CALCIUM FLOURIDE CRYSTAL, OPTICAL ARTICLE AND EXPOSURE APPARATUS FOR PHOTO-LITHOGRAPHY USING THE SAME

摘要

It is an object of the present invention to provide a calcium fluoride crystal in which the light transparency does not deteriorate with consecutive irradiation by high output short wavelength light over long time periods. A calcium fluoride crystal in accordance with the present invention has an internal transmittance of 70% or more for light of 135-nm wavelength or over. A calcium fluoride crystal contains any one of strontium, aluminum, silicon and magnesium, with the strontium content ranging from 1 ppm to 600 ppm, the aluminum content ranging from 1 ppm to 50 ppm, the silicon content ranging from 1 ppm to 50 ppm, or the magnesium content ranging from 1 ppm to 10 ppm. A calcium fluoride crystal has an internal transmittance of 70% or more for light of 135-nm wavelength or over and contains 1 ppm or less of La and 10 ppm or less of Y. An optical system for an excimer laser in accordance with the present invention comprises a lens comprising any calcium fluoride crystal set forth above. An exposure apparatus for photo-lithography in accordance with the present invention comprises an optical system comprising the calcium fluoride crystal and a stage for holding a substrate to be exposed.