Micromachined two-dimensional array piezoelectrically actuated transducers (original) (raw)
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Sensors, 2015
Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of OPEN ACCESS Sensors 2015, 15 8021 PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.
Theory and operation of 2-D array piezoelectric micromachined ultrasound transducers
IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 2008
Piezoelectric micromachined ultrasound transducers (pMUTs) are a new approach for the construction of 2-D arrays for forward-looking 3-D intravascular (IVUS) and intracardiac (ICE) imaging. Two-dimensional pMUT test arrays containing 25 elements (5 × 5 arrays) were bulk micromachined in silicon substrates. The devices consisted of lead zirconate titanate (PZT) thin film membranes formed by deep reactive ion etching of the silicon substrate. Element widths ranged from 50 to 200 μm with pitch from 100 to 300 μm. Acoustic transmit properties were measured in de-ionized water with a calibrated hydrophone placed at a range of 20 mm. Measured transmit frequencies for the pMUT elements ranged from 4 to 13 MHz, and mode of vibration differed for the various element sizes. Element capacitance varied from 30 to over 400 pF depending on element size and PZT thickness. Smaller element sizes generally produced higher acoustic transmit output as well as higher frequency than larger elements. Thicker PZT layers also produced higher transmit output per unit electric field applied. Due to flexure mode operation above the PZT coercive voltage, transmit output increased nonlinearly with increased drive voltage. The pMUT arrays were attached directly to the Duke University T5 Phased Array Scanner to produce real-time pulse-echo B-mode images with the 2-D pMUT arrays.
Piezoelectrically actuated flextensional MUTs
2001 IEEE Ultrasonics Symposium. Proceedings. An International Symposium (Cat. No.01CH37263), 2001
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 up to 4.5 MHz. Such an array could be combined with on-board driving and addressing circuitry for different applications. Classical thin plate theory and Mindlin plate theory are applied to derive two-dimensional plate equations for the transducer, and to calculate the coupled electromechanical field variables such as mechanical displacement and electrical input impedance. In these methods, the variations across the thickness direction vanish by using the bending moments per unit length or stress resultants. Thus, two-dimensional plate equations for a step-wise laminated circular plate are obtained as well as two different solutions to the corresponding systems. An equivalent circuit of the transducer is also obtained from these solutions.
2001 IEEE Ultrasonics Symposium. Proceedings. An International Symposium (Cat. No.01CH37263), 2001
In this paper, we present a technique for the deposition of inks, toners, organic polymers, fuels, small solid particles, biological and chemical fluids, using a fluid ejector. The ejector design is based on a flextensional transducer that excites the axisymmetric resonant modes of a clamped circular plate. It is constructed by depositing a thin piezoelectric annular plate onto a thin, edge clamped, circular plate. Liquids or solid-particles are placed behind one face of the plate which has a small orifice at its center. By applying an ac signal across the piezoelectric element, continuous or drop-on-demand ejection of fluids has been achieved. The ejected drop size ranges in diameter from 4 µm at 3.5 MHz to 150 µm at 7 kHz, the corresponding ejected drop volume ranges from 34 fl to 1.5 nl, and the corresponding flow rate ranges from 117 nl/s to 10 µl/s. The unique features of the device are that the fluid is not pressurized, the fluid container is chemically or biologically compatible with most fluids, and the vibrating plate contains the orifice as the ejection source. The device is manufactured by silicon surface micromachining and implemented in the form of two-dimensional arrays. Individual elements are made of thin silicon nitride membranes covered by a coating of piezoelectric zinc oxide. Classical thin plate theory and Mindlin plate theory are applied to derive two-dimensional plate equations for the transducer, and to calculate the coupled electromechanical field variables such as mechanical displacement and electrical input impedance. In these methods, the variations across the thickness direction vanish by using the bending moments per unit length or stress resultants. Thus, two-dimensional plate equations for a step-wise laminated circular plate are obtained as well as two different solutions to the corresponding systems. An equivalent circuit of the transducer is also obtained from these solutions.
Micromachined Ultrasonic Transducers
Piezoelectric and Acoustic Materials for Transducer Applications, 2008
In this chapter the basic principles, the fabrication process, and some modelling approaches of the novel micromachined ultrasonic transducers (MUTs) are described. These transducers utilize the flex-tensional vibration of an array of micro membranes. They are usually called cMUT (capacitive Micromachined Ultrasonic Transducer) or pMUT (piezoelectric Micromachined Ultrasonic Transducer) depending on the actuation principle, electrostatic or piezoelectric. For water coupling applications both these kinds of transducers offer a better matching to the load compared with the typical piezoelectric transducers and therefore they have a larger intrinsic bandwidth. Here emphasis is given to the cMUTs because they have shown good electroacoustic characteristics, which parallel, or even exceed, those of conventional piezoelectric transducers. Good echographic images of internal organs of the human body have been obtained demonstrating the possibilities of this technology to be utilized in commercial 1D and 2D probes for medical applications. At present pMUTs are in a very early stage of development and the potential advantages over the cMUTs are still to be demonstrated.
