Metrology method and apparatus, computer program and lithographic system

A method of selecting a target from a plurality of candidate targets, the method comprising:

• obtaining a plurality of sets of measurements using a plurality of candidate measurement radiations and the plurality of candidate targets, wherein:
• each candidate target comprises a first structure overlaid with a second structure,
• each set of measurements corresponds to a different combination of one of the plurality of candidate targets and one of the plurality of candidate measurement radiations, and
• each set of measurements comprises:
• a plurality of first measurements comprising measurements of structural asymmetry of the first structure of the candidate target using the plurality of candidate measurement radiations;
• and
• a plurality of second measurements comprising measurements of target asymmetry of the candidate target using the plurality of candidate measurement radiations;
• for each set of measurements, determining a degree of correlation between the plurality of first measurements and the plurality of second measurements;
• selecting the target from the plurality of candidate targets based upon the determined degree of correlation for each set of measurements.

A non-transitory computer program product comprising machine-readable instructions, when run on a processor controlled apparatus, cause the processor controlled apparatus to perform a method, comprising:

• obtaining a plurality of first measurements, wherein the plurality of first measurements comprise measurements of structural asymmetry of a plurality of first structures and the plurality of first measurements are obtained using a plurality of measurement radiations;
• obtaining a plurality of second measurements, wherein:
• the plurality of second measurements comprise measurements of target asymmetry of a plurality of targets,
• each of the plurality of second measurements corresponds to one of the plurality of first measurements,
• each of the plurality of targets comprises one of the plurality of first structures and a second structure overlaid thereon, and
• the measurements of the target asymmetry comprise a first part which is independent of the structural asymmetry and a second part that is due to the structural asymmetry;
• determining a relationship function describing a relationship between the plurality of first measurements and the plurality of second measurements;
• and
• determining, using the relationship function, a corrected overlay value, the corrected overlay value being based on the measurements of the target asymmetry corrected for the structural asymmetry.