Applications of Capacitive Micromachined Ultrasonic Transducers: A Comprehensive Review

The parameter mapping of power ultrasonic transducer model

The vibration and electrical characteristics of transducer is determined by material coefficients and geometry, with material coefficients being susceptible to factors including frequency, pressure, and temperature, which leads to poor repeatability of transducer characteristics. Consequently, it is challenging to provide an accurate theoretical model to predict the characteristics based on the current material coefficients. To achieve a more accurate transducer model, a measurement method is proposed based on the mapping between material coefficients and transducer characteristic parameters to obtain accurate coefficients under working conditions with simple equipment and lower costs. The mapping is analyzed based on the transducer model, identifying five key coefficients. An iterative optimization method is then developed to measure these coefficients. Additionally, the genetic algorithm (GA) method is utilized for cross-checking. Transducers made from seven different materials and with varying lengths are measured, and the coefficients are obtained by both methods. With the obtained coefficients, the vibration and electrical characteristics of multi-material transducers is predicted and found to be in good agreement with the measured values, validating the transducer model and the coefficient measurement method. These coefficients are then compared with results obtained from a dynamic mechanical analyzer (DMA) and reference values. The results demonstrate that theoretical coefficients obtained by the proposed method lead to more accurate predictions for the vibration and electrical characteristics compared to those obtained from the DMA and reference values. Furthermore, the influence of frequency on the coefficients is studied by the method. The iterative method and GA method are compared in terms of their relative errors.

An Investigation into High-Accuracy and Energy-Efficient Novel Capacitive MEMS for Tire Pressure Sensor Application

Tire pressure monitoring systems (TPMSs) are essential for maintaining driving safety by continuously monitoring critical tire parameters, such as pressure and temperature, in real time during vehicle operation. Among these parameters, tire pressure is the most significant, necessitating the use of highly precise, cost-effective, and energy-efficient sensing technologies. With the rapid advancements in micro-electro-mechanical system (MEMS) technology, modern automotive sensing and monitoring systems increasingly rely on MEMS sensors due to their compact size, low cost, and low power consumption. This study presents a novel high-precision capacitive pressure sensor based on a capacitive micromachined ultrasonic transducer (CMUT) structure and a silicon-silicon direct bonding process. The proposed design offers exceptional performance with high accuracy, ultra-low power consumption, and reduced production costs, making it an optimal solution for enhancing the precision and efficiency of TPMS. Leveraging its low power requirements, capacitive sensing technology emerges as a superior choice for energy-efficient systems in the automotive industry.

Flexible PZT-Based Row-Column Addressed 2-D PMUT Array

This article reports a row-column (RC) addressed flexible piezoelectric micromachined ultrasound transducer (PMUT) array with a top-down fabrication process. The fabrication uses a temporary carrier wafer from which the array device is released by deep reactive ion etching (DRIE). About 0.8- m-thick sol-gel processed lead zirconate titanate (PZT) thin film acts as the active piezoelectric. The flexible PMUT membrane includes the PZT film and a 14- m polyimide as a passive layer. A sidewall made of polyimide measuring m in thickness with a cavity of m in diameter is realized by reactive ion etching (RIE). Laser Doppler vibrometer (LDV) characterization of the PMUT indicates 2.7 and 2.1 MHz as the resonance frequency in-air and underwater, respectively. Excitation of a single PMUT element coupled with 5-V direct current (dc) bias results in 1.2-nm/V sensitivity in-air, whereas when the same is excited along with 10-V dc bias, a pressure response of 40 Pa/V at 1 cm is measured underwater using a hydrophone. The presented results under bending to an 8-mm bending radius show the potential for wearable applications in shallow-depth regions subject to further optimization.

