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JOURNALOFMICROELECTROMECHANICALSYSTEMS1
CapacitiveRFMEMSSwitchesWith
Tantalum-BasedMaterials
AnnaPersano,AdrianoCola,GiorgioDeAngelis,AntoniettaTaurino,PietroSiciliano,andFabioQuaranta
Abstract—Inthispaper,shuntcapacitiveRFmicroelectro-
mechanicalsystems(MEMS)switchesaredevelopedinIII–V
technologyusingtantalumnitride(TaN)andtantalumpentox-
ide(Ta
2
O
5
)fortheactuationlinesandthedielectriclayers,
respectively.Acompositional,structural,andelectricalcharacter-
izationoftheTaNandTa
2
O
5
filmsispreliminarilyperformed,
demonstratingthattheyarevalidalternativestotheconven-
tionalmaterialsusedinIII–VtechnologyforRFMEMSswitches.
Specifically,itisfoundthattheTaNfilmresistivitycanbetuned
from0.01to30Ω·cmbychangingthedepositionparameters.
Ontheotherhand,dielectricTa
2
O
5
filmsshowalowleakage-
currentdensityoffewnanoamperespersquarecentimeterfor
E～1MV/cm,ahighbreakdownfieldof4MV/cm,andahigh
dielectricconstantof32.Therealizedswitchesshowgoodactu-
ationvoltages,intherangeof15–20V,aninsertionlossbetter
than?0.8dBupto30GHz,andanisolationof～?40dBatthe
resonantfrequency,whichis,accordingtobridgelength,between
15and30GHz.AcomparisonbetweenthemeasuredS-parameter
valuesandtheresultsofacircuitsimulationisalsopresentedand
discussed,providingusefulinformationontheoperationofthe
fabricatedswitches.[2010-0236]
IndexTerms—Capacitiveswitches,RFmicroelectromechanical
systems(MEMS),tantalumnitride(TaN),Ta
2
O
5
.
I.INTRODUCTION
T
HERFmicroelectromechanicalsystems(MEMS)repre-
sentaviablesolutiontoovercomethelimitationsex-
hibitedbysemiconductor-basedswitches(pindiode,FET
switches,...)owingtotheverylowpowerdissipationandin-
sertionloss,highisolation,andlinearity[1].Moreover,theuse
ofIII–VtechnologyforRFMEMSswitchfabricationmaypave
thewayfortheimplementationoffuture-generationtransmit/
receivemodulesowingtothemonolithicintegrationofMEMS
andHEMTcomponentsinasinglefabricationprocess[2].
However,inspiteoftheattractivecapabilities,RFMEMS
reliabilityisofmajorconcernforlong-termapplications,andit
iscurrentlythesubjectofanintenseresearcheffort[3]–[7].
Todate,thedevicelifetimeisreducedinRFMEMSohmic
switchesbythedegradationofthemetal-to-metalcontactbe-
ManuscriptreceivedAugust4,2010;revisedNovember29,2010;accepted
January3,2011.ThisworkwassupportedinpartbytheItalianMinistryfor
Education,University,andResearch(MIUR)underProjectDM25810ofthe
BasicResearchInvestmentFund(FIRB).SubjectEditorC.-J.Kim.
A.Persano,A.Cola,A.Taurino,P.Siciliano,andF.Quarantaarewiththe
InstituteforMicroelectronicsandMicrosystems,NationalResearchCouncil
(IMM-CNR),UnitofLecce,73100Lecce,Italy(e-mail:anna.persano@le.
imm.cnr.it).
G.DeAngelisiswiththeInstituteforMicroelectronicsandMicrosystems,
NationalResearchCouncil(IMM-CNR),UnitofRome,00133Rome,Italy.
Colorversionsofoneormoreofthefiguresinthispaperareavailableonline
athttp://ieeexplore.ieee.org.
