JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 25, NO. 4, AUGUST 2016 691

Enhancement of the Transmission of PiezoelectricMicromachined Ultrasonic Transducer

With an Isolation TrenchMingjun Wang, Yufeng Zhou, Senior Member, IEEE, and Andrew Randles

Abstract— A new piezoelectric micromachined ultrasonictransducer (pMUT) with an isolation trench between cells wasproposed to improve the output pressure. A 2-D finite-elementmodel was utilized to evaluate and compare the performance ofthe conventional design with fully clamped boundary and thetrench design. It shows that the trench design can improve themembrane displacement or output pressure of pMUTs withouta significant change in the resonant frequency; 8 × 8 aluminumnitride (AlN)-based pMUTs arrays with fully clamped boundarydesign and the isolation trench were fabricated and characterized.An impulse response of the pMUTs array was first employedto determine the resonant frequency. A 200-cycle burst atthe resonant frequency was then delivered to pMUTs and theacoustic output pressure was measured by a hydrophone. Thetrench design could increase the output pressure by ∼76% witha shift of its center frequency by only 0.03 MHz. The nonlinearrelationship between pressure output and applied high voltagestill exists in the trench design. The presence of residual stressin the membrane, and substrate during fabrication was found tohave little impact on the displacement and resonant frequencyof pMUT. In summary, the presence of isolation trench canreduce the deflection-induced tensile stress on the edge ofthe membrane and subsequently improve the performanceof pMUTs. [2016-0001]

Index Terms— Piezoelectric micromachined ultrasonictransducer (pMUT), isolation trench, output pressure, resonantfrequency, aluminum nitride, residual stress.

I. INTRODUCTION

BULK PIEZOELECTRIC ceramic materials have beenused widely in the technology of conventional ultrasonic

transducers which suffer from poor acoustic coupling anda high cost of the two-dimensional (2D) transducer array.In comparison, micromachined ultrasonic transducers (MUTs)have compliant membrane structures using integrated cir-cuit (IC) manufacturing technology which have advantages of

Manuscript received January 6, 2016; revised February 24, 2016; acceptedMarch 11, 2016. Date of publication June 20, 2016; date of current ver-sion July 29, 2016. This work was supported by the Agency for Science,Technology, and Research, Singapore, through the Biomedical EngineeringProgramme under Grant 103 149 0006. Subject Editor G. Stemme.

M. Wang and Y. Zhou are with the School of Mechanical and AerospaceEngineering, Nanyang Technological University, Singapore 639798 (e-mail:wang0683@e.ntu.edu.sg; yfzhou@ntu.edu.sg).

A. Randles is with the Institute of Microelectronics, Agency for Science,Technology, and Research, Singapore 117685 (e-mail: randlesab@ime-a-star.edu.sg).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JMEMS.2016.2577038

the reduced acoustic impedance, improved bandwidth, easyfabrication of large arrays with compact designs, complemen-tary metal-oxide semiconductor (CMOS) compatibility, andthe integration with signal processing and other supportingelectronics [1]. They are especially useful in the micro-scale application, such as intravascular ultrasound (IVUS) andintracardiac echocardiography (ICE) [2], [3].

Piezoelectric micromachined ultrasonic transduc-ers (pMUTs) based on the flexural plate mode vibrationhave several potential advantages over its counterpartcapacitive micromachined ultrasonic transducers (cMUTs)and arise as a promising device for medical imaging,nondestructive evaluation, and proximity detection [4], [5].Unlike cMUTs, which have an inevitable small air gap in thestructure and subsequently challenges in fabrication, pMUTshave a more robust and easier fabrication process because oftheir simple structure and independent sensitivity on the gap.As the total displacement of pMUTs is the result of the strainmismatch between the piezoelectric actuating membraneand the passive supporting layers and not constrained by anarrow gap as that in cMUTs, pMUTs are capable of highpower acoustic transmission [6]. No requirement of high DCpolarization voltage (>100 V) in pMUTs results in fewergeometric, electronic, and design constraints and facilitatesintegration with low voltage electronics [4].

