US Patent 7433018 Pattern alignment method and lithographic apparatus

A method for aligning a substrate in a lithographic apparatus, the method including irradiating a first target portion of the substrate with a first patterned beam to form a first pattern on the substrate. Then, a second target portion of the substrate is irradiated with a second pattern beam to form a second pattern on the substrate. The second target portion at least partly overlaps the first target portion. A diffraction beam is detected during irradiation of the second pattern, due to a diffraction of the second patterned beam on the first pattern. The diffraction beam is compared with a desired diffraction beam, and the substrate is aligned based on a comparison between the detected diffraction beam and the desired diffraction beam.