(Invited) In-Situ Investigation of the Role of Boric Acid during the Electrodeposition of Cobalt

As interconnect dimensions continue to decrease, copper damascene faces significant challenges 1 . As a result, alternative metals such as cobalt are actively being explored. Because the standard reduction potential for cobalt is more cathodic than that of hydrogen evolution, the solution pH is a critical consideration for the plating process. Boric acid is a common component in plating solutions used for cobalt electrodeposition. It is believed that boric acid acts as a buffering agent that helps to stabilize bath pH and to prevent formation of cobalt hydroxide precipitate on the substrate 2-4 . Typically, maximum buffering occurs when the pH equals the pK a for the weak acid. Thus, it is not well understood how an additive such as boric acid, with a pK a value of 9.27 5 , can act as a buffer in typical cobalt electroplating baths 6-9 which often have pH ranges of 2-5. The aim of this work is to provide an improved understanding of the role of boric acid on the electrodeposition of cobalt. It is important to first determine the effects of boric acid on surface pH during cobalt deposition. We present a technique for in-situ monitoring of surface pH as a function of applied plating current density. This is done using a tungsten microelectrode which can function as an indicator electrode for the hydrogen ion activity 10-12 . Results for plating cobalt from a virgin makeup solution (VMS) will be compared to those containing boric acid to demonstrate that this additive acts as a buffer at the electrode surface where pH values drastically increase during metal deposition. Electrochemical impedance spectroscopy (EIS) data will aid in understanding the surface adsorption behavior as well as the coulombic plating efficiency. In addition to surface specific information, potentiometric titration data will be presented to explain buffering effects in the bulk electrolyte. References R. Akolkar, "Current Status and Advances in Damascene Electrodeposition," Encyclopedia of Interfacial Chemistry, pp. 24-31, 2018. N. Zech and D. Landolt, "The influence of boric acid and sulfate ions on the hydrogen formation in Ni-Fe plating electrolytes," Electrochimica Acta, vol. 45, no. 21, pp. 3461-3471, 2000. J. P. Hoare, "On the Role of Boric Acid in the Watts Bath," Journal of The Electrochemical Society, vol. 133, no. 12, pp. 2491-2494, 1986. J. Santos, R. Matos, F. Trivinho-Strixino and E. C. Pereira, "Effect of temperature on Co electrodeposition in the presence of boric acid," Electrochimica Acta, vol. 53, pp. 644-649, 2007. D. R. Lide, CRC Handbook of Chemistry and Physics, Boca Raton, FL: CRC Press, 2005, p. 1274. M. A. Rigsby, L. J. Brogan, N. V. Doubina, Y. Liu, E. C. Opocensky, T. A. Spurlin, J. Zhou and J. D. Reid, "The Critical Role of pH Gradient Formation in Driving Superconformal Cobalt Deposition," Journal of The Electrochemical Society, vol. 166, no. 1, pp. D3167-D3174, 2019. Y. Hu and Q. Huang, "Effects of Dimethylglyoxime and Cyclohexane Dioxime on the Electrochemical Nucleation and Growth of Cobalt," Journal of The Electrochemical Society, vol. 166, no. 1, pp. D3175-D3181, 2019. C. H. Lee, J. E. Bonevich, J. E. Davies and T. P. Moffat, "Superconformal Electrodeposition of Co and Co–Fe Alloys Using 2-Mercapto-5-benzimidazolesulfonic Acid," Journal of The Electrochemical Society, vol. 156, no. 8, pp. D301-D309, 2009. D. Josell, M. Silva and T. Moffat, "Superconformal Bottom-Up Cobalt Deposition in High Aspect Ratio Through Silicon Vias," ECS Transactions, vol. 75, no. 2, pp. 25-30, 2016. S. E. S. E. Wakkad, H. A. Rizk and I. G. Ebaid, "The Electrochemical Behavior of the Tungsten Electrode and the Nature of the Different Oxides of the Metal," The Journal of Physical Chemistry, vol. 59, no. 10, pp. 1004-1008, 1955. J. O. Park, C.-H. Paik and H. C. Alkire, "Scanning Microsensors for Measurement of Local pH Distributions at the Microscale," Journal of The Electrochemical Society, vol. 8, no. 143, p. 174, 1996. E. Webb and R. Alkire, "Pit Initiation at Single Sulfide Inclusions in Stainless Steel," Journal of The Electrochemical Society, vol. 146, no. 6, p. B280, 2002.