Tradeoffs in mask architecture: dealing with future illumination angular diversity

Extending EUV lithography to future nodes will lead to significant mask challenges. Higher-NA systems will come with larger angles of incidence on the mask, which further complicate the well-known shadowing problem. Addressing them may require the use of thinner absorbers. At the same time, however, the larger mean angle along with a larger diversity of illumination and diffraction angles can also lead to image and reflectivity degradation. Here we perform a modeling based study to explore the relative importance of these two effects for the 12- and 11-nm half pitch nodes. Specifically, we sacrifice reflectivity for increased angular bandwidth by reducing the number of layers in the multilayer stack to find solutions. We further explore thinner absorber and chirped multilayer designs in an attempt to mitigate these problems. As of the conclusion of this study, we have not been able to achieve any combination of parameters to meet the lithographic requirement of NILS 2.5 over 50nm defocus. We can also expect that any condition that meets this requirement will lead to throughput loss: in the most optimistic case we found reflectivity at 77o and 85o was reduced by 4%, though the lithographic requirement was not yet satisfied.