The present invention provides a method of depositing a carbon doped silicon oxide film having a low dielectric constant (k). A process gas mixture containing at least a carrier gas, an oxidizer, a carbon gas source, or combinations thereof, is supplied adjacent an edge of a substrate though a purge gas inlet in a substrate support to facilitate deposition of low k carbon doped silicon oxide film having a greater concentration of silicon oxide around the edge of the substrate than an inner portion of the substrate.