A gapfill process is provided using cycling of HDP-CVD deposition, etching, and deposition step. The fluent gas during the first deposition step includes an inert gas such as He, but includes H.sub.2 during the remainder deposition step. The higher average molecular weight of the fluent gas during the first deposition step provides some cusping over structures that define the gap to protect them during the etching step. The lower average molecular weight of the fluent gas during the remainder deposition step has reduced sputtering characteristics and is effective at filling the remainder of the gap.