a novel monolithic piezoelectric sensor

8/7/2019 A NOVEL MONOLITHIC PIEZOELECTRIC SENSOR

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A NOVEL MONOLITHIC PIEZOELECTRIC SENSORM.G. Schweyer, J. A. Hilton*, J. E. Munson and J.C. Andle,BIODE, Inc.,20 Freedom Pkwy, Bangor, ME 04401, USA

*Currently withInnovative Research Opportunities, Brewer, ME04412, USADepartment of Electricaland Computer Engineering,U. Maine, Orono. ME 04469, USAl.M. Hammond, R.M. Lec and Q. Lin,

ABSTRACTA novel Monolithic Piezoelectric Sensor (MPS) is

presented for the detection of physical, chemical andbiochemicalmeasurands. This new sensor overcomesspecific deficiencies associated with the Quartz CrystalMicrobalance (QCM) whilestill employing a well-characterized, temperature-stableThickness-Shearand liquid phase measurements; however, the principalMode (TSM). The sensor is applicable to both gaseousbenefit of the MPS is i n liquid phase measurements. Inthese applications, it offers the ability to operate simple,ye t stable, oscillator circuits in relatively viscous media.pole coupled resonator, inwhich mechanical coupling

The proposed sensor structure is based on a two-between the electrical input and output determines theelectricalproperties. This structure offers approxi-mately 180"of phase shift over its 3dB bandwidth withresonant frequency and approximately 0" of insertionnominally 180" of insertionphaseat the symmetricphase at theantisymmetric resonant frequency.which measure the symmetric frequency, the antisym-

Simple oscillator circuits may be implementedmetric frequency or the nominal center frequency. Thisnovel MPS sensor structure should accelerate the com-mercialization of piezoelecaic sensor technology, par-ticularly in such areas as chemical, biochemicalandenvironmental testing.

INTRODUCTIONOf the availablepiezoelectric sensors, thebest

known is the simplest and oldest Structure, namely theQCM. This configuration is functionally baseduponth e TSM resonance of an AT-quartz or similar crystal.A typical QCM structure consists of two parallel elec-trodes, one on the top and the other on th e bottom of acrystal plate. The resulting parallel plate capacitor ex-hibits resonances associated with the electromechanicalstanding waves in the plate. Well-designed devices ex-hibit a single significant electromechanical resonanceand its odd harmonics.simplicity of manufacture,ability to withstandharsh

The QCM has several positive features, includingenvironments, temperature stability of the series reso-deposited on th e crystal surface. When operated in air,nant frequency and good sensitivity to additional mass

0-7803-4153-81971$10.00 0 1997 IEEE

the mechanical resonance has high Q (wL,,,/R,), whichresults in a very narrow bandwidth resonance.bleenergy into the liquid, the sensor is theoretically

Because the shear wave does not radiate apprecia-applicable to liquidphasemeasurements.However,viscous losses in the liquid result in substantially highervalues of resistance. The addedresistance results inresistance at resonance becomes large compared to th emuch broader bandwidth resonances. In addition, therelatively small shunt reactance of C. for highly viscousliquids and th e shunt capacitance limits therange ofresistance that can be accommodated.

The QCM is a two-terminal device that requires atleast one of the active electrical terminals to be placedin contact with the measurand for sensing. In the gasphase,there are few constraints on the mounting orelectrical connectivity of the resonator within the cir-cuit. In the liquid phase, however,it is critical to ensurethat the fluid does not electrically short the positive andto ensure that only one electrode is perturbed. Sincenegative electrodes. Therefore, a fluid cell is requireda n y stray capacitance between the sensing electrode andRF ground, via the fluid, can significantly alter th e im-pedanceproperties of the crystal, it is important tomaintain the sensingsurface at RF ground.

A more robust and simple TSM based sensor is re-quired. The ideal TSM structure would retain the posi-tive features of the QCM, including simplicity, rugged-ness,temperaturestabilityandmass sensitivity. Ide-ally, the structure would be incorporated into the oscil-lator circuitry with one groundedelectrode and separateinput and output electrodes. Finally, no critical circuitelements are required to derive thenecessaryphaseshift to completethe oscillator loop. This eliminatesthe need for tuning of the circuit and eliminates insta-bilities due to capacitor and inductor temperature coef-ficients.

Such a feedback element is well known in signalprocessing applications and has beenknown as theMonolithic Dual Resonator (MDR), the Coupled Reso-natorFilter(CRF) and other names indicative oftheunderlyingmonolithic crystal filter topology. A greatdealofdesignand analysis literature describes theMDR [ l ] , which may be directly applied to the designof suitable vapor phase detectors. Modification to ac-count for viscous losses is required for accurate fluid

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phase design. The resulting sensor is called the mono- transducer. The resulting frequency was then employedlithic piezoelectric sensor (MPS) [Z]. as the cutoff frequency i n the one dimensional (Z) lon-

MPS PROTOTYPEDESIGNFigure 1 depicts the physical structure of the MPS.

