Micrometer resolved inspection of defects and laser damage sites in UV high-reflecting coatings by photothermal displacement microscopy

Characteristic features of defects in dielectric multilayer UV-mirrors are determined by photothermal displacement imaging. A large number of micron sized absorbing defects are detected, however, their size and absorption strength distribution strongly depends on thin film preparation conditions. It is found that all features well correlate with thin film damage thresholds at 248 nm. An enhanced photothermal amplitude in a halo around ablation spots damaged by high fluence UV light indicates thin film delamination resulting from thermoelastic stress during laser irradiation.