Capacitive Micromachined Ultrasonic Transducers Design Research Articles

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.

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.

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.

Large-Scale Nonlinear Lumped and Integrated Field Simulations of Top-Orthogonal-to-Bottom-Electrode CMUT Architectures.

Capacitive micromachined ultrasonic transducers (CMUTs) promise many advantages over traditional piezoelectric transducers such as the potential to construct large, cost-effective 2-D arrays. To avoid wiring congestion issues associated with fully wired arrays, top-orthogonal-to-bottom electrode (TOBE) CMUT array architectures have proven to be a more practical alternative, using only 2N wires for an N ×N array. Optimally designing a TOBE CMUT array is a significant challenge due to the range of parameters from the device level up to the operating conditions of the entire array. Since testing many design variations can be prohibitively expensive, a simulation approach accounting for both the small and large-scale array characteristics of TOBE arrays is essential. In this paper, we demonstrate large-scale TOBE CMUT array simulations using a nonlinear CMUT lumped-circuit model. We investigate the performance of the array with different CMUT design parameters and array operating conditions. These simulated results are then compared with measurements of TOBE arrays fabricated using a sacrificial release process.

Capacitive micromachined ultrasonic transducers based on annular cell geometry for air-coupled applications

A novel design of an air-coupled capacitive micromachined ultrasonic transducer (CMUT) with annular cell geometry (annular CMUT) is proposed. Finite element analysis shows that an annular cell has a ratio of average-to-maximum displacement (RAMD) of 0.52–0.58 which is 58–76% higher than that of a conventional circular cell. The increased RAMD leads to a larger volume displacement which results in a 48.4% improved transmit sensitivity and 127.3% improved power intensity. Single-cell annular CMUTs were fabricated with 20-μm silicon plates on 13.7-μm deep and 1.35-mm wide annular cavities using the wafer bonding technique. The measured RAMD of the fabricated CMUTs is 0.54. The resonance frequency was measured to be 94.5kHz at 170-V DC bias. The transmit sensitivity was measured to be 33.83Pa/V and 25.85Pa/V when the CMUT was excited by a continuous wave and a 20-cycle burst, respectively. The receive sensitivity at 170-V DC bias was measured to be 7.7mV/Pa for a 20-cycle burst, and 15.0mV/Pa for a continuous incident wave. The proposed annular CMUT design demonstrates a significant improvement in transmit efficiency, which is an important parameter for air-coupled ultrasonic transducers.

An Analytical Model for CMUTs with Square Multilayer Membranes Using the Ritz Method.

Capacitive micromachined ultrasonic transducer (CMUT) multilayer membrane plays an important role in the performance metrics including the transmitting efficiency and the receiving sensitivity. However, there are few studies of the multilayer membranes. Some analytical models simplify the multilayer membrane as monolayer, which results in inaccuracies. This paper presents a new analytical model for CMUTs with multilayer membranes, which can rapidly and accurately predict static deflection and response frequency of the multilayer membrane under external pressures. The derivation is based on the Ritz method and Hamilton’s principle. The mathematical relationships between the external pressure, static deflection, and response frequency are obtained. Relevant residual stress compensation method is derived. The model has been verified for three-layer and double-layer CMUT membranes by comparing its results with finite element method (FEM) simulations, experimental data, and other monolayer models that treat CMUTs as monolayer plates/membranes. For three-layer CMUT membranes, the relative errors are ranging from 0.71%–3.51% for the static deflection profiles, and 0.35%–4.96% for the response frequencies, respectively. For the double-layer CMUT membrane, the relative error with residual stress compensation is 4.14% for the central deflection, and −1.17% for the response frequencies, respectively. This proposed analytical model can serve as a reliable reference and an accurate tool for CMUT design and optimization.

CMUTs with high-K atomic layer deposition dielectric material insulation layer.

