Patent ID: 7230703

Claim:
An apparatus for measuring overlay within a sample, where the sample includes an overlay target that is formed as a series of grating stacks each having an upper and lower grating, each grating stack having a unique offset between its upper and lower grating, the apparatus comprising: an illumination source that generates an optical beam; illumination optics configured to direct a probe beam portion of the optical beam to be reflected by the target; a detector array configured to convert an image into corresponding output signals; collection optics configured to convey an image of the target to the detector by collecting some portion of the reflected probe beam, where the numerical aperture of the collection optics is larger than the numerical aperture of the illumination optics and with the numerical apertures of the illumination and collection optics are selected so that the unit cells of gratings are not resolved, the grating stacks are resolved and they appear to have a uniform color within the image of the overlay target; and a processor configured to measure overlay based on the output signals of the detector.