To ensure the good performance of hyper-numerical-aperture (NA) freeform surfaces lithography objective, not only the aberration should be decreased as much as possible in theory design stage, but also all the tolerances should be allocated reasonably and controlled rigorously in the manufacturing process. Therefore, reasonable tolerance analysis for projection objective is needed to maximally make up for the image quality deterioration caused by manufacture and assembly errors. According to the variation sensitivity between Zernike aberration and the single tolerance, effective compensators for individual aberrations can be chosen during tolerance analysis. As an example the method is applied to the tolerance analysis for an NA1.2 catadioptric projection objective with freeform surfaces designed by us. The results show that, after tolerance analysis using the compensators selected by this method, the root mean square (RMS) wavefront error of the projection objective is less than 0.015λ (λ=193 nm) at 90% probability, which meets the image quality requirement of lithographic projection objective for 10 nm technology node.
Choosing an adequate initial design for optimization plays an important role in obtaining high-quality deep ultraviolet (DUV) lithographic objectives. In this paper, the grouping design method is extended to acquire initial configurations of catadioptric projection objective for DUV lithography. In this method, an objective system is first divided into several lens groups. The initial configuration of each lens group is then determined by adjusting and optimizing existing lens design according to respective design requirements. Finally, the lens groups are connected into a feasible initial objective system. Grouping design allocates the complexity of designing a whole system to each of the lens groups, which significantly simplifies the design process. A two-mirror design form serves as an example for illustrating the grouping design principles to this type of system. In addition, it is demonstrated that different initial designs can be generated by changing the design form of each individual lens group.