Method for focus metrology and associated apparatuses

A method comprising:

• obtaining a first measurement signal relating to measurement of a first sub-target of a target on a substrate, the first sub-target comprising an isofocal structure;
• obtaining a second measurement signal relating to measurement of a second sub-target of the target, the second sub-target comprising a non-isofocal structure;
• obtaining at least one trained relationship and/or model that relates at least the second measurement signal to the focus parameter;
• and
• determining a value for a focus parameter relating to a focus setting of a lithographic exposure process in which the target was exposed, the determining being based on the first measurement signal, second measurement signal and the at least one trained relationship and/or model.

A patterning device comprising:

• target patterning features configured to form a target, the target patterning features comprising:
• isofocal patterning features configured to form a first sub-target such that it comprises an isofocal structure;
• and
• non-isofocal patterning features configured to form a second sub-target such that it comprises a non-isofocal structure.