Bridgman growth and characterization of calcium fluoride crystals

Calcium fluoride (CaF2) single crystal is an important lens material for photolithographic processing of silicon integrated circuits (IC) at wavelengths in the deep ultraviolet (DUV) region. In this paper, CaF2 single crystals have been grown by the Bridgman method using a graphite crucible. Commercial CaF2 raw materials were treated in an electric resistance furnace to remove moisture as well as residual oxides. The scattering particles and absorption near 300 nm were attributed to the existence of residual moisture in the raw materials. The stress distributions were observed and it tended to accumulate at grain boundaries. By optimizing growth conditions, 〈1 1 1〉-oriented CaF2 crystals up to 185 mm in diameter were grown and the transmittance was more than 93% in the range of 380–3500 nm.