BUIR Repository :: Browsing by Subject "CMUT"

Browsing by Subject "CMUT"

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Open Access
Acoustical tuning of CMUT receiver arrays
(IEEE, 2016) Taşdelen, Akif Sinan; Atalar, Abdullah; Enhoş, Kerem; Köymen, Hayrettin
Cell placement in an element and structural modifications on the array baffle significantly change the bandwidth, band shape and signal to noise ratio of a CMUT receiver array. In this paper, optimum receiver performance tailoring by means of cell placement, cell size variation and use of dummy cells in the array elements is discussed. The performance of the array is modified acoustically at the acoustic port of the elements.
Open Access
Batch-compatible micromanufacturing of a CMUT array for optoacoustic imaging of tissue-like phantoms
(2021-08) Özyiğit, Doğu Kaan Buğra
Photoacoustic imaging (PAI), also named optoacoustic imaging, is a technol-ogy for medical imaging that relies on contrast data due to optical stimulation. Capacitive micromachined ultrasound transducers (CMUTs) are previously in-troduced for PAI applications. In this thesis, the provided CMUT array design has been partially micro-manufactured separately from electronics and a laser ﬁber light source while re-serving the necessary chip space for integration with electronics and laser ﬁber light source. Batch compatible wafer-scale microfabrication of CMUT arrays was done by a combination of novel as well as traditional MEMS microfabrication pro-cesses. CMUT array gaps, bottom electrodes, and insulation layer were formed on the Pyrex wafer using three separate photolithography masks. Anodic wafer bonding method is used for the formation of the top electrodes and top side of the gap heights of CMUT arrays. Process development for anodic wafer bonding between Pyrex wafers and SOI wafers has been done, where the Pyrex wafers have been previously processed with plasma etching, wet etching, metal stack de-position, insulation layer deposition, and insulation layer patterning, while SOI wafers have been used as received. Pyrex wafers and SOI wafers were anodically bonded to each other with developed anodic wafer bonding processes. After full completion of the micromanufacturing of the CMUT array chips, these CMUT ar-ray chips will be integrated with ASIC chips. Then, CMUT array chips and ASIC chips will be combined with a traditional printed circuit board (PCB). These in-tegrated CMUT array chips, ASIC chips, and PCB are going to be integrated with a ﬁber laser light source inside a mechanically robust hand-held probe that is planned to be used for optoacoustic imaging. The main goal of this CMUT array micromanufacturing study is to signiﬁcantly contribute to the development of one of the necessary components for imaging of a tissue like-phantom using a hand-held imaging probe.
Open Access
Beam steering in a half-frequency driven airborne CMUT transmitter array
(IEEE Computer Society, 2019) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
An airborne Capacitive Micromachined Ultrasonic Transducer (CMUT) transmit array was designed using electromechanical modelling for unbiased airborne operation. The array elements are designed for maximum swing at 10V p-p unbiased drive, whereas conventional practice is to bias CMUT close to the collapsed voltage to achieve higher swing. The devices were fabricated using a customized single photolithographic process with a combination of wet and dry etching. The wafer level fabrication enabled the usage of 2x2 and 3x3 arrays. Driving CMUTs in an unbiased mode at half frequency drives the ‘static pressure’ depressed silicon membrane at a larger swing without letting it collapse. The 2x2 array displays 3.375 kHz bandwidth when characterized in air. The phase and amplitude differences due to the dispersion of resonance frequencies of the elements are compensated for beamformed and beamsteered airborne operation.
