Scanning diffraction based overlay scatterometry

An overlay metrology system comprising:

• a controller communicatively coupled to a first photodetector and a second photodetector, the controller including one or more processors configured to execute program instructions causing the one or more processors to:
• receive time-varying interference signals from the first photodetector and the second photodetector as an overlay target is scanned along a stage-scan direction by a translation stage when implementing a metrology recipe, wherein the overlay target in accordance with the metrology recipe includes a plurality of measurement cells, wherein each measurement cell includes a grating-over-grating structures including a first-layer grating feature on a first layer of a sample and a second-layer grating feature on a second layer of the sample in an overlapping region, wherein the first-layer grating feature and the second-layer grating feature have a similar pitch;
• determine one or more differential signals between the first photodetector and the second photodetector for each measurement cell of the plurality of measurement cells;
• determine an overlay measurement based on the determined one or more differential signals,
• wherein the sample includes a calibration cell arranged proximate to a measurement cell of the plurality of measurement cells;
• and
• calibrate the time-varying interference signals from the first photodetector and the second photodetector using one or more calibration signals from the calibration cell to calibrate for misalignment between the first photodetector or the second photodetector,
• wherein the calibration cell includes a blank area configured to be scanned as the sample is scanned to generate one or more constant calibration signals associated with the amount of 0-order light captured by each of the first photodetector and the second photodetector,
• wherein the calibration cell includes a calibration grating structure configured to be scanned as the sample is scanned to generated one or more calibration signals including peak-to-peak values of the time time-varying interference signals.

A method comprising:

• illuminating an overlay target with a plurality of measurement cells on a sample having grating-over-grating structures as the sample is translated along a stage-scan direction with an illumination beam, where 0-order diffraction overlaps with first-order diffraction in a collection pupil;
• receiving time-varying interference signals from a first photodetector and a second photodetector as an overlay target is scanned along a stage-scan direction by a translation stage when implementing a metrology recipe, wherein the overlay target in accordance with the metrology recipe includes a plurality of measurement cells, wherein each measurement cell includes a grating-over-grating structures including a first-layer grating feature on a first layer of the sample and a second-layer grating feature on a second layer of the sample in an overlapping region, wherein the first-layer grating feature and the second-layer grating feature have a similar pitch;
• determining one or more differential signals between the first photodetector and the second photodetector for each measurement cell of the plurality of measurement cells;
• determining an overlay measurement based on the determined one or more differential signals,
• wherein the sample includes a calibration cell arranged proximate to a measurement cell of the plurality of measurement cells;
• and
• calibrating the time-varying interference signals from the first photodetector and the second photodetector using one or more calibration signals from the calibration cell to calibrate for misalignment between the first photodetector or the second photodetector,
• wherein the calibration cell includes a blank area configured to be scanned as the sample is scanned to generate one or more constant calibration signals associated with the amount of 0-order light captured by each of the first photodetector and the second photodetector,
• wherein the calibration cell includes a calibration grating structure configured to be scanned as the sample is scanned to generated one or more calibration signals including peak-to-peak values of the time time-varying interference signals.