Extreme ultraviolet lithography (EUVL) is a leading candidate for next-generation lithography for the semiconductor industry. This technology uses EUV light with a wavelength of 13.5 nm (92.5 eV) to be able to produce features as small as 20 nm in size. The short wavelength of EUV means that reflective optics is needed for lithography in the form of Si-Mo multilayer stacks. However, surface contamination by water and hydrocarbons together with EUV light reduces unacceptably the mirror reflectivity with time. In this article, the authors review the material properties of two promising capping layer materials, Ru and RuO2, for protecting the EUVL mirrors against oxidation, carbon uptake, and the permeation of hydrogen and oxygen. Special emphasis is put on the surface properties of these potential cap layer systems. For both materials the microstructure, the morphology, and the stability under oxidizing and reducing environments are reviewed to promote the search for a successful candidate for a capping layer material of EUV optics. (c) 2007 American Vacuum Society.