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.

摘要

随着光刻技术和低温硅退火等大能量、高功率紫外激光系统的发展, 激光装置中窗口材料面临的紫外激光诱导损伤的问题越来越得到重视。MgF₂窗口是紫外波段难以替代的光学窗口材料, 具有比CaF₂ 更优的紫外透光率、更低的折射率和更高的硬度等优点, 因而在高功率紫外激光器、深紫外探测器、光电探测和对抗领域具有重要的应用。研究激光对MgF₂窗口材料的损伤机制对推动其在光学领域的应用至关重要。本文通过扫描电镜分析MgF₂窗口材料在193 nm 紫外激光(2.8 J/cm2)辐照下出现的裂纹和凹坑等损伤形貌, 随着激光脉冲数和能量的增加, MgF₂窗口材料的后表面损伤程度呈指数级增加, 窗口材料的后表面损伤程度比前表面更加严重。利用三维时域有限差分(3D-FDTD)方法对193 nm激光辐照MgF2 窗口材料内部的激光电场分布进行了计算。模拟发现无论缺陷在前表面还是后表面, 电场强度的峰值都出现在后表面附近, 后表面更容易被损伤, 这与实验结果吻合; 缺陷在激光辐照过程中会导致光场的局域调制, 增强吸收, 同时降低材料表面的机械强度。