(Invited) In-Situ Investigation of the Role of Boric Acid during the Electrodeposition of Cobalt
Authors: Miller Mallory A. , Wall Jessica , Sukamto John , Webb Eric G. 
Published: February 2020

As interconnect dimensions continue to decrease, copper damascene faces significant challenges1. 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 substrate2-4. Typically, maximum buffering occurs when the pH equals the pKa for the weak acid. Thus, it is not well understood how an additive such as boric acid, with a pKa value of 9.275, can act as a buffer in typical cobalt electroplating baths6-9 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 activity10-12. 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.

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