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OTMZATN AND NALY PEZELECTRC AND LECTRDE ATERAL' THCKNEEECT N THE ERRMANCE LM BULK CUTC ENATR

S u Ab Apy tudent, M.Tech: chool of Physics2 Project Associate: chool of Physics

chool of PhysicsUniversity ofHyderabadHyderabad, ndia

Email-: srican.e@gmail.comEmail-2: abhykabhybhy@gmail.com

trt this aer reorts the behavior of the FBAR and itserformance due to change of materials and their thicknesses.The resonance of the resonator deends on the thickness of theiezoelectric thin lm acoustic resonator. The connement of theacoustic wave in resonator deends on the electrode thicknessand materials. Here such materials eects and the geometriceffects are studied. t is found that acoustic roerties of all the

layers inuences the resonator erformance with increasedresonance frequency results from thinner electrodes as itdecreases the mass loading eect of resonator's electrodes anduses high longitudinal velocity of the iezoelectric materials.

ywrd; s; zl;h jl;l

. NTRODUCTION

The rapid groth n wreless commications n recentyears has led to the crowdng n equency spectrum andpushed the equencies to higher lmits which ncreased thedemand for design of mniatized high performancemicrowave devices lke mobile phones, iM and To mprove the capability of such systems, development of

mniatized, high performance on-chip lters and resonatorsoperatng n the GHz equency range are needed. They areequently built with the ansmission lnes and ceramicresonators but they become too large n size and hence dicultto mniatize ter. alteative approach to mnmize sizeup to micro meters is to ansform elecomagnetic wave ntoan acoustic wave via piezoelecic eect. The velocity ofacoustic wave is about 10000 tmes lower than the velocity ofan eleomagnetic wave at these equencies. Accordngly theacoustic wavelength is only a few microns at 1 GHz. Therefore

the lters based on acoustic wave resonators like faceacoustic wave (A) and Bulk acoustic wave (BA) devicescan have much smaller size. A devices have severalproblems at equencies above 2 GHz such as lesser powerhandlng and beng temperate dependent. A devices aregenerally manufacted on a LiTa or LiNb sngle crystalsubsate and hence it is dicult to ntegrate these A lterswith silicon. The thn m buk acoustic wave resonator havemany advantages lke smaller n size, higher operatngequencies, higher power handlng capability and itsfabrication process is also compatible with standad siliconbased ntegrated Crcuit(C) fabrication tecnologies.

DC J RuAssociate Professor

chool of Physics

University ofHyderabadHyderabad, ndiaEmail: kcjrsp@uohyd.eet.n

This paper deals with the modellng and optmization ofthn lm bulk acoustic wave resonators usng piezoelectricZnO or AlN with dierent electrodes combination and havngvarious thicesses. has been realized that thicess of

piezoelectric material and electrodes would nuence theequency of the lm bulk acoustic resonator (FBAR) and the

properties of the electrode material detmnes the relatedelectromechanical couplng coefcient.

B COUSTIC WAVE EASONATOR

Bulk acoustic wave devices are used manly n lters,sensors, commication systems and radar systems because of

ther ability to work n microwave equencies above 2GHz.BA resonator is a piezoelectric device it means that theelectromechanical conversion is based on the piezoelectriceffect. Piezoelectric effect is an ability of some materials toconvert elecical energy into mechanical energy and viceversa. Many crystallne phases possess the property of

piezoelectricity. This property comes om the fact that the

positive and negative charges n some crystallne materialsunder applied sess will dergo spatial separation. Thecoupling between electrical and mechanical energes expressed n the following equations [1]:

- ED E

Where, T is sess, is san, E is eleic eld and D iselecic displacement and are the eld variables. The tms E,and are the material parameters: E is the stiess constant, is piezoelecic (sess) constant and is pmittivity of the

material.

A crystal may be strongly piezoelectric for a givendrection, while the effect is completely absent n a dierentdrection. The stiened phase velocity is:

, ' -P p (3)Where is the elastic stiess constant, is the

piezoelectric constant, is the permitivity and is the massdensity. The superscript E and S represent that the electriceld and the san are held constant.

