Atomic Structure of Float Polished Surfaces of Calcium Fluoride Single Crystals for Ultra Violet Applications

Calcium fluoride (111) single crystals for ultra violet applications have been finished to flat by the float polishing process with 7nm-diameter silicon dioxide powder, pure water and a tin lap. The flatness of 32nm P-V on 90-mm-diameter samples that had a surface roughness of 0.72 nm Ry, and 0.077 nm rms were obtained. High-resolution transmission electron microscopy study of the cross-sectioned crystal surface showed that the float-polished surface had (111) plane with small atomic steps, and no subsurface defect. The polished surface roughness depends on the mismatch between the sample surface and the (111) plane. This result was also verified by scanning probe microscope observation.