Experiment and simulation validated analytical equivalent circuit model for piezoelectric micromachined ultrasonic transducers

An analytical Mason equivalent circuit is derived for a circular, clamped plate piezoelectric micromachined ultrasonic transducer (pMUT) design in 31 mode, considering an arbitrary electrode configuration at any axisymmetric vibration mode. The explicit definition of lumped parameters based entirely on geometry, material properties, and defined constants enables straightforward and wide-ranging model implementation for future pMUT design and optimization. Beyond pMUTs, the acoustic impedance model is developed for universal application to any clamped, circular plate system, and operating regimes including relevant simplifications are identified via the wave number-radius product ka. For the single-electrode fundamental vibration mode case, sol-gel Pb(Zr[subscript 0.52])Ti[subscript 0.48]O[subscript 3] (PZT) pMUT cells are microfabricated with varying electrode size to confirm the derived circuit model with electrical impedance measurements. For the first time, experimental and finite element simulation results are successfully applied to validate extensive electrical, mechanical, and acoustic analytical modeling of a pMUT cell for wide-ranging applications including medical ultrasound, nondestructive testing, and range finding.Masdar Institute of Science and Technology (Massachusetts Institute of Technology Cooperative Agreement Grant 6923443)National Science Foundation (U.S.). Graduate Research Fellowshi

High frequency CMUT for continuous monitoring of red blood cells aggregation

Récemment, de nombreuses recherches ont démontré que le transducteur ultrasonore micro-usiné capacitif CMUT peut être une alternative aux transducteurs piézoélectriques dans différents domaines, y compris l’imagerie par ultrasons médicaux. Des travaux antérieurs ont démontré les avantages de CMUT en termes de production à haute fréquence, de sensibilité, de compatibilité avec la technologie complémentaire métal – oxyde – semi-conducteur et de coût de fabrication peu élevé. Ce travail montrera les travaux préliminaires en vue de la fabrication d'un transducteur à ultrasons utilisant des CMUT pour mesurer en continu l'agrégation des globules rouges. Les cellules CMUT ont été conçues et simulées pour obtenir des fréquences de résonance et des dimensions spécifiques répondant à cet objectif, à l'aide de la modélisation par éléments finis avec COMSOL Multiphysics. Des simulations par ultrasons (logiciel Field II) ont été utilisées pour caractériser les faisceaux ultrasonores émis et reçus afin de concevoir la distribution géométrique des cellules. La fabrication a été réalisée en utilisant une photolithographie multicouche et des dépôts. Huit masques ont été conçus pour chaque couche de dépôt. Les masques ont été conçus pour comporter quatre groupes de CMUT, le premier émettant et recevant à 40 MHz, le second émettant à 30 MHz et recevant à 40 MHz, le troisième émettant à 20 MHz et recevant à 30 MHz, et le dernier émettant à 10 MHz. MHz et réception à 30 MHz. La fréquence change avec le rayon de chaque cellule CMUT, mais les dimensions de l'épaisseur sont les mêmes pour toutes les cellules, les épaisseurs des membranes et des couches isolantes sont de 0,3 µm et l'intervalle de vide est de 0,1 µm. Les matrices CMUT ont été fabriquées à l'aide de la technologie de couche de libération sacrificielle du laboratoire Polytechnique LMF.Research has demonstrated that Capacitive Micro machined Ultrasonic Transducer (CMUT) can be an alternative to piezoelectric transducers in different domains including medical ultrasound imaging. Previous work showed advantages of CMUT in terms of high frequency production, sensitivity, its compatibility with complementary metal–oxide–semiconductor technology and its low cost of fabrication. This work will show preliminary work toward fabricating an ultrasound transducer using CMUTs to continuously measure Red Blood Cells aggregation. CMUTs cells were designed and simulated to obtain specific resonant frequencies and dimension that fulfill that purpose using finite element modeling with COMSOL Multiphysics. Ultrasound simulations (Field II software) were used to characterize the emitted and received US beams to design the cells geometrical distribution. Fabrication was done using multilayered photolithography and depositions. Eight masks were designed for each deposition layer. The masks were designed to have four groups of CMUTs, one emitting and receiving at 40MHz, a second emitting at 30 MHz and receiving at 40 MHz, a third one emitting at 20 MHz and receiving at 30 MHz, and a last one emitting at 10 MHz and receiving at 30 MHz. The frequency changes with the radius of each CMUT cell but the thickness dimensions are the same for all the cells, the membranes and insulation layers thicknesses are 0.3 µm and the vacuum gap is 0.1 µm. The CMUT arrays were fabricated using sacrificial release layer technology in Polytechnic LMF Lab

