General rule for predicting surface segregation of substrate metal on film surface

We studied surface segregation of substrate metal on the top of a deposited film in a vacuum. A metallic polycrystalline film of submicron or micrometer thickness was deposited on a substrate and heated in an Auger electron spectroscopy (AES) or x-ray photoelectron spectroscopy (XPS) apparatus. The surface composition was monitored during heating. The specimen was heated until the substrate metal was observed on the surface of the deposited film and the surface concentration of the substrate metal became saturated. The depth profile of the heated specimen was obtained by measuring AES/XPS with Ar ion etching. Al, Ti, Cr, Ni, Cu, Nb, and Fe were used as substrate materials and Al, Ti, Cr, Cu, Zr, Nb, and Fe were used as film materials. More than 25 combinations of film–substrate materials have been investigated. For some film–substrate combinations, substrate metal was observed on the surface by heating at rather low temperature and substrate metal was only observed at the surface (i.e., surface segregation). The substrate metal 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 atom to the grain boundary of a film at the film–substrate interface, the general rule to predict segregation of substrate metal to the film surface is proposed. The rule is 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. As experimental values of adsorption energy for many metals are not available, the adsorption energy is estimated from heat of solution, surface energy, and vaporization energy, and those estimated values are used to predict the segregation behavior.