Interferometer system, method of measuring a movement of a measurement surface with respect to a reference surface, projection system and lithographic apparatus

Abstract

The invention provides an interferometer system, comprising: a light source to provide a measurement beam and a reference beam, interferometer optics to propagate the measurement beam and the reference beam in multiple passes to a measurement surface and a reference surface, respectively, and to combine the reflected measurement beam and the reflected reference beam of a final pass of the multiple passes, and a detector to receive the combined reflected measurement beam and reflected reference beam of the final pass, wherein the interferometer optics comprise at least one optical element to provide an angle difference between the measurement beam and the reference beam of at least one non-final pass of the multiple passes and the measurement beam and the reference beam of the final pass.

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