ArticleFabrication and Sensing Behavior of Ultrasensitive Piezoelectric Microcantilever-Based Precision Mass Sensor

7/27/2019 ArticleFabrication and Sensing Behavior of Ultrasensitive Piezoelectric Microcantilever-Based Precision Mass Sensor

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Journal of the Korean Physical Society, Vol. 49, December 2006, pp. S608S611

Fabrication and Sensing Behavior of Ultrasensitive PiezoelectricMicrocantilever-Based Precision Mass Sensor

Sanghun Shin and Jaichan Lee

School of Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746

(Received 14 February 2006)

We have designed and fabricated a very small microcantilever mass sensor based on a piezo-electric actuation and sensing mechanism for ultrasensitive femtogram detection. We have usedthe resonant-frequency change of the microcantilever upon a mass increase in the microcantileversurface. The microcantilever was fabricated by using micro-electro-mechanical-system (MEMS)

processing. The microcantilever employs a sol-gel-derived Pb(Zr0.52,Ti0.48)O3 (PZT) film capacitorfabricated on a low-stress SiNx elastic layer for piezoelectric actuation. A mass sensitivity of 3 fg/Hzwas achieved with a high mechanical quality factor of 680. The corresponding gravimetric sensitiv-ity factor was 450 cm2/g, which is significantly larger than that of a quartz-crystal microbalance.This electrically-driven ultrasensitive mass sensor enables application for a precision mass sensor todetect biomarker or virus molecules in a femtogram regime.

PACS numbers: 0 7.07.Mp, 85.50.-n

Keywords: Mass sensor, Microcantilever, MEMS, PZT, Resonant frequency

I. INTRODUCTION

A rapid, ultrasensitive and economical method for thedetection of biochemical entities, such as virus particlesor human biomarkers, is becoming an ever more impor-tant technology. Micro- or nano-scale fabrication tech-niques are increasingly being used to create miniaturizedsensory systems. The application of miniaturized sensorysystems is very diverse and ranges from environmentalmonitoring to clinical diagnosis [1].

Generally, a miniaturized chemical sensory system canbe constructed on the basis of direct miniaturization ofconventional chemical instruments, such as a gas chro-matograph or mass spectrometer. Chemical sensors alsoutilize the detection of the physical changes after a chem-

ical substance is absorbed or adsorbed onto the sensoryelement. The physical change due to the absorption oradsorption of a target substance is represented by anincrease of mass or volume, a change in resistivity or op-tical properties, and so on. The detection of mass changehas several advantages over other detection schemes [2].Recently, very small silicon or silicon nitride cantileversutilizing mass detection have been known to have goodsensitivity [1, 3]. In suspended structures consisting ofsilicon or silicon nitride film, signal transduction is typ-ically achieved by employing an optical interferometricsystem to measure the mechanical bending or the fre-quency spectrum resulting from additional loading by

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the adsorbed mass, which leads to many difficulties foran integrated sensor system. On the other hand, the sus-pended microtransducer employing a piezoelectric com-

ponent, i.e., a piezoelectric microcantilever, vibrates atits resonant frequency upon applying an appropriate ACvoltage and provides an electrical signal at the output viapiezoelectric coupling [4,5]. This electrical transductionleads to simplified miniaturization of an integrated de-vice, compared with conventional complex transducers.Therefore, in this study, we have considered a very smallpiezoelectric microcantilever which has significantly im-proved mass sensitivity for a miniaturized sensory sys-tem such as lab-on-a-chip. We report here the fabrica-tion, electromechanical properties and demonstration ofa very small piezoelectric microcantilever as an ultrasen-sitive mass detector.

