Constandinou, T G

Garner, D

Georgiou, P

Liu, Y

Toumazou, C

Constandinou, TG

Spiral - Imperial College Digital Repository

An Auto-Offset-Removal circuit for chemicalsensing based on the PG-ISFETLiu Yan∗†, Pantelis Georgiou†, Timothy G. Constandinou†, David Garner† and Chris Toumazou†∗Dept. of Electrical and Electronic Engineering, †Institute of Biomedical Engineering,Imperial College of Science, Technology and Medicine, London, UK SW7 2AZ United KingdomEmail: {yan.liu06, pantelakis.georgiou00, t.constandinou, c.toumazou}@imperial.ac.ukAbstract—This paper presents a novel readout circuit fora pH sensitive Programmable-Gate Ion-Sensitive Field EffectTransistor (PG-ISFET) to overcome bias issues due to thresholdvoltage variation and increase output-referred sensitivity. Com-pared to other commonly-used ISFET readouts, this circuit usestwo extra programmable nodes which are driven by a feedbackconfiguration. Using the device in a source follower configuration,one node is used to evaluate and cancel the offset of the intrinsicdevice while the other tracks and amplifies changes in pH. Asample and hold protocol has been developed to minimize theleakage effects and improve the pH sensing range. The systemhas been designed and fabricated in AMS 0.35μm, to compensatefor a threshold voltage variation of ±10.5V and provide a pHsensitivity of 200mV/pH.I. INTRODUCTIONSince its introduction in the 1970s, the Ion-Sensitive FieldEffect Transistor (ISFET) [1], has been adopted within severalchemical and biomedical applications [2]–[4]. Convention-ally, the ISFET is regarded as a MOSFET device with thepolysilicon gate replaced by a sensing membrane. Dependingon the membrane, it can be used to detect various analytessuch as hydrogen, sodium and potassium. Originally, theISFET had been established as a pH sensor due to silicondioxide which was used as the initial membrane. Since thenhowever, numerous efforts have been made to realize ISFETsin a CMOS-compatible technology, which is passivated withsilicon nitride that is also pH sensitive [2]. This has the addedadvantage of the low cost of production and potential formonolithic integration with other circuits.However, CMOS ISFETs suffer from many non-idealities,in particular threshold voltage variation and drift [1], whichreduce the measurement accuracy. So far, few efforts haveproved effective enough to minimize these. In [5], UV ra-diation had been used to correctadjust the threshold voltagefrom 10V to 5V. In [6], an exponential drift model had beenproposed based on the dispersive transport to cancel out drift.However, the detailed mechanisms of offset and drift stillremain unclear. Additionally, the read-out interface circuitsfor the ISFET must also be designed to reduce noise inpH measurement. There are two general configurations, thesource drain follower [7], and the gate readout [8]. Currentresearch on these has focused on cancelling various nonlineareffects mentioned but also reducing the power consumptionand hardware complexity [4]. In [9], a simple ProgrammableGate ISFET(PG-ISFET) was proposed which utilises a pro-C-passC-HelmC-Gouy(a) (b)VgB S DElectrodeSiO2MetalSi3N4ElectrolyteVchemTop MetalC_o C_ePositive Ions Negative Ions Charge in the structurePoly2Poly1FETFig. 1. (a) is the structure of CMOS-based device, (b) is the schematic ofthis devicegrammable gate to adjust the devices threshold voltage withoutthe need for any additional post-fabrication processes.In this paper, we propose a novel readout based on aPG-ISFET shown in Fig 1, to cancel out threshold voltagevariation due to trapped charge in addition to amplifyingpH sensitivity. This paper is organized as follows: SectionII describes the theory governing ISFET sensors as well asthe schematic design of the readout. Section III describes thespecific read-out protocol used to sample and readout the pHvariation from the sensor. Finally, simulation results and thesystem fabrication are discussed in Section IV.II. ISFET THEORY AND READOUT SCHEMATICPrincipally, an ISFET is a MOSFET, with its gate exposed tothe electrolyte and the gate bias set using a reference electrode.The overall chemical potential drop through the referenceelectrode-electrolyte-insulator interface is thus given by [4]:Vchem = Eref − ψo + ψlj + χsol − Qox + QssCox(1)where Eref is the contribution of the reference electroderelative to vacuum; χsol is the surface dipole potential.