Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. Ruthenium (Ru) and Cobalt (Co) are used as seed layers for metallization of interconnects. They are also potential materials for the electrode in dynamic random-access memory (DRAM) capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs). Plasma-enhanced ALD (PE-ALD) is used for low-temperature thin film growth by alternating exposures of metal precursors and plasma reactants. During the PE-ALD growth of metals, N-plasma, for example, NH₃ or a mixture of N₂ and H₂, has been applied to avoid surface metal oxidation. The PE-ALD of Ru and Co has been experimentally investigated using metal precursors such as RuCp₂, Ru(EtCp)₂, CoCp₂, and CoCp(CO)₂. However, the reaction mechanism is not clear and theoretical studies on the reaction mechanism is entirely lacking.

In this presentation, we study the PE-ALD growth of Co and Ru by first principle calculations. We first addressed the surface reaction mechanism at the metal precursor pulse and plasma half-cycle on NH_x-terminated Ru and Co surfaces, which corresponds to the steady growth for the PE-ALD. The (001) surface of both metals, with a hexagonal structure, is the most stable and the (100) surface with a zigzag structure is less stable but has high reactivity. These two surfaces allow the study of the influence of the surface facet. The surface saturation coverage was studied by considering individual adsorption and co-adsorption of NH and NH_2­ to terminate both surfaces. The results are then analyzed with ab initio thermodynamics by calculating the Gibbs free energy. Both the ultra-high vacuum (UHV) condition and standard ALD operating condition are used to elucidate the effect of pressure and temperature on the termination of metal surfaces.

The adsorption and reactions of metal precursors (CoCp₂ and RuCp₂) on NH_x terminated metal surfaces were investigated with the inclusion of van der Waals corrections. The plausible reaction pathways include: precursor adsorption, hydrogen transfer, CpH formation and CpH desorption. The direct Cp dissociation mechanism is not considered on these NH_x-terminated metal surfaces due to experimentally observed minimal C impurities at the deposited metal thin films, which indicates that most of the Cp ligand is removed completely. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method. Our results show that (100) surface has higher activity than (001) surface. In addition, the Cp ligand elimination of CoCp₂ has lower barrier than RuCp₂, regardless of surface facet.

At the plasma cycle, we have modelled the reactions of plasma generated radicals ^.N, ^.H, ^.NH, and ^.NH₂ and metal precursor treated NH_x-terminated metal surfaces. We investigated the plausible pathways of eliminating Cp ligands (if any) and surface NH_x species via CpH formation or NH₃ formation. At the post-plasma stage, we applied molecular dynamics (MD) calculations to model the regeneration of surface NH_x species on bare metal surfaces, which is then ready for the next deposition cycle. The reactions at the initial stages on typical H:Si(100) surface are investigated to gain atomic insight on the effect of different substrates on the elimination of Cp ligands. Our work is important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.