Up to date, several strategies have been employed inthe design of pMUTs in order to maximize the membranedisplacement and subsequently a large acoustic output. Onemethod is to modify the initial conditions of the piezoelectricmembrane, such as producing an initial deflection to themembrane by a DC bias [7], [8], fabricating the membranein a shape of dome [9], [10], and applying a static pressureto it [11]. However, adding a DC voltage as cMUTs willcomplicate the driving circuits and shift the resonant frequencybecause the DC bias changes the polarization and stress ofthe diaphragm [12]; fabrication process for the dome-shapedmembrane is not IC compatible; and pressurization of thediaphragm restricts it in the airborne application. Anotherstrategy is to reduce the stress largely on the edge of themembrane, which is due to both residual stress during manu-facture and deflection-induced tensile stress occurring in largemembrane deflection. The residual stress formed in the processof manufacturing is inherent in the thin film structures asa result of the thermal-mechanical fabrication process. The

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plasma-enhanced chemical vapor deposition (PECVD) SiO2has an average residual stress from −100 MPa (compressive)to 150 MPa (tensile). A relatively thin (∼2 μm) membrane inthe pMUT has a relatively high sensitivity to residual stressin the SiO2 and piezoelectric layers, which has a significantinfluence on the resonant frequency in air and may causedifficulties in the design process [13], [14]. The variation in thenatural frequency across a single die is about 3-4% due to thestress variation, and the actual value could be about 10% ona typical die [15]. The significantly lower coupling efficiencybetween electrical and mechanical domain than the predictionfrom analytical or finite element models is mainly attributedto the residual stress of the fabricated devices because theresidual stress may hamper the membrane vibration [15]–[18].

In the conventional pMUT design, cells have fully clampedboundaries, which show an undesired reduction in dynamicrange due to nonlinearity at high driving voltages [15].Although the clamped design attains high displacement atlow voltages, it rapidly exhibits nonlinear stiffness due to thepresence of deflection-induced tensile stress, which is a com-mon phenomenon observed in clamped-clamped MEMS struc-tures. A partially clamped structure with a simply-supportedboundary could partially free the membrane edge for stressrelief [19]. Flexurally-suspended design, membrane being sup-ported by three flexures and a thin oxide layer, has a piston-likedisplacement that results in a higher sound pressure outputby 6 dB, extremely linear voltage-displacement characteris-tics, increased dynamic range, nearly uniform displacementacross the membrane surface, great reduction of the deflection-induced tensile stress, and decrease of resonant frequency from169 kHz to 121.3 kHz [19]. However, these two pMUTs arerestricted to the airborne applications because of the open-uptrench on the membrane and no water-proof for the vacuumcavity. Furthermore, the open-up greatly decreases the resonantfrequency as the effective supporting area has been reduced tothat of flexures and the thin supporting layer.

In this study, a new pMUT configuration was proposedin order to increase the output pressure. An isolating trenchwas introduced between pMUT cells, which could partiallyreduce the tensile stress at the edge of the membrane, achiev-ing a simply-supported plate movement while maintaining ahigh resonant frequency. The performance of the aluminumnitride (AlN) pMUT array without and with the isolationtrench was simulated by finite element method (FEM) andcompared with each other for the membrane displacement,acoustic pressure output, resonant frequency, and transientresponse to the excitation. 8×8 pMUT arrays were fabricatedusing micro-electro-mechanical system (MEMS) technologyand then characterized by a calibrated hydrophone. It is foundthat the new pMUT design can improve the output pressure byabout 76% whilst without affecting the other characteristics ofthe pMUTs significantly, such as the resonant frequency andresponse time to the excitation.

II. DESIGN OF pMUT AND FEM SIMULATION

The structure of the pMUT was illustrated in Fig. 1, andmaterial properties and cell dimensions are listed in Table I.In the trench design, a circular trench with a depth of 6 μm

Fig. 1. (a) The 3D schematic diagram and (b) cross-section of pMUT usedin the finite element simulation with an isolating trench between cells in thenewly proposed design.

TABLE I

MATERIALS AND SIZE OF pMUTs IN THE DESIGN AND SIMULATION

and width of 10 μm is located 5 μm away from the edge ofthe cavity. The performance of pMUT arrays was simulatedand compared with each other (without and with the isola-tion trench) using commercial FEM software (COMSOL 4.4,Burlington, MA, USA). A 2-D model with three cells wasemployed in the simulation. To couple two physics occurringin the pMUTs activation, piezoelectric effect and acousticradiation, the acceleration at the surface of the membranewas used as the initial condition for the acoustic radiationmeanwhile the loadings from the acoustic field were set as theboundary condition for the piezoelectric membrane. The soundhard boundary was specified along the edge of the pMUTmembrane as a rigid baffle, and a semi-circular radiationboundary was used to avoid the wave reflection. Driving signalwas applied to the top surface of the piezoelectric layer whilstthe bottom electrode was grounded.

