Tangential-sagittal decomposition-combination design method for a large-field anamorphic extreme ultraviolet lithography projection objective

Extreme ultraviolet lithography (EUVL) for 2 nm and below node is impacted by the projection objective performance. Anamorphic projection objectives have been employed for high-numerical aperture (NA=0.55) EUVL systems. Existing design methods for anamorphic projection objectives construct initial structures from coaxial subsystems and have been demonstrated for relatively small fields of view. The freeform surfaces during optimization have not matched the optical characteristics of anamorphic systems. A novel tangential-sagittal decomposition-combination (TSDC) design method, to our knowledge, is proposed for an anamorphic initial structure. By adopting an anamorphic XY polynomial freeform surface during optimization, the final system has a root mean square wavefront error of less than 0.92 nm and a maximum distortion of less than 0.48 nm. Its image-side field of view reaches 26 mm×1 mm, doubling the field width reported in previous studies.