Mechanical and electronic approaches to improve the

Acta Mech Sin (2009) 25:1–12DOI 10.1007/s10409-008-0222-6

REVIEW PAPER

Mechanical and electronic approaches to improvethe sensitivity of microcantilever sensors

Madhu Santosh Ku Mutyala ·Deepika Bandhanadham · Liu Pan ·Vijaya Rohini Pendyala · Hai-Feng Ji

Received: 27 September 2008 / Accepted: 8 October 2008 / Published online: 10 January 2009© The Chinese Society of Theoretical and Applied Mechanics and Springer-Verlag GmbH 2009

Abstract Advances in the field of micro electro mechanicalsystems and their uses now offer unique opportunities inthe design of ultrasensitive analytical tools. The analyticalcommunity continues to search for cost-effective, reliable,and even portable analytical techniques that can give reli-able and fast response results for a variety of chemicals andbiomolecules. Microcantilevers (MCLs) have emerged as aunique platform for label-free chem-sensor or bioassay. Sev-eral electronic designs, including piezoresistive, piezoelec-tric, and capacitive approaches, have been applied to measurethe bending or frequency change of the MCLs upon exposureto chemicals. This review summarizes mechanical, fabrica-tion, and electronics approaches to increase the sensitivity ofMCL sensors.

Keywords Microcantilever · Sensor · MEMS · Sensitivity

1 Introduction

Micro electro mechanical systems (MEMS) have shown tre-mendous growth recently. MEMS devices now offer uniqueopportunities in the design of ultrasensitive analytical meth-ods. In 1994, it was realized that microcantilevers (MCLs)can be made extremely sensitive to chemical and physicalchanges [1,2]. To date, physical sensing has been demon-

M. S. K. Mutyala · D. Bandhanadham · L. Pan ·V. R. Pendyala · H.-F. JiInstitute for Micromanufacturing,Louisiana Tech University,Ruston, LA 71272, USA

H.-F. Ji (B)Department of Chemistry, Drexel University,Philadelphia, PA 19104, USAe-mail: [email protected]

strated by detecting thermal energy, strain, magnetic field,electric charge, viscosity, density, and infrared radiation.Extremely sensitive chemical vapor sensors based on MCLshave been demonstrated using selective coatings on theMCLs. MCLs are a sort of transducers which convert phys-ical quantity like force, strain, stress, etc. into measurablequantity such as voltage or current. MCLs are typically madein the form of rectangular or V shaped planks. An Electronmicrograph of a microcantilever (MCL) array is shown inFig. 1. MCLs act as a sensing platform when they are coatedwith a molecular recognition thin film. Absorption or adsorp-tion of corresponding chemicals in or on the coated filmresults in the frequency change or bending of the MCL.

Resonant frequency The resonant frequency, f , of an oscil-lating MCL can be expressed as

f = 1

2π

√K

m∗ , (1)

where K is the spring constant of the lever and m∗ is theeffective mass of the MCL. The effective mass can be relatedto the mass of the beam, mb, through the relation: m∗ = nmb,where n is a geometric parameter. It is clear that the resonantfrequency can change due to changes in mass and springconstant. Resonance frequency of an MCL (dynamic mode)can be used to detect chemical species in air. However, theoscillation of the MCL is significantly dampened in an aque-ous solution, which reduces the quality factor and subsequentdetection sensitivity of MCLs in aqueous solutions. It shouldbe noted that recent results showed that the second resonancefrequency of MCLs remains sharp in aqueous solutions [3]and can be used for detecting biomolecules.

MCL bending One of the unique characteristics of MCLsis that the device can be made to undergo bending due to

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2 M. S. K. Mutyala et al.

Fig. 1 Electron micrographs of an MCL array

molecular adsorption by confining the adsorption to one sideof the MCL. This bending is due to the adsorption-induceddifferential stress or film volume change on the MCL (staticmode).

For the surface stress induced MCL bending, from molec-ular point of view, the binding results in electrostaticrepulsion or attraction, intermolecular interactions, or a com-bination of these that alter the surface stresses on the MCL.Using Stoney’s formula [4], the bending deflection of theMCL due to adsorption can be written as:

�Z =(

3(1 − ν)L2

ET 2

)δs, (2)

where �Z is the observed deflection at the end of the MCL,ν and E are Poisson’s ratio (0.2152) and Young’s modulus(70 GPa for SiO2, 156 GPa for silicon, 300 GPa for Si3N4)

for the substrate, respectively, T is the MCL thickness, L isthe length of the MCL, and δs is the differential stress on theMCL.

