Methods are disclosed for the formation of isolation structures and trenches in semiconductor devices, in which lower corners of an isolation trench are rounded after trench formation using an oxidation process which oxidizes substrate material from the trench sidewalls and bottom faster than from the lower corners of the trench. The oxide formed during the rounding process is then removed prior to performing other etch processes, to expose substrate material having rounded lower corners. Thereafter, a liner is formed and the trench is filled with dielectric material to complete the isolation structure.