Effects of electron beam patterns on melting and refining of silicon for photovoltaic applications

Abstract

An electron beam melting (EBM) system was fabricated to investigate its effectiveness in the refining of metallurgical grade silicon. The metallurgical refining of silicon is a promising process for reducing the production cost of silicon feedstock for solar cells. Especially, EBM is an excellent technology for evaporating not only metal impurities such as Mg and Ca, but also non-metal impurities such as P, C and O. In this study, a Cu cold crucible was placed in a high vacuum chamber equipped with a 100-kW electron-beam (e-beam) gun. Silicon was charged without any pretreatment such as crushing or acid-leaching. Among the various process parameters, the e-beam pattern was a key to determine the purification efficiency. Two patterns of e-beam were applied, circular and spiral, and their effects on the microstructure and impurity removal were investigated. A different interface between the solid and liquid silicon phase was formed during the solidification depending on the e-beam patterns. Those results were identical to those of the numerical analysis. When a spiral pattern was applied, the horizontal interface was formed and grains grew vertically and uniaxially, which enhanced the impurity removal efficiency. The charged silicon purity of 99.806% was enhanced up to 99.996% after only 44 min of EBM and refining. (C) 2012 Elsevier Ltd. All rights reserved.