RÉSUMÉ

A RF thermal plasma process was developed to refine metallurgical grade silicon for photovoltaic applications. We combined a DC bias of the liquid silicon to the process in order to improve the impurities extraction kinetics by inducing electrochemical reactions at the plasma - liquid silicon interface. The silicon sample, molten by the plasma source, was polarized by a dedicated circuit from -30 V to 120 V. The effect of the bias voltage under different plasma compositions (Ar, Ar-H₂, Ar-O₂, Ar-H₂-O₂) on the impurities extraction was followed using several kinds of diagnostic techniques: On-line by Optical Emission Spectroscopy (OES), and ex-situ by Inductively Coupled Plasma (ICP). On-line measurements show that positive bias conditions enhance the evaporation of metallic impurities (Ca, Fe, and Al). These observations were confirmed by analyses of the treated samples with an ICP showing an increase by a factor 10 in the refining effectiveness when a strong positive bias (110V) is applied. In the case of boron elimination, boron vaporization only increases with the oxygen content in the plasma, indeed, there is no apparent effect of the positive silicon biasing. Finally, we showed that it is possible to dope silicon with positive bias under an Ar + 1% H₂ plasma.