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The formation of microtrenches in polysilicon plasma etching over thin gate oxides has been observed and modeled Microtrenches are small trenches formed in the bottom of etching features and are caused by the localized breakthrough of the gate oxide and subsequent rapid etching of the underlying silicon. In contrast to previous reports, the microtrenches were observed a small distance away from the sidewall. Their formation and position were functions of the thickness and angle of the photoresist and polysilicon sidewalls. Simulation of ion scattering from the sidewalls of the etching features indicated that the flux of ions at the bottom of the feature was peaked away from the sidewall under the process conditions of this study. The position of the peak ion flux predicted by the model and the experimentally observed trench varied in a similar fashion as a function of the topography of the etched feature.