Development of new fluoride single crystals as VUV window materials for next-generation optical lighography

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In order to develop new fluoride single crystal materials as window materials for next-generation optical lithography, especially for F_2 laser with 157 nm, crystal growth and VUV optical characterization have been done for different fluorides. Among them, LiCaAlF_6, KMgF_3, BaLiF_3, and Sr:BaF_2 were focused in this work. The advantage of LiCaAlF_6 was its high transparency until 112 nm. Detailed refractive index measurement down to 5 decimal points also showed that the birefringence diminish at 152 nm which is very close to the wavelength of F_2 laser. The advantages of other three crystals were their cubic structure, which enables 0 birefringence, and high transparency as well. Successful growth of LiCaAlF_6, KMgF_3, and BaLiF_3 single crystals with 2 inches in diameter showed that these crystals have possibility to be grown with larger diameter. Especially, 4-inch diameter LiCaAlF_6 single crystal was successfully grown by the Czochralski technique to show the real possibility of diameter extension. For further investigation to extend the diameter, the Bridgman technique was used. KMgF_3 crystal with 10 cm in diameter was grown, although it contained white milky material. Transparent LiCaAlF_6 crystals with 6-inch diameter were grown by the insertion of several plates inside a crucible. The insertion of plates was effective to separate transparent crystal and white material which contained impurity phases. Damage resistance of grown crystals were investigated. Total damage obtained after high power X-ray radiation showed that LiCaAlF_6 and Sr:BaF_2 have very high resistance. These two crystals also showed almost no damage formation after F_2 laser irradiation with 10 times high power than that is used in a practical optical lithography.