Fabrication and characterization of micromachined

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ieee transactions on ultrasonics, ferroelectrics, and frequency control, vol. 52, no. 3, march 2005

Fabrication and Characterization of Micromachined High-Frequency Tonpilz Transducers Derived by PZT Thick Films Qifa Zhou, Jonathan M. Cannata, Member, IEEE, Richard J. Meyer, Jr., David J. Van Tol, Srinivas Tadigadapa, W. Jack Hughes, K. Kirk Shung, Fellow, IEEE, and Susan Trolier-McKinstry, Senior Member, IEEE Abstract—Miniaturized tonpilz transducers are potentially useful for ultrasonic imaging in the 10 to 100 MHz frequency range due to their higher efficiency and output capabilities. In this work, 4 to 10- m thick piezoelectric thin films were used as the active element in the construction of miniaturized tonpilz structures. The tonpilz stack consisted of silver/lead zirconate titanate (PZT)/lanthanum nickelate (LaNiO3 )/silicon on insulator (SOI) substrates. First, conductive LaNiO3 thin films, approximately 300 nm in thickness, were grown on SOI substrates by a metalorganic decomposition (MOD) method. The room temperature resistivity of the LaNiO3 was 6:5 10 6 Ω m. Randomly oriented PZT (52/48) films up to 7- m thick were then deposited using a sol-gel process on the LaNiO3 -coated SOI substrates. The PZT films with LaNiO3 bottom electrodes showed good dielectric and ferroelectric properties. The relative dielectric permittivity (at 1 kHz) was about 1030. The remanent polarization of PZT films was larger than 26 C/cm2 . The effective transverse piezoelectric e31 f coefficient of PZT thick films was about 6.5 C/m2 when poled at 75 kV/cm for 15 minutes at room temperature. Enhanced piezoelectric properties were obtained on poling the PZT films at higher temperatures. A silver layer about 40- m thick was prepared by silver powder dispersed in epoxy and deposited onto the PZT film to form the tail mass of the tonpilz structure. The top layers of this wafer were subsequently diced with a saw, and the structure was bonded to a second wafer. The original silicon carrier wafer was polished and etched using a Xenon difluoride (XeF2 ) etching system. The resulting structures showed good piezoelectric activity. This process flow should enable integration of the piezoelectric elements with drive/receive electronics.

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I. Introduction n recent years, micromachined ultrasonic transducers have received much attention [1]–[3]. The desire for increased resolution in medical imaging has led to the development of higher frequency transducers [4]. The choice of materials has been restricted by the need to produce

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Manuscript received August 4, 2003; accepted August 21, 2004. Financial support was provided by the Office of Naval Research (ONR) and NIH grant number P41-RR11795. Q. F. Zhou, R. J. Meyer, Jr., D. J. Van Tol, S. Tadigadapa, W. J. Hughes, and S. Trolier-McKinstry are with The Pennsylvania State University, University Park, PA 16802 (e-mail: [email protected]). J. M. Cannata and K. K. Shung are with the Department of Biomedical Engineering and NIH Transducer Resource Center, University of Southern California, Los Angeles, California 90089-1451. Q. F. Zhou also is with the University of Southern California.

a piezoelectric material of appropriate thickness to obtain the desired resonance frequency. Due to their large piezoelectric coefficients, thick lead zirconate titanate (PZT) films are one of the best candidate materials for use in ultrasonic transducers for high-resolution medical imaging in the range of 30–100 MHz [5]. To date, a number of groups have successfully fabricated PZT thick films. Sugiyama et al. [6] prepared PLZT thick films by a multiple electrophoretic deposition and sintering process. Barrow et al. [7] reported thick PZT ceramic coatings using sol-gel derived porous 0-3 composites. Chen et al. [8] prepared PZT thick films by a modified solgel process involving an acetic acid route. Kurchania and Milne [9] also fabricated PZT thick films using titanium di-isopropoxide bi-acetylacetonate as a precursor material. In this study, fabrication of microelectromechanical system (MEMS) tonpilz transducers using modified sol-gel PZT thick films was investigated. Conductive LaNiO3 thin films and PZT thick films with the morphotropic phase boundary (MPB) composition of Pb(Zr0.52 Ti0.48 )O3 (PZT 52/48) were deposited on SOI substrates. A new tonpilz structure was designed using silver composites as the tail mass. Fabrication and characterization of MEMS tonpilz will be described.