II. EXPERIMENT

1. Design and fabrication

A schematic diagram and the fabrication process ofthe designed piezoelectric microcantilever are shown inFigure 1. The dimension of the rectangular microcan-tilever is 30 m in length and 10 m in width. P-type (100) 4-in. silicon wafers were used as the startingsubstrate. Low-stress SiNx and low-temperature oxide(LTO) were deposited on both sides of the silicon waferby low-pressure chemical vapor deposition (LPCVD) and

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Fig. 1. Schematic diagram and fabrication process ofpiezoelectric microcantilever: (a) deposition of PZT piezo-electric thin film; (b) top-electrode patterning and PZT etch-

ing; (c) bottom-electrode etching and ILD patterning; (d)top-electrode pad patterning; and (e) back silicon etching andfront-side SiNx etching.

plasma enhanced chemical vapor deposition (PECVD),respectively. Thin (200 A) Ta as an adhesion layer anda Pt (1500-A-thick) metallic layer as a bottom elec-trode were deposited by DC magnetron sputtering onthe LTO/SiNx/Si substrate at 200

C and 350 C, re-spectively. A PZT thin film of 0.5 m was depositedby sol-gel spin coating with a synthesized precursor so-lution, followed by fast annealing at 700 C for 5 min in

a tube furnace (shown in Figure 1(a)) [6]. The detailedsynthesis process of the precursor solution was reportedelsewhere [7]. The PZT capacitor in the microcantileverwas patterned by photolithography and various etchingprocesses. A 1000-A-thick Pt thin-film was depositedon the PZT thin film surface for the top electrode anddefined by a lift-off process. The PZT thin film and bot-tom electrode were etched by inductively coupled plasma(ICP) etcher systems. An interlayer dielectric (ILD) wasused in order to isolate the top and bottom electrodes.A photosensitive polyimide film was used as the ILD andpatterned by a photolithography process. The top elec-trode line and pad were patterned by a lift-off process.

The back-surface silicon was etched by wet chemical etch-ing with KOH solution (KOH : DI water = 6 : 4) at 80

Fig. 2. SEM image of fabricated 30-m-long piezoelectricmicrocantilever.

C for a suspended membrane structure. In order toprotect the PZT capacitors from the KOH solution, thefront side of the wafer was covered with a sacrificial re-sist layer and a glass wafer. Subsequently, the SiNx for amicrocantilever structure was defined with a photoresistmask and etched out by a reactive ion etching system(RIE), followed by a final release process (shown in Fig-ure 1(e)) [8].

2. Mass loading and sensing

In a resonant suspended structure such as a microcan-tilever, a mass increase on the device surface leads toa change of its resonance frequency, proportional to theinitial resonant frequency and the mass increase on unitarea. Metallic-thin-film deposition was used as an artifi-cial mass loading method on the back surface of a verysmall microcantilever for the estimation of mass sensitiv-ity [4,5]. Very small amounts of target substance such asbiomarker or virus particle exist in the human body, aswell as waste matter, i.e., human breath and excretionsproduced in the body metabolism process. Therefore,mass loading was limited to a picogram regime when themetallic thin film was deposited. For the mass loading,

Au was deposited on the back of the microcantilever byelectron-beam evaporation. The resonant frequency ofthe microcantilever was measured from the dispersion ofthe capacitance and dielectric loss of the microcantilevertransducer by using an impedance analyzer (HP4194A).

III. RESULTS AND DISCUSSION

1. Electromechanical properties of a microcan-

tilever transducer

Figure 2 shows a scanning electron microscope (SEM)image of the fabricated piezoelectric microcantilever. No


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Fig. 3. Dispersion of capacitance and dielectric loss ofmicrocantilever with measuring frequency.

distortion of the suspended structure due to residual filmstress was observed. The dielectric constant and lossof the PZT thin films in the fabricated microcantileverstructure were 800 and 2 %, respectively. The PZT ac-tuating layer maintained its initial electrical propertiesthroughout the fabrication process. We have used theresonant-frequency change of the microcantilever as asensing signal. Generally, the accurate resonance can bedetected from the mechanical resonance driven by piezo-electric coupling. An optical measurement system, suchas a laser vibrometer, detects the maximum mechanicaldisplacement at the resonance point and gives a relativelyaccurate value. However, in our previous work, the re-sults of electrical measurements showed a very clear res-onant frequency spectrum which was similar to that ofoptical measurement [4]. Moreover, this electrical mea-surement is appropriate for a miniaturized device. Figure3 shows the variation in the capacitance and dielectricloss of the microcantilever transducer with measuringfrequency. Clear resonance was observed in the spec-tral response. The fundamental bending frequency ofthe microcantilever was in the range of 2.7 MHz. Thequality factor is an important parameter of a resonantsensor. This describes the quality or the width of theresonance frequency peak. The narrower the resonantfrequency peak, the easier and more distinct the peak isto detect. The general relationship for the quality factorcan be written as Eq. (1), where n is the angular fre-quency, m is the dynamic mass of the body, and c is thedamping coefficient:

