High index fluoride materials for 193nm immersion lithography

摘要

We tried to investigate various kinds of metal fluoride materials which have higher gravity than CaF2 and cubic crystal system, and we found out barium lithium fluoride (BaLiF3) and potassium yttrium fluoride (KY3F10) as candidates for the last lens material. We have developed unique Czochralski (CZ) machines and techniques for the growth of large calcium fluoride single crystals. And we applied these technologies to the growth of fluoride high index materials. We have succeeded to grow the large BaLiF3 single crystal with 120mm in diameter and a KY3F10 single crystal, and measured their basic properties such as refractive index, VUV transmittance, birefringence, and so on. As a result of our basic research, we found out that BaLiF3 single crystal is transparent at VUV region, and the refractive index at 193nm is 1.64, and KY3F10 single crystal has the index of 1.59 at the wavelength of 193nm which is slightly higher than fused silica. We expect that these fluoride high index materials are useful for the last lens material of the next generation immersion lithography.