Clamping effect by the substrate on the intrinsic stress in polycrystalline films

Abstract

Mechanical constrictions imposed by a massive substrate to the interactions between coalescing grains within a polycrystalline film deposited on top are investigated by finite element modelling. Such interactions, which underlie the origin of the post-coalescence compression, as suggested recently, induce the twisted zipping (a kind of elastic deformation combining radial and shear strain) of grain boundaries during coalescence. In particular, the substrate clamps the azimuthal interactions that give rise to the crystallography reorientation of the grains (which happens by rotation and/or shear strain) to form pseudocoherent low-angle and coincident-site-lattice grain boundaries. Depending of the mechanical constants of the film/substrate system as well as the aspect-ratio of the structural features in the polycrystalline film, different mechanical responses to the clamping effect are identified, they are: inhibited coalescence of early islands with the formation of a void network between them, coalescence with crystalline defects of larger features growing preferentially along the interface with the substrate, and defect-free coalescence of high aspect-ratio features (columns) by grain boundary twisted zipping.