Removal of Metal Impurities in Molten Silicon by Directional Solidification with Electron Beam Heating

As one of sequential purification processes where high purity silicon for solar grade silicon (SOG-Si) was manufactured by removing impurities from metallurgical grade silicon (MG-Si), a pyrometallurgical method to remove metallic impurities was developed in an industrial scale, utilizing segregation effect of metallic impurities during solidification. The removal of metallic impurities from MG-Si was carried out with electron beam heating equipment. Molten silicon was supplied continuously at a constant mass to a mould made of water-cooled copper and was gradually solidified from the bottom to the upper direction. Iron concentration was presented by Pfann's and Burton's equations, and was removed from 1500 mass ppm of initial iron concentration to below 1 mass ppm. Aluminum was removed in excess, presumably because of vaporization to gas phase. We found that metallic impurities were not removed above a certain height of an ingot by partition during directional solidification and these phenomena depended on iron concentration condensed in the silicon pool. Mechanism for metallic impurity removals was estimated, on the basis of visual examination of structure and EPMA. Iron concentration profile of ingot and critical height for purification were estimated, through experiments using a 20 kg scale, and were verified to be applicable to industrial scale experiments (150 kg scale). Solar grade silicon was made by way of trial with this process and it was verified to have satisfactory quality as a material for a solar cell.