Piezoelectric films for high frequency ultrasonic transducers in biomedical applications

Piezoelectric films have recently attracted considerable attention in the development of various sensor and actuator devices such as nonvolatile memories, tunable microwave circuits and ultrasound transducers. In this paper, an overview of the state of art in piezoelectric films for high frequency transducer applications is presented. Firstly, the basic principles of piezoelectric materials and design considerations for ultrasound transducers will be introduced. Following the review, the current status of the piezoelectric films and recent progress in the development of high frequency ultrasonic transducers will be discussed. Then details for preparation and structure of the materials derived from piezoelectric thick film technologies will be described. Both chemical and physical methods are included in the discussion, namely, the sol-gel approach, aerosol technology and hydrothermal method. The electric and piezoelectric properties of the piezoelectric films, which are very important for transducer applications, such as permittivity and electromechanical coupling factor, are also addressed. Finally, the recent developments in the high frequency transducers and arrays with piezoelectric ZnO and PZT thick film using MEMS technology are presented. In addition, current problems and further direction of the piezoelectric films for very high frequency ultrasound application (up to GHz) are also discussed.     

Surface acoustic wave characteristics and electromechanical coupling coefficient of lead zirconate titanate thin films

Lead zirconate titanate (PZT) thin films were created on ST-X quartz using radio frequency magnetron sputtering deposition. PZT films deposited on quartz are used as a new piezoelectric substrate for surface acoustic wave (SAW) devices. Microelectromechanical technique was used to fabricate interdigital transducers on the surface of the substrate to be used as a SAW delay line device. The results show that the PZT film was successfully deposited on ST-X quartz, and that the PZT film on ST-X quartz can enhance the electromechanical coupling coefficients of SAW.     

High-frequency transducers based on integrated piezoelectric thick films for medical imaging

A screen-printed PZT thick film with a final thickness of about 40 microm was deposited on a porous PZT substrate to obtain an integrated structure for ultrasonic transducer applications. This process makes it possible to decrease the number of steps in the fabrication of high-frequency, single-element transducers. The porous PZT substrates allow high acoustic impedance and attenuation to be obtained, satisfying transducer backing requirements for medical imaging. The piezoelectric thick films deliver high electromechanical performance, comparable to that of standard bulk ceramics (thickness coupling factor over 45%). Based on these structures, high-frequency transducers with a center frequency of about 25 MHz were produced and characterized. As a result, good sensitivity and axial resolution were obtained in comparison with similar transducers integrating a lead titanate (PT) disk as active material. The two transducers were integrated into a high-frequency imaging system, and comparative skin images are shown.     

A 100-MHz ultrasonic transducer array using ZnO thin films

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-μm-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO₃-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 μm wide, 3.2 mm long, and 10 μm thick, and the pitch of an array was typically 100 μm. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 μm at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices     

Polyurea thin film ultrasonic transducers for nondestructive testing and medical imaging

Ultrasonic transducers using polyurea piezoelectric thin film are studied in this paper. Aromatic polyurea thin films, prepared by vapor deposition polymerization, have useful characteristics for use as an ultrasonic transducer. This paper presents the fabrication and experimental evaluation of ultrasonic transducers formed using polyurea films. First, the vapor deposition polymerization process using two monomers is briefly reviewed, and the temperature conditions for higher piezoelectric constants are explored. Second, in order to test the fundamental characteristics of this material as a high-frequency, ultrasonic transducer, a polyurea film of 2.5 microm thickness was deposited on a silicon substrate. In the pulse/echo experiment results, a resonant frequency of about 100 MHz was observed. Third, we fabricated a concave point focus transducer and a cylindrical line focus transducer. To examine the performances of the focus transducers, two-dimensional images of a coin and V(z) curve measurements for an aluminum surface were demonstrated.     

Piezoelectrically actuated flextensional micromachined ultrasound transducers--II: fabrication and experiments

This paper presents novel micromachined two-dimensional array piezoelectrically actuated flextensional transducers that can be used to generate sound in air or water. Micromachining techniques to fabricate these devices are also presented. Individual unimorph array elements consist of a thin piezoelectric annular disk and a thin, fully clamped, circular plate. We manufacture the transducer in two-dimensional arrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85 MHz with 0.15 μm/V peak displacement. The devices have a range of operating resonance frequencies starting from 450 kHz to 4.5 MHz. Such an array could be combined with on-board driving and addressing circuitry for different applications     

