Index of refraction and its temperature dependence of calcium fluoride near 157 nm

We have made accurate measurements near 157 nm of the relative index of refraction, its dispersion. and its temperature dependence for two grades of calcium fluoride in N-2 gas. Accurate measurements of these quantities are needed for the design of lens systems for 157 nm F-2 excimer-laser-based exposure tools for photolithography, These optical properties were measured with precision goniometers on prisms of the materials in a N-2, atmosphere using the minimum deviation method. The dispersion was determined using line emission radiation from a deuterium lamp at several wavelengths near 157 nm. Values of the relative refractive index for two grades of calcium fluoride in N-2 gas corrected to a temperature of 20 degrees C and a pressure of one standard atmosphere are well within our 7 ppm estimated uncertainty for the measurements. The temperature of the samples and the surrounding medium were controlled to 0.05 degrees C, which enabled accurate measurements of the temperature dependencies of the indices around room temperature near 157 nm.