A new detection method for capacitive micromachined ultrasonic transducers

Cataloged from PDF version of article.Capacitive micromachined ultrasonic transducers (cMUT) have become an alternative to piezoelectric transducers in the past few years. They consist of many small circular membranes that are connected in parallel. In this work, we report; a new detection method for cMUTs. We model the membranes as capacitors and the interconnections between the membranes as inductors. This kind of LC net-work is called an artificial transmission line. The vibrations of the membranes modulate the electrical length of the transmission line, which is proportional to the frequency of the signal through it. By measuring the electrical length of the artificial line at a high RF frequency (in the gigahertz range), the vibrations of the membranes can be detected in a very sensitive manner. Far the devices we measured, we calculated the minimum detectable displacement to be in the order of 10(-5) Angstrom/root Hz with a possible improvement to 10(-7) Angstrom/root Hz

Surface micromachined capacitive ultrasonic transducers

Cataloged from PDF version of article.The current state of a novel technology, surface microfabricated ultrasonic transducers, is reported. Experiments demonstrating both air and water transmission are presented. Air-coupled longitudinal wave transmission through aluminum is demonstrated, implying a 110 dB dynamic range for transducers at 2.3 MHz in air. Water transmission experiments from 1 to 20 MHz are performed, with a measured 60 dB SNR at 3 MHz. A theoretical model is proposed that agrees well with observed transducer behavior. Most significantly, the model is used to demonstrate that microfabricated ultrasonic transducers constitute an attractive alternative to piezoelectric transducers in many applications

Radiation Impedance of an Array of Circular Capacitive Micromachined Ultrasonic Transducers

Cataloged from PDF version of article.The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications

High Power CMUTs: Design and experimental verification

Cataloged from PDF version of article.Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with −28 dBc second harmonic at the surface of the array

Fabrication of CMUTS based on PMMA adhesive wafer bonding

Capacitive Micromachined Ultrasonic Transducers (CMUTs) are the potential alternatives for the conventional piezoelectric ultrasonic transducers. CMUTs have been under an extensive research and development since their first development in the mid- 1990s. Initially developed for air-coupled applications, CMUTs have shown far better acceptability in immersion-based applications (i.e. medical ultrasonic imaging, medical therapy, and underwater imaging) when compared to the piezoelectric ultrasonic transducers. CMUTs are parallel-plate capacitors fabricated using the Micro Electro Mechanical Systems (MEMS) technology. Despite of the fact that various CMUT fabrication methods have been reported in the literature, there are still many challenges to address in CMUTs design and fabrication. Standard fabrication techniques are further sub-divided into the Sacrificial Layer Release Process and the Wafer Bonding methods. A number of complications are associated with these techniques, such as optimization of the design parameters, process complexity, sacrificial layer material with the corresponding etchant selection, wafer cost and selection. In particular, the sacrificial release methods consist of complex fabrication steps. Furthermore, structural parameters like gap height and radius have optimization issues during the sacrificial release process. On the other hand, the wafer bonding techniques for the CMUTs fabrication are simple and have a great control over the structure parameters in contrast to the sacrificial release methods. At the same time, the wafer-bonded CMUTs require very high quality wafer surface and have a very high contamination sensitivity. For this purpose, this dissertation aims to develop a simple, low cost and lower constraint thermocompression-based technique for the CMUT fabrication. The proposed wafer bonding technique for the CMUT fabrication in the dissertation uses Polymethyl methacrylate (PMMA) adhesive as an intermediate layer for the thermocompression wafer bonding. The advantages associated with the PMMA adhesivebased wafer bonding over the other wafer bonding methods include low process temperature (usually 200 C or less), high wafer surface defects and contamination tolerance, high surface energy and low bonding stresses. These factors will add cost effectiveness and simplicity to the CMUTs fabrication process. Furthermore, the achieved receive sensitivity with the reported CMUT is found comparable to the commercially available ultrasonic transducer

