The kinetics of high aspect ratio, anisotropic silicon etching in a SF6–O2 plasma is investigated with a combination of Monte Carlo simulations and inductively coupled plasma etching experiments. The spontaneous reaction of atomic fluorine is dominant at room temperature and Knudsen transport of the radicals is the only limitation in narrow structures. At low temperatures (typically between -125 and -95 °C) oxygen passivation becomes effective and anisotropic profiles are obtained because the oxygen passivation can only be removed by the directional ion bombardment. The input parameter settings for the Monte Carlo model are based on measurements with plasma diagnostics. Simulations show that anisotropy is controlled by the oxygen sidewall passivation which depends on the oxygen flux, the oxygen adsorption coefficient, and the aspect ratio. The simulated trench profiles and the aspect ratio dependent etch rate are consistent with the experimental results. Experimentally the etch rate behavior can be tuned from aspect ratio dependent to aspect ratio independent by decreasing the ion flux. This effect can be described well by the recently developed chemically enhanced ion-neutral synergy model. It turns out that aspect ratio independent etching is obtained if the downwards depletion of fluorine radicals due to Knudsen transport is compensated by an increase of the available reaction sites. © 2003 American Institute of Physics.