Silicon oxide is deposited with improved step coverage by first exposing a patterned substrate to a silicon-containing precursor and then to an oxygen-containing precursor or vice versa. Plasma excitation is used for both precursors. Exposing the precursors one-at-a-time avoids disproportionate deposition of silicon oxide near the opening of a high aspect ratio gap on a patterned substrate. The plasma-excited precursors exhibit a lower sticking coefficient and/or higher surface diffusion rate in regions already adsorbed and therefore end up depositing silicon oxide deep within the high aspect ratio gap to achieve the improvement in step coverage.