Provided with an optical system which is applicable to an exposure apparatus used in the manufacture of semiconductors and includes a combination of a spherical mirror having a function of refraction and lenses for astigmation control, using a CaF.sub.2 lens as the last lens, thereby making it possible...http://www.google.fr/patents/US6101047?utm_source=gb-gplus-shareBrevet US6101047 - Catadioptric optical system for lithography
