A process for fabricating silicon oxide filled, shallow trench isolation (STI), regions, in a semiconductor substrate, featuring the use of a disposable boro-phosphosilicate glass (BPSG), layer, used for planarization of various width, silicon oxide filled, STI regions, has been developed. After completely filling all STI shapes with a high density plasma (HDP), silicon oxide layer, resulting in a non-planar, HDP silicon oxide top surface topography, a BPSG layer is deposited. An anneal procedure is then performed resulting in a planar top surface topography of the reflowed BPSG layer. A chemical mechanical polishing procedure is next employed to remove the planar, reflowed BPSG layer, and portions of the underlying HDP silicon oxide, from the top surface of a silicon nitride stop layer, resulting in a planar top surface topography for all silicon oxide filled, insulator regions.