WANG et al.: ENHANCEMENT OF THE TRANSMISSION OF pMUT WITH AN ISOLATION TRENCH 693

Fig. 2. Comparison of (a) resonant frequency, (b) the displacement of themembrane of pMUTs with the fully clamped boundary and an isolating trenchbetween cells, and (c) the deformed edge of the membrane at the maximumdeflection around the trench at their resonant frequencies excited at the voltageof 10 V.

To evaluate the performance of an actuator, resonant fre-quency, average membrane displacement, and output pressureare usually required. Firstly, the resonant frequency of theconventional and isolated pMUTs is found to be 2.43 MHzand 2.25 MHz with a Q factor of 14 and 16, respectively(see Fig. 2(a)). The slight alteration of the resonant fre-quency may be due to the reduced effective stiffness ofthe membrane induced by the isolation trench. Secondly,the average membrane displacement over the surface of cell

Fig. 3. Simulated pressures emitted from the central cell (solid line) andthe first side cell (dash line) in a pMUT with (a) a fully clamped boundaryand (b) an isolating trench between cells when the central cell is excited atthe resonant frequency and at the voltage of 10 V.

with the fully clamped boundary is found to increase from12.8 nm to 20.6 nm (61% improvement) with the presenceof an isolation trench at the resonant frequency at the appliedvoltage of 10 V, which may be due to a reduction of thedeflection-induced tensile stress at the edge of the mem-brane for both actuating and supporting layers and a releasedboundary edge of the passive supporting layer. The totaldisplacement of the membrane in the radial direction could befit by a Gaussian curve (see Fig. 2(b)). In contrast to the fullyclamped boundary, the cell with an isolation trench embraces afree edge without dragging from the adjacent cells. Therefore,it can be bent more than the fully clamped membrane, thevertical edge of membrane titling from 90° to 76.7° and thebottom of the trench also titling slightly (∼2°) (see Fig. 2(c)).The displacement at the edge and the tensile stress inducedby the membrane deflection of the pMUT cell without andwith isolation trench are 0.8 nm and 165.3 MPa, and 1.6 nmand 0.1 MPa, respectively. The simulation results show thatvibration induced stress is on the similar order as that ofresidual stress despite its maximum value at the edge. In addi-tion, the first side pMUT cell was found to have the sameresonant frequency but reduced displacement (7.7 nm and12.2 nm for conventional and isolated pMUT, respectively).Thirdly, a transient study was performed by applying a Han-ning windowed pulse (8 cycles which are sufficiently long toachieve the maximum vibration) at the resonant frequency tothe center element of these two configurations. It is foundthat the average pressure at the interface of the membrane andwater with an isolation trench is larger than that of the fullyclamped cell (136.6 kPa vs. 100.4 kPa in Fig. 3). However, the

694 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 25, NO. 4, AUGUST 2016

Fig. 4. The effect of trench depth (fixed trench width of 10 μm) and trenchwidth (fixed trench depth of 6 μm) on (a) the output pressure, (b) resonancefrequency and bandwidth of pMUTs with isolation trench between cells atthe voltage of 10 V.

cross-talk between neighboring cells of both designs is similar(i.e., ratio of the maximum pressure output from the centercell to that of the first side cell: 53.9% vs. 58.6%), and italso takes similar time to eliminate the membrane vibrationafter the termination of excitation (similar exponential decayof waveform envelope).

The effect of the dimension of isolation trench on the outputof pMUT was also investigated (see Fig. 4a). It is found thatwith the increase of trench depth to 8 μm the output pressureincreases to about 165 kPa. However, further increase of trenchdepth will not result in more improvement, which may bebecause that the soft supporting post causes the significantlynon-uniform movement of the membrane. In comparison,the increase of trench width leads to monotonous and sig-nificant improvement. The change in resonance frequencyand bandwidth is initially more sensitive to the trenchdepth (0–11 μm), but then to the trench width (11–18 μm,see Fig. 4b). The decrease in the resonant frequency(0.43 MHz for 18 μm decrease in the trench width) is largerthan that in the bandwidth (0.07 MHz).

