Patent ID: 8369978

Claim:
A computer-implemented method comprising: receiving a sequence of current spectra of reflected light from a substrate; comparing each current spectrum from the sequence of current spectra to a first plurality of reference spectra from a first reference spectra library, and for each current spectrum from the sequence of spectra determining a first best-match spectrum from the first plurality of reference spectra to generate a first sequence of best-match reference spectra; comparing each current spectrum from the sequence of current spectra to a second plurality of reference spectra from a second reference spectra library, and for each current spectrum from the sequence of spectra determining a second best-match spectrum from the second plurality of reference spectra to generate a second sequence of best-match reference spectra; determining a first value associated with each first best-match reference spectrum to generate a first sequence of first values; determining a second value associated with each second best-match reference spectrum to generate a second sequence of second values; fitting a first function to the first sequence of first values; fitting a second function to the second sequence of second values; determining a first goodness of fit of the first function to the first sequence of first values; and determining a second goodness of fit of the second function to the second sequence of second values; comparing at least one of the first goodness of fit and the second goodness of fit to a threshold value; and determining at least one of whether to adjust a polishing rate or an adjustment for the polishing rate, based on the comparing of the at least one of the first goodness of fit and the second goodness of fit to the threshold value.