Li, Minghao; (2024) An ASIC for in-situ on-chip Fabrication and Amperometric Readout of CMOS Electrochemical Biosensors. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The monolithic integration of electrochemical sensors and readout electronics on semiconductor (e.g. CMOS) technology provides scalability and improves the signal-to-noise ratio of measure ments by removing long interconnects. The integration provides several other advantages such as low-cost sensor redundancy and multi-target detection through embedding potentially large sensor arrays along on a single device. An essential step in developing electrochemical biosensors directly on CMOS integrated circuits (ICs) is to cover the exposed uppermost aluminum layer (a.k.a pads) with a layer of noble metal such as Au, to form the basis of the sensing electrode. In this PhD research, a method has been developed for depositing a smooth, uniform and polycrystalline layer of Au on CMOS microelectrodes. The developed method allows control over Au layer thickness up to about 2 µm and the surface roughness, down to 83 nm rms. The developed microelectrodes were successfully used to measure dopamine in its physiological range, using cyclic voltammetry, chronoamperometry and fast scan cyclic voltammetry techniques within a range of 20 µM to 1 mM (R2= 0.977) with an LoD of 5.3 µM, advancing state of the art in high sensitivity of dopamine measurement. An integrated circuit was designed and fabricated in a 180-nm CMOS process that allows in situ electroplating of CMOS microelectrodes and an integrated amperometric readout to inter face with them. The system consists of five main parts: (i) A 4×4 array of microelectrodes and a 16-to-4 multiplexer; (ii) reference current generators to generate currents from 7.2 pA to 88 µA with insensitivity to the process, supply voltage and temperature (PVT) variations for galvanostatic electrodeposition, (iii) direct digital synthesizer (DDS) and a potentiostat; iv) a switch-capacitor-based amperometric readout circuit for sensor voltage control and measure ment of sensor currents within ±4 µA and a minimum integrated input-referred noise of 4.35 pArms (from f0=7 Hz to f-10 dB=27 Hz), v) followed by a SAR ADC and SPI for digitization and serial communication. The functionality and performance of the entire system were successfully verified by carrying out sequential Au-plating, followed by cyclic voltammetry in ferri/ferrocyanide and measurement of dopamine. The Au-electroplated microelectrodes developed using the on-chip current references demon strate equally repeatable results, compared to using commercially available equipment. The on-chip sensors for dopamine measurement with an Ag/AgCl RE, Au-electroplated WE and Au-electroless plated CE were developed with on-chip current reference generators and suc cessfully tested in dopamine with the on-chip readout circuits.

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