Open Access
Circuit theory based modeling and analysis of CMUT arrays
(2013) Oğuz, Hüseyin Kağan
Many ultrasonic technology applications require capacitive micromachined ultrasonic transducers (CMUTs) to be used in the form of large arrays to attain better performance in terms of powerful, broadband and beam-formed radiated acoustic signals. To entirely benefit from its important characteristics, it is necessary to use analysis tools that are capable of handling multiple CMUT cells. In this regard, finite element analysis (FEA) tools become unfit for use because in arrays with large number of cells it is computationally very cumbersome and often practically impossible. Although, some simplification had been done by assuming long 1-D CMUT array elements as infinitely long, the results of these FEA simulations are misleading. In these models only a single periodic portion is modeled and rigid boundary conditions are applied at the symmetry planes. All the cells are assumed to be electrically driven in phase with the rest of the cells and the solution obtained for this portion is extended over the entire element. However, these simple models are not exact, because they exclude the important effects of mutual acoustic interactions between the cells. In this work, we developed an accurate nonlinear equivalent circuit model for circular uncollapsed CMUT cells. We investigated the effects of mutual acoustic interactions in uncollapsed CMUT arrays and showed that the performance of the array is highly influenced with this phenomenon. These mutual acoustic interactions rise through the immersion medium caused by the pressure field generated by each cell acting upon the others. To study its effects, we connected each cell in the array to a radiation impedance matrix that contains the mutual radiation impedance between every pair of cells, in addition to their self radiation impedances. Hence, analysis of the performance of a large array became a circuit theory problem and can be scrutinized with circuit simulators Surface micromachining technology enables batch fabrication of large CMUT arrays, which resolves cost issues and many physical limitations. Designers have to consider a great number of different array configurations. For nearly two decades, the lack of appropriate design and analysis tools prevented the investigation of array performance. By using the proposed model, one can very rapidly obtain the linear frequency and nonlinear transient responses of arrays with a large number of uncollapsed CMUT cells. Although, we use rapid circuit theory techniques, efficient analysis of very large arrays is still challenging, since a typical CMUT array may contain many tens of elements with hundreds of cells in each, which makes it computationally cumbersome. To partition the problem, we electrically drive a small number of elements in the array and keep the rest undriven but biased and with their electrical ports terminated with a load. The radiation impedance matrix can be partitioned and rearranged to represent these loads in a reduced form. In this way, only the driven elements can be simulated by coupling their cells through this reduced radiation impedance matrix. Under small signal regime, the separately calculated responses of element clusters can be added by using the superposition principle to find the total response. This method considerably reduces the number of cells and the size of the actual radiation impedance matrix, at the expense of calculating the inverse of a large complex symmetric matrix.
Open Access
Deep collapse mode capacitive micromachined ultrasonic transducers
(2010) Olçum, Selim
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.
Open Access
Design and finite element simulation of a novel 3D-CMUT device for simultaneous sensing of in-plane and out-of-plane displacements of ultrasonic guided waves
(MDPI AG, 2023-10-25) Zhang, S.; Lu, W.; Wang, A.; Hao, G.; Wang, R.; Yilmaz, Mehmet
In this study, we introduce a physical model of a three-dimensional (3D) guided wave sensor called 3D-CMUT, which is based on capacitive micro-machined ultrasonic transducers (CMUTs). This 3D-CMUT sensor is designed to effectively and simultaneously obtain 3D vibration information about ultrasonic guided waves in the out-of-plane (z-direction) and in-plane (x and y-directions). The basic unit of the 3D-CMUT is much smaller than the wavelength of the guided waves and consists of two orthogonal comb-like CMUT cells and one piston-type CMUT cell. These cells are used to sense displacement signals in the x, y, and z-directions. To ensure proper functioning of the 3D-CMUT unit, the resonant frequencies of the three composed cells are set to be identical by adjusting the microstructural parameters appropriately. Moreover, the same sensitivity in the x, y, and z-directions is theoretically achieved by tuning the amplification parameters in the external circuit. We establish a transient analysis model of the 3D-CMUT using COMSOL finite element simulation software to confirm its ability to sense multimode ultrasonic guided waves, including A0, S0, and SH0 modes. Additionally, we simulate the ball drop impact acoustic emission signal on a plate to demonstrate that the 3D-CMUT can not only utilize in-plane information for positioning but also out-of-plane information. The proposed 3D-CMUT holds significant potential for applications in the field of structural health monitoring (SHM).
