Citric acid as a complexing agent in CMP of copper investigation of surface reactions using impedance spectroscopy

Polishing slurries used in chemical-mechanical planarization (CMP) of copper typically include a complexing agent and an oxidizer. Our present work investigates the effectiveness of citric acid as a complexing agent for Cu with H2O2 employed as an oxidizer. We show that the rate of copper removal from Cu increases when both citric acid and H2O2 are used in acidic solutions (pH 4.0) and decreases drastically in alkaline solutions (pH 8.0), as well as in citric acid without any H2O2 in the slurry. We investigate the underlying surface reactions of these effects by using Fourier transform electrochemical impedance spectroscopy (FTEIS) in combination with potentiodynamic measurements. We analyze the relative roles of citric acid, H2O2, and solution pH in Cu removal, and develop a reaction scheme describing the surface chemistry of Cu in this system. The results presented here also demonstrate how FTEIS can be used for quantitative investigation of surface reactions in complex CMP systems. (c) 2005 The Electrochemical Society.