Fused Quartz Crucibles for PV Applications: The Role of Czochralski Process Parameters and Sand Quality on the Bubble Formation and Growth

Abstract

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.