Nanosecond laser-induced surface damage and its mechanism of CaF2 optical window at 248 nm KrF excimer laser

Abstract

Calcium fluoride (CaF2) crystals is a kind of important optical material for ultraviolet (UV) and deep-ultraviolet (DUV) lithography and high-power laser-related applications. However, its laser-induced damage threshold (LIDT) directly affects the laser power, so that the above-mentioned applications could be limited. Therefore, the research on the damage characteristics and laser damage resistance of CaF2 crystals is urgent. A 3D Finite-Difference Time-Domain (FDTD) method with Maxwell spinor equation is used, and the results show that the electric field intensity of rear surface is larger than that of front surface, which causes a lower threshold and is consistent with the experimental observations. And a thermo-mechanical coupled finite element model (FEM) of CaF2 with Ce2O3 impurities, which are introduced by polishing process, has semiquantitatively described the damage mechanism of CaF2 by 248 nm-excimer laser.