Direct contact superpolishing of sapphire

We show that sapphire can be superpolished in direct contact with a tin lap by using colloidal silica in water as the polishing liquid. The residual roughness is <0.05 nm rms. No damage to the layer remains. The typical polishing time is 15-30 min. The flatness is determined by prepolishing mainly. Curved surfaces can also be polished by using this method. The least roughness was observed when removal rates were low, i.e., ~ 10 nm/min, which corresponds to approximately half of an atomic layer per second.