Lithographic projection apparatus with an alignment system for aligning substrate on mask

摘要

A lithographic projection apparatus with an off-axis alignment unit for aligning a substrate alignment mark (P1) with respect to a reference (RGP) is described. This unit comprises a structure (WEP) of deflection elements (80-86) which give the sub-beams having different diffraction orders coming from the diffractive substrate mark (P1) different directions so that these sub-beams are incident on separate reference gratings (90-96) and can be detected by separate detectors (DET). This unit also provides the possibility of aligning asymmetrical alignment marks with great accuracy.