Monocrystalline silicon ingots are grown in a Czochralski (Cz) furnace by melting high purity silicon feedstock in a fused quartz crucible. As the standard solar cell size is getting larger, silicon ingots manufacturers have increased their demands on crucibles size and properties. Among other factors, the formation of bubbles and their growth affect the crucible`s properties and performance, which in turn are affected by the process parameters. The mechanisms of these bubbles formation and growth are still not well understood. In this study, we investigate the bubble formation and growth in three different types of crucibles before and after use in a Cz process. The crucibles have different sand qualities, -size and -chemistries. The content of hydroxyl (OH) is measured by Fourier Transform Infrared Spectroscopy (FTIR), while bubble size and distribution are measured by X-ray tomography and optical microscopy, respectively. The results indicate that a reduction in OH content correlates with increasing bubble growth. The reference crucible, which has coarse particle size distribution and standard chemistry, has the largest variation in bubble content and the largest average bubble growth for the samples investigated. The crucible with the finest particle size distribution and high purity seems to be the best choice for silicon ingot production as it experiences, on average, the lowest bubble growth in the bubble free (BF) layer. The results also show that the crucible quality (e.g. the manufacturing process and chemistry) affect the bubble content as well as the OH level.
Cristobalite is one of the quartz crystalline polymorphs that forms at above 1470 °C in its pure form and above 1000 °C for quartz glass. Its formation during the Czochralski process is therefore inevitable, and is usually controlled by doping the quartz sand with barium or barium-based coatings. The formation of cristobalite can lead to significant structural defects in silicon ingots. In this work, we studied the influence of various materials (graphite, silicon carbide and alumina) on the formation and properties of the cristobalite layer. In our study we investigated glass samples extracted from a commercially produced fused quartz crucible. The samples were heat-treated in different furnaces with different contact materials: alumina, silicon carbide and graphite. The furnace with alumina as contact material was an open-air furnace, while the two others were purged with argon. All of the heat treatment experiments lasted for 3 hours at a temperature of 1500 °C, which is the approximate temperature of the Czochralski process. After the heat treatment, the samples were investigated by light microscopy and X-ray diffraction. The results showed that the contact material is the most determining factor for the cristobalite layer’s thickness and morphology. The enhancement of cristobalite formation is the greatest by using graphite as the contact material, followed by alumina. Results indicate a retardation in phase transformation in comparison to other materials. These findings are an important step to further understanding of the cristobalite formation kinetics in fused quartz crucibles during the Czochralski process.
