A method and apparatus for modifying the profile of narrow, high-aspect-ratio gaps on a semiconductor substrate are used to fill the gaps in a void-free manner. Differential heating characteristics of a substrate in a high-density plasma chemical vapor deposition (HDP-CVD) system helps to prevent the gaps from being pinched off before they are filled. The power distribution between coils forming the plasma varies the angular dependence of the sputter etch component of the plasma, and thus may be used to modify the gap profile, independently or in conjunction with differential heating. A heat source may be applied to the backside of a substrate during the concurrent deposition/etch process to further enhance the profile modification characteristics of differential heating.