Metrology method and apparatus, computer program and lithographic system

A method of measuring a parameter of interest relating to a structure having at least two layers, comprising:

• illuminating the structure with measurement radiation and detecting scattered radiation having been scattered by the structure, wherein the scattered radiation comprises normal and complementary higher diffraction orders;
• defining a scatterometry model and an asymmetry model relating to the structure, wherein:
• the scatterometry model relates a scattered radiation parameter to at least a parameter of interest;
• and
• the asymmetry model relates the scattered radiation parameter to at least one asymmetry parameter, the at least one asymmetry parameter relating to one or more measurement system errors and/or an asymmetry in the target other than a misalignment between the two layers;
• using a combination of the scatterometry model and the asymmetry model to determine a system of equations, each equation of the system of equations relating to a different measurement value of a set of measurement values for the scattered radiation parameter, the set of measurement values comprising at least a different measurement value for each of the normal and complementary higher diffraction orders;
• and
• solving the system of equations for the parameter of interest.