Pathfinding the perfect EUV mask: The role of the multilayer

Mitigation of 3D-mask effects is a requirement for pushing high-NA (0.55) EUV lithography to its limits. Both the absorber and the reflective multilayer parts of the EUV mask contribute to the 3D-mask effects. This paper focuses on the investigation and optimization of the multilayer. The impact of different multilayer parameters on the imaging performance is investigated and used to explain the optimization outcome. Multilayer optimization yields better lithographic performance by including imaging metrics in the merit function instead of reflectivity data only. Different geometrical representations of the multilayer are optimized and their performances are compared. The results show a tradeoff among different lithographic metrics with improvements compared to a reference obtained from reflectivity optimization.