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Sensors and Actuators A 195 (2013) 198– 205

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators A: Physical

j ourna l ho me p age: www.elsev ier .com/ locate /sna

esign and implementation of electrostatic micro-actuators in ultrasonicrequency on a flexible substrate, PEN (polyethylene naphthalate)

. Kima,∗, X. Zhanga, R. Daughertya, E. Leea, G. Kunnena, D.R. Alleea, E. Forsytheb, J. Chaea

Department of Electrical, Computer and Energy Engineering, Arizona State University, 650 E.Tyler mall, Tempe, AZ, 85287, USAArmy Research Laboratory, 2800 Powder Mill Road, Adelphi, MD, 20783-1197, USA

r t i c l e i n f o

rticle history:vailable online 15 October 2012

a b s t r a c t

We present an electrostatic actuator fabricated on a flexible polyethylene naphthalate (PEN) substrate,

eywords:ltrasonic actuatorEMS actuator

lectrostatic actuator

which emits acoustic waves at ultrasonic frequencies. The MEMS actuator has a suspended parylenediaphragm which consists of 2–6 mm diameter and 6 �m gap between the diaphragm and substrate.Driving circuitry consists of voltage controlled oscillator (VCO) and output buffer chain, and was fab-ricated by Arizona State University’s Flexible Display Center-IC process. The fabricated actuator emitsultrasonic waves at 25 kHz, and acoustic sound pressure of 27 dB SPL (sound pressure level) driven bythe hydrogenated amorphous silicon (a-Si:H) circuitry.

. Introduction

MEMS (micro-electro-mechanical-systems) acoustic transduc-rs are attractive as they have a potential for economic, sizabletrength in fabrication [1–3]. Ultrasonic systems have broad appli-ations including medical imaging, structure inspection and others.n earlier work, piezoelectric and capacitive technologies wereypically used for MEMS ultrasonic transducers. Muralt et al. [4]escribed fabrication and characterization of piezoelectric micro-achined ultrasonic transducer (PMUT). They used a 2 �m thick

bZr0.53Ti0.47O3 thin film and their PMUT emits ultrasonic acous-ic waves up to 20 cm in air and 2 cm in the test liquid at2 kHz. Mina et al. [5] presented high frequency PMUT using abZr0.52Ti0.48O3 thin film. The operating frequency of their devices 30 MHz to 1 GHz and the PMUT was fabricated using 0.35 �mMOS technology. Aoyagi et al. [6] presented an array of capacitiveicromachined ultrasonic transducers (CMUTs) using 2 �m thick

arylene diaphragm as parylene features conformal deposition,iocompatibility, chemical resistibility, and CMOS compatibility.he emitted acoustic wave of 100s of kHz was detected up to 1 m.heng et al. [7] reported a high density and low parasitic arrayf CMUTs. Their CMUTs operated at 7.5 MHz with 2.76 pF para-itic capacitance. However, these prior arts were made on a rigidubstrate.

Flexible ultrasonic transducers can be used for non-destructivevaluation (NDE). Bowen et al. [8] presented a flexible piezoelec-ric transducer for ultrasonic inspection. Their transducer operates

∗ Corresponding author. Tel.: +1 480 433 5296.E-mail address: skim85@asu.edu (S. Kim).

924-4247/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.sna.2012.10.010

© 2012 Elsevier B.V. All rights reserved.

between 3.2 and 3.5 MHz using 40–100 VPP (peak to peak). Chatil-lon et al. [9] reported the flexible transducer for the inspection ofcomplex geometry components. The flexible transducer operatesat 2 MHz, and can evaluate complex defects at variable deflectionangles (0◦, 45◦ and 60◦). Harvey et al. [10] presented a flexiblecomposite element array transducer. The transducer has piezoelec-tric ceramic composite structure and operates from 3 to 6 MHz.These attractive features motivate our research: building MEMSultrasonic transducers on a flexible substrate [11].

We form MEMS ultrasonic actuators on a flexible material,polyethylene naphthalate (PEN), which is a type of polyester, com-monly used to form an oxygen barrier [12]. The goal is to developan MEMS-based flexible, low power, ultrasonic actuator, which canbe used potentially for equipment inspection.

Fig. 1 illustrates the schematic of MEMS actuators on PEN. Theactuator comprises two electrodes, where the top electrode isembedded in the parylene diaphragm. The diaphragm moves per-pendicular to the substrate by applying DC/AC voltage between thetwo electrodes. The air holes in the diaphragm allow them to moveair in and out of the cavity. We fabricate and test capacitive MEMSultrasonic actuators on PEN with integrated hydrogenated amor-phous silicon (a-Si:H) driving circuitry. The ultrasonic actuators arecharacterized for their output sound pressure level (SPL) as a func-tion of distance between the actuators and a commercial ultrasonicmicrophone in order to demonstrate their performance as wirelessacoustic wave generators.

