The invention pertains to a measurement system for measuring displacement of a moveable object relative to a base in at least a first direction of measurement, the moveable object having at least one reference part that is moveable in a plane of movement relative to the base, the actual movements of...http://www.google.es/patents/US20060139595?utm_source=gb-gplus-sharePatente US20060139595 - Lithographic apparatus and method for determining Z position errors/variations and substrate table flatness
Lithographic apparatus and method for determining Z position errors ...
