Methods are described for forming a dielectric layer on a patterned substrate. The methods may include combining a silicon-and-carbon-containing precursor and a radical oxygen precursor in a plasma free substrate processing region within a chemical vapor deposition chamber. The silicon-and-carbon-containing precursor and the radical oxygen precursor react to deposit a flowable silicon-carbon-oxygen layer on the patterned substrate. The resulting film possesses a low wet etch rate ratio relative to thermal silicon oxide and other standard dielectrics.