ψoand ψlj are the potential drops in the insulator/electrolyte andreference electrode/electrolyte surface. Qox and Qss are thecharge density in the oxide and the surface state [?]. ψo is978-1-4244-3828-0/09/$25.00 ©2009 IEEE 1165C-passpH sense membranSense nodeEvaluationnodeDevice BiasingC_oC_e+-+-RIbiasA1A2A3Vs_refVo_ref+-C_store(a) (b)VoS3S1S2VspH sense DeviceFeedbackReadout Evaluation/CancellationExternalControl/SampleFig. 2. (a) System structure and (b) readout schematicthe only pH dependent term and therefore the potential whichneeds to be monitored.The sensing mechanism of the electrolyte-insulator interfaceis commonly described by a double layer model coupled to anintrinsic surface site model [3]. This double layer is commonlymodelled by two capacitors in series, which describe theequilibrium of charge distribution and exchange. Likewise, theintrinsic surface site model is described by the site-bindingtheory, in which the exchanging and bonding of ions fromthe sites are in equilibrium. However, as a result of these twophenomena, the ideal sensitivity of such an interface is sub-nernstian. The sensitivity is thus given as:S =∂ψo∂pH= 2.3kTqα (2)where α is a scaling down factor which represents theseeffects.There has been a recent drive to implement ISFETs incommercial CMOS technologies to take advantage of a lowmanufacturing cost in addition to exploiting the potentialof monolithic integration. This is possible due to the pHsensitivity of native passivation of CMOS dies, which is siliconnitride on silicon dioxide. Amongst various structures reported,the continuous stacked metal layer approach has been provedto be the most effective [2]. This method uses a floating metalstack to connect the the polysilicon gate of the MOSFETto the top passivation where is can sense changes in pH.However, due to the CMOS process flow, this method has beensusceptible to trapped charge which results in large thresholdvoltage variations.In [5], the threshold variation was shifted by UV illumi-nation, which removed trapped charge in either the surfaceinsulator or the floating layer. By including the effects oftrapped charge we define the floating gate voltage of theCMOS ISFET as:VFG =CpassCT(VG − Vchem) + CGSCTVS +CGDCTVD +QTCCT(3)where QTC models the trapped charge, Cpass is the ca-pacitance of passivation layer; CGS / CGD are the capacitorsbetween gate and source/drain; CT is the total capacitance ofthe floating gate note and Vs and Vd are the source and drainvoltages of intrinsic FET device.In [9] a PG-ISFET was shown to cancel effects of trappedcharge by adding an extra capacitor to the floating gate toscale down the voltage. We can see this effect by introducinga capacitor Ce, which can couple a voltage Ve, to the floatinggate node which now becomes:VFG =CpassCT(VG + Vchem) +CGSCTVS +CGDCTVD+QTCCT+CeCTVe(4)Assuming we set QTC = Ve.Ce, the programmable gateeffectively cancels out the trapped charge. For an N-typeISFET operating in the saturation region, the drain currentis defined as:ID =k′2WL(VFG − VS − Vth)2(1 + λVDS) (5)What we can see from Eq.(5) is that there are five adjustablevalues which can be used to track the pH changes: VG, VS ,VD, Ve and ID. In one configuration we can fix ID andVDS , and track the gate voltage [8] as a voltage follower.In another, we can track the source voltage [7] by groundingthe reference electrode. Grounding the reference electrode ispreferred because first, the changing electrolyte and bubblesin the solution could make the feedback to gate unstable,second, accidently grounding the electrode through electrolyteor encapsulation will cause large leakage current.The circuit schematic of the proposed readout is shown inFig. 2.This is a source follower based on a grounded reference.It includes an evaluation capacitor, Ce which is used to cancelout trapped charge as previously mentioned. This can also beused to bias the device in the same initial region of operationevery time a measurement is required, which is stored oncapacitor Cstore.11662 31 61 2 3 4 5Measurement segmentSingle stageVoVpHTime4 5Fig. 3. A typical operation procedure including segments and phasesHowever, by introducing this capacitor on the floatinggate node, it also scales down the voltage drop across thepassivation capacitance by Cpass/CT , due to the increase ofthe total capacitance on the floating gate node. Although Vecan be used to monitor a change Vchem as is seen in Eq.