GASEOUS REMOVAL OF PHOSPHORUS AND BORON FROM MOLTEN SILICON

The purification of metallurgical grade silicon (MG-Si), especially the removal of phosphorus and boron, was investigated by applying the vacuum evaporation for phosphorus and plasma treatment for boron, respectively, to develop a new production technique of solar grade silicon (SOG-Si) at low cost. The concentration of phosphorus in the MG-Si decreased from 32 mass ppm to 6 to 7 mass ppm after 3.6 ks by vacuum treatment at 0.027 Pa. The concentration of calcium and aluminum also decreased. It has been demonstrated that the overall evaporation of phosphorus and calcium is controlled by the diffusion of these elements through silicon melt to the gas-metal interface. In the experiment of plasma treatment, the MG-Si was melted under various oxidizing plasma conditions by using Ar+O2 or Ar+CO2 as an operating gas for 30 to 1800 seconds. It has been found that the concentration of boron in MG-Si decreased from 28 mass ppm to 2 to 4 mass ppm and boron in MG-Si can be removed as a gaseous boron oxide. A mechanism of boron elimination under the oxidizing plasma conditions is discussed.