力电多场鼓包法测定PZT铁电薄膜的横向压电系数

为表征Pb(Zr_(0.52)Ti_(0.48))O_3(PZT)薄膜的横向压电性能,以纯力场鼓包测试模型和铁电薄膜材料压电方程为基础,推导了PZT铁电薄膜的力电耦合鼓包本构模型。采用溶胶-凝胶法制备了PZT铁电薄膜,并通过化学腐蚀法获得PZT薄膜鼓包样品。在外加电压为0~14V的条件下进行鼓包测试。结果表明,在纯力场作用下,PZT薄膜的弹性模量和残余应力分别为91.9GPa和36.2MPa;随着电压从2V变化到14V,PZT薄膜的横向压电系数d31从-28.9pm/V变化到-45.8pm/V。本工作所发展的力电耦合鼓包测试技术及力电耦合鼓包本构模型为评价铁电薄膜材料的横向压电性能提供了一种有效的分析方法。     

Fabrication and characterization of annular thickness mode piezoelectric micro ultrasonic transducers

Micromachining techniques, in combination with low temperature ceramic composite sol-gel processing, have been used to fabricate annular array thickness-mode piezoelectric micro ultrasonic transducers (Tm-pMUTs). The processing techniques of low temperature (710 degrees C) composite sol-gel ceramic (sol + ceramic powder) deposition and wet etching were used to deposit and structure 27-microm thick lead zirconate titanate (PZT) films on silicon substrates to produce annular array Tm-pMUTs. Using these techniques, high quality PZT materials with near bulk permittivity have been obtained. The Tm-pMUT devices were shown to resonate at approximately 60 MHz in air and 50 MHz in water. From resonance measurements k(t) values ranging between 0.2 and 0.47 have been calculated and shown to depend on the level of porosity within the film. Lower values of kt were observed for films with higher levels of porosity, which was attributed to the relative decrease in the effective piezoelectric coefficient epsilon(33) with respect to stiffness and permittivity as a function of increasing porosity. This paper presents the successful micro-fabrication of a Tm-pMUT device and discusses the optimization of the poling conditions and effect of PZT microstructure on the coupling coefficient k(t). Pulse echo measurements in water, showing a -6 dB center frequency of 53 MHz and 47% -6 dB bandwidth, using a target 15 mm away from the transducer, have been included to demonstrate successful operation of the device. Full analysis of these results will be conducted in later publications.     

Flexible ultrasonic transducers

Flexible ultrasonic transducers (UTs) consisting of a metal foil, a piezoelectric ceramic film, and a top electrode have been developed. The flexibility is realized owing to the porosity of piezoelectric film and the thinness of metal foil. In this paper, the stainless steel (SS), lead-zirconate-titanate (PZT)/PZT composite and silver paste were chosen as metal foil, piezoelectric film, and top electrode materials, respectively. The SS foil serves as both substrate and bottom electrode. The PZT/PZT piezoelectric composite film is made by the sol-gel spray technique. PZT/PZT films of thicknesses from 40 to 70 microm were fabricated onto SS foils. The capability of these flexible sensors operated in the pulse-echo mode for nondestructive testing on flat and curved surfaces of different materials at room temperature and 160 degrees C has been demonstrated. Numerical simulations of the effects of the metal foil thickness on the ultrasonic performance of flexible UTs also were carried out, and the results are in reasonable agreement with experimental data. In addition, a PZT/PZT flexible transducer showed a signal strength comparable with that obtained by a commercial room temperature broad bandwidth transducer.     

Piezoelectric micromachined ultrasonic transducers based on PZT thin films

This paper describes fabrication and characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-microm-thick Pb(Zr0.53Ti0.47O3) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially unclamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-microm plate and amounted to kappa2 = 5.3%. This value compares well with the calculated value based on measured small signal dielectric (epsilon = 1050) and piezoelectric (e3l,f = 15 Cm(-2)) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasi-identical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid.     

80-MHz intravascular ultrasound transducer using PMN-PT free-standing film

[Pb(Mg(1/3)Nb(2/3))O(3)](0.63)[PbTiO(3)](0.37) (PMN-PT) free-standing film of comparable piezoelectric properties to bulk material with thickness of 30 μm has been fabricated using a modified precursor coating approach. At 1 kHz, the dielectric permittivity and loss were 4364 and 0.033, respectively. The remnant polarization and coercive field were 28 μC/cm(2) and 18.43 kV/cm. The electromechanical coupling coefficient k(t) was measured to be 0.55, which was close to that of bulk PMN-PT single-crystal material. Based on this film, high-frequency (82 MHz) miniature ultrasonic transducers were fabricated with 65% bandwidth and 23 dB insertion loss. Axial and lateral resolutions were determined to be as high as 35 and 176 μm. In vitro intravascular imaging on healthy rabbit aorta was performed using the thin film transducers. In comparison with a 35-MHz IVUS transducer, the 80-MHz transducer showed superior resolution and contrast with satisfactory penetration depth. The imaging results suggest that PMN-PT free-standing thin film technology is a feasible and efficient way to fabricate very-high-frequency ultrasonic transducers.     

