A method for forming a buried metal line in a substrate includes forming, at a position between a pair of semiconductor structures protruding from the substrate, a metal line trench in the substrate at a level below a base of each semiconductor structure of the pair. Forming the metal line trench includes etching an upper trench portion in the substrate, forming a spacer on sidewall surfaces of the upper trench portion that expose a bottom surface of the upper trench portion, and, while the spacer masks the sidewall surfaces, etching a lower trench portion by etching the substrate via the upper trench portion such that a width of the lower trench portion exceeds a width of the upper trench portion. The method further includes forming the metal line in the metal line trench.