Patent ID: 8440475

Claim:
A method of calculating alignment residuals in a fabrication unit, the method comprising: providing an alignment model including alignment model parameters; providing an exposure tool suitable for exposing a lot of semiconductor wafers in a plurality of exposure fields; retrieving alignment data comprising alignment values measured by the exposure tool on respective semiconductor wafers of the lot at a plurality of positions of the exposure fields that are used to calculate values for the alignment model parameters of the alignment model; calculating a set of alignment residuals from the alignment data by subtracting effects of the alignment model parameters for each of a plurality of measured positions and for each of the semiconductor wafers in the lot; issuing a warning signal based upon a comparison between the set of alignment residuals and a set of reference values, wherein the set of reference values are calculated by averaging the alignment residuals over each semiconductor wafer of the lot at each of the plurality of positions; subtracting the reference values from the alignment residuals for each of the semiconductor wafers of the lot and at each of the plurality of positions, so as to calculate a resulting set of difference data for each semiconductor wafer of the lot; and calculating a standard deviation from the resulting set of difference data for each of the semiconductor wafer of the lot.