An advanced, time-multiplexed plasma etch process for high aspect ratio structures is presented. Compared to the two pulse Bosch process, the technique consists of a sequence of three pulses. The third pulse is tailored to improved depassivation of the trench bottom prior to each etch pulse. Several depassivation chemistries are explored: O-2, CO2, and SO2. In a further extension the bias voltage is also pulsed, with the aim to balance the radical and ion-enhanced components in the passivation of the sidewalls and trench bottom. The process extensions lead to improved mask selectivity and substantial range for profile control from fully anisotropic,to strongly negatively tapered. The maximum aspect ratio obtained in the Bosch process could not be improved, because the ion angular distribution probably remains the limiting factor. The role of the ions in passivation and etching has been quantified in separate experiments.