Both frequency and bending approaches have been dem-onstrated to detect chemicals with sensitivity as high as parts-per-trillion, and each approach has its own advantages anddisadvantages which can be used for specific applications.MCL sensors hold a position as a cost-effective, sensitivesensor platform for environmental, biomedical, homelandsecurity, and industrial applications. Compared to existingtechnologies for chemical and biomolecular detection in gen-eral, the MCL sensor technology has three key advantages:

Low cost—Electronics for operation and control are rela-tively simple and inexpensive.

Low-power consumption—For static mode, since the MCL’sbending signal is driven by molecular recognition, the onlypower needed is for detection and display, allowing use oflight-weight battery power or photovoltaic cells.

Small size—The entire sensor could fit in an area with itssides less than a few millimeters. The device can be readily

integrated into a robot system without much increase ofweight in order to protect the operator and to guarantee uni-form detection strategies.

Other than those, MCL arrays could be used to simul-taneously detect and identify various chemicals on a sin-gle chip; the analysis time is relatively shorter because theassay does not require extensive sample preparation or signalamplification; it could be used for in-situ detection.

By monitoring changes in bending response or frequencychange of an MCL, mass change and surface stress changesinduced by adsorption or molecular recognition can berecorded precisely and accurately. Optical, piezoresistive,piezoelectric, and capacitive methods have been used for thedetection of MCL’s deflection (Fig. 2).

Each sensing mechanism has its own advantages and draw-backs. Optical method is sensitive and can measure deflectionas small as 1 nm. However, the optical system is complicatedand not user friendly compared to the other methods. Thesensing systems based on piezoresistive, piezoelectric, andcapacitive methods are much simpler and hold the poten-tial for implantable biosensing applications. These methodsdo not require complex adjustment and alignment systemsas needed in optical readout scheme, and they could readilybe integrated with electrical circuit. However, the sensitivityof these methods is generally lower than that of the opti-cal method. Research is underway to either improve theseapproaches or develop new approaches for MCL basedsensing. This review summarizes recent developments inimproving the sensitivity of MCL sensors based on thepiezoresistive, piezoelectric, and capacitive approaches.

2 Piezoresistive readout scheme

The piezoresistive phenomenon is the change in resistivity ofa material when subjected to stress. The piezoresistivity of amaterial depends on its type; the change in its geometry uponapplication of stress over it. Piezoresistive approach is oneof the most widely applied techniques for measuring stressapplied on an MCL. The advantage of this technique is thatthe electrical output can be readily measured and the MCLcan be integrated on integrated circuit (IC) [5].

In a piezoresistive MCL the relationship between the stressand the change in resistance is shown as the following equa-tion:

�R

R= πlσl + πtσt , (3)

where R is initial resistance, �R is change in resistance;πl , πt are longitudinal and transverse piezoresistive coeffi-cients, respectively; σl , σt are lateral and transverse stress,respectively.

The change in the resistivity is typically measured by usinga Wheatstone bridge as shown in Fig. 3. A Wheatstone bridge

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Approaches to improve the sensitivity of microcantilever sensors 3

Fig. 2 Illustration of fourmethods for detecting MCLmotion with sub-nanometerprecision—(left) Laser-beamdeflection method; (middle)Piezoresistive and piezoelectricmethod; and (right) Capacitivemethod

Fig. 3 A Wheatstone bridge scheme[5]

is made up of four piezoresistors: R1, R2, R3, and R4. Oneof these resistors is replaced by an MCL. The differentialvoltage (Vg) from the Wheatstone bridge is proportional tothe change in the resistivity of the MCL [5].

The output voltage, V0, from the Wheatstone bridge isgiven as in Eq. (4).

V0 =(

�R

R

)(Vin

4

), (4)

where Vin is applied input voltage. The output voltage isdirectly related to the change in the resistance �R withrespect to the initial resistance R.

The first piezoresistive MCL showed the resistivity change(�R/R) of 1.2×10−7 when subjected to 1Å deflection [6].It should be noted that this MCL was not originally designedfor sensors, but for atomic force microscopy. Since then, avariety of strategies have been applied to increase the sensi-tivity.

