Aperiodic multilayer masks for M3D mitigation in high- and hyper-NA extreme ultraviolet lithography.

The article focuses on mitigating mask three-dimensional (M3D) phase effects in extreme ultraviolet lithography (EUVL) masks by optimizing multilayer reflectors. It identifies that increasing numerical apertures (NA) in EUVL tools lead to larger illumination angles on mask multilayers, causing phase shifts that degrade imaging metrics such as normalized image log slope (NILS) and image placement error (IPE). Traditional absorber material modifications do not sufficiently address these multilayer-induced phase effects. The authors propose a first-principles, genetic-gradient optimization methodology to design aperiodic multilayer stacks that reduce oblique multilayer M3D effects, improving NILS by up to 40% for hyper-NA patterning and enhancing depth of focus and image placement. While these optimized multilayers show promise for high-NA and hyper-NA applications, manufacturing challenges remain due to increased bilayer counts and layer-specific coating requirements.