Electrochemical monitoring is crucial for both industrial applications, such as microbial electrolysis and corrosion monitoring as well as consumer applications such as personal health monitoring. Yet, state-of-the-art integrated potentiostat monitoring devices have few parallel channels with limited flexibility due to their channel architecture. This work presents a novel, widely scalable channel architecture using a switch capacitor based Howland current pump and a digital potential controller. An integrated, 64-channel CMOS potentiostat array has been fabricated. Each individual channel has a dynamic current range of 120dB with 1.1pA precision with up to 100kHz bandwidth. The on-chip working electrodes are post-processed with gold to ensure (bio)electrochemical compatibility

Ceyssens, Frederik

Li, Peishuo

Molderez, Tom

Rabaey, Korneel

Verhelst, Marian

English

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A 64-channel, 1.1-pA-accurate On-chip Potentiostat for Parallel Electrochemical Monitoring

Ghent University Academic Bibliography

A 64-channel, 1.1-pA-accurate On-chip Potentiostatfor Parallel Electrochemical MonitoringPeishuo Li∗, Tom R. Molderez∗, Frederik Ceyssens∗, Korneel Rabaey† and Marian Verhelst∗∗ESAT-MICAS, KU Leuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium†CMET, UGent, Coupure Links 653, B-9000 Gent, BelgiumEmail: {peishuo.li, tom.molderez, frederik.ceyssens, marian.verhelst}@kuleuven.be, korneel.rabaey@ugent.beAbstract—Electrochemical monitoring is crucial for both in-dustrial applications, such as microbial electrolysis and corrosionmonitoring as well as consumer applications such as personalhealth monitoring. Yet, state-of-the-art integrated potentiostatmonitoring devices have few parallel channels with limitedflexibility due to their channel architecture. This work presentsa novel, widely scalable channel architecture using a switchcapacitor based Howland current pump and a digital potentialcontroller. An integrated, 64-channel CMOS potentiostat arrayhas been fabricated. Each individual channel has a dynamiccurrent range of 120dB with 1.1pA precision with up to 100kHzbandwidth. The on-chip working electrodes are post-processedwith gold to ensure (bio)electrochemical compatibility.I. INTRODUCTIONParallel, accurate electrochemical monitoring is crucial forhigh-throughput and/or widely variable applications in theindustrial and consumer fields. To study individual electro-chemical half reactions, a potentiostat architecture with a 3-electrode setup is used [1]. For the development of novelmicrobial electrochemical reactors, multichannel potentiostatsallow to tune the electrical settings optimally with flexiblestimulation patterns, irrespective of the natural statistical vari-ation of the microorganisms or the environment dynamics [2].Similarly, for the study of novel anti-corrosion coatings, a veryslow aging process, parallelization notably speeds up thoseexperiments [3]. For personal healthcare monitoring, variousbiomolecules act as electrochemical transducers. As such, amultichannel on-chip potentiostat can assess the impact ofdifferent chemicals simultaneously [4] at low cost.Current SotA parallel on-chip potentiostats focus mostly onparallelizing the readout circuitry, by measuring the currentof each individual channel while keeping the potential equalfor all channels. As such only a single, analog potentialcontroller is required. In [5], an active microelectrode array(MEA) for DNA analysis is presented with 16 parallel readoutchannels. Using a more advanced circuit architecture, [6]expanded the number of parallel readout channels to 1024 with93dB dynamic range. Yet, both have only a single stimulationunit. While this is sufficient for studying the spatial effectsof electrochemical reactions, a truly parallel stimulation andsensing potentiostat with large dynamic range is requiredfor monitoring those high-throughput and/or widely variableprocesses.In [7], a multi-functional MEA is presented with 59760electrodes and 2048 parallel readout channels. The numberFig. 