(4), because of this scaling it will be significantly small.Furthermore, if Ve is used to cancel the threshold voltage andsense the pH change, the pH sense dynamic range will belimited by the bias voltage required to compensate for thetrapped charge.Therefore, in order to have rail to rail operation for pHsensing, another programmable gate node consisting of thecapacitor Co, need to be introduced to behave as a sense node,Fig. 2. Using this, the feedback loop will sense the changein Vchem with regards to VS ref and control Vo to track apotential drop due to a pH change.By substituting Eq. (1) into Eq. (4) and including bothevaluation and sense nodes and grouping constant terms inκ, VFG becomes:VFG =CpassCTVchem +CoCTVo +QTCCT+CeCTVe + κ (6)Be holding VFG constant using the circuit proposed, we seethat any pH change is counterbalanced by the voltage Vo andany trapped charge is counterbalanced by the voltage Ve. Thisscheme therefore compensates and tracks at the same time. Aswitch controlled by external signal can choose which nodeshould be on operation at any given time. Any change in Vchemwill now be:ΔVo =CpassCoΔVchem (7)and the pH sensitivity of the entire device becomes:S = 2.3kTqαCpassCo.pH (8)Therefore it can be observed that, the pH sensitivity is scaledby α and the capacitor ratio. However, by changing thecapacitor value of sense node, we can amplify the outputvoltage due to pH.TABLE IDESIGN TARGET SPECIFICATIONTerm ValuesTechnology AMS 0.35 μm CMOS 2 Polys and 4 metalsSupply voltage 3.3VCell size 600 μm× 500 μmStorage capacitor 1pFSense area 30 μm× 100 μmVt handle range ±10.5 VExternal clock frequency 200 KHzBiasing current 100 μAResistor 5 KΩIII. OPERATION AND CIRCUIT IMPLEMENTATIONThe ISFET is biased by fixing the drain current, and sourcevoltage which is defined by Ibias and Vsref , with a fixed VDSset by the resistor R and the feedback of opamps A1 and A2.As described, in this mode the the floating gate voltage VFGwill always be fixed to maintain, Ibias through the feedbackloop using opamp A3. This feedback loop can connect toeither Ve or Vo and be used to sense the offset before anISFET measurement but also measure the pH change duringan experiment. Thus, the measurement consists of two phases,the evaluation phase and the sensing phase.During normal operation it is required that the outputvoltage, Vo is mid-rail to ensure the maximum dynamic rangefor fluctuations in pH. Furthermore, a storage capacitor, Cstorehas been added to prevent discharge of node, Ve, which wouldchange the biasing of the sensor. The circuit operation is thusas follows which can be seen in Fig. 3:Φ1: Switches S1, S2 and S3 are all turned off. Φ2: S2 turnson, connecting Vo to mid-supply to set the sensing node initialoperating point. Φ3: S3 turns on, connecting the evaluationnode, Ve to the feedback loop to cancel out any trapped chargebut also establish a voltage which will keep Vo at mid-supplywhen sensing. Φ4: S3 turns off, with the programmed voltagestored on Cstore. Φ5: S2 turns off allowing Vo to temporarilyfloat. Φ6: S1 turns on, connecting Vo to the feedback loopwhich now tracks and amplifies any changes in pH.Each measurement is executed using a sequence of seg-ments, in which the above phases are included. By doing so weensure that any switch leakage is minimized to reduce sourceof error. Also by segmenting the measurement in this way wealways ensure that the output voltage is reset to mid rail, thusallowing the maximum dynamic range for sensing during asingle segment. This is advantageous for sensing applicationwhere a change in pH is more significant than absolute value.The absolute pH change however can be reconstructed bysumming the voltages at Φ6 for every segment.As we can see in Fig. 3, the measurement phase, Φ6 needsto be longer than the previous phases so that an accuratemeasurement can be acquired. This is possible as we canswitch in phases 1-5 extremely quickly due to the slew rate ofthe Op-Amp used, not allowing any time for discharge alongwith the fact that the discharge time constant is extremely largedue to the switches used.1167V (V)V (V)V (V)PHPH VeVo  VsPH time (us)V (V) CLOCK  1 