Use of high-κ dielectric, atomic layer deposition (ALD) materials as an insulation layer material for capacitive micromachined ultrasonic transducers (CMUTs) is investigated. The effect of insulation layer material and thickness on CMUT performance is evaluated using a simple parallel plate model. The model shows that both high dielectric constant and the electrical breakdown strength are important for the dielectric material, and significant performance improvement can be achieved, especially as the vacuum gap thickness is reduced. In particular, ALD hafnium oxide (HfO2) is evaluated and used as an improvement over plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (Six)Ny)) for CMUTs fabricated by a low-temperature, complementary metal oxide semiconductor transistor-compatible, sacrificial release method. Relevant properties of ALD HfO2) such as dielectric constant and breakdown strength are characterized to further guide CMUT design. Experiments are performed on parallel fabricated test CMUTs with 50-nm gap and 16.5-MHz center frequency to measure and compare pressure output and receive sensitivity for 200-nm PECVD Six)Ny) and 100-nm HfO2) insulation layers. Results for this particular design show a 6-dB improvement in receiver output with the collapse voltage reduced by one-half; while in transmit mode, half the input voltage is needed to achieve the same maximum output pressure.

Performance and reliability of new CMUT design with improved efficiency

The performance and reliability issues of a new capacitive micro-machined ultrasonic transducer (CMUT) are presented in this paper. To enhance the moving efficiency, by obtaining a piston-like motion, an indirectly clamped membrane structure is proposed. The CMUT with indirectly clamped membrane is composed of three parts which are a circular moving plate, a ring-shaped bridge layer and an upper plate. By connecting circular moving plate to upper plate vertically through a ring-shaped bridge layer, this new structure was expected to produce the piston-like and more flexural motion. It also exhibited 1.7 times higher output pressure and 14.7% wider fractional bandwidth when compared to conventional basic structure. Environmental tests were carried out to verify the robustness and resistance to stress from temperature and humidity. For device's durability and lifetime estimation, an accelerated long-term operating test has also been performed. The derived mean time to failure (MTTF) was more than 2.29×1011 vibration cycles, which translates to about 2650h of guaranteed operational time.

An improved analytical method to design CMUTs with square diaphragms

A highly accurate mathematical method has been presented for analytical characterization of capacitive micromachined ultrasonic transducers (CMUTs) built with square diaphragms. The method uses a new two-dimensional polynomial function to more accurately predict the deflection curve of a multilayer square diaphragm subject to both mechanical and electrostatic pressure and a new capacitance model that takes into account the contribution of the fringing field capacitances. Both of the models have been experimentally verified by comparing the models predicted values with measurement results and are found to be in excellent agreement with a maximum deviation of less than 2% for experimentally measured capacitance values. 3-D electromechanical finite element analysis (FEA) for a wide range of material properties, geometric specifications, and loading conditions show that the presented method is highly consistent in accuracy over the typical square-diaphragm CMUT design space.

Capacitive micromachined ultrasonic transducers for generation of highly directional sound with a parametric array

Capacitive micromachined ultrasonic transducers (CMUTs) with vacuum‐sealed cavities have been fabricated and used to project directional sound using parametric arrays. The wafer bonding approach used to fabricate the CMUTs provides good control over device dimensions, a single‐crystal silicon membrane that has predictable mechanical properties, and the capability to fabricate CMUTs with large‐diameter membranes and deep cavities. The fabricated CMUTs are about 8 cm in diameter and comprise 284 circular membranes, each 4 mm in diameter. Testing of CMUTs with 40‐μm and 60‐μm‐thick membranes shows they have center frequencies of 46 kHz and 55 kHz and 3‐dB bandwidths of 1.9 kHz and 5.3 kHz, respectively. With application of DC bias voltages of 380 V and 350 V and an AC excitation of 200 V peak‐to‐peak, the devices generate effective source levels of 139 dB and 131 dB (re 20 μPa), respectively. We used the CMUT design with a 60‐μm‐thick membrane to produce 5 kHz sound at 3 m with a 6‐dB beamwidth of 8.7 deg and a sound pressure level of 58 dB. The results demonstrate that large‐area CMUTs, which produce high pressure ultrasound, can be fabricated for transmitting directional sound with parametric arrays. [Work supported by DARPA.]

The effects of membrane metallization in capacitive microfabricated ultrasonic transducers.

The mechanical effects of the metal layer on the membranes of capacitive micromachined ultrasonic transducers (CMUTs) are analyzed in this paper by means of finite element simulations. The influence of electrode size and thickness on the electrostatic behavior of the single CMUT cell, including diaphragm displacement, cell capacitance, and collapse voltage, is explored. The effect on device sensitivity is investigated through the transformation factor of the cell, that is computed by FEM and compared with the parallel plate model prediction. It is found that for a non-negligible electrode thickness, as in the majority of fabricated devices, both the static and dynamic performance of the cell can be affected in a significant way. Thus, the effects of membrane metallization must be taken into account in CMUT design and optimization.