Patent ID: 7663753

Claim:
A method for determining overlay between a plurality of first structures in a first layer of a sample and a plurality of second structures in a second layer of the sample, the method comprising: providing a plurality of targets that each include a portion of the first and second structures and each is designed to have an offset between its first and second structure portions; illuminating the targets with electromagnetic radiation to obtain spectra for each target, wherein obtaining the spectra comprises acquiring radiation from the targets at a selected one or more wavelengths using an imaging system with wavelength selection and acquiring an image from each of the targets, wherein the spectra are an averaged or summed one or more intensity value(s) of one or more pixels of the corresponding target image; selecting a wavelength using the imaging system such that contrast between the spectra is maximized; and determining any overlay error between the first structures and the second structures using a scatterometry technique based on the obtained spectra for each target.