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

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

Equivalent Circuit-Based Analysis of CMUT Cell Dynamics in Arrays

Cataloged from PDF version of article.Capacitive micromachined ultrasonic transducers (CMUTs) are usually composed of large arrays of closely packed cells. In this work, we use an equivalent circuit model to analyze CMUT arrays with multiple cells. We study the effects of mutual acoustic interactions through the immersion medium caused by the pressure field generated by each cell acting upon the others. To do this, all the cells in the array are coupled through a radiation impedance matrix at their acoustic terminals. An accurate approximation for the mutual radiation impedance is defined between two circular cells, which can be used in large arrays to reduce computational complexity. Hence, a performance analysis of CMUT arrays can be accurately done with a circuit simulator. By using the proposed model, one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with an arbitrary number of CMUT cells. We performed several finite element method (FEM) simulations for arrays with small numbers of cells and showed that the results are very similar to those obtained by the equivalent circuit model

A BCB Diaphragm Based Adhesive Wafer Bonded CMUT Probe for Biomedical Application

This dissertation presents the design methodology, fabrication procedure, and key experimental characterization results of a linear array of capacitive micromachined ultrasonic transducers (CMUT) for possible ophthalmic anterior segment imaging application. The design methodology involves analytical, 3-D electromechanical finite element analysis, and Verasonics Vantage 128 ultrasonic research platform based diagnostic imaging simulations to develop a technique that minimizes electrical charging and center frequency drift while improving the transduction efficiency. In the design, Bisbenzocyclobutene (BCB), a low K polymer from Dow Chemical Company, has been innovatively used for the first time to fabricate the structural layer of the CMUT diaphragm, realize the interelectrode dielectric spacer, and to act as a low temperature adhesive bonding agent. Additionally, the top CMUT electrode has been placed at the bottom of the diaphragm to affect higher capacitance change that increases sensitivity and provides additional decoupling of the electrical charging effects. Several arrays with element count ranging from 8 to 128 elements and a center frequency range of 5 MHz to 40 MHz have been designed and fabricated. Due to an unforeseen adhesion issue during wirebonding, a 32 channel 40 MHz CMUT array has been packaged manually to validate the fabrication process and CMUT operation. Extensive SEM inspections of the CMUT cross-sections show good agreement with the design specifications. Static and dynamic measurements using a Polytec laser Doppler vibrometer, impedance measurement using an Agilent vector network analyzer, and LCR measurement results are in excellent agreement with analytical and FEA analysis using IntelliSuite. The frequency analysis exhibits high electromechanical coupling coefficient of 0.66 at a low bias voltage of 20 V and high uniformity. A successful measurement of the lower drift of the center frequency 0.32% and higher coupling coefficient verifies the hypothesis that the excellent electrical, structural, and processing characteristics of BCB is a viable option to mitigate the dielectric charging and improve the transduction efficiency of CMUTs

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

A Two-Dimensional CMUT Linear Array for Underwater Applications: Directivity Analysis and Design Optimization

Capacitive micromachined ultrasonic transducers (CMUTs) are one of the promising MEMS devices. This paper proposed an integrated vibration membrane structure to design a two-dimensional CMUT linear array for underwater applications. The operation frequencies for different medium have been calculated and simulated, which are 2.5 MHz in air and 0.7 MHz in water. The directivity analyses for the CMUT cell, subarray, and linear array have been provided. According to the product theorems, the directivity function of the complex array is obtained using a combination of the directivity functions of certain simple structures. Results show that the directivity of a CMUT cell is weak due to the small size, but the directivity of the designed linear array is very strong. Influential parameters of the linear array have been discussed, including the cell numbers, the adjacent distance, and the operation medium. In order to further suppress the side lobe interference and improve the resolution and the imaging quality of the imaging system, several weighting methods are used for optimization and comparison. Satisfactory side lobe suppression results are obtained, which can meet the actual requirements

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

Radiation Impedance of Collapsed Capacitive Micromachined Ultrasonic Transducers

Cataloged from PDF version of article.The radiation impedance of a capacitive micromachined ultrasonic transducer (CMUT) array is a critical parameter to achieve high performance. In this paper, we present a calculation of the radiation impedance of collapsed, clamped, circular CMUTs both analytically and using finite element method (FEM) simulations. First, we model the radiation impedance of a single collapsed CMUT cell analytically by expressing its velocity profile as a linear combination of special functions for which the generated pressures are known. For an array of collapsed CMUT cells, the mutual impedance between the cells is also taken into account. The radiation impedances for arrays of 7, 19, 37, and 61 circular collapsed CMUT cells for different contact radii are calculated both analytically and by FEM simulations. The radiation resistance of an array reaches a plateau and maintains this level for a wide frequency range. The variation of radiation reactance with respect to frequency indicates an inductance-like behavior in the same frequency range. We find that the peak radiation resistance value is reached at higher kd values in the collapsed case as compared with the uncollapsed case, where k is the wavenumber and d is the center-to-center distance between two neighboring CMUT cells

A 2D CMUT Array for Liver Elastography

Proper diagnosis of liver disease at an early stage is very crucial for its effective treatment. Chronic liver disease if not treated can often lead to cirrhosis. At present, liver biopsy is the main diagnostic procedure. A non-destructive, patient friendly diagnostic imaging technology is necessary. Piezoelectric transducer used for High Intensity Focused Ultrasound (HIFU) has a narrower bandwidth, thus a single transducer cannot be used for both imaging and HIFU operation. But capacitive micromachined ultrasonic tranducers (CMUTs) can be used for both HIFU and imaging. In this thesis work, a unique design has been proposed where a first attempt has been made to design a 2D CMUT array for dual mode operation to detect liver cancer. First is the HIFU mode, where the device transmits a focused ultrasound to the target region in the liver just to momentarily deform (Strain) the liver tissue. Then the same device is switched to second mode which is an imaging mode. In this mode the ultrasonic image is captured using elastography process. Analytical and 3D finite element analysis has been carried out in MATLAB and Intellisuite software, respectively. Bisbenzocyclobutene (BCB), a low K polymer is used as a primary structural material for the CMUT membrane and also for the structure of the CMUT. Gold is used as a top conductor placed on top of the diaphragm. Analytical calculations have been done on several CMUT cells in an array while designing the final array. Elements ranging from 8x8, 16x16, 32x32, and 64x64 have been analyzed. Finally, 64x64 array have been used for best results with a center frequency of 7.5 MHz which are in great agreement with FEA analysis done using Intellisuite software

Rayleigh-Bloch waves in CMUT arrays

Cataloged from PDF version of article.Using the small-signal electrical equivalent circuit of a capacitive micromachined ultrasonic transducer (CMUT) cell, along with the self and mutual radiation impedances of such cells, we present a computationally efficient method to predict the frequency response of a large CMUT element or array. The simulations show spurious resonances, which may degrade the performance of the array. We show that these unwanted resonances are due to dispersive Rayleigh-Bloch waves excited on the CMUT surface-liquid interface. We derive the dispersion relation of these waves for the purpose of predicting the resonance frequencies. The waves form standing waves at frequencies where the reflections from the edges of the element or the array result in a Fabry-Pérot resonator. High-order resonances are eliminated by a small loss in the individual cells, but low-order resonances remain even in the presence of significant loss. These resonances are reduced to tolerable levels when CMUT cells are built from larger and thicker lates at the expense of reduced bandwidth. © 2014 IEEE