Capacitive Micromachined Ultrasonic Transducer 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.

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.

Investigation of New Accelerometer Based on Capacitive Micromachined Ultrasonic Transducer (CMUT) with Ring-Perforation Membrane.

Capacitive micromachined ultrasonic transducer (CMUT) has been widely studied due to its excellent resonance characteristics and array integration. This paper presents the first study of the CMUT electrostatic stiffness resonant accelerometer. To improve the sensitivity of the CMUT accelerometer, this paper innovatively proposes the CMUT ring-perforation membrane structure, which effectively improves the acceleration sensitivity by reducing the mechanical stiffness of the elastic membrane. The acceleration sensitivity is 10.9 (Hz/g) in the acceleration range of 0-20 g, which is 100% higher than that of the conventional CMUT structure. This research contributes to the acceleration measurement field of CMUT and can effectively contribute to the breakthrough of vibration acceleration monitoring technology in aerospace, medical equipment, and automotive electronics.

Research on CMUTs Design with Different Cell Arrangements

Background: Capacitive micromachined ultrasonic transducers (CMUTs) is a promising component of mechanical-electrical-acoustical conversion, which shows valuable applications in non-distructive testing and obstacle detection fields. In order to study the influence of dimension parameters on the directivity of CMUTs, the three-dimensional multi-physics coupling model is proposed for CMUTs in this paper. Methods: The model combines multiple physical fields through finite element method (FEM), which is more efficient and accurate. The influence of dimension parameters on the transducer is studied from two aspects: the pitch between adjacent cells and cell number, respectively. Results: When increasing the pitch, the directivity becomes better if the pitch is less than half of the wavelength, the grating lobe and side lobe are aroused. However, the bandwidth at -3dB is reduced by up to 71.1% at the same time. When increasing the cell number, the better the direcitvity, while the change of the cell number has little effect on the bandwidth of the array. However, 5.4% frequency shift was caused due to the effective mass change of the transducer. Conclusion: In this study, a 3D model based on FEM is proposed for the influence of dimension parameter on CMUTs. The simulation results indicate that the directivity can be enhanced by changing the dimension. At the same time, it also introduces other performance issues, such as bandwidth decrease and frequency shift through this approach. Therefore, the simulation proposed the guidance of CMUTs performance optimization and patented technology application work in the future.

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.

A 1.11 mm2 IVUS SoC with ±50°-Range Plane Wave Transmit Beamforming at 40 MHz.

Intravascular ultrasound (IVUS) imaging catheters are significant tools for cardiovascular interventions, and their use can be expanded by realizing IVUS imaging guidewires and microcatheters. The miniaturization of these devices creates challenges in SNR due to the need for higher frequencies to provide adequate resolution. An integrated IVUS system with transmit beamforming can mitigate these limitations. This work presents the first practical highly integrated system-on-a-chip (SoC) with plane wave transmit beamforming at 40 MHz for IVUS on guidewire or microcatheters. The front-end circuitry has a 20-channel ultrasound transmitter (Tx) and receiver (Rx) array interfaced with a capacitive micromachined ultrasound transducer (CMUT) array. During each firing, all 20 Tx are excited with the same analog delay with respect to each other, which can be continuously adjusted between ~0 and 10 ns in two directions, generating a steerable plane wave in a range of ±/-50° for a phased array at 40 MHz. The unit delays are generated via a voltage-controlled delay line (VCDL), which only needs two external controls, one tuning the unit delay and the other determining the steering direction. The SoC is fabricated using a 180-nm high-voltage (HV) CMOS process and features a slender active area of 0.3 mm × 3.7 mm. The proposed SoC consumes 31.3 mW during the receiving mode. The beamformer's functionality and the SoC's overall performance were validated through acoustic characterization and imaging experiments.

