A new physical model is described for the plasma anodization of Si. The model is constructed from the continuity equation for the charged oxidizing agent Ominus;, with transport by fieldhyphen;imposed drift and by diffusion. It is argued that at constant total current, the field in the oxide layer is constant in space and in time. A loss term for Ominus;ions is also incorporated in the model; the resulting gradual drop of the Ominus;contribution to the total (constant) current at increasing depth into the oxide explains the observed decrease of the oxidation rate with time. The Ominus;loss can occur, i.a., by detachment Ominus;rarr;O+eminus;or by twohyphen;step mechanisms resulting overall in 2Ominus;rarr;O2+2eminus;. The model predicts an exponential decay of Ominus;in the oxide. At constant current the oxide width as a function of time is given byw=Athinsp;ln(1+Bt), whereAis the characteristic penetration distance of Ominus;in the oxide andABis the initial oxide growth rate, determined by the subsurface Ominus;current density. The twohyphen;parameter model provides excellent fits to available experimental data; standard deviations are sim;1percnt; of the final oxide width. From the parameters, numerical values are derived of underlying physical constants. A lower limit is also deduced for the Ominus;loss rate constant.
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