Dicobalt octacarbonyl Co2(CO)8

Dicobalt octacarbonyl Co2(CO)8[[i],[ii],[iii]]allows to deposit conformal Co films with excellent adhesion and little interdiffusion with Si substrates in hot-wall CVD reactor at temperatures 200 – 400 °C with growth rate 3 nm/min. The resistivities 8 – 20 μΩcm have been achoived but the layers had some surface contamination with C, O, Cl (from synthesis of precursor?). Heating of films resulted in the formation of CoSi2 at low temperatures.

[i] J. Lee, H. J. Park, S. H. Won, K. H. Jeong, H. S. Jung, C. Kim, H. J. Bang, C. M. Lee, J. H. Kim, G.C. Kwon, H. L. Cho, H. S. Soh, J. G. Lee, J. Electrochem. Soc., 2007, 154, H833–H837.

[ii] J. Lee, H. Park and J. Lee, Diffus. Defect Data, Pt. B, 2007, 124–126, 531–534.

[iii] D. X. Ye, S. Pimanpang, C. Jezewski, F. Tang, J. J. Senkevich, G. C. Wang and T. M. Lu, Thin Solid Films, 2005, 485, 95–100

[iv] J. Lee, H. J. Yang, J. H. Lee, J. Y. Kim, W. J. Nam, H. J. Shin, Y. K. Ko, J. G. Lee, E. G. Lee and C. S. Kim, J. Electrochem. Soc., 2006, 153, G539–G542.

Tetracobalt dodecacarbonyl Co4(CO)12

Fig.. Growth rate of Co films from Co2(CO)8 (and in-situ formed Co4(CO)12) vs. deposition temperature (in Arrhenius coordinates)

 Tetracobalt dodecacarbonyl Co4(CO)12is a black solid with melting point 60°C (some decomposion occurs) .  

 Synthesis: Partial decomposition of dicobalt octacarbonyl : 

2 Co2(CO)8 → Co4(CO)12 + 4 CO (45 °C)

 Co4(CO)12 is also formed in situ (during deposition of Co films using Co2(CO)8 precursor),  as the deposition temperature increases above 70°C (regime II), this process is responsible for the rapid increase in the growth rate (by an order of magnitude) with increasing temperature, as compared to regime I (50-70°C) (Fig.1). Pure cobalt films were obtained at higher substrate temperatures [[i]] 

    The decomposition of Co2(CO)8 to Co4(CO)12 and further to metallic Co is actually much more complex process which can be comprised of several mechanisms. The CO group can be removed from Co2(CO)8 forming an electron-deficient complex, Co2(CO)7. This species further reacts with another Co2(CO)8 molecule, with loss of 3 CO molecules yielding an Co4(CO)12. This intermediate can produce a cascade of electron-deficient fragments, which in turn may react to form a variety of higher nuclearity (high Co/CO ratio) cobalt complexes , which have higher sticking coefficient compared with Co2(CO)8 and thus, leading to increased concentration of the adsorbed precursors on the surface and thus resulting in a much higher growth rate of Co in temperature regime II. 

Another driving force is the dipole interaction between the electron-deficient cobalt fragments and the SiO2 surface, in larger extent contributing to the Co–SiO2 surface interaction. In the case of the 4.0 Pa process, there is a transition temperature of 70–80°C, below which the deposition of the Co thin films is limited by the surface reaction and above which it is limited by the gas-phase collisional activation of the reactants. 

[i] J. Lee, H. J. Yang, J. H. Lee, J. Y. Kim, W. J. Nam, H. J. Shin, Y. K. Ko, J. G. Lee, E. G. Lee and C. S. Kim, J. Electrochem. Soc., 2006, 153,  G539–G542.

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