 1 and pure water was added and mixed in a ball mill for 20 hours. After drying, the powder was sintered in the atmosphere at 1000 ° C. for 2 hours, and pulverized again in a ball mill. A composite oxide of Al and Al was prepared. The average particle size of this composite oxide was 0.9 μm.

Using ZnO powder with an average particle size of 0.3 μm and the obtained composite oxide powder of Zn and Al as raw material powders, these powders were weighed so that the Al content would be 1% by mass, and pure water was added to them. In addition, the mixture was mixed with a ball mill for 20 hours, followed by drying and pulverization. Next, powder obtained by classifying and removing coarse particles of 500 μm or more was put into a mold, and press molding was performed at a pressure of 100 MPa, followed by cold isostatic pressing (CIP) at a pressure of 300 MPa to obtain a compact. . This compact was subjected to main sintering at 1400 ° C. for 2 hours while flowing nitrogen gas having an oxygen content of 0.1 ppm to obtain a target. The obtained target (TG1) of the present invention had a sintered density of 5.6 g / cm ³ .

Next, as a comparative example, after obtaining a molded body in the same process as the present invention example, the main sintering was performed in the atmosphere at 1400 ° C. for 2 hours, and further in an Ar atmosphere at 1200 ° C. for 1 hour. Sintering was performed with a hot isostatic press (HIP) of 100 MPa to obtain a target TG2 of a comparative example. The density of the target TG2 was 5.6 g / cm ³ similar to the target TG1 of the present invention. The density before HIP was 5.5 g / cm ³ .
As another comparison, instead of the composite oxide, Al ₂ O ₃ powder having an average particle size of 0.3 μm is used as a raw material powder, and the same process as that of TG2 of the comparative example is applied to set the target TG3 of the comparative example. Obtained. The density of the target TG3 was also 5.6 g / cm ³ .

  For these targets, the oxygen content was analyzed by an inert gas melting infrared absorption method, the state of Al was analyzed by a nuclear magnetic resonance (NMR) method using a CMX-300 apparatus manufactured by Chemanetics, and the resistivity of the target was determined. The measurement was performed by the direct current four-terminal method. Further, these targets were formed by direct current magnetron sputtering, and the resistivity of the thin film was measured by the van der Pau method. The conditions of DC magnetron sputtering are: sputtering gas = Ar, sputtering gas pressure = 0.4 Pa, sputtering power = 200 W, nominal film thickness = 200 nm, and substrate temperature = room temperature.

Table 1 summarizes the evaluation results.
The target TG1 of the present invention has a very high solid solution Al of 10.4%, and the resistivity of the thin film is 6.14 × 10 ⁻⁴ Ωcm and the substrate temperature is room temperature. It is low. On the other hand, the minimum resistivity of the film formed by the target TG2 of the comparative example is close to TG1 of the present invention, but the amount of solid solution Al is small at 7.9% and the resistivity of the target is high, so that the productivity is inferior. It is recognized that In addition, since the target TG3 of the comparative example has a high oxygen content and a small amount of dissolved Al, the minimum resistivity of the film formed of TG3 is high.

  Since the present invention provides a low resistivity ZnO-based thin film, it can be easily applied to large liquid crystal displays that require a low resistivity transparent conductive film and solar cells that require high transmittance.

Claims (2)

A zinc oxide-based target containing 0.2 to 3% by mass of aluminum, with a sintered density of 5.5 g / cm ³ or more, and 8 atomic% or more of aluminum in a solid solution form. A zinc oxide-based sintered target. A zinc oxide powder having an average particle diameter of 1 μm or less and a zinc-aluminum composite oxide powder having an average particle diameter of 1.5 μm or less are adjusted so as to contain 0.2 to 3 mass% of aluminum, and then press-molded. Then, sintering was performed at a temperature of 1200 ° C. to 1500 ° C. in a nitrogen stream with an oxygen concentration of 1 ppm or less, and a sintered density of 5.5 g / cm ³ or more and 8 atomic% or more of aluminum in solid solution A method for producing a zinc oxide based sintered target, comprising obtaining a sintered body present in step (1).