II. Microtonpilz Design and Experimental Procedures The original concept of the tonpilz centered on the efficient use of “precious crystal” [10]. Since then, the strategy of tonpilz design has been to improve the performance of transducers in a continuously refined manner. Tonpilz transducers consist of a heavy tail mass, motor section, and a light head mass. The tail is made of a metal, the motor generally consists of a piezoelectric material, and the head is a light and stiff material [11]. Using the KLM (Krimholtz, Leedom, and Matthaei) model [12], a tonpilz structure for high-frequency ultrasonic transducer in the range of 30–50 MHz was designed as shown in Fig. 1. In this model, the thickness of the p type silicon on insulator layer was about 40 µm, the LaNiO3 thin film was about 0.3 µm, the PZT film was about 7 µm, and the silver layer was in the range of 35- to 40-µm thick. The LaNiO3 was chosen as the electrical contact layer because it can be

c 2005 IEEE 0885–3010/$20.00

zhou et al.: analysis of devices for high-frequency ultrasonic application

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Fig. 1. Materials stack for tonpilz transducers used in the transducer modeling.

grown without reaction on p+ silicon (Si), and act as an electrical contact to the piezoelectric elements in an array. The Pt was not an acceptable choice as it requires a SiO2 layer next to the silicon to prevent reaction with PZT at crystallization temperatures, which would significantly complicate electrical connection to each of the piezoelectric array elements. The SiO2 layer thickness in the SOI layer was about 5-µm thick. Fig. 2 is a flowchart for preparation of micro-tonpilz arrays based on an SOI wafer. The LaNiO3 thin films were derived by a metal organic decomposition (MOD) technique. Lanthanum nitrate and nickel acetate were used as starting materials, and acetic acid plus water were used as the solvents. Nickel acetate was initially dissolved in acetic acid at room temperature. Next, the appropriate quantity of lanthanum nitrate was dissolved in water. The two solutions then were mixed under constant stirring. In order to prevent cracking of films during pyrolysis, formamide was added to the solution with a water/formamide volumetric ratio of 6:1. Acetic acid was added to achieve a final solution concentration of 0.3 M. The SOI substrates (Nagano Electronics Industrial Co., Ltd., Nagano, Japan) were cleaned using a standard silicon cleaning process. The LaNiO3 films were prepared by spinning the solution onto a SOI substrate at 3000 rpm for 30 seconds using a photoresist spinner. After deposition, each layer was annealed at 300◦C for 60 seconds to drive out the solvent and decompose organic compounds. Each layer was then annealed at 700◦ C for 60 seconds using rapid thermal annealing (RTA). This procedure was repeated to achieve the desired film thickness of about 0.3 µm. The film then was furnace-fired at 700◦C for 1 hour to enhance adhesion and conductivity. Lead-based piezoelectric thin films have been successfully prepared by sol-gel processing [13]–[16]. The sol-gel PZT solutions were prepared using 2-methoxyethanol (2MOE) as the solvent. Lead acetate trihydrate was initially dissolved in 2-methoxyethanol at 80◦ C, then refluxed at 115◦ C for 1 hour under argon. The water of hydration was distilled at 115◦ C under a vacuum of about 130 mbar. Meanwhile, appropriate quantities of zirconium n-propoxide and titanium isopropoxide were mixed and stirred in 2-MOE at 25◦ C. Upon completion of the dehydration step, the rotary flask containing the lead acetate complex was cooled well below 100◦ C, and the zir-

Fig. 2. Flowchart for preparation of the microtonpilz actuator.

