Capacitive Micromachined Ultrasonic Transducer Array Research Articles

Development of an Integrated CMUTs-based Resonant Biosensor for Label-Free Detection of DNA with Improved Selectivity by Ethylene-Glycol Alkanethiols

Gravimetric resonant-inspired biosensors have attracted increasing attention in industrial and point-of-care applications, enabling label-free detection of biomarkers such as DNA and antibodies. Capacitive micromachined ultrasonic transducers (CMUTs) are promising tools for developing miniaturized high-performance biosensing complementary metal oxide silicon (CMOS) platforms. However, their operability is limited by inefficient functionalization, aggregation, crosstalk in the buffer, and the requirement for an external high-voltage (HV) power supply. In this study, we aimed to propose a CMUTs-based resonant biosensor integrated with a CMOS front-end interface coupled with ethylene-glycol alkanethiols to detect single-stranded DNA oligonucleotides with large specificity. The topography of the functionalized surface was characterized by energy-dispersive X-ray microanalysis. Improved selectivity for on-chip hybridization was demonstrated by comparing complementary and non-complementary single-stranded DNA oligonucleotides using fluorescence imaging technology. The sensor array was further characterized using a five-element lumped equivalent model. The 4-mm2 application-specific integrated circuit chip was designed and developed through 0.18 μm HV Bipolar-CMOS-Double diffused MOS (DMOS) technology (BCD) to generate on-chip 20V HV boosting and to track feedback frequency under a standard 1.8 V supply, with a total power consumption of 3.8 mW in a continuous mode. The measured results indicated a detection sensitivity of 7.943 × 10−3 μmol∙ L−1∙Hz−1 over a concentration range of 1 to 100 μmol∙L−1. In conclusion, the label-free biosensing of DNA under dry conditions was successfully demonstrated using a microfabricated CMUT array with a 2 MHz frequency on CMOS electronics with an internal HV supplier. Moreover, ethylene-glycol alkanethiols successfully deposited self-assembled monolayers on aluminum electrodes, which has never been attempted thus far on CMUTs, to enhance the selectivity of bio-functionalization. The findings of this study indicate the possibility of full-on-chip DNA biosensing with CMUTs.

Lamination of Capacitive Micromachined Ultrasonic Transducer on a Piezoelectric Array: Process and Evaluation

Harmonic imaging plays a crucial role in medical ultrasound imaging, where the transducer probes are required to emit low-frequency ultrasonic waves into the tissues, and receive reflected signals at the higher, typically the second harmonic of the transmitted wave. We have designed a unique probe with a high-frequency capacitive micromachined ultrasonic transducer (CMUT) on top of a low-frequency piezoelectric array for harmonic imaging. This paper presents a lamination process between CMUT and piezoelectric array using epoxy. The bond-line between the two components was thoroughly investigated using mechanical and electrical methods. The resulting bond-line thickness was less than 2 μm, and shear strength fell within the range 4.8 – 5.1 MPa. Additionally, electrical impedance measurement of all elements in the low-frequency array exhibited resonance shifts before and after bonding, providing further insight into the bonding process.

Design and Fabrication of 1-D CMUT Arrays for Dual-Mode Dual-Frequency Acoustic Angiography Applications.

When microbubble contrast agents are excited at low frequencies (less than 5 MHz), they resonate and produce higher-order harmonics due to their nonlinear behavior. We propose a novel scheme with a capacitive micromachined ultrasonic transducer (CMUT) array to receive high-frequency microbubble harmonics in collapse mode and to transmit a low-frequency high-pressure pulse by releasing the CMUT plate from collapse and pull it back to collapse again in the same transmit-receive cycle. By patterning and etching the substrate to create glass spacers in the device cavity we can reliably operate the CMUT in collapse mode and receive high-frequency signals. Previously, we demonstrated a single-element CMUT with spacers operating in the described fashion. In this article, we present the design and fabrication of a dual-mode, dual-frequency 1-D CMUT array with 256 elements. We present two different insulating glass spacer designs in rectangular cells for the collapse mode. For the device with torus-shaped spacers, the 3 dB receive bandwidth is from 8 to 17 MHz, and the transmitted maximum peak-to-peak pressure from 32 elements at 4 mm focal depth was 2.12 MPa with a 1.21 MPa peak negative pressure, which corresponds to a mechanical index (MI) of 0.58 at 4.3 MHz. For the device with line-shaped spacers, the 3-dB receive bandwidth at 150 V dc bias extends from 10.9 to 19.2 MHz. By increasing the bias voltage to 180 V, the 3 dB bandwidth shifts, and extends from 11.7 to 20.4 MHz. The transmitting maximum peak-to-peak pressure with 32 elements at 4 mm was 2.06 MPa with a peak negative pressure of 1.19 MPa, which corresponds to an MI of 0.62 at 3.7 MHz.

