Glass manufacturing – Processes – With chemically reactive treatment of glass preform
Patent
1986-07-07
1988-07-26
Kellogg, Arthur
Glass manufacturing
Processes
With chemically reactive treatment of glass preform
65 3013, 65 184, 65 11, 65136, 156602, C03C 2300, C03B 1900, C03B 1500
Patent
active
047597879
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of improving the quality of a product made from or made in contact with a body of vitreous silica.
In one aspect, the invention relates to a method of making an improved vitreous silica product, such as a crucible, a tube or a plate, having a reduced impurity content, and to a product produced by the method.
The invention also relates to an improved method for using a vitreous silica vessel for the high temperature processing of a material contained therein. Particularly valuable uses of this method of the invention are found in the treatment of molten semiconductor materials in a vitreous silica crucible (e.g. in the drawing of a single crystal of silicon).
DISCUSSION OF PRIOR ART
An arc moulded crucible (AMC) is normally produced by fusing a powder material under the influence of an electrical arc while the powder material (e.g. quartz powder) is held in place in a rotating mould (e.g. of water cooled metal) by centrifugal force, with or without the application of a vacuum via the mould wall(s).
A vitreous silica crucible is commonly used to contain a melt from which a single crystal is drawn. In the case of semiconductor materials, high purity is of vital importance and much effort has been, and is still being, given to avoiding impurity contamination of the melt during the crystal pulling operation. The crucible is one potential source of such contamination.
In the case of the pulling of a silicon crystal from melt in a vitreous silica crucible, alkali impurities may transfer to the molten silicon from the wall(s) of the crucible and a variety of different, often time-consuming and otherwise expensive, procedures have been proposed for the purification of the starting material used for a vitreous silica crucible to reduce the alkali impurities therein.
It has now been discovered that a reduced impurity content in material fused in a vitreous silica crucible can be obtained by the simple expedient of applying a polarising potential across the wall(s) of the crucible, at least while the latter is at a temperature in excess of 700.degree. C., for such a period and with such a polarity that ions of the or each impurity will migrate across the wall(s) of the crucible away from the inside surface thereof.
Impurity migration can be effected when the crucible is first fused from the starting material and/or by applying a correctly polarised potential across the wall(s) of the crucible when it contains melt. In the case of alkali impurity ions (which so far appear to be the impurities most easily removed from the crucible wall/melt interface) a voltage with positive polarity on the inside and negative polarity on the outside of the crucible will be required. Voltages between 1 and 2000 volts appear to be effective over the temperatures and times typical for pulling a single crystal from a bath of molten semiconductor material. The migration rate is a function of temperature and applied voltage and preferably the temperature of the inside surface of the crucible wall(s) is in excess of 900.degree. C.
The crucible wall(s) is/are not a significant barrier to the diffusion of impurities (e.g. Na, K or Li) which in the absence of an electrolysing potential may migrate through the wall(s) from the crucible holder, and which in the presence of an electrolysing potential can migrate into the wall(s) from melt in the crucible.
Although the invention is thought to have an important commercial impact in the areas of the high temperature production of products from vitreous silica vessels and the manufacture of such vessels with reduced alkali impurity content, it will be appreciated that vitreous silica tubing whose impurity content has been reduced by electrolysis can have other useful applications than as an intermediary in the manufacture of a vitreous silica vessel.
Hence it should be appreciated that the invention also extends to the production of improved vitreous silica products (e.g. tubing) either by a one-stage process in which an ion-migrating potential is applied across th
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Kellogg Arthur
Striker Michael J.
TSL Group PLC
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