In the aforementioned investigation, the residual stress inthe membrane of pMUT was not taken into account. How-ever, a high temperature involved in the pMUT fabricationprocess results in the residual thermal stress built up. Theresidual stress of AlN film after deposition and its influenceon the performance of the trench-isolated pMUT have been

Fig. 5. Comparison of (a) resonant frequency response of pMUT,and (b) sensitivity of the output in response to the residual stress in theMo layer, and (c) silicon oxide layer with and without isolation trench.

studied [20]. The residual stresses for both conventional andisolated pMUTs are assumed the same (470 MPa, 0 MPa,0 MPa, and −200 MPa in Mo, AlN, Si, and SiO2, respectively)because of their same fabrication process except the laststep of adding the isolation trench and from [21], whichutilized a similar structure and dimension as ours. The residualstress was included in the FEM model by adding an initialstress into the material model, and the static deformation wasused as the initial conditions for the subsequent frequencyresponse analysis. At an excitation voltage of 10 V AC, pMUTwith the residual stress shows the almost same displacement(12.97 nm) and resonant frequency (2.42 MHz in Fig. 5(a)).The presence of isolation trench will also slightly increasethe displacement to 21.26 nm and decrease the resonancefrequency to 2.24 MHz, which may be due to the small initialdeflection induced by the residual stress (22.9 nm), whichis mostly from the Mo layer (22.67 nm) rather than from

WANG et al.: ENHANCEMENT OF THE TRANSMISSION OF pMUT WITH AN ISOLATION TRENCH 695

Fig. 6. Fabrication process of the AlN-based pMUTs with an isolating trench between cells. (a) patterning Mo on a hard disk of SiO2, (b) etching ofthe AlN, (c) deposition and patterning of Al, (d) deposition and patterning of a 0.3 μm layer of SiO2 to etch the AlN/Mo stack, (e) etching the oxide layeragain by XeF2, and (f) completely releasing the device.

Fig. 7. The image of 8 × 8 pMUT arrays (a) in a conventional design with fully clamped boundary and (b) with an isolating trench between cells.

the SiO2 layer, and large radius of curvature of the largemembrane (∼50 mm). To further investigate the effect of theresidual stress on the membrane displacement, the values ofresidual stressed in the silicon oxide layer and the electrodelayer were varied from 470 MPa and −200 MPa to 0 MPa(see Fig. 5(b) and (c)). The little influence was found forboth conventional and isolated pMUTs, only about 2% byvarying the residual stress. However, the improvement ofoutput pressure by the introduction of isolation trench is quiteconsistent no matter of the residual stresses in the pMUT.

III. FABRICATION OF THE pMUTs ARRAY

To validate the trench design, AlN-based pMUTs arrayswith a circular membrane were fabricated with either fullyclamped boundary or the isolation trenches in the Insti-tute of Microelectronics, Agency for Science, Technologyand Research (A∗STAR), Singapore on its multiple-projectswafer (MPW) platform. The fabrication process started withan 8-inch custom-fabricated cavity silicon-on-insulator (SOI)wafer of 1 μm buried oxide (BOX) layer and a primequality silicon wafer. The cavities with a depth of 20 μmwere first made on the wafer using plasma etching as the

locations of pMUTs cells. The silicon wafer was chemicallyand mechanically polished (CMP) on the front side, and thealignment marks were put on its backside. The cavity waferwas then bonded to the polished silicon wafer in the vacuum,followed by a grinding and polishing process to form a 5 μmlayer. Then a stack of 0.2 μm molybdenum (Mo), 1 μm AlN,0.2 μm Mo, and 0.3 μm SiO2 was deposited on it consecu-tively. The top Mo was patterned using SiO2 as a hard maskto define the electrodes on the top of the piezoelectric materialfor the excitation of the acoustic wave (see Fig. 6(a)). Then,a 0.4 μm layer of SiO2 was deposited and patterned throughreactive ion etching (RIE). This oxide was used as a hardmask to etch the AlN and open the bottom and top Mo-to-topmetal via (see Fig. 6(b)). A 0.7 μm of aluminum (Al) wasdeposited and patterned for wire connection (see Fig. 6(c)).Subsequently, a 0.3 μm layer of SiO2 was deposited andpatterned as a hard-mask to etch the AlN/Mo stack down to themembrane silicon layer in shaping the actuating piezoelectriclayer (see Fig. 6(d)). Afterwards, the same oxide layer wasetched again by XeF2 to open the top metal-to-pad via(see Fig. 6(e)). The final step was to complete the devicerelease by etching the silicon and oxide to achieve the trenches

696 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 25, NO. 4, AUGUST 2016

Fig. 8. (a) The schematic diagram and (b) photo of experimental setup foracoustic pressure output from pMUT arrays.

down to the supporting post (see Fig. 6(f)). The finalizeddevices are shown in Fig. 7. No acoustic matching layer wascoated on the pMUT array.

