METALLURGICAL GRADE SILICON PURIFICATION BY THERMAL PLASMA PROCESS COMBINED TO A MOLTEN SILICON BATH BIASING

摘要

The goal of this study is the metallurgical grade silicon purification in order to obtain photovoltaic grade silicon. The silicon purification process uses thermal plasma and an applied electrical field to the silicon liquid bath. The aim is to develop electrochemical reactions due to the interaction between the plasma bias and the liquid bath that leads to increase the kinetic of the purification. The solid silicon melted by RF thermal plasma (Argon, P=20 kW) is under a bias DC current generated by the plasma or by independent electrical DC source. The experiments consist in the checking of the impurities extraction process on the liquid bath surface and the understanding of the role of the bath bias on this kinetic according to the nature of the impurity. Indeed, Optical Emission Spectroscopy permits to demonstrate, on line, the role of the sample positive biasing on the evaporation's kinetic of the cationic impurities of the melted silicon sample. Results show that for an applied polarization of 90 V (i = 900 mA), evaporation of calcium during the plasma treatment is increased by a factor 5 compared to a treatment without any current applied. The LIBS analysis confirms the decrease of silicon impurities concentration after a treatment of 30 mn.