conium and titanium precursors were added. This mixture was refluxed for 2 hours at 120◦C. To compensate for lead volatilization during film heat treatments, 20 mol% excess lead was added to the solutions. Following the reflux step, the solution was distilled at 115◦C again with the aid of vacuum. The mixture was allowed to cool near room temperature, and 22.5 vol% acetylacetonate (2,4 pentanedione) was introduced while stirring, which acted as a chelating agent to prevent solution hydrolysis in the presence of atmospheric moisture. Then 2-MOE was added to achieve a final solution concentration of 0.75 M. The PZT solution was deposited on the LaNiO3 -coated SOI substrate by spin-coating at 1500 rpm for 30 seconds. After deposition, each layer was subjected to a two-stage pyrolysis sequence to drive out solvent and decompose organic compounds. This sequence consisted of a 60-second heat treatment at 300◦ C followed by one at 450◦ C (60 seconds). The amorphous layer then was crystallized to phasepure perovskite at 700◦ C for 60 seconds using the RTA. Each layer was about 0.2-µm thick. Thicker PZT films were fabricated by repeating this procedure to achieve the desired thickness, then the PZT films were electroded by sputtering a gold layer of approximately 0.1 µm. A mixture

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of three parts 2–3 µm silver particles (Adrich Chemical Co., Milwaukee, WI) and 1.25 parts Insulcast 501 epoxy (American Safety Technologies, Roseland, NJ) was cast onto the electroded PZT films with the aid of an adhesion promoter (Chemlok AP-131, Lord Corp., Erie, PA). The stack was centrifuged at 3000 rpm for 10 minutes to increase acoustical impedance and to ensure the conductivity. After curing, the composite layer was lapped down to approximately 40-µm thick to form the tail mass of the tonpilz structure. Following deposition of all of the layers, the samples were diced (just to the underlying SiO2 ) using a dicing saw to create individual elements in an array. The dicing blade had a width of about 60 µm, and cuts were about 95-µm deep. Samples were diced with each element having an area of about 180 µm × 180 µm. After optimization of the dicing conditions, no residual silver was observed in the cuts after dicing. Epoxy (EPO-TEK 301, Epoxy Tech., Billerica, MA) was used to fill these gaps under vacuum. Then the sample was put into an oven at 50◦ C to cure the epoxy. After the epoxy cured, the sample surface was lightly polished again. Then the silicon carrier was lightly polished. The original silicon carrier then was etched using Xenon difluoride (XeF2 ). The etching rate was about 10–15 µm/hour, and etching was isotropic. A 10:1 buffered oxide etch (BOE, 10NH4 F:1HF) was used to remove the thin SiO2 layer (the etching rate was about 50 nm/minute at room temperature) with the sample surface protected by the photoresist. Using conductive epoxy, the silver surface was bonded to another Pt-coated Si wafer at 60◦ C for 5 hours. This completed the microtonpilz device. The structure of the thin films was examined using a Scintag (Model XDS2000, Cupertino, CA) x-ray diffractometer (XRD) with Ni filtered CuKα radiation. Patterns were recorded at a rate of 1◦ /minute in the 2θ range of 20◦ to 60◦ . Film thickness was measured using a surface profiler (Tencor Instruments, San Jose, CA). The resistivity of LaNiO3 films was measured using a standard four-probe technique, which was done on a separate wafer that had a thick SiO2 layer. The morphology of the films was observed using a scanning electron microscope (SEM; S-3500N, Hitachi, Tokyo, Japan). The dielectric permittivity was measured using an impedance analyzer (HP4194A, HewlettPackard, Palo Alto, CA) with an oscillation amplitude of 30 mV. High-field hysteresis properties were characterized using an RT66A (Radiant Technology, Albuquerque, NM) ferroelectric test system with a voltage amplifier. The effective transverse piezoelectric (e31,f ) coefficients of the films were characterized using a modification of wafer flexure method described previously [17]. The vibration amplitude of the devices as a function of frequency was measured by interferometry. III. Results and Discussion Fig. 3 shows an XRD pattern of the LaNiO3 thin film deposited on an SOI wafer with a native SiO2 surface layer. The film appears to be well-crystallized, and no

Fig. 3. XRD pattern of LaNiO3 film on an SOI substrate annealed at 700◦ C for 1 hour.