Open Access
Design, fabrication and operation of a very high intensity CMUT transmit array for beam steering applications
(2020-12) Khan, Talha Masood
Several studies have reported airborne ultrasound transmission systems focused on achieving beamforming. However, beam steering and beamforming for capacitive micromachined ultrasonic transducers (CMUTs) at high intensity remains to be accomplished. CMUTs, like other ultrasonic transducers, incorporate a loss mechanism to obtain a wide bandwidth. They are restricted to a limited amount of plate swing due to the gap between the radiating plate and the bottom electrode, along with a high dc bias operation. CMUTs can be designed to produce high-intensity ultrasound by employing an unbiased operation. This mode of operation allows the plate to swing the entire gap without collapsing, thus enabling higher intensity. In this study, we use an equivalent circuit-based model to design unbiased CMUT arrays driven at half the mechanical frequency. This model is cross verified using finite element analysis (FEA). CMUT arrays are produced in multiple configurations using a customized microfabrication process that involves anodic wafer bonding, a single lithographic mask, and a shadow mask. We use impedance measurements to characterize the microfabricated devices. We experimentally obtained the highest reported intensity using a microfabricated 2×2 CMUT array driven at resonance in a pulsed configuration. This array is also capable of beam steering and beamforming at a high intensity such that it can steer the entire half-space. The beam obtained from the array is in excellent agreement with the theoretical predictions. The amplitude and phase compensation for the devices remain constant that makes these arrays attractive for applications involving park assist, gesture recognition, and tactile displays.
Open Access
Designing, fabrication and post- fabrication characterization of half-frequency driven 16 x 16 waterborne transmit CMUT array
(2021-02) Abhoo, Yusuph Abubakar
Capacitive Micromachined Ultrasonic Transducers (CMUT) are micro-scaled electromechanical devices which are used to either transmit or receive pressure signals and applicable for various purposes such as ultrasonic sensor, medical imaging, accurate biometric sensing and parametric speakers. For transmitting CMUT transducer, different sizes and array configurations are used to intensify the transmission power depending on the application. The half-frequency driven waterborne transmitting CMUT array designed in this work is to be used for high resolution volumetric medical imaging purpose. This was accomplished by a design which prioritizes maximizing the power output, achieving a directive radiation pattern with low sidelobes which maximizes the beamformable region. In this work, the issues with steering of the focused beam are also resolved to achieve a focused steerable beam. This work is an advancement from the earlier designed half-frequency driven airborne transmit CMUT to improve power output, introduce the beamforming and focused transmission capabilities and be applicable for high resolution volumetric medical imaging purpose. To improve the power output, the design was made to compensate for the static depression. Compensating for static depression was achieved by designing to operate the CMUT without DC bias voltage which allows for full-gap swing and giving output signal of twice the input frequency. This property allows the cell to produce high power output with low voltage levels but also brings the advantage of operating the cell with very high voltages without collapsing. The CMUT was chosen to be operating at 7.5 MHz and be driven by Digital Phased Array System (DiPhAS) which allowed to have maximum of 256 channels which for volumetric transmission meant a maximum of 16 x 16 array. Since the radiation pattern and Rayleigh distance are both the functions of radius, frequency and the pitch, the design optimization was found while considering all the above preferences simultaneously. The cells’ radii were determined to be 80 µm, the plate thickness was 15 µm, the gap height was found to be 117 nm and the pitch was 192 µm. The array designing was carried out using the large-signal equivalent circuit model and the radiation impedance matrix phenomenon. The simulations showed that with this design, the maximized Rayleigh distance was 45.3 mm and the sidelobe of -17.4 dB. In simulations, very high pressure outputs were achievable with individual cells up to 425 kPa per cell with 150 VPP input while up to 1.5 MPa was emitted by the array plane wave transmission with only 10 VPP input and almost doubles when the transmitted beam was focused at zero degrees. Fabrication was done by the wafer boding and flip-chip bonding techniques where the whole process required only two lithography masks. After fabrication, the tests were performed to identify the yield of the transducer was 18.75% of the array then impedance analysis was done to characterize the functional cells and resonance frequency drift. The transducer was cased in a water-tight manner and the waterborne transmission were done with individual cells to characterize and compare the performance with the design simulations which were in the range of agreement achieving an average of 1625 Pa per cell. The functional cells were then used for plane wave transmission with 10 VPP and the output pressure of 397 kPa was achieved at resonance frequency. The measurement results showed that the design could further be improved by compensating the active area to improve the yield for better results and be able to use it for high resolution 3D medical imaging.
