GRAIN COLLAPSES IN STRAINED ALUMINUM THIN-FILMS

A special stress relaxation effect in thin aluminum films evaporated onto oxidized silicon wafers is reported. The effect is observed at elevated temperature and under tensional stress. It appears as if certain Al grains in the film surface suddenly "collapse" compared to surrounding grains. The phenomenon is observed during the cooling phase of an annealing cycle, or when imposing external strain on the film by bending of the substrate at elevated temperature. Such external strain was imposed by micromechanical technique in situ in a scanning electron microscopy (SEM), and the collapse phenomenon was monitored as it happened. The phenomenon occurred instantaneously and only in films of thickness 1 mu-m or more; thinner films relaxed by hole formation. The grain collapse effect is characterized, and a physical explanation is given, supported by SEM and transmission electron microscopy (TEM) investigations and surface profile measurements.