1. System overviewof independent stimulation units is expanded to 16. In [8] themultiplexed cyclic voltammetry with 16 parallel independentstimulation and sensing potentiostats is further investigated.Further expanding the amount of independent potentiostatsoperating simultaneously is however challenging for the SotAarchitectures due to the limited chip input/output pin numbers.Moreover, the largest dynamic range of the current sensingunit in SotA designs is below 100dB. Therefore, in this papera current stimulation structure is used with a boosted dynamicrange. The potentiostat control is maintained by the digitalfeedback loop running on an external microcontroller. Thelift-off process used to pattern electrodes on chip can bemodified in order to improve the process yield. Compared tothe SotA designs, this paper therefore introduces the followingnovelties: 1) a scalable multiplexing architecture resulting in64 independent stimulation channels with only a single inputpin and a single output pin, and fully software-programmablecontrol with an arbitrary waveform; 2) a switch capacitor basedcurrent pump with bipolar output, 120dB wide dynamic range,and 1.1pA resolution; 3) a post-processing technique withthick metal and an extra protection layer to pattern on-chipworking electrodes (WEs).This paper is structured as follows: First, section II intro-duces the channel architecture. The circuit implementation isdiscussed in section III. Next, section IV details the post-processing steps to pattern the electrodes. Finally, sectionV presents the electrical measurement and chemical cyclicvoltammetry results.978-1-7281-1550-4/19/$31.00 ©2019 IEEE 317Fig. 2. Channel multiplexing architectureII. SYSTEM ARCHITECTUREThe 64-channel potentiostat consists of three parts: the mul-tiplexing architecture, the bipolar current-stimulation/voltage-sensing potentiostat channel and the on-chip working elec-trode, shown in Fig. 1. They are discussed below.A. Multiplexing architectureA flexible, scalable multiplexing architecture is shown inFig. 2. A digital controller is used to close the feedback loop.Only one input/output pair together with a single externalADC/DAC is required to control all channels using time-division multiplexing.Through the selection signal Sel, driving the 1-64 multi-plexer and 64-1 demultiplexer, each channel can be selectedindividually. As such, the ADC can sense the output volt-age of each channel, and send it to an off-chip, low-costmicrocontroller (TI TMS320F28379D). The microcontrollerruns a digital proportional-integral (PI) controller to maintainthe output potential of each channel equal to a set-pointby stimulating the channel’s current. The set-point potentialcan be time-varying, represented by a fully-programmablewaveform.A capacitor at the input of every channel stores the channel-specific current-stimulation input signal when the channelis not selected. In this way, all 64 channels are stimulatedsimultaneously with only one input pin. The interface betweenthe digital controller and the analog chip is composed of asingle 12-bit, 250-kS/s ADC and DAC. This scheme is easilyscalable to a large number of channels by increasing either thestorage capacitor size or the mixed-signal sample rate.B. Potentiostat channelThis work applies a current stimulation, voltage measure-ment architecture. The channel schematic is shown in Fig.3. A Howland current pump is implemented with a switchcapacitor topology. Such scheme has the benefit of a largedynamic current range, because the value of the equivalentresistor of a switch capacitor is inversely proportional to thetunable switch clock frequency. All channels share the sameswitch capacitor clock to set the coarse output range. The inputvoltage of the current pump is channel-specific and used foroutput fine-tuning.Fig. 3. Schematic of a single channelC. On-chip working electrodeThe working electrodes are integrated on chip to allow awide parallelization. The top metal layer of the non-processedchip is aluminum (Al). As Al is not electrochemically stable,each electrode receives a biocompatible gold (Au) coatingduring post-processing.III. CIRCUIT LEVEL DESIGNThe chip is designed in 180nm technology with 3.3V supply.Each channel consists of a memory capacitor, an outputvoltage buffer (Vbuf) and a bipolar current pump (Ipump).The memory capacitance value of 1.2pF limits the voltagedrop to be less than 1mV when cycling through all 64 channelswhile have no impact on the required bandwidth.In order to run a complete cyclic voltammetry with sufficientaccuracy, the targeted voltage sensing swing is set to 2V andthe targeted resolution is 1mV. The output voltage measure-ment unit is implemented with an opamp in unity feedback. Arail-to-rail configuration [9] is adopted to maximize the swingwithout adding much extra area.A modified Howland current pump is used to pro-vide the wide-dynamic bipolar currents [10]. The cur-rent pump uses two opamps, four 10kΩ resistors andone tunable resistor (R tune), generating a bipolar currentIout = (V in− V in ref)/R tune. The first opamp togetherwith all the resistors generates a differential bipolar currentwhile the second opamp isolates the output load from theinternal resistors.The tunable resistor (R tune) is implemented with a switchcapacitor (R = 1/fC), which results in a large dynamicrange by adjusting the switch clock frequency. For the switchcapacitor, a 100fF small capacitor C is chosen to achieve anextremely large equivalent resistor and as such a very lowminimal current resolution. The switches are implemented astransmission gates to maximize the output swing. The switchsize is minimized to reduce the clock feedthrough whichdecreases the output accuracy. Additional switches are addedin series to decrease the static leakage. Non-overlapping clocksare locally generated to prevent leakage during switching.During simulation, the output resistance of the current pumpis about 20 times larger than R tune, leading to a wide loadrange. 