Nanometer-scale-thick metallic films are increasingly used over a broad class of modern technologies. Typical thermo-mechanical processes involved in device fabrication introduce strain in these nanoscale-thick metallic films, causing them to undergo plastic deformation in order to relax the strain. These strain relaxation processes have significant impact on material properties like ductility, hardness, and mechanical strength. In this presentation, we report results of a systematic computational analysis of strain relaxation mechanisms and the associated defect dynamics in ultra-thin (thickness around 5 nm) Cu films subjected to a broad range of biaxial tensile strain (strain levels over the range 0% < ε < 20%). These Cu films are oriented normal to the [111] crystallographic direction. The analysis is based on isothermal-isostrain (canonical) molecular-dynamics (MD) simulations using an embedded-atom-method parameterization for Cu and multi-million-atom slab supercells; the applied strain level, ε, that is maintained during the canonical MD runs is reached from an unstrained state by applying “shock loading” rates. Upon application of tensile biaxial strain to a free-standing ultra-thin Cu film, the response of the thin film is found to be elastic up to a biaxial strain level ε = 5.5%. Beyond this critical ε (yield strain) the strain in the metallic thin film is relieved by plastic deformation. At low levels of the applied biaxial strain beyond the yield strain (ε ~ 6%), threading dislocations are nucleated at the surface of the thin film. The glide of threading dislocations causes shearing of atomic planes that leads to formation of surface pits, while vacancies are generated due to glide and interactions of jogged threading dislocations; due to their supersaturated concentrations, these vacancies aggregate to form clusters. Coalescence of vacancy clusters with surface pits leads to formation of voids extending across the thickness of the film. For 6% < ε < 8%, dislocations are emitted from multiple regions of the thin-film surface, inhibiting the nucleation of voids. For ε > 7%, in addition to nucleation of dislocations from the film surface, dislocation loops are generated in the bulk of the film and grow to intersect the thin-film surface. For ε > 9%, the high density of point defects leads to nucleation of Frank partial dislocations that dissociate to form stacking fault tetrahedra. In addition, the high dislocation density causes dislocation-dislocation interactions that lead to formation of Lomer-Cottrell dislocation locks and complex stable dislocation junctions that act as obstacles to dislocation glide. These defect mechanisms lead to formation of nano-scale domains in the crystalline film with an average domain size of 1.5 nm and low-angle misorientations. Finally, by reloading the strain-relaxed thin-film structures, we characterize these materials in terms of their ductility and elastic properties.

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