Radiation Impedance of Rectangular CMUTs

Recently, capacitive micromachined ultrasound transducers (CMUTs) with long rectangular membranes have demonstrated performance advantages over conventional piezoelectric transducers; however, modeling these CMUT geometries has been limited to computationally burdensome numerical methods. Improved fast modeling methods, such as equivalent circuit models, could help achieve designs with even better performance. The primary obstacle in developing such methods is the lack of tractable methods for computing the radiation impedance of clamped rectangular radiators. This paper presents a method that approximates the velocity profile using a polynomial shape model to rapidly and accurately estimate radiation impedance. The validity of the approximate velocity profile and corresponding radiation impedance calculation was assessed using finite element simulations for a variety of membrane aspect ratios and bias voltages. Our method was evaluated for rectangular radiators with width:length ratios from 1:1 up to 1:25. At all aspect ratios, the radiation resistance was closely modeled. However, when calculating the radiation reactance, our initial approach was only accurate for low aspect ratios. This motivated us to consider an alternative shape model for high aspect ratios, which was more accurate when compared with FEM. To facilitate the development of future rectangular CMUTs, we provide a MATLAB script that quickly calculates radiation impedance using both methods.

A multi-aperture encoding scheme for increased SNR in photoacoustic Imaging

Photoacoustic imaging creates light-induced ultrasonic signals to provide valuable information on internal body structures and tissue morphology non-invasively. A multi-aperture photoacoustic imaging (MP-PAI) system is an improvement over conventional photoacoustic imaging (PAI) systems in terms of resolution, contrast, and field of view. Previously, a prototype MP-PAI system was introduced based on multiple capacitive micromachined ultrasound transducers (CMUTs) with shared channels, such that each element in a CMUT shares its channel with its counterpart in other CMUTs. The system uses the biasing voltages of the CMUTs to switch between them and multiplex the received signals in time. Notwithstanding all the enhancements, the signal-to-noise ratio (SNR) remains limited in PAI. To address this issue, we are proposing a multi-aperture encoding scheme (MAES) to further increase the SNR in a multi-aperture PAI system. The proposed method involves receiving signals with multiple CMUTs simultaneously based on an encoding matrix, instead of switching between individual CMUTs. As a result, shared channels contain a superposition of signals, which are later recovered by applying a decoding matrix. Here, an analytical model for computing SNR with an arbitrary encoding sequence is presented, and the method is validated through numerical simulations and in an experimental study. Bipolar and unipolar encoding sequences were considered for the experiments. The numerical results show, in comparison to conventional MP-PAI, that MAES will obtain an SNR gain of 5.8 and 8.8 dB for S-sequence and truncated Hadamard encodings, respectively, when using 15 transducers. In experiments, three transducers are encoded by S-sequences and show 1.5 dB improvement in SNR over conventional MP-PAI method, which aligns well with the analytical model.

Microelectronic Morphogenesis: Smart Materials with Electronics Assembling into Artificial Organisms

Microelectronic morphogenesis is the creation and maintenance of complex functional structures by microelectronic information within shape-changing materials. Only recently has in-built information technology begun to be used to reshape materials and their functions in three dimensions to form smart microdevices and microrobots. Electronic information that controls morphology is inheritable like its biological counterpart, genetic information, and is set to open new vistas of technology leading to artificial organisms when coupled with modular design and self-assembly that can make reversible microscopic electrical connections. Three core capabilities of cells in organisms, self-maintenance (homeostatic metabolism utilizing free energy), self-containment (distinguishing self from nonself), and self-reproduction (cell division with inherited properties), once well out of reach for technology, are now within the grasp of information-directed materials. Construction-aware electronics can be used to proof-read and initiate game-changing error correction in microelectronic self-assembly. Furthermore, noncontact communication and electronically supported learning enable one to implement guided self-assembly and enhance functionality. Here, the fundamental breakthroughs that have opened the pathway to this prospective path are reviewed, the extent and way in which the core properties of life can be addressed are analyzed, and the potential and indeed necessity of such technology for sustainable high technology in society is discussed.

