Acoustic Wave.docx
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Acoustic Wave.docx
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AcousticWave
AcousticWave
TechnologySensors
Acousticwavesensorsareextremelyversatiledevicesthatarejustbeginningtorealizetheircommercialpotential.Thistutorialaddressesacousticwavesensorphysicsandmaterials,andthevarioustypesofacousticwavesensorsandtheirindustrialapplications.
BillDrafts
Acousticwavedeviceshavebeenincommercialuseformorethan60years.Thetelecommunicationsindustryisthelargestconsumer,accountingfor~3billionacousticwavefiltersannually,primarilyinmobilecellphonesandbasestations.Thesearetypicallysurfaceacousticwave(SAW)devices,andactasbandpassfiltersinboththeradiofrequencyandintermediatefrequencysectionsofthetransceiverelectronics.Severaloftheemergingapplicationsforacousticwavedevicesassensorsmayeventuallyequalthedemandofthetelecommunicationsmarket.Theseincludeautomotiveapplications(torqueandtirepressuresensors),medicalapplications(chemicalsensors),andindustrialandcommercialapplications(vapor,humidity,temperature,andmasssensors).Acousticwavesensorsarecompetitivelypriced,inherentlyrugged,verysensitive,andintrinsicallyreliable.Somearealsocapableofbeingpassivelyandwirelesslyinterrogated(nosensorpowersourcerequired).
AcousticWaveTechnologyOverview
Acousticwavesensorsaresonamedbecausetheirdetectionmechanismisamechanical,oracoustic,wave.Astheacousticwavepropagatesthroughoronthesurfaceofthematerial,anychangestothecharacteristicsofthepropagationpathaffectthevelocityand/oramplitudeofthewave.Changesinvelocitycanbemonitoredbymeasuringthefrequencyorphasecharacteristicsofthesensorandcanthenbecorrelatedtothecorrespondingphysicalquantitybeingmeasured.
Virtuallyallacousticwavedevicesandsensorsuseapiezoelectricmaterialtogeneratetheacousticwave.PiezoelectricitywasdiscoveredbybrothersPierreandPaul-JacquesCuriein1880,receiveditsnamein1881fromWilhelmHankel,andremainedlargelyacuriosityuntil1921,whenWalterCadydiscoveredthequartzresonatorforstabilizingelectronicoscillators[1].Piezoelectricityreferstotheproductionofelectricalchargesbytheimpositionofmechanicalstress.Thephenomenonisreciprocal.Applyinganappropriateelectricalfieldtoapiezoelectricmaterialcreatesamechanicalstress.Piezoelectricacousticwavesensorsapplyanoscillatingelectricfieldtocreateamechanicalwave,whichpropagatesthroughthesubstrateandisthenconvertedbacktoanelectricfieldformeasurement.
PiezoelectricSubstrateMaterials
Figure1.AcousticwavedevicesaremanufacturedbymeansofaphotolithographicprocesssimilartothatusedtomakeICs.Theonlydifferenceisthatnojunctionexistsinacousticwavesensors.
Amongthepiezoelecticsubstratematerialsthatcanbeusedforacousticwavesensorsanddevices,themostcommonarequartz(SiO2),lithiumtantalate(LiTaO3),and,toalesserdegree,lithiumniobate(LiNbO3).Eachhasspecificadvantagesanddisadvantages,whichincludecost,temperaturedependence,attenuation,andpropagationvelocity.Table1listssomerelevantspecificationsforeachmaterial,includingthemostpopularcutsandorientations[2].Aninterestingpropertyofquartzisthatitispossibletoselectthetemperaturedependenceofthematerialbythecutangleandthewavepropagationdirection.Withproperselection,thefirstordertemperatureeffectcanbeminimized.Anacousticwavetemperaturesensormaybedesignedbymaximizingthiseffect.Thisisnottrueoflithiumniobateorlithiumtantalate,wherealineartemperaturedependencealwaysexistsforallmaterialcutsandpropagationdirections.Othermaterialswithcommercialpotentialincludegalliumarsenide(GaAs),siliconcarbide(SiC),langasite(LGS),zincoxide(ZnO),aluminumnitride(AlN),leadzirconiumtitanate(PZT),andpolyvinylidenefluoride(PVdF).
FabricationofAcousticWaveDevices
Figure2.Atypicalacousticwavedeviceconsistsoftwosetsofinterdigitaltransducers.Onetransducerconvertselectricfieldenergyintomechanicalwaveenergy;theotherconvertsthemechanicalenergybackintoanelectricfield.
Thesensorsaremadebyaphotolithographicprocess(seeFigure1).Manufacturingbeginsbycarefullypolishingandcleaningthepiezoelectricsubstrate.Metal,usuallyaluminum,isthendepositeduniformlyontothesubstrate.Thedeviceisspin-coatedwithaphotoresistandbakedtohardenit.ItisthenexposedtoUVlightthroughamaskwithopaqueareascorrespondingtotheareastobemetalizedonthefinaldevice.Theexposedareasundergoachemicalchangethatallowsthemtoberemovedwithadevelopingsolution.Finally,theremainingphotoresistisremoved.Thepatternofmetalremainingonthedeviceiscalledaninterdigitaltransducer,orIDT.Bychangingthelength,width,position,andthicknessoftheIDT,theperformanceofthesensorcanbemaximized.
