Non-metallic particles in Solar Grade Silicon (SoG-Si)

This study investigated the non-metallic inclusions in Solar Grade Silicon (SoG-Si), especially the distribution of inclusions in the top 15mm layer of multicrystalline silicon ingot. The SoG-Si ingot produced from directional solidification process usually pushes the impurities to the top and finally cut off and discarded, which leads to material loss. The hard inclusions lead to wire breakages during the cutting of the ingot into wafers. The main kinds of inclusions found in top-cut silicon scraps from two manufacturers have been investigated using acid extraction, automated feature analysis techniques and SEM-EDS and optical Microscope: they are needle-like Si₃N₄ and lumpy SiC inclusions. Surface observations of the scraps before polishing revealed that, Si₃N₄ inclusions are usually bigger and in some cases can be about a few millimeters. SiC inclusions are usually smaller, ~200μm but can be ~500μm in some cases. For the directional solidified silicon ingot, it was determined that an approximate distance of ~10mm is a good enough cutoff thickness.