Patent ID: 7889357

Claim:
An apparatus comprising: a projection system configured to project a patterned radiation beam onto a target portion of a substrate; and a level sensor configured to perform height measurements of at least part of the substrate to generate height data, for use in positioning a target portion of the substrate with respect to a focal plane of the projection system, wherein the apparatus is adapted to use predetermined correction heights to compute corrected height data for the height data, wherein the level sensor is configured to perform height measurements for a number of gridlines, each gridline comprising a plurality of level sensor spots; wherein, for a first portion of the gridlines, the level sensor is arranged to generate height data with respect to at least part of the substrate that is at least partially outside the target portion that is to be positioned with respect to the projection system, and is further arranged to compute the corrected height data for the height data corresponding to at least one area of the at least part of the substrate located outside the target portion; and wherein, for a second portion of the gridlines, the level sensor is arranged to generate height data with respect to at least part of the substrate that is at least partially inside the target portion that is to be positioned with respect to the projection system, and is further arranged to compute the corrected height data for the height data corresponding to at least one area of the at least part of the substrate located inside the target portion.