Dark field digital holographic microscope and associated metrology method

A method comprises:

• illuminating a target formed by a lithographic process on a substrate with a first illumination beam of radiation;
• capturing, using an imaging branch having a detection NA greater than 0.1, resultant first scattered radiation having been scattered from the target;
• illuminating the target with a second illumination beam of radiation and capturing, using the imaging branch, resultant second scattered radiation having been scattered from the target;
• imposing spatial and/or temporal incoherence between a first beam pair comprising the first illumination beam and a first reference beam and a second beam pair comprising the second illumination beam and a second reference beam, such that:
• the beams of the first beam pair are at least partially spatially and temporally coherent,
• the beams of the second beam pair are at least partially spatially and temporally coherent, and
• any beam of the first beam pair is spatially and/or temporally incoherent to any beam of the second beam pair;
• and
• simultaneously generating a first interference pattern resulting from interference of the first scattered radiation and a first reference beam of radiation;
• and a second interference pattern resulting from interference of the second scattered radiation and a second reference beam.