The invention relates to fingerprint, fitting / estimation in case a process fingerprint will be determined in a two-step approach: 1. Fit a global fingerprint across a full wafer on a sparse layout (typically on every lot)2. Fit a per field fingerprint on a dense layout (done every x lots, where x > 1) In order to prevent fitting twice the same global content (which would lead to overcorrection), a precaution is taken in the second step; first the global fingerprint is removed and then a per field (dense) fit is performed on the residuals of the first modeling. The per field model is not orthogonal to the global model and will therefor capture (part) of the same shape.
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