《现代传感器》课件：Lecture8

ModernSensors

Lecture8X.WuAReviewofLecture7TemperatureSensorsBimetalsensor/switchThermocoupleThermistorSolid-stateIssuesassociatedwithapplicationsSelf-heatingThermalresponseNoiselimitedresolutionLinearityAReviewofLecture7FlowsensorsThermal,ThermaltechniquesaregenerallyleastaccurateandleastexpensiveBernoulli,Bernoullitechniquescanworkwell,butareaccurateonlyfornon-turbulentflow,Dopplertechniques.Dopplertechniquesarepotentiallymostaccurate,butarealsogenerallymostexpensive.PhotosensorsAnoverviewofthebasictypesofsensorsusedtodetectopticalandnearinfraredradiation.ThermodynamiclimitLight:Quantum-mechanicalphenomenonTwocatagories:Quantumdetectors

convertincomingradiationdirectlyintoanelectroninasemiconductordevice,andprocesstheresultingcurrentwithelectroniccircuitry.Thermaldetectors

simplyabsorbtheenergyandoperatebymeasuringthechangeintemperaturewithathermometer.QuantumDevicesLightisaquantum-mechanicalphenomena.Itcomesindiscreteparticlescalledphotons.Photonshaveawavelength,avelocityc=3x108

m/s,afrequency=2c/,energyE=hc/,evenamomentump=h/.therelationshipbetweenenergyandwavelength:E=h/2

Inallcases,theenergyofthephotondetermineshowwedetectit.QuantumDevicesProcess:thephotonisabsorbedandanelectronisliberatedinthestructurewiththeenergyofthephoton.Itisimportanttorecognizethatelectronsareallowedtoexistonlyatcertainenergylevels.ifthephotoncarriesanamountofenergywhichis`allowed'foranelectroninthesemiconductor,itcanbeabsorbed.Onceitisabsorbed,theelectronmovesfreelywithinthedevice,subjecttoelectricfields(duetoappliedvoltages)plicated`bandstructure'inwhichtheallowedenergiesinthestructurechangewithlocationinthedevice.TwomodesofOperationPhotovotaicPhotoconductiveCommercialProductsOpticalInterruptorsProximitydetectors…….

OpticalEncoder

MeasurementoftherotationratePrecisionpositioning…SpectralResponse:silicondetectorMostphototransistorsandphotodiodeshavetheirpeaksensitivityinthenearinfrared

Thepeaksensitivityoccursnearthecutoffwavelength(near1um)andextendstoshorterwavelengths.Thelocationofthispeaksensitivityisduetotheenergyofthe`bandgap'insilicon,andisnoteasilyadjusted.PhotosensorsbyelectronicmaterialsPhotosensorscanbemadefromotherelectronicmaterialswithdifferentband-gapsNoneofthesematerialsareaswidelyavailableassilicon,andcostsfordetectorsmadefromInSbcanbesubstantiallyhigher.PhotosensorSelectionphotocarriers

v.s.thermally-generatedcarriersThedistributionofenergiesgeneratedbythermalprocessesisdependentonthethermodynamicsofthedevice,andonthetemperature.increasingthetemperature anincreaseinthenumber thermallygeneratedcarriers.Conversely,reducingthebandgapofaroom-temperaturedevice anincreaseinthenumberof thermallygeneratedcarriers.Anexample:Silicondetectorsworkwellatroomtemperature,butheatingtomorethan100Cstartstocausesubstantialincreasesin`darkcurrent'.Detectorsmadefrommaterialsotherthansiliconmayofferincreasedcutoffwavelength,butmayalsorequirecoolingbelowroomtemperature.OptimalOperatingTemperatureasdfMaxworkingtempv.s.cut-offIngeneral,thereisanearlylinearrelationshipbetweenthemaximumoperatingtemperatureandthecutoffenergyforthedetector.1/cut-off

kTmaxIfselectingamaterialwithacutoffenergy1/5thatofsilicon(suchasInSb),itisnecessarytocoolthedevicetoabout1/5ofthemaximumoperatingtemperatureofsilicon(coolingto77KisoptimalforInSb).Thistradeoffbetweencutoffandoperatingtemperatureimposesseverecostissuesforoperationofdevicesatfairlylongwavelengths.PhotosensorforNightvisionThetremendousinterestindeviceswithcutoffwavelengthsnear10-20umisadirectresultoftheDODinterestininfrareddetectorsfor`nightvision‘:Sothemaximumcontrastinthermaldetectionisavailablebyproducingdeviceswithsensitivityinthisregion.Ofthematerialsmoststudied,theclearwinnerisMercuryCadmiumTelluride(`MCT').

