Coating apparatus – Gas or vapor deposition – Work support
Reexamination Certificate
2001-07-17
2003-11-18
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
Work support
C118S715000, C118S500000, C427S248100
Reexamination Certificate
active
06648977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The production of free-standing articles by chemical vapor deposition (CVD) can provide such articles with enhanced properties. The use of CVD to provide articles in near-net shape (NNS), such that only minimal finishing is required to provide the article in its finished shape, is especially useful for producing articles having critical dimensional tolerances from materials, such as silicon carbide, which are difficult to shape by conventional machine shaping techniques. The present invention provides an improved method of mounting substrates during the CVD process. It is particularly applicable to the production of near-net shape silicon carbide articles.
2. Description of Related Art
The advantages of silicon-carbide as a fabrication material for astronomical X-ray telescopes and the experimental use of small scale CVD processing to prepare conical silicon carbide shells were recently described by Geril et al. in “Thin Shell Replication of Grazing Incident (Wolter Type I) SiC Mirrors”, SPIE Proc., 2478, 215 (1995).
Free-standing silicon carbide materials produced by CVD processing in applications requiring a high degree of surface smoothness and polishability are described in U.S. Pat. No. 5,374,412. Apparatus and process conditions used to produce such articles are described in that patent. U.S. Pat. Nos. 4,990,374; 4,997,678 and 5,071,596 further describe CVD processes for producing free-standing silicon carbide materials by the pyrolytic deposit of SiC on a mandrel.
Typically, CVD derived articles are produced by CVD deposit of the desired material on a substrate, followed by separation of the article from the substrate. One prior method produces a relatively large sheet of monolithic SiC on a flat graphite mandrel coated with a thin layer of a release coating. Pyrolysis of methyltrichlorosilane in argon and excess hydrogen produced a deposit which, after separation from the mandrel, was cut into multiple susceptor rings for use supporting wafers in semiconductor processing furnaces. While this method produces satisfactory parts, they are not produced in near-net shape and require substantial machining. The production of the deposit in near-net shape is desirable to reduce the amount of waste material generated and reduce the amount of machining required.
Several methods of controlling or isolating the deposit of silicon carbide to one intended side of the substrate during chemical vapor deposition are described in U.S. Pat. Nos. 4,963,393 and 4,990,374. In U.S. Pat. No. 4,963,393, a curtain of flexible graphite cloth is arranged to shield the backside of the substrate from the flowing reacted precursor gases, whereby silicon carbide deposits on the backside of the substrate are avoided. In U.S. Pat. No. 4,990,374 a counterflow of a non-reactive gas flows from behind the substrate past the substrate's peripheral edge whereby the reactive deposition gases and the deposit they produce are confined to the front face of the substrate.
Another prior technique controls the deposition by providing a channel surrounding that portion or zone of the substrate surface where the deposit is desired. The channel functions to restrict flow of the reactive deposition gases to the substrate surfaces surrounding the deposition zone whereby any deposit on the surrounding surfaces is substantially thinner than the deposit formed in the deposition zone.
Still another previous method provided multiple shaped graphite ring mandrels (substrates) mounted along the extent of the deposition chamber by detachable graphite mounts gripping the edge of the rings. Silicon carbide was deposited on both sides of the mandrels, the mandrels removed from the deposition chamber and the edges of the deposits on the mandrels machined to release the bottom and top deposits as two separate silicon carbide articles. This process resulted in relatively heavy deposits of silicon carbide bridging the graphite mandrel and the graphite mounts, necessitating difficult machining in the vicinity of the areas occupied by the mounts, and often resulted in cracks developing in the deposits during separation of the mount from the mandrel. These cracks often propagated through the desired product causing it to be rejected. If sufficient mounts are not used, the increased weight of the deposit on the mandrels sometimes caused the mandrels to slip from the mounts damaging the deposits and adjacent mandrels.
SUMMARY OF THE INVENTION
Chemical vapor deposition (CVD) has been used to produce both free-standing articles and coatings of various materials, such as, silicon carbide. Typically, such a process involves reacting vaporized or gaseous chemical precursors in the vicinity of a substrate to result in silicon carbide depositing on the substrate. The deposition reaction is continued until the deposit reaches the desired thickness. If a coated article is desired, the substrate is the article to be coated and the coating is relatively thin, generally less than 100 microns (0.1 mm) thick. If a free-standing article or silicon carbide bulk material is desired, a thicker deposit, generally greater than 0.1 mm thick, is formed as a shell on the substrate and then separated from the substrate to provide the silicon carbide article.
In a typical silicon carbide bulk material production run, silicon carbide precursor gases or vapors are fed to a deposition chamber where they are heated to a temperature at which they react producing silicon carbide. The silicon carbide deposits as a shell on a solid substrate. The deposition is continued until the desired thickness of silicon carbide is deposited. The substrate is then removed from the deposition chamber and the shell separated therefrom. Monolithic silicon carbide plates and cylinders have been produced by applying such chemical vapor deposition (CVD) techniques on suitably shaped substrates. Some articles require a deposit about one inch thick, which can require deposition processing extending three hundred hours or longer.
Once the silicon carbide precursor gases or vapors are brought to the appropriate conditions to cause them to react, they produce silicon carbide which deposits on any available surface. Such deposit generally is not limited to the intended surface of the substrate and generally extends past such surface to adjoining surfaces as well as depositing on the walls, housing and any other available surfaces associated with the deposition chamber. In prior processes, the silicon carbide deposit has extended past the dimensional limits of the substrate covering adjacent portions of the support structure holding or clamping the mandrel/substrate in position in the deposition chamber. These extraneous deposits not only consume the deposition chemicals, they can be relatively thick requiring either their removal from the production equipment or that the equipment be routinely replaced. The consumption of deposition chemicals and refurbishing or replacement of furnace equipment add considerable expense to the cost of the process. Moreover, it is generally necessary to fracture the deposits to remove the substrate from their mount in the deposition chamber. Fracturing of the relatively thick deposit often results in the formation of cracks which propagate through the deposit. Such cracks are not acceptable in most of the intended applications of the silicon carbide articles, and result in the article being rejected. The prevalence of propagated cracks in relatively thick chemical vapor deposits of silicon carbide have limited the size of articles which can be produced commercially by this method. Moreover, recognition of the potential capacity of CVD silicon carbide deposits to bridge joints between adjacent stacked substrates and the subsequent difficulty of separating and removing individual substrates from such a stack has precluded the use of stacked multiple substrates in the commercial production of silicon carbide articles.
Some previous techniques have sought to reduce the above noted extraneous deposits by contro
Boudreaux Mitch
Goela Jitendra Singh
Pickering Michael A.
Salihbegovic Zlatko
Kackar Ram N
Mills Gregory
Piskorski John J.
Shipley Company L.L.C.
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