Micromachined two-dimensional array piezoelectrically actuatedtransducersGökhan Perçin, Abdullah Atalar, F. Levent Degertekin, and Butrus T. Khuri-Yakub Citation: Appl. Phys. Lett. 72, 1397 (1998); doi: 10.1063/1.121067 View online: http://dx.doi.org/10.1063/1.121067 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v72/i11 Published by the American Institute of Physics. Related ArticlesPiezoelectric and electrostrictive effects in ferroelectret ultrasonic transducers J. Appl. Phys. 112, 084505 (2012) Piezoelectric resonator arrays for tunable acoustic waveguides and metamaterials J. Appl. Phys. 112, 064902 (2012) Focused high frequency needle transducer for ultrasonic imaging and trapping Appl. Phys. Lett. 101, 024105 (2012) A rigid, monolithic but still scannable cavity ring-down spectroscopy cell Rev. Sci. Instrum. 83, 043115 (2012) Acoustic resonator based on periodically poled transducers: Concept and analysis J. Appl. Phys. 111, 064106 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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APPLIED PHYSICS LETTERS VOLUME 72, NUMBER 11 16 MARCH 1998

Micromachined two-dimensional array piezoelectricallyactuated transducers

Gokhan Percin,a) Abdullah Atalar,b) F. Levent Degertekin, and Butrus T. Khuri-YakubEdward L. Ginzton Laboratory, Stanford University, Stanford, California 94305-4085

~Received 16 December 1997; accepted for publication 20 January 1998!

This letter presents micromachined two-dimensional array flextensional transducers that can be usedto generate sound in air or water. Individual array elements consist of a thin piezoelectric ring anda thin, fully supported, circular membrane. We report on an optimum design for an individual arrayelement based on finite element modeling. We manufacture the transducer in two-dimensionalarrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85MHz. Such an array could be combined with on-board driving and an addressing circuitry fordifferent applications. ©1998 American Institute of Physics.@S0003-6951~98!03911-4#

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Two-dimensional arrays of ultrasound transducersdesirable for imaging applications in the fields of medicinnondestructive evaluation, and underwater exploration. Ming arrays of transducers by dicing and connecting individpiezoelectric elements is fraught with difficulty and expennot to mention the large input impedance mismatch probthat such elements present to transmit/receiving electronOur approach is to use micromachined flextensional pieelectric transducers. Individual elements are made ofsilicon nitride membranes covered by a coating of piezoetric zinc oxide~ZnO!. The arrays are made by using silicomicromachining techniques and are capable of operatiohigh frequencies. Inherently, this approach offers the advtage of integrating transducers with transmitter and receelectronics. Thus, we present arrays where elements caindividually addressed for ease of scanning and focusingusing on-board electronics.

We fabricated micromachined piezoelectrically actuaflextensional transducers in a 2-D array by combining cventional IC manufacturing process technology with Zndeposition. Individual array elements consist mainly of a ccular membrane attached to a circular ring of piezoelecmaterial which has optimized dimensions. An ac voltageapplied accross the piezoelectric material to set the cpound membrane into vibration. At the resonant frequencof the compound membrane, the displacement at the cenlarge.

We design the individual array element to have a mamum displacement at the center of the membrane at the rnant frequency~Percin et al.1,2!. Analyses of similiar devicessuch as those of Allaverdiev,3 Vassergiseret al.,4 Okadaet al.,5 and Iuloet al.,6 are helpful in identifying the impor-tant parameters of the device. However, the complexitythe structure and the fact that the piezoelectric we usering rather than a full disk necessitate the use of finite ement analysis to determine the resonant frequencies ofstructure, the input impedance of the transducer, and themal displacement of the surface.

a!Electronic mail: percin@alumni.stanford.orgb!Department of Electrical and Electronics Engineering, Bilkent Univers

Bilkent 06533, Ankara, Turkey.

1390003-6951/98/72(11)/1397/3/$15.00

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It is well known that the transverse displacementj of asimple membrane of uniform thickness, in vacuum, obethe following differential equation:7

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The axisymmetric free vibration frequencies for an edgclamped circular membrane are given by

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whereE is Young’s modulus,h is the membrane thicknessandn is Poisson’s ratio.

By substituting typical dimensions for the individual aray element of the above equations and by using avervalues for h,E, n, andr, we obtain the first resonance frequency at 2.62 MHz, which is a reasonable approximation2.8–3.0 MHz that was measured in our experiments. Tabove equations suggest that the resonant frequency irectly proportional to the thickness of the membrane ainversely proportional to the square of the radius. Howevit is also known that the resonant frequency will be decreaby fluid loading on one or both sides of the membrane. Tshift in the fluid loaded resonant frequency of a simple mebrane is shown by Kwak8 to be

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whereb5rwa/rmh is a thickness correction factor,rw is thedensity of the liquid,rm is the mass density of the circulamembrane, andG is the non-dimensional added virtual maincremental~NAVMI ! factor, which is determined by boundary conditions and mode shape. For the first order axisymetric mode and for water loading on one side of the me

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7 © 1998 American Institute of Physics

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1398 Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998 Percin et al.

brane, G is 0.75. Assuming the composite will behavsimilarly to the single membrane, we expect the resonfrequency to shift down by 63% for one of our devices.

We designed micromachined two dimensional artransducers by modeling one element large scale proto~Percin et al.1,2!. However, the two-dimensional array natuof the device is accomodated by a suitable micromachinprocess. Materials are chosen in accordance with availabof micromachining and IC manufacturing processes. Otpiezoelectric materials, membrane materials, electrode mals, and substrates can be used.