DigitalObjectIdentifier10.1109/JMEMS.2011.2107884
tweentheactuationelectrodeandthebridge.Thisdegradation
canbeduetodifferentphenomena,suchasthecontamination
andtheelectromigrationofmaterialsacrossthecontact,the
creeps,theductile,andthebrittlewearingofthecontact,and
thehardeningofthecontactarea[8].However,thisdrawback
canbemanagedbyproperdesignguidelineswhichallowactu-
ationvoltagesnotexceeding50Vandaswitchlifetimebeyond
onetrillioncycles[9],[10].Incapacitiveswitches,themetal-to-
metalcontactiseliminatedbycoveringtheactuationpadwith
adielectriclayer,whichalsooffersthebenefittoprovideahigh
ratiobetweentheswitchcapacitanceinthedownandupstates
(C
down
/C
up
).However,thisdielectriclayerusuallyundergoes
chargingeffectsinthearea(approximately100μm×100μm
intypicalconfigurations)underthebridge[5],[7],[11]–[13]
whichcancausestictionphenomenabetweenthedielectric
layerandthebridgemetaloranincreaseinthepull-downvolt-
age,dependingonthepolarityoftheinjectedcharge.Dielectric
chargingincreaseswiththeappliedelectricfield[7],andhence,
effortstocontroltheactuationvoltagehavebeenmadebythe
optimizationofthebridgefabricationprocess[14].
Nowadays,mostofthetechnologicalissueslimitingtheRF
MEMSswitchesarestillopen,particularlyinIII–Vtechnol-
ogy,whichismuchlessmaturethantheSi-basedfabrication
process.Hence,theinvestigationofalternativematerialswith
respecttotheonescommonlyusedinIII–Vtechnologyfor
bothdielectricandactuationcomponentsofMEMSswitches
becomesmandatory.
Inthispaper,RFMEMSshuntcapacitiveswitchesincopla-
narconfigurationarefabricatedinIII–Vtechnologymaterials
whicharealternativetothestandardones.Specifically,films
oftantalumnitride(TaN)andtantalumpentoxide(Ta
2
O
5
)
arechosenfortherealizationofactuationlinesanddielectric
layers,respectively.Thispaperisorganizedasfollows.In
SectionII,thecompositional,structural,andelectricalcharac-
terizationofTaNandTa
2
O
5
thinfilmsisreported.Deposition
parameters,suchasthesubstratetemperatureandthesputtering
gasmixturecomposition,havebeenvaried.Theresistivityof
TaNfilmsisobtainedfromHall-effectmeasurements,while
I–Vcharacteristicsallowidentifyingtheconductionmecha-
nismswhichareresponsiblefortheleakagecurrentinthe
Ta
2
O
5
films.Capacitancemeasurementshavealsobeenper-
formed,allowingtheestimationofthestaticdielectricconstant
ofTa
2
O
5
films.InSectionIII,thestepsforthefabricationof
shuntcapacitiveswitchesinIII–VtechnologyusingTaNand
Ta
2
O
5
filmsaredescribed.Finally,inSectionIV,theresultsof
theRFcharacterizationperformedonthefabricatedswitches
arepresentedandcomparedwiththeresultsobtainedbya
circuitsimulation.
1057-7157/$26.00?2011IEEE
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2JOURNALOFMICROELECTROMECHANICALSYSTEMS
TABLEI
COMPOSITIONANDRESISTIVITYOFTaNFILMS
II.MATERIALCHARACTERIZATION
A.TaNThinFilms
ThinfilmsofTaNweredepositedbyreactivemagnetron
sputteringstartingfromaTatarget.Variablemixturesofni-
trogenandargon,withreactivegas(N
2
)varyingfrom20%to
33%,wereusedinordertoproperlycontrolthefilmcompo-
sitionandstoichiometry.Thesubstratetemperaturewasalso
variedfromroomtemperatureto300
?
C.AllTaNfilmswere
depositedatafixedpowerof200Wandare150μmthick.
X-rayphotoelectronspectroscopy(XPS)analysisoftheTaN
filmsindicatesthattheTa/Nratioisclosetounity(0.9)inthe
film,obtainedbyusingthehigherN
2
contentinthesputtering
gasmixtureatasubstratetemperatureof300
?
C.Fortheother
samples,asignificantlylowerTa/Nratioisfound(≤0.7).
Theresistivityofthedepositedfilmswasevaluatedbymeans
ofHall-effectmeasurements,anditresultedtobeontheorder
of10
?2
Ω·cmwhenanN
2
contentof20%wasusedinthe
sputteringgasmixture.ByincreasingtheN
2
contentto33%,
theresistivitysignificantlyincreased.Inparticular,whenthe
filmsweredepositedatroomtemperature,theresistivitywas
foundtoincreasebythreeordersofmagnitude,whereasalower
riseofthesheetresistancewasinducedbythesameincreaseof
theN
2
contentforfilmsdepositedat300
?
