Molecular dynamics simulations of internal stress evolution in ultrathin amorphous carbon films subjected to thermal annealing

The evolution of internal stress in ultrathin amorphous carbon (a-C) films is a complex physical process that is difficult to experimentally analyze due to the very small film thickness (a few nanometers) and the lack of instruments that can perform spatiotemporal stress measurements at sub-nanometer resolutions. Even more challenging is the elucidation of the correlation between internal stress, film activated, and temperature. Molecular dynamics (MD) provides potent computational capability for tracking structural changes activated by stress and temperature at the atomic level. Consequently, the aim of this study was to perform a comprehensive MD analysis that elucidates the origin of internal stress in sub-2-nm-thick a-C films grown on single-crystal silicon under optimal deposition energy conditions and explore its dependence on prevalent structural features (e.g., hybridization state) and temperature. The physical mechanisms of a-C film growth and stress built-up under deposition conditions of energetic particle bombardment and stress relief due to thermal annealing are interpreted in the context of MD results. Simulations of film growth illuminate the correlation between film stress and energy of incident carbon atoms. A significant stress relief occurs mainly in the bulk layer of the multilayered a-C film structure at a critical annealing temperature, which continues to intensify with the further increase of temperature. Simulations of time-dependent variation of stress through the film thickness reveal that the stress relief is a very fast process that accelerates with the increase of temperature. The results of this study provide insight into the spatial and temporal variation of internal stress in ultrathin a-C films due to structure and temperature effects and the film stress-structure interdependence.