Formation of vertical‐walled, high‐aspect‐ratio, narrow trenches in silicon is often required in submicron VLSI semiconductor processing. Present trench etching processes suffer from undercutting of the mask and sloped or bulged sidewalls; and when using chlorine‐based reactive ion etching (RIE), there is frequently enhanced etching at the bottom corners of the trench. We report here the effects of chromium on the RIE of silicon. We found that, compared to using a SiO2 mask only, using a mask consisting of a chromium layer on top of SiO2, in a Cl2:SiCl4:N2 etching gas mixture, improves the trench sidewall angle relative to the vertical from 8° to 2°, and almost eliminates the enhanced etching at the bottom corners. Furthermore, we found that, by etching a sample partially covered with chromium in a Cl2:SiCl4:N2 gas mixture, the trench shape in the areas not covered with chromium depends on how far the trench is from the edge of the chromium layer. Those areas, within 1 cm of the edge of the chromium layer, showed quite steep sidewall angles that degraded (to 8°) at greater distances (few cm). Similarly, the size of the grooves due to the enhanced etching at the bottom corners is reduced as the trench gets closer to the edge of the chromium layer. It appears that the beneficial effect of the chromium is an indirect chemical action, based on the suppression of the concentration of neutrals or the formation of an inhibitor layer, rather than direct physical masking.