Capacitive Micromachined Ultrasonic Transducer Research Articles

Transparent capacitive micromachined ultrasound transducer linear arrays for combined realtime optical and ultrasonic imaging.

Transparent ultrasound transducers could enable many novel applications involving both ultrasonics and optics. Recently, we reported transparent capacitive micromachined ultrasound transducers (CMUTs) and demonstrated through-illumination photoacoustic imaging. This work presents the feasibility of transparent CMUTs for combined ultrasound imaging and through-array white-light imaging with a miniature camera placed behind the array. Transparent CMUT devices are fabricated with an adhesive wafer bonding technique and provide high transparency up to 90% in visible wavelengths. Fabricated linear arrays have a central operating frequency of 9 MHz with 128 active elements. Realtime plane-wave imaging is performed for ultrasound imaging, and lateral and axial resolutions of, respectively, 234 and 338 µm are achieved. Transparent CMUT has demonstrated a high transmit sensitivity of 1.4 kPa/V per channel with a 100 VDC bias voltage. The signal-to-noise ratio for a beamformed image of wire targets is determined to be 28.4 dB. To the best of our knowledge, this is the first report of combined realtime optical and ultrasonic imaging with transparent arrays. This technology may enable one to visually see what is being scanned and scan what one sees without co-registration errors. Future applications could include multi-modality probes for interventional and surgical procedures.

CMUT-Based Sensor for Acoustic Emission Application: Experimental and Theoretical Contributions to Sensitivity Optimization.

The paper deals with a capacitive micromachined ultrasonic transducer (CMUT)-based sensor dedicated to the detection of acoustic emissions from damaged structures. This work aims to explore different ways to improve the signal-to-noise ratio and the sensitivity of such sensors focusing on the design and packaging of the sensor, electrical connections, signal processing, coupling conditions, design of the elementary cells and operating conditions. In the first part, the CMUT-R100 sensor prototype is presented and electromechanically characterized. It is mainly composed of a CMUT-chip manufactured using the MUMPS process, including 40 circular 100 µm radius cells and covering a frequency band from 310 kHz to 420 kHz, and work on the packaging, electrical connections and signal processing allowed the signal-to-noise ratio to be increased from 17 dB to 37 dB. In the second part, the sensitivity of the sensor is studied by considering two contributions: the acoustic-mechanical one is dependent on the coupling conditions of the layered sensor structure and the mechanical-electrical one is dependent on the conversion of the mechanical vibration to electrical charges. The acoustic-mechanical sensitivity is experimentally and numerically addressed highlighting the care to be taken in implementation of the silicon chip in the brass housing. Insertion losses of about 50% are experimentally observed on an acoustic test between unpackaged and packaged silicon chip configurations. The mechanical-electrical sensitivity is analytically described leading to a closed-form amplitude of the detected signal under dynamic excitation. Thus, the influence of geometrical parameters, material properties and operating conditions on sensitivity enhancement is clearly established: such as smaller electrostatic air gap, and larger thickness, Young’s modulus and DC bias voltage.

Broad bandwidth air-coupled micromachined ultrasonic transducers for gas sensing

The present work aims to develop ultra-wide bandwidth air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for binary gas mixture analysis. The detection principle is based on time-of-flight (ToF) measurements, in order to monitor gas ultrasound velocity variations. To perform such measurements, CMUTs were especially designed to work out of resonance mode, like a microphone. The chosen membrane size is 32 × 32 µm2 and gap height is 250 nm. The resonance frequency and collapse voltage were found at 8 MHz and 58 V respectively. As mentioned, the CMUTs were exploited in quasi-static operating mode, in a very low frequency band, from 1 MHz to 1.5 MHz frequencies. The transducer impulse response was characterised, and a −6 dB relative fractional frequency bandwidth (FBW) higher than 100% was measured, enabling to use CMUT for the targeted application. Additionally, a measuring cell has been designed to hold the fabricated CMUT emitter and receiver prototypes facing each other. The volume inside the cell was kept lower than 3 mL and the surface of emitter/receiver was 1.6 × 8 mm2. To validate the general principle of the proposed technique, two binary gas mixtures of CO2/N2 and H2/N2, with varying concentrations, have been tested. The results are very promising with a measured limit of detection (LOD) of 0.3% for CO2 in N2 and 0.15% for H2 in N2.

