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

Abstract

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_x 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_x 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_x-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_x species, are required to deposit high-quality and low-impurity Co thin films using Co metallocene precursors.