In some embodiments, a method of forming a cobalt layer on a substrate disposed in a process chamber, includes: (a) exposing the substrate to a first process gas comprising a cobalt precursor and a hydrogen containing gas to grow a smooth cobalt layer on a first surface of the substrate and on sidewalls and a bottom surface of a feature formed in the first surface of the substrate; (b) purging the first process gas from the process chamber; and (c) annealing the substrate in a hydrogen atmosphere to fill in voids within the cobalt layer to form a void-free cobalt layer. In some embodiments, plasma treating the substrate in gas under low pressure and/or thermally baking the substrate in gas in an atmosphere under a low pressure, may be performed prior to anneal.