A method and apparatus for filling one or more gaps created during manufacturing of a feature on a substrate by: providing a bottom area of a surface of the one or more gaps with a first reactant; providing a second reactant to the substrate; and, allowing the first reactant to initiate reaction of the second reactant in the bottom area of the surface in a stoichiometric ratio of one molecule of the first reactant to multiple molecules of the second reactants leaving a top area of the surface of the one or more gaps which was not provided with the first reactant initially substantially empty.