Production of compensation free SOG silicon feedstock by melallurgical refinement

New metallurgical refining process of Solar Grade Silicon (SOG) feedstock reach to 7N is developed. The process consists of B reduction by oxidation, P reduction by vacuum and metal reduction by directional solidification. Achieved impurity level of B is <; 0.2 ppmw and P is around 0.1 ppmw, and their fluctuations are enough small to say compensation free. Metal impurities are also less than 0.1 ppmw. For the sake of low level of B and P, and its nugget shape, user can use this product as same as crashed-rod by the Siemens method. For quality specification, the product satisfy not only SEMI standard of PV Si feedstock but also NEDO´s project target. To utilize these advantages, NS Solar Material Co., Ltd. starts 500 ton/year plant in 2007 and has continued to produce this SOG Si feedstock.