Embodiments described herein generally related to methods for forming a flowable low-k dielectric layer over a trench formed on a surface of a patterned substrate. The methods include delivering a silicon and carbon containing precursor into a substrate processing region of a substrate processing chamber for a first period of time and a second period of time, flowing an oxygen-containing precursor into a remote plasma region of a plasma source while igniting a remote plasma to form a radical-oxygen precursor, flowing the radical-oxygen precursor into the substrate processing region at a second flow rate after the first period of time has elapsed and during the second period of time, and exposing the silicon and carbon containing dielectric precursor to electromagnetic radiation for a third period of time after the second period of time has elapsed.