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

为了开发新的氟化物单晶材料作为下一代光学光刻的窗口材料，特别是用于157 nm的F_2激光器，对不同氟化物进行了晶体生长和真空紫外光光谱表征。其中LiCaAlF_6、KMgF_3、BaLiF_3和Sr：BaF_2是本文研究的重点。LiCaAlF6的优点是在112 nm以下具有很高的透明性。细到5个小数点的折射率测量也表明，在152 nm处双折射减小，非常接近F2激光的波长。其他三种晶体的优点是它们的立方体结构，可以实现零双折射，并且透明度也很高。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的直径。特别是，用提拉法成功地生长了直径为4英寸的LiCaAlF_6单晶，显示了扩径的真实可能性。为了进一步研究扩大直径，使用了Bridgman技术。生长出直径为10 cm的KMgF_3晶体，但晶体中含有白色乳状物。通过在坩埚内插入几块平板，生长出直径为6英寸的透明LiCaAlF6晶体。插板能有效地分离透明晶体和含有杂质相的白色物质。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具有很高的抗力。这两种晶体在F_2激光照射后也几乎没有损伤形成，激光功率是实际光学光刻的10倍。

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.