A method of depositing a silicon oxide layer over a substrate having a trench formed between adjacent raised surfaces. In one embodiment the silicon oxide layer is formed in a multistep process that includes depositing a first portion of layer over the substrate and within the trench by forming a high density plasma process that has simultaneous deposition and sputtering components from a first process gas comprising a silicon source, an oxygen source and helium and/or molecular hydrogen with highD/S ratio, for example, 10-20 and, thereafter, depositing a second portion of the silicon oxide layer over the substrate and within the trench by forming a high density plasma process that has simultaneous deposition and sputtering components from a second process gas comprising a silicon source, an oxygen source and molecular hydrogen with a lowerD/S ratio of, for example, 3-10.