Novel reticle absorber materials are required for high-NA EUV lithography. TNO and ASML developed an assessment for the compatibility of novel high-NA reticle absorber materials with conditions that mimic the EUV scanner environment^[1]. Four candidate reticle absorber materials were evaluated, TaCo, RuTa, PtMo and Pt2Mo alloys, in a joint research program. For the compatibility tests, dedicated samples with silicon wafer substrates were fabricated. The silicon wafers were coated with a Mo-Si multilayer coating, followed by a Ru capping layer and finally the absorber material.

Chemical outgassing tests, in presence of hydrogen radicals, did not show chemical outgassing for the TaCo and PtMo alloys. RuTa and Pt₂Mo alloys were not tested, based upon their elemental composition chemical outgassing is not expected. Next, all four materials were exposed in a hydrogen plasma resistance test equivalent to an EUV exposure of at least 250 thousand wafers** in the EUV scanner. No plasma-induced defects, like blistering or delamination, were found that were related to the intrinsic absorber properties.

The RuTa and PtMo alloys were selected for EUV exposure in the EBL2 facility at TNO. Both materials were exposed to an 9.6 kJ/mm² EUV peak dose at an EUV peak intensity of 450 mW/mm² in a hydrogen environment. This EBL2 EUV exposure of 6 hours represents about 1-2 months of EUV dose (at least 150 thousand wafers) in a NXE or EXE scanner with a 300 W source. Both materials showed good performance during EUV exposure. Relevant surface defects and chemical outgassing were not observed. The few observed blisters in the low EUV intensity areas are likely provoked by particle contamination or coating defects.

All four absorber materials, TaCo, RuTa, PtMo and Pt₂Mo alloys performed well in the compatibility tests that were executed. Not all compatibility tests could be performed on each absorber material within the scope of our research program. Further testing would be needed to complete the compatibility assessment, including an EUV exposure on a patterned reticle.