Overlay measurements of overlapping target structures based on symmetry of scanning electron beam signals

An overlay metrology system comprising:

• a particle-beam metrology tool configured to scan a particle beam across an overlay target on a sample, the overlay target including a first-layer target element on a first layer of the sample and a second-layer target element on a second layer of the sample fabricated subsequent to the first layer, the particle-beam metrology tool further configured to capture a scan signal associated with interaction of the particle beam with both the first-layer target element and the second-layer target element;
• and
• a controller communicatively coupled to the particle-beam metrology tool, the controller including one or more processors configured to execute program instructions configured to cause the one or more processors to:
• receive the scan signal from the particle-beam metrology tool;
• determine one or more symmetry measurements for the scan signal with respect to one or more symmetry metrics;
• and
• generate an overlay measurement between the first layer and the second layer based on the one or more symmetry measurements, wherein an asymmetry of the scan signal is indicative of a misalignment of the second-layer target element with respect to the first-layer target element, wherein a value of the overlay measurement is based on the one or more symmetry measurements.

An overlay metrology method comprising:

• scanning a particle beam across an overlay target on a sample, the overlay target including a first-layer target element on a first layer of the sample and a second-layer target element on a second layer of the sample fabricated subsequent to the first layer;
• capturing a scan signal associated with interaction of the particle beam with both the first-layer target element and the second-layer target element;
• determining one or more symmetry measurements for the scan signal with respect to one or more symmetry metrics;
• generating an overlay measurement between the first layer and the second layer based on the one or more symmetry measurements, wherein an asymmetry of the scan signal is indicative of a misalignment of the second-layer target element with respect to the first-layer target element, wherein a value of the overlay measurement is based on the one or more symmetry measurements;
• and
• providing overlay correctables based on the value of the overlay measurement to a lithography system to modify exposure conditions of at least one subsequent exposure.