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In case of plasma etch for hard mask dual-doped polysilicon gate application, interferometry endpoint (IEP) technique provided additional margin for protecting the thin gate dielectric as opposed to optical emission spectroscopy (OES) method. This article proposed a theoretical model to simulate the interferometric signal for the etching process control. A good correlation was found between the fitting data and practical IEP signal. This model would be helpful to analyze the potential incoming variations that might affect the endpoint control. The presence of underlaying field oxide in device wafer could be a dominant factor to shift the IEP curve.