A method of forming device isolation regions on a trench-formed silicon substrate and removing residual carbon therefrom includes providing a flowable, insulative material constituted by silicon, carbon, nitrogen, hydrogen, oxygen or any combination of two or more thereof; forming a thin insulative layer, by using the flowable, insulative material, in a trench located on a semiconductor substrate wherein the flowable, insulative material forms a conformal coating in a silicon and nitrogen rich condition whereas in a carbon rich condition, the flowable, insulative material vertically grows from the bottom of the trenches; and removing the residual carbon deposits from the flowable, insulative material by multi-step curing, such as O2 thermal annealing, ozone UV curing followed by N2 thermal annealing.