C(seeTableI).
Hence,thestudiedTaNfilmsshowaresistivitythatishigher
thanthatofNiCr(ρ～1×10
?4
Ω·cm),whichisthestandard
materialusedforactuationlinesintheRFMEMSswitches
fabricatedinIII–Vtechnology.Thisaspectisbeneficialfor
theapplicationofTaNasalternativetoNiCrsincethehigher
resistivitywouldallowabetterseparationofthedcandRF
contributions.Moreover,thepossibilitytotunetheTaNresis-
tivityoverthreeordersofmagnitudecouldofferthepossibility
toovercomesometechnologicallimitationsrelatedtothefilm
thicknessandthelinegeometry.
B.Ta
2
O
5
-BasedMIMStructures
Ta
2
O
5
thinfilmsweredepositedbyRF-magnetronreactive
sputteringfromahigh-puritytantalummetaltarget(4-indiam-
eter).TheTa
2
O
5
filmswerepreparedatafixedpowerof200W
andwithachamberpressureof9mtorr.Aflowratioof1:2in
thesputteringgasmixtureofargonandoxygenwasused.
InordertoperformtheTa
2
O
5
filmelectricalcharacterization,
testcapacitorswithametal–insulator–metal(MIM)structure
werefabricatedon(100)semiinsulatingGaAssubstrates.The
bottomandtopmetallizationsoftheMIMstructureswerede-
positedbymagnetronreactivesputteringinaseparatevacuum
system.ThebottomcontactisacontinuousmultilayerTi/Au/Ti
(10/100/10nm)overthewafer.Thetopelectrodeconsistsof
squareTi/Au(10/300nm)contactswitha180-μmsize,which
wererealizedbystandardopticallithography.TheTa
2
O
5
film
Fig.1.(a)SEMimageofaTa
2
O
5
-basedMIMstructure.Thecrosssection
wasobtainedonasurfacecleavedbyusingaFIB.Theplanviewofthe
Ta
2
O
5
layerandoftheupperTi/Aucontactsurfaceisshownin(b)and(c),
respectively.
Fig.2.TypicalJ?EcharacteristicforTa
2
O
5
-basedMIMstructuresatroom
temperature.A1-mAvalueissetascompliancelevelofthemeasurement.The
insetshowstheplotofln(J/E)versusE
1/2
.Thelineisaguidetotheeyes.
thicknesswasmeasuredbyusinganAlpha-StepIQandresulted
tobe324±5nm.
ThestructureandthecompositionofthedepositedTa
2
O
5
filmswereanalyzedbyusingX-raydiffractionandXPS,
respectively.Thefilmswerefoundtobeamorphouswitha
compositionthatisalmoststoichiometric(ratioO/Ta=2.46).
Ta
2
O
5
filmmorphologywasinvestigatedbyplan-viewand
cross-sectionalscanningelectronmicroscopy(SEM)images.
Inparticular,thecross-sectionalSEMimageswererecordedon
cleavedsurfacesobtainedbyusingafocusedionbeam(FIB).
Atypicalcross-sectionalSEMimageoftheinvestigatedMIM
capacitorsisshowninFig.1(a).Thedifferentmorphologyof
thevariouslayersintheMIMstructureisclearlyobserved.
Specifically,theTa
2
O
5
filmisfoundtobedenseandcontinuous
overthewholesurface,asalsoevidencedbytheplan-view
images[Fig.1(b)].Thesurfaceoftheuppermetalliccontact
isobservedtobecontinuousandgranular,asexpectedforAu
layerdeposition[Fig.1(c)].
Fig.2showsthetypicalJ?Ecurvemeasuredatroomtem-
peraturefortheTa
2
O
5
-basedMIMcapacitors.Intheelectric-
fieldrangeof0.2–1MV/cm,thecurrentdensityincreases
approximatelylinearlywiththefield,andtheconductivity
isverylow.Fortheselowelectric-fieldvalues,thecurrent
inthedielectricfilmsisexpectedtobeduetothehopping
conductionmechanism,i.e.,thethermalexcitationoftrapped
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PERSANOetal.:CAPACITIVERFMEMSSWITCHESWITHTANTALUM-BASEDMATERIALS3
electronsfromonetrapsitetoanotherdominatesthetransport
indielectricfilms[15].Forfieldshigherthanabout1MV/cm,a
kinkinthecurrentdensityisobserved,whichpointsoutthat
aconductionmechanismdifferentfromhoppingdominates.