Itmaybeformulatedtohavecutoffbetween10and20microns,andoffersexcellentpropertiesforinfrareddetection.Inparticular,itofferslowdarkcurrent,highabsorptivity,andlowcarrierscattering.Unfortunately,itisdifficultandexpensivetomanufacture.itsfabricationprocessisanenvironmentalnightmare,thebasicmaterialisnotcompatiblewithelectronics.`bump-bonded'ontosiliconsubstrates,mustbeoperatedatorbelow77K,whichimposesoperationalcomplications.AcommercialimagingsystembasedonMCTdetectorarraysgenerallycostsnear$100,000.(m)InSb(Indiumantimonide)InSbdoesnotoffersensitivityinthe10-20umregion,butismoreeasilymadethanMCT,electronicscompatible,canbeoperatednear100K.ResearchtoextendtheoperationtohighertemperaturesisunderwaythroughoutacademiaandindustrySomepropertiesaboutInSbIndiumantimonide:acrystallinecompoundmadefromtheelements

indiumandantimony.Ithastheappearanceofdarkgreysilverymetalpiecesorpowderwithvitreouslustre.Whensubjectedtotemperaturesover500°C,itmeltsanddecomposes,liberatingantimonyandantimonyoxidevapors.Indiumantimonide

photodiodedetectorsphotovoltaic,generatingelectriccurrentwhensubjectedtoinfraredradiation.

highquantumefficiency(80-90%).

Itsdrawbackisahighinstabilityovertime;thedetectorcharacteristicstendtodriftovertime,andbetweencooldowns,requiringperiodicrecalibrations,increasingthecomplexityoftheimagingsystem.InSbdetectorsalsorequirecooling,astheyhavetooperateatcryogenictemperatures(typically80K).However,largearrays(upto1024x1024pixels)areavailable.NoHopeformaterialsotherthanMCT?Therelationshipbetweencutoffandoperatingtemperatureisprettystrict.MCT,whichhasbeenthefocusofbillionsofdollarsofmaterialsresearchefforthasonlybeenslightlyextendedtohighertemperatures.Thereisnottremendoushopethat

InSborothermaterialswillbenefitfromalargechangeinoperatingrequirements.InSbIRsensorsThesensorisalargeformat(640by512pixels)indiumantimonidefocalplanearraywitha3-5micronbandpasscoldfilter.

TheU-Darter(UpgradedDarter)isa

short/mediumrangeair-to-airmissile.

Theguidanceunitconsistsofadual-bandcooledindium-antimonideIRseeker.Manoeuvrabilityisgreaterthan50g,featuresthreeacquisitionmodes;cage,autoscanandhelmet/radardesignation.ThermaldetectorofferssomehopeforthisproblemThermaldetectorsoperatebyabsorbingtheinfraredradiationandmeasuringthetemperatureriseofthedetectorwithathermometer.theperformanceofthermaldetectorsislimitedbytheavailabilityofsensitiveandsmallheatcapacitythermometers.

Animportantadvantage:Room-tempOperationtheabsenceofanyrelationshipbetweenthewavelengthoftheabsorbedradiationandtheresponseofthedetector.Anyenergywhichisabsorbedcausesaresponseinthedetector.Therefore,itispossibletouseathermalinfrareddetectoratroomtemperaturetodetectradiationfromroomtemperatureblackbodies.WhentousethermalIRsensor?However,itisimportanttonotethatiftheconditionsallowuseofaquantumdetector,suchadetectorwilloutperformathermaldetectorbyseveralordersofmagnitude.Thermaldetectorscomeintotheirowninsituationswhichsimplydon'tallowquantumdetectors.QuantumEfficiency:CCD80%3%ThermalThermalIRSensor:Analysisthermometer+absorberwithafinitethermalconductanceG.ThisfiniteconductanceimposesdynamicconstraintsonthesystembehaviorAssumeathermistorwithatemperaturecoefficientgivenby:ThermalSensor:AnalysisConsiderthethermalcharacteristicsofthissystem.modelthethermometerasafiniteheatcapacityCattachedtoanobjectbyafinitethermalconductanceG.Infraredpowerthermometer,tempofthethermometertooscillateSensitivityToimprovethesensitivity,chooseathermometerwithalargetemperaturecoefficientandasmallheatcapacity.asimple1-poleresponse,itisfrequency-independentbelowthecutofffrequencydecreasesas1/fabovethecutoff.exactlythesameasthatofanelectricallow-passfilter.Thereareseveraldifferentinfrareddetectorswhicharebasedonthisdetectionconcept.Infact,almosteverywell-establishedthermometerhasalsobeenoptimizedasaninfrareddetector.

