Radio-frequency discharge cleaning of silicon-capped Mo/Si multilayer extreme ultraviolet optics

Abstract

Remote oxygen and hydrogen radio-frequency (rf) discharge cleaning experiments have been performed to explore their potential for cleaning carbon-contaminated extreme ultraviolet optics. The samples consisted of silicon wafers coated with 100 Å sputtered carbon, as well as bare Mo/Si multilayer mirrors (Si terminated). The samples were exposed for 3 h to rf plasma discharges at 100, 200, and 300 W. The carbon removal and surface oxidation rates were evaluated using sputter through depth profiling Auger spectroscopy. Reflectivity changes and surface roughness measurements were performed using at-wavelength reflectometry (13.4 nm) and atomic force microscopy, respectively. Data show that excited rf O2 consistently removes carbon at a rate approximately six times faster than excited rf H2 for a given discharge power and pressure. rf O2 also induces loss of reflectivity that is related to the growth of SiO2 on the optic surface. rf H2 shows a much lower oxidation rate of the optic surface. In spite of the lowered oxidation risk, reflectivity loss in Mo/Si multilayers was still observed, although this loss is not related to surface oxidation.