We have developed a semiempirical feature scale model of Si etching in SF6/O-2/HBr plasma. Surface kinetics are modeled using parameters that describe F-based Si etching in SF6 and SF6/O-2 plasmas and Br-based Si etching in HBr plasma. The kinetic parameters in the model are constrained by matching simulated feature profiles with those experimentally obtained at various feed gas compositions. Excellent agreement between experiments and simulations is obtained. The combined experimental and profile simulation study reveals that the addition of HBr to SF6/O-2 plasmas results in improved sidewall passivation and elimination of the mask undercut. The vertical etch rate increases as a result of F and Br fluxes focusing toward the bottom of the feature by reflections from passivated sidewalls. Addition of SF6 to HBr discharge increases the etch rate through chemical etching that produces volatile SiBr4-xFx etch products and ion-enhanced chemical sputtering of fluorinated and brominated Si surfaces by F-containing ions. (c) 2006 American Vacuum Society.