Kinetic phase separation in sputter-deposited aluminium-tin films

Sophisticated multi-source physical vapor deposition processes allow the production of multi-component materials with compositions that are impossible to achieve by conventional casting techniques. The different condensation and solidification mechanisms of these processes may favor the formation of new material structures which are not known from techniques as, e.g. solidification from the melt. In this paper we investigate the structural changes of sputter-deposited aluminium-tin (AI-Sn) in dependence on the Sn content. The Sn-content is varied from 7 to 12 at%. The distribution of the minority phase, Sn, is characterized by selective removal of the Al matrix with aqueous NaOH and subsequent observation in the SEM. With increasing Sn-content this phase appears to be more and more interconnected throughout the entire film. The degree of connectivity of the Sn-backbone remaining after the NaOH-treatment is determined by measuring its electrical conductivity. By these measurements the percolation threshold of the Sn-phase can be determined to be about 8 vol% Sn in Al. From the experimental observations it is concluded that the phase distributions are determined by the different surface mobilities of the two materials in the film growth zone. A simple model is presented which incorporates the basic mechanisms involved in the simultaneous deposition of an immobile material (Al) and a mobile material (Sn). The model reproduces the general trends of the experimental data with respect to the dependence of the percolation threshold on Sn-concentration and film thickness. (C) 1998 Elsevier Science B.V. All rights reserved.