Photoacoustic imaging of visually evoked cortical and subcortical hemodynamic activity in mouse brain: feasibility study with piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) arrays.

This study investigates the feasibility of capturing visually evoked hemodynamic responses in the mouse brain using photoacoustic tomography (PAT) and ultrasound (US) dual-modality imaging. A commercial piezoelectric transducer array and a capacitive micromachined ultrasonic transducer (CMUT) array were compared using a programmable PAT-US imaging system. The system resolution was measured by imaging phantoms. We also tested the ability of the system to capture visually evoked hemodynamic responses in the superior colliculus as well as the primary visual cortex in wild-type mice. Results show that the piezoelectric transducer array and the CMUT array exhibit comparable imaging performance, and both arrays can capture visually evoked hemodynamic responses in subcortical as well as cortical regions of the mouse brain.

Real-time 3D plane-wave imaging using annular capacitive micromachined ultrasonic transducer array

Imaging methods and 2D sparse arrays have been extensively researched to realize real-time 3D volumetric imaging with higher frame rates. In this study, we developed a 3D plane-wave imaging method using an annular capacitive micromachined ultrasonic transducer (CMUT) array as a breakthrough technique for realizing 3D imaging with a limited number of elements. We adopted the double oxidation and wafer bonding processes for creating the cavity of a CMUT with a 2 µm silicon membrane. To ensure surface uniformity before the bonding process, we implemented an additional reactive ion etching process after the second oxidation. The characterization results showed that the resonant frequency of the proposed device was 6.2 MHz with 65 V collapse voltage. The demonstration of real-time 3D volumetric imaging was conducted in oil using a Vantage 64 ultrasound research system. A plane-wave imaging scheme was used to detect wire phantoms immersed in the oil, with 289 plane waves transmitted over a range of 20° from the center of the annular array. The reconstructed data were used to create three orthogonal cross sections of the target region. The − 6 dB lateral and axial resolutions were 0.56 mm and 0.35 mm for the wires positioned at 18 mm from the proposed device, which were narrow enough to detect a wire phantom of 380 µm diameter. The normalized frame rate was calculated and compared with that in a previous study. We verified that plane-wave imaging has advantages in real-time imaging when compared with the classic phased array and synthetic phased array. Thus, the annular CMUT array is a promising mechanism for real-time 3D imaging with higher frame rate than that possible with previous methods.

A Front-End CMOS Interface Circuit With High Voltage Charge Pump and Oscillator for Capacitive Micromachined Ultrasonic Transducers

Label-free biosensors, combined with miniaturized micro-electromechanical sensory platforms, offer an attractive solution for real-time and facile monitoring of biomolecules due to their high sensitivity and selectivity without the need for specifically labeling. Resonators have been acknowledged as an efficient technology for measuring biomolecular binding events including those involving nucleic acid and antibody. Among these, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a promising candidate for biosensing. However, their usage is often limited by the requirement for high voltage supply and continuous frequency tracking, which can result in significant parasitic effects and measurement errors. In this paper, we present a novel front-end interface circuit for a CMUTs-based biosensor. The circuit, fabricated using TSMC 0.18-μX;Jm Bipolar-CMOS-DMOS (BCD) technology, incorporates an on-chip high voltage charge pump and feedback frequency monitoring. The CMUTs array features 20×20 circular cells, fabricated using a low-temperature direct bonding technology, with an experimental parallel-resonant frequency of 1.724 MHz and a high quality factor of up to 40.9. To fit the measured electrical characteristics, a five-element equivalent lumped element model is proposed. The high voltage charge pump provides an output voltage of 20 V, while the feedback oscillator has a ms-level start-up time and a total power dissipation of 3.8 mW. The proposed front-end interface is designed to function as a stand-alone chip for CMUTs-based resonant biodetection.

