Prediction of surface segregation of substrate metal on deposited film

Abstract

The surface segregation phenomena of substrate metal on a deposited film have been studied. A metallic polycrystalline film of 1 µm thick was deposited on a substrate and heated in an AES or XPS apparatus. The surface composition was monitored during heating. The specimen was heated until the substrate element was observed on the surface of the deposited film and the surface concentration of the substrate element became saturated. The depth profile of the heated specimen was obtained by measuring AES with Ar ion etching. For some film–substrate combinations, the substrate element was observed on the surface by heating at a rather low temperature and the substrate element was only observed at the surface (surface segregation). The substrate element is considered to diffuse through grain boundaries of the film to reach the surface. Not all film–substrate combinations show surface segregation of substrate metal. By considering the migration of substrate atoms to the grain boundary of a film at the film/substrate interface, it is concluded that segregation of a substrate atom occurs if the adsorption energy of a substrate atom on a film is larger than that on the substrate itself. Experimental values of adsorption energy for many metals are not available, so the adsorption energy is estimated from the heat of solution, surface energy and vaporization energy; these estimated values are used to predict the segregation behaviour. Copyright © 2000 John Wiley & Sons, Ltd.