Fig. 4. XRD pattern of 7-µm thick PZT 52/48 film on a LaNiO3 coated SOI substrate.

strong texture was observed. Fig. 4 shows an XRD pattern of a 7-µm PZT (52/48) thick film deposited on the LaNiO3 -coated SOI substrate. It was found to be pure perovskite phase with a strong (110) peak. The film was randomly oriented because the LaNiO3 film was randomly oriented. It was likely that the crystallization and growth of the PZT (52/48) film was significantly influenced by the LaNiO3 film structure due to the close matching of lattice parameters between LaNiO3 (a = 0.384 nm) and PZT (a = 0.405 nm) [18]. Fig. 5 shows a SEM cross-sectional picture of the PZT thick films. The thickness was more than 7 µm, and every layer corresponding to one crystallization step was visible; grains are approximately equi-axed with grain sizes ranging from 0.08–0.1 µm. Fig. 6 shows an SEM crosssectional picture of Ag (silver)/PZT/LaNiO3/SOI tonpilz structures. The thickness of Ag, PZT, doped Si, and SiO2


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Fig. 5. SEM cross-sectional view of sol-gel derived PZT thick films. Fig. 7. Resistivity as a function of temperature for 300-nm thick LaNiO3 films.

Fig. 6. SEM cross-sectional picture of Ag/PZT/LaNiO3 /SOI tonpilz stack.

layers were 38, 7, 40, and 5 µm, respectively. These results are in good agreement with the initial design thicknesses. The electrical resistivity as a function of temperature for 0.3-µm thick LaNiO3 films was measured as shown in Fig. 7. The resistivity at room temperature was 6.5 × 10−6 Ω•m, which is comparable to the value of LaNiO3 films obtained by sputtering [19]. The resistivity of LaNiO3 films was smaller than that of the doped silicon in the SOI (2 × 10−4 Ω•m) wafer. Thus, MOD-derived LaNiO3 thin films can be used as an oxide electrode between the PZT film and SOI substrate. The ferroelectric hysteresis loops of PZT 52/48/LaNiO3/ SOI thick films are shown in Fig. 8 (in which the active silicon layer of SOI wafer served as the bottom electrode). The remanent polarization (Pr ) in random PZT films was about 26 µC/cm2 for Emax = 220 kV/cm, the saturation polarization (Ps ) was more than 38 µC/cm2 , and the coercive field was about 45 kV/cm. The dielectric constant at room temperature for this sample was 1030 at 1 kHz, and little dispersion was observed as a function of frequency. Thus, the PZT films possessed good electrical properties [13].

Fig. 8. Polarization as a function of electric field for random PZT 52/48 thick films deposited on a LaNiO3 -coated SOI substrate.

The effective transverse piezoelectric coefficient e31,f of the film was characterized using the wafer flexure technique [17], [20]. An effective e31,f coefficient, e31,f = charge/strain, was calculated from the measured data. A small piece of sample was glued on a 3-inch silicon wafer. This then was suspended over a cylindrical cavity. By controlling the air pressure in the cavity using the output of an audio speaker, a periodic strain in the film was produced. The current was detected using a lock-in amplifier. Prewired strain gages (Omega Engineering, Inc., Stamford, CT) were used to measure the strain in the film. Fig. 9 shows the e31,f coefficient of the PZT thick film deposited on LaNiO3 -coated SOI substrate with different direct current (DC) electric fields (the poling time was kept

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Fig. 9. Room temperature transverse piezoelectric coefficient e31,f of sample as a function of poling electric field. The poling was done at room temperature.

(a)

(b)

Fig. 10. Variation of the normalized e31,f of sample as a function of poling temperature.

constant at 15 minutes). e31,f increased as the poling field increased, and then saturated at a field of about two times the coercive field. The PZT films had effective transverse piezoelectric coefficient of (−6.5 C/m2 ) after poling with an electric field of −75 kV/cm. Fig. 10 shows the variation of the normalized e31,f coefficient as a function of the poling temperature at a constant poling electric field (50 kV/cm) and poling time (15 minutes). The e31,f increased with the poling temperature, and the e31,f of the films can be increased about 50% when the poling temperature is 180◦ C.

Fig. 11. Dicing patterns of Ag/PZT/LaNiO3 /SOI tonpilz structure (a) and (b) after dicing and filling gap.

