A silicon substrate, masked with oxide lines with a spacing of 1 mu m, was etched using a gas-chopping process designed to enhance mask selectivity and produce a highly anisotropic etch profile with ripple-free sidewalls. The gas-chopping process employed a high bias voltage of -200 V and a Faraday cage in the etching step. The use of a high bias voltage relieved the curvature of sidewall ripples that are generated during etch cycles due to the bombardment of energetic ions on the convex portions of the ripples, thus flattening the sidewall surface. The use of a Faraday cage served to suppress the erosion of the mask, thus leading to a high mask selectivity. This can be attributed to an increase in the intrinsic etch selectivity of the Si substrate to the mask due to an increase in the ratio of neutral to ion fluxes on the substrate, and by the suppression of mask faceting due to a reduction in the etch yields of the oxide mask and a CFx film formed on the surface of the mask. (c) 2005 American Vacuum Society.