Methods for rounding the bottom corners of a shallow trench isolation structure are described herein. Embodiments of the present invention provide a method comprising forming a first masking layer on a sidewall of an opening in a substrate, removing, to a first depth, a first portion of the substrate at a bottom surface of the opening having the first masking layer therein, forming a second masking layer on the first masking layer in the opening, and removing, to a second depth, a second portion of the substrate at the bottom surface of the opening having the first and second masking layers therein. Other embodiments also are described.