Self-referencing interferometer, alignment system, and lithographic apparatus

A self-referencing interferometer arranged to be used with a mark and a lithographic apparatus, the self-referencing interferometer comprising:

• an optical arrangement for creating a reference beam and a transformed beam from an alignment beam, for outputting to a detector said reference beam and said transformed beam, for directing the alignment beam onto the mark and for capturing a diffracted beam resulting from diffraction of the alignment beam by the mark, wherein
• the diffracted beam comprises at least one positive diffraction order and at least one
• corresponding negative diffraction order;
• the optical arrangement comprising:
• a beam splitter configured and arranged to split the diffracted beam into a first beam and a second beam, and for combining and outputting to said detector said reference beam and said transformed beam so that the diffraction orders in said reference beam spatially overlap with their corresponding opposite diffraction orders in said transformed beam;
• a referencing system constructed and arranged to create said reference beam from the
• first beam and to direct said reference beam to the beam splitter;
• a transforming system for transforming the second beam into said transformed beam and for directing said transformed beam to the beam splitter;
• and
• a detector system constructed and arranged to receive the spatially overlapping reference beam and the transformed beam from the optical arrangement, and to determine a position signal, the detector system comprising:
• a polarizing system constructed and arranged to manipulate the polarization of the
• reference beam and the transformed beam so that they interfere with each other, and
• to direct the interfering reference beam and the transformed beam to a detector
• thereby forming an interference pattern;
• and
• a detector constructed and arranged to determine a position signal from the interference pattern.