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

Early transition metals ruthenium (Ru) and cobalt (Co)are of highinterest as replacements for Cu in next-generation interconnects.Plasma-enhanced atomic layer deposition (PE-ALD) is used to depositmetal thin films in high-aspect-ratio structures of vias and trenchesin nanoelectronic devices. At the initial stage of deposition, thesurface reactions between the precursors and the starting substrateare vital to understand the nucleation of the film and optimize thedeposition process by minimizing the so-called nucleation delay inwhich film growth is only observed after tens to hundreds of ALD cycles.The reported nucleation delay of Ru ranges from 10 ALD cycles to 500ALD cycles, and the growth-per-cycle (GPC) varies from report to report.No systematic studies on nucleation delay of Co PE-ALD are found inthe literature. In this study, we use first principles density functionaltheory (DFT) simulations to investigate the reactions between precursorsRuCp(2) and CoCp2 with Si substrates that havedifferent surface terminations to reveal the atomic-scale reactionmechanism at the initial stages of metal nucleation. The substratesinclude (1) H:Si(100), (2) NH (x) -terminatedSi(100), and (3) H:SiN (x) /Si(100). The ligandexchange reaction via H transfer to form CpH on H:Si(100), NH (x) -terminated Si(100), and H:SiN (x) /Si(100) surfaces is simulated and shows that pretreatmentwith N-2/H-2 plasma to yield an NH (x) -terminated Si surface from H:Si(100) can promotethe ligand exchange reaction to eliminate the Cp ligand for CoCp2. Our DFT results show that the surface reactivity of CoCp2 is highly dependent on substrate surface terminations, whichexplains why the reported nucleation delay and GPC vary from reportto report. This difference in reactivity at different surface terminationsmay be useful for selective deposition. For Ru deposition, RuCp2 is not a useful precursor, showing highly endothermic ligandelimination reactions on all studied terminations.

Automated summary

In this study, first principles density functional theory simulations were used to investigate the atomic-scale reaction mechanism between RuCp2 and CoCp2 precursors and Si substrates with different surface terminations. The results showed that the surface reactivity of CoCp2 is highly dependent on substrate surface terminations, which explains the variations in nucleation delay and growth-per-cycle reported in different studies. On the other hand, RuCp2 is not a suitable precursor for Ru deposition due to highly exothermic ligand elimination reactions on all studied surface terminations.