Isothermal annealing of hillocks in AlCu films

Abstract

Vapor deposited Al-15 wt.% Cu films were formed on oxidized silicon substrates at room temperature. Subsequent heating of these samples resulted in the formation of copper-rich hillocks on the surface of the films because of compressive, substrate-induced differential thermal expansion strains. Transmission electron microscopy (TEM) showed that hillocks were also present on as-deposited films but they were much smaller and not detected by scanning electron microscopy (SEM). Isothermal annealing studies were carried out in the SEM in the 200–300°C range. The decrease in hillock density and the increase in average hillock size were measured as a function of time. Analysis showed that the activation energies for these two phenomena were 0.29 and 0.28 eV respectively, values that are typical of surface diffusion phenomena. These observations are in agreement with an Ostwald ripening mechanism. TEM of samples thinned from the silicon wafer side were also carried out to elucidate the hillock morphology.