Investigation of Cleaning Process induced CD shift at EUV Mask

EUV lithography is the leading candidate for chip manufacturing at sub 10nm technology node. EUV specifications for mask were derived from those of optical masks and are currently adjusted according to knowledge collected with respect to specific behavior of EUV masks and exposure process. Difference in application of optical and EUV masks - transmission vs. reflection lead to addition of parameters characterizing surface roughness of both capping layer covered multilayer mirror as well as absorber, tight specification of line edge roughness (LER), actinic reflectivity of the mask as well as critical dimension (CD) mean to nominal. Absence of pellicle at EUV masks is expected to result in more frequent cleaning of the mask and hence more pronounced changes of the mask properties due to the mask cleaning process. These assumptions are reflected in tighter specifications for clean process influence. Investigation of the clean process influence on the EUV relevant parameter identifies differences in CD shift between optical and EUV masks. The chemical composition of EUV mask surfaces; absorber and capping layer; not used at optical masks is reason, why change of line width can be observed, and is possibly responsible for differences in CD stability of commercially available EUV absorbers too. Combined with narrowing specifications for CD shift due to cleaning, the material properties may result in the need of material specific clean processes. In our work we investigate the root cause of the differences and check if one process can be used which covers both EUV material stacks at given CD shift specifications.