10% accuracy can be achieved at 500fA output current318Fig. 4. Chip die photograph and post-processing(50mV input, 100Hz clock frequency). The thermal noise isnot an issue since the current pump bandwidth is coupled tothe switch clock frequency and shrinks in proportion with thecurrent range.To decouple the current pump from the memory capacitor,an input voltage buffer stage is added. For testing purposes,additional transmission gates are inserted between the currentpump and the voltage buffer.IV. POST-PROCESSINGThe biocompatible interface between the chip and a(bio)chemical solution is achieved by patterning Au on topof each Al electrode. It was found that the surface of theAl bond pad is not sufficiently flat, especially at the edges.A process is designed to overcome this issue, which consistsof a) gluing the diced chip on a supporting wafer, togetherwith surrounding dummy chips; b) applying and patterninga two-layer liftoff resist stack; c) sputtering 100nm Ti asthick diffusion barrier and then a thick Au layer (400nm); d)dissolving the liftoff resist; e) depositing 5µm of Parylene C;f) patterning the Parylene by lithography and oxygen plasma.Photoresist exposure was done using direct laser writing.An electrode after post-processing is shown in Fig. 4. Theextra Parylene step provides a protection on the Al electrodeedges. To further prevent possible Al corrosion at the edgeafter Au deposition, an edge-shifted pattern is used: The Ti/Aulayer is patterned with 15µm extra margin at each side on topof the Al electrode, while the Parylene opening shrinks with35µm at each side on top of the Ti/Au pattern. The final WEarea is 7×44µm2.With the improved process, not all Al electrodes are com-pletely covered with Au due to the bond pad surface topogra-phy. New processes are being investigated to completely solvethis issue.V. MEASUREMENT RESULTSThe chip is first measured electrically before post-processing, and then electrochemically after post-processing.A. Electrical measurements1) Voltage sensing: The DC output error of the voltagebuffer is measured and calibrated through a two-point cal-ibration: From two consecutive measurement points, a 2-coefficient fitting (gain and offset compensation) is done to cal-culate the voltage buffer input corresponding to the expectedoutput. Then the output is measured again with compensated(a) (b)Fig. 5. Vbuf DC output (a) and distribution at 1V input (b) before and aftercalibration(a) (b)Fig. 6. Ipump DC output (a) and error after calibration (b)input and compared with the expected value. Fig. 5 shows thevoltage buffer output before and after calibration.2) Current stimulation: The current pump DC output ismeasured through the voltage across an external resistor (from100kΩ to 1GΩ). The wide range from 1pA to 1µA forone channel is shown in Fig. 6a, with maximum 0.35%nonlinearity. The output current is configurable by both theclock frequency and the input voltage.A similar calibration procedure is used to calibrate all 64channels. The post-calibration error is below 10% except inthe pA range output (100Hz clock frequency), where themaximum post-calibration error equals 31%, as shown in Fig.6b. This error is much larger than simulation (<1%), probablycaused by the insufficient measurement setup isolation. Notethat only the positive current output is shown, as similarmeasurements were obtained for the negative current.The output resolution with the current measurement set-upis evaluated from the measured noise over the system timeconstant (∼1s for 100Hz clock), resulting in 1.14pA. The1.1µA measured maximum current is limited by the maximumallowed switch-capacitor clock frequency, set by the opampbandwidth.3) Closed loop system: The system bandwidth needs tobe sufficiently large in order to run dynamic measurements.The transfer function of the system is measured with adummy load resistor. The current pump input and the voltagebuffer output are connected to the network analyzer. Theexternal microcontroller is not taken into account because itoperates at a fast clock (200MHz). The results with differentclock frequencies are shown in Fig. 7. 