Patent ID: 7508976

Claim:
A method comprising: providing an overlay pattern having a plurality of measurement locations, each measurement location includes a bottom diffraction grating and a top diffracting grating that overlies the bottom diffraction grating and has a designed in offset from the bottom diffraction grating, wherein at least two measurement locations in the overlay pattern differ from each other at least partially due to an unintentional local process variation created during processing of the overlay pattern; illuminating each of the plurality of measurement locations of the overlay pattern with incident radiation that reacts with the diffraction gratings; detecting the radiation from the measurement locations after reacting with the diffraction gratings; determining a measurement of an overlay error between the bottom diffraction gratings and the top diffraction gratings using the detected radiation from the measurement locations from the overlay pattern; and correcting the determined measurement of the overlay error for effects of the local process variations created during processing of the overlay pattern using the detected radiation from at least one pair of the measurement locations from the overlay pattern.