Design of an objective for a hyper-numerical-aperture immersion lithography tool with a multi-step alternative...

An adequate initial structure plays an important role in achieving a high-quality hyper-numerical-aperture (NA) catadioptric objective for an immersion lithography tool. A multi-step alternative grouping design method is innovated to achieve a feasible aspherical initial structure of the hyper-NA objective. The objective is first divided into three groups: the object-side group (G1), the middle group (G2), and the image-side group (G3), and then each group is designed separately. To guarantee the effective connection of these separate groups, G1 and G2 or G2 and G3 are connected as G12 or G32 on the object or image side with an aspherical structure. Then, through ray tracing in the forward path and the reverse path, G2 will be recalculated to a new aspherical structure, and the hybrid iterating process will be adopted to get a better G2 and to better connect the whole system. Finally, G1 and the new G2 and G3 are connected as a feasible aspherical initial structure of the hyper-NA catadioptric objective, which can be considered a good starting point for further optimization. A high-performance catadioptric objective with a hyper-NA of 1.35 was designed by the proposed method.