Film/substrate interface stability in thin films

We examined the morphological stability of an interface between a misfitting thin film and a substrate within the framework of linear stability theory. An interface instability exists regardless of the mismatch between the elastic properties of the film and the substrate and the magnitude of the interface energy. Stiffer substrates, smaller misfits, and larger interface energies all tend to reduce the range of wave numbers (to smaller values) over which the instability exists. We demonstrate that the film itself is unstable in all circumstances, even under conditions where the free surface remains flat. In other words, a misfitting film on a substrate is always unstable. We also demonstrate that the interface instability is enhanced when the interface can slip than when it cannot.