Patent ID: 8569174

Claim:
A non-transitory computer readable storage medium comprising a computer program product that is operable to cause a data processing apparatus to perform operations comprising: receiving a first spectrum measured from a substrate undergoing polishing during a first scan of an optical monitoring module across the substrate; comparing the first spectrum to at least some of a plurality of reference spectra in a library to determine a best matching first matching spectrum from the at least some of the plurality of reference spectra, each reference spectrum of the plurality of reference spectra in the library having an associated index number; determining a first index number for the first matching spectrum, wherein the first index number is N; receiving a second spectrum measured from the substrate undergoing polishing during a subsequent second scan of the optical monitoring module across the substrate, wherein the second scan is X scans after the first scan; comparing the second spectrum to a subset of the plurality of reference spectra in the library to determine a best matching second matching spectrum from the subset, the subset including at least two spectra and less than all of the reference spectra in the library, wherein Y degrees of freedom is a predetermined number and the subset of the plurality of reference spectra in the library are reference spectra having index numbers within (N+X)±Y; and determining a second index number for the second matching spectrum.