strontium titanate dc electric field switchable and...

Strontium Titanate DC Electric Field Switchable and Tunable BulkAcoustic Wave Solidly Mounted Resonator

George N. Saddik, Damien S. Boescht, Susanne Stemmert, and Robert A. York

Department of Electrical and Computer Engineering, tMaterials DepartmentUniversity of California Santa Barbara, Santa Barbara, CA 93106

Abstract - A voltage switchable/tunable strontium titanatesolidly mounted BAW resonator was implemented using anacoustical Bragg reflector of alternating high and low acousticimpedance layers. In the absence of any bias the device is merelycapacitive, but under a DC bias the material becomespiezoelectrically active, leading to a BAW resonance that iseffectively turned on and off by the applied field. In the resonantstate a voltage-dependent frequency trimming of 1 % isobserved, from 7.05 GHz to 6.98 GHz with an applied bias of 0-9V, respectively. The quality factor at the resonant frequency wasapproximately 100, limited by the simplicity of the device design.The Q was relatively constant with applied bias, with an effectiveelectromechanical coupling coefficient that varied linearly withapplied bias up to a maximum of 3.3 0/0.

Index Terms - Acoustic resonators, bulk acoustic wavedevices, delay filters, ferroelectric films, piezoelectric resonators.

I. INTRODUCTION

Strontium titanate (STO) and barium strontium titanate (BST)are known to have a voltage-dependent dielectric constant [1],and this property has been investigated for many years forpotential use in adaptive and frequency-agile RF components[2]. In recent years it has been demonstrated that thesematerials also have an electrostrictive property [3,4] that canbe exploited to realize voltage-switchable bulk-acoustic wave(BAW) devices for RF applications [5,6,7]. Using thisproperty, high-Q and voltage-activated filters and/or switchescan be created in the microwave frequency range.

Unlike conventional BAW technology, the piezoelectriccoupling coefficient in STO- and BST-based resonators iscontrolled by an applied DC field. In the absence of a DCfield, the piezoelectric coupling is negligible; at fieldsapproaching 0.7 MV/cm DC field, piezoelectric couplingconstants as high as 10% have been measured on thick BSTfilms. This new technology may allow for a new class ofvoltage-selectable high-Q/low-Ioss filter banks, addressing acritical need in modem RF/microwave/mm-wavecommunication systems.

To achieve high Q-factors, the resonator must bemechanically isolated from the damping effects of thesubstrate. In modem BAW devices this is typically doneusing either a suspended membrane approach (film-bulkacoustic resonators, or FBAR), or an acoustic Bragg reflector(solidly-mounted resonator, or SMR). In this paper we

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demonstrate an SMR type resonator on sapphire, using arelatively simple four-layer mirror structure of Pt-Si02•

II. DESIGN AND FABRICATION

A properly designed acoustical Bragg reflector for the SMRstructure consists of multiple quarter wavelength layers ofalternating high and low acoustic impedance. In this work,platinum and silicon dioxide were used. The quarterwavelength and acoustic impedance are determined by [8].The platinum/silicon dioxide acoustical Bragg reflector wasdeposited on a sapphire substrate. A platinum layer wasdeposited on top of the acoustical Bragg reflector for the

Sapphire Substrate

(b)Fig. 1. (a) Schematic diagram ofSMR, (b) SEM crosssection of SMR device.

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Table I. Material acoustic parameters at RT.a

Layer Material Density Longitudinal Elastic(Kg/m3

) Constant (GPa)1,3,5,7 Pt 21500 346.7

2 SrTi03 5123 348.17

schematic representation of the device showing the substrate,acoustic mirror and contact points, Figure 1 (b) is an SEMcross section of the image of the device. Tabulated in Table 1are the material parameters used in the design of the SMRdevice.

4,6 Si02 2649 105.75aD. R. Lide, Handbook ofChemistry and Physics 87th Edition

(CRC, Florida, 2006)

bottom electrode of our solidly mounted resonator (SMR)structure. The platinum and the silicon dioxide weredeposited by electro-beam evaporation and plasma enhancedchemical vapor deposition, respectively. The strontiumtitanate (STO) layer (60 nm) was deposited by radiofrequency magnetron sputtering from a stoichiometric SrTi03

target, on top of the acoustical Bragg reflector structure. TheSTO deposition parameters are described in other publications[9, 10]. The SMR Structure was fabricated using standardprocessing techniques such as photolithography, chemical wetetching, and lift-off for the top electrode. Figure 1 (a) is a

III. EXPERIMENTAL RESULTS

The one port S-parameter data for the SMR structure wasmeasured using a Cascade Microtech probe station, GGBGSG probes and an E836IA vector network analyzer. Figure2 and Figure 3 show the tunability and switchability of theSMR structure, respectively. Figure 2 the resonator is tunablefrom 7.05 GHz at 1 VDC to 6.98 GHz at 9.0 VDC. The dcbias was varied from 0 V to 9.0 V in steps of 1 V.