This smctuse is simple to manufactuse, package andinstrument. It is rugged and reliable. The signal trans-mission is acoustic and is thus both stable and easilyperturbed by addedmass. Sensing occurs on a TTgrounded electrode. Other electrode geometries areallowableandrectangulargeometries are particularlysimpler to design and model.

gitudinal modelof the device.CQ

Figure 2 presents the suggested equivalent circuit,which is obtained by modeling mechanical and electri-cal coupling betweentwo identical resonators. Themechanical coupling occurs due to the overlap of

Figure 2. The MPS equivaiml circuit formists d tw o QC M equivalentcircuit. with mehs nie d coupling Inodeled as mutual inductance) snd np ~ i t krspsrlhncr. Other models h a d on A and I-induetor networks.ICequivdenl totk mutual inductws ma r m-ce

secondpair of longitudinal modes were allowed bu tLongitudinalparameterswere varied suchthat a

would exhibit poor trapping. If this approach were ap-plied to filter design, poorrejectionwouldresult.However, in the current application, theonly consid-erations are that an oscillator only be able to lock onto asinglemodeandthatthesecondsymmetricmode besufficientlyremoved from the first anti-symmetricmode.Thiswasaccomplished by selecting a suffi-cientlylowmutual coupling between th e resonators,such that the bandwidths of th e first and second longi-tudinal mode pairs were small compared to the separa-tion betweenthe pairs of modes.. .acoustic fields in the two resonators. The coupling is

well modeled as mutual coupling between the motionalinductors. The QCM equations describe all of the com-ponents of the equivalent circuit except the mutual cou-pling between the motional inductors, M, and the para-sitic gap capacitance, C,, between the electrodes. I 3 m m IElectrical coupling occurs due to the capacitance, C,between the closely spaced electrodes. While C, is onthe order of 1-10pF, C, is on the order of 0.01-0.1 pF.Therefore, C, is typicallynegligible in narrowband-width models.mation on the properties and applicability of MPS de-

A previous MPS design [Z]provided useful infor-vices;however, it wasnotoptimally designed. Theenergy trapping solutions of Smythe [ l] were employedin a refined Mathcad@lmodel to determine the largestelectrode areas which would allow single mode op-eration. Itwasassumedthatan antisymmetric trans-verse mode (alongX) would not electrically couple to asymmetric device. Thus, the width was maximized andthe metal thickness was selected to critically excludethe third transverse mode. In this manner, only a singletrapped transverse mode was electrically coupled to the

Electrode Length (Z) I 7 m mThickness(Y) I 0.33 mmElectrode Width (X ) I l l m m IArea 77 mmCO

150nm, continuousGold Thickness 150 nm, patternedAluminum Thickness

0.017HLm59.33 fFCm,131Alpha (turnsratio)9.3 pF

Table 1. Dtaip p m e l c r r and e q u i v d m t circuit values for air ladedoperation dthe opiimized design

Due to the interactionbetweenthedesignvari-consideration was to minimize the fixture losses whileables, an iterative procedure was employed. A furtherallowing rubber O-rings to be employed on a standard1 diameter 5 MHz crystal. This places an upper limiton the electrode dimensions. Table 1 presents the de-

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sign data for the optimiz ed sensor. Further reductionsdiameter or thinner plate would be required.in insertion loss are possible; however, a larger crystal

MPS PROTOTYPE PROPERTIESFigure 3. Three curves present the range of electricalThe predicted frequencyrespo nse is presentedintransferefficiency in a 50 oh msystemfor typicalto gassensing.Lowconcentrations of glycerin ap-operating conditions. Air loaded responses are relevantproximate biochemical sensingin dilute plasma orwell-buffered saline solutions. The operation of the sensormachinery applications.in motor oil is also relevant for automotive and other

..4.8) 5.m rtn L a 5m SW hOl

*.glr*WFigure 3. Theoretical response of the optimized MPS inair, 2435% glycerin and 1OW-40motor oil. The Eagle-ware @ model does not current ly include anhsrmo niw.

4 89 5 5.01 5.02 5.03 5.06 5.05F n q u m(W)

Figure 4. Representative experimental d ata for th e mag-nitude of S2:. Responses were measured for air, motor oil(light lines) and various concentrations of glycerin (heavylines).

Figure 4 presents the corresponding experimentaldata for the magnitude of Sz l l . The second longitudinalmode is clearly observable but is well removed fromth efundamental mode.Careful observationalso re-ever, the coupling is extrem ely weak. The mutual cou-veals some effect of the second transverse mode; how-

pling agrees with theory and a traditional two- pole re-coupling exists for liquids more viscousthan 50% glyc-sponse is observedfo rlow viscosity liquids.Undererin,for which a Butterworthresponse is observed.Phase data ispresented in Figure 5.

m S 501 5.02 5m 5m 5112-W)

Figure 5. Representative experimental data for the phaseof S*:. Responses were m easured for air, motor oil (lightIlnes) an d various concentrationsof glycerin (heavy lines).