AcousticWavePropagationModes
Acousticwavedevicesaredescribedbythemodeofwavepropagationthroughoronapiezoelectricsubstrate.Acousticwavesaredistinguishedprimarilybytheirvelocitiesanddisplacementdirections;manycombinationsarepossible,dependingonthematerialandboundaryconditions.TheIDTofeachsensorprovidestheelectricfieldnecessarytodisplacethesubstrateandthusformanacousticwave.Thewavepropagatesthroughthesubstrate,whereitisconvertedbacktoanelectricfieldattheIDTontheotherside.Figure2showstheconfigurationofatypicalacousticwavedevice.Transverse,orshear,waveshaveparticledisplacementsthatarenormaltothedirectionofwavepropagationandwhichcanbepolarizedsothattheparticledisplacementsareeitherparalleltoornormaltothesensingsurface.Shearhorizontalwavemotionsignifiestransversedisplacementspolarizedparalleltothesensingsurface;shearverticalmotionindicatestransversedisplacementsnormaltothesurface.
Figure3.Althoughitistheoldestacousticwavedevice,thethicknessshearmoderesonatorisstillusedformeasuringmetaldepositionrates.
Awavepropagatingthroughthesubstrateiscalledabulkwave.Themostcommonlyusedbulkacousticwave(BAW)devicesarethethicknessshearmode(TSM)resonatorandtheshear-horizontalacousticplatemode(SH-APM)sensor.
Ifthewavepropagatesonthesurfaceofthesubstrate,itisknownasasurfacewave.Themostwidelyusedsurfacewavedevicesarethesurfaceacousticwavesensorandtheshear-horizontalsurfaceacousticwave(SH-SAW)sensor,alsoknownasthesurfacetransversewave(STW)sensor.
Allacousticwavedevicesaresensorsinthattheyaresensitivetoperturbationsofmanydifferentphysicalparameters.Anychangeinthecharacteristicsofthepathoverwhichtheacousticwavepropagateswillresultinachangeinoutput.Allthesensorswillfunctioningaseousorvacuumenvironments,butonlyasubsetofthemwilloperateefficientlywhentheyareincontactwithliquids.TheTSM,SH-APM,andSH-SAWallgeneratewavesthatpropagateprimarilyintheshearhorizontalmotion.Theshearhorizontalwavedoesnotradiateappreciableenergyintoliquids,allowingliquidoperationwithoutexcessivedamping.Conversely,theSAWsensorhasasubstantialsurface-normaldisplacementthatradiatescompressionwavesintotheliquid,thuscausingexcessivedamping.Anexceptiontothisruleoccursfordevicesusingwavesthatpropagateatavelocitylowerthanthesoundvelocityintheliquid.Regardlessofthedisplacementcomponents,suchmodesdonotradiatecoherentlyandarethusrelativelyundampedbyliquids.
Otheracousticwavesthatarepromisingforsensorsincludetheflexuralplatewave(FPW),Lovewave,surface-skimmingbulkwave(SSBW),andLambwave.Beforeturningtoapplicationexamples,itishelpfultobrieflyrevieweachsensortype.
BulkWaveSensors
ThicknessShearModeResonator.TheTSM,widelyreferredtoasaquartzcrystalmicrobalance(QCM),isthebest-known,oldest,andsimplestacousticwavedevice.AsshowninFigure3,theTSMtypicallyconsistsofathindiskofAT-cutquartzwithparallelcircularelectrodespatternedonbothsides.Theapplicationofavoltagebetweentheseelectrodesresultsinasheardeformationofthecrystal.
Figure4.Intheshear-horizontalacousticplatemode(SH-APM)sensor,thewavestravelbetweenthetopandbottomsurfacesoftheplate,allowingsensingoneitherside.
Thisdeviceisknownasaresonatorbecausethecrystalresonatesaselectromechanicalstandingwavesarecreated.Thedisplacementismaximizedatthecrystalfaces,makingthedevicesensitivetosurfaceinteractions.TheTSMresonatorwasoriginallyusedtomeasuremetaldepositionratesinvacuumsystemswhereitwascommonlyusedinanoscillatorcircuit[3].Theoscillationfrequencytracksthecrystalresonanceandindicatesmassaccumulationonthedevicesurface.Inthelate1960s,theTSMresonatorwasshowntooperateasavaporsensor.
TheTSMfeaturessimplicityofmanufacture,abilitytowithstandharshenvironments,temperaturestability,andgoodsensitivitytoadditionalmassdepositedonthecrystalsurface[4].Becauseofitsshearwavepropagationcomponent,theTSMresonatorisalsocapableofdetectingandmeasuringliquids,makingitagoodcandidateforabiosensor.Unfortunately,thesedeviceshavethelowestmasssensitivityofthesensorsexaminedhere.TypicalTSMresonatorsoperatebetween5and30MHz.Makingverythindevicesthatoperateathigherfrequenciescanincreasethemasssensitivity,butthinningthesensorsbeyondthenormalrangeresultsinfragiledevicesthataredifficulttomanufactureandhandle.Recentworkhasbeendoneto
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