A process for forming a silicon oxide-filled shallow trench on the active surface of a silicon chip starts with forming a trench in the silicon chip that has an upper portion with vertical side walls and a lower portion with tapered side walls. Then oxygen is implanted selectively into the walls of the lower portion of the trench and the chip is heated to react the implanted oxygen with the silicon to form silicon oxide. The rest of the trench is then filled with deposited silicon oxide, typically by depositing a layer of silicon oxide over the surface and then planarizing the deposited silicon oxide essentially to the level of the top of the trench. The silicon-filled shallow trench serves to divide the surface portion of the chip into discrete regions, each for housing one or more circuit components of an integrated circuit.