Fully quantitative characterization of CMOS–MEMS polysilicon/titanium thermopile infrared sensors

SensorsandActuatorsB161 (2012) 892–900

ContentslistsavailableatSciVerseScienceDirect

Sensors

and

Actuators

B:

Chemical

jou rn a l h o m e pag e :w w w . e l s e v i e r . c o m / l o c a t e / s n b

Fully

quantitative

characterization

of

CMOS–MEMS

polysilicon/titanium

thermopile

infrared

sensors

Chung-Nan

Chen

∗

InstituteofPhotonicsandCommunications,NationalKaohsiungUniversityofAppliedSciences,Kaohsiung,Taiwan,ROC

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received29August2011

Receivedinrevisedform9November2011 Accepted22November2011

Available online 30 November 2011 Keywords: CMOS Infraredsensor MEMS Specificdetectivity Thermalproperty Thermopile

a

b

s

t

r

a

c

t

ThisstudydemonstratesafullyquantitativecharacterizationofahighlysensitiveCMOS–MEMS polysili-con/titaniumthermopileinfraredsensorbyusingthesimulationsofnon-sequentialraytracingandsolid conduction,andthemeasurementsofvoltageresponseandfrequencyresponseinatmosphereandin vacuum.

Thethermaltimeconstantsof17.0msinairand37.0msinvacuumforthepolysilicon/titanium ther-mopilewithagold-blackabsorberwereestimatedbythemeasurementsoffrequencyresponse.The solidconductance,gasconductance,radiationloss,andheatcapacitanceofthethermopilewere char-acterizedas112␮W/K,141␮W/K,5.88␮W/K,and4.40␮J/Kinatmospherebythesimulationofsolid conductionusingANSYSandthemeasurementsoffrequencyresponse.Thevoltageresponsivity, sen-sornoise,noiseequivalentpower,andspecificdetectivityofthegold-blackcoatedthermopileinair wereestimatedas63.1V/W,27.0nV/Hz1/2_,0.43nW/Hz1/2_,and1.87×108_cmHz1/2_{/Wbythesimulationof} receivedopticalpowerusingLightToolsraytracingsoftwareandthemeasurementsofvoltageresponse. ItshowsthatthesensorhasthehighestspecificdetectivitycomparedtothepublishedCMOS–MEMS thermopilesinatmosphereduetothedesignoflowsolidconductanceandhighemissivity. Eventu-ally,theSeebeckcoefficientofthepolysilicon/titaniumpairwasfirstevaluatedandhasamagnitudeof 170.2␮V/K.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Thermopile infrared sensors were widely used as the sens-ingelementsofnondispersiveinfrared(NDIR)gasdetectors[1,2], thermal imagers [3,4] and non-contact infrared thermometers [5].Thermalimagersandinfraredthermometersarethedevices thattransforminfraredradiationintotemperaturereading.NDIR gasdetectorsconsistofband-passfilters, infraredemitters,and infrared sensors which could detect the concentration of spe-cificgasesbythesignaldifferenceresultedfromtheabsorption of infraredpower by thegases.In order to enhancethe selec-tivityofgassensing,thebandwidthsoftheband-passfiltersare usuallypreferredtobeasnarrowastheabsorptionbandwidths ofthespecificgaseswhichwouldresultin theweakandslight receivedinfraredpowerofthesensors.Itis particularly impor-tantandnecessarytodevelop ahighlysensitiveinfraredsensor forNDIRgassensing.Thermopilesarethermal-typeinfrared

sen-∗ Correspondenceaddress:InstituteofPhotonicsandCommunications,National KaohsiungUniversityofAppliedSciences,415ChienKungRoad,SanminDistrict, Kaohsiung80778,Taiwan,ROC.Tel.:+88673814526x3354;fax:+88673832771.

E-mailaddress:cn [email protected]

