Conventional Capacitive Micromachined Ultrasonic Transducer Research Articles

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.

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.

Vertical cavity capacitive transducer.

The capacitive micromachined ultrasonic transducer (CMUT) has inherent advantages, such as larger bandwidth and monolithic integration capability with electronics, when compared to the piezoelectric transducer. The most significant shortcoming of the device is the trade-off between input and output sensitivities. Adequate receive sensitivity requires an electric field intensity on the order of 105 V/m, which can be achieved by sub-micron gap heights. However, a small gap limits the device stroke and, consequently, the maximum output pressure. This paper addresses this problem by proposing a CMUT with a vertical cavity. The membrane of the device has a piston part that is surrounded by the sidewalls of a vertical cylinder formed in the substrate. The fringing electric field pulls the piston in the vertical direction; hence, the gap height remains fixed, which alleviates the hard limit on device stroke. The performance of the proposed device is compared to that of the conventional CMUT by theoretical and analytical methods, and a micro-fabrication method is devised. Additionally, a millimeter-scale device has been manufactured and tested as a proof of concept.

An Analytical Model for Bandwidth Enhancement of Air-Coupled Unsealed Helmholtz Structural CMUTs

Capacitive micromachined ultrasonic transducers (CMUTs) were reported to own high potential in air-coupled ultrasonic applications such as noncontact nondestructive examination and gas flow measurement. The unsealed CMUTs which utilized the squeeze film effect were reported to overcome the narrow output pressure bandwidth of the conventional sealed CMUTs in air operation. This kind of unsealed CMUTs can also be regarded as Helmholtz resonators. In this work, we present the air-coupled unsealed Helmholtz structural CMUTs which utilize both the squeeze film effect and the Helmholtz resonant effect to enhance the output pressure bandwidth. Based on the mechanism of vibration coupling between membrane and air pistons in membrane holes, we propose an analytical model to aid the design process of this kind of CMUTs. We also use finite element method (FEM) to investigate this kind of CMUTs for our analytical model validation. The FEM results show that the significant bandwidth enhancement can be achieved when the Helmholtz resonant frequency is designed close to the fundamental resonant frequency of the CMUT membrane. Compared with the conventional sealed CMUT cell, the 4-hole unsealed Helmholtz structural CMUT cell improves both the 3-dB fractional bandwidth and SPL-bandwidth product around 35 times. Furthermore, it is found that, with more holes under the same hole area ratio or with a smaller ratio of the cavity height to the viscous boundary layer thickness, the Helmholtz resonant effect becomes weaker and thus the output pressure bandwidth decreases.

High-Efficiency Output Pressure Performance Using Capacitive Micromachined Ultrasonic Transducers with Substrate-Embedded Springs.

Capacitive micromachined ultrasonic transducers (CMUTs) with substrate-embedded springs offer highly efficient output pressure performance over conventional CMUTs, owing to their nonflexural parallel plate movement. The embedded silicon springs support thick Si piston plates, creating a large nonflexural average volume displacement efficiency in the operating frequency range from 1–3 MHz. Static and dynamic volume displacements of the nonflexural parallel plates were examined using white light interferometry and laser Doppler vibrometry. In addition, an output pressure measurement in immersion was performed using a hydrophone. The device showed a maximum transmission efficiency of 21 kPa/V, and an average volume displacement efficiency of 1.1 nm/V at 1.85 MHz with a low DC bias voltage of 55 V. The device element outperformed the lead zirconate titanate (PZT) ceramic HD3203, in the maximum transmission efficiency or the average volume displacement efficiency by 1.35 times. Furthermore, its average volume displacement efficiency reached almost 80% of the ideal state-of-the-art single-crystal relaxor ferroelectric materials PMN-0.33PT. Additionally, we confirmed that high-efficiency output pressure could be generated from the CMUT device, by quantitatively comparing the hydrophone measurement of a commercial PZT transducer.

