Nanosecond laser-induced surface damage and its mechanism of CaF2 optical window at 248 nm KrF excimer laser
Xin Li, Xianan Dou, Hong Zhu, Yue Hu, Xi Wang
Abstract
Abstract Calcium fluoride (CaF 2 ) 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 CaF 2 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 CaF 2 with Ce 2 O 3 impurities, which are introduced by polishing process, has semiquantitatively described the damage mechanism of CaF 2 by 248 nm-excimer laser.