A New Cleaning Technique for X-Ray Masks in Alkaline Solutions by Direct Control of Electrochemical Potential

This paper describes an electrochemical surface cleaning (ECSC) technique newly developed for removal of particulate contamination on X-ray masks employing WNx absorbers. In this technique, the electrochemical potential of absorber films is precisely controlled for preventing corrosion or etching of the films during their immersion in alkaline solutions. The particle removal efficiency and the stress change of WNx absorbers were examined and compared with those after the conventional cleaning operated under electrically floating condition. The etched depth (and thus etch rate) of WNx films in alkaline solutions was measured using an in-situ quartz-crystal-microbalance technique. Furthermore, X-ray photoelectron spectroscopy and atomic force microscopy were employed to characterize WNx film surfaces. Mechanisms responsible for a large stress change observed in the case of conventional cleaning are discussed. Moreover, it is demonstrated that the ECSC operated at cathodic potentials yields high removal efficiencies for a variety of particulates without change in film thickness and stress between before and after cleaning.