Stock material or miscellaneous articles – Hollow or container type article – Glass – ceramic – or sintered – fused – fired – or calcined metal...
Reexamination Certificate
2000-04-12
2003-04-15
Pyon, Harold (Department: 1772)
Stock material or miscellaneous articles
Hollow or container type article
Glass, ceramic, or sintered, fused, fired, or calcined metal...
C428S034100, C428S690000, C428S167000, C428S213000, C428S312600, C428S318600, C428S319100, C428S428000, C118S726000
Reexamination Certificate
active
06548131
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a quartz glass crucible having a crucible body with an inner surface of quartz glass. Furthermore, the invention relates to a process for manufacturing a quartz glass crucible by providing a crucible body with an inner surface of quartz glass.
BACKGROUND OF THE INVENTION
Quartz glass crucibles are employed for example for the pulling of monocrystals from a melt according to the Czochralski method. A crucible of this kind and a manufacturing process therefor are described in U.S. Pat. No. 4,528,163. The known crucible comprises a crucible base body of natural quartz glass with a floor joined to an essentially cylindrical side wall. The crucible has an inner layer of transparent synthetically manufactured quartz glass formed by vitrification of a granulate layer of synthetically manufactured SiO
2
granulate.
To produce the known quartz glass crucible, natural quartz crystalline granulate is added, in a first process step, into a metallic mold rotating about its central axis, to form an evenly thick outer layer in the shape of a crucible. With continued rotation of the mold, synthetically produced SiO
2
powder is then added and it deposits on the inner wall of the outer layer as a granulate layer, and is subsequently melted to form a resistant inner layer firmly joined to the outer layer. In order to avoid formation of crystal seedlings and bubbles in the inner layer, the melting temperature is set high enough so that the added granulate melts completely. The inner surface of the known quartz glass crucible is therefore formed by a smooth amorphous inner layer. A similar process is also known where a crucible-shaped outer layer of crystalline quartz granulate and a granulate of synthetically produced SiO
2
powder are fixed on the inner wall of the mold by vacuum action. The outer layer and the granulate layer are then vitrified together.
When a monocrystal is pulled according to the Czochralski method, for example when a silicon monocrystal is pulled from a silicon melt, a seed crystal of predetermined orientation is dipped into the melt and then pulled slowly upward. The seed crystal and the melt rotate in opposite directions. Surface tension between the seed crystal and the melt causes a small amount of the melt to be pulled along with the seed crystal and as the melt slowly cools it forms a continuously growing monocrystal. However, it may happen in the process that the seed crystal breaks off so that the so-called “starting process” needs to be initiated anew. The period before the actual pulling of the monocrystal can last several hours so that the process becomes correspondingly longer. A long starting process is a considerable expense factor. In addition, as the process time increases so does the thermal and chemical stressing of the quartz glass crucible.
SUMMARY OF THE INVENTION
The object of the invention is to provide a quartz glass crucible which simplifies the starting process, and an inexpensive process for the manufacture of such a crucible.
As regards the quartz glass crucible, the object is achieved according to the invention on the basis of the crucible described above in that the inner surface is roughened at least in the starting zone area by multiple depressions disposed at a distance of no more than 5 mm from one another.
During the pulling of a crystal the inner surface is in contact with the melt. The starting zone comprises that area of the inner surface of the quartz glass crucible where the surface of the melt is located during the starting process. It is an area extending around the inner wall of the crucible. It has been shown that a melt surface free of vibrations and oscillations is decisive for a successful starting process. Oscillations of the melt surface, as caused by for example the rotation of the melt and of the seed crystal, or by the dipping of the seed crystal, are dampened by the crucible according to the invention. This is so because the roughening of the inner surface in the starting zone causes a stabilization and a calming of the melt surface. Roughening the starting zone provides depressions in the surface which prevent in-phase wetting and non-wetting of the surface. This measure prevents oscillations from developing or increasing or at least reduces their intensity, so that fluctuations of the melt surface are reduced or dampened. In order for the depressions to develop a corresponding effect, a roughness profile in at least the starting zone is selected such that it comprises multiple depressions disposed no more that 5 mm from one another. The depressions may overlap. What is of substance is that the inner surface is roughened in the starting zone. The relative geometric position of the depressions in relation to the height of the remainder of the inner surface is not relevant therefor. The depression may be below the remaining inner surface but also on it or above it, if additional quartz glass layers are deposited on the inner surface. The roughened area need not be limited to the starting zone. It is also not required for the success of the inventions' teaching that the surface is roughened without gaps in the circumferential starting zone. It is merely of substance that there is in the region of the melt surface a roughened surface with multiple depressions.
The more pronounced the roughness profile in the starting zone the greater the oscillation dampening effect of the roughness. A particularly well tried roughness has depressions disposed at max. 1 mm from one another, preferably at max. 100 &mgr;m.
An inner surface of synthetic quartz glass is distinguished by its purity so that little contamination passes into the melt when the quartz glass crucible is used in applications as intended. The inner surface of synthetic quartz glass is usually formed by depositing on a crucible base body an inner layer a few millimeters thick.
Different variants of the structural design of the surface roughness in the starting zone region have been shown to be advantageous and they may be present alternatively or cumulatively, as described below in more detail.
In a first variant of the embodiment the surface of the inner surface has an etched structure. A suitable etched structure is described for example in DE A 197 13 014. Such etched surface has irregular raised structural elements extending between a first higher plane and a second lower plane, where a plurality of structural elements has a substantially flat top surface extending to all sides and is delimited on all sides in facet like manner by substantially flat side surfaces which extend between the first and the second plane and where the average depth of roughness R
a
is between 0.1 &mgr;m and 10 &mgr;m and the size of the projections of the structural elements to the first plane on average ranges between 30 &mgr;m and 180 &mgr;m. An etching process and an etching solution suitable for producing such etched structure is also known from DE A 197 13 014. This embodiment is distinguished by a particularly durable surface which in addition substantially reduces the above-described oscillation phenomenon in the melt.
In a further variant of the embodiment the inner surface has a repeating surface impression pattern which is characterized by depressed or raised regularly repeating structures. Due to the regularity of the impression pattern this variant of the embodiment is distinguished by a particularly reproducible effect as regards the stabilization of the starting process.
Another preferred variant of the embodiment of the quartz glass crucible is characterized by an inner surface provided with scores. The scores may be made by for example tools, grinding means, the effect of a particle stream or by laser treatment. In this embodiment the inner surface may be particularly finely structured which can facilitate wetting by the melt and can reduce distortions of the melt surface.
In a further variant of the embodiment the inner surface has a bubble layer with open pores. The open pores provide an additional free volume r
Arndt Martin
Englisch Wolfgang
Fabian Heinz
Gertig Udo
Leist Johann
Heraeus Quarzglas GmbH & Co. KG
Miggins Michael C.
Pyon Harold
Tiajoloff Andrew L.
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