Patent ID: 7262860

Claim:
A method for determining the location of an alignment mark on a stage, the method comprising: directing a measurement beam along a path between an interferometer and a mirror, wherein at least the interferometer or the mirror is mounted on the stage; combining the measurement beam with another beam to produce an output beam comprising information about the location of the stage; measuring from the output beam a location, x 1 , of the stage along a first measurement axis; measuring a location, x 2 , of the stage along a second measurement axis substantially parallel to the first measurement axis; calculating a correction term, ψ 3 , from predetermined information characterizing surface variations of the mirror for different spatial frequencies, wherein contributions to the correction term from different spatial frequencies are weighted differently; determining a location of the alignment mark along a third axis parallel to the first measurement axis based on x 1 , x 2 , and the correction term; and outputting a signal from an electronic controller based on the determined location of the alignment mark.