Open Access
Electrically unbiased and half frequency driven waterborne 16×16-element 2-D phased array CMUT
(2019-08) Enhoş, Kerem
Capacitive micromachined ultrasonic transducers (CMUT) are typically used as arrays consisting of separate cells or interconnected sub arrays, and these cells have several operation modes. Design procedure and measurements have been previously presented for an airborne CMUT cell without a DC bias. In unbiased mode, the plate motion spans the entire gap without collapsing. The frequency of sinusoidal electrical input signal is half of the resulting acoustical output signal. A large plate swing can be obtained at low excitation voltages using the entire remaining gap. In this work, a design procedure of arrays with unbiased operation mode is derived. Designs are validated by means of simulations and measurements on fabricated CMUTs. The problems associated with beamforming are resolved. Use of this operation mode in an array configuration enables larger acoustical power output. A better transmitted signal waveform definition is obtained when pulse width modulation (PWM) is employed. In the design process of CMUTs, large-signal equivalent circuit model is used. In order to have volumetric transmission, a 16×16 (256 elements) phased array configuration is chosen. A design procedure is presented considering the radiation pattern, Rayleigh distance and the parameters of the lumped-element model. This procedure is applied for designing two arrays, with resonance frequencies at 7.5 MHz and at 18.5 MHz. Harmonic balance and transient analyses are carried out with Gaussian enveloped tone burst, transient sinusoidal and PWM signals with different duty cycles. Outputs of these simulations are fed in beamforming toolboxes for further verification. Corresponding experimental measurements are conducted on an ultrasound measurement system. The dimensions of the CMUT elements in the array are determined as 80 μm element radius, 15 μm plate thickness, and 171 nm effective gap height at 7.5 MHz, where the center-to-center inter-element pitch is set at 192 μm. The array is designed such that it has maximum Rayleigh distance of 45.3 mm, while having a maximum sidelobe level of −17.4 dB. According to these specifications, CMUT array is manufactured using wafer scale batch compatible production. Only two lithography masks, which require conventional photolithography steps, are used in production. The vibrating plate is constructed with anodic wafer bonding and the fabricated CMUT array chip is integrated to PCB with flip-chip bonding. Measurements are conducted for this novel device by integrating the CMUT array chip manufactured with MEMS techniques and conventional macro scale manufactured PCB by using impedance analyser.
