Abstract This paper reports on a practical modification of the two-step time-multiplexed plasma etching recipe (also known as the Bosch process) to achieve high aspect-ratio sub-micron wide trenches in silicon. Mixed argon and oxygen plasma depassivation steps are introduced in between the passivation and etching phases to promote the anisotropic removal of the passivation layer at the base of the trench. Argon does not chemically react with polymers and silicon and removes the passivation layer only by physical sputtering. Therefore, it results in a highly anisotropic polymer etching process. This recipe can be easily integrated on conventional ICP equipment and the scalloping on the trench sidewall can potentially be reduced in size to less than 50 nm. To clean up all the passivation residues, a short oxygen plasma step is also added at the end of the cycle that effectively improves the uniformity of the etching profile over various opening sizes. Excellent anisotropy of the inserted argon depassivation step facilitates narrow trenches down to 130 nm wide and gap aspect-ratios as high as 40:1, extending the application of deep reactive ion etching (DRIE) processes into a new broad regime.