BaLiF3 single crystal has been studied as the candidate for the last lens material of the next generation high index
immersion lithography system. Although the refractive index of BaLiF3 is 1.64 at 193nm which is not sufficient for the
requirement, other optical properties such as 193nm transparency and laser durability fulfill the requirement.
It is estimated that the cause of both high SBR part and inhomogeneity of refractive index of BaLiF3 seems to present
along the faces of slip planes which are observed by crossed Nicol observation. As a result of comparative study of
various direction perpendiculars to the growth axis, good crystallinity with less slip planes has been obtained by shifting
the growth axis from <100> which is adequate for the last lens production.
MgF2 single crystal studied as the polarizer material for high power ArF laser oscillator, and crystal with excellent
laser durability and large diameter (>100mm) has been developed by CZ technique. In addition crystals oriented along
both c-axis and a-axis were successfully grown.
BaLiF3 single crystal has been studied as the lens material for the candidate of the next generation high index immersion lithography system. Although the refractive index of BaLiF3 is 1.64 at 193nm which is not sufficient for the requirement, other optical properties such as 193nm transparency and laser durability might fulfill the requirement, and intrinsic birefringence is relatively lower than other candidate materials.
It is estimated that the cause of scattering in the BaLiF3 crystal is aggregation of excess LiF component. The special annealing process to eliminate excess LiF component was applied to improve the transparency. The internal transparency was improved to more than 97%/cm by optimizing growth conditions and annealing conditions.
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.
In this work we present some laser experimental achievements obtained with the Ce:LiCaA1F6 (Ce:LiCAF) active medium. The Ce:LiCAF belongs to the family of fluorides Ce3+ activated, which emit in the near UV with tunable emission. In particular it emits in the band 280-315 nm. The laser action was obtained by pumping Ce:LiCAF with frequency quadrupled Nd:YLF and Nd:YAG lasers. We developed a multi-kHz tunable laser with high quality beam, tunable in the range 280-315 nm and with an average power exceeding hundred of mW, pumping with a pulsed Nd:YLF laser. Due to the high quality of Ce:LiCAF crystal we grew, we were able to obtain the highest ever reported slope efficiency. We achieved also the highest ever reported energy of 26 mJ for a Ce:LiCAF tunable oscillator pumping with a single low repetition rate Nd:YAG IVth harmonic laser. The wavelength coverage is further extended in the range 225-230 nm by means of nonlinear frequency mixing of the Ce:LiCAF output with the residual emission of the fundamental wavelength of the Nd:YAG laser.
For the application of F2 lasers to micro-/nano-fabrication including photolithography, one of the important research subjects is to eliminate the chromatic aberration, and combination of lenses made of two optical materials with different refractive indices is the effective way. However, only CaF2 is widely applied to lenses for a vacuum ultraviolet (VUV) beam, and a "second material" to be paired with CaF2 is strongly requested. Here, we examined two fluoride crystals which are transparent in the VUV region, BaF2 and LiCaAlF6 grown by the Czochralski technique, as candidates for VUV optics. We investigated the change of the VUV transmittance and the onset of optical damage of these fluoride crystals against F2 laser irradiation. These crystals showed good optical tolerance against cumulative F2 laser irradiation, and the damage threshold of LiCaAlF6 was similar to that of VUV grade CaF2 while that of BaF2 was about half of the other two fluoride crystals.