Catadioptric projection lenses for immersion lithography

Recently, the development of high NA lenses for immersion lithography turned from dioptric concepts to catadioptric design forms. The introduction of mirrors involves the new challenge to deal with the inevitable obscuration of either field or pupil. We review the strategies used in this regard for microlithography, while focussing on the two most favored ones, folded and inline concepts. Although the vignetting situation is more complicated for inline systems, we report progress in this field of optical design yielding similar system performance for inline and folded designs. Since inline optical systems are much easier to realize, these are the concept of choice.