Method of producing a quartz glass crucible

Details Method of producing a quartz glass crucible Method of producing a quartz glass crucible

2001-07-10

2004-06-29

Vincent, Sean (Department: 1731)

Glass manufacturing

Processes

With shaping of particulate material and subsequent fusing...

C065S017500, C065S032300, C065S033400, C065S033600, C065S033700

Reexamination Certificate

active

06755049

ABSTRACT:

The present invention relates to a method of producing a quartz glass crucible by providing a crucible base body at least in part with an inner layer in which the formation of cristobalite is induced by using a crystallization promoter.
Such quartz glass crucibles are e.g. used for receiving the metal melt when single crystals are pulled according to the so-called Czochralski method. In this method, a seed crystal with a predetermined direction of orientation is immersed into the melt and then slowly pulled upwards. Seed crystal and melt rotate in opposite directions. The surface tension between seed crystal and melt has the effect that a small amount of melt is also pulled off together with the seed crystal, said melt gradually cooling down, thereby solidifying into the continuously further growing single crystal. In this pulling process the quartz glass crucible is subjected to high mechanical, chemical and thermal stresses that must be withstood by the quartz glass crucible for several hours without any noticeable plastic deformation. In the case of a silicon melt, the melting temperature is e.g. more than 1400° C.
To enhance the thermal stability of the quartz glass crucible, it has therefore been suggested that the crucible should be provided with a surface layer of cristobalite. The melting point of cristobalite is at about 1720° C. Such a method is described in EP-A 748 885. The glassy outer wall of a commercially available crucible of opaque, bubble-containing quartz glass is here treated with a chemical solution containing substances that are conducive to the devitrification of quartz glass into cristobalite. Boron, alkaline-earth and phosphorus compounds are recommended as crystallization-promoting substances (hereinafter also referred to as “crystallization promoters”). Barium hydroxide is preferably used. While the quartz glass crucible is heated up (for instance during the intended use in the pulling process), the pre-treated crucible wall crystallizes under formation of cristobalite, which results in a higher mechanical and thermal strength of the quartz glass crucible.
However, the quartz glass crucibles produced by means of the known method can only withstand long process periods to a certain degree when silicon monocrystals are pulled. The thickness of the crystallized surface layer is normally less than 1 mm and is thus relatively thin. It has been found that after some time the crystallized surface layer starts to detach gradually, with the crystallized surface disintegrating less rapidly than the untreated glassy surface. Cristobalite particles that flake off on account of the bubble growth starting below the thinning cristobalite layer thereby pass into the silicon melt and may lead to dislocations in the silicon monocrystal. That is why the known method has so far not been suited for the production of large quartz glass crucibles which are intended to receive a large melt volume and must therefore withstand long process times according to their intended use and thus strong bubble growth.
Moreover, the crystallization promoter may be abraded during transportation or handling of the quartz glass crucible.
It is therefore the object of the present invention to indicate an inexpensive method of producing quartz glass crucibles with reproducible characteristics for long service lives.
Starting from the above-mentioned method, this object is achieved according to the invention in that the crystallization promoter and a reducing substance are introduced into the inner layer.
When compared with the above-described known method, the method according to the invention shows the following essential differences:
1. The crystallization promoter is introduced into the inner layer. The crystallization promoter is thus contained in the inner layer and has such an effect that upon heating of the quartz glass crucible—e.g. during the intended use—it effects the formation of cristobalite. This is accompanied by the known action of the cristobalite layer, i.e. a solidification of the inner wall and thus an increase in thermal stability and chemical resistance of the crucible. An unintended change in concentration, caused e.g. by abrasion during transportation or handling of the quartz glass crucible, is ruled out.
 Moreover, it is possible with the method of the invention to set a predetermined thickness of the crystallized inner layer in a defined manner through a corresponding distribution and concentration of the crystallization promoter in the inner layer. In particular, a stronger and more stable crystallized layer as compared with the known method is thus obtained in a simple way. Layer thicknesses of several millimeters can be achieved. These withstand bubble growth for a longer period of time.
2. A reducing substance is introduced into the inner layer. At least during introduction into the inner layer the substance develops a reducing effect which, however, may still continue or resume again during the intended use of the quartz glass crucible. This results in a surprising effect with respect to the service life of the quartz glass crucible, which shall be explained in more detail in the following:
 It has been found that during the above-mentioned gradual detachment of the crystallized inner layer the growth of bubbles in the crucible wall is of essential importance to the durability of the crystallized surface. The opaque crucible wall contains a multitude of bubbles in which gases may also be entrapped. On account of the high temperature during use of the crucible and, in particular, in the case of long process times, this results in a growth of gas-containing bubbles, which is facilitated by the low viscosity of the quartz glass at such temperatures. When a growing bubble comes into contact with a thin cristobalite layer; this results in mechanical stresses and local flaking off of the cristobalite layer; this is observed all the earlier the thinner the cristobalite layer is. On the assumption that the bubble growth is considerably influenced by the oxygen formed, the chemical reaction that takes place in this case could be described by the following total reaction equation:
4SiO
2(s)
+C
(s)
+2N
2(g)
→Si
3
N
4(s)
+SiC
(s)
+4O
2(g)
  (1)
s=solid
g=gaseous
Subsequently, with the cooperation of atmospheric nitrogen and carbon which is contained in small amounts in the initial substances or may be charged into the crucible wall during the manufacturing process, each mol of nitrogen forms twice the volume of oxygen.
However, in cases where a reducing substance is present in the inner layer, said substance reacts—at least when charged into the inner layer, but preferably also or again during the pulling process—with oxygen or the introduced nitrogen to form an oxidized solid. The solid does not contribute anything to the total gas volume. Thus this “getter effect” of the reducing substance reduces the formation of bubbles caused by excessive oxygen or by oxygen formed during the pulling process.
According to the invention the reducing substance is produced at least during production of the inner layer so that it develops the above-described bubble-reducing getter effect in the area of the inner wall of the crucible. It is in this area that the reducing substance turns out to be of essential importance, since the flaking off of cristobalite which is caused by the growth of bubbles is thus also avoided and the service life of the quartz glass crucible is thereby prolonged.
The method according to the invention allows, on the one hand, a defined and reproducible formation of cristobalite in the area of the inner wall of the quartz glass crucible and guarantees, on the other hand, that this layer of cristobalite is kept undamaged as much as possible during the intended use of the quartz glass crucible. The quartz glass crucible produced according to the method of the invention therefore withstands long process times. The use of large quartz glass crucibles, which when used must withstand particularly long process t

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