Li-Ion Cell in Operando Monitoring and Prognostication Using CMUT Devices

Capacitive micromachined ultrasonic transducer (CMUT) devices are ideal solutions for in operando battery monitoring applications especially for consumer electronics. Their minute size (2mm x 1mm x 0.5mm) allows direct placement on the battery without affecting the form factor of the device, the volume production techniques used for their manufacture allow for a very low-cost device, and they can be easily integrated with front-end integrated circuits either in a single device or in a discrete multichip form. Together with industry partners we have developed CMUT devices that we use to continuously monitor the behavior of a battery while it is cycling.For this work we used CMUTs (close-up shown in Fig. 1(a)) developed to operate with a low bias voltage (less than 30 V) which reduces the complexity of the driving hardware. The impulse response and its frequency content are shown in Fig. 1(b). In collapse mode, the peak frequency is 4.0 MHz and the one-way –3 dB BW is 76%, which allows flexibility in setting the drive conditions to interrogate the battery. Our testing also shows that the CMUTs show good device-to-device uniformity and they have a very stable performance in variable temperature environments (e.g. 0℃ - 60℃ ).The cell is interrogated in pulse-echo mode. The CMUT is excited by a wide-band unipolar pulse and the returned analog signal is filtered and amplified. The signal is then digitized and processed to extract features that allow us to quantify the state of the battery. Figure 1(c) depicts such signal features change under baseline cell cycling conditions (three cycles at a charge rate of C/4). These changes are the result of mechanical material property changes in the cell e.g., electrode stiffness and density [1] which directly affect the acoustic wave propagation inside the cell. Figure 1(d) depicts signal feature change due cell overcharge (red line). Ultrasound is especially sensitive to gas generation which occurs as the cell voltage extends beyond normal operating range [2]. Signal features provide early warning notice that cell is outside of normal operating conditions.Figure 1. a) Close-up image of CMUT device. b) CMUT impulse response and corresponding frequency content. c) Signal features (RMS and Time of Flight (ToF)) and thickness change during C/4 charge and discharge. d) Signal features during cell constant current overcharge. Cell overcharged after 4.4V (red line). Chang, W. et al. Measuring effective stiffness of Li-ion batteries via acoustic signal processing. Mater. Chem. A 8, 16624–16635 (2020).Appleberry, M. C. et al. Avoiding thermal runaway in lithium-ion batteries using ultrasound detection of early failure mechanisms. Journal of Power Sources 535, 231423 (2022). Figure 1

Transparent Dual-Frequency CMUT Arrays for Photoacoustic Imaging.

The opaque ultrasound transducers used in conventional photoacoustic imaging systems necessitate oblique light delivery, which gives rise to some disadvantages such as inefficient target illumination and bulky system size. This work proposes a transparent capacitive micromachined ultrasound transducer (CMUT) linear array with dual-band operation for through-illumination photoacoustic imaging. Fabricated using an adhesive wafer bonding method, the array consists of optically transparent conductors [indium tin oxide (ITO)] as both top and bottom electrodes, a transparent polymer [bisbenzocyclobutene (BCB)] as the sidewall and adhesive material, and largely transparent silicon nitride as the membrane. The fabricated device had a maximum optical transparency of 76.8% in the visible range. Furthermore, to simultaneously maintain higher spatial resolution and deeper imaging depth, this dual-frequency array consists of low- and high-frequency channels with 4.2- and 9.3-MHz center frequencies, respectively, which are configured in an interlaced architecture to minimize the grating lobes in the receive point spread function (PSF). With a wider bandwidth compared to the single-frequency case, the fabricated transparent dual-frequency CMUT array was used in through-illumination photoacoustic imaging of wire targets demonstrating an improved spatial resolution and imaging depth.

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.

Dynamic ultrasound focusing and centimeter-scale ex vivo tissue ablations with a CMUT probe developed for endocavitary HIFU therapies.

Thermal ablation of localized prostate tumors via endocavitary Ultrasound-guided High Intensity Focused Ultrasound (USgHIFU) faces challenges that could be alleviated by better integration of dual modalities (imaging/therapy). Capacitive Micromachined Ultrasound Transducers (CMUTs) may provide an alternative to existing piezoelectric technologies by exhibiting advanced integration capability through miniaturization, broad frequency bandwidth and potential for high electro-acoustic efficiency. An endocavitary dual-mode USgHIFU probe was built to investigate the potential of using CMUT technologies for transrectal prostate cancer ablative therapy. The USgHIFU probe included a planar 64-element annular HIFU CMUT array (fHIFU = 3 MHz) surrounding a 256-element linear imaging CMUT array. Acoustic characterization of the HIFU array included 3D pressure field mapping and radiation force balance measurements. Ex vivo proof-of-concept experiments consisted in generating HIFU thermal ablations with the CMUT probe on porcine liver tissues. The planar CMUT probe enabled HIFU dynamic focusing (distance range: 32 - 72 mm) while providing acoustic surface intensities of 1 W/cm2 that allowed producing elementary ex vivo ablations in depth of liver tissue (L×W ≈ 10 mm × 5 mm). Combinations of dynamic focusing, along with probe rotation and translation produced larger thermal ablations (L×W ≈ 20 mm × 20 mm) by juxtaposing multiple elementary ablations, consistent with expected results obtained through numerical modeling. The technical feasibility of using a USgHIFU probe, fully-developed using CMUTs for tissue ablation purposes, was demonstrated. The HIFU-CMUT array showed tissue ablation capabilities with volumes compatible with localized cancer targeting thus providing assets for further development of focal therapies.

