Metrology apparatus and metrology methods based on high harmonic generation from a diffractive structure

A method for measurement of a diffractive structure on a substrate, comprising:

• exciting the diffractive structure with first radiation to generate high harmonic second radiation from the diffractive structure and/or substrate;
• and
• detecting the second radiation, at least a portion of that having been diffracted by the diffractive structure to obtain a measured data,
• wherein the measured data corresponding to the second radiation is used to determine a parameter of the diffractive structure or to control a lithography process.

A lithographic cell comprising a metrology apparatus for measurement of a diffractive structure on a substrate for control of a lithography process, comprising:

• a radiation source configured to provide first radiation for excitation of the diffractive structure such that high harmonic second radiation is generated from the diffractive structure and/or substrate;
• a detection system configured to detect the second radiation, at least a portion of that having been diffracted by the diffractive structure;
• and
• a processor configured to use the second radiation to determine a parameter of the diffractive structure or a parameter of the lithographic process.