| CPC G03F 7/70633 (2013.01) [G03F 9/7046 (2013.01); G03F 9/7088 (2013.01); G03F 9/7019 (2013.01)] | 15 Claims |

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1. A metrology system comprising:
a pre-alignment metrology tool configured to measure a plurality of targets on a substrate to obtain measurement data; and
a processing unit configured to:
process the measurement data to determine for each target at least one position distribution that describes variation of the position value over at least part of the target; and
determine a measurement correction from the at least one position distribution that corrects for within-target variation in each of the targets, the measurement correction being used to correct measurements performed by an alignment sensor.
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