The etch profile under the mask of various geometry was simulated with Monte Carlo method applied to scattering phenomena of CI⁺ ions incident on tapered SiO₂ mask slope. The influence of mask geometry on the etch profile was analyzed on the basis of the scattered ion distributions. To observe the influence of mask geometry only, we assumed that CI⁺ ions were incident normal to substrate and that etch rate of spontaneous chemical reaction by chemisorbed CI₂ molecules was negligible compared to the ion-assisted etch rate. Similarity was confirmed between the simulated etch profiles dotained in this study and the experimental ones reported in references. Comparison of the simulated etch profiles for various mask geometry showed that the cavernous undercut increased as the opening width and the slope angle of the mask decreased and as the mask thickness increased. These tendencies were the same as those reported in many references. Therefore, a thin mask with high selec- tivity to substrate and with the vertical side wall should be used to avoid the cavernous un- dercut and to get a vertical etch profile.