Patent ID: 7502096

Claim:
A device manufacturing method using a lithographic apparatus having a projection system and a level sensor system comprising a plurality of level sensor devices, the projection system configured to expose a substrate and each level sensor device configured to project a beam of light patterned by a respective grating of the level sensor device onto a surface of the substrate to be exposed, such that a surface height of the substrate is measured by the level sensor devices by detecting properties of a resulting image of the grating at respective points thereon, the method comprising: obtaining at least one calibration value for the level sensor system, the at least one calibration value compensating for a difference in measured surface heights among said plurality of level sensor devices, the at least one calibration value corresponding to a property of the substrate to be exposed; determining measured and calibrated surface heights of the substrate to be exposed using the plurality of level sensor devices based on the at least one calibration value; and, thereafter, exposing the substrate using the projection system based on the measured and calibrated surface heights.