High purity gas supply for optics purging and cleaning under vacuum is required to be maintained at the output of the
mini-environment gas distribution box in EUV scanners. Typically H<sub>2</sub> gas is used for cleaning and purging while N<sub>2</sub> gas
is used for purging H<sub>2</sub> lines post exposure. An investigation of gas purifier performance for moisture removal is made
under sub-atmospheric pressure conditions. An evaluation of moisture levels as a function of switching between H<sub>2</sub> and
N<sub>2</sub> gas supply states is also conducted. A superior performance (below instrument LDL) is observed for HX (Entegris,
Inc.) gas purifier under various test conditions in the 10-100 kPa pressure range. Our preliminary studies provide a better
understanding of gas purifier related moisture outgassing under vacuum and should facilitate better control and
standardization of tool set-up parameters for environment in EUV lithography.