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For compaison, the -stiened phase velocity is:

V= j (4)Reritng stiened phase velocity Eq.(3):

Where,v

'= v (1 + / (5)

(6)

Eq. (6) is called the piezoelectric couplng constant.The electromechanical couplng coefcient is deed as

2 K2k = (7)t

S D K2+

. WORKNG PlCILE OF FBAR

The core of FBAR is the piezoelecic thn m, which isusually made up of Alumn Niide (AN) or Znc Oxide(ZnO) and the device is conged with a thn m of

piezoelecic material sandwiched between two metalelecodes as shon n Fig.I. Whenever the elecic eld is

applied n the thicess drection, an acoustic wave is excitedand the sucte gets mechanically deformed. order to havea resonant mode n the sucte, the acoustic wave must beconned n the acoustic cavit created by the piezoeleciclayer. This connement can be done with m bulk acousticresonator (FBAR), it consist of an ar cavity below the bottomelecode, n which the excited acoustic wave is aversedbetween top and bottom elecode, so that a resonant mode iscreated. The resonance condition of a BA resonator occswhen piezoelecic material thicess 1 is equal to an ntegermultiple of half a wavelength of the acoustic wave [2].

Fig.I Tee dimensional model ofFB.

FBAR's performance manly depends on eectivecoupling coefcient () and quality factor Q, which ismeased by the parameter called "ge of merit (M)". isdened as [3]

Where and Q-factor are as mentioned below [3]:

I - eries resonance equency at zero impedance

(8)

(9)(10)

- Paallel resonance equencies of nite mpedance.

The sength of the piezoelecic couplng ef determnesthe bandwidth of lters and the mechanical losses n thematerial will determne resonator Q and accordngly lternsertion loss.

Ect o Pizolctric matrials on BThe resonance equency drectly depends on acoustic

velocity of piezo material and resonance equency ncreases

with high acoustic velocity of the piezoelecic material. Theresonant condition of the BA device nversely depends onthe thicess of the piezo materials. The lower the thicess ofthe piezo material more will be the resonance equency of thedevice. The resonance equency of the device can beexpressed by the followng equation [2].

= v' 21 (11)

n order to develope the resonator for higher equencyapplications, ther should be the piezo material, withn thelimitation of mechanical stability. The nnsic property of thepiezoelectric material, electromechanical couplng coefcientrelates the amot of electrical energy that is converted nto

mechanical energy and vice versa [4]. t is mportant to notethe dierence between the piezoelectric material-couplngcoefcient K and the eective coupling coefcient K. Theeffective coupling coefcient K is the property of a device(i.e., resonator). The material property K, nuences what

knd of effective coupling coefcient that could be expectedom a manufacted resonator [5].

B Ect o Elctrod matrials on BThe electrode materials of FBAR manly affect the

resonance equency and elecomechanical couplng. Theresonance equency increases with lower electrode thicessand low density electrode material. Low density and ther

electrode materials decrease the mass loadng eect onelectrodes [6]. The acoustic impedance ratio of the elecodesto piezoelectric material determnes the behaviour of the K2value. For electrode materials with lower acoustic mpedancevalues results n a low value ofK2e.

Therefore resonance and electrical behavior of theresonator depends promnently on the piezo materials andelectrodes. this study, the dependence of the materials andther thicesses have been studied for FBAR based lters andsimulations were performed usng COVETNTORAEsimulator. addition, the application of BA resonators nmicrowave lter design, especially to lters based on

electrically connected ladder-tpe lters with ButerworthVan Dyke (BVD) equivalent model is presented.

. MODELLING OF FBAR

BA resonator can be modeled with the modied BVDequivalent crcuit, this equivalent crcuit has become suitablefor the BA resonator not only for the damental operatng

mode, but also for the higher hamonics that can be modeled byaddng motional arms n sht congation with the staticcapacitance. A modied BVD model is presented n Fig.2, norder to exact the value of the vaious lped components

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om the measement of a BAWresonator [2]. Where corresponds to the static capacitance, p is the associatedmaterial loss resistance, is the resistance associated withmechanical losses and and are the motional nductanceand capacitance.

