Stress relaxation in thin films due to grain boundary diffusion and dislocation glide

PhD ceremony: Mr. C. Ayas, 11.00 uur, Academiegebouw, Broerstraat 5, Groningen

Thesis: Stress relaxation in thin films due to grain boundary diffusion and dislocation glide

Promotor(s): prof. E. van der Giessen

Faculty: Mathematics and Natural Sciences

Thin metallic films are at the heart of electronics and other modern devices. While these films primarily serve electronic purposes, their mechanical properties are a key factor in the reliability and sustainability of the devices.

The research reported on in this thesis is motivated by these issues. It is concerned with the inelastic deformation of films with a thickness of a micrometer or smaller and which are deposited on silicon substrates. The main focus is on the relaxation of thermal as well as growth-induced intrinsic stresses in the film by way of dislocation plasticity inside grains and grain boundary(GB) diffusion. These phenomena are investigated theoretically and numerically with the aid of newly developed methods within the framework of dislocation dynamics and with new continuum models. In the case that stress relaxation occurs by GB diffusion only, the computational results revealed that relaxation is more efficient when the films are made up of slender columnar grains. In the presence of dislocation glide within the grains, however, the microstructure is important because of the competition between the beneficial homogenization of stress with reduced grain size and the increased plastic hardening due to confinement of dislocation glide between impenetrable GBs.

The development of intrinsic stress in films during deposition is governed by the interplay between the efficacy of GB diffusion and film growth rate. Our calculations predict fine microstructures that are growing on slow deposition conditions incorporating the largest average compressive stress in magnitude. The results concerning GB diffusion mentioned above have been partly obtained with a continuum theory and partly by means of a representation in terms of ‘climbing’ dislocations. The latter, numerical technique allows for a seamless coupling of GB diffusion and dislocation glide in a single numerical framework.