Q =mnc

=n

bandwidth(1)

The bandwidth, appearing in Eq. (1), is the width ofthe frequency curve at the half-power point. The halfpower point is defined to be A/

2 of the peak value of

the resonance frequency curve included in the spectralresponse (shown in Figure 3), where A is the maximumamplitude of the vibration at a specific frequency. The

microcantilever in this study showed a quality factor ofca. 680 in air.

Fig. 4. Mass-sensing behavior of 30-m-long piezoelectricmicrocantilever.

2. Mass sensing b ehavior

Figure 4 shows the resonant-frequency shift of the mi-crocantilever with Au films of various thicknesses de-posited on the back of the microcantilever. The mass in-crease in the microcantilever by an Au thin film was con-trolled by the thickness of Au film. Au film with a thick-ness up to 50 A leads to a mass increase of 29 picogramson the back surface of the microcantilever. The resonantfrequency shifted toward a lower frequency range as themass increased, leading to a mass sensitivity of 3 fg/Hz.In a different way, the mass sensitivity is represented

by another sensitivity factor. This can be described bythe gravimetric sensitivity factor (Sm = f/(mf0)),where f is the frequency variation, f0 is the initial res-onant frequency of the transducer, and m is the massincrease per unit area [9,10]. The calculated gravimet-ric sensitivity factor of the microbridge was 450 cm2/g,which is significantly higher than those of conventionalacoustic resonant mass sensors such as TSM and SAW,which are typically 20 140 cm2/g.

IV. CONCLUSIONS

A very small piezoelectric microcantilever transducerwas fabricated for a precision mass sensor. The dielec-tric constant and loss of the PZT capacitor in the fab-ricated microcantilever structure were 800 and 2 %, re-spectively. The microcantilever had a fundamental bend-ing frequency of 2.7 MHz, obtained from the dispersionof the capacitance and loss of the microcantilever. Thequality factor of the resonant frequency was 680 in air.For the first time, we were able to obtain the mass sensi-tivity in femtogram regime with a piezoelectric actuationand sensing mechanism. The microcantilever exhibiteda mass sensitivity of 3 fg/Hz and a gravimetric sensitiv-

ity factor of 450 cm2/g, which are significantly enhancedover other mass-sensing devices. Therefore, the micro-


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Fabrication and Sensing Behavior of Ultrasensitive Piezoelectric Sanghun Shin and Jaichan Lee -S611-

cantilever can be utilized for a highly sensitive nanobal-ance, for example, for biomarker and virus molecules.

ACKNOWLEDGMENTS

This work has been sponsored by the Korea Scienceand Engineering Foundation (KOSEF) through the Na-tional Research Laboratory Program and the Ministry ofCommerce, Industry and Energy (MOCIE) through theClean Production Program.

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D. Park and J. Lee, Jpn. J. Appl. Phys. 42, 6139 (2003).[5] S. Shin, J. K. Paik, N. E. Lee and J. Lee, J. Korean Phys.Soc. 46, 292 (2005).

[6] Caroline S. Lee, H. J. Nam, Y. S. Kim, W. H. Jin and J.U. Bu, J. Korean Phys. Soc. 45, 227 (2004).

[7] J. R. Ahn, S. Jun, D. W. Kim, G. Y. Yeom, J. B. Yoo,J. Lee and T. Sands, Proc. 12th IEEE Int. Symp. Appli-cations of Ferroelectrics, p. 721 (2001).

[8] J. S. Hur, T. Y. Kim, H. J. Shin, S. Moon, I. H. Choiand C. S. Son, J. Korean Phys. Soc. 45, S933 (2004).

[9] J. W. Grate and G. C. Frye, Sensors Update (Wiley-VCH, 1997), Chap. 2.

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ArticleFabrication and Sensing Behavior of Ultrasensitive Piezoelectric Microcantilever-Based Precision Mass Sensor