Minimizing the effects of galvanic corrosion during chemical mechanical planarization of aluminum in moderately acidic slurry solutions

In the fabrication of integrated circuits, certain steps of materials processing involve chemical mechanical planarization (CMP) of Al deposited on thin layers of diffusion barrier materials like Ta/TaN, Co, or Ti/TiN. A specific example of this situation is found in the processing of replacement metal gates used for high-k/metal-gate transistors. Since the commonly used barrier materials are nobler than Al, the Al interface in contact with the barrier can become prone to galvanic corrosion in the wet CMP environment. Using model systems of coupon electrodes and two specific barrier metals, Ta and Co, the present work investigates the electrochemical factors responsible for these corrosion effects in a moderately acidic (pH = 4.0) abrasive-free solution. The techniques of cyclic voltammetry and impedance spectroscopy are combined with strategic measurements of galvanic currents and open circuit potentials. l-ascorbic acid (AA) is employed as a surface modifying agent for controlling galvanic corrosions of Al in the Ta–Al and Co–Al bimetallic combinations. The results elaborate the chemical and electrochemical mechanisms responsible for activating and suppressing the corrosion processes in these systems.