Improvements in CaF2 material properties for next-generation microlithography applications

The key requirements of CaF2 lens blanks are transmission, birefringence and optical homogeneity. While it is obvious that high transmission and resistance to fluence- and time-dependent darkening is critical, the subtleties of single crystals, compared to isotropic glasses, play an important role in the implementation of CaF2 in microlithography steppers. The existence of intrinsic birefringence has caused stepper manufacturers to employ various crystallographic orientations in their lens designs. This fact means that crystal growth and annealing processes must be optimized accordingly. Small-scale defects, known as dislocations and sub-grain mis-orientation, manifest themselves as larger-scale defects known as slip and mosaic. These imperfections may impact birefringence and a critical homogeneity parameter known as residual rms. Residual rms; is directly related to the concentration of asymmetric defects in CaF2 crystals, such as those aforementioned. In the present paper, the author will report on progress made at Corning to meet the stringent requirements of both 193nm and 157nm systems.