The effect of laser reflow on the variation of stress with thermal cycling in aluminum thin films

Abstract

In order to understand the role of film thickness and grain size on the stress-temperature characteristics of aluminum films, we have compared the stress behavior of hot sputter-deposited aluminum films before and after they were subjected to laser reflow, a process which produces a large grained, hillock-free microstructure. For Al thin films, the σ–T curves commonly reported in the literature and those observed by us exhibit a large asymmetry between the tensile and compressive flow stresses induced on thermal cycling. The effect of the laser reflow is to reduce this asymmetry, i.e., reduce the tensile flow stress levels to values that are comparable to the flow stresses in compression. The magnitudes of stress thus obtained agree well with those predicted by considering a misfit dislocation mechanism by which we have observed stress relaxation to occur in Al films in in situ transmission electron microscopy thermal cycling experiments. The model used to calculate flow stresses takes into account the energies of the dislocations to be laid down at the film-substrate and film-native oxide interfaces. As predicted by the model, the stress in the laser reflowed films is inversely proportional to the film thickness in both tension and compression. While the difference between the stress behavior of reflowed and unreflowed films diminishes as film thickness decreases, a final stage of hardening below 200 °C is always seen for the thinner as-deposited films that is absent in their reflowed counterparts.