sorswhichtransferthermalradiationintothermoelectricvoltage. Therefore, theperformances of thermopiles,suchas responsiv-ity,specificdetectivityandresponsespeed,arequitedependent ontheirthermalproperties.Athermopilesensorcomprisesmany thermocouplesconnectedinseriesandtheconnectionsofthe ther-mocouplesdivideintohotjunctionsandcoldjunctions.Ingeneral, thehotjunctionsarethermallyisolatedfromtheirsubstrateand aninfraredabsorberiscoatedontothesurfaceofthethermopile forimprovingthesensitivityoftheinfraredsensor.Asathermopile absorbstheinfraredradiationemittedfromaninfraredsource,the temperaturedifferencebetweenhotjunctionsandcoldjunctionsis formedandthethermoelectricvoltagebetweenthetwoterminals ofthermocouplesinseriesiscreatedatthesametime.The magni-tudeoftheoutputvoltageisdirectlyproportionaltothenumberof thermocouples,theSeebeckcoefficientofthethermocouple mate-rialpair,andthetemperaturedifferencebetweenhotjunctionsand coldjunctions.Andthetemperaturedifferenceisindirectratioto theemissivityoftheinfraredabsorber,receivedinfraredpower, andthereciprocalofthermalconductance.Thevoltage responsiv-ityofaninfraredsensorisdefinedastheratioofoutputvoltageto receivedinfraredpowerwhichspecifiestheinput–outputgainofa sensorsystem.However,specificdetectivityisthemostimportant specificationofaninfraredsensortoindicatethesignal-to-noise 0925-4005/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved.

C.-N.Chen/SensorsandActuatorsB161 (2012) 892–900 893

ratioofanopticalsensor.Themagnitudeofspecificdetectivityfor athermopileinfraredsensorisindirectproportiontotheemissivity oftheinfraredabsorber,theSeebeckcoefficient,andthe recipro-calofthermalconductance.Themainheatlossesforthermopile devicesaresolidconductionandgasconductionsincetheradiation lossisrelativelysmallandnegligible.Forthermalmicrosensors,the gasconductancemayevendominatethebehaviorofheattransfer andtheperformanceinatmosphere[6,7].Inaddition,theresponse timeconstantsofthermaldevices,whicharedefinedastheratio ofheatcapacitancetothermalconductance,arealsohighly depen-dentontheirthermalproperties.It showsthatthequantitative characterizationofthethermalpropertiesisveryusefuland valu-ablefortheanalysisofthesensorperformancesandthedesignof thermalsensors.

For thereasons ofmass production, IC integration,and low cost, a CMOS compatible process was frequently employed as the front-end process of a MEMS thermopile infrared sen-sor [8–11,3,4,12–15]. In past years, polysilicon and aluminum werethe most common materialsfor forming the thermocou-plepairs of CMOSthermopiles since thefabrication process of a polysilicon/metal(poly/metal) thermopile is relativelysimple and compatible. However,the thermal conductivityof a CMOS aluminum film is about 200W/mK [16] which is an order of magnitudegreater than those of polysilicon and titanium [17]. Thehighsolidconductance ofa polysilicon/aluminum(poly/Al) thermopilecouldresultinalowsensitivity.Besides,an interfer-encestructure was used toserve as an infrared absorber of a CMOS–MEMSthermopilewhichhaslowabsorptanceanda nar-rowabsorptionbandwidth[2,18].Instead,aporousgoldblackfilm withabsorptanceof>90%andwideabsorptionbandwasproposed forthermalinfrareddetectors[19–21].Inourwork,thepurpose ofthedesign of polysilicon/titanium(poly/Ti) thermopiles isto improvethespecificdetectivityofsensorsbyhighlyreducingthe solid conductance andincreasing theemissivityof theinfrared sensorwithoutmuch loweringtheSeebeckcoefficientand rais-ingJohnsonnoise.Furthermore,thepostprocessofthethermopile wasdesignedtobeasimplephotomaskless process.Inorderto extremelylowerthesolidconductanceofapoly/metalthermopile, a0.6-␮maluminumfilminthestandardfirstmetallizationprocess wasreplacedbya0.1-␮mtitaniumfilmwithalowthermal con-ductivity.TheSeebeckcoefficientsofapoly/Tipairandapoly/Al pairshouldbealmostthesamesincepolysiliconhasmuchlarger absoluteSeebeckcoefficientthanthoseoftitaniumandaluminum [18,22–24].Besides,theJohnsonnoiseofapoly/Tithermopileis almostequivalenttothatofapoly/Al thermopilewithan iden-ticallayoutsincepolysilicondominatestheelectricresistanceof thesensor.AftertheCMOSprocess,thesuspendedstructureofthe poly/Tithermopilewasformedbyfront-sideanisotropicetching ofsiliconin adual-dopedTMAHsolutionwithlowetchrateof exposedaluminumpads[25].Forfurtherlesseningsolid conduc-tance,acceleratingetchingprocessandachievingahighfillfactor, narrowandlongline-shapedetchingwindowsweredesignedand located between thethermocouples. Eventually,a porous gold blacklayerwasthermallyevaporatedontothesurfaceofthe sen-sorand patterned in situ by metal mask technique toachieve highabsorptionofinfraredpower.Thispaperis focusedonthe characterizationofalltheimportantthermalpropertiesand perfor-mancesofthepoly/Tithermopile,suchasthesolidconductance,gas conductance,radiationloss,heatcapacitance,thermaltime con-stant,emissivity,Seebeckcoefficient,responsivity,Johnsonnoise, shotnoise,temperaturefluctuationnoise,noiseequivalentpower (NEP), and specific detectivity, by adopting the LightTools ray tracingsimulation of receivedinfraredpower,theANSYS finite elementanalysis(FEA)ofsolidthermalconduction,andthe mea-surementsofvoltageresponse,frequencyresponseandvacuum response.

