Atomic-scale cellular model has been developed to simulate the feature profile evolution during poly-Si gate etching in high-density Cl2 and Cl2/O2 plasmas, with emphasis being placed on the formation of passivation layers on feature surfaces. The model took into account the behavior of Cl+ ions, Cl and O neutrals, and etch products and byproducts of SiClx and SiClxOy in microstructural features. The transport of ions and neutrals in microstructures and in substrates was analyzed by the two-dimensional Monte Carlo calculation with three velocity components. The surface chemistry included ion-enhanced etching, chemical etching, and passivation layer formation through surface oxidation and deposition of etch products and byproducts. The computational domain was taken to consist of two-dimensional square cells or lattices of atomic size, and the evolving interfaces were represented by removing Si atoms from and/or allocating them at the cells concerned. Calculations were performed for different line-and-space pattern features of down to 30 nm space width, with an incoming ion energy, ion flux, and neutral reactant-to-ion flux ratio of Ei=50 eV, Γi0=1.0×1016 cm-2 s-1, and Γn0/Γi0=10. Numerical results reproduced the evolution of feature profiles, critical dimensions, and their microscopic uniformity (or aspect-ratio dependence) on nanometer scale, depending on substrate- - temperature, incoming flux of oxygen and etch byproducts, and sticking probability of etch products and byproducts on feature surfaces: the lateral etching on sidewalls is suppressed by surface oxidation thereon. The oxidation also reduces the etch rate on bottom surfaces, leading to a transition from regular to inverse reactive ion etching (RIE) lag with increasing flux of oxygen; in practice, the RIE lag remains almost unchanged for narrow space features owing to reduced oxygen fluxes thereinto, thus leading to regular and inverse RIE lags coexistent in a series of different pattern features. The deposition or redeposition of etch products (desorbed from feature surfaces) onto sidewalls results in the sidewall tapering, which is more significant for narrower space features; in contrast, the deposition of byproducts (coming from the plasma) onto sidewalls results in the tapering, which is more significant for wider features. Synergistic effects between the deposition of etch products/byproducts and surface oxidation enhance the passivation layer formation on feature surfaces, which in turn increases the sidewall tapering and the degree of regular and inverse RIE lags depending on feature width. The present model also enabled the authors to simulate the surface reaction multilayers and passivation layers on atomic scale, along with their chemical constituents and surface roughness.