Patent ID: 7638398

Claim:
A method for fabricating a semiconductor device, the method comprising: forming first and second trenches in a substrate, the trenches being closer in a minor axial direction than in a major axial direction; etching the substrate underneath the first and second trenches to form a first recess connecting the first and second trenches, wherein etching the substrate underneath the first and second trenches comprises: forming spacers on sidewalls of the first and second trenches, and isotropically etching the substrate underneath the first and second trenches using the spacers as an etch mask to form the first recess, wherein the first recess is under-cut from bottom portions of the first and second trenches in the major axial direction such that a pillar of the substrate remains in a center of the first and second trenches, the first recess extending laterally underneath the first and second trenches; forming an isolation structure filling the first recess and the first and second trenches; etching a portion of the isolation structure to form a second recess that defines first, second, third and fourth surfaces of an active region, the active region being shorter in the minor axial direction than in the major axial direction; forming a gate insulation layer over the first, second, third, and fourth surfaces of the active region; and forming a gate electrode around the first, second, third, and fourth surfaces of the active region.