Photochemical polishing of sapphire substrate based on nonadiabatic optical near-field etching

In this study, photochemical polishing based on a nonadiabatic optical near-field etching to sapphire substrate is studied. A sapphire substrate with an initial roughness of Ra=6.00 nm was polished by 100 Pa Cl₂ gas atmosphere under laser irradiation for 60 minutes, and smoothened into Ra= 1.91 nm. Due to the Gaussian intensity distribution in the laser beam section, there was in-plane distribution of surface roughness. Furthermore, etching time got longer, the roughness was increased. This result suggests nonadiabatic optical near-field etching to sapphire substrate has an optimal etching time, which agrees with a result of SiO₂ substrate.