This paper is organized as follows: Section 2 presents the

analytical design of MEMS ultrasonic actuators, Section 3 describesthe design of ASU (Arizona State University) FDC (Flexible DisplayCenter)-IC driving circuitry, Section 4 presents the fabricationdetails of MEMS ultrasonic actuators on a PEN substrate, and

S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205 199

F actuatw

SM

2

psslmtdma

pip

p

wtpub

I

wiwth1b

˛

SsaEdaBa

ig. 1. Schematic illustration of an MEMS ultrasonic actuator array; array of MEMS

ith an integrated driver circuitry.

ection 5 discusses the characterization of the fabricated flexibleEMS ultrasonic actuators with the driving circuitry.

. Ultrasonic actuator design

The ultrasonic acoustic wave generated by MEMS actuatorsropagates through a medium (air) and reaches a sensor; ultra-onic microphone. In order to maximize efficiency of the overallystem, the actuator must generate a sufficiently high SPL, displayow SPL degradation in the traversing medium, and the receiving

icrophone must have low equivalent SPL noise. As we focus onhe design of the MEMS actuator aimed at producing high SPL,esign of the associated microphone is not discussed as a com-ercial ultrasonic microphone is used to characterize the MEMS

ctuator.A single MEMS ultrasonic actuator may be considered as a

oint acoustic source. As a point source, the sound pressure (P) isnversely proportional to the distance between actuator and micro-hone, and may be expressed as [13]

= A

rejwt−kr (1)

here A is a constant for a given point source, r is the distance fromhe actuator to the microphone, and k is the wave number. Theropagating ultrasonic wave is absorbed by the medium. The atten-ation due to absorption of ultrasonic wave in air may be describedy [14]:

= I0e−ar (2)

here ̨ is the attenuation coefficient of air and I0 is the soundntensity near the actuator. I is the sound intensity at distance r,

hich is proportional to the sound pressure squared. The attenua-ion coefficient, ˛, is a function of temperature, pressure, frequency,umidity, and other parameters [14]. At room temperature and

atm pressure, the attenuation coefficient may be approximatedy [15]

= 1.856 × 10−13f 2 (3)

Using the acoustic energy loss described by Eqs. (1)–(3), thePL profiles of actuators having different initial SPLs at 50 kHz arehown in Fig. 2(a). From 1 cm to about a few meters, SPL decayst a rate of 20 dB/dec, which is dominated by the 1/r term inq. (1). Beyond this range, air absorption starts to dominate the

ecay and shortens the achievable maximum distance. Assuming

microphone with an equivalent SPL noise floor of 18 dB (Avisoftioacoustics CM16/CMPA), the maximum distance for a 50 dB actu-tor is approximately 4 cm, while an 80 dB actuator can reach up

ors on a flexible PEN (polyethylene naphthalate) substrate, which can be interfaced

to 7 m. Clearly, to achieve a longer distance, an actuator generat-ing higher SPL is needed. In addition to the high SPL actuator, it isimportant to choose a proper frequency range for specific applica-tions as the distance is a function of frequency as shown in Fig. 2(b).

When a pressure is applied on the diaphragm, the diaphragmdeforms. However, the deflection of diaphragm at each location isdifferent. We use the average deflection to estimate the dynamicsof diaphragm under excitation, which could be expressed by [16]

d = P

ω2 · � · h

(4(k2

1 + k22)J1(k1a/2)J1(k2a/2)

ak1k2(k2J0(k1a/2)J1(k2a/2) + k1J1(k1a/2)J0(k2a/2))− 1

)(4)

where d is the averaged deflection of the diaphragm, P is the pres-sure applied on the diaphragm, � is the density of diaphragm, ais the diameter of the diaphragm, ω is the frequency of excita-tion in radian/s, h is the thickness of the diaphragm, J0() and J1()are the zeroth and first order Bessel function of the first kind,respectively, and P is the amplitude of the total pressure appliedon the diaphragm. c, b, k1 and k2 are mechanical parameters of thediaphragm, defined as

C = (E + �)h2

12(1 − v2)and b = �

p(5)

k1 =√√

b2 + 4cω2 − b

2cand k2 =

√√b2 + 4cw2 + b

2c(6)

where E is the Young’s modulus of the diaphragm, � is the residualstress on the diaphragm, and � is the Poisson ratio. The pressureapplied on the diaphragm (P) consists of two elements: the pressuregenerated by electrostatic force and the pressure caused by squeezefilm damping.