117kHz bandwidth isachieved with 10MHz clock frequency and 1Hz bandwidthcan be extrapolated for the 100Hz clock or 1∼10pA operation.However, the measured bandwidth is actually limited by theload resistor and it is not possible to use smaller load resistors319TABLE ISTATE-OF-THE-ART COMPARISONReference JSSC-08 [5] JSSC-17 [7] TCAS-18 [8] ISSCC-19 [6] This workCMOS fabrication technology[µm] 0.25 0.18 0.35 0.25 0.18Chip area[mm2] 5×3 10.1×7.6 3.79×3.79 7×9 1.66×1.66WE size[µm2] 70×70∼100×100 3×7.5 20×20 ∼30×30 7×44Number of WE 16 59760 256 1024 64Number of independent channels 1 16 16 1 64Imax[µA]/Sensitivity[pA] 0.11/240 ∼1/1000 13/1390 0.0125/0.28 1.1/1.11Current dynamic range[dB] 53.2 ∼60 82.6 93 120Measured voltage swing[V] 1.25 1.0 5.2 1.5 2.5Bandwidth[Hz] 10k 10k 150k 50117k @ 0.1∼1µA∼1 @ 1∼10pAMax. Power consumption[mW] ∼10 86 125 ∼256 1251 0.5 in simulationFig. 7. System transfer function magnitudeFig. 8. Cyclic voltammetry results benchmarked with commercial potentiostatdue to the network analyzer dynamic range limitation.B. Electrochemical measurementsCyclic voltammetry (CV) in chemical solutions is used toverify the close loop performance and quality of the post-processing. The bonded chip was emerged into 0.3mmol/Lpotassium ferrocyanide solution with a commercial Ag/AgClRE (ALS, Japan) and a Pt wire as the CE. A closed loop CVwas run at different scan rates, shown in Fig. 8. The result at50 mV/s is compared with a commercial potentiostat (GamryReference 600+) using a 100×100µm2 dummy electrodearray, normalized to the chip electrode area. The most likelycause of the observed differences are the boundary effectsbecause of the smaller on-chip electrode.VI. CONCLUSIONThis work presents a 180nm CMOS 64-channel poten-tiostat. Compared to SotA (Table I), 4× more independentelectrochemical characterization channels and 30dB largercurrent dynamic range are achieved. These innovations makethe chip feasible to monitor high-throughput, widely-variableelectrochemical processes, such as microbial electrolysis, anti-corrosion studies and biomolecule detection, with fast and flex-ible characterization. Further research on the post-processingis needed to improve the reliability.ACKNOWLEDGMENTThe authors would like to thank the Hercules Foundationfor heavy equipment (AKUL 034 and ZW1115) and FWOVlaanderen for the research funding.REFERENCES[1] A. J. Bard et al., Electrochemical Methods: Fundamentals and Appli-cations, 2nd Edition. Wiley, 2001.[2] T. R. Molderez et al., “A current-driven six-channel potentiostat forrapid performance characterization of microbial electrolysis cells,” IEEETransactions on Instrumentation and Measurement, Mar. 2019.[3] V. N. Rafla et al., “Coupled multi-electrode array with a sinteredAg/AgCl counter/reference electrode to investigate AA7050-T7451 andtype 316 stainless steel galvanic couple under atmospheric conditions,”Journal of The Electrochemical Society, vol. 165, no. 9, pp. C562–C572,Jun. 2018.[4] D. L. Bellin et al., “Electrochemical camera chip for simultaneousimaging of multiple metabolites in biofilms,” Nature communications,vol. 7, no. 10535, Jan. 2016.[5] P. M. Levine et al., “Active CMOS sensor array for electrochemicalbiomolecular detection,” IEEE Journal of Solid-State Circuits, vol. 43,no. 8, pp. 1859–1871, Aug. 2008.[6] A. Manickam et al., “A CMOS biosensor array with 1024 3-electrodevoltammetry pixels and 93dB dynamic range,” in 2019 IEEE Interna-tional Solid-State Circuits Conference-(ISSCC), Feb. 2019, pp. 192–194.[7] J. Dragas et al., “In vitro multi-functional microelectrode array fea-turing 59760 electrodes, 2048 electrophysiology channels, stimula-tion, impedance measurement, and neurotransmitter detection channels,”IEEE Journal of Solid-State Circuits, vol. 52, no. 6, pp. 1576–1590, Jun.2017.[8] C. Giagkoulovits et al., “A 16 x 16 CMOS amperometric microelectrodearray for simultaneous electrochemical measurements,” IEEE Transac-tions on Circuits and Systems I: Regular Papers, vol. 65, no. 9, pp.2821–2831, Sep. 2018.[9] R. Hogervorst et al., “A compact power-efficient 3 V CMOS rail-to-railinput/output operational amplifier for VLSI cell libraries,” IEEE Journalof Solid-State Circuits, vol. 29, no. 12, pp. 1505–1513, Dec. 1994.[10] “A comprehensive study of the Howland current pump,” Texas Instru-ments, Dallas, TX, USA, Tech. Rep. AN-1515, Apr. 2013.320

A 64-channel, 1.1-pA-accurate on-chip potentiostat for parallel electrochemical monitoring

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A 64-channel, 1.1-pA-accurate on-chip potentiostat for parallel electrochemical monitoring

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