Influence of impurities and OH-group content on viscosity, cristobalite formation and bubble evolution in fused quartz crucibles at temperatures for Czochralski process

Monocrystalline silicon production can be made more efficient by extending lifetime of fused quartz crucibles used in the Czochralski process. We investigated how viscosity, OH-group content and metallic impurity content affect cristobalite formation and bubble evolution, and how cristobalite formation affects bubble growth and OH-groups removal by investigating three different crucible types. The different crucible parameters and properties were measured before and after heat treatment of 6, 12 and 18 h at 1500°C. Crucibles were characterized by: viscosity measurements at 1500 °C, micro-computed tomography, IR-microscopy, ICP-MS and optical microscopy. Experimental investigations were complemented by modelling to predict the bubble evolution during heat treatment. The bubble evolution was found to be strongly dependent on OH-group content, viscosity, impurity content and cristobalite formation kinetics, where the OH-dissolution within the crucible during heat treatment was found to be the main contributor to bubble's growth. Cristobalite formation was found to be affected by OH-group content and metallic impurity content, especially barium. In all the samples, irrespective of their sand treatment, the dominating mechanism of bubble evolution during the first hours of heat treatment was the nucleation of new bubbles. In the subsequent hours of heat treatment, the dominating mechanisms were bubble growth and their coalescence. Overall, the optimal combination of different process parameters was found for the chlorinated crucible, which exhibited highest viscosity, lower initial OH-groups content, slower bubble growth and enhanced cristobalite formation at the surface, which would all contribute to a better stability and extended crucible lifetime during the Czochralski process.