微型半导体流量传感器的开发外文文献翻译中英文翻译外文翻译Word文档格式.docx

微型半导体流量传感器的开发外文文献翻译中英文翻译外文翻译Word文档格式.docx

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F.Kohl，R.Fasching，F.Keplinger，R.Chabicovsky,A.Jachimowicz,G.Urban

Abstract

Miniaturizedflowsensorsbasedonthinfilmgermaniumthermistorsweredevelopedofferinghighflowsensitivitiesandshortresponsetimes.Thethermistorsareplacedonasiliconnitridediaphragmcarriedbyasiliconframe.Usingthe3controlledovertemperatureschemethemeasurableairflowraterangesfrom0.6to150000cm/h.Inthispaperwemainlyreportonthedynamicpropertiesofthesensor.Theresponseofthesensortostepchangesoftheheaterpowerwillbecomparedwithitsresponsetoshockwavesforboththeconstantpowermodeandtheconstantovertemperatureoperatingmode.Asimplearrangementforthegenerationofacousticshockwaveswillbepresented.

1．Introduction

Thereisagrowingdemandofmicro-flowsensorsforindustrial,automotive,domesticandmedicalapplications.Themeasuringprinciplecanbebasedonthermistors,thermopiles,pyroelectricelements,pn-junctions,resonatingmicrobridges,Prandtltubesandseveralothereffects[1–10].Micromachiningisadoptedtoachievehighsensitivity,quickresponseandlowpowerconsumption。

Oneimportantapplicationofflowsensorsisthemeasuringoftheinstantaneousairintakeofcombustionengines.Knowledgeofthiscombustionprocessparameterisessentialifonetriestominimizeboththeengine’sfuelconsumptionandthepollutionoftheenvironment.Forthedevelopmentofsuchenginesawidevelocitymeasuringrangeandhighresolutionmonitoringofthetimecourseoftheairvelocityisdesirable.

Theelectrocalorimetricflowsensorpresentedhereisbasedonaheattransferprincipleinwhichaheatedbodyiscooledbyapassingflowandthelocalrateofcoolingdependsontheflowvelocity[11].Thesensorisbasedontheso-called‘hotfilm’flowmeasurementmethod.AverythinsiliconnitridediaphragmsupportedbyamicromachinedsiliconframeismountedflushwiththewallofaflowchannelasshowninFig.1.Athinfilmheatingresistorisembeddedinthediaphragmtoobtainasymmetricsurfacetemperaturedistribution.Twodiaphragmthermistorsmeasurethetemperatureinapositionupstreamanddownstreamoftheheater.

Fig.1.Schematiccross-sectionofatypicalhotfilmflowsensorandtemperaturedistributionalongthediaphragm.H,heater;

DT1,DT2,diaphragmtemperaturesensors.

Atangentialflowdisturbsthethermalsymmetry.Heatiscarriedfromtheheateddiaphragmwhentheinitiallycoldfluidpassesovertheheatedsurface.Sincethefluidtemperatureincreasesinthedirectionofflowthecoolingeffectisreducedinthedown-streamarea.Inthisareathefluidtemperaturemaybecomeevenhigherthanthesurfacetemperatureresultinginalocalheatingofthediaphragm.Thusatemperaturedifferencebetweenthecooledupstreamareaandthelesscooled（orheated）downstreamareaoccurs.Thistemperaturedifferencecanbeconvertedintoanoutputvoltage,whichisusedasameasureforthefluidvelocityormassflow.Theflowrangeandsensitivityisstronglyinfluencedbythedistancebetweentheheaterandthetemperaturesensors[2,12].Withasymmetricarrangementstheshapeoftheoutputcharacteristic（temperaturedifferenceversusflowrate）canbesignificantlychanged[10,13]

Wehavedevelopedasymmetricmicromachinedsemiconductorflowsensorcapableofmeasuringbidirectionalflow.Extensivecharacterizationofthesensorwasdoneexhibitingexcellentflowsensitivityandanextremelywidemeasuringrange.Furthermore,athoroughinvestigationofthedynamicbehaviourofthesensorwascarriedout.

2．Sensorconstructionandtechnology

A（100）siliconwaferhasbeenusedforthefabricationofthesensor.Thechipsizeis234mmandthethicknessis0.3mm.Twothinfilmthermistorsareplacedsymmetricallytoacentralheateronan800-nmthicksiliconnitridediaphragm（Fig.2）.Additionalthermistorsarearrangedattherimofthesiliconchip.Theseso-calledsubstratethermistorsareusedtomeasurethefluidtemperature,whichisclosetothesubstratetemperature.

Fig.2.Schematiccrossofthesensor.Thesizeofthediaphragmis0.5×

1.1mm

Allthermistorsarefabricatedbyevaporationofamorphousgermaniumontocomb-shapedelectrodes[fig.3]

Fig.3

Onemajoradvantageofthistypeofhightemperature-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,thattheelectricalconductivityofamorphousgermaniumisgovernedbyavariablerangehoppingprocess[14].AtroomtemperaturetheTCRvariesonlyslightlywithtemperature,whicheasesappreciabletheburdenforcompensatingofchangesoftheambienttemperature.AlayoutasshowninFig.3anda250-nmthickgermaniumfilmresultinaresistanceof70kVat208C.Ithasbeenprovedthatthelong-termstabilityofthischaracteristicisbetterthan0.5%peryear.Anoiseequivalenttemperaturedifferenceof10mKforabandwidthof10Hzisachievedwiththisthermistortechnology[10].Forcomparison,Johnsonnoiseonlywouldlimittheresolutionto4.75mK.

3Experimental

3.1.Sensormounting

TostudythesensorpropertiesinsituationsthataretypicalforvariousapplicationsthechipwasattachedtodifferentcarrierconstructionsForfreefieldcalibrationinawindtunnelandotherexperimentalmeasurementsthesensorchipisgluedtoa0.15-mmthickprintedcircuitboard（PCB）flushfittedwiththeboardsurface（Fig.4）.ForthispurposetheflexiblePCBwasformedusinganembossingdie.ThedimensionofthisPCBinthedirectionofflowis60mmandthesensorwasplacedmidways.ThegroundplaneofthePCBshieldsthesignalleadsagainstinterferences

Fig.4.Schematiccross-sectionalviewofthesensormounted

onaflexibleprintedcircuitboard

Forflowratemeasurementsthesiliconchiphastobeincorporatedinthewallofaminiaturizedflowchannel.ThiswasachievedusingarigidPCBof0.5mmthicknesswithamilleddeepeningwhichaccommodatesthesensorchipflushwiththesurfaceofthePCB.ThePCBformsonesidewallofarectangularflowchannel.

ThesamePCBmeasuring15mmalongthedirectionofflowwasusedforshockwaveexperiments.TosuppressturbulencesinthisapplicationtheedgesofthePCBwereformedtoshowwedgeshape.ThePCBwasthenplacedinasymmetryplaneofthecylindricalflowchannel.

3.2.Shockwavegenerator

ToinvestigatetheresponsetostepchangesofflowsimpleshockwavegeneratordepictedschematicallyinFig.5wasdeveloped.AcommercialballooncontainedinaPVC-cylinderof70mmdiameterandapproximately250mmlengthisblownuptoitsburstpressurewithnitrogengas.Anorificeoftypical2-mmdiameterlimitstheacoustic

Fig.5.Sketchofthearrangementusedforthegenerationofshockwaves

andtherelatedsensorarrangement

flowoftheresultingshockwave.Thesensorisplacedatasymm