Design of Zeolitic Imidazolate Framework-8-Functionalized Capacitive Micromachined Ultrasound Transducer Gravimetric Sensors for Gas and Hydrocarbon Vapor Detection

A capacitive micromachined ultrasound transducer (CMUT) was engineered and functionalized with zeolitic imidazolate framework-8 (ZIF-8) dispersed in a photoresist AZ1512HS (AZ) matrix to function as a gravimetric gas sensor. The sensor response was recorded in the presence of nitrogen, argon, carbon dioxide, and methane gases as well as water, acetylene, a propane/butane mixture, n-hexane, gasoline, and diesel vapors. The photoresist matrix alone was found to have a negligible response to all the gases and vapors, except for water vapor. No visible difference in sensor response was detected when switching from nitrogen to methane gas. However, a strong shift in the sensor resonance frequency was observed when exposed to higher hydrocarbons, ranging from 1 kHz for acetylene to 7.5 kHz for gasoline. Even longer-chain hydrocarbons, specifically kerosene and more so diesel, had a significantly reduced sensor frequency shift compared with gasoline. Sensors functionalized with a thin film of AZ+ZIF-8 demonstrated higher sensitivity in their response to a hydrocarbon molecular mass than without functionalization.

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.

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).

High-Performance Electrode-Post CMUTs: Fabrication Details and Best Practices.

Capacitive micromachined ultrasound transducers (CMUTs) have been investigated for over 25 years due to their promise for mass manufacturing and electronic co-integration. Previously, CMUTs were fabricated with many small membranes comprising a single transducer element. This, however, resulted in suboptimal electromechanical efficiency and transmit performance, such that resulting devices were not necessarily competitive with piezoelectric transducers. Moreover, many previous CMUT devices were subject to dielectric charging and operational hysteresis that limited long-term reliability. Recently, we demonstrated a CMUT architecture using a single long rectangular membrane per transducer element and novel electrode-post (EP) structures. This architecture not only offers long-term reliability, but also provides performance advantages over previously published CMUT and piezoelectric arrays. The purpose of this article is to highlight these performance advantages and provide details of the fabrication process, including the best practices to avoid common pitfalls. The objective is to provide sufficient detail to inspire a new generation of microfabricated transducers, which could lead to performance gains of future ultrasound systems.

Dual-Backplate CMUTs with Wide Bandwidth and Low Driving Voltage for Airborne Applications.

In this work, novel airborne capacitive micromachined ultrasonic transducers (CMUTs) based on a dual-backplate (DBP) technology are presented. In contrast to conventional CMUTs, these transducers use a three-electrode-based capacitive system, where the membrane is placed between two highly-perforated counter electrodes, enabling enlarged displacement amplitudes in electrostatic actuation and wide and tunable bandwidth due to a ventilated air cavity. Fabricated DBP-CMUT prototypes therefore show exceptionally high receive and transmit sensitivities of -34.5 dB(V/Pa) and 259 nm/V, respectively, in their 84 kHz resonance. The viscous dissipation introduced by ventilating the cavity results in a wide factional bandwidth (FBW) of 29%. An applicability of the developed CMUT for airborne ranging is demonstrated in pulse-echo based ranging measurements, where the distance of a sound reflecting metal plate can be clearly detected by a single CMUT operated in a transceiver mode.

Fabrication of Capacitive Micromachined Ultrasonic Transducer (CMUT) With PMMA/Graphene Membrane

Fabrication of Capacitive Micromachined Ultrasonic Transducer (CMUT) With PMMA/Graphene Membrane

Low-Noise Low-Power Ultrasound AFE With Continuous TGC Built in Both TIA and Beamformer.

