Characterisation of EUV damage thresholds and imaging performance of Mo/Si multilayer mirrors

We studied 1-on-1 and 10-on-1 damage threshold investigations on Mo/Si multilayers with EUV radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. The experiments were performed on different types of Mo/Si mirrors, showing no significant difference in single pulse damage thresholds. However, the damage threshold for ten pulses is ≈ 60 % lower than the single pulse threshold, implying a defect dominated damage process. Using Nomarski (DIC) and atomic force microscopy (AFM) we analysed the damage morphologies, indicating a primarily thermally induced damage mechanism. Furthermore, we studied the radiationinduced change of reflectivity upon damage of a multilayer mirror. Additionally, we characterised transmission and reflection properties of novel Mo/Si multilayer beam splitters performing wavefront measurements with a Hartmann sensor at 13.5 nm wavelength. Such wavefront measurements allow also actinic investigations of thermal lens effects on EUV optics.