Development of Broadband High-Frequency Piezoelectric Micromachined Ultrasonic Transducer Array
Sensors
Piezoelectric micromachined ultrasonic transducers (PMUT) are promising elements to fabricate a two-dimensional (2D) array with a pitch small enough (approximately half wavelength) to form and receive arbitrary acoustic beams for medical imaging. However, PMUT arrays have so far failed to combine the wide, high-frequency bandwidth needed to achieve a high axial resolution. In this paper, a polydimethylsiloxane (PDMS) backing structure is introduced into the PMUTs to improve the device bandwidth while keeping a sub-wavelength (λ) pitch. We implement this backing on a 16 × 8 array with 75 µm pitch (3λ/4) with a 15 MHz working frequency. Adding the backing nearly doubles the bandwidth to 92% (−6 dB) and has little influence on the impulse response sensitivity. By widening the transducer bandwidth, this backing may enable using PMUT ultrasonic arrays for high-resolution 3D imaging.
Silicon Micromachined Ultrasonic Transducers
Japanese Journal of Applied Physics, 2000
This paper reviews capacitor micromachined ultrasonic transducers (cMUTs). Transducers for air-borne and immersion applications are made from parallel-plate capacitors whose dimensions are controlled through traditional integrated circuit manufacturing methods. Transducers for airborne ultrasound applications have been operated in the frequency range of 0.1-11 MHz, while immersion transducers have been operated in the frequency range of 1-20 MHz. The Mason model is used to represent the cMUT and highlight the important parameters in the design of both airborne and immersion transducers. Theory is used to compare the dynamic range and the bandwidth of the cMUTs to piezoelectric transducers. It is seen that cMUTs perform at least as well if not better than piezoelectric transducers. Examples of single-element transducers, linear-array transducers, and two-dimensional arrays of transducers will be presented.
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, 2013
Several micromachining techniques for the fabrication of high-frequency piezoelectric composite ultrasonic array transducers are described in this paper. A variety of different techniques are used in patterning the active piezoelectric material, attaching backing material to the transducer, and assembling an electronic interconnection board for transmission and reception from the array. To establish the feasibility of the process flow, a hybrid test ultrasound array transducer consisting of a 2-D array having an 8 × 8 element pattern and a 5-element annular array was designed, fabricated, and assessed. The arrays are designed for a center frequency of ~60 MHz. The 2-D array elements are 105 × 105 µm in size with 5-µm kerfs between elements. The annular array surrounds the square 2-D array and provides the option of transmitting from the annular array and receiving with the 2-D array. Each annular array element has an area of 0.71 mm 2 with a 16-µm kerf between elements. The active piezoelectric material is (1 − x) Pb(Mg 1/3 Nb 2/3)O 3-xPbTiO 3 (PMN-PT)/epoxy 1-3 composite with a PMN-PT pillar lateral dimension of 8 µm and an average gap width of ~4 µm, which was produced by deep reactive ion etching (DRIE) dry etching techniques. A novel electric interconnection strategy for high-density, small-size array elements was proposed. After assembly, the array transducer was tested and characterized. The capacitance, pulse-echo responses, and crosstalk were measured for each array element. The desired center frequency of ~60 MHz was achieved and the −6-dB bandwidth of the received signal was ~50%. At the center frequency, the crosstalk between adjacent 2-D array elements was about −33 dB. The techniques described herein can be used to build larger arrays containing smaller elements. Changgeng Liu obtained his Ph.d. degree in engineering from Tsinghua University, beijing, china, in 2001, and his b.s. and m.s. degrees in aircraft design and mechanics from nanjing University of aeronautics and astronautics, nanjing, china. He is currently a senior scientist at Geospace research Inc., El segundo, ca, and a visiting scholar at the nIH resource center on medical Ultrasonic Transducer Technology of the University of southern california, los angeles, ca. Prior to joining Geospace research Inc. in 2007, dr. liu held a position as a research associate at the center for advanced microstructures and devices (camd) of louisiana state University, baton rouge, la. dr. liu is a senior member of IEEE. He has published more than 50 journal papers. dr. liu's research interests include high-frequency ultrasonic transducers and arrays, piezoelectric composite materials, mEms/ biomEms, and micromachined biomedical devices.