Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
1999-12-21
2001-07-17
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S192380, C204S298410, C427S580000, C427S122000, C422S186210
Reexamination Certificate
active
06261421
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of the Invention
This invention relates to a method and apparatus for vacuum arc deposition of carbon on a substrate; more especially it relates to cathodic arc carbon ion sources typically used in Arc Ion Plating (AIP) devices, to produce diamondlike protective coatings.
ii) Description of Prior Art
Cathodic arc carbon ion sources are typically used in Arc Ion Plating (AIP) devices to produce diamondlike protective coatings. Diamondlike refers to an amorphous form of carbon with properties similar to diamond protective coatings. In AIP devices, an electric arc burning in a vacuum on a graphite cathode generates very small hot spots called “cathode spots”, these spots being the site of a strong plasma flux which is directed towards the surface of a substrate to be coated. In addition to the carbon ions present in the plasma flux, these sources also generate small graphite particles typically 0.1-10 &mgr;m in diameter, which are deposited with the amorphous carbon film, thereby strongly degrading the coating properties. The small graphite particles are typically formed as a spray of liquid droplets and chunks or irregular non-sperical particles of graphite emitted through thermal shock effects. Preventing these particles from reaching the substrate to be coated is necessary.
Present solutions use various types of filters to eliminate the particle flux between the source and the substrate to be coated. Such filters modify the path of the plasma flux beam, for example, by setting a non-linear or curved ion path by the use of magnetic and/or electric fields. Most particles do not follow the curved path and are collected on the side walls of the vacuum chamber leaving a relatively clean beam arriving at the substrate. Shortcomings of the filters are:
a) a decrease in the output plasma flux and hence in deposition rate and system efficiency;
b) a decrease in the area covered by the beam resulting in smaller surface areas being coated in a given time interval;
c) such systems add to the complexity of the source by imposing a particular source geometry and more importantly an added control of the magnetic/electric field parameters within the filter to be adjusted to the arc source parameters;
d) particles produced by the arc source accumulate in the filter and potentially affect the filter efficiency while imposing shutdown for maintenance in industrial operations; and
e) a portion of the particle flux may still escape into the coating chamber.
The AIP coating systems are currently and widely used industrially, for example, to produce the gold colored titanium nitride films in medical implants, in the automotive industry, decorative industry, and for more resistant cutting tools. In such systems, particles are emitted typically in a smaller quantity compared to carbon sources and the resulting coating degradation has generally been considered not detrimental for most applications. Simple multi-source coating chambers are generally being used industrially to produce these coatings. In an industrial environment, the complexity of the filtered source geometry strongly restrains the transfer from a multi-source AIP chamber device to the deposition of diamondlike. A specific deposition setup is needed for diamondlike, while the multi-source deposition chambers can accommodate various types of coatings by changing the cathode material in the source.
SUMMARY
It is an object of this invention to provide a method and apparatus for vacuum arc deposition of carbon on a substrate; more especially the invention is concerned with such a method and apparatus in which development of the contaminating particles of carbon is inhibited.
In accordance with one aspect of the invention there is provided a method for vacuum arc deposition of carbon on a substrate comprising: establishing an electric arc between an anode and a cathode in a chamber under vacuum, said cathode having a target surface of non-porous graphite; emitting a plasma of carbon ions from said target surface; depositing said carbon ions on a substrate as an amorphous carbon coating and maintaining an elevated local plasma pressure at said target surface effective to minimize the role of heat conduction in said target surface and formation of liquid droplets of carbon, and to promote the electron emission (Nottingham) cooling effects.
In accordance with another aspect of the invention there is provided an apparatus for vacuum arc deposition of carbon on a substrate comprising: a chamber, vacuum means adapted to establish a vacuum in said chamber, a cathode and an anode in spaced apart relationship and means to establish an electric arc between said cathode and said anode, said cathode having a target surface of non-porous graphite; means for supporting a substrate in said chamber for deposition of carbon ions from a plasma developed at said target surface by said electric arc, and means for maintaining an elevated local plasma pressure at said target surface effective to inhibit the formation of liquid droplets of carbon, or limit heat conduction effects.
In accordance with the invention the plasma of carbon ions may be directed along a linear path to the substrate surface and no filters are required to change the path of the plasma, for separation of contaminating carbon particles.
In another aspect of the invention, the method described herein may be adapted to vacuum arc deposition of metal on a substrate by employing a target surface of metal instead of graphite.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the invention the emission of carbon particles is eliminated or inhibited directly at the site of conventional production, i.e., the cathode spots. In other words, the cathode spot emission properties are optimized to enhance the ion flux intensity and energy while simultaneously eliminating the emission of the micro-droplets which form the particles. The chunks of irregular non-spherical graphite particles are also eliminated. Elimination of the particle emission directly at their source means a filtering system is no longer needed, yielding a linear path between the source and the surface to be coated in a manner similar to the usual non-filtered sources used in industry for titanium nitride and other advanced ceramic films. Elimination of the micro-droplet emissions is achieved by maintaining an elevated local plasma pressure at the cathode or target surface so as to decrease the contribution of conduction heat transfer and increase the cooling effects from electron emission in the cathode spots at the target surface, thereby inhibiting the formation of liquid volumes within the spot.
It has been found by theory that within the cathode spot, heat supplied by conduction to the carbon cathode material, which heat supply is responsible for melting and production of liquid droplets, decrease with increase in local spot pressure. An increase in the local spot pressure has also been found by theory to increase the surface cooling mechanisms through an increase of the electron emission, this being referred to in the literature as Nottingham cooling.
Thus in accordance with the invention the local heat load on the cathode surface is reduced by increasing the local plasma pressure in the cathode spot volume. This increases the mobility of the cathode spot. The “local” pressure refers to the plasma pressure in the micrometer size cathode spot area, and not to the pressure in the vacuum chamber where the arc is developed, which latter pressure remains low.
Three different means have been developed to increase the local plasma pressure in the cathode spots and in preferred embodiments of the invention the method and apparatus exploit all three means.
First, higher graphite arc cathode spot mobility is found to be significant in achieving high local plasma pressure and low local heat flux. A magnetic field lines geometry generated from the back of the cathode surface brings the electric arc into rotation at a radial position corresponding to the zero field component in a direction perpendicula
Kandah Munther
Meunier Jean-Luc
Mayekar Kishor
McGill University
Renault Swabey Ogilvy
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