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

为了开发新一代光学光刻窗口材料，特别是157 nm F_2激光器窗口材料，对不同氟化物进行了晶体生长和真空紫外光学特性研究。其中LiCaAlF_6、KMgF_3、BaLiF_3和Sr：BaF_2是本文的重点研究对象。LiCaAlF_6的优点是在112 nm以下具有高的透明性。详细的折射率测量（精确到小数点后5位）也表明，在152nm处双折射减小，这与F_2激光的波长非常接近。其他三种晶体的优点是它们的立方结构，这使得0双折射，以及高透明度。成功地生长了直径为2英寸的LiCaAlF_6、KMgF_3和BaLiF_3单晶，表明这些晶体有可能生长出更大直径的晶体。特别是用提拉法成功地生长了直径为4英寸的LiCaAlF_6单晶，显示了直径扩展的真实的可能性。为了进一步研究以扩大直径，使用Bridgman技术。生长出直径约10 cm的KMgF_3晶体，晶体中含有白色乳状物质。在坩埚中插入几块平板生长出直径为6英寸的透明LiCaAlF_6晶体。板片的插入对分离透明晶体和含有杂质相的白色物料是有效的。研究了生长晶体的抗损伤性能。高功率X射线辐照后的总损伤表明LiCaAlF_6和Sr：BaF_2具有很高的电阻。这两种晶体在10倍于实际光刻功率的F_2激光辐照下几乎没有损伤形成。

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.