Nucleation of Co and Ru Precursors on Silicon with Different Surface Terminations: Impact on Nucleation Delay

The early transition metals Ruthenium (Ru) and Cobalt (Co) are of high interest as replacements for Cu in next generation interconnects. Plasma-enhanced atomic layer deposition (PE-ALD) is used to deposit metal thin films in high-aspect ratio structures of vias and trenches in nanoelectronic devices. At the initial stage of deposition, the surface reactions between the precursors and the starting substrate are vital to understand nucleation of the film and optimize the deposition process through minimising so-called nucleation delay in which film growth is only observed after tens to hundreds of ALD cycles. The reported incubation period (or nucleation delay) of Ru ranges from 10 ALD cycles to 500 ALD cycles and the growth-per-cycle (GPC) varies from report to report. No systematic studies on nucleation day of Co PE-ALD were found in the literature. In this study, we use first principles density functional theory (DFT) simulations to investigate the reactions between the precursors RuCp2 and CoCp2 with Si substrates that have different surface terminations to reveal the atomic scale reaction mechanism at the initial stages of metal nucleation. The substrates include (1) H:Si(100), (2) NHx-terminated Si(100), (3) H:SiNx/Si(100). Ligand exchange reaction via H transfer to form CpH on H:Si(100), NHx-terminated Si(100) and H:SiNx/Si(100) surfaces is simulated and shows that pre-treatment with N2/H2 plasma to yield an NHx-terminated Si surface from H:Si(100) can promote the ligand exchange reaction to eliminate Cp ligand for CoCp2. Our DFT results show that the surface reactivity of CoCp2 is highly dependent on substrate surface terminations, which explains why the reported incubation period and GPC vary from report to report. This difference in reactivity at different surface terminations may be useful for selective deposition. For Ru deposition, RuCp2 is not a useful precursor, showing highly endothermic ligand elimination reactions on all studied terminations.