Microscale electrode array with active CMOS circuits for 2D electrochemical imaging

Abstract

Energy conversion devices make use of thin films and functional materials that exhibit microscopic spatial heterogeneity in their efficiency. The relationship between the distribution of such irregularities and their impact on device performance is not well understood. Hence there is a requirement to map the electrochemical activity in a range of thin films and functional materials. This is termed “electrochemical imaging” [1]. This need is presently addressed by high resolution electrochemical current mapping techniques. One such approach is the use of scanning electrochemical microscopy (SECM) [2]. While high resolution techniques like SECM are used for imaging, they are slow (order of minutes) over a wider area (~cm² scale). Hence there is a need to do 2D spatial electrochemical activity mapping at a faster rate (~ms) than those obtained from the conventional techniques. A potential solution is proposed – the CMOS active-matrix electrochemical imager – an integrated circuit whose high-level architecture is like that of an CMOS optical imager but whose optically sensitive element (photodiode) is replaced by an eletrochemically sensitive element (a working electrode (WE)). For feasibility purposes a CMOS test chip with sequential (passive matrix) readout capability of electrode current has been designed and implemented in a 5V AMS CMOS process. It comprises a readout circuit block (current to time converter with auto-zeroed ping-pong amplifier) and drive circuit (potentiostat) integrated with a 3×3 array of microscale electrodes on the same silicon substrate. The chip has been used to sense electrochemical current in the order of nanoamperes from individual electrodes on the array. The system level architecture to address individual electrodes, electrical characterization of individual circuit blocks, system level electrical characterization and basic electrochemical characterization of microscale electrodes on the 3×3 array are reported in detail. This thesis reports in detail the design and implementation of the following standalone circuit blocks (test structures): current amplifier, current attenuator and current buffer. The need for the same is attributed to the wide range of steady state electrode current whose magnitude depends on the surface area of electrode. This current can range from a few picoamperes to hundreds of nanoamperes. The use of a current amplifier and/or current attenuator as a front end can increase the effective input dynamic range of existing CMOS current sensing circuits. These standalone circuit blocks were electrically tested and characterized for their current gain with input DC currents ranging from few picoamperes to hundreds of nanoamperes.