Oriented growth of large size calcium fluoride single crystals for optical lithography

We report (1 1 1), (1 1 0) and (1 0 0) growth of CaF2 by the vertical Bridgman method. Crystals up to 250 mm diameter were grown and various growth parameters such as growth rate, temperature gradient and post-growth cooling rate were studied. It was found that the growth rate and the cooling rate are slower for the larger diameter crystals with a fixed temperature gradient. These growth parameters were optimized for growing the crystals along specific orientation after realizing that CaF2 has a tendency to grow along an orientation close to 〈1 1 0〉. Degradation in optical transmittance was evaluated by irradiating the crystal to γ-rays up to a dose of 105 rad. Optimized scavenger addition resulted in crystals with better radiation resistance and excellent VUV transmittance.