The electrostatic force generated by applying voltage betweentwo parallel plates is

F = ε0�a2V2

8g2(7)

where ε0 is the air permittivity, V is the voltage applied betweenthe electrodes of the actuator and g is the distance/gap between thetwo electrodes, respectively. The voltage could be further decom-posed to include DC and AC contributions: V = VDC + VAC. In thiscase, the electrostatic force becomes F = (ε0�a2(V2

DC + 2VDCVAC +V2

AC))/8g2. Only the second term generates deflection at the samefrequency ω of VAC. As a result, the pressure applied on thediaphragm that generates diaphragm deflection at ω may be calcu-

lated by

Pac = ε0VDCVAC

g2(8)

200 S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205

Fig. 2. (a) Estimated distance vs. SPL (sound pressure level) of ultrasonic actuatorshaving 50–80 dB SPL at 1 cm (50 kHz) between the actuators and ultrasonic micro-phone. The maximum distance of a 50 dB actuator to reach is ∼4 cm, while an 80 dBactuator can reach up to 7 m, assuming equivalent noise of the microphone is 18 dB.(b) SPL profiles of different frequencies of 50, 100, and 150 kHz. SPL decays with fre-quency as the air absorption of sound increases with frequency around ∼100 kHz.(c) Estimated SPL as a function of the size of diaphragm; two different sets (gap dis-tance of 2 and 6 �m) and three different frequencies (50, 100, and 150 kHz). Largegta

ficeppi

P

wdr

t

Table 1Design specifications of the MEMS ultrasonic actuator.

Young’s modulus (E) 3.2 GPaResidue stress (�) 25 MPaPoisson ratio (�) 0.4Thickness of the diaphragm (h) 4 �mDiaphragm diameter (a) 3 mmInitial gap (g0) 6 �mAir-hole distance (rc) 200 �mRatio of air hole radius to rc (q) 0.16Collapse voltage (Vcollapse) 23.4 VAC input to excite the diaphragm (VAC) 20 VPP

ap helps SPL as it reduces the air dumping while requiring higher electrostatic forceo generate SPL at a given distance. Large diaphragm diameter helps in the first orders sound pressure is proportional to diaphragm area.

When the diaphragm moves, it squeezes the air inside the cavityormed by the two electrodes of the actuator and the counter effects that the air impedes the movement of the diaphragm, whichould be modeled as a pressure in the opposite direction to thexcitation. Introducing perforations on the diaphragm creates freeaths for air and reduces the squeeze film damping. As a result, theressure (Psd) due to squeeze film damping with air-holes in place

s estimated as [17]:

sd = 12 · � · r2c · ω

g3

(12

q2 − 18

q4 − 12

ln q − 38

)d (9)

here � is the viscosity of air, rc could be estimated as half the

istance between air-holes, and q is the ratio of air-hole radius toc.

Following from Eqs. (4)–(9), diaphragm deflection under exci-ation may then be modeled. Diaphragm deflection pressurizes the

Average deflection of the diaphragm (d) 3 nmSPL of single device at 1 cm (SPL) 66.5 dB

air in the front and generates acoustic wave/sound pressure, whichpropagates through the medium and is received by the ultrasonicmicrophone. The sound pressure generated by the movement ofdiaphragm may be analyzed by the piston model [12],

P = ω2�0da2

8r(10)

where P is the sound pressure at distance r and �0 is the den-sity of air. The expression excludes the directionality of the pistonmodel and considers only the maximum sound pressure along itssymmetry axis. As a result, these series of analytical equationsallow us to deduce how much SPL the actuator generates underelectrical excitation VAC, providing VDC has properly biased thediaphragm.

VDC is typically used to pre-stress the diaphragm. However, ifVDC is greater than a critical value, the diaphragm collapses. Thecollapse voltage (Vcollapse) of the diaphragm [18,19] is:

Vcollapse =√

32kg20

27ε0a2(11)

k = 4� · 16Eh3

3(1 − v2)a2(1 + a) and ̨ = 0.817(1 − v2)

�

3E

(a

2h

)2(12)

where g0 is the original distance between the two electrodes and kis the spring constant of the diaphragm. Typically, the diaphragmis biased at 80–90% of Vcollapse to achieve high electromechanicalcoupling. In our calculation, we assume a DC bias of 90% of theVcollapse is applied to achieve about 70% of coupling coefficient [20].

