DIET processes on ruthenium surfaces related to extreme ultraviolet lithography (EUVL)

The aim of this work is to provide insights into desorption induced by electronic transitions (DIET) processes that affect the reflectivity of ruthenium-capped Mo/Si multilayer mirrors working under EUVL (extreme ultraviolet lithography) operating conditions [high vacuum, and 13.5 nm (92 eV) photons]. Critical issues are associated with possible oxidation of the 2 nm thick Ru capping layer due to the inevitable background pressure of H2O, and carbon build up due to background hydrocarbons. In the present work, we discuss aspects of the radiation-induced surface chemistry of Ru irradiated by 100 eV electrons and 92 eV photons. The cross section for electron-stimulated desorption of oxygen from O-covered Ru is ∼6 × 10−19 cm2. Carbon accumulation several nm thick occurs on the Ru surface during electron irradiation in methyl methacrylate (MMA) vapor, a model background impurity hydrocarbon. Radiation damage by low-energy secondary electrons is believed to dominate over direct photoexcitation of adsorbates under EUVL conditions. The secondary electron yield from Ru varies strongly with photon energy, and is ∼0.02 electrons/photon at 92 eV.