Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2000-01-11
2002-05-07
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S030100, C065S032100, C065S033100, C065S111000
Reexamination Certificate
active
06381986
ABSTRACT:
The present application relates to the manufacture of quartz glass products having new or improved properties and to novel processes for making such products from porous silica preforms. One preferred embodiment of the invention involves nitrided vitreous quartz products with outstanding physical properties. Other embodiments involve the use of silicon alkoxides, such as ethyl silicate (TEOS), for impregnation of porous silica preforms prior to nitriding or sintering of the preforms and for gel-casting of quartz glass products. Another embodiment involves the making of quartz glass articles by electrophoretic deposition. Another involves the production of such glass articles by slip casting in special silica molds.
BACKGROUND OF THE INVENTION
It has been known for many years that nitrides of silicon have properties different from silicon dioxide and that some of these properties might be advantageous in certain applications. Silicon nitride and silicon oxynitrides can be produced in various ways as by reaction of silicon and/or silicon dioxide with ammonia, and products of this type would have utility for some special applications.
However, there are many reasons why the commercial use of such products has been very limited, why research relating to nitrided silicon products has not been extensive, and why large capital investment for research and development in this area did not appear to be justified. It is difficult and expensive to produce silicon nitride products or silicon oxynitride products. Silicon dioxide (silica) does not react readily with nitrogen, although it is possible with appropriate reaction conditions to produce oxynitrides by reacting particles of silica with anhydrous ammonia.
In the field of microelectronics, scientists have given some consideration to possible uses of silicon oxynitride films because of the unique dielectric properties and other properties. Such films can be produced by chemical vapor deposition or by nitridation of silicon surfaces or thin silicon-dioxide films. Thin silica films made by a sol-gel process can be penetrated by ammonia, perhaps because of the microporosity and cracking of the dried film. At a temperature of 1000° C. to 1200° C., anhydrous ammonia can react with the silica film to produce oxynitrides with special properties.
Consideration has also been given to the manufacture of glass or glass-ceramic products from compositions containing silica (SiO
2
) and nitrogen (N) as base components as described in Corning Patent No. 4,222,760. However, that patent points out that the practical glass-forming region is quite small in the simple ternary SiO
2
—Al
2
O
3
—N system (FIG. 8) and is essentially non-existent in the simple binary SiO
2
—N system.
Silicon oxynitride glasses can be produced by melting a mixture of oxide and nitride powders at a high temperature, such as 1600° C. to 1700° C. or more. Oxides of aluminum and other metals may be used (i.e., Ca, Li, Mg or Y). The nitrogen source may be Si
3
N
4
or AlN, for example. The oxynitride glass is potentially useful in making special plate glass or glass fibers (See U.S. Pat. No. 4,609,631).
Oxynitrides have some desirable properties which may be superior to those of quartz glass and may have potential value in the semiconductor industry. However, it appears that such potential, if any, has yet to be realized and that the use of oxynitride glass in connection with the commercial manufacture and processing of silicon-wafers and other semiconductor devices has not been found worthwhile.
To date there has been no practical substitute for quartz glass in the commercial manufacture of silicon semiconductors. The modern glass crucibles used in Czochralski (Cz) crystal-growing furnaces have been formed of silica having a very high purity (i.e., a purity of at least 99.99 percent). Substantial amounts of nitrogen cannot be tolerated in Cz crucibles. For more than two decades the manufacturers of silicon crystal have insisted that the crucibles used in crystal-growing furnaces be transparent and free of significant amounts of nitrogen or cristobalite.
Because of the importance of microelectronics and computers, there is a high demand for ultra-pure silica glass in the manufacture of modern micro-chips. The semiconductor industry is becoming increasingly intolerant with respect to contaminants in quartz glass. In order to meet modern requirements for the processing of semiconductor wafers, a glass should contain at least 99.995 percent by weight of silica. The ultra-pure synthetic fused quartz commonly used for this purpose usually has a purity of about 99.999 percent.
Prior to the present invention, the presence of significant amounts of chemically-bound nitrogen in a quartz glass used in semiconductor manufacture would have been considered highly undesirable. Nitrogen heretofore appeared to be an impurity to be avoided.
The percentage of the nitrogen impurity in a commercial quartz glass is low but is not often measured or reported because of the difficulty of ascertaining the nitrogen content with reasonable accuracy. The analytical detection problem is another good reason why the unusual properties and advantages of chemically-bound nitrogen were heretofore not understood nor appreciated in the glass industry.
For several decades vitreous silica products essentially free of crystalline silica have been used extensively because of exceptional thermal shock resistance and other advantageous physical properties. However, these products have a limited useful life when heated above 1200° C. and other disadvantages because of limited resistance to deformation, the devitrification of the glass, and the damage resulting from the crystallographic alpha-beta inversion during heating and cooling of the devitrified glass. There has been a need for a practical solution to these problems for several decades, particularly the devitrification problem, but no simple solution was found prior to the present invention.
There has also been a need to remedy other deficiencies in certain products and processes involving the use of quartz glass or vitreous silica. For example, serious problems have been encountered when attempting to cast elemental silicon in silica molds, making it necessary to tolerate the expense and inefficiency of temporary breakaway casting molds.
In the semiconductor industry, modern epitaxy reactors, diffusion furnaces, CVD equipment and other high-temperature equipment have a great need for effective thermal radiation heat shields. There have been some attempts to meet this need, but they have been crude and generally unsatisfactory.
Gel Casting
There has also been a need for better methods for molding high-purity quartz glass products including improvements in conventional slip casting methods. Because of the limitations of slip casting, other practical casting processes and techniques are sorely needed, such as electrophoretic deposition and gel-casting, (See U.S. Pat. Nos. 4,092,231 and 4,622,056), but this need has not been satisfied. Prior to the present invention, there was no practicable and commercially viable method for gel casting a variety of shaped quartz glass products.
In the field of glass and ceramics, ethyl silicate has been used for more than 50 years as a binder (See U.S. Pat. No. 1,909,008). The use of alkyl silicate binders is a feature of the well-known “Shaw Process” developed by Avnet-Shaw Corporation and disclosed, for example, in U.S. Pat. Nos. 2,795,022 and 3,172,176. Ethyl silicate binders are commonly used in the manufacture of various silica products. Ethyl silicate can be added to a slurry in small amounts and hydrolyzed to serve as a binder and can be used in various injection, extrusion and pressing techniques as described in U.S. Pat. No. 3,423,216 and U.S. Pat. No. 4,789,389, for example. Although the properties and uses of ethyl silicate have been well known for decades, the full potential of this material in the manufacture of glass has not been realized nor appreciated.
It has been known for several years that ultra-pure synthetic quart
Blackmer John F.
Loxley Ted A.
Peters Klaus-Markus
Greene Vincent A.
Vincent Sean
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