Specifically,thecurrentdatashowalinearbehaviorintheplot
ln(J/E)versusE
1/2
forE>1.2MV/cm(seeinsetofFig.2),
whichisindicativethatthePoole–Frenkelemissiondetermines
thecurrentthroughthedielectriclayer[15].Irreversibledielec-
tricbreakdownoccursforE～4MV/cm,consistentwiththe
highestbreakdownfieldvaluesreportedforTa
2
O
5
films[16].
Capacitancemeasurementswerealsoperformedat1MHz
ontherealizedTa
2
O
5
-basedMIMstructures.Withnoapplied
electricfield,thecapacitancevalueis28.77pF.Underthe
assumptionthattheMIMcapacitanceisgivenbythegeometric
expressionC=ε
s
ε
0
A/d,whereε
s
isthestaticdielectriccon-
stantoftheinsulator,ε
0
isthepermittivityoffreespace,Ais
theactivearea(180×180μm
2
),anddisthefilmthickness,
astaticdielectricconstantof32isestimated,inagreement
withthevaluesreportedforTa
2
O
5
films[17].Thisdielectric-
constantvalueismuchhigherthanthatofSi
3
N
4
(6–7),which
isthedielectricmaterialcommonlyusedincapacitiveRF
MEMSswitches.Thehighdielectricconstantisappealingfor
theapplicationofTa
2
O
5
asdielectricmaterialincapacitive
switchessinceitallowssignificantlyincreasingthecapacitance
ratioC
down
/C
up
[18].
Finally,inordertoinvestigatethechargingeffectsinthe
Ta
2
O
5
films,capacitanceandcurrenttransients(notshown
here)weremeasuredundertheapplicationofaconstantelectric
fieldintherangeof0.3–4MV/cm.Itisfoundthatboththe
capacitanceandcurrenttransientsarewelldescribedbylaws
derivedbyachangeinthedielectricpolarization,whichisex-
pressedbyastretchedexponentialrelation[19].Thevariation
inthepolarizationisattributedtothetrappingoftheinjected
chargeinthelargedensityofstatescontainedinthedielectric
film[12].
III.SWITCHFABRICATION
ThedepositedTaNandTa
2
O
5
thinfilmswereusedtofabri-
cateshuntcapacitiveswitchesintheIII–Vtechnology.Tothis
scope,asurface-micromachinedapproachwasfollowed.The
fabricatedswitchesareincoplanarwaveguide(CPW)config-
uration,withasuspendedmetalbridgeconnectingthelateral
groundplanesandadielectriclayeronthecentralconductor
whichprovidesacapacitivecontributionwhenthebridgeison
thedownstate.Aneight-maskprocesswasusedtomanufacture
theswitches.Theprocessflowcanbedescribedasfollows.
1)Ona625-μm-thicksemiinsulatingGaAssubstrate,a
500-nm-thickSi
3
N
4
layerwasdepositedoverallthe
waferasisolationlayer.Next,a120-nm-thickTaNlayer
wasdepositedbysputteringat25
?
Cwitha20%content
ofN
2
inthegasfeedmixture.Inordertodefinethe
actuationelectrodes,theTaNfilmwaspatternedbya
reactive-ion-etchingprocessbasedonfluorinechemistry
thathadbeenoptimized,reachinganetchingratevalue
of28nm/minandagoodselectivitywithrespectto
theunderneathSi
3
N
4
.Theobtainedactuationlinesare
shownasredregionsinFig.3(a).
Fig.3.(a)Designof(inredcolor)theactuationlines,(inyellowcolor)the
underpassandthepadstocontacttheactuationlines,and(indarkcolor)thevia
holesinthedielectriclayercoveringtheunderpass.(b)Designofthecompleted
switch.
2)Then,a400-nm-thickTa
2
O
5
wasdeposited,andviaholes
weredefinedtocontacttheelectrodesbystandardoptical
photolithographyanddryetching(fluorine-basedchemis-
try).Withthedry-etchingprocessbeingnotselectivewith
respecttoTaN,apreviousoptimizationwasperformed.
