Surface characterization of optically polished CaF 2 crystal by quasi-Brewster angle technique

Abstract

With the development of 157nm laser lithography, calcium fluoride (CaF 2 ) has become a very important material for excimer laser components. The optical performance of CaF 2 at 157nm is strongly related to its surface quality. By analyzing the phase change of reflected p-polarization near the Brewster angle at wavelengths between 150nm and 1000nm, a quasi-Brewster angle technique (qBAT) was developed to characterize the surface quality of CaF 2 . This technique suggests that the slope of the quasi-Brewster angle is dominated by top surface roughness, while the displacement of the quasi-Brewster angle is determined by subsurface damage. A simple model developed to describe the surface quality of CaF 2 includes both top surface and subsurface effects. The relationship between subsurface damage and quasi-Brewster angle shift was determined. Using an ultraviolet ozone (UVO) cleaning process, four surface characteristics, e.g., top surface roughness, top surface contamination, subsurface damage, and subsurface contamination could be distinguished.