Determining a structural parameter and correcting an asymmetry property

An apparatus comprising:

• an illumination system configured to illuminate a structure, the structure having an asymmetry property and comprising a sub-structure susceptible to process-induced asymmetry, with radiation with a plurality of illumination conditions, at a first location of a substrate;
• and
• a processor configured to:
• determine a difference between measured asymmetry properties of the structure obtained with each of the plurality of illumination conditions;
• calculate a differential dependence of a difference between modeled asymmetry properties simulated for illumination by each of the plurality of illumination conditions on a structural parameter representing process-induced asymmetry of the sub-structure;
• and
• determine a value of the structural parameter using the determined difference and the calculated differential dependence.

A lithographic apparatus comprising:

• a projection system arranged to project a pattern image on to a substrate;
• and
• an inspection apparatus comprising:
• an illumination system configured to illuminate a structure, having an asymmetry property and comprising a sub-structure susceptible to process-induced asymmetry, with radiation with a plurality of illumination conditions, at a first location of a substrate;
• and
• a processor configured to:
• determine a difference between measured asymmetry properties of the structure obtained with each of the plurality of illumination conditions;
• calculate a differential dependence of a difference between modeled asymmetry properties simulated for illumination by each of the plurality of illumination conditions on a structural parameter representing process-induced asymmetry of the sub-structure;
• and
• determine a value of the structural parameter using the determined difference and the calculated differential dependence.