Plastic and nonmetallic article shaping or treating: processes – Gas or vapor deposition of article forming material onto...
Reexamination Certificate
1998-05-05
2001-05-08
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Gas or vapor deposition of article forming material onto...
C264S334000
Reexamination Certificate
active
06228297
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
Silicon carbide's unique combination of properties make it a particularly suitable material for a variety of applications in the semiconductor, optical, electronic and chemical processing fields. Moreover, chemical vapor deposition (CVD) techniques have been widely used to provide thin films and coatings of a variety of materials on various articles. Silicon carbide articles produced by chemical vapor deposition (CVD) processing are recognized to exhibit superior mechanical, thermal, physical and optical properties. This invention is directed to improvements in a CVD process of producing free standing, self-supporting silicon carbide articles, and is particularly adapted to the production of hollow shells of cylindrical, frustoconical or other shapes. Such shells can be used in x-ray telescopes, semiconductor processing furnaces, heat exchangers, laser tubes and chemical process equipment.
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 was recently described by Geril et al. in “Thin Shell Replication of Grazing Incident (Wolter Type I) SiC Mirrors”, SPIE Proc., 2478, 215 (1995).
The advantages of CVD produced free-standing silicon carbide materials in applications requiring a high degree of surface smoothness and polishability are described in U.S. Pat. No. 5,374,412. The patent describes apparatus and process conditions which are used in the CVD production of free-standing silicon carbide articles. This patent also refers to earlier U.S. Pat. Nos. 4,990,374; 4,997,678 and 5,071,596 as further describing CVD processes of producing free-standing silicon carbide materials by the pyrolytic deposit of SiC on a mandrel.
Several methods of controlling or isolating the deposit of silicon carbide to the 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 a 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 is directed to flow past the substrate's peripheral edge from behind the substrate whereby the deposit is confined to the front face of the substrate.
SUMMARY OF THE INVENTION
Chemical vapor deposition (CVD) has been used to produce both free-standing articles and coatings of silicon carbide. Typically, the 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. If a free-standing article or silicon carbide bulk material is desired, a thicker deposit 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 to produce silicon carbide. The silicon carbide deposits as a shell on a solid mandrel or other substrate provided in the deposition chamber. The deposition is continued until the desired thickness of silicon carbide is deposited on the substrate, or mandrel. The mandrel is then removed from the deposition chamber and the shell is separated from the mandrel. Monolithic silicon carbide plates and cylinders have been produced by applying such chemical vapor deposition (CVD) techniques with suitably shaped substrate or mandrel forms.
Once the silicon carbide precursor gases or vapors are brought to the appropriate conditions to cause them to react, they produce silicon carbide which then deposits on any available surface. The deposit generally is not limited to the intended surface(s) of the mandrel(s) and generally extends past such surfaces to adjoining surfaces as well as depositing on the walls and housing of the deposition chamber. In the past, the silicon carbide deposit has extended past the dimensional limits of the mandrel over adjacent portions of the support structure holding or supporting the mandrel in its position in the deposition chamber. It is then necessary to fracture such deposits to remove the mandrel from the deposition chamber. Fracturing of the deposit often results in the formation of cracks which propagate through the deposit from the point of fracture. Such cracks are not acceptable in the intended applications of the silicon carbide articles, and usually 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 that can be produced commercially by this method. Moreover, recognition of the potential capacity of CVD silicon carbide deposits to bridge any joints between adjacent stacked mandrels and the subsequent difficulty of separating and removing individual mandrels from such a stack has prevented the use of stacked mandrels in the commercial production of silicon carbide articles.
Optimal deposition conditions generally require less than atmospheric pressures, which requires that the deposition be conducted in a vacuum chamber. It is generally less expensive to increase the production volume of vacuum chambers by increasing their vertical dimensions rather than increasing their horizontal, or floor space occupying, dimensions. Accordingly, it would be economically advantageous to provide a commercial technique for creating silicon carbide deposits on a plurality of mandrels, wherein the mandrels are vertically stacked within a single vertically extending deposition chamber. This, however, has not been done in the past, at least in part because of the difficulty in segregating, or isolating, the deposit on one mandrel from the deposit produced on an adjoining mandrel.
The present invention is directed to a process, and associated apparatus, which greatly restricts and, preferably, completely avoids, the formation of deposits extending past the dimensional limits of the mandrel. By limiting, or avoiding, the formation of such deposits, removal of the mandrel from the deposition chamber does not result in cracks which propagate through the deposit. When practice of the invention avoids the formation of a deposit at or adjacent the dimensional boundary of the mandrel, the mandrel can be removed from the deposition chamber without fracturing the deposit. When a greatly restricted deposit forms at the dimensional boundary of the mandrel, it forms a thin coating, substantially thinner than the main body of the deposit, the fracture of which does not result in cracks extending into the main body of the deposit.
The present invention also provides a process wherein silicon carbide deposits are formed on a plurality of substrates, or mandrels, as they are arranged in a vertical stack, one atop another. The mandrels are then removed from the stack and the deposits separated from the mandrels to result in free-standing dense silicon carbide articles.
The invention further provides for the production of rigid, thin-walled cylindrical or frustroconical shells of dense silicon carbide having an aspect ratio, i.e., the ratio of the shell diameter to its wall thickness, of 50 or greater. It also has permitted the commercial production of large diameter, i.e. 18 inch diameter and greater, cylindrical or frustoconical shells of dense silicon carbide.
REFERENCES:
patent: 4963393 (1990-10-01), Goela et al.
patent: 4990374 (1991-02-01), Keeley et al.
patent: 4997678 (1991-03-01), Taylor et al.
patent: 5071596 (1991-12-01), Goela et al.
patent: 5374412 (1994-12-01), Pickering et
Goela Jitendra Singh
Pickering Michael A.
Fiorilla Christopher A.
Rohm and Haas Company
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