In terms of the growth behaviour, the PE-ALD of metal thin films have two regions: initial nucleation region and steady growth region. In the steady growth region, the surface terminations are NHx-terminated Ru or Co surfaces (x = 1 or 2). Our previous studies have investigated the reaction mechanisms of eliminating Cp ligand on these NHx-terminated Ru or Co surfaces in the metal precursor half cycle and found that the Cp ligand is eliminated via H transfer, CpH formation and desorption. The remaining Cp rings after metal precursor cycle are eliminated at the plasma cycle through reacting with plasma generated •H radicals. This explains the low C impurity in the deposited Co thin films, where most of the Cp ligand is removed via CpH and the decomposition of Cp is not preferred.