Fig.1.HeattransferofaMEMSthermopile.

2. Theory,designandsimulation

2.1. Theory

The thermal behavior of a packaged thermopile under an infraredradiationof˚isgovernedbytheheatflowequation[26] CdTh

dt +G(Th−Ta)=ε˚, (1)

whereCistheheatcapacitanceofthethermopilesensor,Thisthe temperatureofhotjunctions,tisthetime,Gisthethermal conduc-tancebetweenhotjunctionanditssurroundingwithatemperature ofTa,andεistheemissivitywhichrevealstheabsorptanceofthe infraredabsorber.ThesolutionofthetemperaturedifferenceT betweenthehotjunctions anditsenvironment inthetransient stateis[26]

T= ε˚_G



1

1+ω2_2, (2)

where ω is the modulated angular frequency of the incident infraredpower,and =C/Gis thethermaltime constant ofthe sensor.

TheoutputthermoelectricvoltageVcausedbytherising tem-peratureofhotjunctionscanbeexpressedby

V=N˛AB(Th−Tc), (3)

inwhichNisthenumberofthermocouplesinseries,˛AB isthe Seebeckcoefficientbetweenthetwothermocouplematerials,Tcis thetemperatureofcoldjunctionswhichisalmostthesame mag-nitudewiththeambienttemperatureTa.Therefore,thevoltage responsivityRVofthethermopilecanbegivenby

RV≡ V ˚= N˛ABε G 1



1+ω2_2 = RV 0



1+ω2_2 , (4)

whereRV0=N˛ABε/Gistheflat-bandresponsivityinthethermal steadystate.ForCMOSpoly/metalthermopileswiththesame lay-out,thenumberofthermocouples isfixedandthevariancesof theSeebeckcoefficientsbetweenpolysiliconandCMOS–metalsare verysmallsincepolysilicondominatesthethermoelectriceffect ofthepoly/metalthermopiles.Therefore,theresponsivityofthe poly/metalthermopilesinthethermalsteadystateisalmostonly indirectratiototheiremissivityandthereciprocalofthermal con-ductance.

Asshownin Fig.1,there arethree typesofheat lossesthat ariseinaMEMSthermopile:solidconduction,gasconductionand radiation.Asthesuspendedstructureofathermopileabsorbsthe incidentinfraredpowerradiatedfromatarget,someheat dissi-patesthroughthesolidsuspendedstructuretosiliconsubstrate.

894 C.-N.Chen/SensorsandActuatorsB161 (2012) 892–900

ThesolidconductanceGsisdirectlyproportionaltothethermal conductivityksofthesuspendedstructureandisexpressedby Gs=kswd

l , (5)

wherew,dandlarethewidth,thicknessandlengthofthe sup-porting leads of the suspended membrane. And some heat is transported to thesubstrate by thecollisions of gasmolecules betweenthehotsuspendedmembraneandthesubstratewithinthe cavity.ThegasconductanceGgisindirectratiotothefree molec-ularconductivityofgaskgandtheareaoftheinfraredsensorAs. Besides,thegasconductanceisafunctionofpressureP[7,27].For athermopilewithone-sideheatsink,thegasconductanceisgiven by Gg=  2−kgAsP



_P t P+Pt



, (6)

whereistheaccommodationcoefficientofgas,Ptisanempirical transitionparameterwhichisdirectlyproportionaltothe recip-rocalofgapdepthbetweenthedeviceandheatsink[27].Asthe pressurePisnegligiblecomparedtothetransitionpressure,thegas conductanceisdirectlyproportionaltothepressureofthecavity. Itmeansthatavacuumpackageisveryusefultodiminishthegas conductance.Radiationlossisthephenomenonofheatexchange betweenathermaldeviceanditsenvironment.Theradiation con-ductanceGrcouldbederivedfromtheStefan–Boltzmannlawand givenby

