Water calibration data (based on reference wafers) is used to control lithographic apparatus (scanner) stability. Scanner stabilization jobs are performed often and regularly (e.g. every 3 days) to update scanner references. Scanner stabilization jobs consist of exposing reference wafers, measuring overlay on the exposed reference wafers with an external metrology tool, and updating the scanner references. From time to time after reference wafers are exposed by the scanner and measured by the external metrology tool, the measured overlay falls out of control limits. In this case the job is rejected and scanner references are not updated, and the reference wafers are re-exposed (rework). If the overlay measurement exceeds the control limits again, the measurements are accepted as it shows a real change that has occurred at the scanner.
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