IV. CHARACTERIZATION

Due to the variations in the shape and dimension of themembranes and the cavities during the manufacture, the res-onant frequency of individual cell shifts, which leads to thepresence and overlap of multiple peaks in the impedance spec-trum of the pMUT arrays. Hence, it is difficult to determinethe resonant frequency using the impedance analyzer [19].An alternative way is to measure either the acoustic impulseresponse or the acoustic output by sweeping the drivingfrequency. The performance of pMUTs arrays without andwith isolation trench was characterized by the experimentalsetup shown in Fig. 8. The fabricated 8 × 8 square arraysof circular membrane pMUTs were mounted to a plastic boxusing super glues and immersed in the vegetable cookingoil to avoid the electrical conduction. Two electronic needles(DCM-210 Precision Positioner, Cascade Microtech, USA)were put on two contacting pads, one grounded and theother for driving signals, under the guidance of a microscope(ZEISS Axioskop 2 MAT, Oberkochen, Germany). A broadbandwidth needle hydrophone (up to 15 MHz, NCS-1, Institute

Fig. 9. Comparison of the impulse response of pMUT arrays (a) with fullyclamped boundary, (b) with an isolating trench between cells, and (c) theircorresponding spectra at the output voltage of 1 V from the function generator.

of Acoustics, Chinese Academy of Sciences, Beijing, China)was placed above the array surface and connected with atranslational stage (MT3/M, Thorlabs, Newton, NJ, USA).The measured signal was registered to a digital oscilloscope(Wavesurfer 44MXs-B, Lecroy, Chestnut Ridge, NY, USA)and then transferred to a personal computer (PC) for dataanalysis. Since all pMUT cells were connected together anddriven in phase, an almost planar wave would be generatedclose to the surface. The hydrophone was manually scannedin the horizontal plane to find the location of the maximumoutput.

Firstly, an impulse signal from a pulse/receiver (5072PR,Panametrics, Waltham, MA, USA) was applied to actuate thepMUTs. To separate the reflected signal from the interfaceof oil and air from the direct propagation signal in the timedomain, the hydrophone tip was placed 1.5 cm deep intothe oil. The signal-to-noise ratio was increased by averagingthe received echoes 50 times. The measured signal has twocomponents in the time domain: the first and large pulseis the driving signal electromagnetically coupled into thehydrophone; the second echo is the impulse response of thepMUTs (see Fig. 9). Thus, the signals within the temporalwindow from 4 μs to 9 μs were selected to calculate thespectrum using fast Fourier transform (FFT). It is foundthat there are two resonant frequencies of the pMUT arrays(around 2.29 MHz and 2.75 MHz), and the presence of iso-lation trenches has slightly changed the resonant frequencies(2.27 MHz and 2.72 MHz). These two resonant frequenciesmay be due to the varied cavity size of 96 μm and 94 μm,respectively, according to the FEM results. So the membrane

WANG et al.: ENHANCEMENT OF THE TRANSMISSION OF pMUT WITH AN ISOLATION TRENCH 697

Fig. 10. Comparison of (a) the representative acoustic pressure waveformsemitted from pMUT arrays with fully clamped boundary and with an isolatingtrench between cells with response to 200-cycle sinusoidal burst and (b) thezoomed in waveforms in the steady state (i.e., 50-56 μs) at the output voltageof 1 V from the function generator.

is not perfectly circular, but a little oval. Such an error(∼1.8%) is within the measured error of MEMS fabrication,−10.5 μm ∼ 5.5 μm [22].

Then a 200-cycle sinusoidal burst at the first reso-nant frequency was generated by a function generator(AFG 3022B, Tektronix, Beaverton, OR, USA) and wentthrough a class A power amplifier (240L, Electronic Naviga-tion Industries, Rochester, NY, USA) before driving the pMUTarrays. To avoid the reflection from the oil surface, a pieceof acoustic absorbing tile was attached to the hydrophone.It shows that both of these two pMUT designs have asimilar dynamic behavior in building up pressure waveform(i.e., about 20 μs), reaching the steady state, and decayingafter the termination (i.e., about 15 μs) in Fig. 10. The muchslower response compared to the transient analysis in Fig. 3may be due to the imperfect impedance match with poweramplifier which requires an input impedance of 50 �. Foreasy comparison, a segment of the signal in the steady state(i.e., 50-56 μs) was extracted. The output from the trenchdesign is much higher than that of conventional design withfully clamped boundary.