3)AmetalmultilayerofTi/Pt/Au(30/30/60nm)wasthen
depositedtorealizeunderpasslinesandthepadsto
contacttheTaNelectrodesbyusingliftofftechnique[see
yellowareasinFig.3(a)].
4)Themetalisnextcoveredwithanother400-nm-thick
Ta
2
O
5
dielectriclayer.Viaholesintheoxidefilmwere
realizedbymeansofphotolithographyanddryetching
inthesameconditionasStep2)[seedarksquaresin
Fig.3(a)].
5)Thesacrificiallayerforthedefinitionoftheairgapunder
thebridgeswasformedbya3-μm-thickphotoresist.In
ordertostabilizethislayerandtoobtainwell-rounded
edgesofbridgeswhicharenecessarytopromotethe
flatnessofthemembraneanchorsandborders,ahard
bakeat200
?
Cwasperformed.Frompreliminarysim-
ulations,anairgapontheorderof3μmfulfilsboththe
electricalisolationbetweenthebridgeintheupposition
andthecentralconductor,aswellastherequirementthat
actuationvoltagesmaynotexceed70–80V.
6)AmetalmultilayerofTi/Au/Ti(5/50/5nm)wasevapo-
ratedontheentiresurface,tobeusedaselectricalcon-
tactfilmandseedlayerforthefollowingelectroplating
process.Theairbridges,theanchorposts,theCPW
lines,andthegroundpadsweredefinedusinga3-μm-
thickphotoresist.TheupperTifilm,whichactsasad-
hesionlayerfortheresist,hastoberemovedinthe
membrane-structureareasbeforethegolddeposition.A
1-μm-thickgoldlayerwasgrownbyelectroplatingusing
agoldcyanidebath.Toobtainflatbridges,thedeposition
parameterswerechoseninordertoachieveaslight
overalltensilestress.Moreover,thebridgesweregrown
withasetofalignedholes,whichallowtheremovalof
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4JOURNALOFMICROELECTROMECHANICALSYSTEMS
Fig.4.SEMplanviewofashuntcapacitiveRFMEMSswitchfabricatedon
aGaAssubstrateusingTaNandTa
2
O
5
thinfilmsfortheactuationlinesand
thedielectriclayers,respectively.
thesacrificiallayerusingdry-etchingtechniquesanda
fasteroperationoftheswitchbyreducingtheairdamping
underneaththebridge.Theholeshaveasquareshapewith
anareaoftypically10×10μm
2
andadistancefrom
eachotherof～10μm.
7)Asecondgold-electroplatingdepositionwitha1.5-μm
thicknesswasusedtothickentheCPWlinesandthe
groundpadsandtobuildupaframetoimprovethe
flatnessofthebridge[seeblueareasinFig.3(b)].A
combinationofselectivewetanddryetchingwasused
toremovetheunwantedTi/Au/Timultilayeramongthe
devices.
8)Finally,theairbridgeswerereleasedbyremovingthe
underneathsacrificialphotoresistbyahighpressureO
2
plasmaprocessperformedinabarreletcherinorderto
preventstickingproblems.Theprocesswasdividedinto
manystepsinordertoavoidoverheatingofmembranes
whichcaninducedistortioneffects.
Profilometeranalysisshowedthatthebridgesarerobust
enoughforreliabilitypurposesandflexibleenoughtobedriven
byreasonablevaluesoftheappliedvoltage.
SEMimageswerealsorecordedinordertocheckthequality
ofthedifferentprocesssteps.ASEMplanviewofthefabri-
catedswitchisshowninFig.4.
IV.RFCHACTERISTICS
AremotelycontrolledHP8510Cvectornetworkanalyzer,to-
getherwithaKarlSüssProbeStationequippedwithgroundsig-
nalgroundRF|Z|-probesanddcprobes,wasusedintheair
forthemeasurementoftheSparametersoftheswitchesin
theupanddownstates.Measurementsweretakenusinga
commercialshort–open–loadthrucalibrationwith801points
inthefrequencyrangefrom1to40GHz.Switchactuation
voltagesof15–20Vwereobtainedbyperformingavoltage
rampwitharateof1V/sec.Finally,theSparametersinthe
downandupstateswererecorded.
TypicalSparametersintheupstateareshowninFig.5.
