Implementations disclosed herein relate to methods for forming and filling trenches in a substrate with a flowable dielectric material. In one implementation, the method includes subjecting a substrate having at least one trench to a deposition process to form a flowable layer over a bottom surface and sidewall surfaces of the trench in a bottom-up fashion until the flowable layer reaches a predetermined deposition thickness, subjecting the flowable layer to a first curing process, the first curing process being a UV curing process, subjecting the UV cured flowable layer to a second curing process, the second curing process being a plasma or plasma-assisted process, and performing sequentially and repeatedly the deposition process, the first curing process, and the second curing process until the plasma cured flowable layer fills the trench and reaches a predetermined height over a top surface of the trench.