Multi-shot damage on Mo/Si multilayer induced by nanosecond EUV radiation

As a vital optical element working in the extreme ultraviolet (EUV) region, the damage mechanism on a Mo/Si multilayer has caught great attention. In this paper, using a nanosecond table-top EUV source, an S-on-1 EUV damage experiment was performed on a Mo/Si multilayer mirror with shot numbers S = 1, 2, 5, and 10. It was demonstrated that the multi-shot damage thresholds follow an inverse-power-law as a function of the pulse number. The incubation effect is explained based on the measured damage morphologies by an atomic force microscope. When the Mo/Si multilayer mirror was exposed to the EUV fluence above the single damage threshold, compaction of the layer structure and melting and resolidification processes are dominating the damage mechanism. It was also found that the crater depth and FWHM at high fluence are independent of the shot numbers, which is explained by the formation of the MoSi2 spacing layer.