VLSnano5

ThermalTestofanImprovedPlatformforSiliconNanowire-BasedThermoelectricMicro-generatorsC.
CALAZA,1,3L.
FONSECA,1,4M.
SALLERAS,1I.
DONMEZ,1A.
TARANCON,2A.
MORATA,2J.
D.
SANTOS,2andG.
GADEA21.
—IMB-CNM(CSIC),CampusUAB,08193Bellaterra,Barcelona,Spain.
2.
—IREC,C/JardinsdelesDonesdeNegre1,pl2a,08930Barcelona,Spain.
3.
—e-mail:carlos.
calaza@imb-cnm.
csic.
es.
4.
—e-mail:luis.
fonseca@csic.
esThisworkreportsonanimproveddesignintendedtoenhancethethermalisolationbetweenthehotandcoldpartsofasilicon-basedthermoelectricmicrogenerator.
Micromachiningtechniquesandsilicononinsulatorsub-stratesareusedtoobtainasuspendedsiliconplatformsurroundedbyabulksiliconrim,inwhicharraysofbottom-upsiliconnanowiresareintegratedlaterontojoinbothpartswithathermoelectricactivematerial.
Inpreviousdesignstheplatformwaslinkedtotherimbymeansofbulksiliconbridges,usedasmechanicalsupportandholderfortheelectricalconnections.
Suchsupportsseverelyreduceplatformthermalisolationandpenalisethefunc-tionalareaduetotheneedoflongersupports.
Anewtechnologicalrouteisplannedtoobtainlowthermalconductancesupports,makinguseofapar-ticulargeometricaldesignandawetbulkmicromachiningprocesstoselec-tivelyremovesiliconshapingathindielectricmembrane.
Thermalconductancemeasurementshavebeenperformedtoanalysetheinuenceofthedifferentdesignparametersofthesuspendedplatform(supporttype,bridge/membranelength,separationbetweenplatformandsiliconrim,)onoverallthermalisolation.
Athermalconductancereductionfrom1.
82mW/Kto1.
03mW/K,hasbeenobtainedontesteddevicesbychangingthesupporttype,eventhoughitslengthhasbeenhalved.
Keywords:Microgenerator,thermoelectricity,harvestingINTRODUCTIONMostofworld'spoweruseisgeneratedbymeansofheatenginesusingfossilfuelcombustion,butalmosttwo-thirdsoftheenergythatisfedintothesesystemsradiatesaway,becomingawasteheatsource.
1Thermoelectricmodules,whichhavethecapabilityofconvertingheatintoelectricity,havebeenproposedasapromisingsolutiontoturnthiswasteheatintousefulpower.
Althoughrecentre-searchhasbeenintenselyexploringnewmaterialsandtechnicalroutestoboosttheefciencyofsuchdevices,thermoelectricenergyconversionstillrep-resentsamajorscienticchallengetowardsaneffectivewasteheatrecovery.
Severalhigh-perfor-mancethermoelectricmaterials,suchasBi-Tebasedalloys,skutteruditecompounds,Ag-Pb-Sb-Tequaternarysystemsandhalf-Heuslercompounds2–6havebeenlatelyreportedasefcientthermoelectricmaterials,buttheyareknowntobescarceandexpensive,toxicinsomecases,aswellasdifculttointegrateinmicroelectronics.
Alternativedevelop-mentsfocusonthesmartstructuringofmainstreammicroelectronicmaterialsasaroutetoachievesili-con-basedthermoelectricgenerators.
Individualsiliconnanowires(SiNWs)haveshownanen-hancedthermoelectricperformanceoverthatofthebulksilicon(ZT0.
01).
7,8However,eventhoughimprovedZTvalueshavebeenreported,thedis-cussionaroundwhetherthisnanomaterialwillen-abletheproductionofcompetitivethermoelectricdevicesisstillopen.
