Methods and systems are provided for depositing silicon oxide in a gap on a substrate. The silicon oxide is formed by flowing a process gas into a process chamber and forming a plasma having an overall ion density of at least 10¹¹ ions/cm³. The process gas includes H₂, a silicon source, and an oxidizing gas reactant, and deposition into the gap is achieved using a process that has simultaneous deposition and sputtering components. The probability of forming a void is reduced by ensuring that the plasma has a greater density of ions having a single oxygen atom than a density of ions having more than one oxygen atom.