Patent ID: 8149420

Claim:
A metrology system, comprising: an object movable in a first direction and in a second direction different from the first direction; a diffraction grating mounted on the object; a interferometer head operable to direct a measurement light beam towards the grating at a non-zero angle of incidence and to generate a first signal in response to movement of the object; a separation change detector operable to generate a second signal in response to changes in separation between the object and the interferometer head; stored calibration values pertaining to respective known displacements, the calibration values derived from displacement measurements made with a single first separation in the second direction between the interferometer head and the grating, each of the calibration values pertaining to a respective one of the known displacements; and a processor connected to receive the first and second signals generated as the object moves at least in the second direction to a second separation from the interferometer head, the second separation different from the first separation, the processor operable in response to the first signal and the second signal to measure a displacement of the object in the first direction as a measured displacement and to measure a separation change, the processor additionally operable to calculate an offset from the separation change and to determine at least one offset displacement from the measured displacement and the offset, the processor additionally operable to derive from at least one of the original calibration values an offset calibration value pertaining to the at least one offset displacement, and to correct the measured displacement using the offset calibration value.