Design parameters of the fabricated MEMS actuators are listedin Table 1. Output SPL is not a strong function of the thickness of thediaphragm; SPL improves by only 4 dB as the thickness increasesfrom 2 �m to 8 �m. However, thicker diaphragms require higherDC voltage biasing to induce efficient electromechanical coupling.As a result, we select 4 �m as the thickness of the diaphragm tobalance SPL and DC voltage operation. In Fig. 2(c), actuators withvarious diameters and gaps generate SPLs at frequency from 50 kHzup to 150 kHz. The SPL is calculated at 1 cm away with a 90% Vcollapsefor VDC and 20 VPP for VAC. For the same device, higher frequencytends to deliver higher SPL. Nevertheless, choice of the operationfrequency is set by ASU FDC-IC specifications. At a given frequency,an actuator with larger diameter produces higher SPL. However,larger diaphragms are prone to fabrication failure, multi-modeexcitation and other non-idealities, which compromise the SPLimprovement gained by increasing diameter. The last parameterwe discuss here is the gap distance between electrodes. Although alarger gap helps to reduce the squeeze film damping and increaseSPL, it requires higher DC voltage to bias the diaphragm. Increasing

the gap from 4 to 6 �m improves SPL by about 10 dB, while increas-ing gap from 6 to 8 �m improves SPL by only 5 dB. Consequently,the initial gap between the two electrodes of the actuator is set at6 �m.

S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205 201

Fig. 3. Hydrogenated amorphous silicon TFT driver. (a) System block diagram of the TFT driving circuit. The ring oscillator (VCO) is used to generate oscillating voltageoutput at an ultrasonic frequency, which drives the MEMS actuators. The output of the VCO itself is not robust enough to directly drive the MEMS actuators. Addi-tionally, the finite parasitic resistance of the MEMS actuators requires that the output resistance of the driver be sufficiently low such that a significant portion of thedriving voltage drops across the load. To achieve this, this circuit introduces a low output impedance, self biasing output buffer which is capable of sustaining the elec-trical signal required to produce the necessary acoustic pressure at the output. (b) Photo of the fabricated driver; it consists of 5–7 stages ring oscillator with bufferchain. Tunable voltage range is 3–32 V with 35 V DC supply. Size is 3 mm × 3 mm. VDD is the power supply voltage for the buffers. The buffer chain includes the phaseb is the

c

3

Tttr

aiTsfmg

ilnwTcwmts

aDcricttrvoteal

owc

uffers and output buffers; VDD is omitted in the diagram to avoid clutter. VVCO

larity.

. Driving circuitry design

Fig. 3 shows the IC circuitry used to drive the MEMS actuators.he IC driver was fabricated by ASU FDC-IC process [21]. The a-Si:Hhin film transistor (TFT) driver consists of 5–7 stage voltage con-rolled ring oscillator (VCO) and buffer chain. The tunable voltageange of this driver is 3–32 V and size is 3 mm × 3 mm.

The VCO is made up of a chain of inverters connected in series;n odd number of inverters are used, and the output is fed backnto the input such that neither a high nor a low output is stable.he instability of the output coupled with the delay through eachtage causes sustained oscillations which have output swing andrequency dependent on the supply voltage. Due to the measure-

ent, the VCO is used for driving circuitry, and it allows the signaleneration to transistor based circuit.

The VCO in Fig. 3 is the primary signal generating portion of thentegrated circuitry. The basic operating principle involves estab-ishing a logical contradiction on each node; by placing an oddumber of inverters in a “ring” configuration, the input and output,hich are internally shorted, have a contradictory logical value.

his fact, coupled with the delay through each stage allows for thisircuit to achieve sustained oscillation. An alternate state exists,hich occurs with an output voltage between the minimum andaximum voltage supply, yet this state is unstable, and a small per-

urbation, such as noise, causes the circuit to enter its oscillatorytate.

Each inverter in the ring is coupled to ground through andditional current starving transistor. A current source or even aC voltage bias at the drain of the current mirror establishes aurrent through each of the current starving transistors. As the cur-ent through the inverters is restricted, the delay for each stagencreases, thus reducing the frequency and allowing for frequencyontrol. Additional frequency control can be achieved via varyinghe power supplied to the inverters by varying the voltage suppliedo the VCO (VVCO) [22]. The principal is the same as that of the cur-ent starving; however the output swing is limited by the supplyoltage. This effect is mitigated by the phase control buffers andutput buffers, both of which are rail-to-rail buffers, powered byhe IC power supply voltage (VDD). A chain of buffers, increasing inffective size from input to output, makes up the output buffer. Thisllows for the output of this circuit to drive a much larger capacitiveoad than the VCO output.