2730(a)(b)(c)(d)(e)10050 150 200-1.00.01.02.0-0.50.51.52.5113 5 674-0.50.51.52.503Fig. 4. pH change response: (a) clock, (b) evaluation voltage, (c) sensevoltage, (d) source voltage and (e) pH change.IV. SIMULATION AND DISCUSSIONThe circuit has been designed and fabricated in AMS0.35μm according to the specifications shown in Table I. Thecircuit has been designed to compensate for offsets caused bysurface trapped charge in the range of ±10V , based on resultsreported in [5] and observed in our own measurements andthe offset caused by trapped charge in the floating gate in therange of ±0.5V .Simulation results has been shown in Fig. 4 whereby thesecharges are modelled using DC voltage sources with initialvalues of 10V and 0.5V respectively. The ratio of Cpass/Cois set to 4, which will give a pH sensitivity 200mV/pH , andthe Ce is set to 1pF, in order to compensate throughout theentire offset range whilst also reduce errors due to leakage.The ISFET pH sensitivity is modeled using a voltage sourceto represent Vchem which is based on silicon nitride or silicondioxide with a maximum sensitivity of 50mV/pH [2]. Weuse a test waveform, which combines a single sinusoidalperiod (with 200mV amplitude and 20μS period), with a seriesof voltage steps from -200mV to 0V then to 200mV. Thisrepresents a pH change stimulus of a sinusoid change(with4pH amplitude and 20μS period) and a series of pH stepsfrom 7 to 3 then to 11, each pH change lasting 10uS.Initially, the source voltage will be driven low to keep thetransistor in operation; In Fig. 4, point (1), the offset voltageis applied to model a change due to trapped charge. In Fig. 4period (2), we initialize all the nodes as in Φ1; In (3), Vo isset to a reference value 1.5V as in Φ2. In (4), Ve was raised to1.6V to compensate the offset as in Φ3 and stored as in Φ4; In5, we disconnect both nodes as in Φ5, no voltage changed; In(6) the sense node was activated and the measurement beginsas in Φ6; In (7) a pH change is simulated, which is trackedand amplified by a factor of 4 at Vo.The full layout of the system is shown in Fig. 5 with anoverlayed floorplan clearly indentifying each block.Current andVoltage sources PG-ISFETDigital logic and switchesInstrumentationAmplifierStoragecapacitorFig. 5. Circuit layout measuring 600μm×500μm in AMS 0.35μmCMOSV. CONCLUSIONThis paper has presented a novel solution to a timelychallenge in CMOS ISFET sensor interface. An ISFET devicewith two external capacitively coupled terminals has beendeveloped to minimize the effects of trapped charge foundin ISFETs fabricated in commercial CMOS. By using feed-back and a specific readout protocol, the circuit can providecompensation for threshold voltage and also amplification ofthe pH sensitivity. Fabricated in AMS 0.35μm, the circuit hasgreat potential for making monolithic CMOS based chemicalsensing systems.ACKNOWLEDGEMENTThe authors would like to acknowledge Dr. Leila Shepherdfor useful discussion.REFERENCES[1] P. Bergveld, “Thirty years of ISFETOLOGY What happened in the past30 years and what may happen in the next 30 years,” Sensors & Actuators:B. Chemical, vol. 88, no. 1, pp. 1–20, 2003.[2] J. Bausells, J. Carrabina, A. Errachid, and A. Merlos, “Ion-sensitive field-effect transistors fabricated in a commercial CMOS technology,” Sensors& Actuators: B. Chemical, vol. 57, no. 1-3, pp. 56–62, 1999.[3] R. van Hal, J. Eijkel, and P. Bergveld, “A general model to describe theelectrostatic potential at electrolyte oxide interfaces,” Advances in Colloidand Interface Science, vol. 69, no. 1-3, pp. 31–62, 1996.[4] L. Shepherd and C. Toumazou, “A biochemical translinear principle withweak inversion ISFETs,” Circuits and Systems I: Regular Papers, IEEETransactions on [see also Circuits and Systems I: Fundamental Theoryand Applications, IEEE Transactions on], vol. 52, no. 12, pp. 2614–2619,2005.[5] M. Milgrew and D. Cumming, “Matching the Transconductance Charac-teristics of CMOS ISFET Arrays by Removing Trapped Charge,” ElectronDevices, IEEE Transactions on, vol. 55, no. 4, pp. 1074–1079, 2008.[6] S. Jamasb, S. Collins, and R. Smith, “A physical model for drift in pHISFETs,” Sensors & Actuators: B. 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An Auto-Offset-Removal circuit for chemical sensing based on the PG-ISFET

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