Thereturnlossislowerthan?10dBbelow28GHz,while
Fig.5.Insertion(S
21
)andreturn(S
11
)lossesforashuntcapacitiveswitch
intheupstate:(Solidline)Experimentalresultsand(dashedline)circuitsim-
ulations.Thebridgeoftheswitchhasalengthandawidthof550and150μm,
respectively,beingwiderinthecentralpartwithanareaof150×250μm
2
.
theinsertionlossis?0.2dBat20GHzandbetterthan
?0.8dBupto30GHz.TheexperimentalSparametersare
comparedwiththeresultsofacircuitsimulationcarriedout
usingthecommercialsoftwarepackageAgilentAdvancedDe-
signSystem(seeFig.5).Forthesesimulations,theequiva-
lentresistor–inductor–capacitor(RLC)circuitmodel,together
withtherelatedformulaspreviouslyproposedin[20],wasused.
ThemeasuredSparametersarefoundtobewelldescribedby
theresultsprovidedbythecircuitsimulations,witha0.07-pF
equivalentupcapacitance(C
up
).
TheSparametersinthedownstate,togetherwiththesimu-
latedresults,areshowninFig.6.Thereturnlossisbetterthan
?0.4dBinalmostallthemeasuredfrequencyrange,anda
resonantfrequency(f
0
)of23GHz(isolationof?38dB)is
observed.Specifically,circuitsimulationsareabletodescribe
theexperimentaldataprovidedthatatunableseriescapacitance
(C
s
)isaddedtotheRLCmodel.Asreportedin[20],therea-
sonforthisadditionalseriescapacitanceislikelyaresidualair
gapbetweentheactuatedbridgeandthedielectricunderneath
itduetoapoorflatnessofthebridgecausedbybothresidual
stressandstressgradientinthesuspendedgoldmembrane.
Acurvatureinthecentralregionofthebridgecanbeindeed
noticedinFig.4.Thevaluesofthelumpedelementsinthe
equivalentRLCcircuit,whichbestdescribetheexperimental
data,arefoundtobeasfollows:R=0.39Ω,L=34.52pH,
C=18.14pF,andC
s
=1.43pF.ThevalueofC
s
isconsistent
withthecapacitancewhichiscalculatedtobeassociatedtothe
effectiveareaofthebridgeportionoverthecentralconductor
(150×250μm
2
lessthetotalareaofholes)whena200-nmair
gapispresent.Moreover,the200-nmairgapisobtainedby3-D
electromechanicalsimulations(notshownhere)performedon
thetestedswitchesusingAnsoftHFSSsimulator.
DuetothecontributionofC
s
,thetotaldowncapacitance
(C
down
)resultstobeof1.32pF,andhence,thecapacitance
ratioC
down
/C
up
turnsouttobe～19.
Theinsetshowstheresonantfrequencyforswitcheshaving
bridgesofthesamewidthanddifferentlengths.Itisobserved
thatf
0
reducesfrom～30to15GHzbyincreasingthebridge
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PERSANOetal.:CAPACITIVERFMEMSSWITCHESWITHTANTALUM-BASEDMATERIALS5
Fig.6.Insertion(S
21
)andreturn(S
11
)lossesforthesameswitchofFig.5
inthedownstate:(Solidline)Experimentalresultsand(dashedline)circuit
simulations.Theinsetshowstheresonantfrequencyasafunctionofthebridge
length.
lengthintherange450–750μm,inagreementwiththeincrease
oftheinductivecontribution[1].
Reliabilitytestswereperformedontherealizedcapacitive
switchesbyapplyingvoltagepulsesof15–20V(dutycycleof
50%withaperiodof1ms).ItisfoundthattheSparameters
exhibitnovariationevenafteronemillionofcycles,butthe
curvedshapeofthebridgeaffectstherepeatabilityofresults.
Ontheotherhand,evidentstictionphenomenamainlyinduced
bythedielectricchargingwerefoundtooccuronlyforvoltages
ashighas～100V.Hence,forappliedvoltagesnearactuation,
thepoorflatnessofthebridgeisthemostlimitingfactorforthe
reliabilityoftheswitchesstudiedhere.Consequently,before
performingasystematictestingoftheswitchreliability,the
optimizationofthemembranedepositionhastobecarried
out.Furthermore,theair-gapcontributioncanbeeliminated
byusingtheso-calledfloating-metalsolution.Insuchacase,
theportionofthedielectricunderneaththebridgeismetallized
onthetoptoobtainaMIMcapacitorindependentofthe
shapeofthebridgewhentheswitchisactuated[21].Parasitic
contributionsduetotherealshapeofthecollapsedbridgeare
expectedtobeeliminatedwithinthemicrowaverangewhenthe
floatingmetalisused,butontheotherhand,thedrawbackof
stiction,typicalofametal-to-metalcontact,canoccur.