Ouraimistoworktowardsanall-Sithermalgeneratorbydesigningthermally(ReceivedJune12,2015;acceptedOctober22,2015;publishedonlineNovember24,2015)JournalofELECTRONICMATERIALS,Vol.
45,No.
3,2016DOI:10.
1007/s11664-015-4168-82015TheAuthor(s).
ThisarticleispublishedwithopenaccessatSpringerlink.
com1689efcientsiliconmicroplatformssuitableforthesubsequentmonolithicintegrationofbottom-upSiNWsasthermoelectricactivematerial.
DESIGNANDFABRICATIONTheplanarcongurationproposedforthesilicon-basedthermoelectricmicrogeneratorusesasilicononinsulator(SOI)substrateandsiliconmicroma-chiningtechniquestodeneathermallyisolatedsuspendedSiplatformsurroundedbyabulkSirim.
BothpartsaresubsequentlylinkedbymeansofSiNWarraysthataregrownonaCVDreactorusingabottom-upvapour–liquid–solid(VLS)process.
9Inourpreviouswork,thesuspendedplatformwaslinkedtothebulkSirimbymeansofbulkSibridges,inadditiontotheSiNWarrays,actingasmechanicalsupportandasaholderfortheelectricalconnections.
9–11However,thiskindofsupportse-verelyreducestheplatformthermalisolationduetothehighthermalconductivityofthebulkSi,limit-ingthedeviceabilitytogetalargetemperaturegradientfromawasteheatsource.
Hence,longbridgesupportsareneededtodeveloplargethermalgradientsandsignicantdeviceareaiswastedgivingrisetopoorpowerdensities.
Inthiswork,anewtechnologicalroutehasbeenset-uptoincreasetheplatformthermalisolationbyreplacingsuchsiliconbridgesbythindielectricmembranes,withamuchlowerthermalconductivity,whichareusedtosupportthemetallicelectricalconnections.
Apar-ticulargeometricaldesignisproposedtoetchtheSiunderthemembraneareausingashortwetbulkmicromachiningprocess,whichisenoughtoshapethesesuspendedlowthermalconductancethinmembranesand,atthesametime,improvethesurfacequalityoftheh111iverticalwallswheretheSiNWswillbegrown.
AsketchofbothdesignsisshowninFig.
1.
Theyconsistofasuspendedsiliconplatform(S1)thatwillbelaterconnectedtoabulksiliconrim(S2)withSiNWsarrays.
Informerdesigns,theelectri-calconnectionswereplacedontopofbulksiliconbridgeswhileinthenewdesignproposedasanalternativetheyarelayingonathindielectricmembrane.
TheSiNWswillbegrownperpendiculartotheh111iwallsthathavebeenusedtodenethedifferentSipartsontheSOIdevicelayer.
Thetemperaturedifferenceattainableacrosssuchde-viceswhenoperatedinharvestingmode(placedontopofaheatsource)willessentiallydependonthelengthofthethermoelectricmaterialconnectingthehigh-andlow-temperatureareas,whichistechno-logicallylimitedbythetaperingeffectduringNWsgrowth.
12–15TrenchesforthesuccessivelinkageofmultipleSiNWarrayshavebeendevelopedinordertoovercomethisproblem,providinglargereffectiveSiNWlengths.
AdetailedschematicoftheintendednalstructureisshowninFig.
2.
ThefabricationisperformedonSOIwafers,withthicknessesoftheSidevicelayer,buriedoxidelayerandhandleSiwaferof15lm,0.
5lm,and500lm,respectively.
DuetothepeculiarityofSiNWsgrowth,whichtakesplacepredominantlyalongtheh111idirection,a(110)surfaceorientationisselectedfortheSOIdevicelayersurface,sothath111iplanescanbeexposedonverticallyetchedtrenches.
Incontrast,theorienta-tionoftheSOIsiliconhandlewaferdoesnotplayanyroleandastandard(100)orientationisused.
Thefabricationprocessstartswiththedepositionofa300nmthickLPCVDSi3N4layer,tobeusedasmechanicalsupportforthemetals.
