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Cleaning of low thermal expansion material (LTEM) substrates for mask blanks in EUV lithography

Low thermal expansion material (LTEM) substrates were cleaned with recipes developed to clean blank quartz substrates. These recipes were capable of cleaning the LTEM without damaging the LTEM substrate. No effect of etching doped metals in LTEM was observed in these experiments. However, LTEM substrates currently require multiple cleaning cycles to obtain the same removal or cleaning efficiencies as quartz substrates. In addition, no change in the surface roughness or degradation of the backside choromium layer was observed.