Patent ID: 8718810

Claim:
A method of performing polishing, comprising: measuring an initial spectrum of light reflected from a substrate; identifying a selected extrema in the initial spectrum and determining an initial wavelength or an initial width of the selected extrema in the initial spectrum; polishing the substrate with a polishing pad; while the substrate is being polished and after measuring the initial spectrum, irradiating the substrate with light from a light source to generate light reflected from the substrate, measuring, using a processor, a current spectrum of the light reflected from the substrate, identifying, using the processor, the selected extrema in the current spectrum and determining a current wavelength or a current width of the selected extrema in the current spectrum, determining, using the processor, a difference between the initial wavelength or initial width and the current wavelength or current width, respectively, determining, using the processor, a value of a parameter, which is associated with the difference from a lookup table; and changing, using a controller, the polishing of the substrate depending upon the parameter.