Group design for the initial structure of an extreme ultraviolet lithography projection system

Commercial EUV lithography projection systems are designed with six-mirror, using high order aspherical surfaces or freeform surfaces, which require high processing and inspection requirements. When designing the initial structure of the projection system, it is important to consider the balance between the system's aberration and the difficulty of processing and manufacturing. EUV lithography optical systems minimize the number of mirrors in order to improve light energy utilization. Freeform surfaces or aspherical surfaces are used to provide more degrees of freedom for optimization. For a 6-mirror projection system, there are several ways for group design. There are 2-4, 3-3 and 4-2 combinations when divided into two groups, or into 2-2-2 triple groups. According to the design parameters of the EUV lithography projection optical system, the initial structure obtained by different group methods is analyzed. And choose more reasonable initial structure for further optimization.