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.