A method is described for filling a high-aspect-ratio feature, in which compatible deposition and etching steps are performed in a sequence. The feature is formed as an opening in a substrate having a surface; a fill material is deposited at the bottom of the feature and on the surface of the substrate; deposition on the surface adjacent the feature causes formation of an overhang structure partially blocking the opening. The fill material is then reacted with a reactant to form a solid reaction product having a greater specific volume than the fill material. The overhang structure is thus converted into a reaction product structure blocking the opening. The reaction product (including the reaction product structure) is then desorbed, thereby exposing unreacted fill material at the bottom of the feature. The depositing and reacting steps may be repeated, with a final depositing step to fill the feature. Each sequence of depositing, reacting and desorbing reduces the aspect ratio of the feature.