Feature profiles of poly-Si etched in Cl2/O2 plasmas have been analyzed through a mechanistic comparison between experiments and simulations. The emphasis was placed on a comprehensive understanding of the formation mechanisms for profile anomalies of tapering, microtrenching, and footing (or corner rounding near the feature bottom). Experiments were conducted in a commercial etching reactor with ultra-high-frequency plasmas by varying O2 percentage, wafer stage temperature, rf bias power, and feed gas pressure. Simulations of the feature profile evolution were done by using a semiempirical, atomic-scale cellular model based on the Monte Carlo method that we have developed. The experiments indicated that sidewall profiles become more tapered with increasing O2 addition to Cl2 plasmas, while microtrenching and footing are pronounced in pure Cl2 plasma, being suppressed with increasing O2. A comparison with the simulations indicated that the tapered profiles are caused by the deposition of etch products/by-products on feature sidewalls from the plasma, being enhanced with increasing oxygen flux (due to synergistic effects between deposition of products/by-products and surface oxidation) and being reduced with increasing ion energy and neutral reactant flux. On the other hand, the footing is attributed to the redeposition of etch products on sidewalls from the feature bottom being etched, being reduced with increasing oxygen flux, ion energy, and neutral reactant flux. Microtrenching is caused by the ion reflection from feature sidewalls on incidence, being reduced with increasing oxygen flux (partly due to surface oxidation of the feature bottom) and being enhanced and then reduced with increasing ion energy and neutral reactant flux. The tapering, footing, and microtrenching were found to be closely related to each other: the footing near the feature bottom fades away under conditions of increased tapering of sidewalls, and the microtrenching is affected significantly by the degree of footing as well as the taper angle of the sidewalls.