Lakin 1995

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SOLIDLY MOUN TED RESONATORS ND FILTERSK.M. Lakin, K.T. McCarron, and R.E. Rose

TFR Technologies, Inc.701 SE Salmon Ave.Redmond. OR 97756

ABSTRACTconfine waves to a finite volume in an efficient manner.AcousticesonatorsequirematerialnterfaceshatConventionally this is achieved by using air or vacuuminterfaces the lectrodes.Anotherechniquesofabricate the resonator onto a set of quarter wavelengththick ayersattached to a ubstrate o orma olidlyused to fabricate ow nsertion loss filtersfor GPS andmounted esonator SMR). The SMR concept hasbeenother applications. Filters for GPS having less than 3 dBinsertion loss and 40 dB out-of-band rejection have beendemonstrated.Theseillers are composed of laddernetworks with series and shunt resonators.

I. INTRODUCTION

from 500 MHz to 6 GHz. These systems include pager,Wireless networks are growingrapidly n hespectrum

various forms of data communication. The need for highcellular phone, navigation, satellite communication, andperformance filters has becomemorepparent asspectrum crowding increases with the deploymentof newfront-end filters thatrotecteceiversromdjacentsystems.narticularheresrowingeedorchannelnterference ndoutput iltershatimithebandwidthfransmitteroise.urther,ubsystemoccupy the same or similar packages as high frequencyminiaturization may requirehighperformancefilters tointegrated circuits.

the substrate to form the membrane and thereby define theresonator. Substrates such as silicon and gallium arsenidehave been used with some success.BASIC RESONATOR CONFIGURATIONS

VIA ISOLATED RESONATORElectrodes7, Piezoelectric

Etched VIA Interlace.J

aAIR GAP ISOLATED RESONATOR

WSOLIDLY MOUNTED RESONATOR SMR)

Elecl rodes ~ 4 Piezoelectric\

The configuration of Fig.2a samembranestructurefabrication involves depositionof a piezoelectric filmon asupporting substrate followed by removal of a portion of

by the edge Of the I . underheesonator [6 ] This maye accomplished by firstThe second configuration involves fabrlcating an air gapdepositing and patterning an area of temporary support

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film,extepositing and patterning an overlaypiezoelectric resonator with electrodes, and finally usingan under cutting etch to remove the temporary support.11. SOLIDLYMOUNTE RESONATORSThe solidly mounted resonator (SMR) in Fig. 2c is of aconsiderably different form than the membrane structuresdescribed above. Sinceheiezoelectricsolidlyacoustically isolate the piezoelectric from the substratef amounted to the substratesomemeansmust be used tohigh Q resonance is obe obtained.In 1965, Newell [7] escribed a method of transformingthe mpedanceofa esonatormountingsubstrate oalower value at the crystal that gave partial isolation. Thetechnique used quarter wavelength sections of materialshaving arge mpedance atios hat avorably ransformthesubstrate mpedance.Newellshowed hatboth reeand clamped nterfaces could be obtained using quarterwavelength thick transformation layers.The reflectioncharacteristicsofaquarter wJavelengthreflector are shown in Fig. 2 .

1 . 00 . 90 . 80 . 7

00 . 6

LL 0.50 . 40.3

I0 . l0.01 I l I I0 600 l 2 0 0 l000 2 4 0 0 3000

F r e q u e n c y , MHzFig. 2. Rej ectio n coejjicient of two multi layer reJectors.having 2 : l impedance ratio between a4acent layers. andThe narrower freque ncy response i sf or a sei of layersthe wider response s fo r approximately 1O:l impedanceratio.The effect of the reflector on mechanical displacement isshown in Fig. 3 . for the wide bandwidth reflector of Fig.2.Anmportantffectfheeflectorayers, asdemonstrated by Newell, is the partial lateral stiffeningofthe piezoelectriclatehatminimizesisplacementsassociatedwithplatewavegeneration and consequentspurious resonances normally observed in free plates.

