Mechanism of shape formation of three-dimensional periodic nanostructures by bias sputtering

We previously demonstrated a process for fabricating three-dimensional (3D) periodic nanostructures composed of corrugated $\mathrm{a-}{\mathrm{Si/SiO}}_{\mathrm{2}}$ multilayers, which behave as 3D photonic crystals. In this process, bias sputtering is a key technique by which the pattern is self-forming. This letter clarifies the mechanism of the self-shaping effect of bias sputtering by comparing deposition simulation and experiments. The mechanism is decomposed into three main effects: diffuse incidence of neutral particles of film material, sputter etching by normally incident rare-gas ions, and subsequent redeposition of sputtered film material. Specifically, redeposition has a self-adjusting effect on the depth of holes or valleys, and is the key of formation of stable patterns.