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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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
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