Patent ID: 7251042

Claim:
A lithographic apparatus comprising: (a) an illumination system configured to condition a beam of radiation; (b) a support structure configured to support a patterning device, the patterning device serving to impart the beam of radiation with a pattern in its cross-section; (c) a substrate table configured to hold a substrate; (d) a projection system configured to project the patterned beam onto a target portion of the substrate, and (e) an interferometer configured to measure a position of a mirror of the lithographic apparatus, the interferometer comprising (i) a modulator configured to modulate a position of an input beam of the interferometer; (ii) a synchronous detector configured to synchronously detect a position modulated interfered beam of the interferometer using a modulation frequency reference signal; and (iii) a calculator configured to calculate an unflatness of an area of the mirror from an effect of a modulation of the position of the input beam on an output signal of the synchronous detector.