Chemistry: electrical and wave energy – Processes and products – Vacuum arc discharge coating
Reexamination Certificate
2001-04-06
2002-12-17
VerSteeg, Steven H. (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Vacuum arc discharge coating
C204S298410
Reexamination Certificate
active
06495002
ABSTRACT:
FIELD AND HISTORICAL BACKGROUND OF THE INVENTION
The present invention is directed to depositing ceramic films on a substrate, and more particularly to a method and apparatus for depositing ceramic films by vacuum arc deposition.
Vacuum (or cathodic) arc deposition is a method for depositing a thin film from a solid source (the consumable cathode) with a very high growth rate (References
1
and
2
). In some forms, several solid sources may be used to stoichiometrically mix the corresponding species leading to compound films (References
3
and
6
). The vacuum arc can also be run in a background gas for reactive deposition of a compound film, e.g., TiN and CrN for coating steels (References
7
and
8
). Vacuum arc deposition has in general been limited to metal cathodes, which typically sustain discharge current densities above 1MA/cm
2
in non-stationary spots several &mgr;m in diameter (Reference
9
). One or more spots may be sustained (~50A per spot). At these cathodic spots, cathode material is efficiently converted to fully ionized plasma, which can be guided (e.g. magnetically) to a substrate for deposition. This process, however, has the potential of creating tremendous, localized stresses in the cathode material, especially if the thermal and electric conductivities are low, as they will be in a non-metal. This would clearly be the case for, for example, a boron carbide (B
4
C) cathode, typically produced via pressed powder metallurgy.
Recently, there has been some research activity in the development of vacuum arc deposition from non-metal (ceramic or semiconducting) cathodes. Researchers in England and Israel have independently succeeded in the operation of a continuous vacuum arc on a polycrystalline silicon cathode and have deposited a-Si films of near-electronic quality (References
10
-
13
). Both groups, however, reported that problems remained regarding cathode fracturing. A group in Germany made the first published successful run of a vacuum arc on a pure boron cathode (References
14
-
15
). The process was run reactively in a nitrogen background to make c-BN or in an Ar background because of problems running in vacuum in a stable fashion.
Encouraged by these developments, the inventors of the present invention pursued research to develop vacuum arc deposition of boron and boron carbide. Both of these materials make films that are of great interest in Fusion as well as in non-Fusion applications. Developments in the processing and handling of the cathodes, before and during the deposition, have allowed the inventors of the present invention to demonstrate continuous vacuum arc operation on both boron and boron carbide cathodes in a true vacuum mode (<1 mPa). The present invention is believed to be the first successful deposition of stoichiometrically correct boron carbide from a boron carbide cathode. Although this technology can be potentially applied to other non-metals (as long as powders are available to consolidate into cathodes), this development was motivated by a need for improved coating methods for radio-frequency (rf) antennas used in magnetic fusion energy (MFE). Boron carbide is a material traditionally used in this application, primarily because of the low atomic number of the materials in this compound.
The recent experience from tests of rf antennas in tokamak experiments at Tore Supra and DIII-D has been that B
4
C is a desirable coating, but needs to be thick (due to erosion) and yet have good thermal conductivity, so that the underlying cooling channels can rapidly remove the heat from the surface of the coating (Reference
16
). Unfortunately, the commercial sources for appropriate boron carbide coating are very limited and fall between two extremes. Plasma sprayed coatings can be made quite thick (>10 &mgr;m). However their inherent porosity substantially limits the thermal conductivity. On the other hand, sputtered boron carbide generally produces dense films that are limited in the thickness due to the high cost and low deposition rates, as well as to stresses in the film.
Vacuum arc deposition offers the possibility of dense films, deposited at a lower cost and much higher rate. Also, independent substrate bias and heating can lead to deposition of oriented crystalline film with good thermal conductivity in the direction of crystalinity. As mentioned, this invention entails the first successful deposition of boron carbide using vacuum arc technology. However, the films were produced without biasing and were amorphous. The stoichiometry of the deposited film matched that of the B
4
C cathode. Heating up to 600° C. did not produce any crystallinity, but is believed to have helped in making films as thick as 350 nm without significant stresses.
Despite the absence of crystallinity, the coatings obtained by the present invention are attractive for fusion applications beyond the antennas. As a boron-rich, low-Z material, B
4
C would be attractive as a first wall coating in a fusion reactor. Like any other carbonaceous material, one might expect some co-deposition of sputtered material with potential retention of radioactive tritium in a reactor environment (Reference
17
). However, contrary to pure carbon materials, B
4
C has much lower chemical and high-temperature sputtering rates, and has the added benefits of oxygen gettering due to the large boron content (Reference
18
). The low carbon content suggests lower hydrogen isotope retention. Of course, for an application to relatively smaller structures, such as the antennas, this issue is even less of a concern.
Finally, one can benefit from the good vacuum compatibility of the vacuum arc process by considering using the method of the present invention for in-situ repair of in-vessel components in a fusion reactor or a reactor-relevant experiment, such as ITER (References
19
-
21
).
Outside fusion, boron carbide is a valuable metallurgical coating. It is particularly desirable in its amorphous phase, as obtained by the present invention, because of the combination of high hardness and low friction coefficient. A good example application would be as a coating for bearings or for cutting tools.
Various methods and apparatus for depositing coatings are disclosed in U.S. Pat. Nos. 3,944,683; 4,452,686; 4,496,868; 4,551,221; 4,645,895; 4,716,083; 5,072,992; 5,306,408; 5,582,874; 5,635,254; 5,896,012; 5,962,288; 6,054,187; 6,087,069; 6,120,640 and H. Shinno, T. Tanabe, M. Fujitsuka and Y. Sakai, “Characterization of Carbon-Boron Coatings Prepared on Molybdenum by a Vacuum Arc Deposition Method”, Thin Solid Films 189 (1990) 149-159. However, conventional techniques do not deposit a stoichiometrically correct coating of a ceramic material, such as boron carbide, titanium diboride, and lanthanum hexaboride.
OBJECTS AND SUMMARY OF THE INVENTION
The principal object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which overcome the drawbacks associated with conventional techniques.
An object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which assure long-term survival of the cathode and reduce the production of macroparticlaes that negatively impact the quality of the deposited films.
Another object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum deposition arc which produce thick films and yet have good thermal conductivity.
Yet another object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which produce dense films that are deposited at a lower cost and a much higher rate than conventional techniques and devices.
An additional object of the present invention is to provide a method and apparatus for depositing ceramic films by vacuum arc deposition which deposits a stoichiometrically correct coating of an electrically conductive ceramic material, such as boron, boron carbide, lanthanum hexaboride, etc., on a substrate.
Yet an addit
Hazelton Robert C.
Keitz Michael D.
Klepper C. Christopher
Niemel John
Yadlowsky Edward J.
Dinesh Agarwal, P.C.
Hy-Tech Research Corporation
VerSteeg Steven H.
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