We used ANSYS to optimize an individual array elment made with piezoelectric ZnO on a silicon nitride mebrane. Several iterations were run to maximize the displament of the membrane as a function of the dimensions ofpiezoelectric ring. Maximum displacement was obtainwhen the piezoelectric ring had an inner diameter of 30mmand outer diameter of 80mm with a thickness of 0.3mm, andthe silicon nitride had a diameter of 100mm with a thicknessof 0.3mm. The dc displacement is 2.27 Ao/V. The resonance

FIG. 1. Realized micromachined device process flow.

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frequency 3.46 MHz obtained from ANSYS simulationvacuum is a good approximation to 3.07 MHz that was msured in vacuum with one of our devices.

The fabrication process for micromachined twdimensional array flextensional transducers is given in F1. At the right side of the figure, actual pictures of two ajacent elements from a two-dimensional array are givalong with the process flow. The process starts with growa sacrificial layer, chosen to be silicon oxide~8% phosphorusdoped densified LTO!. A membrane layer of low-pressurchemical vapor deposition~LPCVD! silicon nitride is grownon top of the sacrificial layer. The bottom Ti/Au electrodlayer is deposited on the membrane by e-beam evaporatio228 °C. The degree to which the ZnOc-axis, ^0002&, is ori-ented normal to the substrate surface is very sensitive todegree to which the Au film is111& oriented. The quality ofthe ZnO is measured by an x-ray rocking curve scan. TZnO had a 5.5°-wide rocking curve above Au with a 5°-wirocking curve. The bottom metal layer is patterned by wetch, and access holes for sacrificial layer etching are driin the membrane layer by plasma etch. Later, the bottelectrode layer is patterned by wet etch, and a piezoelecZnO layer is deposited on top of the bottom electrode. TZnO is deposited by dc planar magnetron reactive sputtefrom a 127-mm-wide target consisting of 99.99% Zn. Tdeposition is made in an 20%–80% argon-oxygen ambwith a flow rate of 26.6 sccm, a pressure of 7 mTorr,substrate temperature of 145 °C, and a dc power of 350The separation between the substrate and the target imm. The depositon rate is 9.0 Ao/s. The top Cr/Au electrodelayer is formed by e-beam evaporation at room temperaand patterned by liftoff. The last step is etching the sacrificlayer by wet etch, and this concludes the front surfacecromachining of the devices. Figure 2 shows final 60360two-dimensional array devices. The die size is 1 cm31 cm.

Figure 3 shows the real part of the electrical input impedance of only one row of 60 elements of devices showFig. 2. Operating in air, the transducers have a resonantquency of 3.0 MHz and a fractional bandwidth of abo1.5%. The real part of the electrical input impedance ha280 V base value, and it was determined by SPICE simution that this base value is caused by the bias lines conn

FIG. 2. Realized micromachined device.

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1399Appl. Phys. Lett., Vol. 72, No. 11, 16 March 1998 Percin et al.

ing individual array elements. This can be avoided by uselectroplating to increase the thickness of the bias lines.ure 3 also shows the existence of acoustical activity indevice, and an acoustic radiation resistanceRa of 150 V.Figure 4 presents the change of the electrical input impance in vacuum of a device consisting of one row ofelements. The resonance frequency is 3.00 MHz in air3.07 MHz in vacuum~at 50 mTorr!. This result is in accor-dance with our expectations, since the resonant frequeand the real part of electrical input impedance at resonashould increase in vacuum. The acoustic activity in vacureflects energy coupling to the structure. This is a ma

FIG. 3. Electrical input impedance real part of the realized micromachidevice.

FIG. 4. Electrical input impedance real part change of the device in vacand air.

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source of loss in the device and is a present topic of resein our group. Figure 5 shows the result of an air transmissexperiment where an acoustic signal is received followthe electromagnetic feedthrough. The insertion loss isdBs. In the transmit/receive experiment, the receiver hadrow of 60 elements, and the transmitter had two rows of 1elements. Loss due to eletrical mismatches is 34.6 dOther important loss sources are alignment of receivertransmitter, and structural losses.

In summary, we have developed a novel ultrasonic traducer which is silicon micromachined into two-dimensionarrays. The individual array element is based on a variaof a flextensional transducer. The transducer design wastimized using finite element analysis, and the ultrasotransmission was demonstrated in air.

This research was supported by the Defense AdvanResearch Projects Agency of the Department of Defensewas monitored by the Air Force Office of Scientific Researunder Grant No. F49620-95-1-0525.

1G. Percin, L. Levin, and B. T. Khuri-Yakub, IEEE Ultrasonics Symposium, November 3–6, 1996.

2G. Percin, L. Levin, and B. T. Khuri-Yakub, Rev. Sci. Instrum.68, 4561~1997!.

3A. M. Allaverdiev, N. B. Akhmedov, and T. D. Shermergor, Prikl. MaMekh. 23, 59 ~1987!.

4M. E. Vassergiser, A. N. Vinnichenko, and A. G. Dorosh, Sov. PhAcoust.38, 558 ~1992!.

5H. Okada, M. Kurosawa, S. Ueha, and M. Masuda, Jpn. J. Appl. PhPart 133, 3040~1994!.

6A. Iula, N. Lamberti, G. Caliano, and M. Pappalardo, IEEE UltrasonSymposium, November 1–4, 1994.

7A. W. Leissa,Vibration of Plates~Scientific and Technical InformationDivision, Office of Technology Utilization, NASA, 1969!, pp. 1–10.

8M. K. Kwak, J. Sound Vib.178, 688 ~1994!.

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FIG. 5. Air transmission/receive experiment of the realized micromachidevice.

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