Model for predicting thermal stresses in thin polycrystalline films

Abstract

This paper reports the development of a numerical model to analyze thermal stresses induced in thin polycrystalline films deposited on thick substrates. The model accounts for viscoplastic deformations due to crystallographic slip in each grain of the polycrystal. The microstructural processes, which constrain dislocation slip due to the substrate, surface layer, and grain boundaries as well as due to hardening on the slip systems themselves, are considered and incorporated into the model. Using this model, the predicted stress-temperature curves for aluminium films with a natural oxide layer are in good agreement with the measured curves, especially for thicker films. It was found that the predictions of stress-temperature curves for 〈111〉 fiber textured films are not substantially affected by reasonable deviations from a perfect fiber texture. Therefore, one can globally model the deformation of a 〈111〉 textured polycrystalline film by assuming it to be a single crystal provided the film is in a state of equal biaxial strain. However, it is also shown that for an equal biaxial thermal strain, the local stress state at the grain level is more complicated due to the effect of the orientation of the individual grains, although the global stress state will be equal biaxial for a 〈111〉 textured film. In fact, very high local stress gradients might have to be accommodated.