Focus measurement in projection exposure apparatus

摘要

Disclosed is a method of detecting focus information about an image projecting optical system, in which mark projection is made through the optical system and based on illumination lights having different chief ray incidence directions, the mark projection being carried out so that mark images formed by the illumination lights, respectively, are superposed one upon another approximately upon an imaging plane, and focus information about the optical system is detected on the basis of information related to a deviation between the mark images superposed.