Extreme ultraviolet induced oxidation of Ru(0001)

13.5 nm EUV photons generate a significant amount of secondary electrons at the surface of the multilayer mirrors (MLM) in the lithography tool, inducing photochemical reactions between residual gas molecules and the mirror surfaces [1]. An understanding of the catalytic role of the mirror surface and the driving force behind these reactions is required for controlling mirror degradation. Ruthenium is used as a capping layer on the MLM, therefore, we investigate the EUV photochemistry on a Ru(0001) crystal surface. We use temperature programmed desorption (TPD), and reflection-absorption infrared spectroscopy (RAIRS) in a UHV chamber to characterize the binding structure and changes to the chemical composition of molecular adlayers on the surface. A thin adlayer with a few monolayers thickness was first adsorbed onto the clean surface of the crystal, held at a temperature less than 100 K. The surface was then irradiated with EUV photons. To detect any change of the Ru surface and the adlayer, the pre- and post-radiation TPD and RAIRS spectra are compared. In the case of water adlayers, the intact water and partially dissociated water have different TPD and RAIRS spectra, which can be considered as signatures for water dissociation [2]. We have observed that dissociation of water on the Ru crystal is enhanced by EUV radiation. This also leads to the presence of oxygen adatoms on the surface, which is a sign of fully dissociated water. The atomic oxygen was found to oxidize the surface of Ru crystal. This work is part of the research programme CP3E of the Foundation FOM with support from NWO. CP3E is co-financed by Carl Zeiss SMT and ASML, and AgentschapNL through the EXEPT programme.