Bufferlayer and caplayer engineering of Mo/Si EUVL multilayer mirrors

Bufferlayer and caplayer engineering strategies are getting progressively important for improving crucial properties of EUVL multilayer optics and EUV reflection masks. While bufferlayers modifying the contact between the reflecting interference multilayer and the superpolished substrate aim for a partial smoothing of the residual substrate roughness or for a mitigation of local substrate defects as well as for multilayer film stress relaxation, surface caplayers are capable to enhance the multilayer reflectivity of EUV mirrors and masks. We present experimental results on various bufferlayer systems (singlelayer and multilayer) applied to different substrate materials (ULE, Zerodur, silicon). The bufferlayers have been deposited by e-beam. evaporation and ion-polishing techniques at UHV conditions and substrates with and without bufferlayer have been coated with standard Mo/Si multilayers (50 doublelayers, d-spacing 6.8 nm) in the same deposition run. The samples have been analyzed exsitu by means of AIM, TEM, X-ray scattering., and reflection and normal-incidence EUV reflectance measurements. We have found significant improvement (+ 0.7 %) of the Mo/Si multilayer EUV reflectivity for some bufferlayer systems applied to substrates with 0.2 - 0.3 nm r.m.s. high spatial frequency roughness (HSFR), while no effect was found on superpolished substrates exhibiting 0.1 nm r.m.s HSFR. The effect of caplayer modification applied to Mo/Si multilayers has been examined regarding EUV reflectivity. The native siliconoxide layer ontop of the Mo/Si multilayer coatings has been replaced by an ultrathin (2 nm.) chemically inert caplayer. We have found a I % - 1.5 % improvement in EUV peak reflectance for one cap material applied to several Mo/Si multilayer in comparison to a native siliconoxide cap.