Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
1997-08-18
2002-03-12
Marcheschi, Michael (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S056000, C065S111000, C065S017300, C065S017600, C065S022000, C065S030100, C065S032100, C065SDIG008, C264S653000, C264S654000, C264S660000, C264S666000, C264S674000, C264S681000
Reexamination Certificate
active
06355587
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. Another embodiment involves the use of the quartz glass products of this invention in connection with furnaces or high-temperature equipment, particularly in the semiconductor industry. Other embodiments relate to opaque or porous silica glass products that can function as insulation or radiation heat shields. One favored embodiment of the invention involves the use of refractory dopants, such as silicon carbide, silicon nitride, silicon oxynitride or other suitable metal nitrides.
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, as pointed out in more detail in said copending application, 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.
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.
From the early 1960's to about 1990, horizontal tube furnaces were mainstay equipment in the semiconductor industry for oxidation of silicon, diffusion, heat treating and various deposition processes. They are sometimes called simply “diffusion furnaces” but the more correct generic term is a tube furnace.
An important part of processing equipment is the reaction chamber. The associated process tube and wafer carriers can be formed of silicon carbide or high-purity quartz glass. Silicon carbide is a superior material because it is structurally stronger, has a longer useful life, and does not break down with repeated heating and cooling; but the use of silicon carbide tubes and wafer boats has been slowed down because of cost and weight.
Quartz is highly purified glass favored for its inherent stability at high temperatures and its cleanliness. Drawbacks to this glass are its fragility and the tendency to break up and sag after extended use at temperatures above 1200° C. The breakup or devitrification of the quartz results in small particles or flakes of the quartz tube falling onto the silica wafers. Sagging can impede the placement of the wafer holders or cassettes in and out of the tube.
Vertical tube furnaces have smaller cleanroom foot prints, are better suited to automation, and offer other advantages over horizontal furnaces. The vertical furnaces are preferred for a number of wafer treatment processes including chemical vapor deposition (CVD).
The semiconductor industry is continually striving for greater uniformity and higher process yields. In order to achieve more precision and greater efficiency, the furnaces, reactors and other high-temperature equipment need proper non-contaminating insulation and more efficient means for reducing radiation heat losses. Prior to the present invention, the attempts to meet these needs have been crude and generally unsatisfactory.
SUMMARY OF THE INVENTION
One preferred embodiment of the present invention relates to the nitriding or nitridation of porous silica preforms and involves new technology which appears to be a giant step forward and a breakthrough of potentially great importance in the field of nitrogen-containing silica or silicon oxynitrides. Incredible improvement in the physical properties of a high-purity quartz glass can be obtained by incorporating a minute amount of chemically-bonded nitrogen in the silica.
That first embodiment is remarkable not only because of the difficulty in forming substantial amounts of chemically-bound nitrogen but also because of the difficulty in measuring or detecting the amounts being formed or in ascertaining any benefits therefrom. The improvement obtained in the resistance of quartz glass to devitrification was quite unexpected.
In describing said first embodiment, said copending application Ser. No. 08/269,002 abandoned sets forth different methods for nitridation of silica preforms as described hereinafter. One favored method can be carried out in a single induction furnace wherein the silica preform is heated in nitrogen from a temperature below 1400° C. to a temperature preferably above 1700° C. Another method comprises a two-stag process wherein the silica preform is first nitrided at a lower temperature, such as 1000° C. to 1200° C., and is later sintered to high density.
Use of Refractory Dopants
A second preferred embodiment of the invention relates to the use of special refractory dopants or additives to alter and modify the molecular structure and improve the physical properties of sintered quartz glass. Nitrogen-modified and carbon-modified silica glass made according to the invention can be provided with improved resistance to devitrification and increased high-temperature viscosity or sag resistance. The preferred refractory dopants are carbides and nitrides of silicon, such as silicon carbide, silicon nitride or silicon oxynitride. Silicon nitride is a preferred dopant, but nitrides of calcium, aluminum, chromium, titanium and other metals may also be advantageous.
A suitable refractory composition for making nitrided quartz glass articles could, for example, consist essentially of micronized particles of vitreous silica and a small amount (e.g., 0.01 to 0.3 percent by weight) of submicron particles of silicon nitride or other metal nitride that can release nitrogen for reaction with silica during the sintering operation.
Nitridation of the quartz glass in an induction furnace can be effected as the silica preform is heated from 1400° C. to 1700° C. or above in helium or in a vacuum because of the nitrogen released by the metal nitride. However, it is usually desirable to sinter the silica preform in nitrogen or a predetermined mixture of nitrogen and helium.
Gel Casting
In the practice of the present invention, it may be desirable to employ a gel-casting process in making the porous silica preform. Such a
Blackmer John F.
Loxley Ted A.
Peters Klaus-Markus
Greene Vincent A.
Marcheschi Michael
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