To further illustrate the improvement of the acoustic pres-sures, the input voltages from the function generator wasincreased from 0.1 V to 1.0 V, and the stabilized pressureoutput of 200-cycle sinusoidal burst was compared in thesteady state (Fig. 11(a)). The improvement within the wholerange of output voltage is almost consistent (76%). The nonlin-ear relationship between output pressure and applied voltage,which is commonly in fully clamped pMUT design [15],

Fig. 11. Comparison of the output of pMUT arrays (mean±std) with fullyclamped boundary and an isolating trench between cells (a) with responseto the input voltage of function generator at their resonant frequenciesand (b) with respect to the driving frequency at the output voltage of 1 Vfrom the function generator (n = 6).

becomes apparent when the input voltage is larger than 0.6 V,and the presence of isolation trench has no significant changeto this phenomenon. Furthermore, the frequency response ofthe pMUT array from 1.5 MHz to 3 MHz was measured whilemaintaining the input voltage as 1 V. The two resonant fre-quencies found using this approach, 2.30 MHz and 2.75 MHzin Fig. 11(b), are close to the values shown in the impulseresponse (see Fig. 9(b)). The relatively large variations in themeasurement are due to the heterogeneity of manufacturedcells, the initial substrates, and etch stops. The Q valuescalculated here are a little higher than those in Fig. 9, whichmay be due to the low signals picked up by the hydrophoneand imperfect impulse in the excitation.

V. DISCUSSION

The effective response of piezoelectric thin films is differentfrom that of bulk material, particularly because of the in-planeclamping of the film by the substrate and the residual stress inthe film. Strongly intrinsic stress in the membrane generatedduring fabrication can dominate over the flexural rigidity andaffect the resonant frequency dramatically. By releasing thesubstrate beneath the thin film, the mechanical constraintsnear the bottom interface of the film are reduced and thepiezoelectric film can respond to the electric field more freely

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Fig. 12. The effect of top electrode radius of pMUT on the average membranedisplacement without and with isolation trench.

and contribute more to the dielectric and piezoelectric proper-ties [23], [24]. To improve the performance of the pMUTs,a new configuration was introduced and studied by bothFEM simulation and experimental measurement in this study.The introduction of high aspect-ratio of trenches in the oxidelayer can reduce the residual stress as measured by bent-beam strain sensors in micromechanical resonators [23]. Thesimulation showed that the introduction of an isolation trenchbetween pMUT cells is able to partially release the deflection-induced tensile stress, allow more motion of membrane at theedge, decrease the equivalent stiffness of the membrane [17],and subsequently improve the acoustic output by about 76%.The trench could isolate the passive supporting layer and theactuating piezoelectric film. A deeper trench will soften thesupporting post, cause significantly non-uniform movement ofthe membrane, and reduce the output as shown in Fig. 4(a).Furthermore, it is not easy to control the trench depth in thefabrication if no stop layer is used. In contrast, the trench widthis much easier to control and has a monotonous relationshipwith the output pressure.

The membrane displacement of pMUT is also depen-dent on the electrode size on its top. Partially covered topelectrode (∼70%) was found to have the optimal performancefor fully clamped pMUT [21]. However, for the design ofsuspended membrane by four bridges, the fully covered topelectrode would achieve the best outcome [17]. The effect oftop electrode radius of our pMUT on the average membranedisplacement without and with isolation trench was also inves-tigated (see Fig. 12). It is found that both designs have similartrends because of the similar bounding condition of membranewith the surrounding posts. The optimal electrode radius is 36μm and 38 μm with the increase of the average membranedisplacement from 12.5 nm to 15.5 nm (1.24 fold) and from20.9 nm to 22.5 nm (1.08 fold), respectively. The pMUTwith isolation trench is still much better than the conventionaldesign with fully clamped boundary (1.45 fold). The similaroptimal radii of the top electrode of these pMUTs could resultin easy design work.

