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 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 the nucleation of the film and optimize the deposition process by minimizing the so-called nucleation delay in which film growth is only observed after tens to hundreds of ALD cycles. The reported 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 delay of Co PE-ALD are found in the literature. In this study, we use first principles density functional theory (DFT) simulations to investigate the reactions between 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), and (3) H:SiNx/Si(100). The 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 pretreatment with N2/H2 plasma to yield an NHx-terminated Si surface from H:Si(100) can promote the 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, which explains why the reported nucleation delay 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.


A. Reaction mechanism of RuCp2 and CoCp2 on bare Si substrate
On a bare Si(100) surface, the reaction mechanism involves metal-carbon bond breaking yielding an adsorbed metal atom and two adsorbed Cp rings. The configurations along the reaction pathways are shown in Figure S1. We see that the metal-C bonds are first partially broken, resulting in the formation of metal-Si bond with surface Si atom. After the metal-C bonds are completely broken, the reactions of direct dissociation mechanism on bare Si(100) surface are overall exothermic, with computed reaction energies at -6.80eV for CoCp2 and -7.39eV for RuCp2. Figure S1. The plotted reaction pathway of direct dissociation of MCp2 (M=Ru or Co) on bare Si(100) surface. Si, C and H atoms are represented by dark yellow, black and white colors, respectively. Ru and Co atoms are represented by green and orange colors.

B. Generation of surface NHx-terminations with active plasma radicals
Before generating the NHx-terminations, we first show the structures of bare Si and H-terminated Si(00) in Figure  S2. A full monolayer (ML) coverage of hydrogen, i.e. 4 H atoms for (2×2), is placed on top of surface Si atoms to form hydrogen terminations. After hydrogen passivation, the surface Si atoms have formed the well-known Si-Si dimer with bond length at 2.41Å, while for bare Si(100), the Si-Si distances of surface Si atoms are at 3.84Å.

S3
These NHx-terminations play an important role in the reaction mechanism of Cp ligand elimination on NHxterminated Ru and Co surfaces as analysed in our previous studies 1-2 and in previous experimental studies [3][4] . Essentially, we exchange the Si-H termination with Si-NHx (x = 1 or 2) terminations. We first address the removal of surface H terminations on H-Si(100) surface. A (2×2) supercell is applied to study the removal of surface H terminations. Surface bound H species can be removed by reacting with H * radical from the plasma and forming H2 which desorbs. Another possibility is that surface H reacts with NH2 * radicals from the plasma, resulting in formation and desorption of NH3. Further, NHx radicals can then terminate the Si(100) surface. As analysed in our recent work on the reaction mechanism the plasma cycle 5 , we consider the plasma generated radicals •H, •N, •NH and •NH2 as the predominant species in the ALD chamber. Simulating the charged ions and molecules will be performed in a follow-up work and is excluded in this current paper.
To examine these reactions, one •H radical is placed successively near one surface H atom with initial distance at 1.2Å for •H radical and substrate H. The plotted reaction pathways for removal of surface H species with •H radical is shown in Figure S3. The configurations along the pathway is shown in Figure S4. The overall reaction energy for successive H2 formation and desorption with plasma •H radicals is endothermic, suggesting that this process is thermodynamically unfavourable. At each H2 formation step, the energy cost is around 1.2eV per step.

Elimination of surface H terminations with plasma generated H radicals
Additionally, one •NH2 radical is placed successively near one surface H atom with initial distances at 1.5Å for N of •NH2 radical and substrate H termination. The plotted reaction pathways for removal of surface H species with •NH2 radical is shown in Figure S5. The configurations along the pathway is shown in Figure S6.  The reaction energy for NH3 formation and desorption using the reaction of •NH2 radicals with surface bound hydrogen is negative, with the desorption energies of NH3 more than compensated by the energy gain from the NH2(g) + H(s) reaction. At each NH3 formation step, the energy gain is around -2.5eV per step. We can therefore infer that the preferred elimination mechanism of surface hydrogen from Si(100) is the reaction with plasma generated •NH2 radicals via NH3 formation and desorption.

Structure and stability of surface NH/NH2 terminations on bare Si(100) surface
After complete removal of surface H species, the resulting bare Si(100) surface can then be terminated with plasma •NH and •NH2 radicals to produce NHx-terminations after the plasma cycle, which is similar to the NHx-terminated Ru and Co surfaces after the plasma cycle in our previous studies.
We now address the structure of preferred NHx-terminations on Si(100) surface. A (2×2) supercell is applied to study the NHx-terminations. We first investigated the case of terminating the Si(100) surface with a single NH or NH2 species. The energy of NH and NH2 uses N2 and H2 as references, as in our previous studies of NHxterminations on Ru and Co surfaces. 6 The adsorption sites are illustrated in Figure S7 and the results are summarized in Table S1 and we see that NH prefers bridge(II) site and NH2 prefers top site. When contributing to the Cp ligand elimination via H transfer, these two types of H, i.e. bridge H from NH and surface H from NH2, are both considered and analysed.  We then considered the mixed terminations of NH and NH2 on Si(100) surface. This is analysed with computing the changes of Gibbs free energy (ΔG). A (2×2) supercell is applied to study the NHx-terminations. A full monolayer (1ML) corresponds to 4 adsorbates on the surface, i.e. 4 NH or 4 NH2 for single terminations. Since NH and NH2 have different preferred binding sites, we can set the maximum surface coverage up to two monolayers (2ML) with 1ML NH occupying bridge site and 1ML NH2 occupying top site. NH2 has more exothermic adsorption energy than NH. We can regard the mixed terminations as NH2-dominated, which means that we can set the initial coverage of one full layer of NH2 and then add NH one by one. This is analogous to the study of NHx-terminations on Ru and Co surfaces. 6 A typical deposition temperature in the range of 300K to 800K is applied and the plotted ΔG is shown in Figure S8. Mixed terminations with coverage up to 2ML, i.e. 4NH and 4NH2 on the surface, are the final structure of NHx-terminations on Si(100) surface, which are shown in Figure  S9. Figure S8. The changes in Gibbs free energy (ΔG) of single terminations and mix terminations with NH and NH2 on bare Si(100) surfaces. Figure S9.