Open Access
High-Intensity airborne CMUT transmitter array with beam steering
(IEEE, 2020) Khan, Talha Masood; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
A 2×2 high-intensity CMUT transmit array that is capable of two-dimensional beam steering is presented. The device uses an ac drive voltage at half the ultrasound frequency without any dc bias, enabling the usage of the entire gap height. The device is designed using a large signal equivalent model approach. A fabrication method that requires a single lithographic mask has been used. The fabricated devices are operated at 76 kHz to beam steer at various angles. An equivalent element pressure of 144 dB// 20 μ Pa at the transducer surface was measured. The entire half-space can be steered without any sidelobes and the beam obtained from the array is in excellent agreement with the theoretical predictions. [2020-0253]
Open Access
Mic-in-CMOS: CMUT as a sealed-gap capacitive microphone
(IEEE, 2020) Köymen, Hayrettin; Ahiska, Y.; Atalar, Abdullah; Köymen, I.; Taşdelen, A .Sinan; Yılmaz, Mehmet
The design and production of a CMOS compatible, watertight and ingress-proof CMUT (capacitive micromachined ultrasonic transducer) microphone, mic-in-CMOS, with vacuum-gap is described. We present an analytical model-based approach for the design of mic-in-CMOS, where a basis for quantitative comparison of performance trade-offs is provided. The sealed vacuum gap of the mic-in-CMOS is basically a lossless sensor, free of mechanical noise. Its SNR is determined by the noise of the pre-amplification electronics (the noise contributor in a CMUT with vacuum gap is essentially the radiation resistance, which is less than 0 dBA for audio band for a 1 mm2 device). The design of mic-in-CMOS involves many multilateral trade-offs such as gap height vs membrane thickness vs sensitivity vs need for linear operation vs bias voltage and atmospheric depression, to name few. The mic-inCMOS design can be mass produced using CMOS film stacks only, as such the fabrication process can be carried out entirely in a CMOS processes production line complemented with CMOS compatible post-processing approaches. Mic-inCMOS has the advantage of low production cost with minimal packaging requirement and on-die EMI / EMC.
Open Access
A micromachined pressure sensor
(2017-09) Karaca, Hasan
Capacitive Micromachined Ultrasonic Transducer (CMUT) is a microelectromechanical device that is basically formed by a moving top electrode, a stable bottom electrode and a gap in between. In spite of its this simple mass-spring construction, CMUT is a nonlinear device and its working principles have been formulated. According to these studies, the top electrode can be set in motion by the applied pressure on it and by depending on the amount of that pressure, the resonant frequency of the CMUT can be altered. Therefore, it is possible to use CMUT to obtain a pressure sensor. In this respect, what we have to do is keep tracking of its resonant frequency to deduce the pressure. The most e ective way of doing it, on the other hand, is using an oscillator circuit which also provides us the capability of tracking the resonant frequency in real time. Also, to design an integrated circuit that works with the CMUT, the best way is utilizing a Colpitts oscillator. In this thesis, we design a pressure sensor with CMUT based Colpitts oscillator. In order to achieve our design, rst of all, we examine the small signal equivalent circuit model of an uncollapsed mode CMUT and investigate the related analytical equations that models the behavior of it. To simplify the equations, we liken the small signal equivalent circuit model to a crystal oscillator by making necessary transformations. After that, we investigate the \feedback system approach" and \negative resistance concept" methods that help us to analyze the oscillator circuits; and we determine the Colpitts oscillator circuit as the oscillator circuit part of our device. We evaluate the CMUT based Colpitts oscillator circuit and derive the limitations on the circuit parameters for achieving a power e cient device. In addition to that, we discuss the dc biasing of the oscillator circuit that does not cause any loading e ect on the oscillator circuit and design a ring oscillator and a charge pump circuit which help us to obtain bias voltage on the CMUT. Finally, we calculate the sensitivity (in Hz/Pa) and the temperature sensitivity of a CMUT in addition to the Quality factor of our circuit; and by being based on these calculations, we obtain the optimum CMUT parameters for the best available sensitivity and conclude the design. At the end, we design a CMUT based Colpitts oscillator that works as a pressure sensor which measures pressure between zero atm and one atm with sensitivity of 14.6 Hz/Pa at 1 atm. The selected CMUT parameters, on the other hand, for the radius of the CMUT cell, the gap height of the CMUT cell, the thickness of the insulator layer and the thickness of the top plate are 44 m, 100 nm, 100 nm and 3 m respectively. The Quality factor of the circuit is 5 and the inherent Quality factor of the CMUT is 432.
