Design of an extreme ultraviolet lithography projection objective with a grouping design method through forward and reverse real ray tracing

As the logic node gets more and more advanced, the performance of extreme ultraviolet (EUV) objective projection is required to be higher and higher in a large field of view. It is known that a good initial structure can greatly reduce the dependence on the experience of optical designers. In this paper, a grouping design method through forward and reverse real ray tracing is proposed to design the aspheric initial structure for the EUV objective system. The system is first divided into three groups, and each spherical group is designed separately. Then, the three groups are connected as a whole spherical initial objective system. Through forward and reverse real ray tracing, each spherical group is recalculated to an aspheric structure in turn. Finally, an iterative process is applied to improve the performance of the aspheric initial structure. The aspheric initial structure calculated by this method can be taken as a good starting point for further optimization. As verification of the design method, a six-aspheric-mirror EUV lithography objective with a numerical aperture of 0.33 has been designed, whose root mean square (RMS) wavefront error is less than 0.2 nm and distortion is less than 0.1 nm.