Gr=(ε+εb)As(Th2+Ta2)(Th+Ta)≈4(ε+εb)AsTa3, (7) whereεbistheemissivityofthebottomsurfaceofthesensor,is theStefan–Boltzmannconstant,andthetemperatureofhot junc-tionsisconsideredtobeclosetotheambienttemperaturesince thetemperaturerisingofhotjunctionsunderthermalradiationis tiny.Thetotalthermalconductanceofathermopilesensorisgiven by

G=Gs+Gg+Gr. (8)

Ingeneral,theradiationconductanceofathermopileis negli-giblesincethetemperatureofhotjunctionsisonlyslightlyhigher thantheambienttemperature.However,thegasconductanceofa MEMSthermaldeviceinatmosphereissignificantduetothe minia-turedimensionofcavitygap.Forreducingthegasconductance,a vacuumorafilling-gasenvironmentwasoftenusedbypublished papersandmanufacturers[18].

ThemainnoisesourceofathermopilesensorisJohnsonnoise duetonobiassupplyrequirementduringthesensor’soperation [28].Johnsonnoiseisexpressedas

vJ

=



4kBTsRsf, (9)

wherekBistheBoltzmannconstant,Tsistheabsolutetemperature ofthethermopile,Rsistheresistanceofthesensor,andfisthe noisebandwidth.Inaddition,shotnoiseisgivenby[26]

vs

=Rs·



2qIf, (10)

inwhichqistheelementarychargeofanelectron,and Iisthe current.Thetemperaturefluctuationnoiseresultedfromthe tem-peraturefluctuation



T2



1/2_{inbackgroundiswrittenby[26]}

vTF

=N˛AB·



T2



1/2 = RV 0 ε



4kBT2Gft, (11)

whereft=1/4isthebandwidthoftemperaturefluctuationnoise. Thetotalsensornoiseofathermopilesensorisexpressedas

vn

=



v

2

J+

v

2s+

v

2TF. (12)

NoiseequivalentpowerNEPandspecificdetectivityD*arethe mostsignificantspecificationsofaninfraredsensortoqualifythe signal-to-noiseperformanceofthesensor.NEPisdefinedasthe receivedinfrared powerwhich hasa signal-to-noise ratio of1. TheNEPofathermopileinthesteadystatecanbeexpressedand approachedas NEP≡ _R

vn

V ≈ G N˛ABε



4kBTsRsf. (13)

Forapoly/metalthermopile,thepolysiliconlinesdominatethe resistanceandthethermoelectriceffectofthesensor.Considera simplepolysiliconcantileverwhichhasadimensionoflpinlength, wpinwidth,dpinthickness,andaresistivityofp,theresistance rofthecantileveralongthelengthofpolysiliconlineisgivenby r=plp/wpdp.IfthepolysiliconcantileverisdividedintoNequal lineswiththesamewidthofwp/Nbyconsideringthattheintervals betweenthepolysiliconlinesarenegligible.Thentheresistanceof thepolysiliconlinesinseriescouldbewrittenas

Rs≈N·p lp (wp/N)dp =

N2_{r. (14)}

AndtheNEPcanberewrittenas

NEP= G

˛ABε



4kBTsrf. (15)

ItshowsthatthedependenceofNEPonthethermocouple num-berNdisappearsduetotherapidlyincreasingofsensorresistance. D*isdefinedasthereciprocalofNEPandnormalizedtothesensor areaAsandthebandwidthf

D∗≡



Asf NEP = ˛ABε G



As 4kBTsr . (16)

ItimpliesthatthemagnitudeofD*isalmostonlydependent ontheemissivityandthethermalconductanceofthesensorfora poly/metalthermopilewiththesameprocessandsensorsize.

Inthiswork,theabsorptionspectrumoftheabsorberlayerwas measuredbyusingFouriertransforminfraredspectrometry(FTIR) in order toestimatethe emissivityε of thepoly/Ti thermopile infraredsensor[5].AccordingtothestatementofKirchhoff’slawof thermalradiation,theemissivityofabodyequalsitsabsorptivity atthermalequilibrium.Therefore,theemissivityoftheabsorber layercouldbeevaluatedbythecalculationofaverageabsorptivity ofinfraredradiationoveraspecifiedwavelengthrange.

