Local stress relaxation phenomena in thin aluminum films

Abstract

This article reports a systematic study of local stress relaxation effects in thin Al films, deposited on silicon substrates by sputtering or electron beam evaporation. The stresses were induced thermally during an annealing cycle, as well as thermomechanically by micromechanical technique. The latter experiments were performed in situ in the vacuum chamber of a scanning electron microscope (SEM), which made it possible to monitor the reconstruction of the film surface as it happened, and simultaneously register applied forces, strains, and temperatures. Three fundamental, local stress relaxation phenomena were observed. Compressive film stresses were, at elevated temperature, relaxed by hillock formation. The in situ experiments in the SEM, combined with cross-sectional transmission electron microscopy of hillock structures, provided important information on the growth mechanism of hillocks. Tensional film stresses at elevated temperature were relaxed by two modes of local stress relief. One mode of tensional stress relief occurred in films of thickness 1 μm or more. Singular Al grains were observed to ‘‘collapse’’ and become wider and thinner than neighboring grains. Such grain collapses are scarcely reported by previous workers, and were in the present investigation seen to occur very suddenly, in a second or less, as the strain and the temperature reached critical levels. A collapse mechanism, based on local plastic yield and interfacial slip, is suggested. Films of submicron thickness relaxed by a second mode: hole formation. Essentially two types of holes were observed: small, circular holes and large branched-out holes. A hole formation mechanism, based on creep cavitation and interfacial slip, is suggested. The observed holes resemble the ‘‘voids’’ many other workers have reported in narrow, passivated Al lines. The present results show that passivation coatings and narrow geometries are not necessary prerequisites for holes in Al films; a high tensional film stress is enough.