Oxidation of silicon on substrate induced bubble-like damage of Mo/Si mirror irradiated by femtosecond EUV pulses

Mo/Si multilayers are commonly used as extreme ultraviolet (EUV) reflection coatings owing to their high reflectivity at EUV wavelengths. This study utilized Shanghai Soft X-ray Free-Electron Laser (SXFEL) to generate 13.5 nm, ∼300 fs pulse lasers, and the laser damage behaviors of Mo/Si multilayers were investigated. The designed Mo/Si multilayer achieved high reflectivity at an incident angle of 20°. After exposing the surface to an average of 20 laser pulses per point at normal incidence, bubble-like damage was observed. Monte Carlo method was employed to obtain the energy absorption distribution of Mo/Si multilayers under EUV laser radiation. It was confirmed that the cause of EUV laser damage to Mo/Si multilayers was the enhancement of energy absorption, leading to the melting of the layers. High-energy absorption occurred at a place close to the substrate induced by the oxidation of Si and on the top of the multilayers, which resulted in multilayers detaching from the substrate and forming bubble-like morphology. This intensified interlayer diffusion and altered crystal orientation, resulting in irreversible damage to the multilayers.