Toward the in situ remediation of carbon deposition on Ru-capped multilayer mirrors intended for EUV lithography: Exploiting the electron-induced chemistry

The realization of next-generation extreme ultraviolet (EUV) lithography depends on the application of Ru-capped 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 (similar to 10(-5) mbar) of NO or O-2 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 similar to 4.5 similar to 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.