Fitting the collected data to the modified Butterworth-VanDyke (MBVD) [11,13] model the quality factor of theresonator was calculated (1) to be 101 over the bias range(Fig. 4).

(1)

(2)k2 _;r2 fa - f,.

t,eff - 4 fawhere {Or is the resonant frequency, Lm is the motionalinductance, Rm is the motional resistance, and fr and fa are theresonance and antiresonance frequencies, respectively. Thereturn loss at 9.0 V was -9.5 dB. The effectiveelectromechanical coupling coefficient (kt~efJ) was extractedfrom the MBVD parameters to be 3.3 % and near linear, ascan be seen from Figure 4. The measured resonant frequencywas normalized to the maximum measured frequency at 1V dcand plotted in Figure 5 against the applied dc voltage. FromFigure 5 we can see that the frequency is decreasing as afunction of increasing applied dc voltage.

8.58.07.0·12..................-.-..........,r-""I"'........~r--r~-r-.....,-r .............

&.5

0

·2 0 ,...-.. ·4m-c .& Increasing ..""'-"'".... DC Bias ·1....UJ ·8 ·10

·12·10 0 5 10 15 20 25 30 35 40

7.5Freq (GHz)

Fig. 2. Return loss of tunable SMR showing the tunabilityand the inset is a plot of the main resonance over a widefrequency range.

1.0

o.s

---+

120 ---....---......--.........----.-----... ...

3.5

3.0110

20

..80

~a!cJ•

O~-----_.......--........-----------a..O'Oo 2 4 • • i.

DCVoltageMFig. 4. Plot of the resonator quality factor (blue diamonds)and the electromechanical coupling coefficient (red circles)vs applied dc voltage.

Red : Off State~ Blue: On State

6.0 6.5 7.0 7.5 8.0

Freq (GHz)Fig. 3. Plot of the return loss of switchable STO SMR,showing the off state in red and on state in blue.

·1z-t ...

·1

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1.000 ~--f~-----------------, 0 mT

·10 I~ ~m .20-c~

N ·30(J)

·40

·50 --+---.---.-----T-----.---r---.---.-----T-----.---r---.---.-----T-----.---r---.---..---.-----.-----j

o 2 4 6 8 10 12 14 16 18 20

Freq (GHz)Fig. 7. Simulated insertion loss ofa single STO SMR.

contact via a thin cross-over metal. In addition, higherpiezoelectric coupling coefficients can be obtained by usingBST films instead of STO. Devices using these advances arecurrently in development.

1084 6

DC Voltage (V)2

.....

..........

.......... ,

....., , ,

o 0 "',?,"'~"'"

o""~""""o '0

0.988 -+-.--T---r-""T""""""'1--r---r---"1r"'"""""T"""--r--r"'"""""T""--r-r--r---r-T""""""T'"--r-~--.--r---r-,---l

o

0.990

0.998

....... 0.996"CCI)N

~ 0.994

Eoz 0.992

Fig. 5. Plot of the resonant frequency normalized to the maximum measured frequency at 1V DC vs. applied DC voltage.

IV. DISCUSSION

Fig. 6. A simple shunt resonator circuit structure used tosimulate STO SMR insertion loss.

ACKNOWLEDGEMENT

The research was supported in part by the U.S. ArmyResearch Office (W91INF-0601-0431) and National ScienceFoundation (NSF-CCF 0507227).

REFERENCES

[I] D. R. Chase, L.-Y. Chen, and R.A. York, "Modeling the C-VNonlinearity in Thin-Film BST Capacitors," IEEE Trans.Microwave Theory and Tech., vol. 53, pp. 3215-3220, Oct 2005.

[2] R. York, "Tunable Dielectrics for RF Circuits", Chap 6 ofMultifunctional Adaptive Microwave Circuits and Systems," M.Steer, ed., Wiley 2008.

