Electroless Au Plating of CMOS Microelectrodes: Fabrication, Characterization, and Electrochemical Measurement

An essential step in developing amperometric sensors directly on CMOS integrated circuits (ICs) is to cover the exposed uppermost metal layer (aluminum pads) with a thin layer of noble metal to form the basis of the sensing electrode. A simple and scalable method to achieve the gold layer is through electroless plating. Despite the popularity of electroless plating in, e.g., PCB manufacturing, there is a lack of information on how it can be applied to Al microelectrodes and what the electrochemical performances of Au-coated microelectrodes are. This letter presents a detailed process for electroless gold plating of CMOS microelectrodes, with a step-by-step characterization of the surface roughness, thickness, and elemental composition to optimize the deposition parameters (e.g., deposition time and temperature) for achieving a smooth and uniform gold coverage of the microelectrodes. A gold layer with an rms surface roughness of 53.6
$\pm$
7.9 nm is achieved on the microelectrodes and successfully characterized by cyclic voltammetry in a ferri/ferrocyanide solution. Sonication, oxygen plasma, and continuous cyclic voltammetry are applied to the Au-coated microelectrodes to determine their mechanical and electrochemical stability.