Transport mechanisms of ions and neutrals in high aspect ratio contact holes during low-pressure, high-density plasma etching were investigated by both experiments and numerical simulations. Etching experiments were performed in electron cyclotron resonance plasmas with a C4F8/O-2 gas mixture, together with measurements of plasma parameters. The SiO2 etch rate decreased, and the deposition rate of polymer film during over-etching increased with decreasing hole diameter. The trajectories of ions and neutrals were investigated by numerical simulations that took into account the local charging effect for ions and Knudsen transport for neutrals. The etch rate distribution on the hole bottom was estimated by an etching model that included ion sputtering and ion-assisted etching. A sub-trench profile of the Etch rate was obtained for a low aspect ratio hole, while a flat profile was observed for a high aspect ratio hole. This numerical result was in good agreement with the experimental results. Moreover, the dependence of polymer film thickness on over-etch duration was explained by a deposition model that included both the polymer formation by ion-assisted deposition and polymer removal by oxygen atoms.