Embodiments of the present invention generally relate to methods for forming a flowable carbon-containing film on a substrate. In one embodiment, an oxygen-containing gas is flowed into a remote plasma region to produce oxygen-containing plasma effluents, and a carbon-containing gas is combined with the oxygen-containing plasma effluents in a substrate processing region which contains the substrate. A carbon-containing film is formed in trenches which are formed on the substrate and a low K dielectric material is deposited on the carbon-containing film in the trenches. The carbon-containing film is decomposed by an UV treatment and airgaps are formed in the trenches under the low K dielectric material.