Nanometer-scale-thick metallic films are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. Processing of such films generates voids and other defects in the material's structure. Relaxation of processing related thermomechanical stresses is mediated by the dynamics of these defects. Such defect dynamical phenomena 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 order to establish links between structural evolution and mechanical behavior of such thin-film materials.

In this presentation, we report a comprehensive computational analysis of atomistic mechanisms of relaxation of biaxially applied tensile strain over a range of strain levels in free standing ultra-thin copper films with their surface plane oriented normal to the [111] crystallographic direction. Our study is based on isothermal-isostrain large-scale molecular dynamics (MD), using an embedded-atom-method parameterization to describe the interatomic interactions. In the MD simulations, the slab supercells employed may or may not contain a central cylindrical void that is oriented along [111] and extends throughout the thickness of the film. Our analysis reveals three deformation regimes with increasing applied strain level. In the first regime (strain < 1-2%), the film's response is purely elastic. In the second regime (strain < 8%), strain relaxation is governed by ductile void growth mediated by dislocation emission from the void surface, dislocation glide, dislocation jogging, vacancy generation, and pipe diffusion; ductile void growth is accompanied by the formation and expansion of a plastic zone around the void. These mechanisms of strain relaxation result in substantial film surface roughness leading to formation of threading dislocation loops that propagate from the film's surfaces into the bulk of the film (at strains > 4%); the nucleation and dynamics of threading dislocation loops is analyzed systematically. In the third regime (strain > 8%), strain relaxation is governed by formation of a dislocation population uniformly distributed throughout the film. These defects pin the dislocations emitted from the void surface and inhibit the expansion of the plastic zone around the void; consequently, the role of void growth is not dominant in this deformation regime and is found to become negligible at higher strain levels. More importantly, under such deformation conditions, the defect distribution triggers a transition in the film's structure: from single-crystalline to nanocrystalline. Detailed structural characterization yields a narrow unimodal distribution of grain sizes with an average size of 1.5 nm; the lattice structure remains face-centered cubic for all the grains, which are bounded by low-angle grain boundaries.

In addition to the structural characterization, we characterize the roughness of the film surfaces, as well as the void surface roughness, over the entire range of strain levels examined. The effects of nanocrystalline structure formation on the surface roughness are discussed in detail. The evolution of the dislocation population for the various dislocation types that are formed also is analyzed over the strain range examined and is correlated with the evolution of surface roughness, void volume, and system energy. Finally, in the third deformation regime, by relaxing and subsequently reloading biaxially the obtained nanocrystalline structure, we calculate the nanocrystalline films' mechanical properties including their ductility, elastic moduli, and fracture toughness and we compare them with those of the single-crystalline material.