Scanning overlay metrology with high signal to noise ratio

An overlay metrology method comprising:

• generating illumination with an illumination source;
• splitting the illumination from the illumination source into primary illumination and auxiliary illumination;
• directing the primary illumination to an overlay target on a sample, wherein the overlay target includes gratings in two or more layers;
• overlapping a first portion of the auxiliary illumination with at least one diffraction lobe associated with diffraction of the primary illumination from the gratings in each of the two or more layers on one or more first photodetectors in a collection pupil plane to generate a first interference pattern;
• overlapping a second portion of the auxiliary illumination with at least one additional diffraction lobe associated with the diffraction of the primary illumination from the gratings in each of the two or more layers on one or more second photodetectors in the collection pupil plane to generate a second interference pattern;
• modulating phases of the first and second interference patterns during a scan of the overlay target with a scanning sub-system, wherein the scanning sub-system includes at least one of a translation stage to scan the sample with respect to the primary illumination, beam-scanning optics to scan the primary illumination with respect to the sample, or a phase modulator to modulate a phase of the auxiliary illumination;
• generating time-varying interference signals with the one or more first photodetectors and the one or more second photodetectors based the modulated phases of the first and second interference patterns;
• and
• generating one or more overlay measurements of the overlay target based on the time-varying interference signals.

An overlay metrology system comprising:

• an illumination source configured to generate an illumination;
• one or more beamsplitters configured to split the illumination from the illumination source into primary illumination and auxiliary illumination;
• one or more illumination optics configured to direct the primary illumination to an overlay target on a sample, the overlay target having gratings in two or more layers when implementing a metrology recipe, wherein the overlay target in accordance with the metrology recipe includes a first set of one or more cells with gratings having a first grating direction and a second set of one or more cells with gratings having a second grating direction, wherein the primary illumination has a rotated quadrupole distribution with respect to the first and second grating directions;
• an objective lens to collect at least one positive diffraction lobe and at least one negative diffraction lobe associated with diffraction of the primary illumination from the gratings in each of the two or more layers when implementing the metrology recipe;
• a first collection channel comprising:
• a first set of one or more photodetectors in one or more collection pupil planes;
• one or more first collection optics configured to implement the metrology recipe by:
• overlapping a first portion of the auxiliary illumination with at least one diffraction lobe of the primary illumination along a first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a first interference pattern;
• and
• overlapping a second portion of the auxiliary illumination with at least one additional diffraction lobe of the primary illumination along the first diagonal of the rotated quadrupole distribution by the overlay target on at least one of the first set of one or more photodetectors to generate a second interference pattern;
• a second collection channel comprising:
• a second set of one or more photodetectors in one or more collection pupil planes;
• one or more second collection optics configured to implement the metrology recipe by:
• overlapping a first portion of the auxiliary illumination with at least one diffraction lobe of the primary illumination along a second diagonal of the rotated quadrupole distribution by the overlay target on the at least one of the second set of one or more photodetectors to generate a third interference pattern;
• and
• overlapping a second portion of the auxiliary illumination with at least one additional diffraction lobe of the primary illumination along the second diagonal of the rotated quadrupole distribution by the overlay target on at least one of the second set of one or more photodetectors to generate a fourth interference pattern;
• a scanning sub-system configured to modulate phases of the first, second, third, and fourth interference patterns during a scan of the overlay target when implementing the metrology recipe, wherein the scanning sub-system includes at least one of a translation stage to scan the sample with respect to the primary illumination, beam-scanning optics to scan the primary illumination with respect to the sample, or a phase modulator to modulate a phase of the auxiliary illumination;
• and
• a controller communicatively coupled to the first and second photodetectors, the controller including one or more processors configured to execute program instructions causing the one or more processors to implement the metrology recipe by:
• receiving time-varying interference signals from the first and second collection channels during the scan of the overlay target;
• and
• generating one or more overlay measurements of the overlay target along the first and second grating directions based on the time-varying interference signals.