US Patent 7321432 Measurement and compensation of errors in interferometers

In general, in one aspect, the invention features a method for determining the location of an alignment mark on a stage including measuring a location, x1, of a stage along a first measurement axis using an interferometer, measuring a location, x2, of the stage along a second measurement axis substantially parallel to the first measurement axis, and determining a location of the alignment mark along a third axis substantially parallel to the first measurement axis based on x1, x2, and a correction term, ψ3, calculated from predetermined information including information characterizing imperfections in the interferometer.