The driver was powered by VDC of 30 V and generates 25 kHzscillation output. The ultrasonic microphone reads ultrasonicaves from the actuators and the output of the microphone is

oupled to a spectrum analyzer.

variable supply voltage for the ring oscillator; VVCO is shown in the diagram for

4. Fabrication

The MEMS actuator is fabricated on a flexible PEN sub-strate. The bottom electrode sits on the substrate while the topelectrode is embedded in a suspended diaphragm. When elec-trostatic excitation is applied between the two electrodes, thediaphragm vibrates and generates acoustic pressure at ultra-sonic frequency. Air holes are designed to form air paths, whichreduce the air squeeze-film damping effect to achieve higherSPL for the actuator. Parylene is chosen as the diaphragmmaterial because it is flexible and non-brittle. It is compat-ible with CMOS [23] and could be uniformly deposited atroom temperature, which is favorable for ASU FDC-IC process[21].

The fabrication process flow of the MEMS ultrasonic actu-ator is illustrated in Fig. 4(a). A 150 �m thick PEN sheet wasmounted on a silicon substrate. A bottom metal layer of 200 nmaluminum was deposited with a sputter, which was subsequentlypatterned by lift-off. A sacrificial layer was patterned on top ofthe bottom aluminum using AZ9260 photoresist of 6 �m. Then,the first parylene layer of 0.25 �m was coated to serve as theisolation between bottom and top electrode. Next, the top alu-minum layer was deposited and patterned. To enhance adhesionbetween parylene and the top aluminum layer, the parylene surfacewas plasma-cleaned before aluminum deposition using an oxygenplasma tool (Harrick Plasma, PDC-001). The second parylene layerof 3.75 �m was then deposited. The two parylene layers and topaluminum form the diaphragm. The parylene layers were etchedto form the air holes, which are useful when removing the sacrifi-cial layer, by reactive-ion etching (RIE). After etching the parylenelayers, the PEN substrate was separated from the Si wafer and cutinto dies. Finally, the sacrificial layer was removed using acetone.Fig. 4(b) shows a fabricated die, having 8 devices and various hole-configuration. Devices were fabricated on the flexible substrate, sothey can be bent as shown in Fig. 4(c). Fig. 4(d) is a snap shot of 4 mmdiameter device which has two electrodes and several holes for airventing.

The fabricated device was mounted in a dual inline package(Spectrum Semiconductor Materials, Inc., HYB02415) with driv-ing circuitry as shown in Fig. 5. To compare the performanceof actuators on silicon vs. on PEN, MEMS actuators were alsofabricated on a silicon substrate. Both actuators were coupled

with driving circuitry. The IC driver and MEMS actuators weremounted using double-sided tape. Aluminum wire bonding and aconductive epoxy were used to form contacts to the driver andactuators.

202 S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205

F ral lal m diam

5

5u

boaC2ms

Fo

ig. 4. (a) Fabrication process of the MEMS ultrasonic actuator on a PEN: the structuayer. (b) A fabricated die (8 devices). (c) A die bent by hand. (d) Snap shot of a 4-m

. Result

.1. SPL characterization as a function of distance and frequencysing a signal generator

We measured SPL from two actuators (6-mm diameter silicon-ased and 3-mm diameter PEN-based actuators) as a functionf distance and frequency. The acoustic wave was measured by

commercially-available ultrasonic sensor (Avisoft-Bioacoustics

M16/CMPA). Both actuators were driven by VDC of 30 V and VAC of0 VPP at 50 kHz from a signal generator and a spectrum analyzereasured acoustic signal as shown in Fig. 6. The spectrum analyzer

howed results of dBm scale, so we should convert dBm to dB SPL.

ig. 5. Hybrid system: IC driver coupled to MEMS actuators on silicon (a) and on PEN

scillation.

yer is a 4 �m thick parylene/metal composite and photoresist is used as a sacrificialeter device having top/bottom electrodes and several air-venting holes on a PEN.

The dBm to dB SPL conversion is derived from calibration. In orderto perform the calibration, a commercial SPL meter, CM-130 fromGalaxy Audio, was used in conjunction with an ultrasonic micro-phone, CM16/CMPA from Avisoft Bioacoustics. The output of themicrophone is an AC voltage, which was then amplified and sentto a spectrum analyzer. The peak in the spectrum analyzer corre-sponds to SPL at a given frequency and was mapped with the valuesfrom the SPL meter.

