Nanometer-scale-thick metallic films are increasingly used in modern technologies, from microelectronics to various areas of nanofabrication. Processing of such materials generates voids and structural defects and may induce structural transformations. These materials also evolve structurally during the relaxation of electro- and thermo-mechanical stresses. All of these effects underlie many materials reliability problems in the highly complex structures of nanometer-scale devices in modern electronic and nanofabrication technologies. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms of strain relaxation in the thin films and the associated structural changes in order to establish links between structural evolution and mechanical behavior. In this presentation, we report a comprehensive computational analysis of atomistic mechanisms of relaxation of biaxially applied tensile strain over a broad range of strain levels in free standing ultra-thin copper films with the film surface oriented normal to the <110> crystallographic direction; the films studied contained voids extending throughout the thickness of the film with the void axes oriented along <110>. Our study is based on isothermal-isostrain large-scale molecular-dynamics simulations, using an embedded-atom-method parameterization to describe the interatomic interactions. Our analysis reveals four regimes of structural response and mechanical behavior as the applied biaxial strain level increases. In the first regime (ε < 2%), the films respond elastically. In the second regime (2% < ε < 8%), strain relaxation is governed by ductile void growth that is mediated by dislocation emission from the surface of the void and formation of a plastic zone around the void. Dislocation glide and dislocation-dislocation interactions lead to formation of multi-layered microtwin boundaries and Lomer-Cottrell locks in the thin-film material. In addition, dislocation arrays are nucleated at the surface of the void and lead eventually to lattice rotations and nanometer-sized grain formation in the thin film that was originally single crystalline. In the third regime (ε ≥ 8%), dislocations and dislocation arrays are nucleated at the surface of the thin film in addition to those that are nucleated at the void surface. For applied biaxial strain levels higher than 10%, strain relaxation mechanisms in the thin-film material are not dominated by void growth. The fourth regime (ε ≥ 12%) is characterized by the nucleation of a high density of dislocation arrays, leading to a phase transformation of the thin film from a face-centered-cubic (fcc) lattice structure to a hexagonal-close-packed (hcp) lattice structure. Throughout the strain range examined, we characterize the surface roughness of the strain-relaxed films and calculate their elastic properties, ductility, and hardness. Furthermore, we examine the validity of static lattice stability criteria in capturing the phase transformation onsets in the metallic thin films as the applied biaxial strain level increases and construct the underlying free-energy landscapes in order to determine accurately these transformation onsets.

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