Development of Au Microelectrodes on CMOS chips and Application for On-chip Dopamine Measurement

A 7-step process to develop a polycrystalline Au layer on the Al microelectrodes of CMOS (Complementary Metal Oxide Semiconductor) microchips is presented. The process is based on electrochemical deposition techniques, thus naturally scalable and low-cost while also allowing control over the thickness and surface roughness of the deposited Au layer. The optimisation of key process steps and parameters such as the deposition time, temperature, current density, and duty cycle are presented along with thorough characterisation of the Au surface as well as repeatability and stability tests for use in electrochemical measurements. The proposed method allows control over the thickness of the Au layer within 0 to 2 μm and can achieve surface roughness as low as 84 nm rms. The Au-coated CMOS microelectrodes were used to measure dopamine in a buffer solution without further modifications. A sensitivity of 11.4 mA mM^-1 cm^-2 was achieved with a detection limit of 5.3 μM. The Au microelectrodes are stable and can undergo over 20 cycles in cyclic voltammetry before showing any signs of degradation.