MEMSSensorsforUnderwaterApplications

MEMSSensorsforUnderwaterApplicationsV.Natarajan,M.Kathiresan,K.A.Thomas,RajeevR.Ashokan,G.Suresh,E.VaradarajanandShinyNairAbstractMicroelectromechanicalsystem(MEMS)-basedsensorsformarineenvironmenthelptorealizenewsystemsthatbringenhancedlevelsofperception,control,andperformancetosonarsystemsandsensorsrelatedtomarineenvi-ronments.Processing,assembly,packaging,testing,andmanufacturingmethodsareallhighlydictatedbytheintendedapplicationofMEMSdevices;hence,thesedisciplinesarebeinghoneduptomeetthedemandswithnewmaterialsandperformancerequirementsacrossawidespectrumofunderwaterapplications.Fivebasicparametersaremeasuredintheoceantodeneitsphysicalstate:tempera-ture,salinity,pressure,density,andvelocityofsound.Thesecanbeobtainedusingapressuresensor,temperaturedetector,andaconductivitysensor.BiologicallyinspiredMEMSshearstresssensorscomprisingapiezoresistiveoatingelementofferthepotentialtomakeowmeasurementsinuidwithunprecedentedsen-sitivity,andspatialandtemporalresolution.Inordertogetnerresolutionofunderwaterobjectsinturbidwaters,itisimperativetoworkatMHzfrequencies.DifferenttypesoftransducerssuchasCMUT,PMUT,andHelmholtzresonatorarealsorealizedbyMEMSfabricationandarereadilyscalableinsize.Inaddition,multiplexing,pulsing,andpre-amplifyingelectronicscanbeeasilyintegratedonthesamechipwiththetransducersoronaseparatechipviaip-chipbonding.Thisallowsfor1Dand2Darraysofelementstobeeasilysteeredelectronically.Thus,fabricationofalargenumberoftransducerswithbuilt-inpre-ampliersrequiredinaplanararraycongurationispossiblewithMEMS-basedtechnology.KeywordsMOSFETacousticsensorsCTDsensorsMEMSshearstresssensorsBiaxialMEMSaccelerometerV.Natarajan(&)M.KathiresanK.A.ThomasR.R.AshokanG.SureshE.VaradarajanS.NairNavalPhysicalandOceanographicLaboratory,Thrikkakara,Kochi682021,Indiae-mail:natarajan_K.J.Vinoyetal.(eds.),MicroandSmartDevicesandSystems,SpringerTractsinMechanicalEngineering,DOI:10.1007/978-81-322-1913-2_29,SpringerIndia2014487488V.Natarajanetal.1IntroductionMicroelectromechanicalsystems(MEMS)istheintegrationofmechanicalele-ments,sensors,actuators,andelectronicsonacommonsubstratethroughtheutilizationofmicrofabricationtechnology.MEMSdeviceshaveledtothedevelopmentofproductsthatsense,think,act,communicate,self-power,andnavigate.Ithasgreatpotentialintheareaofmarineenvironmenthydrophones,tiltsensors,Conductivity,Temperature,andDepth(CTD)sensors,owsensors,micro-uidics,bio-MEMS,micro-powerandsecurity/defenceapplications.Inte-gratingMEMSdevicesismorecomplexthanmanufacturingthem.ThekeytorapidresearchanddevelopmentofMEMSisduetobothtechnologyandfabricationadvancesthroughinnovativemodeling,design,fabrication,andcharacterizationandtesttoolsthathaveevolvedoverthelastfewyears.MEMS-basedsensorsformarineenvironmenthelptorealizenewsystemsthatbringenhancedlevelsofperception,control,andperformancetosonarsystemsandsensorsrelatedtomarineenvironments.MEMSsensorsndapplicationswheresize,weight,andpowermustbereducedsimultaneouslywithanincreaseinfunctionalityandlowcostachievedbybatchfabricationandhighprecisionintheirmanufacture.Processing,assembly,packaging,testing,andmanufacturingmethodareallhighlydictatedbytheintendedapplicationofMEMSdevices;hence,thesedis-ciplinesarebeinghonedtomeetthedemandswithnewmaterialsandperformancerequirementsacrossaverywidespectrumofapplications.PiezoelectricmaterialsformthebasisofadvancedMEMSsensorsandenergyharvestingtechniquesthataregainingprominenceinindustrial,medical,andinunderwaterapplications.Researchersarelayingemphasistoperfectaleadzir-conatetitanate(PZT)depositionprocesscompatiblewithmassproductionandintegrateitintoestablishedMEMSprocessesfortheaboveapplications.Fivebasicparametersaremeasuredintheoceantodeneitsphysicalstate.Thesearetemperature,salinity,pressure,density,andvelocityofsound.Thesecanbeobtainedusing(a)pressuresensortomonitordepth,(b)platinumresistancetemperaturedetector,and(c)conductivitysensorutilizingplatinumelectrodesorinductivetypecells;thelasttwoparametersarederivedfromCTDdata.Apartfromthesensoritisofequalimportancetodesignasignalconditioningcircuittoconverttheresponsesofvarioussensorsintousefulform.MEMSshearstresssensorsofferthepotentialtomakeowmeasurementsinuidwithunprecedentedsensitivity,alongwithspatial,andtemporalresolution.MostMEMSshearstresssensorshavebeendevelopedformeasurementsinairandutilizeindirectmethods.Thedevelopmentofmicromachined,distributedowsensorsbasedonabiologicalinspiration,theshlaterallinesensors,isthetechnologyoftheday.Itcomprisespiezoresistiveoatingelementshearstresssensorsfordirectdynamicmeasurement.Acousticsensorscanbescalarorvector,meaningtheycanmeasureonlythemagnitudeofacousticpressureatapointorcanmeasureanyoftheacousticMEMSSensorsforUnderwaterApplications489vectors,i.e.,displacement,velocity,oraccelerationintermsofitsmagnitudeanddirection.Scalarsensorshaveomnidirectionalpatternandlargearraysofscalarsensorsareusedfordirectionofarrival(DOA)estimation.Vectorsensorswiththeirinherentdirectionalpatternofgureofeightoffergreateradvantageintermsoftheirsmallerdimensionsandselectivenoiserejectioncapabilities.Inordertogetnerresolutionoftheunderwaterobjectsinturbidwaters,itisimperativetoworkatMHzfrequencies.Alargenumberoftransducersarerequiredinaplanararrayconguration.ThisispossiblewithMEMS-basedtechnology,whichhasabuilt-inpre-amplierincloseproximitytothesensor.Differenttypesoftransducersarebeingpracticed:CMUT,PMUT,andHelmholtzresonator.MUTsarerealizedbybatchfabricationtechniquesemployedbythesemiconductorindustryandarereadilyscalableinsize.Inaddition,multiplexing,pulsing,andpre-amplifyingelectronicscanbeeasilyintegratedonthesamechipwithtransducersoronaseparatechipviaipchipbonding.Thisallowsfor1Dand2Darraysofelementstobeeasilysteeredelectronically.2TypesofSensors2.1AcousticSensorAnovelMEMSdesignforanunderwateracousticsensor,hydrophone,hasbeenattempted.Ahydrophoneisadevicethatpicksuptheunderwateracousticenergyandconvertsitintoelectricalenergy.Generallyahydrophoneisusedtomeasuretheintensitiesatpointswithinacousticelds.Forthispurpose,itisdesirablethatthehydrophoneisrelativelysmallcomparedtothewavelength,sothattheeffectofthehydrophoneupontheeldisnegligible.ManyLeadZirconateTitanate-polymer(PZT)compositetransducerswithvariousconnectivityhavebeendeveloped,whichconsistofsmallPZTelementsembeddedinalow-densitypolymermatrix.ThesecompositeshaveloweracousticimpedancethanconventionalPZT,leadingtoanimprovedbandwidth1.Oneofthemajorproblemsassociatedwiththesepiezoelectriccompositematerials,however,isthehighfabricationcost.ThepiezoelectricPVDF,polyvinylideneuoride(PVDF)lmhasbeenfoundtobeespeciallysuitedforunderwateracousticeldmeasurementsbecauseofitslowacousticimpedanceandlateralcouplingcoefcient.WiththedevelopmentofsiliconICtechnology,itisdesiredtointegratethemechanicalelementswithmicroelectronicdevicesonacommonsiliconsubstratetofabricatelowcost,highlysensitiveandminiaturizedsensors.InthisstructureFig.1a,asheetofPVDFlmisbondedtotheextendedgateofaMOSFET,resultinginastructurenamedaspiezoelectric-oxide-semiconductoreld-effecttransistor(POSFET)2.SincethePOSFETstructurecombinestheactiveelectronicswiththesensingelementonthesamechip,ithasbeenwidelyusedandstudiedasanessential490V.Natarajanetal.Fig.1aSchematicofMOSFET-Acousticsensor.