EUV soft X-ray characterization of a FEL multilayer optics damaged by multiple shot laser beam

We have investigated the damaging effects of a femtosecond pulsed laser beam with 400 nm wavelength on a Mo/Si EUV multilayer. The exposures have been done in vacuum with multiple pulses (5 pulses/mm2) of 120 fs varying the laser fluence in the 38-195 mJ/cm2 range. The analysis of the different irradiated regions has been performed ex-situ by means of different techniques, including specular and diffuse reflectivity, X-ray photoemission spectroscopy (XPS) and total electron yield (TEY) in the EUV and soft X-ray range. Surface images have been acquired by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Results clearly indicate a progressive degradation of the EUV multilayer performances with the increase of the laser fluence. Spectroscopic analysis allowed to correlate the decrease of reflectivity with the degradation of the multilayer stacking, ascribed to Mo-Si intermixing at the Mo/Si interfaces of the first layers, close to the surface of the mirror.