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A contour-mode film bulk acoustic resonator of high quality factorin a liquid environment for biosensing applications

Wencheng Xu,a Seokheun Choi, and Junseok ChaeSchool of Electrical, Computer and Energy Engineering, Arizona State University, Tempe,Arizona 85287, USA

Received 4 September 2009; accepted 14 January 2010; published online 3 February 2010

This letter reports an acoustic resonator of high quality factors Qsoperating in liquid media. Thefilm bulk acoustic resonatorFBARis made of a ring-shaped piezoelectric aluminum nitride thinfilm, and is excited in a contour mode. By having a low motional resistance upon coupling withliquids, the contour mode FBAR achievedQs up to 189, more than 12over the state-of-the-artFBARs in liquids. The resonator was characterized by an aptamerthrombin binding pair for abiosensor and showed a mass resolution of 1.78 ng /cm2. 2010 American Institute of Physics.doi:10.1063/1.3309586

Film bulk acoustic resonators FBARs have been con-sidered as attractive devices in microwave circuits, and re-cently find increasing interests for biosensing applications.13

Compared to quartz crystal microbalances, one of the mostcommonly used nonlabeling sensors,4,5 FBARs have a simi-lar structure, but the physical miniaturization provided by themicromachining techniques significantly improves the sensi-tivity and allows for batch fabrication and integration intolarge-scale sensor arrays. An FBAR typically consists of asuspended piezoelectric thin film, such as ZnO or AlN, sand-wiched by two metal electrodes. The very thin piezoelectriccomposite1 mresults in a high resonant frequency upto 10 GHz, and thus offers a largefrequency sensitivity to amass loading 1000 Hz cm2 /ng.6,7 FBARs typically ex-hibit a high quality factor Q in a gaseous environment,where the acoustic energy is well entrapped inside the FBAR

body due to the large impedance mismatch between the gasand solid. The high Q allows distinguishing a very smallfrequency shift and, consequently, detecting an extremelyminute amount of mass change on the sensor surface. How-ever, longitudinal-mode FBARs L-FBARs experience vis-cous damping in the presence of liquids. When L-FBARscontact with liquid, the compressional vibration waves dissi-pate into the lossy media and cause significant Q degrada-tion. Previously reported Q of 15 in water indicates a 95%reduction inmass resolution compared with the one operat-ing in the air.8,9 We have previously demonstrated a methodto improve the Q of L-FBARs in liquid environments byintegrating a microfluidic channel to the resonator;10 Qof up

to 120 has been achieved. However, the unavoidable fact ofsqueeze damping and the critical height requirements of themicrofluidic channel limited further improvements.

High Q is obviously the key requirement for a high res-olution FBAR sensor in liquids. In this letter, we report acontour-mode FBARC-FBARshowing a substantially im-proved Q in a semi-infinite depth liquid environment. Asshown in Fig. 1a, the C-FBAR has a suspended circular-shaped AlN ring sandwiched between the top and bottom Auelectrodes. A liquid droplet is in direct contact with the topelectrode. The AlN ring is excited in its radial directions, inwhich the vibration displacement is parallel to the resonator/

liquid interface. The shear viscous damping, instead of thesqueeze damping in L-FBARs, alleviates the acoustic energyloss and consequently results in high Qs. Compared to thetraditional L-FBARs, C-FBARs have lower resonant fre-quencies 100200 MHz, yet they exhibit significantlyhigherQs and, consequently, notably improved mass sensingresolutions. Additionally, C-FBARs do not require havingintegrated microfluidic channels on top of FBARs, whichsubstantially reduces the complexity of fabrication. Figure1bshows the scanning electron microscope SEMpictureof an AlN C-FBAR, fabricated with a two-mask process us-ing the standard silicon micromachining technology. 0.6 mthick low-stress silicon nitrideSiNwas first deposited on asilicon wafer as an isolation layer, followed by 1500 thickCr/Au bottom electrode formation. AlN was then sputteredusing a single-module AMS physical vapor deposition sput-tering tool. The 1 m thick AlN film was c-axis orientedand showed the x-ray diffraction rocking curve value of2.18. Top electrode of another 1500 thick Cr/Au wasformed and then the AlN and SiN layers were dry etched toform the resonator and to expose the Si substrate. The ring isfinally released from the substrate by dry etching the siliconusing XeF2.

The unperturbed in the air ring-shaped resonator vi-brating in a pure radial-extensional mode can be modeledwith a lumped-element equivalent circuit Fig. 2a. Re isthe series resistance of electrodes, C0 is the static parallelcapacitance of the device; R1, L 1, and C1are, in accordancewith the Butterworth Van Dyke BVDmodel, the motional

aElectronic mail: wencheng.xu@asu.edu.

FIG. 1. Color online aSchematic figure of the C-FBAR biosensor con-tacting with a liquid droplet and b the SEM photo of a fabricated AlN

C-FBAR.

