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|Author(s):||Ulf Griesmann; J H. Burnett; R Gupta;|
|Title:||Absolute Refractive Indices and Thermal Coefficients of CaF2, SrF2, BaF2, and LiF Near 157 nm|
|Published:||May 01, 2002|
|Abstract:||We present accurate measurements near 157 nm of the absolute index of refraction, the index dispersion, and the temperature dependence of the index, for the cubic-symmetry, ultraviolet optical materials CaF2, SrF2, BaF2, and LiF. Accurate values of these quantities for these materials are needed for designs of the lens systems for F2 excimer-laser-based exposure tools for 157 nm photolithography. These are expected to use CaF2 as the primary optical material and possibly one of the others for correcting chromatic aberrations. These optical properties were measured using the minimum deviation method in a nitrogen atmosphere. Using the known index of refraction of nitrogen gas, the absolute refractive indices were obtained with an absolute accuracy of 5-6 x 10-6. Measurements on CaF2 and BaF2 samples from different suppliers showed sample variations outside the measurement uncertainty|
|Keywords:||BaF2,barium fluoride,CaF2,calcium fluoride,index of refraction,lithium fluoride,refractive index,SrF2,strontium fluoride|