Atomistic analysis of strain relaxation in [110]-oriented biaxially strained ultrathin copper films

摘要

Results are reported of a systematic atomic-scale computational analysis of strain relaxation mechanisms and the associated defect dynamics in nanometer-scale thin or ultrathin Cu films that are subjected to a broad range of biaxial tensile strains. The films contain pre-existing voids and the film planes are oriented normal to the [110] crystallographic direction. The analysis is based on isothermal-isostrain molecular-dynamics simulations according to an embedded-atom-method parameterization for Cu and employing multimillion-atom slab supercells. In addition to an initial elastic response for an applied biaxial strain level ε<2%, our analysis reveals three regimes in the thin-film mechanical response as ε increases. For 2%≤ε≤6%, biaxial strain relaxation is dominated by emission and propagation of dislocations (plastic flow) from the surface of the void accompanied by ductile void growth. For 6% < ε < 10%, the biaxial strain in the thin film is relaxed by both ductile void growth and emission of dislocations from the surfaces of the thin film. For ε ≥10%, strain relaxation is dominated by dislocation emission from the surfaces of the thin film, leading to a structural transformation from the face-centered cubic to a hexagonal close-packed phase. The defect nucleation mechanisms and the high-strain response of the thin films are found to be significantly different from those observed in <111>-oriented Cu thin films [M. R. Gungor and D. Maroudas, J. Appl. Phys. 97, 113527 (2005); M. R. Gungor and D. Maroudas, Appl. Phys. Lett. 87, 171913 (2005)].