Patent ID: 7602489

Claim:
A lithographic apparatus comprising: a support structure constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in the cross-section of the radiation beam to form a patterned radiation beam; a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and a displacement measuring system configured to measure a position of a moveable object with respect to a reference frame of said lithographic apparatus, in at least three coplanar degrees of freedom (x, y, Rz) of an orthogonal x-y-z coordinate system centered on said moveable object, said moveable object comprising one of said support structure or said substrate table, said displacement measuring system comprising at least three sensor heads, each sensor head being positioned along a measuring direction associated with said sensor head that is substantially coplanar with an x-y plane of said coordinate system, said measuring direction associated with said sensor head being substantially perpendicular to a line connecting said sensor head to a center of said movable object and extending coplanar with said x-y plane, wherein the measuring direction of each of the at least three sensor heads defines a non-zero angle with the x and y axes of the x-y-z coordinate system.