Thermodynamics of deposition flux-dependent intrinsic film stress.

Amirmehdi Saedi,Marcel J Rost

doi:10.1038/ncomms10733

Abstract

Vapour deposition on polycrystalline films can lead to extremely high levels of compressive stress, exceeding even the yield strength of the films. A significant part of this stress has a reversible nature: it disappears when the deposition is stopped and re-emerges on resumption. Although the debate on the underlying mechanism still continues, insertion of atoms into grain boundaries seems to be the most likely one. However, the required driving force has not been identified. To address the problem we analyse, here, the entire film system using thermodynamic arguments. We find that the observed, tremendous stress levels can be explained by the flux-induced entropic effects in the extremely dilute adatom gas on the surface. Our analysis justifies any adatom incorporation model, as it delivers the underlying thermodynamic driving force. Counterintuitively, we also show that the stress levels decrease, if the barrier(s) for adatoms to reach the grain boundaries are decreased.

Highlights

Vapour deposition on polycrystalline films can lead to extremely high levels of compressive stress, exceeding even the yield strength of the films
Whereas several of these mechanisms rely on kinetically limited processes, the grain boundaries (GBs) adatom insertion model suggests that the compressive stress is generated via adatoms that are forced into the GBs by the enhanced chemical potential (CP) of the surface that is set-up by the deposition flux
From a thermodynamic point of view, the most fundamental question has never been addressed, probably due to conceptual difficulties in calculating the CP of the surface during the growth: ‘how can a flux as low as B0.1 monolayer per second (ML per s) lead to stress jumps as high as B150 MPa?’ Our study focuses exactly on this question and we show that these low fluxes can generate such huge stresses and that the driving force for the stress jumps is decreased, if it is easier for the adatoms to diffuse towards and into the GBs