Removal of Boron from Metallurgical-grade Silicon by Applying the Plasma Treatment

Abstract

The removal of boron from metallurgical-grade silicon was investigated by applying an Ar/H₂O plasma treatment to develop a new production technique for low cost solar-grade silicon (SOG-Si).
The concentration of boron in MG-Si decreased from 35.7 to 0.4 ppmw, satisfying the requirement for SOG-Si, using Ar+1.24vol%H₂O plasma gas for 25 min of melting time. It has been demonstrated that the overall rate of elimination of boron is controlled by the diffusion of boron in silicon melts. The removal of boron is affected by the kind of plasma operating gas with the highest elimination rate of boron by Ar/H₂O plasma. The experimental findings support that boron in silicon reacts with oxygen in gas only at the plasma-impinging area.
It may be said that only this technique has the possibility to lower boron content of silicon down to the required 0.1 ppm for SOG.