Influence of absorption distribution in Mo/Si multilayers on 13.5 nm femtosecond laser damage morphologies

Abstract

13.5nm femtosecond laser induced damage behaviors of Mo/Si multilayer were investigated. In this repo rt, we designed and prepared the Mo/Si mirrors for high reflectivity at 20 degrees. The laser damage test for EUV femtosecond pulses was executed at domestic soft x-ray free-electron laser facility in Shanghai. The EUV central wavelength is 13.5 nm and pulse duration is 300 fs. To understand the influence of absorption distribution in the Mo/Si multilayers, the sample was irradiated at normal incident angle for no reflection status. The damage morphologies were characterized by SEM and TEM. It is found that the most severe absorption area appears bubble-like damage, in which the first ten pairs of Mo/Si bilayer merged thoroughly and the bubble under the multilayers is located at the interface close to the substrate. The absorption distribution in the multilayers was simulated by Monto Carlo method, and that was compared with the electric field distribution simulation. We can conclude from the simulation that the damage locations are consistent with the high absorption layers in the coating stacks.