The mechanism for the cavernous undercuts appearing on submicron‐wide Si trench sidewalls etched in Cl2 plasma is investigated. Etching with the inclined Si substrate implies that the ions scattered from the opposing mask edge plane are responsible for the undercut. Angular and energy distributions of ions scattered from the mask edges are simulated with a Monte Carlo method. The number and energies of the scattered ions are calculated as being high enough to etch sidewalls. Etching profile simulation incorporating the ion scattering effect shows good agreement with experimental results concerning the amount and the location of the undercut. These results apparently show that ion scattering at a sloped mask edge is the dominant origin of the cavernous undercut.

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