The invention relates to a method for producing articles of high-purity synthetic quartz glass, especially quartz crucibles for use in the crucible pulling of silicon crystals according to Czochralski.
High quality quartz glass is an indispensible material in semiconductor technology in producing and processing materials such as germanium, indium phosphide, gallium arsenide and particularly silicon, because of its purity, temperature stability and chemical resistance. Thus, for instance, the precipitation of very high-purity polycrystalline silicon by decomposition of trichlorosilane on heated substrates is often carried out in quartz bells (see, for instance, DE-AS No. 31 07 421). Also for high-temperature processes, such as oxidation, to which silicon wafers are subjected in the course of being processed into semiconductor components, reaction chambers of quartz glass have proven themselves.
The highest requirements are demanded of quartz glass if used as the melting crucible material in the crucible pulling of silicon crystals according to Czochralski. Because of the direct contact between the aggressive silicon melt and the wall of the quartz crucible, the danger of contamination of the melt and of softening of the crucible wall is particularly great. For this reason, the resistance of the pulled silicon crystals can be increased only to a certain limit if quartz crucibles are used which are made of natural quartz, i.e., quartz crystals or quartz sand because of the large content of the doping substances boron, phosphorus and aluminum in the crucible material. Crystals with higher resistance, on the other hand, must be made by the more expensive float-zone-pulling method. Crucibles of natural quartz are therefore coated with the purer synthetic quartz according to DE-AS No. 29 28 089. However, the coating process must be carried out in a cumbersome manner under continuous rotation of the workpiece. There is the further danger that parts of the synthetic inner coating may burst off the outer wall of natural quartz.