Shape transition in nano-pits after solid-phase etching of SiO₂ by Si islands

We study the nano-pits formed during the etching of a SiO₂ film by reactive Si islands at T≈1000 °C. Combining low energy electron microscopy, atomic force microscopy, kinetic Monte Carlo simulations, and an analytic model based on reaction and diffusion at the solid interface, we show that the shape of the nanopits depend on the ratio R/x_s with R the Si island radius and x_s the oxygen diffusion-length at the Si/SiO₂ interface. For small R/x_s, nanopits exhibit a single-well V-shape, while a double-well W-shape is found for larger R/x_s. The analysis of the transition reveals that x_s∼60 nm at T≈1000 °C