From High Density Nanoparticle Deposits to Nanometer Thin Continuous Films By Growth-Inhibited Nucleation Enhancement

In our group, we have been working on the electrochemical nucleation and growth of high island density deposits of copper, indium, nickel, cobalt and iron deposits for about a decade. From simple electrolyte solutions, all metal systems seem to saturate at an island density of few times 10¹¹ islands per cm² at most or about 20-30nm spacing per particle. From direct copper plating for damascene processes, we know that the island density can be augmented by the introduction of growth inhibiting species such as the chloride-polyether suppressor additive combo. In this case, the Cl-suppressor adsorbs on the copper island as soon as it is formed. As the current-potential curve for the Cl-suppressor covered copper is polarized more negative with respect to the non-suppressed copper, the overpotential is driven more negative during galvanostatic deposition allowing higher island density [1]. As such the island density can be driven higher than 10¹³ cm^-2 (spacing of about 3nm per particle) on oxide-poor noble metals such as Pt and up to 10¹² cm^-2(spacing of about 10nm per particle) for oxide-prone surfaces like RuTa alloys [2]. Note that these values are extrapolated as islands can no longer be distinguished from films at this point (coalescence thickness of 5nm and 2nm for hemispherical particles, respectively). The adsorbed suppressor typically also introduces flattened particle shapes helping to bring the effective coalescence thickness even lower. On platinum and cleaned RuTa surfaces, closed copper films with thicknesses down to 2-3nm could be obtained. From our study, it became apparent that the surface condition and overpotential were the two drivers.

For the application of dense CNT forests, we were interested in fabrication of densely packed Ni, Co and Fe nanoparticles. We have shown that the density of the ECD catalyst particles is directly translated to the CNT density; the highest density of 2x10¹¹cm^-2 from an alkaline NiNO₃-NH₄Cl bath [3]. Based on the know-how from copper, several typical growth inhibiting additives (e.g. saccharine, 2-butyne-1,4-diol, PEG) were tried to increase the Ni and Co nanoparticle density. However, with no avail, as the adsorption and inhibition was typically too slow. In copper deposition, the additives already form complexes with the cuprous intermediate formed during copper deposition. As such the adsorption is very fast, whereas for Ni and Co, the additives need to adsorb via a physisorption which happens too late to affect the nucleation stage. Recently we found that the Ni particle density is strongly affected by the boric acid concentration [4]. The formation of growth inhibiting hydroxides gives the desired effect for growth-inhibited nucleation. However, the local pH cannot rise to fast as otherwise precipitation from solution already occurs in the depletion layer. Therefore a small concentration of boric acid was still needed at 5mM NiCl₂ concentration to allow nickel growth to proceed long enough. At the best trade-off conditions a density of 4x10¹¹ cm^-2 Ni nanoparticles on TiN was obtained. By tuning the NiCl₂concentration, the growth-inhibited nucleation process could be extended to higher Ni island densities resulting in nanometer thin nickel films [5]; similar to what was achieved for copper. In this paper, we will go in on the mechanism of growth-inhibited growth for the fabrication of high density magnetic metals such as Ni, Co and Fe.

1. “The Effect of Polyether Suppressors on the Nucleation and Growth of Copper on RuTa Seeded Substrate for Direct Copper Plating” M. Nagar, A. Radisic, K. Strubbe, P.M. Vereecken Electrochim. Acta, 127, 315-326 (2014).
2. “Systematic studies of electrochemical nucleation and growth of copper on Ru-based substrates for damascene process”, Magi Margalit Nagar, PhD thesis, Universiteit Gent.
3. "Electrochemical tailoring of catalyst nanoparticles for CNT spatial-dimension control" A. Romo-Negreira, D. J. Cott, K. Maex, M. M. Heyns and P. M. Vereecken, JES 157, K47 (2010).
4. “Enhanced nucleation of Ni nanoparticles on TiN through H₃BO₃-mediated growth inhibition” Johannes Vanpaemel, Marleen H. van der Veen, Stefan De Gendt, Stefan and Philippe Vereecken, Electrochimica Acta 109, 411-418 (2013).
5. “Electrochemical deposition of sub-nanometer Ni films on TiN” Johannes Vanpaemel, Masahito Sugiura ,Daniel Cuypers, Marleen H. van der Veen, Stefan De Gendt, Stefan and Philippe M. Vereecken, Langmuir, 30 (8), 2047–2053 (2014).