Visualizing tactile feedback: an overview of current technologies with a focus on ultrasound elastography

Tissue elasticity remains an essential biomarker of health and is indicative of irregularities such as tumors or infection. The timely detection of such abnormalities is crucial for the prevention of disease progression and complications that arise from late-stage illnesses. However, at both the bedside and the operating table, there is a distinct lack of tactile feedback for deep-seated tissue. As surgical techniques advance toward remote or minimally invasive options to reduce infection risk and hasten healing time, surgeons lose the ability to manually palpate tissue. Furthermore, palpation of deep structures results in decreased accuracy, with the additional barrier of needing years of experience for adequate confidence of diagnoses. This review delves into the current modalities used to fulfill the clinical need of quantifying physical touch. It covers research efforts involving tactile sensing for remote or minimally invasive surgeries, as well as the potential of ultrasound elastography to further this field with non-invasive real-time imaging of the organ's biomechanical properties. Elastography monitors tissue response to acoustic or mechanical energy and reconstructs an image representative of the elastic profile in the region of interest. This intuitive visualization of tissue elasticity surpasses the tactile information provided by sensors currently used to augment or supplement manual palpation. Focusing on common ultrasound elastography modalities, we evaluate various sensing mechanisms used for measuring tactile information and describe their emerging use in clinical settings where palpation is insufficient or restricted. With the ongoing advancements in ultrasound technology, particularly the emergence of micromachined ultrasound transducers, these devices hold great potential in facilitating early detection of tissue abnormalities and providing an objective measure of patient health.

A Review of Ultrasound Neuromodulation Technologies

The invasiveness of neuromodulation technologies that require surgical implantation (e.g., electrical and optical stimulation) may limit their clinical application. Thus, alternative technologies that offer similar benefits without surgery are of paramount importance in the field of neuromodulation. Low-intensity ultrasound is an emerging modality for neural stimulation as ultrasound can be focused in deep tissues with millimeter resolution. Transcranial focused ultrasound stimulation (tFUS) has already been demonstrated in a wide range of animals and even humans at different sonication frequencies (mostly in the sub-MHz range due to the presence of the skull). This article first provides some fundamental knowledge in ultrasound, and then reviews various examples of successful tFUS experiments in animals and humans using different stimulation patterns, as well as available tFUS technologies for generating, focusing, and steering ultrasound beams in neural tissues. In particular, phased array technologies for the ultrasound stimulation application are discussed with an emphasis on the design, fabrication, and integration of ultrasound transducer arrays as well as the design and development of phased array electronics with beamformer and high-voltage driver circuitry. The challenges in tFUS, such as its underlying mechanism, indirect auditory response, and skull aberration effects, are also discussed.

A Digital-Twin Framework for Predicting the Remaining Useful Life of Piezoelectric Vibration Sensors with Sensitivity Degradation Modeling

Piezoelectric vibration sensors (PVSs) are widely applied to vibration detection in aerospace engines due to their small size, high sensitivity, and high-temperature resistance. The precise prediction of their remaining useful life (RUL) under high temperatures is crucial for their maintenance. Notably, digital twins (DTs) provide enormous data from both physical structures and virtual models, which have potential in RUL predictions. Therefore, this work establishes a DT framework containing six modules for sensitivity degradation detection and assessment on the foundation of a five-dimensional DT model. In line with the sensitivity degradation mechanism at high temperatures, a DT-based RUL prediction was performed. Specifically, the PVS sensitivity degradation was described by the Wiener-Arrhenius accelerated degradation model based on the acceleration factor constant principle. Next, an error correction method for the degradation model was proposed using real-time data. Moreover, parameter updates were conducted using a Bayesian method, based on which the RUL was predicted using the first hitting time. Extensive experiments on distinguishing PVS samples demonstrate that our model achieves satisfying performance, which significantly reduces the prediction error to 8 h. A case study was also conducted to provide high RUL prediction accuracy, which further validates the effectiveness of our model in practical use.

cMUT technology developments

Capacitive micromachined ultrasonic transducer (cMUT) technology has steadily advanced since its advent in the mid-1990's. Though cMUTs have not supplanted piezoelectric transducers for medical ultrasound imaging to date, researchers and engineers are continuing to improve cMUTs and leverage unique cMUT characteristics toward new applications. While not intended to be an exhaustive review of every aspect of cMUT state-of-the-art, this article provides a brief overview of cMUT benefits, challenges, and opportunities, as well as recent progress in cMUT research and translation.