D i s t a n c e W MFig 3 . Mechanical displacement as a fun tion of depthJrom the top of the resonotor into thr reJlector de~e lorehigh impedance and L low impedance regions.The absence of a V I A or any special substrate preparationactive circuit wafers. It is necessary, in this case, to firstshowsconsiderablepromise ordirect ntegrationontopassivate he C'sand hen abricate S R devices, nareas provided, after all IC processing has taken place.111. SMR FILTERSThevariousormsof rystalilters and their use atwell known. The use of crystal filters at system operatingintermediate frequency locations n wireless ystems sfrequencies,nd articularlyloseohe antenna, isrelativelynewbecauseuntilrecentlycrystalfiltershavenot had the low insertion loss, bandwidth. and impedancelevels necessary for these applications.Thin film resonator filters have been under developmentfor some ime [ l -6,8-10] using he membrane approach.yieldswithragilemembranes is lowndpuriousThese filters have shown some promise but manufacturingresponses from plate waves have been a problemS]The use of piezoelectric films on thick substrates to createovermoded esonators and filtersallowed train in theresonatorswithoutfracturedmembranes and showed anabsenceofspurious esonances [ I l l . Inorder oobtainhigh performance and high manufacturing yield the S M Rapproach has been applied to ladder filter configurations[l21 with recent results reported here.Experimental results for a ladder filter composed of threeseries and two shunt SMR's is shown in Fig. 4 The deepnotches are used to increase the filtcr skin selcctivit?. Thehigh frequency notch s due to parallel resonance in theseries esonatorsand he low frequencynotch sdue toseries resonance in the hunt esonators.The S I , wide

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. 30.0Nm - 4 0 . 0 tl I

1 . 4 7 5 0 1.5150 1 . 5 5 5 0 1 . 5 9 5 0 1 . 6 3 5 0 1 . 6 1 5 0

mUNm

-607 0

-anr e q u e n c y , GHz IO00 1600 2200 2800 3400 4000Fig. 4 . Experimental results fo r a ladderjilte r made using Frequency MHzS M R s in a simple configuration.band response of a ladder filter is illustrated in Fig. 5 for a a high our of band rejecrion. The inserfion uss is 5 dB.Fig. 5 . Wide response /or a 2 488 GHzjilrer designedfbrdifferent design.There is in eeneral a trade off between in-band insertion U ................................................- - - -- - - - - I - - - - - - - - - - - .loss and out-of-band ejection or adder iltershavingfinite Q resonators. This is illustrated in the comparisonof the filters in Fig. 5 andFig. 6 designedfor hesame m ~ ocenter frequency. The experimental results show that thepass-band, can be ncreasedmuchmore han he oss ofrejection, s measuredromheenterfheilter -40in-band insertion loss. Thus the filter in Fig 5 has a 5 dBinsertion loss withearly 70 dB out-of-bandejection -60while0 dElthef rejection. filtern Fig. 6 has 3 dB oss with approximately -70d- -

ilters,such as used for ransmitteroutput,needonly 0 l200 2 4 0 0 3600 4800 6haveut-of-bandejectiontevelecessaryor Frequencg MHz

10

-20

- 80

harmonic or noise attenuation. Frequently that is a muchlessevereequirementhat eededoreceiver orsynthesizer clean-up filters.One of he mportant features of the S M R filter s heabsence fpparentpurious latewaveesonancesanywhere in the filter response. The ladder filters reportedusingmembraneesonators [S] showed considerablespurious esponsedue oplatewavesgenerated by theresonator and formedntoateral tandingwaves bydiscontinuities at the edges f the electrodes or plate.The third harmonic response of the filter is suppressed byas a result of resonator and filter design. The out-of-bandrejection is very favorable compared with high dielectriccanstant filters.Some systems, such as GPS, require front end filters withan insertion loss of ess than 1.5 dB inordernot odegrade the signal-to-noise ratio. Therefore, techniques oimprovehensertion loss of ladderiltershrough

0

Fig. 6 Filter similar t the one in Fig 5 excepr designedwith lower out of bandrejection.improved ilmqualityand hicknesscontrol are underinvestigation.IV. SUMMARY AND ACKNOWLEDGMENTThe solidly mounted resonator S M R ) is a form of bulkwave acoustic resonator that is isolated from the substrateusing a sequence of quarter-wavelength thick layers thatform anefficient eflector. The S M R concepthas beenreduced lo practice using thin films and film depositiontechniques to form the bonds between the various layers,The demonstrated advantage of the S M R , over other thinfilm bulk wave resonator approaches, is that the resultingtype of substrateused. The resonance esponse sveryresonator response can be made nearly independent of theclean, showing a lack of spurious effects. The fabricationprocess does not require a substrate backside IA.