AlN is a promising piezoelectric material, and its highelastic modulus, low density, and low dielectric constantand post-CMOS compatible fabrication make it ideal formany applications [25]. The lower processing temperature(<400 °C) allows AlN-based pMUT to be monolithically

fabricated with the ICs and have a low residual stress.Although the integration of zinc oxide (ZnO) MEMS deviceswith circuitry has been successfully demonstrated, ZnOfilms have higher conductivity than AlN (resulting in morepower loss) and the fact that Zn is a fast-diffusing ionmay pose contamination issues for CMOS manufacturing.In comparison to lead zirconate titanate (PZT), the lowerpiezoelectric coefficient of AlN are mitigated by a significantlyreduced dielectric constant (i.e., e31, f = −1.0 C/m2 and−9.6 C/m2, ε33, f = 10.7 and 650–1300 for AlN and PZT,respectively) [26]. Thus, the transmission efficiency of AlNpMUTs was smaller than the other designs [10], [24],[27]. The reduced capacitance of AlN pMUTs comparedwith PZT pMUTs can result in improved signal-to-noiseratio (SNR), but implies a significantly higher sensitivity toparasitic capacitance. The difficulties in the manufacture ofhigh-performance piezoelectric thin films are attributed to thelimited application of pMUTs. However, the isolation trenchis independent on the choice of membrane materials andwould have consistent improvement on the pressure output.

The residual stress was deemed to worsen the output ofthe pMUT in the air and result in non-optimal performanceand deviations from model predictions [17], such as reducedcoupling coefficient and bandwidth. For AlN deposited onthe metal layer, the residual stress is thickness dependentand could change from compressive (−4 GPa at 10 nm) totensile stress (0.46 GPa at 3.1 μm) [20]. Therefore, a stress-free AlN layer could be achieved at the appropriate thickness.However, thickness tuning is a both time-consuming and costlyprocess. Recently, both recent experimental and FEM studieshave shown that pMUTs with an initial deflection caused bythe residual stress can enhance both the transmission [28], [29]and receiving [30]. Engineering the residual stress could beachieved by applying static pressure or exciting an AC voltageto the piezoelectric layer on the membrane. However, largecompressive stress and upwards buckling would make themembrane very fragile for the low yielding, which is notpreferred for mass fabrication. In addition, stress distributionalso changes because of buckling, and the stress-free lay-ers may resume the stress after buckling. When the pMUTmembrane has a significant deformation on its surface (i.e.,the radius of curvature of 1400–4400 μm for a 140 μmmembrane) the resonant frequency increases linearly and themaximum displacement at the center increases exponentiallywith the decrease of the radius of curvature [31]. The largeradius of curvature (50 mm) of our 96 μm membrane dueto residual stress induced initial deflection (22.9 nm) resultsin almost no change of displacement and resonant frequency.So the residual stress was found to have little influenceon pMUT performance here, and its effect on more curvedpMUT needs further investigation. Altogether, considering thecomplicated fabrication process and the little improvementachieved by engineering the residual stress [32], the isolationtrench method is preferable.

The bandwidth of pMUT is usually limited (or Q > 100)in the wave excitation, even worse than the conventionalbulk PZT transducer mainly due to the residual stress. Thishigh Q factor is beneficial in the transmission mode of

WANG et al.: ENHANCEMENT OF THE TRANSMISSION OF pMUT WITH AN ISOLATION TRENCH 699

ultrasound. However, broad bandwidth is preferred in theultrasonic receiver for a short electric pulse to achieve goodaxial resolution. In addition, harmonic imaging is also fea-sible for ultrasound transducer with sufficient bandwidth.To increase the bandwidth, a rectangular membrane withlarge length/width aspect ratio was used in pMUT designto merge several resonant modes within a narrow frequencyrange. As a result, a −6 dB bandwidth could be extendedto 95% at a central frequency of 1.24 MHz with no useof matching layer [32]. The introduction of isolation trenchon such rectangular membrane may enhance both output andbandwidth in sonography.

Usually, the conventional pMUTs (including our isolationdesign) has a Gaussian-like mode shape, which has the max-imum displacement amplitude at the center of the membrane.Minimizing overall membrane tension and enabling flat vibra-tion may be another effective way to enhance the transmittingsensitivity of pMUT. Theoretically, the displacement of acircular clamped membrane under uniform pressure loadingp is [36]:

w (r) = pr40

64D

[1 −

(r

r0

)2]2

(1)

where r is the radial coordinate, r0 is the membrane radius,and D is the flexural rigidity. The maximum bending momentoccurs at the center of the membrane, r = 0.

