Methods for fabricating device substrates are provided where the device substrates have rounded trench corners in medium voltage (MV) and high voltage (HV) regions thereof to minimize interference with performance of MV or HV devices adjacent thereto. The fabricating methods involve thermally oxidizing a trench-forming area in an MV or HV region on a semiconductor substrate to form a silicon oxide layer having narrowed birds beak edges that create rounded trench shoulders semiconductor substrate. An isolation trench is then formed through the silicon oxide layer, into the semiconductor substrate, removing portion of the silicon oxide layer and leaving the birds beak edges. After removing the birds beak edges, an oxide layer is formed lining the trench and shoulders to create rounded trench corners in the MV or HV region. Trenches having rounded corners may be formed simultaneously with forming trenches in low voltage regions that don't have rounded trench corners.