V.CONCLUSION
ShuntcapacitiveRFMEMSswitchesaredevelopedinIII–V
technologyusingTaNandTa
2
O
5
asmaterialsforactuation
linesanddielectriclayers,respectively.Theelectricalcharac-
terizationindicatesthattheTaNandTa
2
O
5
filmsarevalidal-
ternativeswithrespecttothestandardmaterials,whichareNiCr
andSi
3
N
4
,foractuationlinesanddielectriclayers,respectively.
Thefabricatedswitchesshowprettylowactuationvoltages
of15–20V,aninsertionlossthatisbetterthan?0.8dBup
to30GHz,andanisolationof～?40dBattheresonant
frequencywhichisintherangeof15–30GHz,accordingto
thebridgelength.ThemeasuredSparametersareinagreement
withthoseobtainedbyacircuitsimulationperformedusing
thelumped-elementequivalent-circuitmodel,whichhasbeen
previouslyproposedin[17].Specifically,inordertoaccurately
describetheSparametersinthedownstate,anadditionalseries
capacitanceisinsertedtotheRLCcircuit,whichisattributedto
aresidualairgapbetweentheactuatedbridgeandthedielectric
underneathit.
TheresultspresentedhereprovethepotentialofTa-based
materialsfortherealizationofcapacitiveRFMEMSswitches.
Theoptimizationofthebridgefabricationisinprogressin
ordertoimprovethedeviceperformanceandreliability.
ACKNOWLEDGMENT
TheauthorswouldliketothankR.Marcelliforthefruit-
fuldiscussionsontheRFcharacteristics,M.C.Martuccifor
thetechnicalsupportintheswitch-fabricationprocess,and
theFondazioneBrunoKessler,MEMSResearchUnit,Povo,
Italy,forprovidingthedesignoftheswitchesrealizedin
thiswork.
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AnnaPersanoreceivedtheLaurea(withhonors)de-
greeinphysicsfromtheUniversityofLecce,Lecce,
Italy,in2003,andthePh.D.degreeinmaterial
engineeringfromtheUniversityofSalento,Lecce,
in2008.
ShehasbeenwiththeInstituteforMicroelectron-
icsandMicrosystems,NationalResearchCouncil
(IMM-CNR),Lecce,since2008,wheresheiscur-
rentlyaContractResearcher.Hercurrentinterestsin-
cludethefabrication,electricalcharacterization,and
reliabilityofRFmicroelectromechanicalsystems
switchesrealizedwithinnovativetechnologiesonGaAsandGaN/Sisubstrates.
Shehasalsobeenworkingonthephotoelectricalcharacterizationoflow-
dimensionalsystems(quantumdots,quantumwires,nanorods,andtetrapods)
foroptoelectronicapplicationsand,morerecently,onthedevelopmentand
electricalcharacterizationofmicro-andnanodevicesforterahertzapplications.
AdrianoColawasborninLaSpezia,Italy,in
1961.HereceivedthePh.D.inphysicsfromthe
UniversityofBari,Bari,Italy,in1991,withathe-
sisondeeplevelsandSchottkybarriersonIII–V
compounds.
AfterhispostdoctoralstudiesattheLabora-
toiresd’étudesdespropriétésélectroniquesdes
solides,CentreNationaldelaRechercheScientifique
(LEPES-CNRS),Grenoble,France,andattheNa-
tionalInstituteofNuclearPhysics(INFN),Pisa,
Italy,hejoinedtheInstituteforMicroelectronicsand
Microsystems,NationalResearchCouncil(IMM-CNR),Lecce,Italy,in1994,
whereheiscurrentlyaSeniorResearcher.Heisthecoauthorofabout100
peer-reviewedjournalpapers.HehasbeenworkingonGaAsandCdTeX-ray
detectors,high-speedheterostructurephotodetectors,and,morerecently,on
low-dimensionalstructures(2DEG,quantumdots,nanowires,andnanorods)
forterahertzandphotovoltaics.Hismainresearchinterestsarethetransport
propertiesofsemiconductormaterialanddevices.
