Zero-order overlay metrology

A metrology system comprising:

• an illumination source configured to generate one or more illumination beams;
• an imaging sub-system comprising:
• one or more lenses configured to image a sample onto a detector located in a field plane conjugate to the sample, wherein the sample includes overlapping periodic features on two sample layers, wherein the overlapping periodic features on the two sample layers have a common pitch, wherein the detector generates zero-order double diffraction signals associated with zero-order double diffraction of the one or more illumination beams by the overlapping periodic features;
• and
• one or more ellipsometry optics including at least one of a polarizer or a waveplate in an optical path associated with at least one of the one or more illumination beams or the zero-order double diffraction;
• and
• a controller including one or more processors configured to execute program instructions causing the one or more processors to implement a metrology recipe by:
• receiving the zero-order double diffraction signals associated with the overlapping periodic features from the detector and generated with one or more configurations of the one or more ellipsometry optics, wherein constituent features of the overlapping periodic features are unresolved in the zero-order double diffraction signals;
• and
• generating one or more spatially-resolved metrology measurements of the sample based on spatial variations of the zero-order double diffraction signals generated with the one or more configurations of the one or more ellipsometry optics.