Effect of substrate roughness on Mo/Si multilayer optics for EUVL produced by UHV-e-beam evaporation and ion polishing

Our current status of Mo/Si multilayer extreme ultraviolet coatings fabricated by UHV e-beam evaporation and ion polishing is presented. Standard Mo/Si multilayer coating deposited onto 4 inch Si-wafer substrates showing peak reflectivities of 67.4% with a lateral homogeneity in coating reflectance of approximately 0.3% and a variation in peak wavelength of approximately 0.05 nm over the full aperture have been achieved. At.-wavelength reflectivities achieved for comparable multilayer coatings deposited onto Zerodur substrates with an enhanced microroughness (σ=0.55 nm) is shown to drop severely by almost 10%. A partial smoothing could be achieved by applying thin a-C buffer layers to these substrates, which in turn enhance the at-wavelength reflectivity of the Mo/Si multilayer by 0.7%