The effects of alloying material on regrowth-layer structure in silicon power devices

Abstract

The effects on silicon regrowth-layer structure of pure aluminium are compared with those of aluminium-silicon alloy of eutectic composition when these are used as alloying material in joining silicon discs to metal contacts. Non-uniform dissolution of silicon by pure aluminium is prevalent, and the configuration of the regrowth and undissolved silicon surfaces shows dissolution in preferred crystallographic directions. Measurements of recesses in the surfaces show that penetration into the silicon to depths of the order of 100 μm may occur during alloying. This is sufficient to penetrate diffused junctions resulting in the formation of local alloyed junctions of irregular shape with disastrous effects on the electrical characteristics of the device. Deep penetration is caused by a high solubility for silicon being suddenly presented locally when melting of the aluminium is initiated at points where conditions are most favourable. In contrast aluminium-silicon eutectic alloy does not dissolve further silicon immediately on melting, and deep local penetration can thereby be avoided. Measurements made on devices incorporating aluminium-silicon eutectic alloy show reductions in mean penetration depths of over 50%, and the elimination of those regrowth configurations that indicate dissolution in preferred directions.