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 H2 gas is used for cleaning and purging while N2 gas
is used for purging H2 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 H2 and
N2 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.