Self-referencing interferometer, alignment system, and lithographic apparatus

摘要

A self-referencing interferometer includes an optical system to split an alignment beam to create a reference beam and a transformed beam. The optical system includes a beam splitter to combine the reference beam and the transformed beam so that the diffraction orders in the reference beam spatially overlap with their respective opposite orders in the transformed beam. A detector system receives the spatially overlapping reference beam and transformed beam from the optical system and determines a position signal. The detector system includes a polarizing system for manipulating the polarization of the beams so that they interfere, and for directing the interfering reference beam and transformed beam to a detector for determining a position signal from the variation of intensity of the interfering beams.