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21 March 2008 Surface phenomena related to degradation of EUV mirrors: interaction of ethyl alcohol with ruthenium surfaces
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We characterize the interaction of ethyl alcohol (C2H5OH) vapor with a crystalline Ru(1010) surface, a model surface for Ru capping layers used for extreme ultraviolet lithography (EUVL) optics. Our main objective is to provide fundamental insights into the basic physical processes that affect the reflectivity of Ru-coated Mo/Si multilayer EUV mirrors that are exposed to EUV radiation. While ethanol has been used previously as a gaseous hydrocarbon in EUV mirror lifetime tests, there appear to be no published reports describing the basic surface chemistry of ethanol on Ru. We use ultrahigh vacuum surface science methods, and find that fractional monolayers (ML) of ethanol dissociate almost completely on clean Ru(1010); H2 and CO desorb upon heating to ~500 K, and leave a fractional ML of carbon on the surface. At coverages greater than a few tenths of a ML, ethanol desorbs in molecular form at temperatures below 300 K. Weakly chemisorbed ethanol in the first ML desorbs at ~210 K with a binding energy of 0.55 eV. Multilayer ethanol desorbs at ~155 K. Repeated dosing of ethanol followed by heating to ~600 K causes accumulation of a carbon monolayer which becomes unreactive to ethanol. Also, we find that an oxygen monolayer is unreactive, with little or no dissociation of ethanol observed. Electron bombardment of Ru at 300 K in 5×10-8 Torr ethanol can lead to accumulation of a carbon film >1nm thick.
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B. V. Yakshinskiy, Iwao Nishiyama, Andrea Wüest, and Theodore E. Madey "Surface phenomena related to degradation of EUV mirrors: interaction of ethyl alcohol with ruthenium surfaces", Proc. SPIE 6921, Emerging Lithographic Technologies XII, 69213E (21 March 2008); doi: 10.1117/12.774012;

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