Tolerance analysis of hyper numerical aperture lithography objective with freeform surfaces

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.