Plasticity-related phenomena in metallic films on substrates

Plastic deformation due to thermal stresses has been investigated for different metallic films deposited on Si or α-alumina substrates. We conducted post-mortem TEM and SEM investigations of samples that underwent thermal cycles in order to capture the microstrucrural changes imposed by thermal stresses. The ultimate goal is to determine the dominant plasticity mechanisms responsible for such changes. In-situ thermal cycles performed inside the TEM allowed direct and real-time observations of dislocation behaviour under stress. It is shown that dislocation density drops in Al/Si, Au/Si and in Cu/α-alumina thin film systems. Except in the case of pseudo-epitaxial Cu on sapphire, the interaction of dislocations with the interfaces (passivation, oxide, adhesion layer) is attractive and leads to the disappearance of interfacial dislocations. In this light, the generalized observation of high tensile stresses that arise in metallic films at the end of cooling is explained in terms of insufficient dislocation sources instead of classic strain hardening. Diffusional processes can substitute for a lack of dislocation, but the low relaxation strain rate that would be expected should lead to high stresses during the cooling stages of thermal cycles.