The form of the conversion is

dB SPL(dBm, f ) = s(f ) × dBm + i(f ) and dB SPL(dBm, f )

= dBm + CdBm × f c (13)

(b) in DIP (dual inline package); the driver produced 3.7 VDC and 7 VAC at 25 kHz

S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205 203

F ctuatV , respP

wla

a

S

wipttc

a(ttabii2bbEa

Ftfmd

ig. 6. Emitted acoustic pressure in SPL from (a) silicon-based and (b) PEN-based aPP; the silicon-based and PEN-based actuators have 6 mm and 3 mm in diameterEN-based actuator can reach up to 2 m having −112.95 dB/dec.

here s(f) is the frequency dependent coefficient of the slope, calcu-ated as s(f) = 1 and i(f) is the coefficient of the y-intercept, calculateds i(f) = CdBm × fc, respectively.

After several calibrations, the empirical conversion between SPLnd dBm is:

PL(dB) = 1 × dBm + 64.96 × (fkHz)0.053 (14)

here fkHz is the operating frequency in kHz. This simple empir-cal relation shows that SPL is linearly proportional to the outputower at a given frequency. We first characterized output SPL fromhe MEMS actuators as a function of distance, frequency, and exci-ation voltage using a signal generator and DC power supply, thenharacterized the actuators driven by IC drivers.

At 1 m, the silicon-based and PEN-based actuators produce 60nd 35 dB SPL, respectively. The silicon-based actuator is larger6 mm in diameter) than the PEN-based actuator (3 mm in diame-er); yet the difference in SPL is fairly large. The maximum distanceo reach using the silicon-based and PEN-based actuators is 8 mnd 2 m, respectively. The acoustic amplitude decay of the silicon-ased actuator is −45.5 dB/dec and that of the PEN-based actuator

s −112.95 dB/dec, respectively. PEN-based device marks approx-mately 55% of SPL, at a distance of 1 m, of Si-based device. At

m, SPL of PEN-based device is much less: only 45% of SPL of Si-

ased device. Theoretically, the SPL of a diaphragm-based actuatorecomes higher as the size of the diaphragm increases. Based uponqs. (1)–(3), SPL of a 3 mm PEN actuator reaches 60, 42 and 37 dB SPLt 10 cm, 1 m, and 2 m, respectively. However, our measurements

ig. 7. The frequency response of (a) silicon-based and (b) PEN-based actuators. The maxhat of PEN-based actuator demonstrates up to ∼83 dB SPL at 200 kHz when supplied by VD

rom 10 kHz to 50 kHz, but above 50 kHz, it shows relatively constant. This is somewhatainly because our initial prediction is based upon 3 mm diameter and 4 �m thick dia

iameter and 4 �m thick diaphragm.

ors as a function of distance at 50 kHz when supplied by VDC of 30 V and VAC of 20ectively. The silicon-based actuator can reach up to 8 m having −45.5 dB/dec and

show 48, 32, and 24 dB SPL at 10 cm, 1 m, and 2 m, respectively. Thediscrepancy may be due to partial collapse of diaphragm, result-ing from stiction and stressed diaphragm. Also, when we estimatethe acoustic pressure, we assume ∼70% coupling coefficient fromeffective DC biasing. However, it was difficult to control the biasingof the diaphragm when we characterized the actuators, sometimesresulting in partial collapse of the diaphragm. This non-optimal DCbiasing becomes more problematic for the flexible substrate.

The frequency response of PEN-based actuator was character-ized at 1 cm distance from 10 kHz to 200 kHz using VDC of 30 Vand VAC of 20 VPP. Our calculation shows that the highest SPL isproduced at around 50 kHz when the actuator has a 3 mm diam-eter and 4 �m thick diaphragm, which is largely different fromwhat we have measured. The discrepancy between calculation andmeasurement is due to several issues, including stiction, stresseddiaphragm and non-optimal DC biasing. Especially, the partial stic-tion of diaphragm may impact significantly the produced acousticpressure. Our frequency response measurement is shown in Fig. 7.