Using the processing techniques described above, microtonpilz arrays were obtained. An SEM micrograph of a diced sample before and after filling the cuts with epoxy is shown in Fig. 11. No residual silver was observed in the gaps after dicing, which eliminate shorting of the top and bottom electrodes of the elements. Fig. 12 shows a backside micrograph of the dicing pattern after removing the Si carrier and SiO2 layer. Contact to individual microtonpilz elements was obtained using ultrasonic wire bonding. It is expected that the wire bonding step could be eliminated if the tonpilz elements were bonded to a Si wafer prepatterned to contact individual pixels. Blanket contact then could be made to the top surface.


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Fig. 14. Vibration amplitude of microtonpilz device as a function of frequency. Fig. 12. Backside surface view of Ag/PZT/LaNiO3 /SOI tonpilz structure after removing original Si carrier and SiO2 layer.

sons for this. First, the epoxy used to bond the device to the final Pt-coated Si wafer did not provide sufficient mechanical decoupling. As a result, even when only a single element was excited, oscillation was observed on adjacent elements. Second, it is possible that the p+ Si conductivity was insufficient to drive the piezoelectric element well near 50 MHz. The detailed ultrasonic characterization of microtonpilz based on PZT films for high-frequency application is on-going.

IV. Conclusions

Fig. 13. Polarization as a function of electric field for microtonpilz device.

The ferroelectric hysteresis loops of tonpilz devices are shown in Fig. 13 (in which the active silicon layer served as the top electrode). The Pr was about 25 µC/cm2 for Emax = 225 kV/cm, the Ps was about 40 µC/cm2 , and the coercive field was about 50 kV/cm. The ferroelectric properties are very similar to that of PZT thick films in Fig. 8. Then, each element was poled at room temperature using three times the coercive electric field. The vibration amplitude as a function of frequency was measured by interferometry as shown in Fig. 14. A resonant frequency peak of about 47 MHz was observed. The peak as a function of frequency is not as clear as the design suggested. It is believed that there are two major rea-

Conductive LaNiO3 thin films deposited directly on SOI substrates were obtained by a MOD technique. Randomly oriented PZT (52/48) films up to ∼7-µm thick were fabricated using a modified sol-gel process on LaNiO3 -coated SOI substrates. The dielectric constant (at 1 kHz) of the films was 1030. The remanent polarization of PZT film was about 26 µC/cm2 . The effective transverse piezoelectric coefficient (e31,f ) of PZT thick films was about −6.5 C/m2 when poled at −75 kV/cm for 15 minutes. The piezoelectric properties are enhanced about 50% when the PZT films were poled at 180◦ C. Using dicing, XeF2 , and wet etching process, a microtonpilz structure was successfully fabricated. A resonant frequency peak at about 47 MHz was observed by interferometry. The devices have a potential application in high-frequency ultrasound.

Acknowledgments The authors would like to thank Dr. B. D. Huey for his kind help to measure the vibration, and Mr. G. Gerber for dicing our sample.