High-Resolution Ultrasound Characterization of Local Scattering in Cancer Tissue

Ultrasound (US) has afforded an approach to tissue characterization for more than a decade. The challenge is to reveal hidden patterns in the US data that describe tissue function and pathology that cannot be seen in conventional US images. Our group has developed a high-resolution analysis technique for tissue characterization termed H-scan US, an imaging method used to interpret the relative size of acoustic scatterers. In the present study, the objective was to compare local H-scan US image intensity with registered histologic measurements made directly at the cellular level. Human breast cancer cells (MDA-MB 231, American Type Culture Collection, Manassas, VA, USA) were orthotopically implanted into female mice (N = 5). Tumors were allowed to grow for approximately 4 wk before the study started. In vivo imaging of tumor tissue was performed using a US system (Vantage 256, Verasonics Inc., Kirkland, WA, USA) equipped with a broadband capacitive micromachined ultrasonic linear array transducer (Kolo Medical, San Jose, CA, USA). A 15-MHz center frequency was used for plane wave imaging with five angles for spatial compounding. H-scan US image reconstruction involved use of parallel convolution filters to measure the relative strength of backscattered US signals. Color codes were applied to filter outputs to form the final H-scan US image display. For histologic processing, US imaging cross-sections were carefully marked on the tumor surface, and tumors were excised and sliced along the same plane. By use of optical microscopy, whole tumor tissue sections were scanned and digitized after nuclear staining. US images were interpolated to have the same number of pixels as the histology images and then spatially aligned. Each nucleus from the histologic sections was automatically segmented using custom MATLAB software (The MathWorks Inc., Natick, MA, USA). Nuclear size and spacing from the histology images were then compared with local H-scan US image features. Overall, local H-scan US image intensity exhibited a significant correlation with both cancer cell nuclear size (R2 > 0.27, p < 0.001) and the inverse relationship with nuclear spacing (R2 > 0.17, p < 0.001).

Aluminum Nitride Piezoelectric Micromachined Ultrasound Transducer Arrays for Non-Invasive Monitoring of Radial Artery Stiffness

An aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array was proposed and fabricated for non-invasive radial artery stiffness monitoring, which could be employed in human vascular health monitoring applications. Using surface micromachining techniques, four hexagonal PMUT arrays were fabricated within a chip area of 3 × 3 mm². The mechanical displacement sensitivity and quality factor of a single PMUT were tested and found to be 24.47 nm/V at 5.94 MHz and 278 (in air), respectively. Underwater pulse-echo tests for the array demonstrated a -3 dB bandwidth of 0.76 MHz at 3.75 MHz and distance detection limit of approximately 25 mm. Using the PMUT array as an ultrasonic probe, the depth and diameter changes over cardiac cycles of the radial artery were measured to be approximately 3.8 mm and 0.23 mm, respectively. Combined with blood pressure calibration, the biomechanical parameters of the radial artery vessel were extracted using a one-dimensional vascular model. The cross-sectional distensibility, compliance, and stiffness index were determined to be 4.03 × 10^-3/mmHg, 1.87 × 10^-2 mm²/mmHg, and 5.25, respectively, consistent with the newest medical research. The continuous beat-to-beat blood pressure was also estimated using this model. This work demonstrated the potential of miniaturized PMUT devices for human vascular medical ultrasound applications.

