Study of CaF 2 samples using DUV birefringence measurement and x-ray diffraction techniques

Abstract

In this paper we report measurement results of optical lithography grade calcium fluoride samples using deep ultraviolet (DUV) birefringence and X-ray diffraction methods. Linear birefringence maps of a variety of calcium fluoride samples were generated from measurements at both optical lithography wavelengths (157 nm, 193 nm and 248 nm) and at 632.8 nm. Comparing the respective wavelength results for birefringence in certain samples showed significant differences in birefringence patterns observed at 157 nm and 633 nm for a light beam propagating along the [111] crystal axis. Such differences cannot be explained from the dispersion of stress birefringence at those wavelengths. Our interpretation is that the discrepancy in the birefringence patterns observed at 157 nm and 633 nm is due to crystal defects in those calcium fluoride samples. The crystal quality of those calcium fluoride samples was subsequently determined by X-ray diffraction techniques. The results obtained from both birefringence and X-ray data substantiate each other qualitatively for judging the crystal quality of calcium fluoride samples.