Methods and apparatus for filling gaps on partially manufactured semiconductor substrates with dielectric material are provided. In certain embodiments, the methods include introducing a first process gas into the processing chamber and accumulating a second process gas in an accumulator maintained at a pressure level substantially highest than that of the processing chamber pressure level. The second process gas is then rapidly introduced from the accumulator into the processing chamber. An excess amount of the second process gas may be provided in the processing chamber during the introduction of the second process gas. Flowable silicon-containing films forms on a surface of the substrate to at least partially fill the gaps.