Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
1997-11-19
2001-05-01
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C501S012000, C065S017200, C423S338000
Reexamination Certificate
active
06225245
ABSTRACT:
The present invention relates to a synthetic quartz glass powder suitable for use in a semiconductor field, particularly for a high purity quartz glass part material suitable for use in a high temperature region of at least 1000° C. and for the production thereof.
Heretofore, a crucible, a jig or the like which is useful for production of a semiconductor single crystal, has been produced by melting a natural quartz powder obtained by pulverizing natural quartz. However, natural quartz contains various metal impurities even when it is of good quality, and, as such, has not been fully satisfactory from the viewpoint of the purity. Especially for a high purity single crystal required along with the progress in high performance in the semiconductor industry, inclusion of metal impurities adversely affects the performance of a semiconductor, and a crucible, a jig or the like which is feared to bring about inclusion of metal impurities or the like, can not be used.
Accordingly, for glass products to be used in the semiconductor industry or the like, a very strict control is carried out with respect to their quality. Such high purity glass is produced mainly by e.g. (1) a method in which natural quartz is highly purified for use, (2) a gas phase synthetic method wherein a fume formed by hydrolysis and thermal decomposition of a gaseous silicon compound such as silicon tetrachloride in an oxyhydrogen flame, is used, and it is subjected to melt-forming, or (3) a method wherein a synthetic quartz powder is used, which is obtained by calcining a silica gel powder which is in turn obtained by hydrolysis, gelation or the like of a silicon alkoxide or the like in a liquid phase.
However, the method (1) has a problem that there is a limit in reduction of the content of minor impurities, and the method (2) has a problem that the production cost is extremely high. By the method (3) wherein a silica gel, particularly a silica gel powder prepared by using a silicon alkoxide as the staring material, is used, it is possible to obtain a synthetic quartz glass having a low content of minor impurities inexpensively as compared with the method (2), but this method (3) does not necessarily satisfy the required level.
For example, a crucible for pulling up a silicon single crystal, is used at a high temperature of from 1400 to 1500° C. for a long period of time. Particularly, in recent years, it has become common to employ a continuous pulling up method which is carried out while continuously supplying a starting material silicon, whereby the period of time of using the crucible is further prolonged. The glass phase (amorphous) constituting the crucible is a metastable phase. Accordingly, if held in the above mentioned temperature range for a long period of time, it tends to convert into crystalline (crystballite) as a stable phase. Especially when metal impurities, etc. exist, they serve as crystal nuclei and further promote crystallization. If this phenomenon (generally called devitrification) takes place during pulling up of a single crystal, a crystalline portion formed along the inner wall of the crucible is likely to peel off and enter into the silicon melt, thereby to hinder single crystallization (“Science of Silicon” compiled by UCS Semiconductor Substrate Technology Study Group, 1996). On the other hand, also with respect to a core tube used for e.g. heat treatment of a silicon wafer, although the temperature of its use is low, if devitrification takes place due to its use for a long period of time, such devitrification causes a decrease in strength or breakage due to repetition of heating and cooling, since the thermal expansion coefficient differs between the crystalline portion and the amorphous portion. A synthetic quartz glass is considered to be hardly devitrified, since its content of metal impurities is very low as compared with a natural rock crystal powder, but a devitrification phenomenon of a problematic level has still been sometimes observed.
In view of the above problems, the present inventors have conducted an extensive study to obtain a synthetic quartz glass powder which undergoes no devitrification when used at a high temperature in the form of a shaped product. As a result, it has been found that a synthetic quartz glass powder obtained by applying certain specific treatment to a silicon alkoxide, water and an organic solvent such as an alcohol, used as starting materials, is able to prevent devitrification of a shaped product obtained by melt-forming such a synthetic quartz glass powder. The present invention has been accomplished on the basis of this discovery. Namely, it has been found that a substantial effect for suppressing crystallization can be obtained when a silicon alkoxide, water and an organic solvent such as an alcohol used as the starting materials, are preliminarily passed through a filter prior to their introduction into the reaction system.
Further, it has been found that by the commonly employed method for measuring minor metal impurities in glass wherein the residue obtained by dissolving a quartz glass powder with hydrofluoric acid, is dissolved and extracted by nitric acid, sulfuric acid or the like, followed by ICP analysis or an atomic absorption analysis, the degree of the influence over the devitrification phenomenon can not be distinguished. Accordingly, as described in detail hereinafter, the devitrification phenomenon is considered to be dependent not only on the absolute amount of inclusion of the impurities but also on the form in which the impurities exist. Accordingly, unless an analytical method capable of distinguishing the form of impurities which causes this devitrification, can not be found out, it is difficult to properly deal with the problem of this devitrification. The present inventors have conducted an extensive study also on this problem and as a result, have found it possible to predict the degree of occurrence of this devitrification phenomenon in a shaped product of quartz glass obtained by melt-forming, by evaluating the synthetic quartz glass powder by means of a specific evaluation method. The present invention has been accomplished on the basis of this discovery.
Namely, the present invention provides a synthetic quartz powder obtained by calcining a powder of silica gel, characterized in that white devitrification spots having sizes of larger than 20 &mgr;m in diameter formed in an ingot obtained by vacuum melting the synthetic quartz powder at a temperature of from 1780 to 1800° C. to form an ingot, followed by maintaining the ingot at a temperature of 1630° C. for 5 hours, are at most 10 spots/50 g, etc.
Now, the present invention will be described in detail with reference to the preferred embodiments.
The synthetic quartz glass powder of the present invention is one obtained by calcining a silica gel powder, whereby white devitrification spots having sizes of larger than 20 &mgr;m in diameter formed in an ingot obtained by vacuum melting the synthetic quartz glass powder at temperature of from 1780 to 1800° C. to obtain an ingot, followed by maintaining it at a temperature of 1630° C. for 5 hours, are at most 10 spots/50 g. Namely, 50 g of the synthetic quartz glass powder is put into a heat resistant container such as a crucible and vacuum-melted at a temperature of from 1780 to 1800° C., followed by cooling, to obtain an ingot. This ingot is maintained at a temperature of 1630° C. for 5 hours, and then cooled. When the inside of this ingot is inspected, white spherical portions having sizes of from a few tens &mgr;m to 1 mm are sometimes observed, and they are problematic “devitrification spots”. As a result of extensive researches by the present inventors, it has been found that there is a significant interrelation between the number of such “devitrification spots” formed and the crystallization occurring during use of a finally molded product, and that when a quartz glass part to be used at a high temperature, such as a crucible for pulling up a silicon single crystal, is prepared by using a synthetic quartz glass
Katsuro Yoshio
Moriyama Takashi
Takazawa Akihiro
Utsunomiya Akira
Group Karl
Mitsubishi Chemical Corporation
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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