Utilization of wave front sensing techniques for EUV damage studies

Current optics damage and materials processing by EUV radiation are mainly relying on the free-electron-laser (FEL), synchrotron and laser produced plasma (LPP) light sources. The damage profiles are evaluated by the SEM, TEM, AFM and XPS etc. approaches. Subsequently, the damage threshold are given according to the pulse energy and the sample damage profiles, which remains some uncertainty. We have developed a high numerical aperture (NA~0.15) Hartmann wave front sensor for the EUV spectral region. This sensor has been successfully used for the EUV microscopy and sample damage study at FLASH-II. It firstly characterizes the beam spatial quality. The real-time measurement of the FEL wave front directly conducts the Schwarzschild optics alignment. With the benefit of capturing intensity and phase simultaneously, the EUV focus and beam profiles at the sample location can be precisely retrieved via retro-propagation. The wave-front sensing technique improves the EUV focusing quality, ensures sufficient power density, accurately provides full dose map of the sample damage area. The FEL-based EUV damage study is lack of the important dynamic information from femtosecond to picosecond time scale due to the synchronization limitations. Although there are a few EUV pump-IR/green-light-probe experiments have been performed, the long wavelength probe beam is inadequate to access the ultrafast process and the coating depth. By utilizing the wave-front sensing technique, the table-top EUV sources, such as gas-/surface-plasma-based high harmonics and plasma-based soft-x-ray lasers, could reach brilliant focus allowing to damage the samples. With the advantage of laser driven EUV sources, the EUV pump-EUV probe scheme can be realized with femtosecond-level delay. The wave-front sensing techniques are applicable for both pump and probe beams, have the potential to open a new era for EUV damage studies.