Glass manufacturing – Processes – With shaping of particulate material and subsequent fusing...
Reexamination Certificate
2001-08-31
2002-06-18
Derrington, James (Department: 1731)
Glass manufacturing
Processes
With shaping of particulate material and subsequent fusing...
C065S017300, C065SDIG008, C428S034600, C428S312600, C428S428000
Reexamination Certificate
active
06405563
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to an opaque silica glass article and a process for producing the same. More particularly, it relates to an opaque silica glass article comprising a transparent portion and an opaque portion, and having good heat insulating property and good surface smoothness, and to a process for producing the opaque silica glass article by melt-forming together a raw material for the opaque portion and a raw material for the transparent portion into an article of an arbitrary shape.
(2) Description of the Related Art
An opaque silica glass article has good heat-insulating property, i.e., is capable of cutting-off heat rays transferring as radiant heat. In the case where the silica glass article contains a salient amount of fine bubbles uniformly distributed therein, its heat-insulating performance is superior.
One example of the opaque silica glass article is a flange provided at the base of a furnace tube used as a furnace for heating a silicon wafer, as illustrated in
FIG. 1. A
heating furnace as illustrated in
FIG. 1
has heretofore been used widely for heating a silicon wafer, which comprises a heating element
1
, a furnace tube
2
, a boat
4
for supporting silicon wafers
3
, an insulating cylinder
5
and a base
6
. A flange
9
is provided at the base of the furnace tube
2
. Also shown is port
8
. The flange
9
is made of opaque silica glass and welded together with the furnace tube
2
by an oxyhydrogen flame. The flange
9
has a function of heat insulation for cutting off heat transferring to the base
6
and a packing
7
, which have a poor heat resistance. A desired atmosphere can be kept within the furnace tube
2
by the seal by means of packing
7
between the flange
9
and the base
6
. Opaque silia is widely used in many field including the flange of a heating surface.
The opaque silica glass article is usually made by a method of heating a powdery siliceous raw material to melt and vitrify the raw material. The method for heating the raw material includes, for example, Verneuil's method wherein the raw material is subjected to flame fusion by using an argon-oxygen plasma flame or an oxyhydrogen flame, and a vacuum melting method wherein a vessel is charged with the raw material and the raw material is heated and melted in vacuo.
As the raw material for the opaque silica glass article, natural silica rock or stone, and rock crystal of a low quality level have heretofore been widely used. These raw materials contain a multiplicity of fine bubbles therein, and, when the raw materials are melted for vitrification, the bubbles remain within the glass to yield opaque silica glass articles.
In recent years, LSI is being highly integrated in the field of a semiconductor, and thus a raw material with a high purity for an opaque silica glass article is eagerly desired. A most typical example of the silica glass article is the above-illustrated flange of a furnace tube used in a furnace for heating a silicon wafer. However, natural raw materials used for the production of an opaque silica glass article contain a salient amount of impurities as well as a salient amount of fine bubbles, and the bubbles are very difficult to remove. Namely, it is difficult to obtain a raw material with a high purity by purification. On the other hand, a rock crystal with a relatively high purity contains a minor amount of fine bubbles therein in the crystal, and therefore, even when the rock crystal is melted, the degree of opaqueness is not enhanced and the resulting silica glass article is translucent.
To solve the above-mentioned problems of the prior art, many proposals have been made. For example, a process has been proposed wherein an amorphous silica with a high purity which contains reduced amounts of an alkali metal, an alkaline earth metal, iron and aluminum, and a salient amount of fine bubbles, and has a silanol group as a vaporizable ingredient contained uniformly at a specific concentration is subjected to flame fusion (Japanese Unexamined Patent Publication (abbreviated to “JP-A”) H6-24711). However, only silica glass articles having a simple shape such as an IC (Integrated circuit)-sealing silica filler and a matrix ingot for silica glass powder can be directly produced, and after-treatments such as after-shaping by lathing are necessary for the production of silica glass articles with a complicated shape such as a flange-form, a ring-shape, column, square pillar or hollow-square pillar. Utilization of the raw material is low in the production of silica glass articles with a complicated shape, and thus, the production cost is inevitably increased.
As another process for producing an opaque silica glass article, a process has been proposed wherein a highly purified crystalline silica powder is heated in an ammonia atmosphere and then the thus-ammoniated silica powder is heated and melted in an inert gas atmosphere to give an opaque silica glass article having an increased number of very fine bubbles, i.e., having a large total cross-sectional area of bubbles per unit volume of the opaque silica glass, and thus exhibiting an enhanced heat insulation (JP-A H7-61827 and JP-A H7-300341). However, this process has problems such that the density of opaque silica glass, and the diameter and amount of bubbles contained therein, greatly vary depending upon the particle diameter and particle diameter distribution of raw material powder and the state of raw material powder charged in a vessel for fusion, and thus, the diameter and amount of bubbles in the surface portion and those in the central portion greatly differ from each other, and an opaque silica glass article having bubbles uniformly distributed therein is difficult to produce with good reproducibility.
As still another process for producing an opaque silica glass article, a process has been proposed wherein a finely divided powder of a foaming agent such as carbon or silicon nitride is incorporated in a siliceous raw material such as silica rock or stone, &agr;-quartz or cristobalite, and the mixture is subjected to a flame fusion using an oxyhydrogen flame (JP-A H4-65328). The above-mentioned problems can be solved by this proposed process. However, the use of an oxyhydrogen flame invites introduction of a hydroxyl group within silica glass which leads to reduction of the viscosity of molten glass and results in an opaque silica glass article not suitable as articles used for a long period of time at a high temperature, such as a jig for the production of semiconductor devices. Further, in this flame fusion step, the residence time of finely divided particles in the flame is very short, and the completion of reaction in the flame is difficult and it is possible that the foaming agent incorporated remains in the molten material as a foreign matter, and further that the siliceous raw material reacts with the foaming agent with the result of undesirable coloration of the molten material.
It is said that when a silica glass jig for the production of a semiconductor is cleaned after the use thereof, the bubbles exposed on the surface are removed, i.e., the surface is partly scraped down. To solve this problem, a procedure has been adopted for adhering a protective transparent silica glass film of a predetermined shape on the surface by heating with an oxyhydrogen flame or in an electric furnace.
For the flange provided at the base of a furnace tube of a heating furnace for a silicon wafer, a heat insulating property as well as a sealing property are required to stably control the atmosphere within the furnace tube. Conventional opaque silica glass flanges have a rough surface due to the presence of bubbles and thus, even where a packing is used, a complete seal cannot be attained. For overcoming this defect, a flange having an opaque portion with good heat insulating property and a transparent portion with good sealing property is suitable.
Several processes have been proposed for producing the flange having an opaque portion with good heat insulati
Akiyama Tomoyuki
Kikuchi Yoshikazu
Kudo Masayuki
Nagata Hiroya
Tsukuma Koji
Derrington James
Tosoh Corporation
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