This article presents a low-noise high-power-efficiency analog front-end (AFE) for capacitive-micromachined-ultrasonic transducers (CMUT). Implemented in 28-nm CMOS technology, the proposed AFE features three-stage continuous time-gain compensation (TGC) embedded in both trans-impedance amplifiers (TIAs) and an analog beamformer to provide a large compensation range with no extra power-consumption cost. The use of noise cancellation and capacitive feedback optimizes the noise performance of TIAs. The first stage of the TGC is built in the TIA by adjusting the positive and negative resistance loads, which are composed of voltage-controlled transistor arrays. An all-pass passive network is used as the delay unit of the analog beamformer, meanwhile achieving the second TGC stage. Phase shift for all frequency components in the ultrasound pass-band is manifested as a delay to the echoes. The third stage of the TGC is merged with a summing unit, which is a closed-loop amplifier with variable resistance feedback. The design takes into account the ability to handle large signals and power consumption, with TIA and beamforming operating at voltages of 2.5 V and 0.9 V, respectively. Experimental results show that the proposed AFE achieves a 2.11 pA/√Hz input-referred noise (IRN) at the 5 MHz center frequency of the echoes while consuming only 1.02 mW/channel. A total exponential TGC range of 60 dB with continuous ranges of 12 dB, 24 dB, and 24 dB assigned to three stages has been verified for this work.

Thermal analysis and SNR comparison of CMUT and PZT transducers using coded excitation

Coded excitation (CE) has the ability to enhance image quality and penetration depth by improving the signal-to-noise ratio (SNR). Their usefulness has been extensively proven in the literature, however, there are very few publications that have discussed the practicality of using CEs, as they can increase the operating temperature of a transducer beyond the safety limits. In this paper, the potential for capacitive micromachined ultrasonic transducers (CMUTs) to handle CEs is investigated and compared to a geometrically similar Lead Zirconate Titanate (PZT) probe. It is hypothesized that CMUTs are comparatively advantageous for CE and can generate CE signals more effectively while operating within the safety limits of temperature specified by the International Electrotechnical Commission (IEC). Simple chirp signals were used for excitation and the temperature of the transducer assembly was measured under two different test conditions following the IEC standards. The still air test showed that at the pulse repetition frequency of 4 kHz and a signal duration of 8μs, the PZT probe reached the safe limit of 27 °C within 3 min of acquisition, whereas the CMUT probe showed an increase of only 4–5 °C over 30 min of scan time. The phantom experiments showed that for the maximum temperature rise allowed at the object-transducer interface by the IEC, the gain in SNR possible for the CMUT probe was 13–14 dB and the increase in penetration depth was 30 mm, whereas no gain in SNR and penetration depth was achieved for the PZT probe. The peak mechanical index and the derated spatial peak temporal averaged intensity were found to be less than half the FDA limits for both probes in all cases, proving temperature to be the first limiting factor when using CE. Therefore, the CMUT has shown to be thermally more efficient than the PZT probe and a good candidate for CE imaging.

A Comparative Study between Silicon Carbide and Silicon Nitride based Single Cell CMUT

This research explores the design and conducts a comparative analysis of a non-insulated Capacitive Micromachined Ultrasonic Transducer (CMUT) featuring an innovative asymmetric electrode configuration to improve the performance of the device. Specifically, this configuration involves the utilization of a top electrode with a smaller radius in comparison to the bottom electrode. The study encompasses an investigation into the effects of varying biasing voltage within the range of 40 V to 100 V. The materials employed in this study are carefully selected to optimize the CMUT's performance. The substrate material is silicon, and the bottom and top electrodes are made from aluminium. Additionally, silicon dioxide is utilized as the foundation material within the device's structure.

Ultrasound Fill-Level and Touch Sensors Through Acoustic Coupling for CMUTs

Capacitive micromachined ultrasonic transducers (CMUTs) are the next-gen ultrasound transducers for diverse applications. CMUTs provide high bandwidth and sensitivity and have excellent integrability with CMOS processes. Additionally, our CMUT design enables mobile and battery driven devices with a small form factor and requiring voltages as low as < 30 V. Here, we present a novel coupling approach and demonstrate two sensor applications: fill level sensing and touch sensing. With a package containing a stack of silicone and epoxy, we achieve excellent coupling into different materials such as mold plastics and aluminum via direct or intermediate acoustic impedance matching. This enables an universal solution for coupling into almost any kind of material. Fill level sensing is demonstrated through a ca. 4 mm-thick aluminum containment wall with levels from 2.3 mm to 600 mm. Furthermore, a touch sensor is realized via detecting a reduced echo amplitude due to the presence of a finger. Measurements through a commercial decor panel and a ca. 4 mm-thick aluminum plate show a maximum signal difference of 6.5 % and 19.2 % in the echo, respectively. Additionally, we investigate several interference and coupling phenomena in the layered system for the touch scenario. By that, we enable a wide field of applications, where CMUTs are coupled to hard materials such as plastics and metals, as found in touch sensors in e.g. consumer electronics, as well as liquid (flow / fill level) monitoring in chemistry, pharmaceutical and food industry. Furthermore, structural health monitoring for smart industry and non-destructive testing are feasible with our approach as well.