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We show that by using various in situ etch process signals (e.g., reflected power, induced direct current bias) as the coefficients in an orthogonal polynomial expansion we can determine endpoint for processes which heretofore have not had strong endpoint signals. The technique essentially amplifies weak signals. We have used the orthogonal expansions and the integral of the expansions to determine endpoint in, among others, via etch processes. The method is reproducible from wafer-to-wafer and can be used by inexperienced operators and simple threshold algorithms to determine endpoint in plasma processes. © 2000 American Vacuum Society.