Hard carbon thin film and method of forming the same

Details Hard carbon thin film and method of forming the same Hard carbon thin film and method of forming the same

2000-02-10

2003-03-04

Meeks, Timothy (Department: 1762)

Coating processes

Coating by vapor, gas, or smoke

Carbon or carbide coating

C427S249110, C427S249140, C427S255280, C427S249600, C427S249700, C427S569000, C427S587000, C427S593000

Reexamination Certificate

active

06528115

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a hard carbon thin film and a method of forming the hard carbon thin film.
2. Description of the Related Art
Hard carbon thin films exhibit excellent hardness, resistivity, chemical stability and the others, and have gathered expectations for their applications to functional thin films for electronic devices and semiconductors, e.g. protective coatings on sliding parts for compressors such as rotary compressors, protective coatings on blades such as electric shaver blades, protective coatings on masks for screen deposition as well as on squeegees, constituent film layers of solar cells, protective coatings on thin film magnetic heads, and protective coatings on SAW devices.
In the above applications, poor adhesion of the hard carbon thin film to an underlying layer becomes problematic occasionally. A technique to improve its adhesion to the underlying layer such as a substrate has been proposed which provides a silicon interlayer between the underlying layer and the hard carbon thin film (See, for example, Japanese Patent Laying-Open No. Hei 1-317197(1989)).
Although the conventional techniques such as mentioned above have a potential advantage of imparting increased adhesion, delamination of the hard carbon thin film from the underlying layer is disadvantageously occasioned when influenced by the internal stress of the hard carbon thin film which becomes greater as a thickness thereof increases. Also, the interlayer must be formed in a separate step which results in a complicated fabrication.
In view of the above, there has been a continuing need for a hard carbon thin film which is capable of exhibiting an improved adherence to an underlying layer such as a substrate.
A crystalline hard carbon thin film, as generally called a diamond thin film, is typically formed through thermal decomposition of a material gas such as methane using a hot filament. Such a technique is however accompanied by the elevation of a substrate temperature up to about 1000° C. which limits the material type of a substrate to be used. Also, the diamond thin film thus formed generally exhibits a large surface irregularity, which necessitates post-polishing thereof to smooth the surface, such as for use as a surface acoustic wave device.
A diamond-like thin film mainly consisting of non-crystalline or amorphous components has also been known as illustrative of the hard carbon thin film. Such a diamond-like thin film is generally formed using a plasma CVD technique which permits the formation thereof at a reduced substrate temperature around a room temperature. The diamond-like thin film thus formed is superior in surface smoothness but is generally inferior in surface hardness to the diamond thin film.
Accordingly, there remains a need for a technique which is capable of forming diamond thin films having smaller surface irregularities at reduced substrate temperatures, and another need for a technique which is capable of forming diamond-like thin films having increased surface hardnesses. Such needs would be met if a technique is provided which can control to some extent those mechanical properties of the diamond and diamond-like thin films to form hard carbon thin films with tailored properties. However, such a technique has not been reported up to date.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a hard carbon thin film which exhibits a satisfactory hardness as well as a good adhesion to an underlying layer such as a substrate, and a method of forming the hard carbon thin film.
It is another object of the present invention to provide a method for forming a hard carbon thin film which can control a proportion of crystalline and non-crystalline components in the thin film as well as its film properties such as hardness and surface smoothness.
A hard carbon thin film in accordance with a first aspect of the present invention characteristically has a graded structure in which a ratio of sp
2
to sp
3
carbon-carbon bonding (hereinafter referred to as “sp
2
/sp
3
ratio”) in thethin film decreases in its thickness direction from a film/underlayer interface toward a surface of the thin film.
A hard carbon thin film in accordance with a second aspect of the present invention comprises two or more layers having respective individual sp
2
/sp
3
ratios different from each other, so that the sp
2
/sp
3
ratio in the thin film decreases in a stepwise manner in its thickness direction from a film/underlayer interface toward a surface of the thin film.
A hard carbon thin film in accordance with a third aspect of the present invention characteristically has a graded structure in which the sp
2
/sp
3
ratio in the thin film in its thickness direction decreases from a film/underlayer interface to a minimum and increases therefrom toward a surface of the thin film.
A hard carbon thin film in accordance with a fourth aspect of the present invention characteristically comprises three or more layers having respective individual sp
2
/sp
3
ratios different from each other, so that the sp
2
/sp
3
ratio in the thin film decreases in a stepwise manner in its thickness direction from a film/underlayer interface to a minimum and increases therefrom in a stepwise manner toward a surface of the thin film.
The sp
2
and sp
3
carbon-carbon bondings indicate different forms of chemical bonding between carbon atoms. It is generally known that the carbon-carbon bonding in the diamond thin film is predominantly sp
3
while that in a graphite is predominantly sp
2
. It is also known that an amorphous diamond-like carbon thin film, as well as a partially crystalline diamond-like carbon thin film, may have a structure in which both sp
2
and sp
3
carbon- carbon bondings are distributed therethrough. In the present invention, such a sp
2
/sp
3
ratio is characteristically varied in a film thickness direction as described earlier.
In the present invention, the sp
2
/sp
3
ratio is varied preferably in the range of 0-3. Accordingly, the present invention is intended to include the case where the sp
2
/sp
3
ratio is zero, i.e., the carbon-carbon bonding in the thin film is essentially devoid of sp
2
and predominantly of sp
3
.
In general, the increased sp
2
/sp
3
ratio, accordingly the increased proportion of sp
2
carbon-carbon bonding tends to cause a decrease in internal stress to provide better adhesion to an underlying layer such as a substrate. On the other hand, the reduced sp
2
/sp
3
ratio, accordingly the increased proportion of sp
3
carbon-carbon bonding tends to produce a film with increased hardness and internal stress.
The sp
2
/sp
3
ratio as specified in the present invention can be determined such as by an electron energy loss spectroscopy (EELS).
In the present invention, the hard carbon thin film is contemplated to include a crystalline diamond carbon thin film, an amorphous diamond-like carbon thin film, and a diamond-like carbon thin film having a partial crystalline structure. Accordingly, the change of sp
2
/sp
3
ratio in a thickness direction of a thin film may be accompanied by the change in proportion of crystalline and non crystalline components in the thickness direction of the film.
The hard carbon thin film according to the present invention can be formed using generally-employed film-forming techniques. Foremost among those techniques are plasma CVD techniques including an ECR plasma CVD technique. A hot-filament CVD technique may also be used. Such techniques as to physically form thin films may also be applicable which include a sputtering technique and an ion beam deposition technique using an ion gun. Furthermore, the thin film may be formed using any combination of the above-mentioned plasma CVD, hot-filament CVD, sputtering and ion-beam deposition techniques.
The hard carbon thin film of the present invention may be formed on an underlying layer such as a substrate through an interlayer interposed therebetween. The material types of the interlayer include Si, Ti, Zr, W, Mo, Ru,

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