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References [1] J. J. Bernstein, S. L. Finberg, K. Houston, L. C. Niles, H. Chen, L. E. Cross, K. Li, and K. Udayakumar, “Micromachined high frequency ferroelectric sonar transducers,” IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 44, no. 5, pp. 960–969, 1997. [2] M. Lukacs, M. Sayer, and F. S. Foster, “Single element and linear array PZT ultrasound biomicroscopy transducers,” in Proc. IEEE Ultrason. Symp., 1997, pp. 1709–1712. [3] J. Baborowski, N. Ledermann, and P. Muralt, “Piezoelectric micromachined transducers (PMUT’s) based on PZT thin films,” in Proc. IEEE Ultrason. Symp., 2002, pp. 1051–1054. [4] M. Lukacs, M. Sayer, D. Knapik, R. Candelaa, and F. S. Foster, “Novel PZT films for ultrasound biomicroscopy,” in Proc. IEEE Ultrason. Symp., 1996, pp. 901–904. [5] Q. F. Zhou, H. L. W. Chan, and C. L. Choy, “PZT ceramic/ceramic 0-3 composite thick films for ultrasonic transducer applications,” Thin Solid Films, vol. 375, no. 1-2, pp. 95– 99, 2000. [6] S. Sugiyama, A. Takagi, and K. Tsuzuki, “(Pb, La)(Zr, Ti)O3 films by multiple electrophoretic deposition sintering processing,” Jpn. J. Appl. Phys., vol. 30, pp. 2170–2173, 1991. [7] D. A. Barrow, T. E. Petroff, and M. Sayer, “Thick ceramic coating using a sol gel based ceramic-ceramic 0-3 composite,” Surf. Coatings Technol., vol. 76, pp. 113–137, 1995. [8] H. D. Chen, K. R. Udayakumar, C. J. Gaskey, and L. E. Cross, “Fabrication and electrical properties of lead zirconate titanate thick films,” J. Amer. Ceram. Soc., vol. 79, no. 8, pp. 2189–2192, 1996. [9] R. Kurchania and S. J. Milne, “Characterization of sol-gel Pb(Zr0.53 Ti0.47 )O3 films in the thickness range 0.25–10 µm,” J. Mater. Res., vol. 14, no. 5, pp. 1852–1859, 1999. [10] F. V. Hunt, Electroacoustics: The Analysis of Transduction, and Its Historical Background. College Park, MD: American Institute of Physics, 1982. [11] R. J. Meyer, Jr., T. C. Montgomery, and W. J. Hughes, “Tonpilz transducers designed using single crystal piezoelectrics,” presented at IEEE Oceans ’02 Marine Tech. Soc., vol. 4, pp. 2328– 2333, 2002. [12] R. E. McKeighen, “Design guidelines for medical ultrasonic transducers,” Proc. SPIE, vol. 3341, pp. 2–18, 1998. [13] Q. F. Zhou, E. Hong, R. Wolf, and S. Trolier-McKinstry, “Dielectric and piezoelectric properties of PZT 52/48 thick films with (100) and random crystallographic orientation,” in Proc. Mater. Res. Soc. Symp., vol. 655, cc11.7, 2000. [14] Q. Q. Zhang, Q. F. Zhou, and S. Trolier-McKinstry, “Structure and piezoelectric properties of sol-gel-derived 0.5Pb[Yb1/2 Nb1/2 ]O3 -0.5PbTiO3 thin films,” Appl. Phys. Lett., vol. 80, no. 18, pp. 3370–3372, 2002. [15] Q. F. Zhou, Q. Q. Zhang, T. Yoshimura, and S. TrolierMcKinstry, “Dielectric and transverse piezoelectric properties of sol-gel-derived (001)-oriented Pb[Yb1/2 Nb1/2 ]O3 -PbTiO3 epitaxial thin films,” Appl. Phys. Lett., vol. 82, no. 26, pp. 4767– 4769, 2003. [16] Q. F. Zhou, Q. Q. Zhang, and S. Trolier-McKinstry, “Structure and piezoelectric properties of sol-gel-derived (001)-oriented Pb[Yb1/2 Nb1/2 ]O3 -PbTiO3 thin films,” J. Appl. Phys., vol. 94, no. 5, pp. 3394–3402, 2003. [17] J. F. Shepard, Jr., P. J. Moses, and S. Trolier-McKinstry, “The wafer flexure technique for the determination of the transverse piezoelectric coefficient (d31 ) of PZT thin films,” Sens. Actuators A, vol. 71, pp. 133–138, 1998. [18] S. Y. Chen and I. W. Chen, “Texture development, microstructure evolution, and crystallization of chemically derived PZT thin films,” J. Amer. Ceram. Soc., vol. 81, no. 1, pp. 97–105, 1998. [19] Z. S. Zhang, “LiNiO3 bottom electrodes for ferroelectric thin films,” M.S. thesis, The Pennsylvania State University, University Park, PA, 2000. [20] F. Shephard, Jr., F. Xu, I. Kanno, and S. Trolier-McKinstry, “Characterization and aging response of the d31 piezoelectric coefficient of lead zirconate titanate thin films,” J. Appl. Phys., vol. 95, no. 9, pp. 6711–6716, 1999.

