Freeforms in EUV Lithography Projection Optics

Freeform surfaces, mostly in the form of Zernike polynomials, have been used in Projection Optics for Microlithography since the earliest days of the Perkin Elmer Micralign in the 1970’s. This is because the optical tolerances are so small that one cannot rely on statistical accumulation and cancellation. No manufactured optical surface can be considered to be spherical or even rotationally symmetrical - they have to be measured and compensated. Article not available.