Compressed sensing approach for CMUT sparse array in multi-element synthetic transmit aperture

In order to improve the imaging quality of capacitive micromachined ultrasonic transducer (CMUT) sparse array, the authors propose an imaging strategy combining multi-element synthetic transmit aperture (MSTA) imaging mode and compressed sensing (CS) framework. The imaging strategy consists of four steps: First, use the genetic algorithm (GA) to sparse the CMUT array and obtain the position of the receiving active array elements. Second, the CMUT sub-arrays are used to transmit ultrasonic signal in turn, and the sparse array obtained in the first step is used to receive ultrasonic echoes. Third, the received under-sampling channel data are used to recover the full channel data with the CS reconstruction algorithm. Finally, an MSTA image is beamformed from the recovered full channel data. This imaging strategy is called MSTA-CS mode. Due to the efficient sparse recovery capability of CS technology, the MSTA-CS imaging strategy not only reduces the number of transmitting hardware channels, but also improves the imaging clarity of CMUT sparse array. Both the Field II simulation and the breast phantom experiments demonstrate that the MSTA-CS mode achieves higher contrast and structural similarity under the same receiving sparse array conditions. • In this paper, Multi-element Synthetic Transmit Aperture (MSTA) method is used in ultrasound imaging of CMUT sparse array, this method reduces the number of transmitting hardware channels. • Under the condition of lateral under-sampling of CMUT sparse array, the closed array elements echoes data are recovered by using compressed sensing framework. • Take the full array imaging results as the standard, and verify the compressed sensing theory through breast phantom experiments.

New Insights for Parasitic Effects of Label-Free Biosensors Based on Capacitive Micromachined Ultrasonic Transducers

Capacitive micromachined ultrasonic transducers (CMUTs) are regarded as an attractive candidate in bio-applications such as imaging and molecule monitoring. However, the previous researches on biochemical sensing are mostly air-coupled application based on a CMUTs array because cell-to-cell mutual radiation and large motional loss in liquid environment are able to produce nonignorable noise. For a CMUTs cell, its characteristics (e.g., electrical, mechanical, and acoustic) are susceptible to parasitic effects owning to the physically capacitive dielectric dispersion, motional damping, and connection loss. Neglecting the parasitic effects on multidomain characteristics of CMUTs leads to significant robustness errors. Furthermore, finite element method (FEM) is not sufficient enough to model such parasitic effects when considering an integrated circuit interface and the evaluation of system responses. This article highlights a lumped element model (LEM) to analyze the behaviors of a label-free biosensor based on a single CMUTs cell directly operating in liquid. We successfully explore the performance of the biosensor with different types of parasitic effects in transient and frequency domains through LEM and FEM. The parasitic effects on mass loading of sensing layer and biomolecule such as deoxyribonucleic acid (DNA) and Immunoglobulin G (IgG) probes are predicted by LEM based on their surface mass densities. The proposed approach is capable of analyzing the variation’s margin of CMUTs-based biosensors to improve robustness for parasitic extraction in liquid.

Design and Implementation of Low-Voltage Tunable Capacitive Micro-Machined Transducers (CMUT) for Portable Applications.

Capacitive micromachined ultrasonic transducers (CMUT) are MEMS-based transducers with advantages over conventional ultrasonic transducers, such as their small size, the ease of integration with semiconductor electronics, and batch fabrication. In this study, the effect of different membrane topologies on the displacement, resonant frequency, and output pressure of the CMUT membrane is investigated in the transmission mode in an air environment. A novel structural-support feature, the rocker stem, is introduced, where the membrane is weakly held to the substrate in order to minimize mechanical constraints. Four different CMUT topologies are designed and assessed to analyze the impacts of topological variations. A new CMUT array configuration is also designed to provide an approach for maximizing CMUT density. This study aims to contribute to efficient CMUT design and the determination of optimum structural parameters for portable applications in air.

Design of an Integrated Micro-Viscometer for Monitoring Engine Oil.

