Generation of optical absorption bands in CaF2 single crystals by ArF excimer laser irradiation:: Effect of yttrium impurity

Generation of optical absorption bands was observed in CaF2 single crystals with different Y impurity concentrations, excimer grade (yttrium impurity; <1 ppm) and UV grade (5 ppm), by irradiation with ArF excimer laser light at room temperature. The transmittance at the wavelength of 193 nm gradually decreased from 100% down to a saturation level of similar to 95% fur the UV grade specimen during the ArF excimer laser irradiation. This degradation process was found to obey first order kinetics. On the other hand, no significant decrease in transmittance was observed for the excimer grade specimen. "Four-band structure" which was a characteristic of Y-associated F centers (YFCs) and a band peaking at similar to 6.6 eV were observed in the spectrum of the irradiated UV grade CaF2. The 6.6 eV band, which substantially controlled transmission loss at the wavelength of 193 nm and was generated via one photon process, was tentatively attributed to YFCs from growth kinetics on the laser irradiation and decay kinetics upon annealing. Neither intrinsic F centers nor H centers were observed by the ArF laser light irradiation at room temperature. These results demonstrated that a trace amount of Y impurity enhanced the formation of F centers perturbed by a nearest Y impurity and that an absorption band of the resulting YFCs controlled the transmission loss at the wavelength of 193 nm. (C) 1998 American Vacuum Society. [S0734-2101(98)06205-8] .