This paper investigates the interest of synchronous pulsed plasmas to etch silicon in HBr/O 2 chemistries. Using mass spectrometry, ellipsometry, and angle resolved X-ray photoelectron spectroscopy, we demonstrate that the radical concentration in the plasma, the etch rate, and formation of passivation layers, can be controlled by the RF pulse parameters (frequency and duty cycle). Accordingly, pattern profiles are improved, selectivity to the mask is increased and the silicon recess is reduced from 4 to 0.8 nm when the HBr/O 2 plasma lands on thin silicon oxide.