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This paper describes a methodology based on advanced metrology to quantify the overlay performance of individual wafer steppers and a means to optimize their matching. Use of accurate two point through the lens alignment directly referenced to the reticle and a three axis interferometrically controlled X -Y -O stage permits full characterisation of the individual machine overlay errors. With the help of parameters determined in an extended metrology model of the microlithographic lens distortions and the wafer stage grid distortions one can automatically adjust corresponding machine servo -mechanisms for optimum matching. The model can be used to calculate and predict the systematic errors between random pairs of machines rather than having to directly measure all pairs, so ensuring efficient, high quality matching. Results of an eight machine matching experiment demonstrating the model's validity and effective use on PAS 2500 wafer steppers are described.

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