Metrology method and apparatus and associated computer product

A metrology apparatus comprising:

• an illumination system configured to illuminate one or more targets formed on a substrate by a lithographic process with measurement radiation comprising a plurality of measurement profiles;
• a detection system configured to detect scattered radiation arising from illumination of the one or more targets;
• and
• a processor operable to:
• derive measured target response sequence data from the detected scattered radiation, wherein the measured target response sequence data describes a variation of a measurement response of the one or more targets in response to variations of the plurality of measurement profiles;
• compare the measured target response sequence data to reference target response sequence data relating to a measurement response of the one or more targets as designed to the measurement radiation, wherein the reference target response sequence data describes an optimal measurement response of the one or more targets in response to a designed plurality of measurement profiles without un-designed variation;
• and
• determine a measurement validity based on the comparison of the measured target response sequence data and reference target response sequence data.

A non-transitory computer program product comprising machine readable instructions which, when run on a suitable processor of a lithographic system, cause the processor to perform a method of metrology in a metrology apparatus for monitoring a measurement process, the method comprising:

• obtaining measured target response sequence data relating to a measurement response of one or more targets formed on a substrate by a lithographic process to measurement radiation comprising a plurality of measurement profiles, wherein the measured target response sequence data describes a variation of the measurement response of the one or more targets in response to variations of the plurality of measurement profiles;
• obtaining reference target response sequence data relating to a measurement response of the one or more targets as designed to the measurement radiation, wherein the reference target response sequence data describes an optimal measurement response of the one or more targets in response to a designed plurality of measurement profiles without un-designed variation;
• comparing the measured target response sequence data and reference target response sequence data;
• and
• determining a measurement validity based on the comparison of the measured target response sequence data and reference target response sequence data.