(a)(b)bFabricatedMOSFET-Acousticsenorwithpre-ExtendedGate/SensoramplierandrabbitearsformountingSourceGateDrainstructureforpiezoelectricorpyroelectricsensors3,4.However,thereisonedisadvantagewiththisstructure,whichisthecapacitanceexistingbetweentheextendedgateelectrodeandthesemiconductingsiliconsubstrate,withtheinsu-latingsilicondioxidelayeractingasadielectricmaterial.DuetothelimitationsofICtechnology,thesilicondioxidelayeristhinnerthanthesensor.Thiscapacitanceshuntsalargepartofthepiezoelectricsignalgeneratedbythesensorandresultsinasignicantreductionofthedevicesensitivity.Somemethodshavebeendevelopedtominimizetheparasiticcapacitanceusinghighresistivitypolysiliconorsapphiresubstratetoreplacethesiliconunderthelowerelectrode.Usingthistechnique,theeffectivesensitivityispredictedtobeincreasedbyafactorofveormore.Anintegratedtransducerstructurewiththeextended-gateelectrodeoftheMOSFETpaddedupwithapolyimidedielectriclayerisreportedandasensitivityimprovementofover13dBwasachieved5.SomehavereportedtheuseofSU-8insteadofpolyimidebelowthepiezoelectricsensorduetoitscompatibilitywithphotolithographyprocesses6,7.Thesche-maticandfabricateddeviceisshowninFig.1a,b.2.2Conductivity,Temperature,andDepthSensorsInSONAR,soundisusedforthedetectionandclassicationofunderwaterbodies.TheSONARperformancedependsonthesoundspeedproleinwaterwhichvarieswithlocationandseasons.Soundproleinaregioninoceanishighlyinuencedbythedepth,temperature,andsalinity(conductivity)ofseawater.MicroCTDsarebeingattemptedtobemanufacturedusingMEMSpolymertechnology8.Thecruxoftheproblemwithpressuresensorsforunderwaterapplicationsistheirpackaging.Temperaturesensorshavebeendeveloped.Theconductivitysensoristhemostdifcultofthemall.TheuidhastopassthroughthemicrochannelsofthesensorandushedatasteadyratesoastorecordCTDparameters.Apartfromthesensoritisofequalimportancetodesignasignalconditioningcircuittoconverttheresponsesofvarioussensorsintousefulform.MEMSSensorsforUnderwaterApplications4912.2.1ConductivitySensorConductivityisanintrinsicpropertyofseawaterfromwhichsalinityanddensitycanbederived.Fromthemeasureofconductance,conductivitycanbecalculatedaftertakingintoconsiderationthecellconstantthatreectstheratiooflengthandcross-sectionalareaofthesampledwatervolumeinwhichtheelectricalcurrentactuallyows.Theowofcurrentthroughelectrolyticconductors(liquid)isaccomplishedbythemovementofelectriccharges(positiveandnegativeions)whentheliquidisundertheinuenceofanelectricaleld.Theconductanceofaliquidcanbedenedbyitselectricalpropertiestheratioofcurrenttovoltagebetweenanytwopointswithintheliquid.Asthetwopointsmoveclosertogetherorfurtherapart,thisvaluechanges.Conductivityismeasuredbytwodesignapproaches:electrodesortransformers(inductive).TheelectrodemethodhasfourelectrodesofsufcientlylowresistanceFig.2a.Thetransformer(inductive)methodusesatransformertocoupleaknownvoltagetothewateranddetecttheresultingcurrentowwithasecondtransformercore.Theelectrodemethodissimplerandaccurate.Themeasurementcanbecarriedoutusingaconstantcurrentsourceandmeasuringthevoltage.Thecon-ductanceYismeasuredbyreadingthevoltagedropacrossthesensingelectrodesinthepresenceofaconstantcurrentowingthroughitusingOhmsLaw.Thisismultipliedbythecellconstanttoobtaintheconductivity.Thetemperaturecom-pensationtobeapplied.Maintenanceofstablecellgeometryisthelimitingdesignchallengeandiscrucialforhighaccuracywhenconsideringtheeffectsofseawatersuchascoatingsduetomineraldepositionsandwaterpollutantsduetooilslick,biofouling,industrialwastes,andmarinegrowth.Anti-foulingcoatingscanbegivenneartheentry/exitportsoftheelectrodecelltoenhancethelifeoftheconductivitycell.AdesigntoovercomethiseffecthasbeendevelopedandseveralsensorshavebeenfabricatedandusedatseaTemperatureSensorTheelectricalresistanceofaconductorvariesaccordingtoitstemperatureandthisformsthebasisofresistancethermometry.Theeffectismostcommonlyexhibitedasanincreaseinresistancewithincreasingtemperature,apositivetemperaturecoefcientofresistance.Whenutilizingthiseffectfortemperaturemeasurement,alargevalueoftemperaturecoefcientisideal;however,itsstabilityovertheshortandlongtermisequallyvital.Therelationshipbetweenthetemperatureandtheelectricalresistanceisusuallynonlinear.Forthemeasurementoftemperature,athinlmofplatinumprovidesanextremelystableandsensitivethermometer.TemperatureismeasuredindirectlybyreadingthevoltagedropacrossthesensingresistorinthepresenceofaconstantcurrentowingthroughitusingOhmsLaw.Foraccuratemeasurementofresistance,aWheatstonebridgecircuitisincorporated.492V.Natarajanetal.Fig.2aConductivity.bTemperatureandcPressuresensorPlatinumlmresistoronaceramic/polymersubstrateisaneasyapproachFig.2b.Thinlmsensorshavefastthermalresponseandtheirsmallthermalmassminimizesintrusioninthemediabeingtested.Theresistancethermometersensorisprotectedfromtheenvironmentbyathinlmofthermallyconductiveoxide.Theshowndeviceisonaexiblesubstrateandseveralvariantshavebeenpro-ducedandusedPressureSensorSiliconmicromachinedpressuresensorsarethemostmaturecommercialsensorstechnologyavailabletoday.Thepressuresensorsusedformeasurementofhighpressuresarebasedonpiezoresistivesensingtechnique11.ThebasicstructureofapiezoresistivepressuresensorconsistsoffoursensingelementsinaWheatstonebridgecongurationtomeasurestresswithinathin,crystallinesiliconmembrane.Thestressisadirectconsequenceofthemembranedeectinginresponsetoanappliedpressuredifferentialacrossthefrontandbacksidesofthesensor.Thesensorssensitivitycanbeimprovedbyndingtheoptimummembraneshapeandresistorcongurationusingniteelementanalysis.Thebridgeismadeoffourpiezoresistorslocatedonthefouredgesofthesensormembrane,closetotheedgeswherethestressisthemaximumwhenverticalpressureisappliedtothecenterofthemembraneFig.2c.Twooftheresistorsarepositionedparalleltothedirectionofthestress,andtheirresistanceincreaseswithpressure.Theothertworesistorsareorientedperpendiculartothedirectionofthestressandtheirresis-tancedecreaseswithpressure.Intheabsenceofappliedpressure,thebridgeisbalancedandtheoutputiszero.Onapplicationofpressure,thereisachangeintheresistorRa,Rb,RcandRdvalues,resultinginabridgeoutputvoltageproportionaltotheinputpressure.Forthepoly-siliconresistoracrossthediaphragm,theresistancechangecausedbypressureduetochangeinthedimensionsoftheresistorscanbeexpressedasMEMSSensorsforUnderwaterApplicationsDR=R12tDl=lDr=r4931whereDRandRarethechangeinresistanceandoriginalresistanceoftheresistortisthePoissonsratioofthematerial,Dlisthechangeinlengthoftheresistorduetopressure;listheoriginallengthoftheresistor,Dqisthechangeindensity,andqistheoriginaldensity.Thefabricateddeviceshavebeenpackagedforunderwateroperationswithdataloggingelectronicsandusedupto1,000mdepthinthemarineenvironment.2.3ShearStressSensor2.3.1IntroductionMicroElectromechanicalSystemsshearstresssensorsofferthepotentialtomakeowmeasurementsinuidwithunprecedentedsensitivity,andspatialandtem-poralresolution12,13.MostMEMSshearstresssensorshavebeendevelopedformeasurementsinair14andutilizeindirectmethods.Substantialworkonthermal-basedsensors(hotwire/lmanemometry)hasbeenreported15,butthesedevicesrequireaprioriknowledgeofowproles,insitucalibrationunderidenticalconditions,andarelimitedbyheattransferinwater.Thesesensorsaredesignedtostudytheeffectofhydrodynamicsandsurfaceroughnessonowprolesandmasstransfer.Arraysofthesesensorsallowtherstdirectmeasurementofshearstressprolesunderunsteadywave-drivenowoveracoralreefcanopy(naturalroughsurfaces),aswellasinoscillatoryowingcell-culture,andcardiovascularmockups.Robustunderwatershearstresssensorsarerequiredformeasurementswithnespatial,*100lm1mm,andtemporal(110kHz)resolution,aswella