Patent ID: 8223347

Claim:
A method comprising: using a lithographic process to form a calibration structure comprising two periodic structures of a same pitch on each of two successive layers of a substrate, such that an overlaid pair of periodic structures is formed, the periodic structures being parallel, but offset relative to each other by an overlay amount; measuring a spectrum produced by directing a beam of radiation onto the calibration structure; comparing, using a processing unit, the measured spectrum with one or more stored spectra so as to determine values of the grating parameters for the calibration structure from the measured spectrum; using the lithographic process to form further overlaid periodic structures on the same or on one or more subsequent substrates; using the determined grating parameter values to compute a series of modeled spectra from overlaid periodic structures having a range of overlay amounts; measuring one or more further spectra produced by directing a beam of radiation onto the further overlaid periodic structures on the same or one or more subsequent substrates; performing a matching process using the computed series of modeled spectra and the one or more further spectra; and determining, using the processing unit, overlay amounts via the matching process for the further overlaid periodic structures.