Imaging overlay with mutually coherent oblique illumination

An overlay metrology system comprising:

• one or more illumination sources configured to generate one or more pairs of illumination beams when implementing a metrology recipe;
• one or more illumination optics configured to direct the one or more pairs of illumination beams to an overlay target on a sample when implementing the metrology recipe, wherein the illumination beams within a particular one of the one or more pairs of illumination beams are directed to the overlay target with common altitude incidence angles and symmetrically opposed azimuthal incidence angles along a selected illumination direction, wherein the overlay target in accordance with the metrology recipe includes two or more grating structures distributed along one or more measurement directions;
• one or more detectors;
• one or more imaging optics configured to image the overlay target onto the one or more detectors when implementing the metrology recipe, wherein an image of a particular one of the two or more grating structures includes a sinusoidal interference pattern generated exclusively by interference of a single non-zero diffraction order of light from each of the illumination beams within the particular one of the one or more pairs of illumination beams;
• and
• a controller communicatively coupled to the one or more detectors, the controller including one or more processors configured to execute program instructions when implementing the metrology recipe causing the one or more processors to determine overlay measurements along the one or more measurement directions based on one or more images of the overlay target received from the one or more detectors.