The important performances of thermopiles wereevaluated bythesimulationofreceivedopticalpower,themeasurementof thermoelectricvoltage,andthetheoreticalcalculationofsensor’s noises.Inordertoestimatethereceivedpower˚ofaTO5-packaged thermopileatvarioustemperaturesofablackbodyinfraredsource andatambienttemperature,araytracingsimulationwascarried outbyadoptingLightToolssoftware.Theresponsivityofthe ther-mopileisdefinedastheratiooftheoutputthermoelectricvoltage Vtothereceivedopticalpower˚,inwhichtheoutputvoltagewas obtainedbythemeasurementofsensor’svoltageresponseunder infraredradiationandthecorrespondingreceivedopticalpower wassimulatedbyLightTools.Thesensornoise

vn

ofthethermopile iscalculatedbyusingEqs.(9)–(12)andtheNEPofthesensorwas evaluatedbycalculatingtheratioofsensornoisetoresponsivity. Subsequently,thespecificdetectivityD*couldbefurthergivenby utilizingitsdefinitioninEq.(16).

Thethermalpropertiesofthermopileswerefurtherfoundand analyzed bythe simulation of solid conductance, the measure-mentsoffrequencyresponsesinairandinvacuum.Toestimate thesolidconductanceofthermopilesensors,thetemperature dis-tributionsofhotjunctionsunderinfraredradiationwassimulated byadoptingANSYSFEAsoftware.Inthesimulation,anexact3D modelofapoly/metalthermopilechipwasconstructedandthe

900 C.-N.Chen/SensorsandActuatorsB161 (2012) 892–900

4. Conclusion

ACMOS–MEMSgold-blackcoatedpoly/Tithermopileinfrared

sensorwithahighspecificdetectivitywasfabricatedbyusinga

0.8-␮msix-maskCMOSprocessandapostprocesswithoutany

lithographystep.Thethermalpropertiesandperformancesofthe

poly/Tisensorhavebeenfullycharacterizedbysimulationsand

measurements in this study. The characterization resultsshow

that thepoly/Tithermopilehasexcellent responsivity and

spe-cificdetectivityof63.1V/Wand1.87×108_cmHz1/2_/Wduetothe

lowthermalconductivityoftitanium.Thespecificdetectivityof

the CMOSpoly/Ti thermopile withthe designs of low thermal

conductanceandhighemissivityis6.1timeshigherthanthatof

standardCMOSpoly/Al thermopileswithoutgoldblackcoating.

Thecontributionsof solidconduction, gasconduction and

radi-ationlossontheheatlossesofthethermopilewereabout43%,

55%and2%.Andthegasconductancecouldbefurtherneglected

and the specific detectivity of the sensor would be raised to

4.06×108_cmHz1/2_{/W by operating under a pressure less than}

0.02Torr.Moreover,theSeebeckcoefficientofn+-polysiliconand

titaniumwasfirstestimatedandhasamagnitudeof170.2␮V/K.It

showsthatpolysiliconmaterialdominatestheSeebeckcoefficient

ofCMOS–MEMSpoly/metalthermopiles.

Acknowledgements

TheauthorwouldliketothanktheNationalScienceCouncil

oftheRepublicofChina,Taiwan,forfinanciallysupportingthis

researchunderContractNo.NSC97-2221-E-151-006.Theauthor

wouldalsoliketothanktheNationalNanoDeviceLaboratoriesfor

thesupportingofprocessfabrication.

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Devices45(1998)1896–1902. Biography

Chung-NanChenwasborninTaiwan.HereceivedtheB.S.degreeinphysicsfrom ChineseCultureUniversity,Taipei,Taiwan,in1988,theM.S.andPh.D.degreesin electro-opticalengineeringfromNationalChiaoTungUniversity,Hsinchu,Taiwan, in1993and2000,respectively.HisPh.D.dissertationisthetechnologydevelopment ofmicrobolometerIRFPAdetectors.HeservedfortwoyearsintheTaiwanArmy.He waswithOptoTechCorporation,Hsinchu,Taiwan;hewasinvolvedintheR&Dof MEMSproductsandbecameadeputymanageroftheDivisionofMicroelectronics from1996to2005.In2005,hebecameanAssistantProfessorwiththeInstitute ofPhotonicsandCommunications,NationalKaohsiungUniversityofApplied Sci-ences,Kaohsiung,Taiwan.Hisresearchinterestsincludethermalsensors,MEMS andsemiconductorprocess,andinfraredengineering.