1 Abstract—With the application of microfabrication

techniques, phononic crystals have been transformed over the past decade: from hand assembled millimeter-to-meter scale crystals consisting of metal balls in water or epoxy, to precisely machined crystals with sub-micron features operating at frequencies in excess of 1 GHz. This paper reviews the contributions of Sandia National Laboratories to micro and nano scale phononic crystal devices including: the integration of piezoelectric transducers, the choice of phononic crystal materials, phononic crystal design, and the application of phononic crystals to radio frequency and thermal management applications.

Index Terms—Acoustic Bandgap, Elastic Bandgap, Microelectromechanical Systems, Microfabrication, Phononic Bandgap, Phononic Crystal, Phononic Crystal Cavity

I. INTRODUCTION hononic crystals are the elastic wave equivalent of photonic crystals, where a periodic lattice of scattering inclusions properly arranged in a host or matrix material

causes certain frequencies to be completely reflected by the structure [1]. This creates bandgaps in the frequency response of a phononic crystal. Over the past decade phononic crystals have been scaled from large hand assembled balls in acoustically lossy materials such as water and epoxy to micro and nano fabricated 2D structures realized in low loss materials operating at frequencies from 10-5,000 MHz [1-12]. Micro/Nano fabrication of phononic crystals has presented many opportunities and challenges. Batch fabrication and the ability to co-integrate piezoelectric transducers with phononic crystals for rapid electrical interrogation have lead to experimentation on a wide variety of geometries and devices [1-6, 9-12]. The ability to suspend 2-D phononic crystals above the substrate using micromachining techniques has allowed for low loss phononic waveguides [1-3] and cavities [4-5] to be demonstrated. The wide range of materials

This work was supported by the Laboratory Directed Research and Development (LDRD) program at Sandia National Laboratories and by the DARPA Chip Scale Spectrum Analyzers (CSSA) Program. Sandia National Laboratories is a multiprogram laboratory operated by the Sandia Corporation, Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

available [1-12] and the implications of finite membrane thickness [13-14] require careful consideration when designing a phononic crystal for a specific application. Phononic crystals and devices have been fabricated and studied theoretically and experimentally in several material systems including Si-air [5,7,8,9,11], SiC-air[4], Si-W[6] and SiO2-W[1-3]. Phononic band gaps at GHz frequencies have been demonstrated in each of these material systems [8,9,4,6,3] with complete phononic bandgap widths exceeding 10% of the center frequency and band gaps for longitudinal waves in excess of 50%. Solid-Solid phononic crystals realized in Si-W and SiO2-W material systems have demonstrated less sensitivity to lithography when compared to Solid-Air phononic crystals [13], allowing scaling to higher frequency operation important to radio frequency (RF) and thermal applications. Certain devices, such as cavities, requiring very low material damping have been best realized in Solid-Air materials systems using high-Q materials such as Si [5] and SiC [4]. Applications of micro-phononic crystals have been in the areas of acoustic isolation [15], RF cavities and filters [4-5, 15-16], acoustic focusing and imaging [17-19] and in thermal management [7-8]. Phononic crystals operating as acoustic mirrors are applicable to mechanically vibrating structures such as gyroscopes and microresonators [4-5, 15-16]. By strategically placing defects in a phononic crystal, devices such as acoustic waveguides [1-3], cavities [4-5, 15-16] and filters can be formed. For applications in RF communications, phononic crystals can realize cavities with high quality factor, low insertion loss and small size [4-5, 15-16], properties not readily achieved together in existing technologies. Phononic crystals also offer unique methods [1] for coupling phononic cavities together to form higher order filters that can improve filter performance metrics such as insertion loss and shape factor. Phononic crystals have demonstrated acoustic focusing and negative refraction [17-19] which may lead to miniaturization or improved performance in applications such as ultrasound and nondestructive testing. Recently, a nano-fabricated Si-Vacuum phononic crystal with its tailored phonon dispersion [7-8] has demonstrated a >50% thermal conductivity reduction of a Si slab while leaving the electrical conductivity nearly unchanged, potentially leading to improved performance in thermoelectric devices.

Micro and Nano Fabricated Phononic Crystals: Technology and Applications

Roy H. Olsson III1, Maryam Ziaei-Moayyed1, Bongsang Kim1, Charles Reinke1, Mehmet F. Su2, Patrick Hopkins1, Yasser M. Soliman2, Drew F. Goettler2, Zayd C. Leseman2 and Ihab El-Kady1,2

1Sandia National Laboratories, Albuquerque, NM, USA 2University of New Mexico, Albuquerque, NM, USA

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983978-1-4577-1252-4/11/$26.00 ©2011 IEEE 2011 IEEE International Ultrasonics Symposium Proceedings

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984 2011 IEEE International Ultrasonics Symposium Proceedings

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985 2011 IEEE International Ultrasonics Symposium Proceedings

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cromachined phoormed from W inuency operation.nonic crystal. Thwaves centered a

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2.25 GHz. xplored in [1], y high acousticc transducers us achieving a onse of the air-

ononic crystal nclusions in a b) Modeling he crystal has at ~ 1.4 GHz.

