12165
2007, 111, 12165-12168 Published on Web 07/31/2007
Toward the In Situ Remediation of Carbon Deposition on Ru-Capped Multilayer Mirrors Intended for EUV Lithography: Exploiting the Electron-Induced Chemistry David J. Davis,† Georgios Kyriakou,† Robert B. Grant,‡ Mintcho S. Tikhov,† and Richard M. Lambert*,† Chemistry Department, Cambridge UniVersity, Cambridge CB2 1EW, U.K., and Lithography Subsystems, BOC Edwards, Manor Royal, Crawley, West Sussex, U.K. ReceiVed: June 19, 2007; In Final Form: July 16, 2007
The realization of next-generation extreme ultraviolet (EUV) lithography depends on the application of Rucapped multilayer mirrors. Under EUV irradiation, carbon deposition due to the presence of hydrocarbons in the vacuum environment rapidly degrades mirror reflectivity, thus preventing implementation of this technology. We show that in the presence of low-energy electron irradiation, which corresponds to the photoelectron energy distribution encountered in practice, very low pressures (∼10-5 mbar) of NO or O2 are effective for oxidative removal of carbon from contaminated ruthenium surfaces at ambient temperature. This procedure leads to the net accumulation of oxygen on and beneath the metal surface. Subsequent exposure of the resulting Ru surface to CO under electron irradiation leads to efficient removal of this oxygen, again at ambient temperature. Carbon removal rates on the order of ∼4.5 × 10-5 nm s-1 are achievable, showing that sequential (or simultaneous) application of electron-induced oxidation and reduction reactions provides a viable strategy for remediation (or mitigation) of EUV mirror contamination under operating conditions.
Carbonaceous deposits produced on Ru-capped multilayer mirrors under extreme ultra violet (EUV, 91.8 eV, 13.5 nm) irradiation in the presence of adventitious gaseous hydrocarbons are a major obstacle to process implementation of EUV lithographysthe key to fabrication of next-generation semiconductor chips. By means of synchrotron and laboratory-based experiments,1 we have shown that mirror contamination is due to the slow secondary electrons induced by EUV radiation; these initiate electron-impact-induced dissociation of organic adsorbates, resulting in growth of a graphitic film on the Ru surface. We showed that the rate of graphitic film growth is strongly dependent on the chemical nature of the organic species and provided an explanation in terms of the relative surface lifetimes of the various types of adsorbates. Others have shown that water vapor within the process environment can cause significant oxidation of multilayer mirror surfaces under EUV irradiation, which also constitutes a serious threat to optics lifetime.2 The mechanisms underlying these processes have been reviewed by Madey and co-workers.3 From a practical viewpoint, the key issue is as follows. Contamination layers significantly thicker than ∼0.5 nm reduce EUV reflectivity by >0.6%; given that a typical optical stack incorporates a sequence of 10 mirrors, such degradation in performance would be sufficient to render the system useless.4 Contaminant layers
