Inspection method and apparatus

A non-transitory computer readable medium coupled to a metrology apparatus, and having stored thereon computer-executable instructions, execution of which by a processing device causes the processing device to perform operations associated with the metrology apparatus, comprising:

• defining one or more first target profiles of one or more targets;
• storing initial calibration data of the metrology apparatus;
• compiling a library of modeled spectra that would be observed from inspection of the one or more targets using the metrology apparatus calibrated according to the initial calibration data;
• obtaining current calibration data of the metrology apparatus;
• modeling one or more second target profiles of the one or more targets at least partially based on the current calibration data and the library of spectra;
• determining any differences between one or more parameters of the one or more first target profiles and the one or more second target profiles;
• and
• using any of the determined differences between the one or more parameters to determine a drift in the one or more parameters, such that subsequent measurement stability of the metrology apparatus is improved when subsequently taking measurements of a physical target with the metrology apparatus.

A metrology apparatus comprising:

• a projection system configured to project a beam of radiation onto a target;
• and
• a processing device configured to:
• define a first target profile of the target;
• store initial calibration data of the metrology apparatus;
• compile a library of modeled spectra that would be observed from inspection of the target using the projection system of the metrology apparatus calibrated according to the initial calibration data;
• obtain current calibration data of the metrology apparatus;
• model a second target profile of the target at least partially based on the current calibration data and the library of spectra;
• determine any differences between one or more parameters of the first target profile and the second target profile;
• and
• use any of the determined differences between the one or more parameters to determine a drift in the one or more parameters, such that subsequent measurement stability of the metrology apparatus is improved when subsequently taking measurements of a physical target with the metrology apparatus.