Differential interferometer system and lithographic step-and-scan apparatus provided with such a system

摘要: A differential interferometer system for measuring the mutual positions and movements of a first object (WH) second (MH) is described, which comprises unit (1, 2, 3, 4) with reflector (RW) (5, 6, 7, 8) (RM). Since beam (bm) passes through both reflected by reflector, since reference (br) traverse same path at least between two units, accurate measurements can be performed very rapidly. The may used to great advantage in step-and-scan lithographic projection apparatus.