A method and apparatus are disclosed for depositing a dielectric film in a gap having an aspect ratio at least as large as 6:1. By cycling the gas chemistry of a high-density-plasma chemical-vapor-deposition system between deposition and etching conditions, the gap may be substantially 100% filled. Such filling is achieved by adjusting the flow rates of the precursor gases such that the deposition to sputtering ratio during the deposition phases is within certain predetermined limits.