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F r e q u e n c y , MHza

8 0 . 0 a7 0 . 06 0 . 0 715 0 . 0 p11

N -12n -1314-1s-16-17-18 10.0-19-20 0 . 0

3 0 . 0 .2 0 . 0 2

-15.0 - 10 . 0 -5.0 0.0 S . 0 1 0 . 0 15 .0Frequency, MHz

bFig. 7 S R adder f i l ter for a f requency around 900 MHz.a) Wide band response, and b) close- in response showinggroup delay across the 3 B bandwi d t h o f t he f i l t e r .Miniature i l tershavebeendemonst rated wi th nser t ionlosses l ess t han 3 dB for appl icat ion to wireless systems.These f i l ters have been synthesized using sol id ly mountedresonators f rom approximately 900 MHz t o over 2 .5 GHz.Des i gnnder fo rmanceradeof f sa v ee e ndemonst rated or n-band nser t ion loss andout-of-bandrejection.Appl icat ions of he SMR and adder f i l ter concept to awide range of wireless systems are being invest igated andl arge o l umemanu factur ing f SMR fi l ters is beingp l anned fo r i mp l emen t a t i on i n l a t e 1996 wi t h sma l l sca l epi lo t l ine product ion in ear ly 1996.The au t ho r s wi sh o acknowl edge con t r ac t suppor t fromt h e U.S. Army SBIR p rog ram, t he U.S. Army Advancedp r o g r a m , h e U.S. NavySBIRprogram,andother USConcep t sandTechno l ogyprogram,andD A R P ASBIRG o v e r n m e n t support

V . R E F E R E N C E S

Cul l en , and R .A. Wagne r , Fundam en t a l Mode UHF/ VH1. T . W .Grudkowsk i ,.F .B l ack , .M.Reeder ,D .E.Miniature Resonators and Fi l ters . Appl ied Physics Lt rs . .Vol . 39, no. 11 Nov. 1980, pp. 993-995.2 . K . M .Lak i n nd J.S. Wang , A cous t i cBu l kW a v eComposi teResonators ,Appl iedPhysicsLtrs,Vol .39,no . 3, Feb. 1981, pp. 125-128.3 . C. Vale, l Rosenbaum. S Horwi tz, S Kr i shnaswamyand R. Moore, FBAR Fi l ters at GHz Frequencies , 45 thAnnual Symp. o f F r q . C on t . P roc ., 1991 , pp . 332-336 .

Kawabat a , VHF/ UHF Compos i t e Resonat or on a S i l icon4. M. Ki t ayama,.ukuichi ,.h iosaki .nd .

pp.139-141Substrate , I I Appl .Phys.Vol .22 1983)Suppl22-3.

5 K . N a k a m u r a , Y Ohash i and H. Sh i mi zu . UHFBulkAcous t i cavei l terst i l izinghulnOi Si O,Di aphragmsonSi l icon . J J ApplPhys. Vol 25 No 31986 , pp , 371-3756. H. Satoh, Y. Ebata, H. Suzuki .andC .Narahara , AnAir Gap Type Piezoelect r ic Comp osi te Resonator , 39thpp. 361-366.AnnualSympos i umonFrequencyControl Proc.. 1985

7. W.E.ewel l ,Face-Moun t ediezoelect r icResonators ,roc. IEEE, Vol3,une965,p.575-5818 K . M .L a k i n ,G . R .K l i n ea n dK . T .M c C a r r o n . T h i nFi lmBulkAcoust icFil ters or GPS . l992 UltrasonicSymp . Proc. , pp . 471-476.9 . R.B. Stokesand J.D. Crawford . X-BandTh i nFi l mAcoust ici l tersn aAs , IEEE Trans .Mi crowaveTheo ry Tech . Vol. 41 no. 617, Dec 1993. pp. 1075-1080.

Fil ters ,nProc.EEEMTT-Snt.MicrowaveSymp.IO. K .M. Lak i n Model i ng o f Th i n F i lm Resonat o r s andDi g . , l une 1992 , pp . 149-152 .11. K.M. Lakin , G.R . K l i n e a n d K.T. McCar ron . Hi gh QMicrowave Acoust ic Resonalors and Fi l lers . IEEE T r a n sMi crowaveTheoryTech .Vol . 11 no 12. Dcc. 19Y3. pp2139-2146.12. K . M .ak i n ..R .l m en d K T McCar ronDevel opmen tfin iaturei l tersori relessAppl icat ions to be pub l i shed i n . IEEE Trans . Mi crowaveTheory Tech. Vol. 1 3 no . 12 , Dec . 19 .

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