Spatial anisotropy of the exciton level in CaF2 at 11.1 eV and its relation to the weak optical anisotropy at 157 nm -: art. no. 233101

CaF2 is the basic optical material for the next step in photolithography to produce nanostructures at a wavelength of 157 nm by the semiconductor industry. Recently an optical anisotropy on CaF2 has been observed at 157 nm which implies serious consequences for the design of the precision optics. In the present work we demonstrate that this optical anisotropy originates from a spatial anisotropy of the exciton level at 11.1 eV as a fundamental effect of the CaF2 crystal with cubic symmetry. For this purpose we have investigated the excitonic state by precision reflection measurements using dispersed synchrotron radiation on oriented surfaces of CaF2 with extremely low impurity concentrations. The results are discussed in terms of the complex dynamic dielectric function.