ModernthermalinfrareddetectorsInrecentyears,theDODhasinvestedagreatdealofR&Dfundsintodetectiontechniqueswhichallowlong-wavedetectionfromuncooledplatforms.Anadditionalfocusofthisworkhasbeentechniqueswhicharecompatiblewiththeformationofdensearrays.MicrobolometerarraysmadebyHoneywellPyrobyTIThesedetectorsarebasedonthesimplestthermaldesign-aresistancethermometer.Whatisnovelaboutthisdeviceisthatitcombinesthebestmicrofabricationtechnologywithgoodthermometertechnologyandelectronicsintegration.MicrobolometerTechnologyIndividualsensorelementsusethechangeinelectricalresistanceofavanadiumoxideresistordepositedontothetiny“platelets”fabricatedbysiliconmicro-machiningIncomingtargetradiationheatsthevanadiumoxidecausingachangeinresistance80,000andmoresensorscanbefabricatedtogetherintoatwo-dimensionalarray.Thestructuredimensionedtooperateat30Hz.thethermalconductanceoftheisolatinglegscanbeadjustedtomatchthetime-constantfor30-hertzoperation320by240or160by120microbolometerarraySampleImagesTheTechnologyusesiliconmicrofabricationtechniquestomakeanisolatedthermalstructurewithverylittleheatcapacity.Inthefinaldevice,aflakeofsiliconnitridewithdimensionsof50umx50umx0.5umisfloatedaboveasiliconsubstrate.Thisflakeissupportedbyapairoflegs,andiscoatedwitharesistivematerialwithagoodthermalcoefficientofresistanceUnderneaththeflakeisatransistorwhichisusedtoconnectthecurrent-measuringcircuittothedeviceInthisdevice,muchresearchwentintodevelopingatechniquefordepositingthenitrideontopofatransistor,forreleasingthedeviceswithveryhighyield,andforobtainingasensitivethermometerintheformofadepositedmetalfilm.Thisresistorismadefromvanadiumoxide,whichoffersaTCRofabout1%nearroomtemperature.Theresistancechangeisaresultofastructuralphasetransitioninvanadiumoxideaboveroomtemperature,sothisdevicemustbeheldnearroomtemperaturetoallowoperationwithgoodsensitivity.MilThermalImagingSensor:ahistoryInvolvedCountriesTypicalStructurePicturesMicrobolometersonawaferTimelinesofsomeuncooledIRimagingsensorsAnexample:160x120/35mCircuitBlockDiagramCameraCostAnalysisPackagingDevelopmentApplicationsNightviewerTempMeasurement……FLIR:AWorldLeaderinThermalImagingInfraredCameras

Foundedin1978,originallyprovidinginfraredimagingsystemsinstalledonvehiclesforuseinconductingenergyaudits.laterexpandedtootherapplicationsandmarketsforthermalimagingtechnology,stabilizedthermalimagingcamerasforlawenforcementaircraft,radiometrydevicesthermalimagingsystemsforuseinground-basedsecurityandsearchandrescue.Havingsincegrownsubstantiallyduetoincreasingdemandforinfraredproductsacrossagrowingnumberofmarketscombinedwiththeexecutionofacquisitions.

TauUncooledCoreSpecificationsThermalImagerUncooled

VOx

MicrobolometerFocalPlaneArray(FPA)DigitalVideoDisplayFormats640×512PixelPitch17μm25μmSpectralBand7.5-13.5μmFullFrameRates30Hz(NTSC)

25Hz(PAL)25/50Hz(PAL)Sensitivity(NEdT)<50mKatf/1.SceneRange(HighGain)-40°Cto+160°CSceneRange(LowGain)N/A-40°Cto+550°CTimetoImage~3.5sec~2.0secSize(w/olens)1.75"x1.75"x1.18”HowisNEdTmeasuredNET(noiseequivalentdifferentialtemperature):keyfigureofmeritwhichisusedtoqualifymidwave(MWIR)andlongwave(LWIR)infraredcameras.Itisasignal-to-noisefigurewhichrepresentsthetemperaturedifferencewhichwouldproduceasignalequaltothecamera’stemporalnoise=approximatelytheminimumtemperaturedifferencewhichthecameracanresolve.Itiscalculatedbydividingthetemporalnoisebytheresponseperdegree(responsivity)inunitsofmilliKelvins.Thevalueisafunctionofthecamera’sf/number,itsintegrationtime,andthetemperatureatwhichthemeasurementismade.

PyroelectricplasticmaterialAsfor