Absolute index of refraction and its temperature dependence of calcium fluoride, barium fluoride, and strontium fluoride near 157 nm

Abstract

We have made accurate measurements near 157 nm of the absolute index of refraction, the index of refraction, the index dispersion, and the temperature dependence of the index, for the cubic-symmetry, group-II fluorides: calcium fluoride, strontium fluoride, and barium fluoride. Accurate measurements of these quantities for calcium fluoride are needed for designs of lens systems for 157 nm excimer-laser- based exposure tools for photolithography. Measurements of these isotropic materials high ly transmissive near 157 nm, such as strontium fluoride and barium fluoride, are needed for designs which include secondary materials in addition to calcium fluoride to correct for chromatic aberrations. These optical properties were measured with precision gonimeters on prisms of the materials in a nitrogen atmosphere using the minimum deviation method. The relative refractive indices were converted to absolute indices by using values of the index of nitrogen obtained by independent interferometric measurement near 157 nm. Values of the absolute refractive indices for these materials were obtained with an accuracy of 6 X 10<SUP>-6</SUP>. A key result is that the dispersion of barium fluoride near 157 nm is approximately 68 percent larger than that of calcium fluoride, which makes barium fluoride a potentially useful material for combination with calcium fluoride for chromatic aberration correction near 157 nm.