Ultra-Precision Float Polishing of Calcium Fluoride Single Crystals for Deep Ultra Violet Applications

High purity calcium fluoride (111) single crystals were float polished to a flatness of 32 nm p-v on 90-mm-diameter samples that had a surface roughness of 0.72 nm Ry, and 0.077 nm rms, as measured with a Scanning Probe Microscope (SPM). By observing an area of a float-polished surface with a high resolution transmission electron microscope, it was determined that the float-polished surface had a perfect (111) lattice with small atomic steps, and no subsurface damage. The polished surface roughness depends on the mismatch between the sample surface and the (111) plane. This result was also verified by SPM observation.