Off-axis through the lens mutually coherent dark field imaging system with incoherent light for overlay metrology

A metrology method comprising:

• generating, with an incoherent illumination source, incoherent illumination;
• diffracting, with a diffraction grating, the incoherent illumination into one or more pairs of mutually coherent illumination beams;
• directing, with an objective lens, the one or more pairs of mutually coherent illumination beams to a metrology target on a sample, wherein the metrology target is configured in accordance with a metrology recipe to include periodic features associated with two or more lithographic exposures;
• collecting, with the objective lens, sample light associated with diffraction of the one or more pairs of mutually coherent illumination beams;
• passing, with one or more masks, a single nonzero-order diffraction beam associated with each of the mutually coherent illumination beams of the one or more pairs of mutually coherent illumination beams;
• blocking, with the one or more masks, a zero-order diffraction beam associated with each of the mutually coherent illumination beams of the one or more pairs of mutually coherent illumination beams;
• generating, with a detector, an image of the metrology target based on light passed by the one or more masks;
• and
• generating one or more metrology measurements of the sample based on the image in accordance with the metrology recipe.

A metrology system comprising:

• a diffraction grating comprising one or more diffraction grating cells having a direction of periodicity configured to split incoherent illumination from an incoherent illumination source into one or more pairs of mutually coherent illumination beams;
• an objective lens to direct the one or more pairs of mutually coherent illumination beams to a metrology target on a sample, wherein the metrology target is configured in accordance with a metrology recipe including two or more lithographic exposures to generate periodic features having a direction of periodicity arranged into one or more target cells, wherein the direction of periodicity of a target cell corresponds to the direction of periodicity of a diffraction grating cell, wherein the objective lens further collects sample light associated with diffraction of the one or more pairs of mutually coherent illumination beams;
• one or more masks, wherein the one or more masks are configured in accordance with the metrology recipe to pass a single nonzero-order diffraction beam and block a zero-order diffraction beam associated with each of the mutually coherent illumination beams of the one or more pairs of mutually-coherent illumination beams;
• a detector configured to generate an image of the metrology target based on light passed by the one or more masks;
• and
• a controller communicatively coupled to the detector, wherein the controller includes one or more processors configured to execute program instructions causing the one or more processors to generate one or more metrology measurements of the sample based on the image in accordance with the metrology recipe.