Alignment sensor based on wavelength-scanning

An alignment method includes directing an illumination beam with a varying wavelength or frequency towards an alignment target, collecting diffraction beams from the alignment target and directing towards an interferometer. The alignment method also includes producing, by the interferometer, two diffraction sub-beams from the diffraction beams, wherein the diffraction sub-beams are orthogonally polarized, rotated 180 degrees with respect to each other around an alignment axis, and spatially overlapped. The alignment method further includes measuring interference intensity of the diffraction beams based on a temporal phase shift, wherein the temporal phase shift is a function of the varying wavelength or frequency of the illumination beam and a fixed optical path difference between the diffraction beams. The alignment method also includes determining a position of the alignment target from the measured interference intensity.