A Row-Column (RC) Addressed 2-D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array on a Glass Substrate.

This article presents a row-column (RC) capacitive micromachined ultrasonic transducer (CMUT) array fabricated using anodic bonding on a borosilicate glass substrate. This is shown to reduce the bottom electrode-to-substrate capacitive coupling. This subsequently improves the relative response of the elements when top or bottom electrodes are used as the "signal" (active) electrode. This results in a more uniform performance for the two cases. Measured capacitance and resonant frequency, pulse-echo signal amplitude, and frequency response are presented to support this. Biasing configurations with varying ac and dc arrangements are applied and subsequently explored. Setting the net dc bias voltage across an off element to zero is found to be most effective to minimize spurious transmission. To achieve this, a custom switching circuit was designed and implemented. This circuit was also used to obtain orthogonal B-mode cross-sectional images of a rotationally asymmetric target.

Design of a hexagonal air-coupled capacitive micromachined ultrasonic transducer for air parametric array

An air parametric array can generate a highly directional beam of audible sound in air, which has a wide range of applications in targeted audio delivery. Capacitive micromachined ultrasonic transducer (CMUTs) have great potential for air-coupled applications, mainly because of their low acoustic impedance. In this study, an air-coupled CMUT array is designed as an air parametric array. A hexagonal array is proposed to improve the directivity of the sound generated. A finite element model of the CMUT is established in COMSOL software to facilitate the choice of appropriate structural parameters of the CMUT cell. The CMUT array is then fabricated by a wafer bonding process with high consistency. The performances of the CMUT are tested to verify the accuracy of the finite element analysis. By optimizing the component parameters of the bias-T circuit used for driving the CMUT, DC and AC voltages can be effectively applied to the top and bottom electrodes of the CMUT to provide efficient ultrasound transmission. Finally, the prepared hexagonal array is successfully used to conduct preliminary experiments on its application as an air parametric array.

An Investigation of Silica Aerogel to Reduce Acoustic Crosstalk in CMUT Arrays.

This paper presents a novel technique to reduce acoustic crosstalk in capacitive micromachined ultrasonic transducer (CMUT) arrays. The technique involves fabricating a thin layer of diisocyanate enhanced silica aerogel on the top surface of a CMUT array. The silica aerogel layer introduces a highly nanoporous permeable layer to reduce the intensity of the Scholte wave at the CMUT-fluid interface. 3D finite element analysis (FEA) simulation in COMSOL shows that the developed technique can provide a 31.5% improvement in crosstalk reduction for the first neighboring element in a 7.5 MHz CMUT array. The average improvement of crosstalk level over the −6 dB fractional bandwidth was 22.1%, which is approximately 5 dB lower than that without an aerogel layer. The results are in excellent agreement with published experimental results to validate the efficacy of the new technique.

Design and Experiment of Capacitive Micromachined Ultrasonic Transducer Array for High-frequency Underwater Imaging

Background:: Ultrasound is widely used in the applications of underwater imaging. Capacitive micromachined ultrasonic transducer (CMUT) is a promising candidate for the traditional piezoelectric ultrasonic transducer. In underwater ultrasound imaging, better resolutions can be achieved with a higher frequency ultrasound. Therefore, a CMUT array for high-frequency ultrasound imaging is proposed in this work. Methods:: Analytical methods were used to calculate the center frequency in water and the pull-in voltage for determining the operating point of CMUT. A finite element method model was developed to finalize the design parameters. The CMUT array was fabricated with a five-mask sacrificial release process. Results:: The CMUT array owned an immersed center frequency of 2.6 MHz with a 6 dB fractional bandwidth of 123 %. The pull-in voltage of the CMUT array was 85 V. An underwater imaging experiment was carried out with the target of three steel wires. Conclusion:: In this study, we have developed CMUT for high-frequency underwater imaging. The experiment showed that the CMUT could detect the steel wires with a diameter of 100 μm and the axial resolution was 0.582 mm, which was close to one wavelength of ultrasound in 2.6 MHz

3D cone-beam breast ultrasonic tomography imaging for capacitive micromachined ultrasonic transducer cylindrical array.

In this paper, a 3D cone-beam ultrasonic transmission tomography method for capacitive micromachined ultrasonic transducer (CMUT) cylindrical phased array was proposed. The performance of the proposed method was evaluated by numerical simulation experiments. After beam focusing of the transmitting array, the deviation of reconstructed sound attenuation distribution and sound speed distribution were respectively reduced by at least 71.4% and 46.4%. The research results in this paper will provide a good theoretical and practical foundation for the realization of ultrasonic imaging based on CMUT cylindrical array in the future.