GiorgioDeAngeliswasborninRome,Italy,
in1980.HereceivedtheLaureadegreeinelec-
tronicengineeringfromtheUniversityofRome
“TorVergata,”Rome,in2006,whereheiscurrently
workingtowardthePh.D.degreeinintegratedcir-
cuitsandtelecommunications.
Hehasbeentherecipientofaresearchfellowship
ondesign,testandreliabilityoffrequencytunable
RFmicrodevicesattheInstituteforMicroelectron-
icsandMicrosystems,NationalResearchCouncil
(IMM-CNR),Rome,since2006.Hisresearchinter-
estsincludethedesignandtestofmicrowavecircuitsandmicroelectromechan-
icalsystemsdevicesforspaceandEarthapplications.
AntoniettaTaurinoreceivedtheLaurea(withhon-
ors)andPh.D.degreesinphysicsfromtheUni-
versityofLecce,Lecce,Italy,in1995and1999,
respectively.
Since2001,shehasbeenaResearcherwiththe
InstituteforMicroelectronicsandMicrosystems,Na-
tionalResearchCouncil(IMM-CNR),Lecce.Her
competencesarerelatedtotheanalysisofthemor-
phological,structural,andcompositionalproperties
ofsemiconductingmaterialsbytransmissionelec-
tronmicroscopy(TEM)techniques.Hermainfields
ofinterestarerelatedtonanostructuredthinfilmsforgassensorsandquantum
confinedsemiconductorsforoptoelectronicapplications.Recently,heractivity
hasbeenextendedtoadvancedSEMtechniques,likescanningTEMand
electron-beam-inducedcurrent,aswellastofocused-ion-beamtechniquesfor
thenanofabrication,nanodeposition,andnanomanipulationofmaterialsand
devices.
PietroSicilianoreceivedtheLaureadegreein
physicsfromtheUniversityofLecce,Lecce,Italy,
in1985,andthePh.D.degreeinphysicsfromthe
UniversityofBari,Bari,Italy,in1989.
HeiscurrentlyaDirectorofResearchatthe
InstituteforMicroelectronicsandMicrosystems,Na-
tionalResearchCouncil(IMM-CNR),Lecce,where
hehasbeenworkingformanyyearsinthefield
ofsensors,microelectromechanicalsystems,andmi-
crosystems,asLeaderoftheSensorsandMicrosys-
temsgroup.HeistheDirectorofIMM-CNR,Lecce,
responsibleforseveralnationalandinternationalprojects.HeisaRefereeand
MemberoftheAdvisoryBoardsofinternationaljournalsandhasauthored
about250scientificpapers.
Dr.SicilianoisamemberoftheSteeringCommitteeoftheItalianAssocia-
tiononSensorsandMicrosystems(AISEM),andisthePresidentoftheItalian
Associationon“AmbientAssistedLiving”(AitAAL).HehasbeenaChairman
andamemberoftheorganizingcommitteesofseveralinternationalconferences
andschools.
FabioQuarantawasborninBari,Italy,in1961.
HereceivedtheLaureadegreeinphysicsfromthe
UniversityofBari,Bari,Italy,in1988.
From1988to1994,hewaswiththePhysicsDe-
partment,UniversityofBari,andwiththeMaterial
ScienceDepartment,UniversityofLecce,Lecce,
Italy.In1994,hejoinedtheInstituteforMicroelec-
tronicsandMicrosystems,NationalResearchCoun-
cil(IMM-CNR),Lecce,Italy,becomingapermanent
ResearcherStaffMemberin1998.Heiscurrently
inchargeasLeaderoftheMicroelectronicTech-
nologiesLaboratoryofIMM-CNR,Lecce.Heisthecoauthorofover80
peer-reviewedjournalpapers.Hisresearchinterestsincludethefabrication
technologyandcharacterizationofdevicesbasedonIII–VandSiCcompound
semiconductorsformicro-andnanoelectronicsand,morerecently,thedevelop-
mentofinnovativetechnologiesforthefabricationofmicroelectromechanical
systemsonGaAsandGaN/Sisubstratesandofsolid-statechemicalsensorsfor
gasdetection.
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