A method for filling a trench within a silicon substrate. There is first provided a silicon substrate having a trench formed therein. There is then oxidized thermally the silicon substrate to form within the trench a thermal silicon oxide trench liner layer. There is then treated the thermal silicon oxide trench liner layer by exposure to a plasma formed from a gas composition which upon plasma activation simultaneously supplies an active nitrogen containing species and an active oxygen containing species to form a plasma treated thermal silicon oxide trench liner layer. There is then formed upon the plasma treated thermal silicon oxide trench liner layer a conformal silicon oxide intermediate layer formed through a plasma enhanced chemical vapor deposition (PECVD) method employing a silane silicon source material. Finally, there is then formed upon the conformal silicon oxide intermediate layer a gap filling silicon oxide trench fill layer through an ozone assisted sub-atmospheric pressure thermal chemical vapor deposition (SACVD) method employing an ozone oxidant and a tetra-ethyl-ortho-silicate (TEOS) silicon source material.