Overlay metrology system with direction-isolated imaging

An overlay metrology method comprising:

• illuminating an overlay target on a sample with illumination comprising a first illumination dipole and a second illumination dipole, wherein the overlay target includes two or more cells, wherein each cell of the two or more cells comprises first-direction features having periodicity along a first direction and second-direction features having periodicity along a second direction;
• generating first image on a first detector based on first-order diffraction of the first illumination dipole by the first-direction features, wherein the first-direction features are resolved and the second-direction features are unresolved in the first image;
• generating a second image of the overlay target based on first-order diffraction of the second illumination dipole by the second-direction features, wherein the second-direction features are resolved and the first-direction features are unresolved in the second image;
• and
• determining a first overlay measurement along the first direction based on the first image and generate a second overlay measurement along the second direction based on the second image.

An overlay metrology target comprising:

• two or more cells on a sample, wherein each cell of the two or more cells comprises first-direction features having periodicity along a first direction and second-direction features having periodicity along a second direction, wherein the two or more cells comprise:
• one or more first cells, wherein the first-direction features and the second-direction features in the one or more first cells are located in a first layer of the sample;
• and
• one or more second cells, wherein the first-direction features in the one or more second cells are located in a second layer of the sample, wherein the second-direction features in the one or more second cells are located in a third layer of the sample.