Electroless Deposition of High Quality Co Films on Cu Polycrystalline Substrate Assisted By Pb UPD Monolayer
Authors: Wu Dongjun , Joi Aniruddha , Dordi Yezdi , Brankovic Stanko 
Published: February 2020

In the last several decades, the electrochemical thin film growth was proven to be one of the enabling fabrication methods behind the train of hi-tech enterprise [[i],[ii]]. Today, the electro- and electroless- depositions are recognized as standard processing operations in magnetic disk drive [[iii],[iv]], and microprocessor and MEMS technologies [[v],[vi],[vii],[viii]].
The focus of our talk is the electroless growth of Co ultra-thin films on highly textured Cu substrate. The Co/Cu system has been already studied before using different deposition approaches [[ix]]. Although layer-by-layer growth was initially reported [[x],[xi]] it is now known that, at room temperature, the growth of cobalt on Cu(111) is dominated by island formation due to kinetically limited interlayer transport [[xii]].
The electrolyte for electroless growth of Co is based on CoSO4 as a metal salt and TiCl3 as a reducing agent [[xiii]]. Prior to deposition experiments, the Cu substrates are annealed at 270 ○C using reducing atmosphere containing H2+CO mixture. To improve the interlayer transport of Co during the growth, the monolayer (ML) of Pb is pre-deposited using electroless plating on Cu surface before the Co growth. [[xiv]]. The benefit of Pb ML presence on Co interlayer kinetics during the growth is evident by high quality of Co films obtained, Figure 1. They are characterized by large 2D grains having the width to height ratio in excess of 500. The ultrathin Co films are continuous, highly conformal and very smooth yielding a 2D-like appearance, Figure 1. The saturation roughness measured over the range of 10 x10 microns for 10 nm thick Co films is only 3 nm. The Co films show extremely soft properties and very high electrical conductivity which indicates a very small concentration of defects and high quality of the grain boundaries. The benefit and application of this Co deposition protocol is discussed emphasizing its simplicity, fidelity of control and low capital cost of equipment.
The authors acknowledge the support from Lam Research Corporation and NSF Chemistry division under the contract # 0955922.

REFERENCES

[i] Nanomaterials: Synthesis, Properties and Applications. IOP Publishing, Bristol, (1996) UK.
[ii] Nanostructures and Nanomaterials: Synthesis, Properties and Applications. Imperial College Press, London, UK (2004).
[iii] E. Cooper et al. IBM J Res & Dev
49 103 (2005).
[iv]S.R. Brankovic et al.. IEEE Trans Magn
42 132 (2006).
[v]M. Data et al. J Electrochem. Soc.
142 3779 (1995).
[vi]P. Andricacos, Electrochem. Soc. Interface
8 32 (1999).
[vii] H. Deligianni, Electrochem. Soc. Interface
15 33 (2006).
[viii] W. Ehrfeld, Electrochim Acta
48 2857 (2003).
[ix] M.T. Kief and W.F. Egelhoff, Phys. Rev. B
47 10 785 (1993)
[x] Gonzalez, R. Miranda, M. Salmeron, J.A. Verges, and F. Yndurain, Phys. Rev. B
24, 3245 (1981).
[xi] 3Q. Chen, M. Onellion, and A. Wall, Thin Solid Films
196, 103 (1991).
[xii] J. de la Figuera, J.E. Prieto, C. Ocal, and R. Miranda, Phys. Rev.
B 47, 13 043 (1993).
[xiii] Y. Dordi, A. Joi et al, US Patent application # 14/264,994
[xiv] N. Dimitrov et al, Electrochemistry Communications, 44 19 (2014).

Figure 1