Defects in calcium fluoride generated by 157 nm laser and low-energy electrons

Single crystal calcium fluoride (CaF2) is currently being studied for use as an optical material for vacuum ultra-violet applications. It is also being investigated as a material for lithographic masks. Metal halides have a strong tendency to form point defects under energetic particle and photon irradiation. Defect generation is the basis for mask exposure, but is highly detrimental to optics. Here we examine the consequences of exposing single crystal CaF2 to 2 keV electrons and pulsed 157-nm laser light.;Low-energy electrons and 157-nm laser light produce metallic colloids in CaF2, which causes absorption in the visible region. Electron induced colloid growth has been studied by others. We study effect of 157-, 193-, 248-, 532-, and 1064-nm laser radiation and heat on colloids in electron-irradiated CaF2. Transmission measurements were used to probe the bleaching process as a function of treatment.;Chemical reactions with hydrochloric acid and/or water are strongly enhanced by many defects and can be used to probe the distribution of defects produced by electron- and laser-irradiation. Imaging the treated surface with atomic force microscopy allows us to compare relative defect densities versus depth into the sample. Samples treated with 2-keV electrons were damaged at depths as deep as 600 nm. Electron irradiation also appeared to produce line defects that intersect the surface. 157-nm laser irradiation produced significant crystal damage only to a depth of about 100 nm; line defects were not observed.;The stability of CaF2 at the excimer laser wavelengths of 157 and 193 nm is an important issue primarily due to defects formed by electron-hole recombination. In addition to producing lattice defects, electron-hole recombination can proceed radiatively to yield a luminescence centered at 280 nm. Pre-existing defects produced by electron irradiation, polishing, mechanical indentation, or heating change the luminescence intensities.;Single crystal CaF2 optical components have become part of newer optical systems for lithography at 193 nm. Soon the laser community would like to see 157 nm lithography become a reality, where CaF2 optical components are among the few alternatives. The study of the defect generation at 157 nm should allow for a better understanding of the inherent issues with using CaF2 as an optical material at this wavelength.