A semiconductor device manufacturing method is presented. The method entails providing a semiconductor structure comprising a substrate, one or more semiconductor fins on the substrate, and a trench isolation structure around each semiconductor fin, wherein the trench isolation structure comprises a first component intersecting an extension direction of the semiconductor fin and a second component parallel with the extension direction; etching the trench isolation structure to expose a portion of the semiconductor fin; forming a patterned buffer layer on the semiconductor structure covering the second component and having an opening exposing the first component; forming an insulation layer in the opening, with upper surfaces of the insulation layer and the semiconductor fin substantially on the same horizontal level; and removing the buffer layer. This inventive concept reduces, if not eliminates, oxide loss in Single Diffusion Break (SDB) region.