A chemical vapor deposition method for forming a dielectric material in a
trench formed on a substrate. The method includes flowing a
silicon-containing precursor into a process chamber housing the
substrate, flowing an oxidizing gas into the chamber, and providing a
hydroxyl-containing precursor in the process chamber. The method also
includes reacting the silicon-containing precursor, oxidizing gas and
hydroxyl-containing precursor to form the dielectric material in the
trench. The ratio of the silicon-containing precursor to the oxidizing
gas flowed into the chamber is increased over time to alter a rate of
deposition of the dielectric material.