This paper proposes a novel integrated micro-viscometer for engine-oil monitoring. The final solution consists of a capacitive micromachined ultrasonic transducer (CMUT) and an application-specific integrated circuit (ASIC). The CMUT is used to generate and capture acoustic waves while immersed in engine oil. The low power transceiver ASIC is interfaced with the CMUT structure for actuation and reception. An integrated charge pump boosts the supply voltage from 3.3 to 22 V to generate the DC polarization voltage of the CMUT. The receiver has a power consumption of 72 µW with an input-referred noise current of 3.2 and a bandwidth of 7 MHz. The CMUT array occupies an area of 3.5 × 1 mm, whereas the ASIC has a chip area of 1 × 1 mm. The system was tested using engine oils of different types and ages at different temperatures. Measurement results show a significant frequency shift due to the dynamic viscosity change that occurs as oil ages. A shift of −1.9 kHz/cP was measured, which corresponds to a shift of 33 Hz/mile. This work paves the way for high accuracy-integrated solutions for oil condition monitoring and is expected to play a significant role in a more economic and environmentally friendly usage of oil.

Fabrication of 2-D Capacitive Micromachined Ultrasonic Transducer (CMUT) Array through Silicon Wafer Bonding.

Capacitive micromachined ultrasound transducers (CMUTs) have broad application prospects in medical imaging, flow monitoring, and nondestructive testing. CMUT arrays are limited by their fabrication process, which seriously restricts their further development and application. In this paper, a vacuum-sealed device for medical applications is introduced, which has the advantages of simple manufacturing process, no static friction, repeatability, and high reliability. The CMUT array suitable for medical imaging frequency band was fabricated by a silicon wafer bonding technology, and the adjacent array devices were isolated by an isolation slot, which was cut through the silicon film. The CMUT device fabricated following this process is a 4 × 16 array with a single element size of 1 mm × 1 mm. Device performance tests were conducted, where the center frequency of the transducer was 3.8 MHz, and the 6 dB fractional bandwidth was 110%. The static capacitance (29.4 pF) and center frequency (3.78 MHz) of each element of the array were tested, and the results revealed that the array has good consistency. Moreover, the transmitting and receiving performance of the transducer was evaluated by acoustic tests, and the receiving sensitivity was −211 dB @ 3 MHz, −213 dB @ 4 MHz. Finally, reflection imaging was performed using the array, which provides certain technical support for the research of two-dimensional CMUT arrays in the field of 3D ultrasound imaging.

Lumped-Parameter Equivalent Circuit Modeling of CMUT Array Elements

Design and modeling are key steps in the value chain of Capacitive Micromachined Ultrasonic Transducer (CMUT) arrays. Although CMUT array element models are very powerful, most of them are still limited in their use as tools for electronic design assistance. The electroacoustic equivalent circuits developed are mainly based on a distributed-element approach while lumped-parameter electrical circuits are better suited for electronic software design tools interoperations. To meet this need, the present study aims to implement an electroacoustic equivalent scheme of a full array element, based on a two-port network representation made of lumped-parameters. After an extensive bibliographical review of CMUT models, the new model is set-up from a fully distributed approach using Foldy’s electroacoustic definitions at the element level. Transmit and receive modes are implemented using scalar equations given by the lumped parameters. Moreover, based on a reciprocity analysis, the performance of the complete measurement chain in emission and reception will be defined using the relevant transfer functions. Finally, to help one design CMUT array elements for a given application, a method based on the computation of membranes thickness-size master curves is proposed. The two-port network representation of a full CMUT-based array element allowed by the new lumped-parameter modeling opens a wide range of possibilities regarding array design, electronic integration, operations with acoustic propagation simulation tools and more.

A Wearable Ultrasonic Neurostimulator-Part II: A 2D CMUT Phased Array System With a Flip-Chip Bonded ASIC.

