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.
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