2.1 Geometry optimization

The dimensions of the MCL and the piezoresistor play a crit-ical role in the sensitivity of MCL sensors. By using FiniteElement Method (FEM) analysis, Chivukula et al. haveshown that optimizing the device dimensions is useful toa greater extent in increasing the sensitivity of the device [7].FEM analysis was done to investigate the optimized devicedimensions by varying the MCL and piezoresistor geometriesand the doping concentration of the piezoresistive device.The simulation results showed that both MCL deflection

displacement and the �R/R change increase with thedecrease in the thickness of piezoresistors; the highest sen-sitivity can be obtained when the piezoresistor’s length isapproximately 2/5 of the SiO2 MCL’s length (Fig. 4); anincrease of both Si and the leg width result in a decreasein MCL’s deflection and its sensitivity; the sensitivity of theMCLs with lower doping concentrations is more significantthan those with higher doping concentrations. Temperaturecontrol is critical for thin piezoresistor in lowering the S/Nratio and increasing the sensitivity.

2.2 Material selection

Most of the MCLs are made of silicon or silicon nitride(Si3N4) materials. Li et al. showed that piezoresistive MCLsmade of silicon dioxide were more sensitive than the silicon-based MCLs due to lower Young’s modulus of SiO2

(57–70 GPa) when compared to that of Si (170 GPa) andSi3N4 (290–380 GPa). The embedded piezoresistor is madeof single crystal silicon and is fully insulated by SiO2 (Fig. 5).This reduces the electronic noise of the MCL since it is iso-lated from the surrounding environment [8].

2.3 Stress concentration method

Concentrating the stress to a designated area is anotherapproach to enhance the sensitivity. Yu et al. reported amethod to concentrate the stress by introducing holes in theMCLs. These holes caused the stress to be concentrated inits vicinity. The size of the holes is 10µm × 5µm [5]. Theresults showed the MCLs with holes were 1.3 times moresensitive than those without holes in response to the sameMCL deflection (Figs. 6 and 7). This increase in the surfacestress is attributed to the discontinuity of the MCL structuredue to the holes.

Decreasing the thickness of the MCLs is a common strat-egy to increase the sensitivity. However, thin MCLs are morefragile and harder to handle than the thicker ones. Naeli et al.[9] reported with another stress-concentrating design thatmade it possible to achieve both high sensitivity and tough-ness. In this design, they separated the stress in the piezore-sistors from the stress on the beam structure. In fabrication,the piezoresistive strings were laid on the wafer surface with

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Fig. 4 Change is the resistance of the piezoresistor vs. the thickness and length of the piezoresistor when a 2 N/m surface stress is applied on thesurface of a SiO2 MCL [7]

Fig. 5 Schematic of a piezoresistive SiO2 MCL [8]

Fig. 6 The layout of a Wheatstone bridge design of piezoresistiveMCLs with stress concentrating holes [5]

a cavity underneath. Two thin piezoresistive clamped siliconbeams were released on the MCL surface. The new piezo-resistive design showed an increase in the relative resistancechange by a factor of 5.2, as shown in Fig. 8, compared to anMCL with the same thickness and conventional piezoresistordesign.

Fig. 7 Stress distribution vs. vertical displacements of MCLs with andwithout holes [5]

2.4 Integration with piezoactuators

Piezoresistive MCL can also detect mass change by integra-tion with piezoactuators. Sone et al. [10] introduced an MCLdevice that consists of a piezoresistive design, a Wheatstonebridge circuit, a positive feedback controller, an excitingpiezoactuator, and a phase-locked loop (PLL) demodulator(Fig. 9). In the design, R1 represents the piezoresistive MCLsin the Wheatstone bridge. The oscillator includes a positivefeedback system, a preamplifier, and a piezoactuator driver.The phase locked loop circuit is used for demodulating thefrequency signals from the circuit. Results showed that amass sensitivity of 190 fg/Hz and a mass resolution of about500 fg were obtained in water. The mass sensitivity is 100times greater than that of a quartz crystal oscillation system.

