中英文文献翻译-微型半导体流量传感器的开发

DevelopmentofminiaturizedsemiconductorflowsensorsF.Kohl，R.Fasching，F.Keplinger，R.Chabicovsky,A.Jachimowicz,G.UrbanAbstractMiniaturizedflowsensorsbasedonthinfilmgermaniumthermistorsweredevelopedofferinghighflowsensitivitiesandshortresponsetimes.Thethermistorsareplacedonasiliconnitridediaphragmcarriedbyasiliconframe.Usingthe3controlledovertemperatureschemethemeasurableairflowraterangesfrom0.6to150000cm/h.Inthispaperwemainlyreportonthedynamicpropertiesofthesensor.Theresponseofthesensortostepchangesoftheheaterpowerwillbecomparedwithitsresponsetoshockwavesforboththeconstantpowermodeandtheconstantovertemperatureoperatingmode.Asimplearrangementforthegenerationofacousticshockwaveswillbepresented.1IntroductionThereisagrowingdemandofmicro-flowsensorsforindustrial,automotive,domesticandmedicalapplications.Themeasuringprinciplecanbebasedonthermistors,thermopiles,pyroelectricelements,pn-junctions,resonatingmicrobridges,Prandtltubesandseveralothereffects110.Micromachiningisadoptedtoachievehighsensitivity,quickresponseandlowpowerconsumption。Oneimportantapplicationofflowsensorsisthemeasuringoftheinstantaneousairintakeofcombustionengines.Knowledgeofthiscombustionprocessparameterisessentialifonetriestominimizeboththeenginesfuelconsumptionandthepollutionoftheenvironment.Forthedevelopmentofsuchenginesawidevelocitymeasuringrangeandhighresolutionmonitoringofthetimecourseoftheairvelocityisdesirable.Theelectrocalorimetricflowsensorpresentedhereisbasedonaheattransferprincipleinwhichaheatedbodyiscooledbyapassingflowandthelocalrateofcoolingdependsontheflowvelocity11.Thesensorisbasedontheso-calledhotfilmflowmeasurementmethod.AverythinsiliconnitridediaphragmsupportedbyamicromachinedsiliconframeismountedflushwiththewallofaflowchannelasshowninFig.1.Athinfilmheatingresistorisembeddedinthediaphragmtoobtainasymmetricsurfacetemperaturedistribution.Twodiaphragmthermistorsmeasurethetemperatureinapositionupstreamanddownstreamoftheheater.Fig.1.Schematiccross-sectionofatypicalhotfilmflowsensorandtemperaturedistributionalongthediaphragm.H,heater;DT1,DT2,diaphragmtemperaturesensors.Atangentialflowdisturbsthethermalsymmetry.Heatiscarriedfromtheheateddiaphragmwhentheinitiallycoldfluidpassesovertheheatedsurface.Sincethefluidtemperatureincreasesinthedirectionofflowthecoolingeffectisreducedinthedown-streamarea.Inthisareathefluidtemperaturemaybecomeevenhigherthanthesurfacetemperatureresultinginalocalheatingofthediaphragm.Thusatemperaturedifferencebetweenthecooledupstreamareaandthelesscooled(orheated)downstreamareaoccurs.Thistemperaturedifferencecanbeconvertedintoanoutputvoltage,whichisusedasameasureforthefluidvelocityormassflow.Theflowrangeandsensitivityisstronglyinfluencedbythedistancebetweentheheaterandthetemperaturesensors2,12.Withasymmetricarrangementstheshapeoftheoutputcharacteristic(temperaturedifferenceversusflowrate)canbesignificantlychanged10,13Wehavedevelopedasymmetricmicromachinedsemiconductorflowsensorcapableofmeasuringbidirectionalflow.Extensivecharacterizationofthesensorwasdoneexhibitingexcellentflowsensitivityandanextremelywidemeasuringrange.Furthermore,athoroughinvestigationofthedynamicbehaviourofthesensorwascarriedout.2SensorconstructionandtechnologyA(100)siliconwaferhasbeenusedforthefabricationofthesensor.Thechipsizeis234mmandthethicknessis0.3mm.Twothinfilmthermistorsareplacedsymmetricallytoacentralheateronan800-nmthicksiliconnitridediaphragm(Fig.2).Additionalthermistorsarearrangedattherimofthesiliconchip.Theseso-calledsubstratethermistorsareusedtomeasurethefluidtemperature,whichisclosetothesubstratetemperature.Fig.2.Schematiccrossofthesensor.Thesizeofthediaphragmis0.51.1mmAllthermistorsarefabricatedbyevaporationofamorphousgermaniumontocomb-shapedelectrodesfig.3Fig.3Onemajoradvantageofthistypeofhightemperature-resolutionthermistorsisthatreliableflowsensingoperationispossiblewithonlyasmalltemperaturedifferencebetweentheheaterandthefluid.Thepresentedsensoroperateswithheaterovertemperatureslessthan25K.Fullresolutionisalreadyobtainedwithaheaterovertemperatureof10K.However,theincreaseofthefluidtemperaturecausedbytheheaterismuchsmallerthantheseovertemperatures.Sothesensorisespeciallyapplicableinsuchcaseswheretheheatermustnotcauseasignificantincreaseofthefluidtemperature.Themaximumelectricalpowerratingoftheheateris40mWifthefluidisair.However,thetypicaloperatingpowerisabout4mW,whichcorrespondstoaheatervoltageof3V.Bothplatinumandnichromehavebeenappliedastheheatermaterial.Furthermore,narrowpairtolerancesofthethermistorcharacteristicsareimportanttoachievehighresolutionintemperaturedifferencemeasurements.Nonethelessahighprecisionofthesensorgeometryisnecessaryforanoffsetfreebi-directionalsensorcharacteristic.Boththeheaterandthethermistorsexhibitsmalldimensionsinthenominalflowdirectionandlargeextentsperpendiculartothisdirection.Theseextremeaspectratiosofheaterandthermistorareawerechosenforfourreasons:(a)toachieveapronounceddirectionalcharacteristicfortheflowsensitivity,(b)toavoiddelayofresponseduetothermalpropagationtimes,(c)toachievesuitableresistancevaluesofthethermistorandtheheater,and(d)toensureauniformlocaltemperaturethroughouttheamorphousgermaniumareaunderstaticanddynamicheattransportconditions.Furthermore,duetothehighaspectratioaonedimensionalmodelingofheatconductioninthediaphragmandtwo-dimensionalmodelsforheatconvectionaresufficientforbasicconsiderations.Thethinfilmstructureswereproducedonawafer,whichhasfirstbeencoveredbyasiliconnitridelayer.ThenalowstresssiliconnitrideprotectivefilmisdepositednearlyatroomtemperatureusingaPECVDprocess.Thelowdepositiontemperaturepreventsthegermaniumfilmfromrecrystallization.Bothsiliconnitridelayersformthediaphragmofthemicromachinedsensor.SiliconnitrideexhibitsalowthermalconductivityresultinginhighflowsensitivityThethermalconductivityofsiliconnitrideisabout2.3W/m?Kascomparedto150W/m?Kforsilicon.Afurtheradvantageofthesiliconnitridediaphragmisitssmallthicknessresultinginasmallthermalconduction.The800-nmthickdiaphragmusedinoursensorhasbeenprovedtobeverystableinatangentialflow(Fig.2).Amorphousgermaniumexhibitshighvaluesofboththeresistivityanditstemperaturecoefficient.Thetemperaturecoefficientofresistance(TCR)isapproximately22%/Kandtheresistivityisabout5Vmatroomtemperature.Measurementsofthetemperaturedependenceofthethermistorresistancebetween77and330Krevealed,thattheelectricalconductivityofamorphousgermaniumisgovernedbyavariablerangehoppingprocess14.AtroomtemperaturetheTCRvariesonlyslightlywithtemperature,whicheasesappreciabletheburdenforcompensatingofchangesoftheambienttemperature.AlayoutasshowninFig.3anda250-nmthickgermaniumfilmresultinaresistanceof70kVat208C.Ithasbeenprovedthatthelong-termstabilityofthischaracteristicisbetterthan0.5%peryear.Anoiseequivalenttemperaturedifferenceof10mKforabandwidthof10Hzisachievedwiththisthermistortechnology10.Forcomparison,Johnsonnoiseonlywouldlimittheresolutionto4.75mK.3Experimental3.1.SensormountingTostudythesensorpropertiesinsituationsthataretypicalforvariousapplicationsthechipwasattachedtodifferentcarrierconstructionsForfreefieldcalibrationinawindtunnelandotherexperimentalmeasurementsthesensorchipisgluedtoa0.15-mmthickprintedcircuitboard(PCB)flushfittedwiththeboardsurface(Fig.4).ForthispurposetheflexiblePCBwasformedusinganembossingdie.ThedimensionofthisPCBinthedirectionofflowis60mmandthesensorwasplacedmidways.ThegroundplaneofthePCBshieldsthesignalleadsagainstinterferencesFig.4.Schematiccross-sectionalviewofthesensormountedonaflexibleprintedcircuitboardForflowratemeasurementsthesiliconchiphastobeincorporatedinthewallofaminiaturizedflowchannel.ThiswasachievedusingarigidPCBof0.5mmthicknesswithamilleddeepeningwhichaccommodatesthesensorchipflushwiththesurfaceofthePCB.ThePCBformsonesidewallofarectangularflowchannel.ThesamePCBmeasuring15mmalongthedirectionofflowwasusedforshockwaveexperiments.TosuppressturbulencesinthisapplicationtheedgesofthePCBwereformedtoshowwedgeshape.ThePCBwasthenplacedinasymmetryplaneofthecylindricalflowchannel.3.2.ShockwavegeneratorToinvestigatetheresponsetostepchangesofflowsimpleshockwavegeneratordepictedschematicallyinFig.5wasdeveloped.AcommercialballooncontainedinaPVC-cylinderof70mmdiameterandapproximately250mmlengthisblownuptoitsburstpressurewithnitrogengas.Anorificeoftypical2-mmdiameterlimitstheacousticFig.5.Sketchofthearrangementusedforthegenerationofshockwavesandtherelatedsensorarrangementflowoftheresultingshockwave.Thesensorisplacedatasymmetryplaneoftheflowchannelof16mmdiameter,optionally300mmfromtheorificeorattheendofan8.5-mlong,3/40diameterflowchannel,whichprovidessufficientdelayofacousticreflections.Thearrangementproducesshockwavessuperimposedbyanacousticwavewhosefrequency(680Hz)correspondstothefundamentalresonanceoftheballooncontainment.Aminordisadvantageofthissimplearrangementisthepoorreproducibilityoftheflowstepamplitude,whichdependsontheactualburstpressureofeachballoonOptionallytheacousticflowoftheshockwavecanbelimitedbyanorifice.Foroperationofthesensorintheconstantpowermodeorifice,diameterslessthan2mmwereusedtoavoidexcessivecooldownbytheconvectionaccompanyingtheshockwave.3.3.DynamicpropertiesAconstantbiasof0.5Visappliedtoallgermaniumthermistors.Thecurrentflowingthrougheachthermistorisconvertedtoasignalvoltagewiththeaidofacurrenttovoltageconvertercircuit.Thisbiasingtechniquehastheadvantagethatforsmalltemperaturevariationstheconverteroutputsignalisdirectproportionaltothetemperaturechangesoftherespectivethermistor.ThecourseoftheindividualtemperatureofeachdiaphragmthermistorcanberecordedbymonitoringtheoutputofthesignalconditioningcircuitwithadigitalstorageoscilloscopeAvariationoftheheatervoltagechangesthepowerdissipatedbythecentralresistor,whichleadstocorrespondingchangesoftemperatureatthediaphragmthermistorsites.Fig.6showsatypicalrecordofsuchanexperiment,whereastepchangeoftheheatervoltagewasapplied.Aslightdecreaseoftheresponsetimeaswellasthemagnitudeofthechangewithincreasingflowvelocitywasobserved.Onemayconcludefromtheseexperimentsthatatmoderateflowvelocitiesconvectiveheattransferplaysonlyaminorroleconcerningthedynamicsofdiaphragmtemperaturechanges.Fig.6.Uppertraces:temperatureresponsetoastepchangeofthepowerdissipatedbytheheateratadistanced=100mfromtheheaterforafreefieldvelocityof15m/s.Lowertrace:heatervoltageTheobservedtimeconstantrangesabout5mswhichvalueisclosetothevalueofd/2a,deducedfromaone-dimensionalmodeloftheheattransport,wheredisdistancefromtheheatsourceandathethermaldiffusivityofthediaphragm.Usinga1.510-/sforsiliconnitrideleadstodelaytimesofabout1and6msforthesmallandlargedistancesdofheateranddiaphragmthermistors,respectively.Onthecontrarythereisnoperceptibletemperaturechangeatthesiliconrim.Thisisanimportantfeaturetoavoidthermalrunawayifthedeviceisoperatedinaconstantovertemperaturemode.Figs.7and8showthemeasuredtimecoursesoftheindividualtemperaturesoftheupstreamanddownstreamdiaphragmthermistorinresponsetoashockwaveforconstantheatervoltage.Thefiguresalsoshowthecalculatedtimecoursesofthetemperaturemeanandofthetemperaturedifference.Fromthelatterquantitytheoutputsignalofaflowsensorisderivedbyelectronicmeans.TheFigs.7and8differbytheinterdistanceofheateranddiaphragmthermistors,measuring35and100mm,respectively.Thesensorwiththesmallerdimensionshowsclearlythebetterdynamicproperties.ThecommonfeaturesofFigs.7and8are:(a)theupstreamthermistortemperatureshowsalargertemperaturechangecomparedtothedownstreamthermistor;(b)thedownstreamthermistortemperatureincreasesinitiallyandsettlesfinallyatalowervalue;(c)themeanoftemperaturessettlesmuchslowerthanthedifferenceofthesetemperatures;and(d)theacousticoscillationsthataresuperimposedtotheflowsteparewellresolvedbybothdevices.Fornormalflowsensoroperationtheoutputsignalisderivedproportionaltothetemperaturedifferenceofthediaphragmthermistors.Thefastsettlingofthisquantityinresponsetoastepchangeoftheflowcanbeunderstoodbyassumingafastadaptationoftheheattransferprocessbetweenthediaphragmandtheflowingfluid.Thefrontoftheshockwavepropagateswiththevelocityofsoundc=340m/sandsneedsabout1mstopassthediaphragmwidth,whichisunresolvableintheFigs.7and8.Behindthisfront,theboundarylayeroftheflowisbuiltupstartingwithzerothickness=0withthearrivaltimet=ta=x/Csoftheshockwavefrontatsomelocationx.Ifthefluidtemperaturedifferssignificantlyfromthelocalsurfacetemperaturetheconvectionbecomeseffectiveimmediatelyatt=tawithaveryhighrateofconvectiveheattransport.Subsequentlyincreaseaccordinglytoifmoderatecompression=0,=0canbeassumed.Theparameter0isthevalueofthecinematicviscosityand0themassdensityofthefluidinfrontoftheshockwave.Thevaluesnandrarethecorrespondingrepresentativesafterthestepchange,takenfromoutsidetheboundarylayer15.Asincreasestheefficiencyofconvectiondecreasesaccordingly.Forminiaturizedheaterdimensionsthetemperaturefieldgeneratedbythedissipatedpowerdoesnotpenetrateverydeepintotheflowingfluid,evenatmoderateflowrates.Alsothecomponentofthesurfacetemperaturegradientpointingparalleltotheflowmayassumeaveryhighvalue.Thechangeoftheheattransferinducedbyconvectionvarieslocallyduetothelocalincreaseordecreaseofthediaphrafmsurfacetemperaturealongthedirectionofflowandwiththelocalthicknessoftheboundarylayer.Initiallytheboundarylayerisverythin,whichmeansthatthefluidtemperatureinthevicinityofthesurfacesuddenlybecomesclosetotheupstreamsurfacetemperature.Thusintheupstreamareatheconvectionalwaysenhancesthecoolingofthediaphragmsurface.Inthediaphragmabruptchangesoftheflowprimarilyenhancestheheatconductionnormaltothediaphragmsurfaceandthenthecharacteristiclengthrelatedtotheheatconductionprocessisthedistanceofthediaphragmthermistorsfromthesurfaced*,i.e.,thethicknessofthepassivatingSiNfilm.Sinced*deachthermistortemperatureexhibitsafasttransientinreactiontosuddenchangesofthelocalconvectiveheattransferrate.Amorecomplicatedresponsetotheshockwaveisobserveddownstream.Inthisareaalocalwarmupofthediaphragmbythepassingfluidmayoccur.Thispossibleincreaseintemperatureisdependentontheheattransfersituationintheupstreamarea.Ifthefluidtemperaturenearthesurfaceexceedsthesurfacetemperaturethenheattransferfromthefluidtothesurfaceoccurs.ThisismorelikelyforhighersurfacetemperaturegradientsalongtheflowdirectioninthedownstreamareaHowever,theaveragetemperatureofthewholediaphragmdecreaseswithtimeduetocoolingbyconvection.ThiscomparableslowtransientissignalledbythemeantemperaturecurveofFigs.7and8.Thiseffectdominatesintheperiodoffallingdownstreamtemperature.Sinkingdiaphragmtemperaturemeansthatsurfacetemperaturedistributionalsobecomesflatterwithincreasingtime.Reductionofsurfacetemperaturegradientsmeansthatthelocalconvectivewarmupeffectvanisheswithelapsedtime.Tworeasonsmaycontributetothefinalriseofdownstreamtemperatures.First,thethicknessoftheboundarylayerincreasescontinuouslywithtimethusreducingtheefficiencyoftheconvectivecoolingingeneralandsecond,theflowvelocityaccompanyingtheshockwaveamplitudemaydecreaseduetothelimitedvolumeofgasstoredintheburstingballoonFig.7.Fig.8.AreductionoftheinterdistancebetweenheateranddiaphragmthermistorsleadstoshorterheatpropagationdelaytimesalongthediaphragmplaneThustheamplitudeandrisetimeofthetemperaturedifferencearereducedaswellasthesettlingtimeofthetemperaturemean.AcomparisonofFig.7withFig.8showsthattheenhancedbandwidthresultsalsoinahigheramplitudeoftheoscillatingsignalrelativetothestepheight.However,itshouldbementionedthatthetransientbehaviourshowninFigs.7and8isonlyonetypicalexample.Thebehaviourdependsstronglyontheheightoftheflowstepanddiffersbetweenincreasingordecreasingflow.Operatingtheminiaturizedflowsensorswithconstantheatingpowerusuallyresultinaverylimitedflowmeasuringrange.Becauseofefficientconvectivecoolingatveryhighflowratestheovertemperaturesoftheheaterandthediaphragmthermistorsaswellasthesensoroutputsignaldecreasewithincreasingflow.Thusthesensorsignalisnotamonotonicfunctionoftheflowifthisoperatingmodeisused.Inordertoobtainawidemeasuringrangeaconstanttemperaturedifferencebetweenthediaphragmandthefluidisdesirableandanelectroniccontrollerisneededtoensurethisoperatingmode.However,thistemperaturecontrollerhastobedisabledinthecaseofinvestigationsofthedynamicpropertiesofthesensor.3.4.ConstantovertemperatureoperationAblockdiagramoftheusedelectronicset-upisshowninFig.9.AnelectronicPI-controllerisusedtoestablishaconstantdifferencebetweenthetemperaturemeanofthetwodiaphragmthermistorsandthetemperaturemeanofbothsubstratethermistors16.Thiscorrespondscloselytoaconstantovertemperatureconditionoftheheaterwithrespecttothefluidtemperatureoverthewholeflowrange.AsmentionedabovethevariationofthedissipatedpowerdoesnotchangethetemperatureofthesubstratethermistorsFig.9.Blockdiagramoftheelectroniccircuit.S,sensorchip;SC,signalconditioningunit;C,PI-controller;PL,powerlimiter;DL,digitallinearizationunit;VNL,nonlinearvelocityoutputsignal;HV,heatervoltagesignalTREF,overtemperaturesetpoint;VL,linearizedvelocityoutputsignal;,summingamplifier;A,differentialamplifier;appreciable.Thisisanimportantfeaturetoavoidthermalrunawayinaconstantovertemperaturemode.Thedynamicbehaviorofthecontrolledsystem,whichisdeterminedbythethermalpropertiesofthesensordiaphragm,restrictsthedynamicbehaviorofthepresentedtemperature-trackingcontroller.Thetemperaturemeanofthediaphragmthermistors,currentlyusedtogeneratetheinputsignalforthetemperaturecontrollerdeterminestheachievablebandwidthduetoitscomparableslowresponse.Amoresophisticatedcontrolstrategyexploitingthetemperaturedifferencesignalmayleadtowiderbandwidthforthesensoroperationintheconstantovertemperaturemodealthoughthesensitivitytothermaloverloadofthesensorlimitstheachievablegain.ButtotakeadvantageofthefullbandwidthofthesensorconstantpoweroperatingmodecanbeappliedifareducedflowmeasuringrangeisacceptableThetemperaturedifferencebetweenthetwodiaphragmthermistors,usedforthegenerationoftheoutputsignal,isalwaysanonlinearmeasurefortheflowvelocityofthemediuminboththeconstantpowermodeaswellasfortheconstantovertemperaturemodeofoperation.Alinearizationofthecharacteristicisofgreatadvantageforanypracticalmeasurement.Anoutputsignaldirectproportionaltothevalueoftheflowvelocitycanbeaccomplishedusingadigitallookuptable