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Ru is a promising element for coating Mo/Si multilayer mirrors for extreme ultraviolet (EUV) lithography because it is highly resistant to oxidation. However, during long exposure to EUV light, a Ru thin film is eventually oxidized in the presence of oxygen and/or water molecules, resulting in lower mirror reflectivity. In the lithography system, oxygen radicals can form by the EUV-induced photolysis of oxygen and/or water molecules, thereby accelerating the oxidation.
Plasma process may help us to understand oxidation process of Ru layers by O atoms and also reduction of the once formed oxide layers with hydrogen plasma. In this paper the oxidation and reduction of Ru thin films grown on a Si(100) surface are studied by X-ray photoemission spectroscopy (XPS). Ru thin films are oxidized with O2 plasma generated by an RF discharge, and their XPS spectra were measured. The spectra are decomposed into several components for Ru suboxides attributable to different stages of oxidation. After sufficient exposure to oxygen, a stoichiometric rutile RuO2 layer was found to have formed near the surface. Thermal annealing at 500 K results in a thicker RuO2 layer. The Ru oxide layer can be removed by exposure to hydrogen plasma. We measure desorbed species under D beam irradiation with a QMS installed in a differentially evacuated chamber and observe desorption of D2O molecules.
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