Fabrication and characterization of micromachined high-frequency tonpilz transducers derived by PZT thick films

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-microm 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 x 10(-6) omega x m. Randomly oriented PZT (52/48) films up to 7-microm 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 microC/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-microm 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.     

Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.     

Two-dimensional ultrasound receive array using an angle-tuned Fabry-Perot polymer film sensor for transducer field characterization and transmission ultrasound imaging

A 2-D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically induced changes in the optical thickness of a thin polymer film acting as a Fabry-Perot sensing interferometer (FPI). By illuminating the sensor with a large-area laser beam and mechanically scanning a photodiode across the reflected output beam, while using a novel angle-tuned phase bias control system to optimally set the FPI working point, a notional 2-D ultrasound array was synthesized. To demonstrate the concept, 1-D and 2-D ultrasound field distributions produced by planar 3.5-MHz and focused 5-MHz PZT ultrasound transducers were mapped. The system was also evaluated by performing transmission ultrasound imaging of a spatially calibrated target. The "array" aperture, defined by the dimensions of the incident optical field, was elliptical, of dimensions 16 x 12 mm and spatially sampled in steps of 0.1 mm or 0.2 mm. Element sizes, defined by the photodiode aperture, of 0.8, 0.4, and 0.2 mm were variously used for these experiments. Two types of sensor were evaluated. One was a discrete 75-microm-thick polyethylene terephthalate FPI bonded to a polymer backing stub which had a wideband peak noise-equivalent pressure of 6.5 kPa and an acoustic bandwidth 12 MHz. The other was a 40-microm Parylene film FPI which was directly vacuum-deposited onto a glass backing stub and had an NEP of 8 kPa and an acoustic bandwidth of 17.5 MHz. It is considered that this approach offers an alternative to piezoelectric ultrasound arrays for transducer field characterization, transmission medical and industrial ultrasound imaging, biomedical photoacoustic imaging, and ultrasonic nondestructive testing.     

25 MHz ultrasonic transducers with lead- free piezoceramic, 1-3 PZT fiber-epoxy composite, and PVDF polymer active elements

This paper presents the fabrication and characterization of single-element ultrasonic transducers whose active elements are made of lead-free piezoceramic, 1-3 PZT/polymer composite and PVDF film. The lead free piezoelectric KNNLT- LS(K_0.44Na_0.52Li_0.04)(Nb_0.84Ta_0.10S_0.06b)O₃ powders and ceramics were prepared under controlled humidity and oxygen flow rate during sintering. Due to its moderate longitudinal piezoelectric charge coefficient (175 pC/N) and k_t of 0.50, the KNN-LT-LS composition may be a good candidate for highfrequency transducer applications. PZT fibers with 25 μm diameter formed by the viscose suspension spinning process were incorporated into epoxy to fabricate 1-3 composites with the averaged k_t = 0.64 and d₃₃ = 400 pC/N. Using KNN-LS-LT ceramic, 1-3 PZT fiber composite, and PVDF film, 3 different unfocused single element transducers with center frequencies of 25 MHz were fabricated. The acoustic characterization of the transducers demonstrated that wideband and low insertion loss could be obtained employing KNN-LS-LT ceramic. The ?6 dB bandwidth and insertion loss were 70% and ?21 dB, respectively. In comparison, the insertion loss of the ceramic transducer was much smaller than those made with 1-3 composite and PVDF film. This was attributed to closer electrical impedance match to 50 Ω and higher thickness coupling coefficient of the ceramic transducer.     

Hybrid multi/single layer array transducers for increased signal-to-noise ratio

In medical ultrasound imaging, two-dimensional (2-D) array transducers are necessary to implement dynamic focusing in two dimensions, phase correction in two dimensions and high speed volumetric imaging. However, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance, which decreases the transducer signal-to-noise ratio (SNR). We have previously shown that SNR is improved using transducers made from multi-layer PZT, due to their lower electrical impedance. In this work, we hypothesize that SNR is further increased using a hybrid array configuration: in the transmit mode, a 10 Omega electronic transmitter excites a 10 Omega multi-layer array element; in the receive mode, a single layer element drives a high impedance preamplifier located in the transducer handle. The preamplifier drives the coaxial cable connected to the ultrasound scanner. For comparison, the following control configuration was used: in the transmit mode, a 50 Omega source excites a single layer element, and in the receive mode, a single layer element drives a coaxial cable load. For a 5x102 hybrid array operating at 7.5 MHz, maximum transmit output power was obtained with 9 PZT layers according to the KLM transmission line model. In this case, the simulated pulse-echo SNR was improved by 23.7 dB for the hybrid configuration compared to the control. With such dramatic improvement in pulse-echo SNR, low voltage transmitters can be used. These can be fabricated on integrated circuits and incorporated into the transducer handle.     