Open Access
Nonlinear modelling of an immersed transmitting capacitive micromachined ultrasonic transducer for harmonic balance analysis
(2009) Oğuz, Hüseyin Kağan
Finite element method (FEM) is used for transient dynamic analysis of capacitive micromachined ultrasonic transducers (CMUT), which is particularly useful when the membranes are driven in the nonlinear regime. A transient FEM analysis shows that CMUT exhibits strong nonlinear behavior even at very low AC excitation under DC bias. One major disadvantage of FEM is the excessive time required for simulation. Harmonic Balance (HB) analysis, on the other hand, provides an accurate estimate of the steady-state response of nonlinear circuits very quickly. It is common to use Mason’s equivalent circuit to model the mechanical section of CMUT. However, it is not appropriate to terminate Mason’s mechanical LC section by a rigid piston’s radiation impedance, especially, for an immersed CMUT. We studied the membrane behavior using a transient FEM analysis and found out that for a wide range of harmonics around the series resonance, the membrane displacement can be modeled as a clamped radiator. We considered the root mean square of the velocity distribution on the membrane surface as the circuit variable rather than the average velocity. With this definition the kinetic energy of the membrane mass is the same as that in the model. We derived the force and current equations for a clamped radiator and implemented them in a commercial HB simulator. We observed much better agreement between FEM and the proposed equivalent model, compared to the conventional model.
Open Access
Optimization of a collapsed mode CMUT receiver for maximum off-resonance sensitivity
(Institute of Electrical and Electronics Engineers, 2018-07-28) Khan, M.; Khan, T. M.; Taşdelen, A. S.; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
We propose an airborne collapse capacitive micromachined ultrasonic transducer (CMUT) as a practical viable ultrasound transducer capable of providing a stable performance at the off-resonance frequencies. Traditional practice is to bias the CMUT plate close to collapse voltage to achieve high coupling coefficient and sense the incoming ultrasound as an open-circuit receive voltage signal of the transducer or short-circuit receive current (SCRC). Maintaining CMUT plate in the vicinity of collapse threshold is rather difficult. In this paper, an analytic approach to design an airborne collapsed-mode CMUT for maximum off-resonance sensitivity is presented. We use small-signal circuit model to evaluate the performance of a collapsed CMUT for varying operating conditions. CMUT operational parameters that yield the highest off-resonance SCRC are directly obtained from performance design curves. Collapsed CMUT plate is then biased in a critical biasing region that produces a stable and maximum off-resonance sensitivity. We experimentally verify and measure a stable sensitivity of a fabricated collapsed CMUT cell of -60 dB V/Pa at 100 kHz when biased between 50 to 65 V. We characterize our linear circuit model performance against the measured performance of collapsed CMUT in air within 4-dB tolerance. [2018-0058]
Embargo
Process development for microfabrication of phase reversal CMUT devices for structural health monitoring and development of dynamic characterization processes for MEMS applications
(2024-08) Küçük, Merve Mintaş
If appropriately designed, Capacitive Micromachined Ultrasonic Transducers (CMUTs) offer advantageous properties such as low cost, small size, low impedance, and environmental friendliness, over piezoelectric transducers. These advantageous properties of CMUTs enable the CMUT devices to be employed in a large area of applications, such as medical applications and non-destructive testing (NDT) applications. CMUT devices and technologies that are heavily developed for medical applications also shed light on the development of CMUT devices to be used in Structural Health Monitoring (SHM) applications for civil infrastructures. Continuous monitoring of the signals produced by the sudden changes happening within civil infrastructures such as bridges or railways may give crucial information about the health of these structures. The rapid release of localized strain energy, which generates Acoustic Emission (AE) waves, is an important indicator of the state of the health of a structure. Detecting AE wave signals may give significant clues about damage formation such as impact, crack initiation, or crack growth. Because AE waves are scattered among a broad range of frequencies, sensing