Meniscus-mode: A novel operation mechanism for signal amplification in capacitive micromachined ultrasonic transducers

This paper presents a novel operation mechanism for ultrasonic testing in-water. The mechanism employs the surface-tension forces between the water and ultrasonic probe surfaces to cause an elongation (meniscus) of the water column and thus focuses the ultrasonic beam. The proposed meniscus-mode operation mechanism exploits this phenomenon to amplify the ultrasonic signal in comparison to the traditional fully-immersed setup. The visualization of the proposed mechanism is presented with the aid of finite-element simulations. In addition, the ultrasonic signal amplification due to operation in meniscus-mode is demonstrated experimentally using a capacitive micromachined ultrasonic transducer (CMUT) fabricated in PolyMUMPs technology. A factor of ∼2.24× improvement in the ultrasonic signal strength is measured.

Encapsulation of Capacitive Micromachined Ultrasonic Transducers (CMUTs) for the Acoustic Communication between Medical Implants.

The aim of this work was to extend conventional medical implants by the possibility of communication between them. For reasons of data security and transmitting distances, this communication should be realized using ultrasound, which is generated and detected by capacitive micromachined ultrasonic transducers (CMUTs). These offer the advantage of an inherent high bandwidth and a high integration capability. To protect the surrounding tissue, it has to be encapsulated. In contrast to previous results of other research groups dealing with the encapsulation of CMUTs, the goal here is to integrate the CMUT into the housing of a medical implant. In this work, CMUTs were designed and fabricated for a center frequency of 2 MHz in water and experimentally tested on their characteristics for operation behind layers of Polyether ether ketone (PEEK) and titanium, two typical materials for the housings of medical implants. It could be shown that with silicone as a coupling layer it is possible to operate a CMUT behind the housing of an implant. Although it changes the characteristics of the CMUT, the setup is found to be well suited for communication between two transducers over a distance of at least 8 cm.

Design, Development, and Characterization of a Low Frequency CMUT-Based Anemometer

The design fabrication and development of a 67.5 kHz capacitive micromachined ultrasonic transducer (CMUT) suited for Martian anemometry is presented in this paper. To have low signal attenuation under Martian conditions, the device operating frequency is limited to 100 kHz. This is due to the low-density carbon dioxide (CO2) atmosphere and acoustic impedance mismatch transduction losses. CMUTs capable of generating frequencies less than 100 kHz need either large Silicon area or higher operating voltages. This is a problem for the battery operation and portability of devices. The devices presented in this paper are designed and fabricated using low cost commercially available surface micromachining technique. COMSOL Multiphysics and MATLAB simulations were used to analyze the device critical design parameters and investigate the operability of devices. Simulation results show that the designed single cell 170- $\mu \text{m}$ radius membrane has a resonant frequency ~65 kHz. The device exhibits a static displacement of 105nm under 20 V DC bias. Using the developed single cell model, a $3\times 10$ array CMUT anemometer was fabricated and evaluated that generates a ~65 kHz acoustic signal in lab environment. This proposed CMUT anemometer can operate for a supply < 38 V. The device performance was evaluated using a commercial air-coupled capacitive microphone named CAP1. Successful transmit-receive of ultrasound from the developed 2D array to CAP1 for separation in the range of 1–15 cm was performed. The experiment results performed in lab environment show the speed of sound and the atmospheric attenuation can be accurately measured using this developed technology with a ±5% accuracy.

Improvement of Sensitivity of Capacitive Micromachined Ultrasound Transducer

CMUT (Capacitive Micromachined Ultrasonic Transducer) has a wild range of applications in medical detecting and imaging fields. However, operating under self-generating-self-receiving (SGSR) method usually results in poor sensitivity. But the sensitivity cannot be improved simply by increasing the resonant frequency since the frequency of a specific kind of CMUT is designed for specific usage. In this paper, based on one specific type of CMUT, a mechanical model is built and the simulation analysis is demonstrated. A brand-new method one-generating-multiple-receiving (OGMR) is introduced and a special circuit model has been designed to improve the signal to thermal noise ratio. By increasing the number of receiving capacitors from 1 to 8, we increased the signal-noise ratio to 2.83 times.

