Large Size single Calcium Fluoride(CaF2) crystal are being used as lens material in optical lithography exposure tools for manufacturing semiconductor integrated chips.Their excellent transparency,optical homogeneity, and laser quantities of them with diameters greater than 200mm are needed. Large crystals are used because they increase thenumerical aperture, which in turn enhances the imageresolution of printed features. Crystals with an extremelylow level of stress birefringence and high refractive indexhomogeneity are essential since any residual stress orinhomogeneity will directly affect the image projected on thewafer. To reduce exposure time and increase the life of the lens, the lens material must be highly laser-durable andresistant to radiation damage with prolonged use.
CaF2 single crystals are mainly grown using the Bridgman method, although the Czochralski method is also reported.However, growing large, high purity single crystals along aselected orientation is very difficult, mainly due to the formation of grain boundaries during the growth of the cone part and cracks during the cooling process. Moreover, the crystal is exposed to a strong temperature gradient during growth,which induces thermal stress in the grown crystal. This induced stress causes high birefringence and low homogeneity. Many researchers now use the annealing process to eliminate the stress induced in the crystal in order toreduce birefringence and improve homogeneity.