O2 phononic between two a normalized the low loss r waveguides hown in Fig.

normalized because the very narrow rows of the

cies.

er frequency me limiting crystals, air-en very high the phononic crystal cavity crons and the mum feature The general to isolate an

c loss of the required to very high-Q

-SiC

986 2011 IEEE International Ultrasonics Symposium Proceedings

i

cSf

phachandcav

Figure 8. a) Winclusions from awaveguide show

Figure 9. (a) SEcavity with drivcavity by 5 layerSEM of the SiC/filling fraction of

ononic crystalhieves a Q of 2d has a normalvity from the p

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EM of a fabricatve and sense AlNrs of phononic c/air phononic cryf 0.44 (c) SEM

sides w

l cavity is sho2,000 at 2.25 Glized transmisspiezoelectric tra

formed by removtal. b) Measureded transmission

ted silicon carbidN transducers sep

rystals on each systal with a pitchof one of the howalls.

wn in Fig. 10GHz in a very ssion of -10 dB. ansducer and m

ving 2-rows of d response of theof 1 at 68 MHz.

de 10th overtone parated from the side (b) close-uph of 1.83µm andoles with vertica

0. The resonatsmall form fact By isolating t

metal electrode

e

p d al

tor tor the

es,

Figure in Fig. 9

smal a Q facthan a min the results crystal overtonphononmaterialayers quality advantathat maproblemresonatcrystal This isinvestigloss filt

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The Sanjay

10. Measured re9. The resonatoll form factor and

ctor of 2000 wmore traditionasame fabricatireported in [resonator in

nes in the cavinic crystal lattial is achieved, a much more factor. This p

age of eliminaake utilizationmatic. Experimtor is seen to periods isolat

s not predictgation before ptering.

ver the past decand assembledals, with dimenncies < 1 MHmiting dimensiess of 1 GHz. Md suspension aic loss in a 2ucers for rapid number of latng phononic crmance in resonw being reporte

Aauthors woulRaman and

esponse of the por achieves a Q od has a normaliz

was achieved al overtone SiCion process. T16], where th

ncreases linearity and exponeice constants umaking the adarea efficient

phononic crystating many ofof the resona

mentally, the increase with

ting the cavityted from theophononic crys

IV. CONCLUS

cade, phononicd balls arrangensions greater

Hz, to preciselyions of 200 nmMicromachinin

above the subst2D crystal, incharacterizatio

ttices and marystals for minnators, lenses ed.

ACKNOWLEDG

ld like to thanDr. William

phononic crystal of 2,000 at 2.25 Gzed transmission

in ~10 times C resonator [2This is consiste Q factor of rly with the

entially with thuntil the f.Q prddition of phon

method of intal approach hf the overtone

ator in a filter loss of a phon

h the number y from the tranory and requtals can be ap

SIONS c crystals have

ed in lossy fluithan 1 mm an

y microfabricam and operatingng of phononictrate, removingntegration of pon and experimaterial systemniaturization an

and thermal

GMENT nk Dr. DenniChappell of

cavity shown

GHz in a very n of -10 dB.

smaller size 6] fabricated tent with the f a phononic

number of he number of roduct of the nonic crystal

ncreasing the has the added e resonances or oscillator

nonic crystal of phononic

nsducers [5]. uires further pplied to low

e been scaled id and epoxy nd operating ated crystals g frequencies c crystals has g a source of piezoelectric mental study s. Devices nd improved management

is Polla, Dr. DARPA for

987 2011 IEEE International Ultrasonics Symposium Proceedings

funding under the Chip Scale Spectrum Analyzers Program (CSSA). The authors would like to thank Bob Newgard, Dr. Bob Potter and Chris Conway of Rockwell Collins and Dr. Amy Duwel of Charles Stark Draper Laboratories for discussions on high-Q resonators. The authors would like to acknowledge the staff of the Microelectronics Develop Laboratory at Sandia National Laboratories for fabrication of the phononic crystal devices. Finally, we would like to acknowledge our late friend and colleague Dr. James (Jim) G. Fleming who founded the Sandia phononic crystal effort prior to his passing in 2007.

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[3] R. H. Olsson, M. Su, S. X. Griego, Y. Soliman, D. Goettler, Z. Leseman and I. El-Kady “Ultrahigh frequency (UHF) phononic crystal devices operating in mobile communication bands,” Tech. Dig. IEEE Intl. Ultrasonics Symp., pp.1150-3, 2009.

