Methods and apparatus for depositing a cobalt layer in a feature, such as, a word line formed in a substrate, are provided herein. In some embodiments, method of processing a substrate includes: exposing a substrate at a first temperature to a cobalt containing precursor to deposit a cobalt layer within a word line feature formed in the substrate, wherein the word line feature is part of a 3D NAND device; and annealing the substrate to remove contaminants from the cobalt layer and to reflow the cobalt layer into the word line feature, wherein the substrate is at a second temperature greater than the first temperature during the annealing.