Patent ID: 7564555

Claim:
A method of measuring the overlay error of a substrate, comprising: projecting a first beam of radiation onto a first target of the substrate, the first target comprising at least two superimposed patterns having a bias of +d between a first of the patterns arranged in or on a first layer and a second of the patterns arranged in or on a second layer, and measuring the asymmetry of that first beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a first signal indicative of the measured asymmetry; projecting the first beam of radiation onto a second target of the substrate, the second target comprising at least two superimposed patterns having a bias of −d between a first of the patterns arranged in or on a first layer and a second of the patterns arranged in or on a second layer, and measuring the asymmetry of that first beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a second signal indicative of the measured asymmetry; projecting a second beam of radiation onto the first target and measuring the asymmetry of that second beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a third signal indicative of the measured asymmetry; projecting the second beam of radiation onto the second target and measuring the asymmetry of that second beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a fourth signal indicative of the measured asymmetry, wherein the first and second beams have different polarizations, or different wavelengths, or both, and the overlay error is determined on the basis of the first, second, third and fourth signals.