2.5 Minimizing the thermal effect

One concern in the field of piezoresistive MCL sensors is thatthe sensor performance is readily influenced by the thermal

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Fig. 8 Left Comparison between conventional and stress-concentrating MCLs. Right SEM images of stress-concentrating beams isolated fromMCL base [9]

Fig. 9 A harmonic vibration mass biosensor system based on MCLs[10]

stress during operation. Yang et al. developed a double-MCLdesign to overcome the thermal stress effect which improvesthe sensitivity as shown in Fig. 10 [11]. The double MCL iscomposed of a top immobilized MCL and a bottom sensingMCL. These two MCLs are connected in a way that the biax-ial surface stress in the former can be converted to uniaxialstrain in the latter. The sensitivity of the double MCL couldbe increased by increasing the length ratio or by decreasingthe thickness ratio of the two MCLs (Fig. 11). More than twoorders of sensitivity enhancement have been achieved andthe induced thermal effects are minimized.

3 Capacitive readout scheme

A capacitive readout scheme consists of an MCL over anelectrode which is ground biased as shown in Fig. 12. Ina capacitive sensing scheme, an AC voltage of frequency

higher than that of the mechanical bandwidth of the MCLis applied. The output current measured with respect to theapplied voltage gives the capacitance value which is a mea-sure of the displacement of the MCL.

The capacitive current, IC(t), at the output [12] due to theMCL displacement is given as:

IC(t) = ∂

∂t(C · V ) ∼= Cp

∂VAC

∂t+ VDC

∂c(t)

∂t, (5)

where, C is the capacitance between the MCL beam and thedriver.

This capacitance consists of static component, Cp, and thecapacitance because of the MCL displacement, c(t). The firstterm in the above equation, Cp

∂VAC∂t , indicates the parasitic

current, and the second term, VDC∂c(t)∂t , indicates the current

due to the MCL oscillation.The main advantages of capacitive microsensors are

low-power, simple structures, low temperature sensitivity andlow cost integration of the read out to the portable systems.It outdraws peizoresistive systems because of its low powerconsumption. The drawback is that the bending amplitudeis limited by the space between the MCL and the bottomelectrode.

3.1 Size, shape, and materials of the MCL

Li et al. [13] developed an MCL that is actuated to resonanceby in-plane electrodes as shown in Fig. 13. The resonant fre-quency is detected by the capacitive approach with on-chipcircuitry. For resonance frequency measurement, the sensi-tivity improvement can be realized by reducing the MCLdimensions in order to increase stiffness and subsequent res-onant frequency.

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Fig. 10 a A conventional MCLsystem with only one MCL;b Double MCL design with oneMCL on the top and another atthe bottom [11]

Fig. 11 Surface stress sensitivity of double and single MCL systemswith respect to the length ratio [11]

Fig. 12 Basic schematic of a capacitive MCL [12]

However, for bending mode, higher sensitivity will beachieved with lower stiffness MCL and larger electrode area[14]. In this work, Lee et al. compared the sensitivity of threeMCL shapes: single beam, double beam, and microbridge asshown in Fig. 14.

The three sensors were tested in a closed chamber withhumidity level ranging from 45% Relative humidity (RH)to 95% RH. Figure 15 shows the single beam MCL sensorprovides a high degree of humidity sensitivity. Although the

Fig. 13 Circuit diagram of an electrostatically driven MCL to theresonant frequency shift [13]

results suggest that the micro-bridge type of humidity sensorprovides a lower sensitivity, it is a more robust device thanthe single beam MCL type.

3.2 Elimination of the parasitic effect

It is shown from Eq. 6 that the sensitivity of the capaci-tive MCL sensors improves as the parasitic capacitance isdecreased [12].

Vg = Ic

ωCpa

∼= Cp

Cp + CpaVac + 0.39

Cp

Cp + Cpa

Vdc

δ0A

= Vp + Vd, (6)

where, Vp is the parasitic voltage, which is due to the appliedAC voltage, Vd is corresponding to the MCL displacementand is proportional to both the DC voltage and the oscillationamplitude, Vg is the resulting voltage.

Furthermore, reducing the dimensions typically results inan increase of the mass responsibility and subsequent sensi-tivity. However, reducing the dimensions will also raise prob-lems due to low-level current and the parasitic capacitance.Electrostatic transduction in the nanometer-size regimerequires the minimization of the parasitic capacitance toincrease the sensitivity. Several CMOS circuitry [12,15,16]were designed and placed very close to the transducer struc-ture for such purpose to increase the sensitivity.

