A method for filling a trench within a substrate. First a substrate is provided having a trench formed therein. The trench has a bottom surface and opposing side walls. An undoped silicon glass liner is then thermally grown to coat the bottom surface and side walls of the trench. An undoped silicon oxide layer is then deposited over the undoped silicon glass liner. A boron doped silicon oxide layer is then deposited over the undoped silicon oxide layer, filling the trench. The boron doped silicon oxide layer is then heated to reflow the boron doped silicon oxide to fill any void initially formed within the boron doped silicon oxide layer within the trench, thereby eliminating any void so formed.