A Monte Carlo method which determines the evolution of a Si etched surface under an SF6/O2 plasma mixture in a high-density low-pressure reactor is presented. The approach consists of a two-dimensional discretization of the initial target (mask and substrate) in a series of same size cells, to determine the substrate time-space evolution according to plasma-surface interactions. One ion transport model through the sheath is introduced to determine the angular and energetic distribution function of ions near the substrate surface. Spontaneous chemical etching by SiF4 desorption and preferential sputtering are studied. Etch profile simulations were performed to interpret the role of certain parameters such as mask geometry and F sticking probability. The simulation results show that the neutral reemission in the trench is implicated in the undercut formation. Moreover, the predicted etch profiles let a bowing appear at the sidewalls. The model reveals that these zones are sputtered by an incident ion flux reflected from the mask sides.