Patent ID: 8860928

Claim:
A lithographic apparatus comprising: an illumination system configured to condition a radiation beam; a substrate table constructed to hold a substrate; an image sensor; a support constructed to support a patterning device, said apparatus being arranged to transfer a pattern from said patterning device onto said substrate, the patterning device further comprising at least one alignment structure for aligning the patterning device to the image sensor; and a projection system for projecting the radiation beam, said apparatus being operable to impart the radiation beam through the projection system onto said alignment structure so as to obtain a resultant aerial image, and comprises a non-transitory medium containing machine executable instructions to cause the apparatus to scan the image sensor in accordance with a scanning scheme, through a target volume containing said resultant aerial image so as to measure features of said image and thereby enable determination of the location of the alignment structure relative to the image sensor, wherein said scanning scheme comprises scanning the image sensor through the target volume to obtain a single alignment measurement the scanning scheme further comprising a plurality of scans, each scan comprising a plurality of traverse passes at successive levels spaced through at least a portion of the target volume and in which at least two of the scans at least partially overlap, traversing a same portion of the target volume while measuring said features of said image.