Real-Time Coded Excitation Imaging Using a CMUT-Based Side Looking Array for Intravascular Ultrasound.

Intravascular ultrasound (IVUS) is a well-established diagnostic method that provides images of the vessel wall and atherosclerotic plaques. We investigate the potential for phased-array IVUS utilizing coded excitation (CE) for improving the penetration depth and image signal-to-noise ratio (SNR). It is realized on a new experimental broadband capacitive micromachined ultrasound transducer (CMUT) array, operated in collapse mode, with 96 elements placed at the circumference of a catheter tip with a 1.2- mm diameter. We characterized the array performance for CE imaging and showed that the -6-dB device bandwidth at a 30-V dc biasing is 25 MHz with a 20-MHz center frequency, with a transmit sensitivity of 37 kPa/V at that frequency. We designed a linear frequency modulation code to improve penetration depth by compensating for high-frequency attenuation while preserving resolution by a mismatched filter reconstruction. We imaged a wire phantom and a human coronary artery plaque. By assessing the image quality of the reconstructed wire phantom image, we achieved 60- and 70- μm axial resolutions using the short pulse and coded signal, respectively, and gained 8 dB in SNR for CE. Our developed system shows 20-frames/s, pixel-based beam-formed, real-time IVUS images.

Fabrication and Characterization of a Wideband Low-Frequency CMUT Array for Air-Coupled Imaging

The capacitive micro-machined ultrasonic transducer (CMUT) has a good acoustic matching with air. In this paper, a 16-element CMUT array for airborne application is designed and fabricated by SOI wafer bonding process. All the CMUT cells have the same geometric parameters with circular and sealed membrane, which vibrate uniformly for good transmission efficiency. And the thin membrane with small mass is proposed to broaden the bandwidth of CMUT. The sensor array operates at a 215.6 kHz resonance frequency with a large −3dB fractional bandwidth of 15.7% and a good consistency. The selection of the driving parameters (i.e. the DC bias voltage and AC excitation signal) has been analyzed comprehensively, which can be used to optimize the driving strategy of the CMUT for better transmission and reception performance. The CMUT array proposed in this paper has been used to demonstrate a two-dimension non-contact air-coupled imaging by through-transmission mode and pitch-catch mode. The images of two kinds of objects have been reconstructed to exhibit the capability of the CMUT array in air-ultrasonic imaging, suggesting broad applications including 3D imaging, gesture recognition, and other human-machine interactions in air.

Closed-Form Expressions on CMUTs With Layered Anisotropic Microplates Under Residual Stress and Pressure.

Capacitive micromachined ultrasonic transducers (CMUTs) are promising in the emerging fields of personalized ultrasonic diagnostics, therapy, and noninvasive 3-D biometric. However, previous theories describing their mechanical behavior rarely consider multilayer and anisotropic material properties, resulting in limited application and significant analysis errors. This article proposes closed-form expressions for the static deflection, collapse voltage, and resonant frequency of circular-microplate-based CMUTs, which consider both the aforementioned properties as well as the effects of residual stress and hydrostatic pressure. These expressions are established by combining the classical laminated thin plate (CLTP) theory, Galerkin method, a partial expansion approach for electrostatic force, and an energy equivalent method. A parametric study based on finite-element method simulations shows that considering the material anisotropy can significantly improve analysis accuracy (~25 times higher than the theories neglecting the material anisotropy). These expressions maintain accuracy across almost the whole working voltage range (up to 96% of collapse voltages) and a wide dimension range (diameter-to-thickness ratios of 20-80 with gap-to-thickness ratios of ≤2). Furthermore, their utility in practical applications is well verified using numerical results based on more realistic boundary conditions and experimental results of CMUT chips. Finally, we demonstrate that the high accuracy of these expressions at thickness-comparable deflection results from the extended applicable deflection range of the CLTP theory when it is used for electrostatically actuated microplates.

Capacitive micromachined ultrasonic transducers leak detection by dye penetrant test

Dye penetrant test has been used to detect non hermetic capacitive micromachined ultrasonic transducers (CMUTs). The method has proven its ability to highlight faulty CMUTs membranes among more than hundreds of CMUTs membranes. The fluorescence microscopy image highlights the defective CMUTs with a strong contrast and respecting the shape of the CMUTs, which makes it possible to design robust faulty CMUT automatic identification algorithms. Scanning electron microscopy has been performed in order to check the reliability the detection of the non-hermetic CMUTs. Observations confirmed the reliability of the method.