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Fig. 14 SEM images of the fabricated: a Single beam MCL; b Double beam MCL; c Microbridge [14]

Fig. 15 Temperature effect on sensitivity of the three sensors [14]

One CMOS circuitry to eliminate the parasitic capacitanceinduced by the external bonding pads and wires [12] is shownin Fig. 16. This CMOS circuitry is integrated with an MCLby using a monolithic technology that combines a standardsCMOS processes and a nanofabrication method. The inte-grated system demonstrated a high sensitivity (10−18g) anda high spatial resolution (300 nm) and also had less noisecompared to resistive and transimpedance methods.

Ghatnekar-Nilson et al. [16] used electron beam lithogra-phy to define the resonator system comprising of an MCL, anactuator, and a capacitive readout circuit on a CMOS circuit(Fig. 17). This has improved the MCL sensitivity by reduc-ing the effects caused by parasitic capacitance. An MCL of420 nm in width, 20µm in length, and 600 nm thickness isfabricated on the pre-processed CMOS chip. The mass sen-sitivity is 17 ag per Hz.

Davis et al. [17] shows, by reducing the size of MCLto nanometer scale, the Q-factor has dramatically increasedfrom 50 to 100 in air and from 20 to 30,000 in vacuum.This system has shown a reduction in parasitic capacitanceto approximately 3fF, which is sufficiently low for the capac-itive readout system to function.

4 Piezoelectric readout technique

Another recently booming technique to measure the MCLdeflection or its resonant frequency change is the piezoelec-tric design. Piezoelectricity is the property of a material inwhich a potential is developed across a material when it issubjected to strain [18]. A piezoelectric MCL can be usedfor actuation as well as sensing.

The piezoelectric materials, such as ZnO, are usuallydeposited at the fixed ends of an MCL as shown in Fig. 18.The deposition of the piezoelectric films onto the MCL iscarried out by several methods which include metal organicchemical vapor deposition (MOCVD) [19], hydrothermalmethod [20], sputtering [21], pulsed laser deposition [22],and Sol-gel method [23]. Among these methods, sol-geldeposition method is considered to be most cost-effectiveand convenient. The formation of a highly homogenous pie-zoelectric layer and its composition can be readily controlled.

The factors that determine the sensitivity of the piezoelec-tric MCLs are discussed below.

4.1 Piezoelectric materials

Materials with higher piezoelectric constant normally resultin higher sensitivity. A wide variety of piezoelectric mate-rials can be chosen for fabricating piezoelectric MCLs. Themost commonly used materials are ZnO and Lead Zirco-nium Titanate (LZT, also known as PZT) [18–24]. PZT hasa higher piezoelectric constant than ZnO, thus the sensitivityof the MCL is improved when PZT is used as the piezoelec-tric material for the force sensor, instead of ZnO [25]. It isalso believed that this PZT sensor is easy to use in severeenvironments such as low temperature and ultrahigh vac-uum. The orientation of these layers also plays a pivotal rolein determining its sensitivity [26]. A PZT piezoelectric layerwith an orientation of <100> is used because its piezoelec-tric constant is 1.7 times of that of <111> orientation. The

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Fig. 16 Left Schematic drawing of the NEMS system based on a lat-erally vibrating MCL (s direction) electrostatically excited and withcapacitive readout. The structural layer for the MCL is one of the poly-silicon layers used in CMOS technology. Right Optical images of thenanomechanical resonator integrated monolithically with the CMOS

voltage amplifier. The large image shows the CMOS circuit and thearea where the nanomechanical device is fabricated. The inset ia a zoomimage of it, where it can be better appreciated the MCL, the driver andthe integrated capacitor for proper circuit polarization [12]

Fig. 17 Optical images ofcombined EBL and DWL [16]

thickness of the film and roughness of the surface also playsa role in determining the piezoelectric constant [27] and thesubsequent sensitivity.

4.2 MCL geometry

Piezoelectric MCL dimensions play a role in determiningthe piezoelectric constant and resonance frequency which inturn determines the sensitivity of the MCL. Yi et al. inves-tigated the effect of piezoelectric MCL’s length, width andresonance mode on its sensitivity by determining the reso-nant frequency shift per mass change (� f/�m) [18]. Thetip of the piezoelectric unimorph MCL (made up of PZTand stainless steel layer) was loaded with point mass. Theresulting resonance frequency change was determined and

compared experimentally and theoretically. Figure 19 showsthe � f/�m is larger when the MCL dimensions are smaller.The influence of the MCL’s dimensions and the material’sproperty on the resonance frequency shift, � f , with respectto the loaded mass, �m is given by Eq. 7:

� fn/�m = (v2n/4π)(1/L3w)(1/0.236

√12ρ)

√E/ρ, (7)

where, the � f/�m is directly proportional to eigen valueυ2

n and inversely proportional to length, L , and width, w;E is the effective Young’s modulus and ρ is the effectivedensity of the unimorph MCL. By keeping the MCL thick-ness, Young’s modulus and density constant and decreasingthe MCL length and width by a factor of α, � f/�m can beincreased by a factor of α4.

