Analysis of current transients for voltage pulse-modulated surface processing: Application to anodic electro-dissolution of copper for electrochemical mechanical planarization

The voltage pulse modulation technique is useful for electrochemical processing of metal and alloy surfaces by utilizing faradaic reactions like electro-deposition and electro-dissolution. A theoretical framework is presented here to facilitate quantitative analysis of experimental data (current transients) obtained in this approach. A typical application of this analysis is demonstrated for an experimental system involving electrochemical removal of copper surface layers, a relatively new process for abrasive-free electrochemical mechanical planarization of copper lines used in the fabrication of integrated circuits. Voltage pulse-modulated electro-dissolution of Cu in the absence of mechanical polishing is activated in an acidic solution of oxalic acid and hydrogen peroxide. The current generated by each applied voltage step shows a sharp spike, followed by a double-exponential decay, and eventually attains the rectangular shape of the potential pulses. The formulas presented here show excellent fits to these current transients, and help to probe the detailed electrochemistry of surface layer removal using a simple circuit model of the reactive interface.