ArF laser-induced damage of calcium fluoride windows with protected anti-reflection coatings

High-power ArF lasers for high-volume production of mobile consumer microelectronics demand low loss and laser durable calcium fluoride (CaF2) laser-windows. Increased laser durability is realized by F-SiO2 protective coatings on CaF2 windows with subsurface-damage-free (SSD-free) surface finishing. Surface reflection loss is reduced by fluoride-based anti-reflection (AR) coatings. In this contribution, we integrated the protective coating with the fluoride-based AR coatings on SSD-free CaF2 windows and assessed the laser damage resistance of the window using an ArF laser with a pulse length of 20 ns and a rep rate of 20 Hz. Damage initiation sites were detected on the entrance surface at a fluence of 3.5 J / cm2. A 100% damage probability was determined at a fluence of 5 J / cm2. An absorption-driven thermal damage mechanism was indicated by the damage morphology. Similar laser damage resistance was realized on the protected AR-coated windows with more than 9% transmission gain when compared to that of the F-SiO2 protected CaF2 laser-windows. The results suggest that two-photon absorption is a dominant factor when nonlinear absorption occurs in the laser-irradiated CaF2 windows. The thermally induced damage could be further simulated using the experimentally determined two-photon absorption coefficient based on the ArF laser calorimetry technique.