Journal of Applied Physics vol:74 issue:1 pages:639-644
Subsequent to previous work on modeling the plasma anodization of silicon at constant current, a model is presented here for the plasma anodization at constant voltage. The continuity equation for the charged oxidizing agent O- in the SiO2 film was constructed considering O- ion transport by field-imposed drift and a loss process for O- ions by electron detachment. The resulting gradual drop of the O- current density with distance in the SiO2 layer, together with the decrease of the total current as the film grows thicker, explains the decrease of the oxidation rate with time. Fits of the resulting three-parameter model equation to experimental data, from several sources, are excellent, with standard deviations on average < 1% of the final oxide width. Parameter values obtained from different sources for similar experimental conditions are mutually consistent. Useful information can be deduced from the extracted parameters; the ratio of the O- mobility over the O- loss rate coefficient mu/k obtained for constant voltage Si anodization is in good agreement with the mu/k value derived from constant current anodization data-for which a different model equation applies-thus confirming the validity of the underlying physical concepts. Finally, a rationalization is given for the drop-off of the total current density and of the subsurface O- ion current density as the oxide layer becomes thicker.