C _ L

C

Fig.2: Modied BVD model of a BAW resonator

this case, n addition to the basic equivalents, Rs is aseries resistance, which models the elecical losses due to theelecodes. This equivalent crcuit resonates for two particularequencies:

_1

s - LmCm

p

The simplicity of the model, due to the fact that it onlycontans lumped elements, makes it very suitable for obtainngelectrical models for more sophisticated suctes. Thestructe of these equivalent crcuits leads us to state a seriesof analogies between the electromagnetic and mechanicaldoman, which is also suitable for obtanng the electricalcharacteristics of a BAWresonator.

V. SULTS ND ISCUSSION

This section provides the results of simulation obtanedwith Zo of Im thicess as the piezo electric material andthn m Alni (A) of thicess 0.3 m as the electrode

layer of a BAWresonator usng Coventerware. DC analysisgives disibution of the mechanical displacement, where the

top electrode is applied with 1V and the bottom electrode isgroded, i.e. V = 0 V. Frequency response analysis is

performed by using the drect hamonic analysis, n which, aharmonic potential is applied to the top elecode with loadvalue of 1V and material dampng coefcients are given. Theequency response simulation is used to estimate thedisplacement and impedance variation with signal equency.

Fig 3.a Fig 3.bFg 3.a: DC Anaysis resut of dispacement magnitude (Jm)Fg 3.b: Frequency anaysis- ode dispacement at 2291GHz equency

t is appaent that the disibution is iform at the cene ofthe sucte, but this is not lly extended to the whole

surface. This is due to the clapng of the BAWresonator atthe edges.

G raphical results of impedance with respect to equencyobtaned with ZnO of thickness 1 m and Al electrode of

thicess .3m and .2m are given below Fig. 4a and b.

,.

Fa FbFg 4.a Impedance Vs Frequency with eectrode thickness as 03umFg 4.b Iedance Vs Frequency with eectrode thickness as 02 u

t can be noticed om the above gures that as the thicessof the elecode decreases, the series equency and parallelequency moves towards the ncreasng drections ofequency scale. Thus it decreases the mass loadng eect on

piezo plate.

n order to stdy the eect of the piezoelectric materialand electrode eects on the behaviour of the BAWresonator,

the simulations have been carried out by varying the thicessfor two dierent piezoelectric materials viz. Zo and AN.

Table 1 Summary of the resul ts obtained by the simul ation fordierent thickness of piezoel ectric ofZnO andA1N.

Zo ANhick / Q ', / Q

1 229146 216 825 351429 174 52312 201313 254 847 321836 186 58014 182020 287 854 292454 192 59116 160121 322 875 268019 208 62118 148447 374 851 243643 214 6442 143970 427 879 224271 229 660

The value of the effective electromechanical couplngcoefcient is also plotted at each piezoelectric thicess forZnO and AlN as shon n Fig Sb. this case, as thepiezoelectric thickness ncreases, the value of K2 alsoslightly increases for Zo and AN i.e., the mechanical loss npiezoelectric lms has no signicant eect on K2j

Thckne(JFg S.a Fg S.b

Fg S.a Resonance equency of FBR with dierent thicknessFg S.b: kitt vaue for ZnO and N

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Th valus of Keachivd n BAW rsonators a about6.6% at 2.24 GHz usng AIN. Compard to AIN, th ZnOgivs a bttr couplng cofcint of K2e= 8.8% at 1.43 GHz.This couplng is songly affctd by th lctrod matrialswhich will b studid n th nxt sction. Howvr, ZnO is notcurrntly a viabl altativ to AIN for high volums du toth highr manufactng dicultis and wors prformancn trms of acoustic losss at highr quncis.

Ect o th Elctrod Thicnss

Th BA W rsonator smulation has bn carid out fordirnt thicsss of Aln lcod, whr ZnO is thpizolcic matrial with thicss of1 m.

