Evaluation of coated and uncoated CaF2 optics by variable angle spectroscopic ellipsometry

CaF2 is one of the dominating optical materials used for ArF excimer laser optics. Surface quality of optically finished CaF2 plays an important role in the components' lifetime. A variable angle spectroscopic ellipsometry was employed to evaluate surface quality of optically finished CaF2 in terms of top surface and subsurface damage. The subsurface damage was revealed by removing the top surface. Combining plasma ion assisted deposition and ellipsometric measurement, a dense smooth F-SiO2 film was developed to prevent fluorine loss of CaF2 optics under ArF excimer laser irradiation, leading to an extended lifetime. In addition, an integrated protective coating approach was established on fluoride multilayer coated CaF2 optics, resulting in environmentally stable optical performance.