[3] K. Morito, Y. Iwazaki, T. Suzuki, and M. Fujimoto, "Electricfield induced piezoelectric resonance in the micrometer tomillimeter waveband in a thin film SrTi03 capacitor," J.Applied Phys., vol. 94, pp. 5199-5205, Oct 2003.

[4] S. Tappe, U. Bottger, and R. Waser, "Electrostrictiveresonances in B'l(nSro.3Ti03 thin films at microwavefrequencies," Appl. Phys. Lett., vol. 85, pp. 624-626, 2005.

[5] J. Berge, A.Vorobiev, W. Steichen, S. Gevorgian, "TunableSolidly Mounted Thin Film Bulk Acoustic Resonators Based onBaxSrl_xTi03 Films," IEEE Microw. Wireless Compon. Lett, vol.17, no. 9,pp.655-657, Sept. 2007.

[6] Z. Xinen 1. Phillips, A. Mortazawi, "A DC Voltage DependantSwitchable Thin Film Bulk Wave Acoustic Resonator UsingFerroelectric Thin Film," in IEEE MTT-S Int. Microwave Symp.Dig., pp.671-674, June 2007.

V. CONCLUSION

In conclusion, a dc electric field tunable and switchableSTO solidly mounted bulk acoustic wave resonator has beendemonstrated. The return loss at 9.0 volts is -9.5 dB; thequality factor is 101 and kt~eff is 3.3 % at a bias voltage of 9.0V. With further advances in the device design, thistechnology could allow for the creation of high-Q switchablefilters for RF/microwave front-ends.

1Term

C Term2C6 Num=2C=1.0 pF _ Z=50 Ohm

CC5C=1.0 pF

Note that the low resonance frequency of our device withrespect to the half-wavelength resonance frequency of theSTO layer alone ('"70 GHz), is due to the thick metalelectrodes that are in the acoustical path of the resonator [12].In future designs, thicker STO or BST films and thinnerelectrodes would be used instead, since the thick metalelectrodes have a damping effect on the Q-factor. The thinSTO film used in this work was chosen merely for simplercomparison of the device results with earlier varactorstructures of similar design, but without the mirror structure[7].

Nevertheless, this work (Fig. 3 in particular) demonstratesthe potential for voltage-switchable resonators. UsingAgilent's ADS software, the one-port reflection coefficientdata was fit to an MBVD model, and using this model we canthen predict the behavior of the structures in other circuittopologies. A simple example is the capacitively coupledshunt resonator transmission-structure in Figure 6. In Figure7 the simulated insertion loss at 7 GHz is -6.5 dB.

In addition to the excessively thick electrodes and relativelythin STO layer, two other Q-limiting factors can be easilyidentified in this work: the use of only four layers in ourBragg mirror, and our use of a direct probe measurement onthe top electrode of the device. Future devices would use alarger number of mirror layers, and would move themeasurement contacts away from the device, with electrical

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[7] G. N. Saddik, D. S. Boesch, S. Stemmer, R. A. York "dc electricfield tunable bulk acoustic wave solidly mounted resonatorusing SrTi03," Appl. Phys. Lett. 91, 043501, July 2007

[8] W. E. Newell, "Face-Mounted Piezoelectric Resonators,"Proceedings ofthe IEEE, vol. 53, no. 6, June 1965.

[9] 1. W. Lu, S. Schmidt, Y.-W. Ok, S. P. Keane, and S. Stemmer,"Contributions to the dielectric losses of textured SrTi03 thinfilms with Pt electrodes," J. Appl. Phys., vol. 98,054101,2005.

[10] S. Schmidt, D. O. Klenov, S. P. Keane, J. Lu, T.E. Mates, and S.Stemmer, HAtomic structure of (Ill) SrTi03/Pt interfaces,"Appl. Phys. Lett., vol. 88, 131914, 2006.

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[11] J. D. Larson III, P. D. Bradley, S. Wartenberg, and R. C. Ruby,"Modified Butterworth-VanDyke Circuit for FBARResonators, and Automated Measurement System," IEEEUltrasonic Symposium, vol. 1, pp. 863-868, Oct. 2000.

[12] J. D. Larson III, P. D. Bradley, S. Wartenberg, and R. C. Ruby,"Modified Butterworth-VanDyke Circuit for FBARResonators, and Automated Measurement System," IEEEUltrasonic Symposium, vol. 1, pp. 863-868, Oct. 2000.

[13] 1. Rosenbaum, Bulk Acoustic Wave Theory and Devices,Massachusetts: Artech House, 1988.

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