Grouping design method with real ray tracing model for extreme ultraviolet lithographic objective

Choosing an adequate initial design for optimization plays an important role in obtaining high-quality extreme ultraviolet (EUV) lithographic objectives. A grouping design method with real ray tracing model is developed to acquire initial configurations of high numerical aperture (NA) objective for EUV lithography. In this method, the objective system is first divided into three mirror groups. The initial parameters of each mirror group are then determined by real ray calculation under design constraints. Finally, the three mirror groups are connected directly into a feasible initial system. Due to real ray calculation, the discrepancy of the ray path induced by paraxial approximation and the exhaustive search of variables is avoided in a high-NA objective design. In addition, the incidence angles on reflective mirrors can be controlled in the design of each group, which makes the initial configuration suited to further optimization and compatible multilayer design. An NA 0.33 six-mirror EUV lithographic objective is designed as an example to implement this method.