Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
1999-12-30
2001-05-01
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C156S345420, C118S7230ER, C118S7230IR, C118S7230AN
Reexamination Certificate
active
06223686
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for forming a thin film and more particularly to such an apparatus by plasma chemical vapor deposition (CVD) suitable for use in the production process for a magnetic disc serving as a memory medium for a hard disc memory device.
The magnetic disc for a hard disc memory device is usually produced by forming a magnetic layer on the surface of a disc such as a glass disc and further providing a protective layer thereon. An apparatus for forming a thin film by plasma CVD may be used to form a carbon film as a protective layer on both surfaces of a disc at the same time. The carbon film comprises amorphous carbon, and such a film formed by plasma CVD is very hard and has superior characteristics such as resistance against abrasion, corrosion, peeling, voltages and insulation. Thus, carbon films are most suitable as a protective layer for a magnetic disc. The hardness of these films can be controlled by adjusting the bias voltage applied to the substrate at the time of the plasma CVD.
With a prior art apparatus for forming a thin film by plasma CVD, however, a negative charge-up may take place when a bias voltage is applied to the substrate, resulting in an abnormal discharge and causing particles to be emitted from the chamber walls and to become attached to the substrate. There may also occur peeling on the substrate surfaces.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved apparatus for forming a thin film adapted to optimize the bias voltage to be applied to the substrate such that a plasma can be produced in a stable manner and that no abnormal discharge will take place.
An apparatus embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a vacuum chamber inside which the substrate is disposed, ECR plasma generating means connected to this vacuum chamber, field generating means for generating a specified magnetic field inside the vacuum chamber, gas supply means for introducing a reaction gas and an inert gas into the vacuum chamber, and a voltage applying device for applying to the substrate a negative DC voltage superposed with a high-frequency pulse with frequency 25-250 kHz. The frequency of the superposed pulse is selected by using an ammeter to measure the load current of the voltage applying device to thereby determine an optimum frequency for minimizing the load current of the voltage applying device, and the voltage of the substrate is not constantly negative but reaches a positive value instantaneously.
Thus, electrons are pulled towards the substrate and charge the outer surface of the substrate negatively, thereby improving the efficiency of attracting plasma ions to its surface. Although a plasma normally becomes unstable when a voltage superposed with a pulse is applied, the frequency of the pulse to be superposed is selected by monitoring the load current of the voltage applying device to determine an optimum frequency at which the plasma impedance reaches a maximum.
REFERENCES:
patent: 4970435 (1990-11-01), Tanaka et al.
patent: 5160397 (1992-11-01), Doki et al.
patent: 5221416 (1993-06-01), Kishi et al.
patent: 6030667 (2000-02-01), Nakagawa et al.
Akita Noritaka
Hashimoto Yoshihiro
Suzuki Masayasu
Coudert Brothers
Hassanzadeh P.
Mills Gregory
Shimadzu Corporation
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