Patent ID: 8710619

Claim:
A semiconductor device, comprising: an SOI substrate having a support substrate, a BOX layer formed over the main surface of the support substrate and made of an insulator, and an active layer formed over the upper surface of the BOX layer; an LOCOS insulating film formed over the upper surface of the active layer and annularly surrounding an element region in a planar view; a deep trench annularly surrounding the element region in a planar view, formed continuously in a portion of the LOCOS insulating film and the active layer therebelow, and reaching the BOX layer; and an insulating film filled in the deep trench, wherein the trench width of the deep trench formed in the portion of the LOCOS insulating film is smaller than a trench width of the deep trench formed in the active layer, wherein the insulating film also functions as an interlayer insulating film covering therewith a semiconductor element formed in the element region, wherein the deep trench defines a hollow portion, wherein, in a cross-section view, a width of the hollow portion is larger at a lower portion of the deep trench than at an upper portion or a middle portion of the deep trench, wherein said interlayer insulating film in the deep trench is disposed in direct contact with the active layer, and wherein the trench width of the deep trench formed in the portion of the LOCOS insulating film is smaller than 1.2 μm.