We have studied the etching of very high aspect ratio contact holes in hydrocarbon materials SiLK(TM), potential dielectric candidates for the next generation of interconnections. During the etch process, slight deformation of the etch profiles such as bowing are observed. Experiments described in this article suggest that bowing originates from the deflection of ions on the sidewalls of the polymer, generating some etching. The mechanisms leading to the ion deflection on the sidewalls are presented and discussed. This study also shows how the process can be tuned to minimize the bow formation.