Mmax = Epz p

16D(1 − v)(2)

where E is Young’s modulus, z p is the distance from theneutral axis of deflection to the middle of the piezoelectriclayer, v is the Poisson’s ratio. For the circular membrane witha simple-supported under uniform distributed loading, the outof plane displacement and the maximum bending moment are,

w′ (r) = pr40

64D(1 + v)

[2 (3 + v)

(1 −

(r

r0

)2)

− (1 + v)

(1 −

(r

r0

)4)]

(3)

M ′max = Epz p

16D(1 − v2)(3 + v) (4)

It shows that the maximum displacement and bending momentcould be enhanced by (5 + v)/(1 + v) and (3 + v)/(1 + v),respectively, which depend on the value of Poisson’s ratioand are always larger than 1. The piston-like vibration, whichcould push more acoustic medium back and forth for higherpressure generation, has already been realized in cMUT with amaximum output pressure exceeding 1 MPa at the surface [33].Mode shape in the pMUT could also change from Gaussian-like to piston-like using many tiny etching holes (5 μm) inthe membrane (200 μm) [34]. When the etching holes cover35% of the membrane area, the resonant frequency increasedslightly from 2.24 MHz to 2.31 MHz. Although the rela-tively smaller displacement was found due to larger dampingattributed to the larger effective vibration area, the directivityof piston-like membrane motion in the far field was improved

and the pressure level was increased by 5.3 dB. pMUT withboth isolation trench between cells and etching holes on themembrane may be able to generate high displacement as wellas directivity, which will be investigated later.

High intrinsic acoustic crosstalk occurs almost in all MUTarrays that have continuous acoustic pathways among ele-ments, which is due to the Stoneley wave at the membrane-water interface and guided wave (Lamb wave) propagation onthe thin substrate and mode conversion at the interface of fluidand solid [35], [36]. The Stoneley and Lamb wave was foundto be responsible for the cross-talk pressure and displacementfield, respectively. Isolation trenches or extra separation wallsbetween CMUT element have less than 2 dB reduction inthe crosstalk level [37], [38], which is similar to our FEMsimulation results. An acoustic band gap in the substratecould reduce the crosstalk without loss of the pressure fromthe transmitting element [39]. Separated diaphragm elementwith no substrate underneath each element (the substrate onlyproviding the least support between neighboring elements)has the least crosstalk [35]. Altogether, it suggests that themain mechanism of crosstalk is the dispersive guided modepropagating in the substrate with very low loss.

VI. CONCLUSION

In this paper, it was found that the introduction of a trenchto isolate the edge of the pMUT cells from passive supportinglayer can effectively reduce the deflection-induced tensilestress of the piezoelectric membrane, allow more motion ofmembrane edge, and improve the output of the AlN-basedpMUT without significantly affecting other operating parame-ters, such as the resonant frequency and nonlinear performanceat high voltage, which has been proven by both FEM modelsimulation and experimental measurement.

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Mingjun Wang received the B.S. degree fromNorthwestern Polytechnical University, China, in2010. He is currently pursuing the Ph.D. degree inmechanical and aerospace engineering at NanyangTechnological University, Singapore. His researchinterests are MEMS ultrasound device and its appli-cation in high-intensity focused ultrasound ablation.

Yufeng Zhou (M’06–SM’11) received the B.S. andM.S. degrees from the Department of ElectricalScience and Engineering, National Key Laboratoryof Modern Acoustics, Nanjing University, China,in 1996 and 1999, respectively, and the Ph.D.degree in bioacoustics from Duke University, USA,in 2003. He received post-doctoral training fromDuke University and the Applied Physics Labora-tory, Center for Industrial and Medical Ultrasound,University of Washington. He joined the School ofMechanical and Aerospace Engineering, Nanyang

Technological University, Singapore, as an Assistant Professor in 2010. Hisresearch interests are mostly focused on biomedical ultrasound, includingthe high-intensity focused ultrasound for solid tumor ablation, extracorporealshock wave lithotripsy, sonothrombolysis, ultrasound-mediated drug deliv-ery, bubble cavitation, and its interaction with an acoustic burst for tissuefragmentation.

Andrew Randles received the B.S. andM.S. degrees from the Rochester Institute ofTechnology in 1999 and 2002, respectively, andthe Ph.D. degree in nano-mechanics from TohokuUniversity, Japan, in 2007. He was a Post-DoctoralResearcher with Tohoku University for a year.He is currently the Scientist 1 with the Institute ofMicroelectronics, A*Star, Singapore. His interestsare MEMS design, fabrication, simulation, andtesting.