of such AE waves should also be done in broadband, and sensors are preferred to be highly sensitive among such band. For real-life applicable developments, it should be also considered that the environment of the real-life application may be very noisy due to many unrelated reasons, which makes employment of the CMUTs developed in a tightly controlled laboratory environment unpractical for the real-life applications. The noise may often be induced by the noise interferences that are produced by a variety of events that are not needed to be detected. To prevent misjudgments, it is important to differentiate between noise interferences and relevant AE signals, as the presence of significant noise can hinder the detectability of AE waves associated with structural damage. In this process development for CMUT prototype microfabrication study, we collaborated with a group of researchers who have introduced a new approach to designing broadband CMUTs, as well as a unique type of CMUT combination that uses phase-reversal (PR) of generated electrical current for detecting a wide range of mechanical vibration wave frequencies and reducing unwanted noise. By considering the simplest combination of two CMUT cells, the theoretical study, supported by FEM simulations, demonstrated that reversing the electrical current phase of one cell can create low-frequency and high-frequency stopbands for noise rejection, which is applicable for CMUTs operating in air damping. The primary objective of this thesis study is to develop microfabrication processes to microfabricate PR-CMUT devices to bridge the gap between theoretical design and real-world application of PR-CMUT devices. These PR-CMUT arrays that are designed for wafer-scale batch-compatible manufacturability have a flat passband in the 200-250 kHz and 200-300 kHz frequency ranges and two improved stopbands on both sides of the relevant frequency ranges. The photolithography masks, compatible material selections, and microfabrication process flows (integration processes) required for the microfabrication of these PR-CMUT devices were designed considering the capabilities of our cleanroom facility. Microfabrication of the devices was tried multiple times, and in line with the problems encountered in these processes, the microfabrication process flows were updated and the PR-CMUT devices were tried to be produced in multiple iterations. Unit processes, and multiple integration processes were developed and completed. Possible solutions to be implemented in the future microfabrication studies were determined. Additionally, dynamic characterization of individual circular geometry CMUT membranes were explored using a ZYGO Optical Profilometer. With this measurement tool (ZYGO), it is possible to measure CMUT device membrane displacements precisely when the membrane of the CMUT device is moving (vibrating) dynamically. Results obtained from ZYGO Optical Profilometer tool were compared with the impedance analyzer results. It was shown that the resonance frequency of a circular membrane CMUT device can be observed with the ZYGO Optical Profilometer. Furthermore, based on the conclusions from the studies in this thesis, future studies are suggested for further development towards realization and characterization of these PR-CMUT MEMS (MicroElectroMechanical System) devices.
Open Access
A transimpedance amplifier design for capacitive micromachined ultrasonic transducers operating at 7.5MHz
(2021-01) İlhan, Giray
Capacitive Micromachined Ultrasonic Transducers (CMUTs) are MEMS devices used in ultrasound imaging, e.g. ultrasound mammography. CMUT proved to be a viable transducer solution in ultrasound mammography without the hazardous effects of conventional X-ray mammography. The CMUTs have a very high electrical impedance, where a transimpedance amplifier (TIA) is most appropriate for preamplification during reception. A TIA with 25MHz bandwidth and 120kΩ transimpedance gain is designed on Cadence Virtuoso using XFAB XC06M3 process. The CMUT small signal model is used for 50μm radius and 7.5MHz operating frequency. This model is incorporated into TIA circuit simulations. Two design options are proposed for the TIA, one with the passive feedback resistor and the other with a MOSFET feedback resistor. The latter enabled us to save space in layout design compare to the former. Transient and noise simulations are conducted and compared for schematic and layout views. The input referred current noise of the TIA is simulated to be 0.5pA/√Hz and the power consumption is simulated to be approximately 3.3mW for both designs.