A 2D ultrasonic array is the ultimate form of a focused ultrasonic system, which enables electronically focusing beams in a 3D space. A 2D array is also a versatile tool for various applications such as 3D imaging, high-intensity focused ultrasound, particle manipulation, and pattern generation. However, building a 2D system involves complicated technologies: fabricating a 2D transducer array, developing a pitch-matched ASIC, and interconnecting the transducer and the ASIC. Previously, we successfully demonstrated 2D capacitive micromachined ultrasonic transducer (CMUT) arrays using various fabrication technologies. In this paper, we present a 2D ultrasonic transmit phased array based on a 32 × 32 CMUT array flip-chip bonded to a pitch-matched pulser ASIC for ultrasonic neuromodulation. The ASIC consists of 32 × 32 unipolar high-voltage (HV) pulsers, each of which occupies an area of 250 μm × 250 μm. The phase of each pulser output is individually programmable with a resolution of 1/fC/16, where fC is less than 10 MHz. This enables the fine granular control of a focus. The ASIC was fabricated in the TSMC 0.18- μm HV BCD process within an area of 9.8 mm × 9.8 mm, followed by a wafer-level solder bumping process. After flip-chip bonding an ASIC and a CMUT array, we identified shorted elements in the CMUT array using the built-in test function in the ASIC, which took approximately 9 minutes to scan the entire 32 × 32 array. A compact-form-factor wireless neural stimulator system-only requiring a connected 15-V DC power supply-was also developed, integrating a power management unit, a clock generator, and a Bluetooth Low-Energy enabled microcontroller. The focusing and steering capability of the system in a 3D space is demonstrated, while achieving a spatial-peak pulse-average intensity ( ISPPA) of 12.4 and 33.1 W/ cm2; and a 3-dB focal volume of 0.2 and 0.05 mm3-at a depth of 5 mm-at 2 and 3.4 MHz, respectively. We also characterized transmission of ultrasound through a mouse skull and compensated the phase distortion due to the skull by using the programmable phase-delay function in the ASIC, achieving 10% improvement in pressure and a tighter focus. Finally, we demonstrated a ultrasonic arbitrary pattern generation on a 5 mm × 5 mm plane at a depth of 5 mm.

Electronic nose system based on a functionalized capacitive micromachined ultrasonic transducer (CMUT) array for selective detection of plant volatiles

Here, a small, low-power, wireless gas sensor platform for selective detection of volatile organic compounds (VOCs) released from plants under different abiotic or biotic stress conditions is described. This sensor platform is implemented based on a capacitive micromachined ultrasonic transducer (CMUT) array, in which elements were functionalized with a variety of materials including polymers, phthalocyanines, and metals to improve selectivity. Input impedance measurements of the functionalized CMUT array were compared to pre-coating measurements to analyze the mechanical loading. The CMUT arrays were then exposed to VOCs known to be emitted by plants with different concentrations under dry air flow at room temperature. The results demonstrated that 1-Octanol created the strongest response across different channels and a resolution of 3-ppb was calculated for the CMUT element functionalized using silver ink when exposed to 1-Octanol. The relative responses of different channels to tested volatiles were observed to be different. The k-nearest neighbor (k-NN) algorithm was used for the gas classification by dividing the data to training and test groups. The k-NN results showed that the gases at low concentrations were successfully classified with better than 97 % accuracy. Finally, to emulate the ambient atmosphere for plants, the gas tests were repeated by adding different levels of humidity to the gas flow. With a minimum 98 % accuracy, the k-NN classifier demonstrated that the functionalized CMUT array can be used for selective detection of the group of plant VOCs used in this study, even at different relative humidity levels in the ambient atmosphere.

A Wearable Ultrasonic Neurostimulator - Part I: A 1D CMUT Phased Array System for Chronic Implantation in Small Animals.

In this work, we present a wireless ultrasonic neurostimulator, aiming at a truly wearable device for brain stimulation in small behaving animals. A 1D 5-MHz capacitive micromachined ultrasonic transducer (CMUT) array is adopted to implement a head-mounted stimulation device. A companion ASIC with integrated 16-channel high-voltage (60-V) pulsers was designed to drive the 16-element CMUT array. The ASIC can generate excitation signals with element-wise programmable phases and amplitudes: 1) programmable sixteen phase delays enable electrical beam focusing and steering, and 2) four scalable amplitude levels, implemented with a symmetric pulse-width-modulation technique, are sufficient to suppress unwanted sidelobes (apodization). The ASIC was fabricated in the TSMC 0.18- μm HV BCD process within a die size of 2.5 × 2.5 mm2. To realize a completely wearable system, the system is partitioned into two parts for weight distribution: 1) a head unit (17 mg) with the CMUT array, 2) a backpack unit (19.7 g) that includes electronics such as the ASIC, a power management unit, a wireless module, and a battery. Hydrophone-based acoustic measurements were performed to demonstrate the focusing and beam steering capability of the proposed system. Also, we achieved a peak-to-peak pressure of 2.1 MPa, which corresponds to a spatial peak pulse average intensity ( ISPPA) of 33.5 W/cm2, with a lateral full width at half maximum (FWHM) of 0.6 mm at a depth of 3.5 mm.

