Inspection Methods, Inspection Apparatuses, and Lithographic Apparatuses

An inspection apparatus comprising:

• a detector configured to detect radiation reflected from a point on a substrate;
• and
• a processor configured to:
• measure asymmetry of the radiation reflected from the point on the substrate;
• and
• determine the overlay error of the point on the substrate as a function of the measured asymmetry of the radiation reflected from the point on the substrate,
• wherein the function limits an effect of linearity error, and
• wherein the function is determined by fitting a polynomial or a Fourier series to a comparison of measured asymmetry of radiation reflected from each of a plurality of targets comprising a predetermined overlay offset on the substrate to the corresponding predetermined overlay offset of the respective target.

A non-transitory computer readable medium comprising one or more sequences of machine-readable instructions enabling an apparatus to perform operations comprising:

• measuring asymmetry of radiation reflected from each of a plurality of targets on a substrate, each of the plurality of targets comprising a predetermined overlay offset;
• comparing the measured asymmetry of the radiation reflected from each of the plurality of targets to a corresponding predetermined overlay offset of the respective target;
• and
• determining an overlay error of a point on the substrate as a function of measured asymmetry reflected from the point,
• wherein the function is determined by fitting a polynomial or a Fourier series to a comparison of the measured asymmetry of the radiation reflected from each of the plurality of targets to the corresponding predetermined overlay offset of the respective target, and
• wherein the function limits an effect of linearity error.