Open Access
A transimpedance amplifier for capacitive micromachined ultrasonic transducers
(2015-12) Kansu, Yavuz
In this thesis a design of a CMOS transimpedance amplifier (TIA) for a capacitive micro-machined ultrasonic transducer (CMUT) is presented. CMUT’s have a high electrical impedance when used as receivers. Any capacitance between the CMUT and a high impedance amplifier will degrade the frequency response. So, we need to amplify the current rather than the voltage. This approach requires a transimpedance amplifier. The designed TIA has a 30 MHz bandwidth and 400 kΩ transimpedance gain. The total input referred current noise of the TIA is 270 fA/√Hz at 10 MHz. The noise figure of the TIA is 2.7 dB at 10 MHz when connected to the CMUT with 200 kΩ source resistance. The power consumption of the TIA is 10.5 mW and the size of the TIA layout is 133µm x 45µm. The TIA chip will be fabricated in AMS C35B4C3 (0.35µm) process.
Open Access
Transmitting CMUT arrays without a DC bias
(IEEE Computer Society, 2019) Enhoş, Kerem; Taşdelen, Akif Sinan; Yılmaz, Mehmet; Atalar, Abdullah; Köymen, Hayrettin
This study focuses on design, simulation, fabrication and measurement of transmitting CMUT arrays with unbiased mode of operation. We presented a design procedure considering the radiation pattern, Rayleigh distance and the parameters of the large signal equivalent circuit, which can be applied for different operation frequencies and applications. Large signal equivalent circuit model is used for lumped-element simulations. Harmonic balance and transient analyses are carried out with Gaussian enveloped tone burst, sinusoidal and pulse width modulation (PWM) signals with different duty cycles. Outputs of these simulations are fed in beamforming toolboxes for further verification. According to the design specifications transmitting CMUT arrays are fabricated. Corresponding experimental impedance measurements are conducted.
Open Access
Tunable Q matching networks for capacitive ultrasound transmitters
(Springer New York LLC, 2021-05-06) Khan, M.; Khan, Talha Masood
Airborne capacitive micromachined ultrasonic transducers (CMUTs) have predominantly large input capacitive reactance with small series radiation resistance. To maximize the acoustic power radiation at resonance we employ low cost LC matching networks between the low output impedance driving source and CMUT transducer. Conventional LC networks, including pi and T-network topologies are employed to provide a high-Q match. An important free parameter that controls the bandwidth of match in pi and T-networks is the center impedance. Since a simple pi-network can be formed from two basic L-sections, the center impedance of two L-sections provides control over the bandwidth of match. Hence these are tunable-Q matching networks. We use the lumped circuit model of CMUT to derive the input impedance of CMUT at resonance. At resonance the lumped equivalent circuit of transducer can be reduced to a series RC circuit consisting of radiation resistance and transducer’s equivalent capacitance only. From the input impedance of CMUT we determine the lumped inductance and capacitances of various LC matching networks. The aim of these matching networks is to cancel the large input capacitive reactance of CMUT cell and to match the cell’s radiation resistance to source resistance at resonance. We report significant improvement in the radiated power when CMUT is driven at resonance using the proposed matching schemes.
Open Access
Unbiased charged circular CMUT microphone: lumped-element modeling and performance
(Institute of Electrical and Electronics Engineers, 2018) Köymen, Hayrettin; Atalar, Abdullah; Güler, S.; Köymen, I.; Taşdelen, A. S.; Ünlügedik, A.
An energy-consistent lumped-element equivalent circuit model for charged circular capacitive micromachined ultrasonic transducer (CMUT) cell is derived and presented. It is analytically shown and experimentally verified that a series dc voltage source at the electrical terminals is sufficient to model the charging in CMUT. A model-based method for determining this potential from impedance measurements at low bias voltages is presented. The model is validated experimentally using an airborne CMUT, which resonates at 103 kHz. Impedance measurements, reception measurements at resonance and off-resonance, and the transient response of the CMUT are compared with the model predictions.