Dual-Frequency CMUT Arrays for Multiband Ultrasound Imaging Applications.

Dual-frequency capacitive micromachined ultrasonic transducers (CMUTs) are introduced for multiscale imaging applications, where a single array transducer can be used for both deep low-resolution imaging and shallow high-resolution imaging. These transducers consist of low- and high-frequency membranes interlaced within each subarray element. They are fabricated using a modified sacrificial release process. Successful performance is demonstrated using wafer-level vibrometer testing, as well as acoustic testing on wirebonded dies consisting of arrays of 2- and 9-MHz elements of up to 64 elements for each subarray. The arrays are demonstrated to provide multiscale, multiresolution imaging using wire phantoms and can span frequencies from 2 MHz up to as high as 17 MHz. Peak transmit sensitivities of 27 and 7.5 kPa/V are achieved with the low- and high-frequency subarrays, respectively. At 16-mm imaging depth, lateral spatial resolution achieved is 0.84 and 0.33 mm for low- and high-frequency subarrays, respectively. The signal-to-noise ratio of the low-frequency subarray is significantly higher for deep targets compared to the high-frequency subarray. The array achieves multiband imaging capabilities difficult to achieve with current transducer technologies and may have applications to multipurpose probes and novel contrast agent imaging schemes.

Research on optimization sparse method for capacitive micromachined ultrasonic transducer array: heuristic algorithm

PurposeBecause of the small size and high integration of capacitive micromachined ultrasonic transducer (CMUT) component, it can be made into large-scale array, but this lead to high hardware complexity, so the purpose of this paper is to use less elements to achieve better imaging results. In this research, an optimized sparse array is studied, which can suppress the side lobe and reduce the imaging artifacts compared with the equispaced sparse array with the same number of elements.Design/methodology/approachGenetic algorithm is used to sparse the CMUT linear array, and Kaiser window apodization is added to reduce imaging artifacts, the beam pattern and peak-to-side lobe ratio are calculated, point targets imaging comparisons are performed. Furthermore, a 256-elements CMUT linear array is used to carry out the imaging experiment of embedded mass and forearm blood vessel, and the imaging results are compared quantitatively.FindingsThrough the imaging comparison of embedded mass and forearm blood vessel, the feasibility of optimized sparse array of CMUT is verified, and the purpose of reducing the hardware complexity is achieved.Originality/valueThis research provides a basis for the large-scale CMUT array to reduce the hardware complexity and the amount of calculation. At present, the CMUT array has been used in medical ultrasound imaging and has huge market potential.

Discrimination of Binary Gas Mixture Using CMUT Based Sound Attenuation Spectrum Gas Sensor

