During bottom filling of high aspect ratio gaps and trenches in an integrated circuit substrate using HDP-CVD, a pulsed HF bias is applied to the substrate. In some embodiments, pulsed HF bias is applied to the substrate during etching operations. The pulsed bias typically has a pulse frequency in a range of about from 500 Hz to 20 kHz and a duty cycle in a range of about from 0.1 to 0.95.