Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1994-07-28
2002-10-22
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192220, C204S192230
Reexamination Certificate
active
06468403
ABSTRACT:
The present invention relates to a method for producing a transparent conductive film, whereby a transparent conductive film useful for e.g. a transparent electrode film for a liquid crystal display, a plasma display, an EL display, a touch panel or an anti-fogging glass for vehicles, or. an optical thin film useful for a building window having various optical properties, is formed by sputtering.
The present invention also relates to a method for forming a silicon oxide film having an alkali barrier property or a silicon nitride film having various functions.
As a method for forming the above transparent conductive film, there has been proposed a chemical film-forming method such as a sol-gel coating method, a spraying method or: a chemical vapor deposition (CVD) method, or a physical film-forming method such as a vacuum deposition method or a sputtering method.
However, for the industrial production of such transparent electrode films, the physical film-forming method such as a vacuum evaporation method or a sputtering method has been mainly employed for the reason that a transparent conductive film excellent in electrical properties and optical properties can thereby be readily obtained. Especially, in recent years, the sputtering method has been widely used for the-reason that a film can be formed uniformly on a substrate having a large area, and yet the film can be constantly formed even on a relatively low temperature substrate.
The sputtering method is a film-forming method, wherein argon gas is ionized by direct current (DC) discharge or radio frequency (RF) discharge and bombarded to a negatively biased metal or oxide target, and the substance sputtered from the target is precipitated on a substrate.
For example, when a tin oxide film is to be formed, tin oxide containing from 0.1 to 10 wt % of antimony or a tin alloy containing from 0.1 to 10 wt % of antimony may be used as the target material.
Likewise, when an ITO film is to be formed, indium oxide containing from 5 to 10 wt % of tin oxide (ITO target) or an indium-tin alloy containing from 5 to 10 wt % of tin (IT target) may, for example, be used as the target material.
Likewise, when a zinc oxide film is to be formed, zinc-oxide containing from 0.1 to 10 wt % of aluminum, zinc oxide containing from 0.1 to 15 wt % of gallium, a zinc alloy containing from 0.1 to 10 wt % of aluminum, or a zinc alloy containing from 0.1 to 15 wt % of gallium, may, for example, be used as the target material.
As the sputtering gas, an inert gas such as argon, which may contain oxygen, as the case requires, may be employed.
However, when the conventional sputtering method is used for the production of a transparent conductive film, in either case of using an oxide target or a metal target as the target material, there has been a problem such that during continuous sputtering, black fine protrusions (so-called nodules).which are believed to be a sub oxide having a remarkably poor sputtering rate as compared with the target material, are formed on the target surface, whereby the deposition rate of the transparent conductive film tends to gradually decrease, and at the same time arcing tends to occur frequently, whereby the target material spitted by the arcing is likely to deposit on the substrate and form defects of the transparent conductive film.
For example, if continuous sputtering is carried out for a long period of time by using a tin oxide target having a thickness of 6 mm, the deposition rate of tin oxide will decrease to a level of from 60 to 70% of the initial rate and at the same time, the frequency of arcing sharply increases, immediately prior to the completion of the use of the target.
Accordingly, in an industrial mass production process, it is common to gradually increase the sputtering electric power or gradually prolonging the deposition time, as the deposition rate decreases, to cope with the problem by experience. When the arcing frequency or the decrease of the deposition rate is especially remarkable, it is common to open the apparatus to the atmospheric air and to mechanically scrape off the nodules.
On the other hand, it has been known that formation of nodules can be suppressed by sputtering with a large electric power density. However, if the power density is increased, arcing occurs more frequently, and once such arcing occurs, the damage is likely to be more substantial.
With the above-mentioned target which is capable of forming a transparent conductive film, the target is likely to undergo cracking due to inadequate cooling of the target. Further, the sputtering speed tends to be high, whereby the film properties of the transparent conductive film tend to be poor.
To solve the above problems, it has been proposed to conduct cleaning for removal of the above nodules by plasma using nitrogen or a gas having a nitrogen component, as disclosed in Japanese Unexamined Patent Publication No. 293767/1992. This method has a merit that cleaning can be conducted without opening the apparatus to the atmospheric air, but it also has drawbacks such that, to conduct the above cleaning, the sputtering of transparent conductive oxide has to be once stopped, and if formation of nodules is remarkable, such nodules can not be completely removed even if cleaning is carried out for a long period of time.
It is an object of the present invention to provide a method for producing a transparent conductive film excellent in the productivity, which does not require cleaning by plasma or mechanical cleaning under the atmospheric air and whereby the above-mentioned conventional problems such as formation of nodules, generation of arcing and decrease of the deposition rate, can be solved.
In a liquid crystal display which has been rapidly progressed in recent years, it is known that alkali metal ions such as Na
+
or K
+
contained in the substrate glass diffuse through various thin layers formed on the surface of the glass substrate during the assembling process of the liquid display cell or during the use for a long period of time and reach the liquid crystal layer to deteriorate the performance of the device. Further, if such alkali metal ions reach the transparent conductive film such as ITO formed on the glass substrate, they deteriorate the electrical conductivity, which in turn deteriorates the response of the liquid display device or the quality of the Display. Therefore, when soda lime glass produced by a float method is to be used as the substrate for a liquid crystal display cell, it has been common to form a thin film of silica of a few tens nm as the first layer on the glass to let it have an alkali barrier property.
The ability to prevent the diffusion of alkali metals (i.e. the alkali barrier property) depends largely on the nature of the material to be used, and not only that, when a certain material is selected, the alkali barrier property depends largely on the density of microscopic defects or the impurity concentration in the material.
On the other hand, it is commonly known that if the substrate temperature is increased, the density of the thin film deposited on the substrate increases, and the density of the microscopic defects decreases. Accordingly, it is common to conduct film deposition on the heated substrate in order to obtain a thin film having a high alkali barrier property.
For these reasons, silica has been used frequently as a material for the alkali barrier film to prevent diffusion of alkali metals, and it has been common to conduct film deposition on the heated substrate, at a high temperature in order to obtain a high level of the alkali barrier property. Therefore, the substrate has been limited to a material having heat resistance.
The film-forming method may, for example, be a dipping method, a CVD method, an EB evaporation method or a sputtering method. From the viewpoint of costs and performance, the CVD method or the sputtering method is selected in many cases. As the CVD method, a so-called pyrolytic CVD method is employed, whereby the substrate temperature tends to be naturally high
Ando Ei'ichi
Osaki Hisashi
Oyama Takuji
Shimizu Jun-ichi
Takaki Satoru
Asahi Glass Company Ltd.
McDonald Rodney G.
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