An Integrated Front-end Circuit Board for Air-Coupled CMUT Burst-Echo Imaging

To conduct burst-echo imaging with air-coupled capacitive micromachined ultrasonic transducers (CMUTs) using the same elements in transmission and reception, this work proposes a dedicated and integrated front-end circuit board design to build an imaging system. To the best of the authors’ knowledge, this is the first air-coupled CMUT burst-echo imaging using the same elements in transmission and reception. The reported front-end circuit board, controlled by field programmable gate array (FPGA), consisted of four parts: an on-board pulser, a bias-tee, a T/R switch and an amplifier. Working with our 217 kHz 16-element air-coupled CMUT array under 100 V DC bias, the front-end circuit board and imaging system could achieve 22.94 dB signal-to-noise ratio (SNR) in burst-echo imaging in air, which could represent the surface morphology and the three-dimensional form factor of the target. In addition, the burst-echo imaging range of our air-coupled CMUT imaging system, which could work between 52 and 273 mm, was discussed. This work suggests good potential for ultrasound imaging and gesture recognition applications.

MEMS Ultrasound Transducers for Endoscopic Photoacoustic Imaging Applications.

Photoacoustic imaging (PAI) is drawing extensive attention and gaining rapid development as an emerging biomedical imaging technology because of its high spatial resolution, large imaging depth, and rich optical contrast. PAI has great potential applications in endoscopy, but the progress of endoscopic PAI was hindered by the challenges of manufacturing and assembling miniature imaging components. Over the last decade, microelectromechanical systems (MEMS) technology has greatly facilitated the development of photoacoustic endoscopes and extended the realm of applicability of the PAI. As the key component of photoacoustic endoscopes, micromachined ultrasound transducers (MUTs), including piezoelectric MUTs (pMUTs) and capacitive MUTs (cMUTs), have been developed and explored for endoscopic PAI applications. In this article, the recent progress of pMUTs (thickness extension mode and flexural vibration mode) and cMUTs are reviewed and discussed with their applications in endoscopic PAI. Current PAI endoscopes based on pMUTs and cMUTs are also introduced and compared. Finally, the remaining challenges and future directions of MEMS ultrasound transducers for endoscopic PAI applications are given.

Evaluation of capacitive micromachined ultrasonic transducers for passive monitoring of microbubble-assisted ultrasound therapies.

Passive cavitation detection can be performed to monitor microbubble activity during brain therapy. Microbubbles under ultrasound exposure generate a response characterized by multiple nonlinear emissions. Here, the wide bandwidth of capacitive micromachined ultrasonic transducers (CMUTs) was exploited to monitor the microbubble signature through a rat skull and a macaque skull. The intrinsic nonlinearity of the CMUTs was characterized in receive mode. Indeed, undesirable nonlinear components generated by the CMUTs must be minimized as they can mask the microbubble harmonic response. The microbubble signature at harmonic and ultra-harmonic components (0.5-6 MHz) was successfully extracted through a rat skull using moderate bias voltage.

Broadband phononic frequency comb generation in a parametrically excited capacitive micromachined ultrasonic transducer

Optical frequency combs consisting of evenly spaced discrete frequencies have been used for a number of applications such as frequency metrology, range finding and molecular fingerprinting. Recently, the acoustic equivalent of these frequency combs, known as phononic frequency combs (PFCs), have been demonstrated in micro and nanoscale resonators. However, such demonstrations are restricted to PFC generation in high-Q mechanical resonators under carefully controlled operating conditions with limited practical applications. In this work, we demonstrate a novel method of generating PFCs with a parametrically driven capacitive micromachined ultrasonic transducer (CMUT) operating in a fluid medium. The proposed system consists of an electrically driven CMUT array that forms part of a resonant RLC circuit of frequency w0. By applying two drive tones of frequency w0 and w0+Δ, we are able to generate broadband acoustic frequency combs via a parametric four-wave mixing process. The intensity and number of combs generated is strongly dependent on the relative strength of the two driving tones whereas the comb spacing is determined by the frequency difference Δ between the drive signals. We also briefly discuss a potential application of PFCs in extended non-ambiguous range (NAR) distance metrology with interferometric resolution.