A micromechanical structure is described. A region of semiconductor material has a first surface, a second surface opposite to the first surface, and a lateral surface that surrounds the region of semiconductor material. Insulative material covers the first surface and the lateral surface of the region of semiconductor material to provide electrical isolation to the region of semiconductor material by forming a boundary. To form the micromechanical structure, a trench is etched in a semiconductor substrate to surround a region of the semiconductor substrate. A surface of the semiconductor substrate and the trench are oxidized to form a top oxide and a lateral oxide region. A backside of the semiconductor substrate is etched to expose a backside of the region of the semiconductor substrate and a portion of the lateral oxide.