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Fig. 18 Schematic of a typical piezoelectric MCL [18]

Fig. 19 � f/�m versus length for all the modes and widths [18]

Figure 20 shows that for the same MCL width the res-onance frequency increases as the MCL length increases.This was proved both theoretically (solid lines) and experi-mentally (symbols). The effect of the width on the frequencyshift (Fig. 20 right) shows the frequency shift is larger when

Fig. 21 Experimental and theoretical � f with respect to �m of theMCL with 1.03 cm in length, 0.4 cm in width at mode 1 (squares) andmode 2 (circles) [18]

the MCL is narrower. The influence of the resonance mode ofthe MCL on its sensitivity (Fig. 21) shows that the frequencyshift is larger for the mode 2 operating MCL than the mode1 with respect to the same mass loading.

4.3 Substrate effect on sensitivity

The electrical properties of one piezoelectric material dependmainly on the crystallographic orientation of the material.One factor affecting the sensitivity of the piezoelectric MCLsis the substrate on which the piezoelectric layer is deposited.Wang et al. [24] investigated the sensitivity of PZT thin filmson Pt/Ti and Ir/IrO2 substrates and the X-ray diffractometer(XRD) results showed that the PZT film fabricated on Pt/Tisubstrates has a highly preferred orientation in the directionof <100> and no preferred orientation on the Ir/IrO2 sub-strate. This is because Ir undergoes oxidation during anneal-ing and the PZT deposition on the randomly oriented IrO2

does not produce the <100> orientation. The PZT film wascrystallized with columnar grains on the Pt/Ti substrate andwithout columnar grains on Ir/IrO2 and the PZT layer on thePt/Ti surface was smoother and more uniform than that on theIr/IrO2 (Fig. 22). The PZT deposited on the Pt/Ti substrate

Fig. 20 Left Experimental andtheoretical � f with respect to�m of the MCL with, Leftvarying lengths at a fixed 0.4 cmwidth, Right length 1.05 cm,width 0.4 cm (squares) andlength 0.99 cm, width 0.2 cm(circles) [18]

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Fig. 22 SEM images of PZTfilms deposited on (left) Pt/Tiand (right) Ir/IrO2 [24]

Fig. 23 Charge change versus displacement before and after poling[18]

has a higher piezoelectric constant than that on the Ir/IrO2

substrate.Further studies showed that Pb diffused through the Pt/Ti

layer. On the other hand IrO2 has a very good barrier effect.So, Pt/IrO2 is a suitable substrate to achieve higher piezo-electric constant and less defective microstructures.

4.4 Improving sensitivity by poling

Dong et al. developed [28] a new fabrication techniquewherein the PZT layer is subjected to poling. The PZT layeris polarized at 100◦C with an electric field of 125 kV/cm forone hour. The MCL sensibility was 1.61 pC/mm before polingand rose to 3.25pC/mm after poling (Fig. 23). This sensitiv-ity increase was attributed to the increase in the piezoelectricconstant of the PZT layer upon poling.

4.5 Optimized positioning of the piezoelectric layer

Du et al. [20] fabricated a piezoelectric MCL sensor thatcontains two PZT layers (Fig. 24). The PZT layers were syn-thesized by hydrothermal method and placed on a titanium

Fig. 24 Schematic of an MCL sensor unit with two PZT layers [20]

Fig. 25 Optimized positioning of the PZT layer for maximum sensi-tivity. L is the MCL length, Lp is the length of the sensing and actuationlayer, and a is the length between the fixed end of the MCL and the leftedge of the actuation/sensing layer [20]

substrate. The top layer acted as an actuation layer and thebottom one acted as a sensing layer.

This work showed that the location of the actuation andsensing layer played a role in improving the MCL sensitivity.The MCL is most sensitive when the two ends of the actua-tion and sensing layers were placed at the points where thestrain is zero (Fig. 25).