Table 2: Summary of the resul ts obtained by the simul ation withdierent thickness of el ectrodes

AI Electrode C(pF) (MHz) Q K2efj%)

0.4 0.175 2291.46 216 8.240.2 0.174 2616.33 194 8.76

0.1 0.173 2842.86 175 8.600.05 0.174 2955.10 162 8.40

W can notic om th abov Tabl 2 that whn th mtallcod thicss is lowrd, th allocation of th rsonantquncis is highr du to lssr mass loadng ct on pizolm and Q-valu is also dgradng bcaus for lowr valus oflcod thicss got highr lcic losss. By obsrvingK2lvalus om th abov Tabl, th lctrod thicss hasno visibl ct on th charactrization of th ctivlcomchanical couplng factor.

B Ect oth Elctrod matrial this sction, th BA W rsonator was analyzd n thprsnc of dirnt mtal lcods. ordr to study thbhavior of th BA W rsonator with various tp of mtallcods, th sam smulations hav bn carid out withmtals such as Alnum(AI), Coppr(Cu), Gold (Au) andTgstn (). Fig 6. a shows th variation of quncywith dirnt mtal lcods and Fig 6. b, variation K2efor dirnt lod matrials.

Table 3: Summary of the simul ation resul ts for dierent el ectrodematerial s

Electrode fp- f, fo Q m%Materials (MHz)

Alumnum(AI) 101.54 2616.33 216 8.76Coppr(Cu) 119.72 2195.42 183 12.32

Gold (Au) 153.99 1498.54 81 22.53Tgstn (W) 168.51 1407.51 159 25.98

n Fig 6. a, it can b noticd that, Alnum lcodsyilds highr rsonant quncis than Coppr, Gold orTgstn bcaus Alumnum has th lowst dnsity and hncgivs lss mass loadng for th FBAR dvic.

Howvr, dgradation of K2e dpnds on th acousticimpdanc of th mtal lcods usd. For th havistmatrial, n this cas, Tgstn (W) has high acousticimpdanc and th valu of K2e achivd for BA W rsonatorswith it is about 26% at 1.5 GHz usng pizo ZnO.

Fg6.a Fg6.b

Fgure 6.a FBAR resonant equency with dierent eectrode materiasFgure 6.b Variation ofK24J with dierent eectrode materias

Alumnum (AI) lcods hav good lctricalconductivity and poor acoustic charactristics bcaus oflowr acoustic mpdanc, which rsults n a low

lctromchanical couplng cocint of about 8.7% andthrfor its us s ar limitd to th lowr bandwidth ltrs.t can b noticd om th Tabe that th rsonanc

quncy will b shid to lowr valus and th quncydirnc btwn th sris and paralll quncy will bhancd for dirnt lcod matrials. Hnc, it can bconcludd that high acoustic impdanc mtal lcods arprfrrd for highr bandwidth ltrs, du to th high ctivlctromchanical couplng constant associatd with thm.

CKNOLEDGENT

Th authors acknowldg National Program on Micro andSmart Systms (NPMSS) for ndng th rqurd toolsundr th NMDC program for th succssl compltion of thproct.

EFERENCES

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[2] Jose, S, Hueting, RE and Jansman, AB "Modelling of bulkacoustic wave resonators for microwave jlters. 11th Annua orkshop onSemiconductor Advances for Future Eectronics and Sensors, SAFE 2008,Vedhoven, The Netherands

[3] Q Chen and Q ang, "The eective electromechanical couplingcoecient86_022904_,13 _2005[4] IEEE Standard on iezoeectricity 17601987, IEEE Trans UtrasonFerroeectr Freq Contro 43, 717 _1996[5] O enendez, J Verdu, E Corraes and deaco, review of recent

patents on bulk acoustic wave resonators and jlters Recent atents onEectrica Engineering, Vo 2,92108, 2009[6] Tao Zhang, Hui Zhang, Zuo-qing ang, and Shu-yi Zhang Eects ofelectrodes on permance jgures of thinjlm bulk acoustic resonatorsInstitute of Acoustics, China