The molecular-flow conductance of a high aspect ratio feature can limit the etching species arriving at the bottom of the feature and thus limit the etch rate. A simple conductance model can predict the etch rate of the time-domain multiplexed etch process with good results at moderate-aspect ratios (5-20) for trenches, but at very high aspect ratios (>20) the conductance model breaks down. Other mechanisms are needed to explain the deceleration of etch rate and almost complete etch stop. In this article the reasons for etch stop at the bottom of deep features are discussed. Measurement results of deep silicon etching are presented. Very deep holes and trenches were etched into silicon to study the effect of process parameters. At moderate aspect ratios the bottom of the hole is nearly flat and the sidewalls are vertical. At high aspect ratio the sidewalls stare to bow and the feature bottom turns into a sharp spearhead. The shape of the feature can have an impact on the step coverage of the passivation layer deposition during the passivation step and on passivation removal during the etch step, leading to excessive sidewall etching and reduced etch rate at the feature bottom. The nonzero sidewall reaction probability and flow conductance of tapered tubes were studied by Monte Carlo simulation. The main reason for deceleration of etching seems to be the loss of etchant species due to sidewall reactions combined with feature closure by ion-limited passivation polymer etching.