4.6 Anti-cracking

The piezoelectric constant of the PZT film increases withthe thickness of the PZT layer but may result in cracks afterreaching certain limit due to the residual stress developed.The cracks will result in lower sensitivity. In a conventio-nal sol-gel process, the PZT films are patterned after the

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Fig. 26 Piezoelectric MCLfabricated with (left)conventional and (right)modified method [29]

Fig. 27 SEM image of thesurface undergoing (left) threeannealing cycles; (right) fourannealing cycles [23]

completion of the annealing process. In a modified processdeveloped by Xie et al. [29], patterning is done before eachpost-annealing process to remove unwanted PZT film.Figure 26 compared the PZT surfaces fabricated by theconventional method and the modified process.

Liu et al. also developed [23] a similar method to avoidcracks of the PZT layer by modifying the layer coating pro-cess (Fig. 27).

5 Conclusion

MCLs provide an ideal platform for chemical and biomolec-ular detection. We can foresee the upcoming of more MCLsensors. However, there is still a long way to go to con-vert the proof-of-concept results to robust, commercially-available products. This review focuses on enhancing thesensitivity of MCL sensors operating based on piezoresis-tive, capacitive, and piezoelectric principles by optimizing itsdimensions, choosing proper material, and adopting novelfabrication techniques. It is seen that these factors play asignificant role in the MCL sensitivity.

References

1. Chen, G.Y., Warmack, R.J., Thundat, T., Allison, D.P., Huang, A.:Resonance response of scanning force microscopy cantilevers.Rev. Sci. Instrum. 65, 2532 (1994)

2. Gimzewski, J.K., Gerber, C., Meyer, E., Schlittler, R.R.:Observation of a chemical reaction using a micromechanical sen-sor. Chem. Phys. Lett. 217, 589 (1994)

3. Campbell, G.A., Mutharasan, R.: Escherichia coli O157:H7 detec-tion limit of millimeter-sized PZT cantilever sensors is 700Cells/mL. Anal. Sci. 21, 355 (2005)

4. Stoney, G.G.: The tension of metallic films deposited by electrol-ysis. Proc. R. Soc. (Lond.) 82, 172 (1909)

5. Yu, X., Tang, Q., Zhang, H., Li, T., Wang, W.: Design of high-sensitivity cantilever and its monolithic integration with CMOScrcuits. IEEE Sens. J. 7, 489–495 (2007)

6. Tortonese, M., Yamada, H., Barrett, R.C., Quate, C.F.: Atomicforce microscopy using piezoresistive cantilever. Solid State Sens.Actuators 448–451 (1991)

7. Chivukula, V., Wang, M., Ji, H.F., Khaliq, A., Fang, J.,Varahramyan, K.: Simulation of SiO2-based piezoresistiveMCLs. Sens. Actuators A 125, 526–533 (2006)

8. Li, P., Li, X.: A single-sided micromachined piezoresistive SiO2cantilever sensor for ultra-sensitive detection of gaseous chemi-cals. J. Micromech. Microeng. 16, 2539–2546 (2006)

9. Naeli, K., Brand, O.: Cantilever sensor with stress-concentratingpiezoresistor design, sensors, 2005 IEEE, pp. 592–595 (2005)

10. Sone, H., Ikeuchi, A., Izumi, T., Okano, H., Hosaka, S.: Femtogrammass biosensor using self-sensing cantilever for allergy check. Jpn.J. Appl. Phys. 45, 2301–2304 (2006)

11. Yang, S.M., Yin, T.L., Chang, C.: Development of a double-MCLfor surface stress measurement in microsensors. Sens. ActuatorsB 121, 545–551 (2007)

12. Verd, J., Abadal, G., Teva, J., Gaudo, M.V., Uranga, A., Borrise, X.,Campabadal, F., Esteve, J., Costa, E.F., Perez-Murano, F., Davis,Z.J., Forsen, E., Boisen, A., Barniol, N.: Design, fabrication, andcharacterization of a submicroelectromechanical resonator withmonolithically integrated CMOS readout circuit. J. Microelectro-mech. Syst. 14, 508–519 (2005)

123


13. Li, Y.C., Ho, M.H., Hung, S.J., Chen, M.H., Lu, M.S.C.: CMOSmicromachined capacitive cantilevers for mass sensing. J. Micro-mech. Microeng. 16, 2659–2665 (2006)

