Patent ID: 7318206

Claim:
A method of determining an offset for each of a plurality of measurement systems, the method comprising the steps of: obtaining a set of test measurements of an artifact on a monitor wafer for each of the plurality of measurement systems, and a current offset for each measurement system; measuring the artifact on a measurement system under test (MSUT) to obtain a new test measurement; adding the new test measurement to the set of test measurements for the MSUT; calculating an exponentially weighted moving average (EWMA) of each measurement system based on their respective sets of test measurements; calculating for each measurement system a prime value equal to the EWMA of a respective measurement system plus the current offset of the respective measurement system; calculating a median of the prime values for the plurality of measurement systems as a group; calculating for each measurement system, a delta-to-median value equal to the prime value for the respective measurement system minus the median; and resetting the current offset for each respective measurement system to a reset offset equal to the current offset for a respective measurement system minus the delta-to-median for the respective measurement system in response to the delta-to-median value for the respective measurement system exceeding a predetermined threshold.