Metrology of multi-layer stacks

A system comprising:

• a low coherence light source configured to emit light over a plurality of wavelengths;
• an interferometer objective configured to receive light from the light source, to direct a portion of the light as a reference beam along a reference beam path to a reference surface, to direct another portion of the light as a measurement beam along a measurement beam path to a test sample, and to combine the reference beam and the measurement beam after reflection from the reference surface and the test sample, respectively, into an output beam;
• a detector array arranged to receive the output beam from the interferometer objective and to produce sample interferometry data comprising information about the test sample, the sample interferometry data comprising a plurality of interference signals, each interference signal of the plurality of interference signals corresponding to a different location on the test sample;
• an electronic processor in communication with the detector array, the electronic processor being configured to transform the sample interferometry data to a frequency domain,
• wherein the electronic processor is further configured to identify a non-linear phase variation from the sample interferometry data in the frequency domain, the non-linear phase variation being a result of dispersion introduced into the measurement beam by the test sample, and to remove the identified non-linear phase variation from the sample interferometry data to produce compensated interferometry data.