A method of removing contaminants from Ru-capped Mo/Si multilayer mirrors for extreme ultraviolet lithography has been developed. It employs atomic hydrogen generated by a heated catalyzer consisting of a W wire. A new experimental system was designed and constructed to examine the cleaning capability of atomic hydrogen transported through a quartz tube. The chemical state of an oxidized Ru surface was investigated by X-ray photoelectron spectroscopy before and after cleaning; and it was found that transported hydrogen radicals deoxidized the surface and reduced the amount of oxide to the level before oxidation. Although the time needed was longer, transported atomic hydrogen was found to be capable of deoxidizing an oxidized Ru surface. The dependence of the density of atomic hydrogen on W catalyzer temperature and hydrogen gas pressure was measured by a vacuum ultraviolet absorption technique; and the potential to increase the density, and thereby to reduce the treatment time, was demonstrated. (c) 2007 Elsevier B.V. All rights reserved.