The Multi-Functional Role of Boric Acid in Cobalt Electrodeposition and Superfill

Although alternative metals such as cobalt are becoming increasingly important as candidates for filling interconnects, questions remain regarding the role of boric acid, one of the primary bath components, in the electroplating process. This work demonstrates that boric acid has multiple functions in cobalt electroplating and superfill. Raman spectroscopy was used to determine speciation of boric acid in solution as a function of concentration and pH. Combining this with titration experiments, it was found that polyborate formation coincides with an increase in buffering capacity in the range of pH that is typical for cobalt electroplating. Atomic force microscopy and secondary ion mass spectrometry confirmed the correlation between hydroxide/oxide in the deposited film and the amount of boric acid in solution. Electrochemical experiments demonstrated that boric acid adsorbs to the electrode surface under certain conditions. Interconnect fill quality was also shown to be directly related to boric acid concentration. Boric acid is shown to be a necessary bath component due to its role in pH buffering and surface adsorption, affecting both blanket film plating and superfill of interconnects in the presence of a suppressor.