Disclosed are methods for fabricating semiconductor devices incorporating a composite trench isolation structure comprising a first oxide pattern, a SOG pattern and a second oxide pattern wherein the oxide patterns enclose the SOG pattern. The methods include the deposition of a first oxide layer and a SOG layer to fill recessed trench regions formed in the substrate. The first oxide layer and the SOG layer are then subjected to a planarization sequence including a CMP process followed by an etchback process to form a composite structure having a substantially flat upper surface that exposes both the oxide and the SOG material. The second oxide layer is then applied and subjected to a similar CMP/etchback sequence to obtain a composite structure having an upper surface that is recessed relative to a plane defined by the surfaces of adjacent active regions.