Noninvasive fluid bubble detection based on capacitive micromachined ultrasonic transducers

Ultrasonic fluid bubble detection is important in industrial controls, aerospace systems and clinical medicine because it can prevent fatal mechanical failures and threats to life. However, current ultrasonic technologies for bubble detection are based on conventional bulk PZT-based transducers, which suffer from large size, high power consumption and poor integration with ICs and thus are unable to implement real-time and long-term monitoring in tight physical spaces, such as in extracorporeal membrane oxygenation (ECMO) systems and dialysis machines or hydraulic systems in aircraft. This work highlights the prospect of capacitive micromachined ultrasonic transducers (CMUTs) in the aforementioned application situations based on the mechanism of received voltage variation caused by bubble-induced acoustic energy attenuation. The corresponding theories are established and well validated using finite element simulations. The fluid bubbles inside a pipe with a diameter as small as 8 mm are successfully measured using our fabricated CMUT chips with a resonant frequency of 1.1 MHz. The received voltage variation increases significantly with increasing bubble radii in the range of 0.5-2.5 mm. Further studies show that other factors, such as bubble positions, flow velocities, fluid medium types, pipe thicknesses and diameters, have negligible effects on fluid bubble measurement, demonstrating the feasibility and robustness of the CMUT-based ultrasonic bubble detection technique.

Multiperspective Photoacoustic Imaging Using Spatially Diverse CMUTs

Photoacoustic imaging (PAI) is a promising technique to assess different constituents in tissue. In PAI, the propagating waves are low-amplitude, isotropic, and broadband. A common approach in PAI is the use of a single linear or curved piezoelectric transducer array to perform both PA and ultrasound imaging. These systems provide freedom, agility, and versatility for performing imaging, but have limited field of view (FOV) and directivity that degrade the final image quality. Capacitive micromachined ultrasonic transducers (CMUTs) have a great potential to be used for PAI since they provide larger bandwidth and better cost efficiency. In this study, to improve the FOV, resolution, and contrast, we propose a multiperspective PAI (MP-PAI) approach using multiple CMUTs on a flexible array with shared channels. The designed array was used to perform MP-PAI in an in vitro experiment using a plaque mimicking phantom where the images were compounded both incoherently and coherently. The MP-PAI approach showed a significant improvement in overall image quality. Using only three CMUTs led to about 20% increase in generalized-contrast-to-noise ratio (gCNR), 2-dB improvement in peak signal-to-noise ratio (PSNR), and double the structural coverage in comparison to a single CMUT setup. In numerical studies, the MP-PAI was thoroughly evaluated for both the coherent and incoherent compounding methods. The assessments showed that the image quality further improved for increased number of transducers and angular coverage. For 15 transducers, the improvement for resolution and contrast could be up to three times the amount in a single-perspective image. Nonetheless, the most prominent improvement of MP-PAI was its ability to resolve the structural information of the phantoms.

An Ultrasonic Energy Harvesting IC Providing Adjustable Bias Voltage for Pre-Charged CMUT

Ultrasonic wireless power transmission (WPT) using pre-charged capacitive micromachined ultrasonic transducers (CMUT) is drawing great attention due to the easy integration of CMUT with CMOS techniques. Here, we present an integrated circuit (IC) that interfaces with a pre-charged CMUT device for ultrasonic energy harvesting. We implemented an adaptive high voltage charge pump (HVCP) in the proposed IC, which features low power, overvoltage stress (OVS) robustness, and a wide output range. The ultrasonic energy harvesting IC is fabricated in the 180 nm HV BCD process and occupies a 2 × 2.5 mm² silicon area. The adaptive HVCP offers a 2× - 12× voltage conversion ratio (VCR), thereby providing a wide bias voltage range of 4 V-44 V for the pre-charged CMUT. Moreover, a VCR tunning finite state machine (FSM) implemented in the proposed IC can dynamically adjust the VCR to stabilize the HVCP output (i.e., the pre-charged CMUT bias voltage) to a target voltage in a closed-loop manner. Such a closed-loop control mechanism improves the tolerance of the proposed IC to the received power variation caused by misalignments, amount of transmitted power change, and/or load variation. Besides, the proposed ultrasonic energy harvesting IC has an average power consumption of 35 μW-554 μW corresponding to the HVCP output from 4 V-44 V. The CMUT device with a local surface acoustic intensity of 3.78 mW/mm², which is well below the FDA limit for power flux (7.2 mW/mm²), can deliver sufficient power to the IC.