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. Use of simple conductance model to predict etch rate of pulsed inductively couple plasma etch process gives good results when applied to linewidths typical in microelectromechanical systems (MEMS) at moderate aspect ratios, but at very high aspect ratio the conductance model does not predict the observed almost complete etch stopping. Other mechanisms are needed to explain it. In this paper the reasons of etch stop at the bottom of deep features are discussed. Measurement results of deep silicon etching are presented. Very deep holes of 30 and 60 µm diameter are etched into 1.3 mm thick silicon wafers to study the effect of process parameters. At moderate aspect ratios the bottom of the hole is nearly flat and side-walls are nearly vertical. At high aspect ratio the side-walls start to bow and the feature bottom turns into sharp spear head like. After long etch time the passivation breaks down near the top of the feature. The shape of the feature can have impact on step-coverage of passivation layer deposition during passivation step and passivation removal during etch step, which can cause the excessive sidewall etching and reduced etch rate at the feature bottom. Simple Monte Carlo calculation results of the effects of non-zero sidewall reaction probability and flow conductance of tapered tubes are presented. Main reason for etch stopping seem to be the loss of etchant species due to sidewall reactions.