Deep collapse mode capacitive micromachined ultrasonic transducers

Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 2010.Thesis (Ph. D.) -- Bilkent University, 2010.Includes bibliographical references leaves 107-117.Capacitive micromachined ultrasonic transducers (CMUTs) are suspended microelectromechanical membrane structures with a moving top electrode and a rigid substrate electrode. The membrane is actuated by electrical signals applied between the electrodes, resulting in radiated pressure waves. CMUTs have several advantages over traditional piezoelectric transducers such as their wider bandwidth and microfabrication methodology. CMUTs as microelectromechanical systems (MEMS), are fabricated using CMOS compatible processes and suitable for batch fabrication. Low cost production of large amount of CMUTs can be fabricated using already established integrated circuit (IC) technology infrastructure. Contrary to piezoelectrics, fabricating large 2-D arrays populated with transducer elements using CMUTs is low-cost. The technological challenges of CMUTs regarding the fabrication and integration issues were solved during the past 15 years, and their successful operation has been demonstrated in many applications. However, commercialization of CMUTs is still an overdue passion for CMUT community. The bandwidth of the CMUTs are inherently superior to their piezoelectric rivals due to the nature of the suspended membrane structure, however, their power output capability must be improved to achieve superior signal-to-noise ratio and penetration depth. In this thesis, we gave a comprehensive discussion about the physics and functionality of CMUTs and showed both theoretically and experimentally that their power outputs can be increased substantially. Using the conventional uncollapsed mode of CMUTs, where the suspended membrane vibrates freely, the lumped displacement of the membrane is limited. Limited displacement, unfortunately, limits the power output of the CMUT. However, a larger lumped displacement is possible in the collapsed state, where the membrane gets in contact with the substrate. By controlling the movement of the membrane in this state, the power output of the CMUTs can be increased. We derived the analytical expressions for the profile of a circular CMUT membrane in both uncollapsed and collapsed states. Using the profiles, we calculated the forces acting on the membrane and the energy radiated to the medium during an applied electrical pulse. We showed that the radiated energy can be increased drastically by utilizing the nonlinear forces on the membrane, well beyond the collapse voltage. Using the analytical expressions, we developed a nonlinear electrical equivalent circuit model that can be used to simulate the mechanical behavior of a transmitting CMUT under any electrical excitation. Furthermore, the model can handle different membrane dimensions and material properties. It can predict the membrane movement in the collapsed state as well as in the uncollapsed state. In addition, it predicts the hysteretic snap-back behavior of CMUTs, when the electric potential across a collapsed membrane is decreased. The nonlinear equivalent circuit was simulated using SPICE circuit simulator, and the accuracy of the model was tested using finite element method (FEM) simulations. Better than 3% accuracy is achieved for the static deflection of a membrane as a function of applied DC voltage. On the other hand, the pressure output of a CMUT under large signal excitation is predicted within 5% accuracy. Using the developed model, we explained the dynamics of a CMUT membrane. Based on our physical understanding, we proposed a new mode of operation, the deep collapse mode, in order to generate high power acoustic pulses with large bandwidth (>100% fractional) at a desired center frequency. We showed both by simulation (FEM and equivalent circuit) and by experiments that the deep collapse mode increases the output pressure of a CMUT, substantially. The experiments were performed on CMUTs fabricated at Bilkent University by a sacrificial release process. Larger than 3.5 MPa peak-to-peak acoustic pulses were measured on CMUT surface with more than 100% fractional bandwidth around 7 MHz using an electrical pulse amplitude of 160 Volts. Furthermore, we optimized the deep collapse mode in terms of CMUT dimensions and parameters of the applied electrical pulse, i.e., amplitude, rise and fall times, pulse width and polarity. The experimental results were compared to dynamic FEM and equivalent circuit simulations. We concluded that the experimental results are in good agreement with the simulations. We believe that CMUTs, with their high transmit power capability in the deep collapse mode can become a strong competitor to piezoelectrics.Olçum, SelimPh.D