APPLIED PHYSICS LETTERS 96, 053703 2010

0003-6951/2010/965/053703/3/$30.00 2010 American Institute of Physics96, 053703-1

Author complimentary copy. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586http://dx.doi.org/10.1063/1.3309586

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components of the resonator, respectively, resistance, induc-tance, and capacitance,11

C0= 0332ravew

tR1=

2

8

t

2rave

1/2

EP3/2

Qd312 ,

C1=8

2wravet

d312

Ep L1=

8

wt

2rave

1

d312 EP

2,

where 33 is the relative dielectric constant of AlN in the zdirection,tis the thickness, is the density of AlN, w is thering width, rave is the average radius of the ring, d31 is thepiezoelectric coefficient, and EP is the equivalent Youngsmodulus of AlN, respectively. As shown in Fig. 2b,the topsurface perturbation caused by a fluid contact can be de-scribed by a motional inductance L 2and resistance R 2

12,13

L2=L1

4fww

1/2 R2= 2fL2,

where fis the resonant frequency, is the shear stiffness ofAlN, w and w are the liquid density and shear viscosity,respectively. The theoretical Qs in the air and in liquid canbe calculated by Qair=2fL1 / R1+Re and Qwater=2fL1+L2 / R1+R2+Re, respectively.

13

A C-FBAR20 m width and 90 m inner radius ringwas experimentally characterized in the air and in water.We measured the one-port reflection coefficient S11 usinga network analyzer HP 8510C. The experimental Q valuesof the series resonance were obtained from Qs = fs /2dz /df f=fs, where fsis the frequency of the series reso-nance, Z is the phase of the impedance, and Z, extracted

from the recorded S11data. The theoretical and experimentalvalues ofQs in the air and in contact of deionized water areshown in Fig.2c.

Figure3 shows an impedance plot of a C-FBAR in theair and in water. Measured Qs in the air range from 250to310. These values were lower than previously reported Qs,11

partially because of the relatively large resistanceReof ourlong electrodes which were designed to leave rooms for the

droplet. A water droplet was dispensed directly on top of thedevice. As the diameter of droplet was significantly largerthan the acoustic decay length in water, 45 nm at 160MHz, the depth of droplet was treated as semi-infinite. Theresonant frequency shifted downwards by 310 kHz and Qbecame 189 due to the effects from the motional compo-nents,L2and R2, respectively; yet the Qof 189 demonstratesa 1319 improvement over the previously reportedL-FBARs in liquids.

We also characterized the C-FBAR sensor withaptamerthrombin binding pairs. The top gold electrode ofthe C-FBAR was modified with COOH-terminated self-assembly monolayerSAM, and then washed the electrode

with ethanol and water. Then, a custom-designed aptamerwas immobilized on top of the COOH-terminated SAM. Theaptamer sequence was designed at the 5-terminus with anamine based linker 5-NH2-CH26-TTC CAA CGG TTGGTG TGG TTG G-3. After washing the surface with abuffer solution 1phosphate buffered saline PBS, adroplet of thrombin solution 2 M was dispensed to letthe thrombins bind to the aptamers and the unboundedthrombins were washed away with PBS. Figure4shows themajor test sequence and the resonant frequency changes. Thefrequency shifts were measured in the washing steps aftereach chemical process in order to minimize the frequency

variations resulted from the device being contacted withdifferent liquids. The aptamerthrombin bindings results afrequency shift of 28.4 kHz which corresponds to a massloading of 252.8 ng /cm2 via a mass sensitivity of112 Hz cm2 /ng. The measured noise floor is approximately200 Hz, which suggests a mass resolution of 1.78 ng /cm2.

In summary, a ring-shaped AlN C-FBAR has been de-signed and fabricated for mass sensing in a liquid environ-ment. By exciting in its radial-extensional mode, the resona-tor experiences the shear viscous damping instead of thesqueeze damping, which significantly alleviates the acousticenergy dissipation to the contacting liquid. Q of up to 189 inliquid has been experimentally demonstrated, which is at

least 1319 times higher than the state-of-the-art L-FBAR inliquid. This improvement directly enhances the mass reso-lution of the FBAR sensors.

FIG. 2. Lumped-element equivalent-circuit models for a the C-FBAR inthe air;bperturbed C-FBAR with one-sided liquid contact; ccomparisonbetween theoretical Q from this model and experimentally measured Qvalues.

FIG. 3. Color online Measured impedance plot of a fabricated C-FBAR20 m width and 90 m inner radius ring-shapedin the air and in watershowing Q of 317 and 189, respectively.

053703-2 Xu, Choi, and Chae Appl. Phys. Lett. 96, 053703 2010

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This work was supported by National Science Founda-tion under Grant No. 741834. The authors would like tothank Mr. Xu Zhang for his assistance in the measurements.

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FIG. 4. Color online Measured resonant frequency shifts f of theC-FBAR sensor due to the mass loading on the top gold electrode; itheC-FBAR is in contact with water,iiCOOH-terminated SAM are formed,iii the aptamer is immobilized, and iv thrombins are bound to theaptamers.

053703-3 Xu, Choi, and Chae Appl. Phys. Lett. 96, 053703 2010

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