Thermodynamics of Deposition Flux dependent Intrinsic Film Stress

Vapor deposition on polycrystalline films can lead to staggering levels of compressive stress, exceeding even the yield strength of the films. Mysteriously, a significant part of this stress has a reversible nature: it disappears when the deposition is stopped and re-emerges upon resumption. Although numerous mechanisms have been proposed to explain these effects, the origin of the phenomenon remains still unclear. The incorporation of atoms in the grain boundaries is, to our opinion, the most likely mechanism, but the required driving force for it has not yet been addressed. Therefore, instead of searching for a specific (atomic) incorporation process, we analyze, here, the entire film system using thermodynamic arguments only. We find that the tremendous stress levels observed in the experiments can be surprisingly explained by the flux induced entropic effects in the extremely dilute adatom gas on the surface alone. This thermodynamic evaluation delivers not only the necessary basis for the adatom incorporation models, it even shows that the measured stress values can be reached with low deposition fluxes. Finally, fully counterintuitive, we also show that the stress levels decrease, if the barrier(s) for adatoms to reach the grain boundaries are decreased!