Interferometric quality checking system for a mirror used in EUV-lithography, whereby a limited surface area is checked for non-rotationally symmetric errors so that rotationally symmetric parent body errors can be located

摘要

System comprises an interferometer arranged in a plane perpendicular to the longitudinal axis of the parent body (1), whose surface footprint (2) is to be tested. Light generated by the interferometer passes through a diffractive optical element (5) and is incident normally with the footprint surface. Reference light is generated by a reference surface (4) in the path of the light from the interferometer. Prior to testing, non-rotationally symmetric interferometer errors are detected, then the footprint is measured and non-rotationally symmetric errors detected which are related to rotationally symmetric errors in the parent.