Development and improvement of methods for reducing contamination of silicon-kerf from wafer slicing

Abstract

International audienceThe work, carried out within the framework of the European project ICARUS, concerns the quality improvement of silicon powder resulting from wafer slicing, in photovoltaic industry. Wafering commonly uses diamond-wire sawing and causes the loss of about 35% of silicon which could be valued as raw material. This silicon kerf is contaminated by light elements , in particular oxygen and carbon, and metals in the amounts of several percent for the firsts and hundreds or even thousands of ppm for the seconds, with preponderant elements such as aluminium and nickel. They come from cutting liquid, wire, brick holder and spontaneous oxidation of silicon in water. Therefore, the purity of raw silicon-kerf from wafering is 2N (99%) at best.The aim of the present study is to reach at least 4N (99.99%) silicon-kerf purity with an approach combining a slight adaptation of the cutting process, with an emphasis on industrially acceptable solutions, and light chemical and thermal treatments. Firstly, it is shown that the use of a coolant with low Carbon Oxygen Demand (COD) allows reducing carbon content by half, compared with usual liquid, to approach 1% C by weight. Secondly, alternative beams made of polyester resin filled with silicon powder allow to drastically decrease aluminium content. Then, a hydrochloric acid treatment, followed with a heating step at 500°C under argon flow, allows dividing carbon concentration by six and that of metals by almost three. The final silicon purity effectively reaches 4N, excluding oxygen and carbon. This quality could be further improved by segregation processes to get solar grade silicon