Qifa Zhou received his Ph.D. degree from the Department of Electronic Materials and Engineering at Xi’an Jiaotong University, Xi’an, China in 1993. He is currently a research assistant professor at the National Institutes of Health (NIH) Resource on Medical Ultrasonic Transducer Technology, Los Angeles, CA, and the Department of Biomedical Engineering at the University of Southern California (USC), Los Angeles, CA. Before joining USC in 2002, he worked in the Department of Physics at Zhongshan University of China at Guang Zhou, the Department of Applied Physics at Hong Kong Polytechnic University at Kowloon, and the Materials Research Laboratory at the Pennsylvania State University, University Park, PA. His current research interests include the development of ferroelectric thin films, microelectricalmechanical (MEMS) technology, modeling and fabrication of high-frequency ultrasound transducers and arrays for medical imaging applications. He has published more than 70 papers in this area. He is a member of the Materials Research Society.

Jonathan M. Cannata (S’01–M’04) was born in Carson, CA, on August 4, 1975. He received his B.S. degree in Bioengineering from the University of California at San Diego in 1998, and his M.S. and Ph.D. degrees in Bioengineering from the Pennsylvania State University, University Park, PA, in 2000 and 2004, respectively. Since 2001 he has served as the engineering manager of the NIH (National Institutes of Health) Resource Center for Medical Ultrasonic Transducer Technology at Penn State University (2001 to 2002) and currently at the University of Southern California (USC). Dr. Cannata also is a Research Assistant Professor of Biomedical Engineering at USC. Dr. Cannata’s current research interests include the design, modeling, and fabrication of high frequency single element ultrasonic transducers and transducer arrays for medical imaging applications. Dr. Cannata is a member of the Institute of Electrical and Electronics Engineers (IEEE).

Richard J. Meyer, Jr. is a senior research associate and associate professor of Materials Science and Engineering in the Transducer Division of the Systems Engineering Department at Penn State University, University Park, PA. He joined the Applied Research Laboratory at Penn State in November 2000. His undergraduate degree was completed in 1993 in ceramic science and engineering within the Materials Science and Engineering Department at Penn State and was the student marshal for the graduating class. A masters degree was conferred in 1995 in the materials program at the Materials Research Laboratory at Penn State under the direction of Thomas Shrout and Shoko Yoshikawa. He completed his Ph.D. degree in October 1998 with the development of high-frequency 13 composite transducers. After graduation, he studied 1 year as a post-doctoral scholar under the supervision of Dr. R. E. Newnham, then was promoted to research associate at the Materials Research Laboratory, Pennsylvania State University, University Park, PA. His research interests include ceramic processing, development of undersea and medical sonic and ultrasonic devices, and composites materials for actuators and transducers. He was the recipient of the 2003 Office of Naval Research Young Investigator Award for the study of high-power, piezoelectric materials. He currently has more than 40 published papers and three patents in the field of ultrasound and actuator material technology.

zhou et al.: analysis of devices for high-frequency ultrasonic application David J. Van Tol works in the Transducer and Arrays Group at the Applied Research Laboratory, Penn State University, University Park, PA. He has been involved in a broad range of transducer development projects. Examples of low frequency work (1–10 kHz) include an underwater acoustic intensity probe and a 2 kHz projector/hydrophone pair used in a subsoil imaging system. In the middle frequencies (20–500 kHz), he designed and built transducers for many applications. These include acoustic lens systems, rock locators for a dredging barge, an acoustic intensity probe, and an ocean velocimetry system. He also has been active in the higher frequencies (0.5– 5 MHz), having built imaging systems using acoustic lenses as well as building some prototype transducers for other sonar imaging systems. He acquired a B.S. degree in electrical engineering from Iowa State, Ames, IA, in 1991 and an M.S. degree in acoustics from Pennsylvania State University, College Park, PA, in 1996, and is currently working toward his Ph.D. degree at Pennsylvania State University.