Afterpatterningthisnitridelayerusingphotolithographyandadryetchprocess,themetallizationusedtosimultane-ouslyobtaintheelectricalconnectionswiththesili-condevicelayerandabuilt-inheaterelement(electricallyisolatedfromthesiliconbythenitridelm)wasperformedusinga30nmthicktita-nium/tungsten(Ti/W)(10/90%)adhesionlayeranda200nmthickWlayerdepositedbysputtering.
Asecondphotolithographyandawetetchwereusedtopatternthemetal.
Oncethedifferentmetalstruc-turesarepatternedthesurfaceisprotectedwitha1lmthickSiO2layerdepositedbyPECVD.
ThelaststepontheSOIdevicelayeristodenethesiliconstructures,i.
e.
theisolatedplatformandthetren-chesthatwillenclosetheSiNWs.
ThisisdonewithaphotolithographicstepandadryetchprocessthatsequentiallyremovestheSiO2andthesilicondevicelayer,untiltheburiedoxidelayerisreached.
Next,ashort(150s)KOHetchstepwasperformedonthewafertopsidetoreleasethenitridebridge.
Thisnewstepiscritical,asitmustremovetheexposedSiFig.
1.
Illustrationofthemicrogeneratordesigns,classic(a)andproposedalternative(b).
Theisolatedsiliconmass(S1)islinkedtobulkSi(S2)bymeansofaSibridge(a)oralternativelybyathindielectricmembrane(b)withlowerthermalconductance.
Forbothdesigns,theareaofthesuspendedplatformis1mm91mm.
Bridgeandmembranelengthsare200lmand100lm,respectively.
Fig.
2.
DetaileddeviceillustrationshowingtheintegrationoftheSiNWsontheSOIbasedstructure.
Thefeaturedareaisamagni-cationofthe[supportmembrane-platform-rim]regiononFig.
1(right).
ThelengthoftheSiNWsis10lm.
Calaza,Fonseca,Salleras,Donmez,Tarancon,Morata,Santos,andGadea1690devicelayeronlyunderthenitride/metal/oxidebridge,whilepreservingtheotherdevicefunctionalparts.
Inviewofthat,themembranestructureandtheetchholeshavebeendesignedwithaspecicangletoallowafastSiunder-etch,whilepermanentSipartsarepreservedasverticalwallshavebeenalignedwithh111iplanes,whichpresentamuchsloweretchratewhenexposedtoKOH.
TheSEMimageofthebridgeinFig.
3clearlyshowsthatonlysmallSiislands,whichareisolatedfromeachother,remainunderthebridgeafterthisshortKOHstep.
Thiswetetchprocessplaysandadditionalrole,asithelpstorestorethesurfacequalityoftheh111iver-ticalwallswhereSiNWswillbegrown,removingthescallopingeffectofthepreviousRIEetch(Boschprocess).
Devicesarecompletedbyprocessingthebackside,usinga1lmthickpatternedaluminumlayerthatactsasahardmaskforaDRIEprocessthatetchesthehandlewaferandtheburiedoxidelayer.
Thisprocesssequenceisintendedtobuildthedifferentpartsofthethermoelectricgenerator,maintainingallmetalsandsiliconsurfacescoveredbySiO2,excepttheSiverticalwallsthatexposetheh111iplanesforthesiliconnanowiregrowth.
RESULTSANDDISCUSSIONAsetofdifferentdeviceshasbeenproducedusingthedescribedfabricationroute.
Inaddition,deviceswiththeformerbulkSibridgesupportshavebeenproduced(Fig.
4)tobeusedasreferencetoevaluatetheimprovementattainedinthethermalisolationofthesuspendedplatforms.
Deviceswithtwodif-ferentbridgelengths(100lmand200lm)andwithdifferentnumberoftrenches(1–4)havebeenfab-ricatedusingthenewmembrane-likesupports.
Figure5showsadetailofthemultipletrenchesusedtoincreasetheeffectiveNWlength.
