Probing the Mechanism of Electroless Gold Plating Using an EQCM: II. Effect of Bath Additives on Interfacial Plating Processes

Abstract

This work describes results of a mechanistic investigation of electroless gold plating from a dimethylamine borane ͑DMAB͒ bath, which was performed with the aid of the electrochemical quartz crystal microbalance ͑EQCM͒. It was found that the interaction of DMAB with the gold surface during deposition is dependent on bath composition. The extent, or strength, of this interaction is detected as an anodic current increase coinciding with a mass increase at the gold surface on the cathodic scan in the vicinity of the gold oxide reduction potential. Electrochemical impedance spectroscopy of a gold electrode in DMAB solutions shows a decrease in double-layer capacitance (C dl ) with an increase in DMAB concentration at the reactivation potential, suggesting a strong concentration-dependent binding capacity upon gold oxide reduction. The C dl reaches a limiting value at approximately 1 g/L, implying that sufficient reactive intermediate is formed at this concentration, which binds to all available free gold sites upon gold oxide reduction. Measurements of the plating rate using EQCM on industrial end-of-line wirebond monitors showed that an increase in plating rate is associated with a greater extent of interaction between the reducing agent and the catalytic gold surface. These results were confirmed with X-ray fluorescence, scanning electron microscopy, and energy-dispersive spectrophotometry. The usefulness of the EQCM as a screening tool to predict electroless gold plating rates in baths of varying formulation is presented. This technique may be applicable to other electroless baths where interfacial adsorption/desorption processes are expected to play an important role during deposition.

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