A case study of thin film stress evolution at a dissimilar material interface via molecular dynamics simulations

Abstract

Evolution of deformation and stress in growing thin films has been studied in this work using computational simulations that resolve matter at atomic length and time scales. For the surface layers of films laying on the substrate of a dissimilar material, the stress distribution analysis around defects becomes more challenging. Herein, spatial and temporal distribution of deformation and associated stress evolution are presented for different thin film formation events including (1) sub-monolayer growth during an early film nucleation stage and (2) coalescence of adjacent monolayer “islands.” Validity of the stress computed via local computations of the virial expression for stress in a system of interacting particles was checked by comparing to results obtained from considerations of local atomic deformation in conjunction with existing expressions for epitaxial thin film growth stress. For the geometries studied here, where a monolayer of film with a highly characterized linear defect, as in the case of a stacking fault, was simulated for coalescence, fairly good agreement was found. This result demonstrates that, for similar defects at the surface layer, with sufficient sub-ensemble averaging of the standard virial expression for stress, semiquantitative spatial stress distribution information can be obtained from atomic scale simulations. Using our validated stress computation method, we reveal significant stress localization during thin film growth processes, leading to pronounced differences in maximum and minimum stress observed over very small spatial extent (of order multiple GPa over 3–6 nm distances). One prominent mechanism of stress localization revealed here is coalescence between adjacent growing islands. For geometries explored here, stress manifesting during coalescence is highly localized.