Eachtrenchis10lmwideandmidwaysiliconbars(3lmwide)areusedtodeneconsecutivetrenches.
Con-gurationsfortestpurposeshavebeencreatedincludingabuilt-inheater(isolatedfromSibytheSi3N4layer)tocharacterizethethermalisolationbyforcingacontrolledthermalgradientbyJouleheating.
Thethermalisolationachievedwiththedifferentdesignshasbeenassessedbymeasuringthetotalthermalconductancebetweenthebulksiliconandtheisolatedplatform,whichaccountsforthether-malconductivityofthedifferentheatpathsthatconnectbothelements,i.
e.
thesupport(Sibridgeordielectricmembrane),theSibarsthatdenetheSiNWtrenchesandthesurroundingair.
Thermalconductancehasbeenobtainedusingtheintegratedheatertodissipateaknownpowerontheisolatedplatformandtosimultaneouslymeasurethedevel-opedtemperaturegradient.
Forthispurpose,thetemperaturecoefcientoftheresistance(TCR)waspreviouslymeasuredfortheheatermaterial(1950±25ppm/°C)tocalibratetheheaterasathermometer.
Firstofall,theperformanceofthenewsupportswascomparedwiththatofformerbulkSibridges.
Aclassicdesignusingtwo200lmlongbridgeshasbeencomparedwithanewdesignusingashorter100lmlongdielectricmembrane,withasingletrench(T1)forbothdevices.
Figure6showsthetemperaturereachedintheisolatedplatformasafunctionofthepowerdissipatedintheheaterele-ment.
Despitethereducedlength,themembraneoutperformsthebridgesupportsintermsofthermalisolation.
Thermalconductanceisalmosthalved,from1.
82mW/Kto1.
03mW/K,pointingoutthatbridgeconductancewasthemaincontributiontototalthermalconductanceinolddesigns,turningintoalimitingfactorforthermoelectricperfor-mance.
Next,theinuenceofthedistancebetweentheisolatedplatformandthebulksiliconrimintheactivearea(theeffectiveNWlength)hasbeenanalyzedusingasetoffourdevicesfeaturinga100lmlongdielectricmembranesupportandthefourdifferenttrenchdesigns(T1–T4).
Figure7showsthetemperaturereachedintheisolatedplatformasafunctionofthepowerdissipatedintheheaterelement.
Asanticipated,thenumberoftrencheshasasignicanteffectonthermalisolationsincethermalconductanceisreducedfrom1.
03mW/KforT1to0.
68mW/KforT4,themorenumerousthetrenches,thebetterthethermaliso-lation.
Theobservedtrendandvaluespointoutthattheconductanceofthesebarsisthemaincontri-butiontototalthermalconductanceinthenewmembrancedesigns.
However,thethermalconduc-tanceofthesetrenchesoncelledwithNWsinrealthermoelectricgeneratorswilldependalsoontheparametersusedforNWgrowth,whichdetermineNWsizeanddensity.
AcompleteoptimizationwillbenecessarytondaNWdistributionandanum-Fig.
3.
SEMimageofthesupportingmembraneaftertheKOHetch.
Metalconnectionsaresandwichedinathindielectricmembrane,whichisreleasedbythesiliconunder-etch.
OnlysmallisolatedSiislandsremain.
ThermalTestofanImprovedPlatformforSiliconNanowireBasedThermoelectricMicro-generators1691Fig.
4.
SEMimagesshowingthemicrofabricatedplatforms,withbulkSi(a)orthindielectric(b)supports.
Botharesingletrenchdevicesandincludeaheaterelementforcharacterizationpurposes.
Fig.
5.
SEMimagesofthesiliconstructuresusedtoincreasetheeffectivenanowirelength,from10lm(T1)to40lm(T4),withsuccessivetrenchestobelledwithSiNWs.
Theimagesareamagnicationofthebottom-rightregionoftheplatform-rimareafeaturedinFig.
4.
