Significant advances have been made in recent years in the development of lithography optics. The resolution of
structures smaller than 45 nm demanded a new class of catadioptric objective lenses whose design and construction is the
result of decades of development in lithography optics. This article describes the best-performing lens types and design
principles of the catadioptric concept of these lenses.
"Toward the global optimum in lithographic lens design", Proc. SPIE 7652, International Optical Design Conference 2010, 76522B (9 September 2010); doi: 10.1117/12.868530; https://doi.org/10.1117/12.868530