Surfaces of Calcium Fluoride Single Crystals Ground with an Ultra-Precision Surface Grinder

Calcium fluoride single crystals for next-generation optical lithography were fabricated with surfaces corresponding to the (001), (111), and (110) crystalline planes. The grinding process utilized an ultra-precision surface grinder and was optimized for resin-bonded SD3000-75-B diamond wheels. A premium crystalline surface had a measured surface roughness of 0.89 nm Ra, 6.99 nm Ry and 1.10 nm rms, with no microcracks. We show that the surface roughness of these samples depends on the crystalline plane and the orientation, as well as on the grinding conditions. Measurement of the sub-surface damage layer and the laser-induced damage threshold are also presented herewith.