Patent ID: 8860932

Claim:
A method of polishing, comprising: receiving an identification of a selected spectral feature and a spectral characteristic of the selected spectral feature to monitor during polishing; polishing a substrate having a second layer overlying a first layer; measuring a sequence of groups of spectra of light from the substrate while the substrate is being polished, each group of the groups of spectra including spectra from different locations on the substrate; for each group, calculating a value for a dispersion parameter of the spectra in the group to generate a sequence of dispersion values, the dispersion parameter being a measure of variability of the spectra in the group; detecting exposure of the first layer based on the sequence of dispersion values; after detecting exposure of the first layer, measuring a sequence of spectra of light from the substrate while the substrate is being polished; determining a first value of the spectral characteristic of the feature in a spectrum from the sequence of groups of spectra at a time that exposure of the first layer is detected; adding an offset to the first value to generate a second value; and monitoring the spectral characteristic of the feature in the sequence of spectra and halting polishing when the spectral characteristic of the feature is determined to reach the second value.