Eliminating nanometer-scale asperities on metallic thin films through plasma modification processes studied by molecular dynamics and AFM

We report the effects of reducing surface asperity size at the nanometer scale on metallic surfaces by plasma-assisted surface modification processes using simulations and experiments. Molecular dynamics (MD) simulations were conducted by irradiating various inert gas ions (Ne, Ar, Kr, and Xe) onto a cobalt slab with nanoscale asperities on the surface. The MD simulations showed that as the atomic number of the inert gas increased the surface asperity size was reduced more efficiently, while the etching rate decreased. The dependencies of the scattering behaviors on the inert gas ions originated from the mass exchange between the working gas ions and the slab atoms. Atomic force microscopy and X-ray fluorescence measurements were performed on hard disk media subjected to the surface modification processes. These measurements experimentally demonstrated that the density of nanoscale asperities was reduced with a lower etching rate as the atomic number of the inert gas increased, consistent with the simulation results. Through this study, we clarified that heavier working gases were more effective in reducing surface asperity size without significantly reducing the thickness of the material, which can contribute to better control of surface morphologies at the nanometer scale.