Patent ID: 7764377

Claim:
A computer implemented method comprising: selecting a reference spectrum, the reference spectrum being a spectrum of white light reflected from a film of interest on a first substrate and that has a thickness greater than a target thickness, the reference spectrum being empirically selected for particular spectrum based endpoint determination logic so that calling endpoint when a second substrate has a thickness within a predetermined threshold of the thickness of the film of interest on the first substrate causes the target thickness to be achieved when a polishing step is concluded on the second substrate by applying the particular spectrum based endpoint logic; obtaining a plurality of current spectra in a sequence, each current spectrum being a spectrum of white light reflected from the film of interest that is on the second substrate and that has a current thickness that is greater than the target thickness, the film of interest on the second substrate being subject to the polishing step, wherein each current spectrum is obtained at a different time; and determining, for the second substrate, whether the endpoint of the polishing step has been achieved, the determining being based on the reference spectrum and the plurality of current spectra and including calculating a scalar difference value between the reference spectrum and each current spectrum of the plurality of current spectra in the sequence to obtain a sequence data points, wherein each data point represents the scalar difference value between the reference spectrum and a single current spectrum, generating a difference trace from the sequence of data points, wherein the sequence of data points forms the difference trace which is a scalar as a function of time and determining whether the difference trace has reached a minimum, wherein the determining step is performed by an endpoint determining computer.