14. Lee, C.Y., Lee, G.-B.: Micromachine based humidity sensorswith integrated temperature sensors for signal drift compensation.J. Micromech. Microeng. 13, 620–627 (2003)

15. Teva, J., Abadal, G., Torres, F., Verd, J., Pérez-Murano, F.,Barniol, N.: A femtogram resolution mass sensor platform, basedon SOI electrostatically driven resonant cantilever. Part I: Elec-tromechanical model and parameter extraction. Ultramicroscopy106, 800–807 (2006)

16. Ghatnekar-Nilsson1, S., Fors’en, E., Abadal, G., Verd, J.,Campabadal, F., Pérez-Murano, F., Esteve, J., Barniol, N., Boisen,A., Montelius, L.: Resonators with integrated CMOS circuitry formass sensing applications, fabricated by electron beam lithogra-phy. Nanotechnology 16, 98–102 (2005)

17. Davis, Z.J., Abadal, G., Helbo, B., Hansen, O., Campabadal, F.,Pérez-Murano, F., Esteve, J., Figueras, E., Verdb, J., Barniol, N.,Boisen, A.: Monolithic integration of mass sensing nano-canti-levers with CMOS circuitry. J. Sens. Actuators A 105, 311–319(2003)

18. Yi, J.W., Shih, W.Y., Shih, W.-H.: Effect of length, width, andmode on the mass detection sensitivity of piezoelectric unimorphcantilevers. J. Appl. Phys. 91, 1680–1686 (2002)

19. Shimizu, M., Okaniwa, M., Fujisawa, H., Niu, H.: Ferroelectricproperties of Pb(Zr, Ti)O3 thin films prepared by low-temperatureMOCVD using PbTiO3 seeds. J. Eur. Ceram. Soc. 24, 1625–1628(2004)

20. Du, L., Kwon, G., Arai, F., Fukuda, T., Itoigawa, K.,Tukahara, Y.: Structure design of micro touch sensor array. Sens.Actuators A 107, 7–13 (2003)

21. Thomas, R., Mochizuki, S., Mihara, T., Ishida, T.: Effect of sub-strate temperature on the crystallization of Pb(Zr, Ti)O3 films onPt/Ti/Si substrates prepared by radio frequency magnetron sputter-ing with a stoichiometric oxide target. Mater. Sci. Eng. B 95, 36–42(2002)

22. Husmann, A., Wesner, D.A., Schmidt, J., Klotzbucher, T.,Mergens, M., Kreutz, E.W.: Pulsed laser deposition of crystallinePZT thin films. Surf. Coat. Technol. 97, 420–425 (1997)

23. Liu, M., Wang, J., Wang, L., Cui, T.: Deposition and characteriza-tion of Pb(Zr, Ti)O3 sol-gel thin films for piezoelectric cantileverbeams. Smart Mater. Struct. 16, 93–99 (2007)

24. Wang, Z.J., Chu, J.R., Maeda, R., Kokawa, H.: Effect ofbottom electrodes on microstructures and electrical proper-ties of sol-gel derived Pb(Zr0.53 Ti0.47)O3 thin films. ThinSolid Films 416, 66–71 (2002)

25. Lee, C., Itoh, T., Sasaki, G., Suga, T.: Sol-gel derived PZT force sen-sor for scanning force microscopy. Mater. Chem. Phys. 44, 25–29(1996)

26. Gong, W., Li, J., Zhu, X., Gui, Z., Li, L.: Preparation and character-ization of sol-gel derived (100)-textured Pb (Zr, Ti) O3 thin films:PbO seeding role in the formation of preferential orientation. ActaMater. 52, 2787–2793 (2004)

27. Udayakumar, K.R., Schuele, P.J., Chen, J., Krupanidhi, S.B.,Cross, L.E.: Thickness-dependent electrical characteristics of leadzirconate titanate thin films. J. Appl. Phys. 77, 3981–3986 (1995)

28. Dong, W., Lu, X., Liu, M., Cui, Y., Wang, J.: Measurement onthe actuating and sensing capability of a PZT MCL. Meas. Sci.Technol. 18, 601–608 (2007)

29. Xie, J., Hub, M., Ling, S.-Fu, Dua, H.: Fabrication and charac-terization of piezoelectric cantilever for micro transducers. Sens.Actuators A 126, 182–186 (2006)

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