According to Eqs. (4)–(12), the diaphragm presents no obvi-ous resonant peak due to the significant damping effect of the airbetween the diaphragm and substrate. Such a diaphragm responsehas a 180◦ phase shift with respect to the actuation. Thoughdiaphragm movement is reduced at higher frequency and has the

constant phase as shown in Eq. (4) the overall SPL is proportionalto frequency squared as expressed in Eq. (10). The measurementshows that the SPL is a function of frequency and increases slightlyabove 50 kHz, up to 200 kHz. Silicon-based actuator showed up to

imum SPL of silicon-based actuator reaches up to ∼100 dB SPL at 180 kHz whereasC of 30 V and VAC of 20 VPP. Both curves show that the measured SPL increases rapidly

different from our initial prediction (peak at ∼50 kHz) of the frequency responsephragm and the measured devices have 6 mm (silicon-based), 3 mm (PEN-based)

204 S. Kim et al. / Sensors and Actuators A 195 (2013) 198– 205

Fig. 8. Emitted acoustic pressure from (a) silicon-based and (b) PEN-based actuators in SPL as a function of distance and angle; the silicon-based and PEN-based actuatorshave 6 mm and 3 mm in diameter, respectively. Both actuators show significant directive acoustic responses.

F (SPL)

t ween tt

∼trs

5

uiautsstts

atm

5

FhHoVP

ig. 9. The hybrid system measurements. (a) Measured output sound pressure levelrend follows −18.54 dB/dec. (b) Measured output SPL as increasing the distance bethan that of the silicon-based data.

100 dB SPL at 180 kHz and PEN-based actuator demonstrated upo ∼83 dB SPL at 200 kHz. This suggests that the MEMS actuators caneach a longer distance if ASU FDC-IC can drive at higher frequencyuch as 100–200 kHz.

.2. Directionality of emitted acoustic waves

We characterized two actuators as a function of angle to theltrasonic microphone; again, the silicon-based actuator has 6 mm

n diameter and the PEN-based actuator has 3 mm in diameter. Bothctuators were driven by VDC of 30 V and VAC of 20 VPP at 50 kHz. Theltrasonic sensor was set to be angled to the actuators from −90◦

o 90◦ to measure the amplitude of emitted acoustic waves. Fig. 8hows the measured SPL as a function of distance and angle for theilicon-based and PEN-based actuators. Eccardt et al. [24] presentedhe history of micromachined transducer and described its acous-ical properties. Their work shows the directivity pattern of ultra-onic micromachined transducers, similar to what we measured.

The silicon-based actuator has larger lobes than PEN-based onend allowed detection of acoustic signals at ±90◦. The result showshat both types are very sensitive to the angle and the directivity is

ore sensitive at short distance.

.3. Hybrid system

For the hybrid system, MEMS actuators are interfaced with ASUDC-IC driving circuitry instead of a signal generator. The driveras output VDC of 11.98 V and VAC of 24 VPP when it is not loaded.

owever, when the driver is loaded with the MEMS actuators, itsutput substantially lowers to VDC of 7.8 V and VAC of 8 VPP andDC of 2.84 V and VAC of 2 VPP for 6-mm silicon-based and 3-mmEN-based actuators, respectively. This reduction is because the

as increasing the distance between the silicon-based actuator and microphone. Thehe PEN-based actuator and microphone. The slope is much steeper (−66.51 dB/dec)

MEMS actuators are not purely capacitive, and possess undesirablereal resistances. The silicon-based and PEN-based actuators have aparallel resistance of 1.3 M and 310 k, respectively.

The hybrid system using the silicon-based actuator and ultra-sonic microphone were mounted on a gauged rail to characterizethe output SPL along the distance between the actuator and micro-phone. Using a conversion formula between power and SPL, a plot ofthe distance vs. output SPL is generated as shown in Fig. 9(a). Theoutput SPL is −18.54 dB/dec slope that roughly follows the pre-dicted −20 dB/dec slope. The hybrid system using the PEN-basedactuator is also characterized. Fig. 9(b) shows the plot of the outputSPL by varying the distance of PEN-based actuator with IC driver. Itshows that output SPL slope is −66.51 dB/dec, which is significantlyworse than the predicted −20 dB/dec slope, which is probably dueto near field effect. Another source of discrepancy may be due tothe non-optimal DC biasing, as described in section 5.1.

6. Conclusion

An ultrasonic actuator on a flexible substrate has been success-fully implemented using MEMS technology. The hybrid systemwith PEN-based ultrasonic actuator emits ultrasonic acousticwaves effectively up to 3 cm when driven by a-Si:H driving cir-cuitry. Preliminary results show that small parallel resistance of100s k of the MEMS actuators load the IC driver to lower thedriving voltage. We plan to fine tune the fabrication process of theactuators to increase the resistance, which is expected to improvethe driving amplitude and consequently the working wireless

distance. If the MEMS actuators are implemented in a large arrayform, it is feasible to generate acoustic pressure waves up to 10 s ofmeters. Future work includes forming the array and monolithicallyintegrating the array with the driver on a PEN substrate. Moreover,

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e plan to fabricate a new In–Ga–Zn–O based IC driver as it hasigher mobility than that of a-Si:H based IC driver [25,26]. Theigher mobility can reinforce the discrepancy of MEMS actuator.

cknowledgements

The authors thank CSSER (Center for Solid-State Electronicsesearch) staff at Arizona State University. This work was sup-orted by Army Research Laboratory under Cooperative Agreement911NG-04-2-0005.