Calaza,Fonseca,Salleras,Donmez,Tarancon,Morata,Santos,andGadea1692beroftrenchesenhancingthethermoelectricper-formance,whichshowsanoppositedependencyonthermalandelectricalconductions.
Finally,theinuenceonthermalconductanceofthelengthofthemembranesupporthasbeenana-lyzedusingasetoftwodeviceswith100lmand200lmlongdielectricmembranes(B1,B2),andthefourtrenchesdesign(T4).
Figure8showsthetem-peraturereachedintheisolatedplatformasafunctionofthepowerdissipatedintheheaterele-ment.
Thesmallchangeobservedintotalthermalconductance,from0.
68mW/KforB1to0.
65mW/KforB2,afterhavinghalvedthecontributioncomingfromthemembranesupport,conrmsthatmaincontributiontothermalconductanceinnewdesignsislinkedtothesiliconbarsusedtodenethetrenchestobelledwithSiNWs,asanticipatedinthepreviousmeasurement.
Inthelightofabovementionedimprovementinthermalconductance,thenewplatformdesignsareexpectedtogeneratehigherpowerdensitiesthancurrentdevicesusingbulkSibridges,whichgen-eratedamaximumpowerdensityof9lW/cm2forDT=27°C.
9Fig.
6.
Temperatureincreaseintheplatformasafunctionofdissipatedpowerfortwodeviceswithasingletrench,onewith200lmlongbulkSisupports(black)andotherwitha100lmlongSi3N4membrane(red)(Colorgureonline).
Fig.
7.
Temperatureincreaseintheplatformasafunctionofdissipatedpowerfordeviceswitha100lmlongSi3N4membraneanddifferentnumbersofconsecutivetrenches(T1–T4)(black,red,blue,green)(Colorgureonline).
ThermalTestofanImprovedPlatformforSiliconNanowireBasedThermoelectricMicro-generators1693CONCLUSIONSANDFUTUREWORKAnewtechnologicalroutehasbeenproposedtointegratelowthermalconductancesupportswiththesiliconmicromachinedsuspendedplatformsusedtobuildall-Sithermoelectricmicrogenerators.
Asetofdevicesbasedonthisprocesshavebeensuccessfullyfabricatedandthermalmeasurementshaverevealedthatasignicantthermalconductancereductionisattainedwiththismembrane-likesupports,eventhoughshorterlengthsareused.
Thisresultpavesaroutetofurtherimprovethepowerdensityattainedwiththeall-SimicrogeneratorsbasedonSiNWs.
ThecompatibilityofthesupportswiththeSiNWsgrowthprocesshastobeconrmed,andthethermalcon-ductanceofthesupportshastobecontrastedwiththatoftheNWsarraysinordertoestablishtheoptimumlengthforthisnewtypeofsupport(i.
e.
,theattainablesupportareareduction).
ACKNOWLEDGEMENTSThisworkhasbeensupportedbytheEUFP7-NMP-2013-SMALL-7,SiNERGY(SiliconFriendlyMaterialsandDeviceSolutionsforMicroenergyApplications),undercontractn.
604169,theSpan-ishMinistryofEconomyandCompetitiveness(TEC-2010-20844)andthe''GeneralitatdeCatalu-nya''(AdvancedMaterialsforEnergyNetwork(XaRMAE),2009-SGR-440).
C.
CalazaandA.
Tar-anconwouldliketothankthenancialsupportoftheRamonyCajalpostdoctoralprogramoftheSpanishMinistryofEconomyandCompetitiveness.
OPENACCESSThisarticleisdistributedunderthetermsoftheCreativeCommonsAttribution4.
0InternationalLicense(http://creativecommons.
org/licenses/by/4.
0/),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedyougiveappropriatecredittotheoriginalauthor(s)andthesource,providealinktotheCreativeCommonslicense,andindicateifchangesweremade.
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Fig.
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Calaza,Fonseca,Salleras,Donmez,Tarancon,Morata,Santos,andGadea1694