[4] M. Ziaei-Moayyed, M. F. Su, C. Reinke, I. F. El-Kady, and R. H. Olsson, "Silicon Carbide Phononic Crystal Cavities for Micromechanical Resonators," 2011 IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), pp.1377-1381, Jan. 2011.

[5] S. Mohammadi, A. A. Eftekhar, A. Khelif, H. Moubchir, W. D. Hunt, and A. Adibi, “High Q micromechanical resonator in a two dimensional phononic crystal slabs,” Appl. Phys. Lett., vol. 95, 051906, 2009.

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[10] S. Benchabane, A. Khelif, J.-Y. Rauch, L. Robert, V. Laude, “Evidence for complete surface wave band gap in a piezoelectric phononic crystal,” Phys Rev. E, vol. 73(6), pp. 065601, 2006.

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[12] N. Kuo, C. Zuo, G. Piazza, “"Microscale inverse acoustic band gap structure in aluminum nitride," Applied Physics Letters, Vol.95, Issue 9, 2009.

[13] C. M. Reinke, M. F. Su, R. H. Olsson III, and I. El-Kady, “Realization of Optimal Bandgaps in Solid-Solid, Solid-Air, and Hybrid Solid-Air-Solid Phononic Crystal Slabs,” Appl. Phys. Lett. 98, 061912 (2011).

[14] A. Khelif, B. Aoubiza, S. Mohammadi, A. Adibi, V. Laude, “Complete band gaps in two-dimensional phononic crystal slabs,” Phys. Rev. E, vol. 74,046610,2006.

[15] T. T. Wu, W-S. Wang, J-H. Sun, J-C. Hsu and Y-Y. Chen, “Utilization of phononic crystal reflective gratings in a layered surface acoustic wave device,” Appl. Phys. Lett., 94, 101913, 2009.

[16] D. Goettler, M. Su, Z. Leseman, Y. Soliman, Roy H. Olsson, and I. El-Kady, “Realizing f.Q product limit in silicon via compact phononic crystal resonators,” J. App. Phys., in press.

[17] M. Ke , Z. Liu, C. Qiu ,W. Wang ,J. Shi ,W. Wen, and P. Sheng, “Negative-refraction imaging with two-dimensional phononic crystals,” Phys. Rev. B, vol. 72, 064306, 2005.

[18] S. Yang , J. H. Page, Z. Liu , M. L. Cowan, C. T. Chan, and P. Sheng, “Focusing of sound in a 3D phononic crystal,” Phys. Rev. Lett, vol.93, 024301, 2004.

[19] N. Kuo, G. Piazza, “Evidence of Acoustic Wave Focusing in a Microscale 630 MHz Aluminum Nitride Phononic Crystal Waveguide”, IEEE Frequency Control Symposium, pp. 530 - 533, June 2010.

[20] H. Kando, M. Watanabe, S. Kido, T. Iwamoto, K. Ito, N. Hayakawa, K. Araki, I. Hatsuda, T. Takano, Y. Nagao, T. Nakao, T. Toi and Y. Yoshii, “Improvement in temperature characteristics of plate wave resonator using rotated Y-cut LiTaO3 / sin structure,” 2011 IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), pp.768-771, Jan. 2011.

[21] M. Eichenfield, J. Chan, R. M. Camacho, K. J. Vahala and O. Painter, “Optomechanical crystals,” Nature, 462, pp. 78-82, 2009.

[22] R. H. Olsson III, J. G. Fleming, I. F. El-Kady, M. R. Tuck, and F. B. McCormick, “Micromachined Bulk Wave Acoustic Bandgap Devices,” International Conf. on Solid-State Sensors, Actuators, and Microsystems, pp. 317-321, June 2007. [23] R. H. Olsson III, I. El-Kady and M. R. Tuck, “Microscale Phononic Crystals and Devices,” EUROSENSORS 2008, pp. 3-8, Sept. 2008. [24] Y. M. Soliman, M. F. Su, Z. C. Leseman, C. M. Reinke, I. El-Kady, R. H. Olsson III, “Effects of Release Holes on Microscale Solid–Solid Phononic Crystals”, Applied Physics Letters, vol. 97, 081907, 2010. [25] M. F. Su, R. H. Olsson III, Z. C. Leseman and I. El-Kady, “Realization of a Phononic Crystal Operating at Gigahertz Frequencies,” Applied Physics Letters, 96, 05311, (2010). [26] S. Gong, N. Kuo and G. Piazza, “A 1.75 GHz SiC Lateral Overtone Bulk Acoustic-Wave Resonator”, 2011 Solid-State Sensors, Actuators and Microsystems Conference (TRANSDUCERS), pp.922-925, June 2011.

988 2011 IEEE International Ultrasonics Symposium Proceedings