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Biographies

Sangpyeong Kim received the B.S. degree from the Department of Electrical andComputer Engineering from Ohio State University, Columbus, OH, in 2007, and M.S.degree in electrical engineering from Arizona State University, Tempe, AZ, in 2011.He is currently a Ph.D. candidate in Electrical Engineering at Arizona State Univer-sity. His research interests are MEMS biosensor and transducers including design,fabrication and analysis of microsensors and transmitters.

Xu Zhang received the B.S. degree in electronic engineering from Tsinghua Uni-versity, China, in 2006. He is currently a PhD candidate in the School of ElectricalComputer and Energy Engineering at Arizona State University. His research inter-ests include mixed-signal IC design, wireless sensing circuitry, interface circuitry forMEMS and micro-packaging.

Robin Daugherty is currently studying at the Fulton School of Electrical, Com-puter, and Energy Engineering at ASU. He will finish his master’s degree in electricalengineering in December 2012. His primary focus in academics is faculty assistedresearch. He has experience in circuit design, layout and integration as well as solidstate electronics, with interests extending into hardware prototyping and signalprocessing.

Edward H. Lee (S’11) is currently working toward the B.S.E.E. degree in electricalengineering from Arizona State University, Tempe. He is currently a researcher atthe Flexible Display Center at the ASU Research Park. He has been leading the design,computer-aided optimization, and experimentation of low noise integrated activepixel sensors on flexible substrates. He is a Goldwater scholar and is a member ofEta Kappa Nu.

George R. Kunnen received the B.S.E degree in electrical engineering from ArizonaState University, Tempe, in 2009. After 2009, he served a year of active duty in theMarine Corps subsequent to serving 4 years in the reserves. He is currently workingtoward the Ph.D. degree in electrical engineering at Arizona State University. He is atpresent a research scientist at the Flexible Display Center at ASU Research Park. Hehas been leading the design and testing of large area sensing arrays using amorphoussilicon and mixed oxide TFTs for neutron detection.

David R. Allee (BS in electrical engineering, University of Cincinnati, 1984; MS andPhD in electrical engineering, Stanford University, 1986 and 1990 respectively, post-doctoral fellow at Cambridge University, 1990–1991) is a professor of electricalengineering at Arizona State University. David is currently eirector of Research forBackplane Electronics for the Flexible Display Center (flexdisplay.asu.edu) fundedby the Army, and he is investigating a variety of flexible electronics applications. Hehas been a regular consultant with several semiconductor industries on low voltage,low power mixed signal CMOS circuit design. He has co-authored over 100 archivalscientific publications and U.S. patents.

Eric Forsythe is the Flexible Electronics Team Leader at the Army Research Labora-tory. Prior to ARL, he was a post-doctoral fellow at the University of Rochester, inboth the physics Department working with Kodak in organic light emitting diodetechnology. Dr. Forsythe received his Ph.D. in Physics from Stevens Institute of Tech-nology on silicon nanoparticle spectroscopy and device research. Dr. Forsythe isengaged in transitioning flexible display technology to the Army while develop-ing research areas in flexible electronics. He has coauthored more than 40 refereedpapers and (4) patents in the field of flexible electronics.

Junseok Chae received the B.S. degree in metallurgical engineering from Korea Uni-versity, Seoul, Korea, in 1998, and the M.S. and Ph.D. degrees in electrical engineeringand computer science from the University of Michigan, Ann Arbor, in 2000 and 2003,respectively. He joined Arizona State University, Tempe, in 2005, as an assistant pro-fessor, and he is currently an associate professor of electrical engineering. He haspublished more than 100 journal and conference articles, two book chapters, andone book, and he holds two U.S. patents. His areas of interest are microdevices for

bioenergy, implantable microdevices, and integrating MEMS with readout/controlelectronics. He was awarded the first place prize and the best paper award in theDesign Automation Conference (DAC) Student Design Contest in 2001. He is a recip-ient of National Science Foundation CAREER Award for an MEMS protein sensorarray.