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

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.

Experiment on the temporal evolution characteristics of a CCD multilayer structure irradiated by a 1.06 µm continuous laser.

In this study, an experiment of a CCD detector irradiated by a 1.06 µm continuous laser is established. Factors including the microlens focusing beam, microlens carbonization at high temperature, shading aluminum film opening rate, low shading aluminum film absorption rate, and different materials' absorption to the laser are comprehensively considered. By combining the irradiation time, the output image of the CCD detector, surface morphology of the damage area, the optical micrograph, and regional energy spectrogram, the damage mechanism of the CCD is explored. In addition, the threshold time and the damage morphology of the multilayer structure of the CCD detector are investigated. The results show that when the irradiation time increases, the damage starts from the microlens due to melting, which is represented as point damage. Subsequently, the aluminum film melts and is separated from the SiO2 by stress and melting damage, causing vertical bright linear damage. Without the protection of the shading aluminum film, the silicon electrode heats up and reaches the melting point, causing damage to the wiring circuit, which is represented as horizontal dark linear damage. Eventually, the N-Si layer in the silicon substrate melts and the clock signal is destroyed, which means that the optical signal is not converted into an electrical signal. The CCD detector gets completely damaged.