Self-limiting nitrogen/hydrogen plasma radical chemistry in plasma-enhanced atomic layer deposition of cobalt

Cobalt (Co) is a potential candidate in replacing copper for interconnects and has been applied in trenches in the semiconductor industry for over twenty years. A non-oxidizing reactant is required in the plasma-enhanced atomic layer deposition (PE-ALD) of thin films of metals to avoid O-contamination. PE-ALD of Co has been demonstrated experimentally with plasma sources of NH₃ or a mixture of N₂ and H₂, but the growth mechanism and key reactions are not clear. In this study, we have investigated the reactions of plasma-generated predominant species, i.e. radicals ˙H, ˙N, ˙NH and ˙NH₂, at metal precursor (CoCp₂) treated Co(001) and Co(100) surfaces using static DFT calculations at 0 K and molecular dynamics simulations at 600 K. The proposed reaction mechanisms are (1) ˙N radicals play an important role in eliminating the surface-bound Cp ligand (if any) via pyridine (C₅H₅N) formation and desorption, whereas ˙H radicals have endothermic reactions for eliminating the Cp ligand via CpH formation and desorption; (2) the surface NH species are eliminated by ˙H radicals via NH₃ formation and desorption. The simulations of these key reactions show that on the Co(001) surface, the remaining Cp ligand and surface NH species after the metal precursor pulse will be completely removed with ˙N and ˙H radicals, resulting in Co atoms deposited on the Co(001) surface at a coverage of 3.03 Co nm^-2. However, on the Co(100) surface, the surface NH₂ species cannot be completely removed via NH₃ formation and desorption due to overall endothermic reactions. Instead, ˙H radicals react with trench N species, resulting from H transfer in the metal precursor pulse, to form NH. These trench N species cannot be eliminated completely on the Co(100) surface, which will be the source of N impurities in the deposited Co thin films. At the post-plasma stage, the metal surface will be covered with NH-terminations with plasma generated ˙NH radicals and is then ready for the next deposition cycle. Our DFT results highlight and explain why ammonia or H₂/N₂ plasma, which produce NH species, are required to deposit high-quality and low-impurity Co thin films using Co metallocene precursors.