The authors are with the National Institute of Standards and Technology, Building 221, Room B228, 100 Bureau Drive, Gaithersburg, Maryland 20899-8421. USA
John H. Burnett, Rajeev Gupta, and Ulf Griesmann, "Absolute refractive indices and thermal coefficients of CaF2, SrF2, BaF2, and LiF near 157 nm," Appl. Opt. 41, 2508-2513 (2002)
We present high-accuracy measurements for wavelengths near 157 nm of the absolute index of refraction, the index dispersion, and the temperature dependence of the index for the ultraviolet optical materials with cubic symmetry: 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 tools are expected to use CaF2 as the primary optical material and possibly one of the others to correct for chromatic aberrations. These optical properties were measured by the minimum deviation method. Absolute refractive indices were obtained with an absolute accuracy of 5 × 10-6 to 6 × 10-6.
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Absolute index of refraction (column 1), dispersion (column 2), and temperature dependence of the index (column 3) at 157.63094 nm. The temperature is 20.00(5) °C. For CaF2 and BaF2 three samples were measured, and data are given for one sample each, along with the averages and spreads. The coefficients of quadratic polynomials that were fit to the refractive-index data between 156.8 and 158.6 nm are listed in the rightmost column for one sample of each material. The formula is valid in this range with the standard uncertainties given in column 1.
Table 2
Uncertainty Budget for Refractive-Index Measurement
Absolute index of refraction (column 1), dispersion (column 2), and temperature dependence of the index (column 3) at 157.63094 nm. The temperature is 20.00(5) °C. For CaF2 and BaF2 three samples were measured, and data are given for one sample each, along with the averages and spreads. The coefficients of quadratic polynomials that were fit to the refractive-index data between 156.8 and 158.6 nm are listed in the rightmost column for one sample of each material. The formula is valid in this range with the standard uncertainties given in column 1.
Table 2
Uncertainty Budget for Refractive-Index Measurement