Aspects of the disclosure pertain to methods of preferentially filling narrow trenches with silicon oxide while not completely filling wider trenches and/or open areas. In embodiments, dielectric layers are deposited by flowing a silicon-containing precursor and ozone into a processing chamber such that a relatively dense first portion of a silicon oxide layer followed by a more porous (and more rapidly etched) second portion of the silicon oxide layer. Narrow trenches are filled with dense material whereas open areas are covered with a layer of dense material and more porous material. Dielectric material in wider trenches may be removed at this point with a wet etch while the dense material in narrow trenches is retained.