Introduction In order to overcome the long term stability issues caused by functionalized films in gas sensing [1], uncoated sensors have become increasingly attractive for applications where the selectivity is not a major concern such as industrial gas monitoring. Despite their poor selectivity, discrimination can be achieved by measuring different properties of the gas mixture [2]. The sound attenuation of a gas depends on several of its physical properties such as mass density, viscosity and sound velocity among several others [3]. Its value depends on the frequency in a non-linear manner which makes measuring large parts of its spectrum interesting for gas discrimination.In this abstract, an uncoated sensor capable of measuring the attenuation spectrum continuously over a frequency range is presented. Measurements on binary mixtures such as nitrogen (N2) with either hydrogen (H2), carbon dioxide (CO2) or methane (CH4) are presented. Then, a simple method based on the construction of a preliminary mixture signature allowing to distinguish each type of mixture demonstrates the potential of this sensor to be used in smart gas sensors as a perspective to future work. Although attenuation sensors can be found in the literature [3], to the best of our knowledge, it is the first of this kind with easy integration thanks to the use of capacitive micromachined ultrasonic transducers (CMUTs) and allowing discrimination of binary mixtures. Setup The manufacturing process of the CMUT arrays is similar to the one used in reference [4] and their characteristics are reported in Table 1. Schematics of the experimental setup are shown in Figure 1. An electrical signal (1) is sent to an emitter CMUT array (2) which generates a continuous ultrasonic wave at a given frequency f. The wave travels a distance d through the gas and is attenuated by an amount that depends on the gas composition, through the attenuation coefficient α, before reaching the receiver (3) which is connected to a charge amplifier (4). Both the emitter and receiver signals are fed to a network analyzer (5) in order to measure the total transfer function of the setup |H| as a function of frequency. Far from the resonant frequency of the CMUT array (fr = 9.6MHz), |H| is given by Equation 1, where |He | is the transfer function of the setup which is independent of the gas. Thus, by measuring first |H| under pure N2, |HN2 |, it is possible to know the shift in attenuation Δαaccording to Equation 2. Results Tests under H2, CO2 and CH4 in N2 at different concentrations were performed at 20°C and 1atm for f ranging from 2MHz to 4.5MHz. The normalized measurements are shown in Figures 2, 3 and 4, respectively. The normalization consists in multiplying Δα by the wavelength in N2, λN2 , at the optimal frequency, fopt , for each mixture. Theses frequencies correspond to the best theoretical limit of detection LOD (Equation 3). The noise standard deviation σ increases with the frequency from 1.7x10-5 at 2MHz to 6.1x10-5 at 4.5MHz. For each mixture, the optimal frequency, fopt , sensitivity, S, and theoretical limit of detection, LOD, are reported in Table 2. The calculated values of the LOD were then verified for the three binary mixtures at lower concentrations. The results are shown in Figures 5, 6 and 7 and are consistent with the calculations and even better in the case of CH4 where the step at 0.25% is still visible. These values of LOD correspond approximately to an attenuation of 1m-1,which corresponds to the state of the art on attenuation sensors [3]. Finally the normalized attenuation spectra for the three mixtures, shown in Figure 8, makes the discrimination of the three mixtures possible. In this study, it is done by simply considering both the sign of λN2 Δα and the one of the mean slope Σ in the region between 2MHz and 3.6MHz defined by Equation 4 (both signs are reported in Table 2 for each mixture). Conclusion A CMUTs based device is used to measure the attenuation spectrum of a gas. Its characterization shows performances comparable to the state of the art of attenuation sensors. Such measurements allow to determine the concentration of binary mixtures of N2 with either H2, CO2 or CH4. Finally, a signature of each of the three binary mixture is introduced to show the potential of this sensor as part of a smart sensor network. Possible perspectives include increasing the frequency range and testing in other gases.

Analysis of Negative Capacitance-Based Broadband Impedance Matching for CMUTs.

Tight integration of capacitive micromachined ultrasonic transducer (CMUT) arrays with integrated circuits can make active impedance matching feasible for practical imaging devices. In this article, negative capacitance-based impedance matching for CMUTs is investigated. Simple equivalent circuit model-based calculations show the potential of negative capacitance matching for improving the bandwidth along with electrical power transfer and acoustic reflectivity, but the model has limitations especially for acoustic reflectivity evaluation. For more realistic results, an experimentally validated CMUT array model is applied to a small 1-D CMUT array operating in the 5-15 MHz range. The results highlight the difference between electrical power transfer and acoustic reflectivity as well as the tradeoffs in signal-to-noise ratio (SNR). According to the results, ideal negative capacitance termination matched to the CMUT capacitance provides the broadest bandwidth and highest SNR if acoustic or electrical reflections are of no concern. On the other hand, negative capacitance and resistance matching to minimize acoustic reflectivity provides both lower reflection and closer to ideal SNR as compared with electrical power matching. It is observed that acoustic matching also reduces acoustic crosstalk and improves array uniformity. While several challenges for integrated circuit implementation are present, negative capacitance-based impedance matching can be a viable broadband active impedance matching method for CMUTs operating in conventional and collapsed mode as well as other ultrasound transducers with mainly capacitive impedance.

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.