W. Jack Hughes is a senior scientist at the Applied Research Laboratory at Pennsylvania State University, University Park, PA, and is a professor of acoustics. He was in charge of the Transducer and Arrays Group from 1980 to 2003. Since 1965 he has been responsible for the design, fabrication, and calibration of transducers and arrays used in many Navy research and development sonar systems for weapons and mine hunting. He is active in the areas of linear, two-dimensional, planar, and cylindrical arrays, including theory, fabrication, and testing. He has over 38 years experience in the design of tonpilz transducers and arrays, and has developed a very broad bandwidth magnetostrictive/piezoelectric element and array. He also has developed shaped sensor technology with PVDF and 1-3 composite materials for use in advanced conformal arrays, and continues to be involved in transducer array self-noise reduction. Dr. Hughes also was involved in medical ultrasonic transducers and concepts for a diver-held sonar system. His graduate students have generated theses in many areas, and encompassing frequencies from ultrasonic arrays down to subaudio frequencies. Dr. Hughes received his B.S. degree in physics in 1964 from Rensselaer Polytechnic Institute, Troy, NY, and his Ph.D. degree in acoustics in 1978 from The Pennsylvania State University. He is a Fellow in the Acoustical Society of America. He has over 40 papers, seven patents, and numerous symposium and workshop presentations in his field.

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K. Kirk Shung (S’73–M’75–SM’89–F’93) obtained a B.S. degree in electrical engineering from Cheng-Kung University in Tainan, Taiwan in 1968, a M.S. degree in electrical engineering from the University of Missouri, Columbia, MO, in 1970, and a Ph.D. degree in electrical engineering from University of Washington, Seattle, WA, in 1975. He did postdoctoral research at Providence Medical Center in Seattle, WA, for 1 year before being appointed a research bioengineer holding a joint appointment at the Institute of Applied Physiology and Medicine, Seattle, WA. He became an assistant professor at the Bioengineering Program, Pennsylvania State University, University Park, PA, in 1979 and was promoted to professor in 1989. He was a Distinguished Professor of Bioengineering at Pennsylvania State until September 1, 2002, when he joined the Department of Biomedical Engineering, University of Southern California, Los Angeles, CA, as a professor. He has been the director of National Institutes of Health (NIH) Resource on Medical Ultrasonic Transducer Technology since 1997. Dr. Shung is a Fellow of the IEEE, the Acoustical Society of America, and the American Institute of Ultrasound in Medicine. He is a founding Fellow of the American Institute of Medical and Biological Engineering. He has served for two terms as a member of the NIH Diagnostic Radiology Study Section. He received the IEEE Engineering in Medicine and Biology Society early career award in 1985 and co-authored a best paper published in IEEE Transactions on UFFC in 2000. He was the distinguished lecturer for the IEEE UFFC society for 2002–2003. He was elected an outstanding alumnus of Cheng-Kung University in 2001. Dr. Shung has published more than 160 papers and book chapters. He is the author of a textbook, Principles of Medical Imaging, published by Academic Press in 1992. He co-edited a book, Ultrasonic Scattering by Biological Tissues, published by CRC Press in 1993. Dr. Shung’s research interest is in ultrasonic transducers, highfrequency ultrasonic imaging, and ultrasonic scattering in tissues.

Susan Trolier-McKinstry is a professor of ceramic science and engineering and director of the W. M. Keck Smart Materials Integration Laboratory at the Pennsylvania State University, University Park, PA. Her main research interests include electroceramic thin films for actuator and dielectric applications, the development of texture in bulk ceramic piezoelectrics, and spectroscopic ellipsometry. Her B.S., M.S., and Ph.D. degrees were obtained at Pennsylvania State University in ceramic science. She has held visiting appointments at Hitachi Central Research Laboratory, Tokyo, Japan, the Army Research Laboratory, Fort Monmouth, NJ, and the Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland. She is a member of the American Ceramic Society, the Materials Research Society, and IEEE. She is past-president of Keramos and the Ceramics Education Council, and is co-chair of the committee revising the IEEE Standard on Ferroelectricity. She is vice-president for ferroelectrics of the IEEE UFFC. She is the recipient of the Robert Coble Award of the American Ceramic Society, the Wilson Award for Outstanding Teaching in the College of Earth and Mineral Sciences, the Materials Research Laboratory Outstanding Faculty Award, and a National Science Foundation (NSF) Career grant.