Low-K dielectric gapfill by flowable deposition

Methods are described for forming a flowable low-k dielectric layer on a patterned substrate. The film may be a silicon-carbon-oxygen (Si-C-O) layer in which the silicon and carbon constituents come from a silicon and carbon containing precursor while the oxygen may come from an oxygen-containing precursor activated in a remote plasma region. A similarly deposited silicon oxide layer may be deposited first to improve the gapfill capabilities. Alternatively, or in combination, the flow of a silicon-and-carbon-containing precursor may be reduced during deposition to change the properties from low-k to high strength roughly following the filling of features of the patterned substrate.