Reaction Mechanism of the Metal Precursor Pulse in Atomic Layer Deposition of Ruthenium and Cobalt and the Role of Surface Facet NH/NH2 Coverage

Ruthenium and Cobalt are potential candidate in replacing copper for interconnects and have been applied in the trenches and vias in semiconductor industry. A non-oxidizing reactant is required in atomic layer deposition (ALD) of thin films of these metals to avoid O-contamination. ALD of Ru and Co has been demonstrated experimentally, but the growth mechanism and key reactions are not clear. In this paper, the reaction mechanism of metal cyclopentadienyl (Cp, C5H5) precursors (RuCp2 and CoCp2) and NHx-terminated metal surfaces (Ru and Co) is studied by density functional theory (DFT) calculations. The Cp ligands are eliminated by CpH formation via a hydrogen transfer step and may desorb from metal surface. The nature of the NHx-termination plays an important role in the reaction energies and barriers as does the surface facet on Ru and Co, with (001) and (100) surfaces showing different reaction energetics. The results show that on the NHx-terminated surfaces corresponding to ALD operating condition (temperature range 550K to 650K), the two Cp ligands can be eliminated completely on both Ru and Co (100) surface during the metal precursor pulse, resulting in Ru or Co atom deposited on the (100) surface. But the second Cp ligand reaction of hydrogen transfer is thermodynamically unfavourable on the (001) surface, resulting in RuCp or CoCp fragment termination on (001) surface, along with the possibility of surface boned CpH. CoCp2 always has lower reaction barriers than RuCp2, regardless of surface facets or NHx coverage. These final structures after metal precursor pulse are essential to model the reaction during the following N-plasma step.