Method of making <200nm light transmitting optical fluoride crystals for transmitting less than 200nm light

摘要

The invention relates to optical fluoride crystals, and particularly to optical fluoride crystals such as calcium fluoride, which have high transmission levels to below 200 nm light, such as produced by excimer lasers. In particular the invention relates to making optical fluoride crystals with improved transmission surfaces. The invention relates to the elimination of mid-spatial frequency roughness 1-1000 mum spatial wavelengths and high-spatial frequency<1 mum spatial wavelengths from optical fluoride crystal surfaces. The removal of the mid-spatial frequency 1-1000 mum spatial wavelengths and high-spatial frequency<1 mum spatial wavelengths from the optical fluoride crystal surfaces provides improved transmission at below 200 nm optical lithography wavelengths such as 193 nm and 157 nm. The invention includes final finished optical fluoride crystals having final finish high optical transmission surfaces free of mid-spatial frequency roughness 1-1000 mum spatial wavelengths and high-spatial frequency roughness<1 mum spatial wavelengths with high transmission at lambd<200 nm.