The effect of adding SF6or C2H6to Cl2plasma to control the pattern profile in poly‐Si etching has been studied using a magnetron reactive ion etching system. It is found that the sidewalls of patterns etched with pure Cl2plasma are either sloped or vertical depending on whether a radio‐frequency (rf) power of 700 W (0.9 W/cm2) or 600 W(0.77 W/cm2) is applied. By increasing the content of SF6the angle of the sidewall taper of the pattern etched at 700 W rf power reaches 90°. Adding C2H6makes it possible to adjust the slope of the sidewall over a range from 77° to 90° at a rf power of 600 W. These observed results are attributed to plasma polymerization.