Outgassing Characteristics of Structural Materials and the Removal of Contaminants from EUVL Masks using 172-nm Radiation

Under EUV radiation, the surfaces of the imaging optics and mask, which are coated with a Mo/Si multilayer, become contaminated with organic compounds. This study examined the rate of outgassing and the species that desorb from materials used in vacuum equipment. The rate of outgassing for metals was found to depend on surface roughness. And most of the desorbed hydrocarbons, which are a primary contaminant, come from the wiring and some other components rather than from the metals used for the stages, even though the metals have a large surface area. Furthermore, since an efficient way of removing carbon contamination from masks and the imaging optics is required, a method of cleaning finished EUVL masks with 172-nm irradiation was investigated. This method restores the reflectivity of a Mo/Si multilayer to its original level, and removes contaminants at the rate of 2 nm/min in a low-pressure environment, which in this study was an initial backpressure in the chamber of 500 Pa provided by a scroll pump, and an O2 flow to maintain the pressure at 2X103 Pa. Irradiation with 172-nm light in a low-pressure environment was found to be an effective way to clean masks.