Micromachined high frequency ferroelectric sonar transducers

Millimeter-sized ferroelectric monomorph sonar transducers have been built using sol-gel PZT on micromachined silicon wafers. First generation transducer arrays with diaphragms varying in size from 0.2 to 2 mm were tested. Second generation 8×8 arrays have also been built and tested in water in the frequency range of 0.3 to 2 MHz. Improvements to the sol-gel process have yielded high-quality, crack-free PZT films up to 12 μm in thickness, which leads directly to higher sensitivity and figure of merit for acoustic transducers. The longitudinal piezoelectric coefficient d₃₃ is 140 to 240 pC/N, measured through a double beam laser interferometer. Remanent polarization of 28 μC/cm², a coercive field of 30 kV/cm, and dielectric constant of 1400 were measured on 4-μm thick films. Test results are presented, including frequency response, beam patterns, and sensitivity. High-resolution acoustic images have been generated using these transducers and a four-element underwater acoustic lens. Potential applications for these transducers include high-frequency imaging sonars, medical ultrasound, ultrasonic communication links, and flaw detection (NDT)     

Transducer design for a portable ultrasound enhanced transdermal drug-delivery system

For application in a portable transdermal drug-delivery system, novel transducers have been designed to enhance insulin transmission across skin using ultrasound. Previous research has shown transdermal delivery of insulin across skin using commercial sonicators operating at 20 kHz with intensities ranging from 12.5 to 225 mW/cm/sup 2/. The goal of this research was to design and construct a small, lightweight transducer or array that could operate with a similar frequency and intensity range as a commercial sonicator used in previous transdermal ultrasound insulin experiments, but without the weight and mass of a sonicator probe. To obtain this intensity range, a cymbal transducer design was chosen because of its light, compact structure and low resonance frequency in water. To increase the spatial ultrasound field for drug delivery across skin, two arrays, each comprising of four cymbal transducers, were constructed. The first array, designated the standard array, used four cymbals transducer elements in parallel. A second array (named the stack array) used four cymbal transducers that used stacked piezoelectric discs to drive the titanium flextensional caps. Under similar driving conditions, the standard array produced intensities comparable to those achieved using a commercial sonicator.     

PIMNT单晶弯曲圆盘换能器设计

新型弛豫铁电单晶材料铌铟酸铅-铌镁酸铅-钛酸铅晶体(Pb(In_1/2Nb_1/2) O₃-Pb(Mg_1/3Nb_2/3) O₃-PbTiO₃, PIMNT)的压电系数是陶瓷材料的 6 倍以上,应变量高出压电陶瓷 10 倍以上,具有较高的机电耦合系数,压电性能优于传统 PZT 材料。文章将单晶材料应用于带空气腔的弯曲圆盘换能器中,利用 ANSYS 仿真软件优化换能器结构,确定换能器尺寸,设计制作 PIMNT 压电单晶换能器和PZT4压电陶瓷换能器,并进行了水池实验。换能器实测结果与仿真结果保持一致,单晶换能器的谐振频率为 2.85 kHz,最大发送电压响应为 136.3 dB。结果表明,相比于同尺寸的陶瓷换能器,将 PIMNT 压电单晶应用于弯曲圆盘换能器可降低谐振频率,提高发送电压响应,提升换能器的工作性能,为进一步改善单晶换能器综合性能提供参考。     

Time Reversal Based Piezoelectric Transducer Self-diagnosis Under Varying Temperature

Conventional structural health monitoring (SHM) studies using surface-mountable wafer-type piezoelectric transducers such as Lead Zirconate Titanate (PZT) mainly have focused on structural damage identification, but not so much on functionality of the PZT transducers although the transducers themselves could be often the weakest link in the entire system. In this study, piezoelectric transducer self-diagnosis schemes are developed based on a time reversal process and explicitly considering temperature effects. Several transducer self-diagnosis indices are proposed so that debonded and cracked PZT conditions can be identified and distinguished from temperature variation. First, the proposed self-diagnosis schemes are theoretically formulated, and then the effectiveness of the proposed schemes under varying temperature is verified through numerical simulations and experimental tests.