Patent ID: 7187431

Claim:
A system to determine an optical characteristic of a lithographic apparatus having a projection system, comprising: an interferometric sensor configured to measure a wavefront of a radiation beam at a level of a substrate, the interferometric sensor including a detector; an actuator configured to displace the interferometric sensor in a direction along an optical axis of the projection system; a first optical module configured to determine a change of phase of the wavefront at each of a plurality of locations on the detector of the interferometric sensor, the change of phase resulting from displacement of the interferometric sensor by the actuator between a first position and a second position; and a second module configured to determine, for each of the plurality of locations on the detector, a corresponding pupil location at a pupil plane of the projection system traversed by the radiation beam, using the change of phase determined by the first module and a value of the displacement of the interferometric sensor by the actuator, to produce a mapping between locations on the detector and corresponding pupil locations.