Damage characteristics of 193 nm laser irradiated on MgF₂ window

Abstract

MgF₂ crystal windows for the weakness part of the optical system have obtained much attention due to the rapid development of photolithography and low-temperature silicon annealing. Investigating the mechanism of laser-induced damage on MgF₂ windows is much more important to push forward its optical applications, which has a few advantages, including lower light transmission, refractive index and higher hardness than CaF₂ crystals. In this work, the damage morphology induced by the 193 nm excimer laser under a threshold energy of 2.8 J/cm² shows obvious cracks and craters, which were observed by scanning electron microscopy (SEM). As the number and energy of laser pulses increase, damage to the rear surface increases exponentially. Interestingly, the rear surface of the window material was much more severely damaged than the front surface. Here, the electric field distribution of window material under 193 nm excimer laser irradiation was proposed to illustrate the damage physical mechanism, which is calculated by the 3D finite-difference time-domain (FDTD) method. In conclusion, the electric field intensity of the rear surface is stronger than that of the front surface due to defects in the window materials. Therefore, the improvement of optical crystal quality and optical geometry for the high power laser system could be considered to solve the damage problems for the application of MgF₂ optical windows.

Keywords

193 nm excimer laser / laser induced damage / MgF₂ window / finite-difference time-domain (FDTD)

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