O I Z 3 1 5 ~ 1 1 P ISq"nna(dRo.E. .

liquid property, sqrt(p.11)(lines) and experimental dataFigure 6. Theoretical dependence of frequency on the(points). Data is shown for the0,90 an d 180 degree phasepoints.The asy mme tric resonanc e is insensitive to viscousor density properties of the liquid while the symmetricresonanceexhibits a strongerdependence thanpre-dicted by thicknessshearmode(TSM) theory [31.Theoretical and experimentaldataar ecompared inFigure 6. There is a consistentoffsetby whichthetheoretical model underestimates the device frequency,which is probablyanerror inthickness. Theexperi-mental data are in excellent qualitative agreement withthe Eagleware @ model andare in goodquantitativeagreement within experimental error. In particular, theantisymmetricdata is in excellentagreement.The

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center frequency data (90 point) is in excellent agree-ory [3]. This is also true of th e symmetric resonancement with both the Eaglewa re @ model and TSM the-(180)data.

C ONC LUSIONSQuartz crystal technology has demonstrated excel-temperature stability and high dynamic range of thelent promise for numerous sensor applications. Theresulting sensors have made this technology extremelycommercialized, very few liquid-based sensors haveattractive. While gas-phase sensor solutions have beenbeen ableto reliably operate in field-portable instru-mentation.The current work presents a novel approach to em-ploying thickness shear m ode (TS M) piezoelectric sen-sors to both gas-based and liquid-based sensing. Thewell-known monolithic dual resonator (MDR) offersTS Msensors in oscillator circuits. In particular, theattractive alternatives to the instrumentation of quartzinclusion of a switched phase inverter would allow thetisymmetric resonant frequencies.sequential measurement of both th e symmetric and an-

tive to mass loading by a thin solid film.The sensitiv-Both resonant modes are seen to be equally sensi-ity is identical to other TSM devices for low-viscosityfilms. Thetwo resonant modes exhibit different be-havior to viscous liquid loading. By measuring bothfrequencies, it is possible to distinguish liquid proper-ties from those of the solid film. As is the case for th eQCM , the MP S cannot distinguish liquid density fromviscosity. The use of twosensors, one with a corn-gated surface and the other with a smooth surface over-comes this limitation [4].It is possible to construct a single-frequency oscil-lator which is sensitive to bound mass but immune tosolution effects by selecting 0 oscillation conditions.While this technique is not rigorously immune to vis-cosity, experiment and theory indicate very little sensi-tivity of the 0 resonance to viscosity.

F U T U R E W O R Kbe addressed before viable sensors can be developed.While the MPS is promising, several issues must

compressional wave generation [5,6]. Low noise meth-Optimized devices must be designed which minimizeods of altering the amplifier phase between 0and 180are desirable in order to create a dual mode oscillator.Frequency-sensitivity tradeoffs must be evaluatedwhich compare the MPS sensitivity to its noise per-formance as afunction of frequency [7].applications should be pursued in order to verify the

Once optimized devices are designed, real Sensorapplicability of the technique to improved biochemical,electrochemical and other liquid-phase sensor applica-tions.

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A C K N O W L E D G E M E N T SBIODE wishes to acknowledge the State of Mainefor funding through the Center for Innovation in Bio-technology and the Center for Technology Transfer.BIODE wishes to acknowledge th e US Army for sup-port under contract DAMD17-95C-5033. The views,opinions, and/or findings contained in this report arethose of the authors and should not be construed as anofticial Depa rtment of the Army po sition, policy ordecision unless so designated by other documentation.lino of the University of Maine and Dr. S teve Martin ofThe authors wish to acknowledge Dr. John Vete-Sandia National Labs for useful discussions.and Jason Ouellette of the Laboratory for Surface Sci-Most im portantly, the authors thank Reichl H askellence and Technology (LASST) at the University ofMaine for fabricating prototype sensors.

R E F E R E N C E SR. Smythe, Crystal Filters, in Miniaturized andInterrated Filters, Mitra and Kurth, editors, pp.280-329 (1989).M. Schweyer, I. Hilton*. J. Munson, I. Andle, J .M. Hammond, and R.M. k c , A Novel M onolithicPiezoelectric Sensor, Proc. 1997 Frequency Con-trol Sym posium (in press).Characterization of a Quartz Crystal M icrobalanceS. J. Martin, V.E. Granstaff and G. C. Frye,

Anal Chem 1991,63,2272-2281.with Simultaneous Mass and Liquid Loading,S. I. Martin, K. 0. Wessendorf, C. T. Gerbert, G.C. Frye, R. W. Cernosek, L. Casaus and M. A.Mitchell, Measuring Liquid Properties withSmooth and Textured Surface Resonators, Roc.1993 IEEE Intl Freq. Control Symp, pp. 603-608.T. Schneider and S. Martin, Influence of Com-pressional Wave Generation on Thickness-ShearMode Resonator Response in a Fluid, Anal Chem1995,61,3324-3335.L. Tessier, F. Patat, N. Schmitt, G. Feuillard andM. Thompson. Effect of Compressional Waveson the Response of Thickness-Shear Mode Acous-tic Wave Sensor in Liquids, Anal Chem 1994, 66.3569-3574.J. R.Vig, On Acoustic Sensor Sensitivity, IEEETrans. UFFC, May 1991.

a novel monolithic piezoelectric sensor

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