A three slit interferometric focus control device capable of focal positioning accuracies on the order of 300 angstroms. The device includes a zero path Michelson interferometer with reference and movable legs comprising, respectively, an annular reference mirror and the surface to be brought into position...http://www.google.de/patents/US4493555?utm_source=gb-gplus-sharePatent US4493555 - High sensitivity focal sensor for electron beam and high resolution optical lithographic printers