Practical CMUT Fabrication With a Nitride-to-Oxide-Based Wafer Bonding Process

The introduction of wafer-bonding technique to capacitive micromachined ultrasonic transducer (CMUT) fabrication offered advantages over the surface micromachining technique, such as simplified fabrication, better membrane uniformity, and increased active area. The conventional wafer-bonding CMUT process employs silicon-on-insulator (SOI) wafers to produce the vibrating membrane layer. However, the use of SOI wafers can lead to poor membrane thickness uniformity despite the high cost. Furthermore, an additional local-oxidation (LOCOS) process should be used for SOI-based wafer bonding process to improve breakdown voltage and reduce parasitic capacitance. This paper reports a nitride-to-oxide bonding process for CMUT fabrication, designed to achieve similar results as the LOCOS-added process reported by Park et al. , but without the need for SOIs and fabrication complexity. A fully functional CMUT designed for collapse-mode operation was fabricated and an immersion center frequency was measured to be 4.2 MHz with a 90% bandwidth. [2016-0210]

Fabrication of High-Efficiency CMUTs With Reduced Parasitics Using Embedded Metallic Layers

The transmit and receive sensitivity of the capacitive micromachined ultrasonic transducer (CMUT) is proportional to the active device capacitance formed by the vacuum gap of the device, and an insulation layer between the gap and the device electrode. In the sacrificial release process of CMUT fabrication, this insulation layer cannot be made arbitrarily thin due to conformality issues. In this paper, we propose and prove the applicability of a micromachining technique by which metallic sacrificial islands are embedded inside grooves etched on the substrate, yielding topology free surfaces. This obviates the conformality requirement, and enables the growth of a thinner insulation layer which reduces the effective gap height, and, hence, improves sensitivity. Embedded metalic layers, which provide a flat surface for subsequent process steps, have also been used as the back electrode of the CMUT, which facilitated the manufacturing of devices with reduced stray capacitance on thermally oxidized wafers. CMUT devices were fabricated using the proposed technique, and their parameters were measured to justify the performance improvement. While the dc bias requirement is reduced by 19%, the output sensitivity of the device is 10% higher than that of the conventional CMUT, and spurious capacitance is decreased by 70%.

Lumped element modeling of air-coupled capacitive micromachined ultrasonic transducers with annular cell geometry

Air-coupled capacitive micromachined ultrasonic transducers (CMUTs) based on annular cell geometry have recently been reported. Finite element analysis and experimental studies have demonstrated their significant improvement in transmit efficiency compared with the conventional circular-cell CMUTs. Extending the previous work, this paper proposed a lumped element model of annular-cell CMUTs. Explicit expressions of the resonance frequency, modal vector, and static displacement of a clamped annular plate under uniform pressure were first derived based on the plate theory and curve fitting method. The lumped model of an annular CMUT cell was then developed by adopting the average displacement as the spatial variable. Using the proposed model, the ratio of average-to-maximum displacement was derived to be 8/15. Experimental and simulation studies on a fabricated annular CMUT cell verified the effectiveness of the lumped model. The proposed model provides an effective and efficient way to analyze and design air-coupled annular-cell CMUTs.

Bilateral CMUT Cells and Arrays: Equivalent Circuits, Diffraction Constants, and Substrate Impedance.

We introduce the large-signal and small-signal equivalent circuit models for a capacitive micromachined ultrasonic transducer (CMUT) cell, which has radiating plates on both sides. We present the diffraction coefficient of baffled and unbaffled CMUT cells. We show that the substrate can be modeled as a very thick radiating plate on one side, which can be readily incorporated in the introduced model. In the limiting case, the reactance of this backing impedance is entirely compliant for substrate materials with a Poisson's ratio less than 1/3. We assess the dependence of the radiation performance of the front plate on the thickness of the back plate by simulating an array of bilateral CMUT cells. We find that the small-signal linear model is sufficiently accurate for large-signal excitation, for the purpose of the determining the fundamental component. To determine harmonic distortion, the large-signal model must be used with harmonic balance analysis. Rayleigh-Bloch waves are excited at the front and back surfaces similar to conventional CMUT arrays.

Improved Analytical Modeling of Membrane Large Deflection With Lateral Force for the Underwater CMUT Based on Von Kármán Equations

This paper is to develop analytical models for the underwater capacitive micromachined ultrasonic transducer (CMUT) to understand its large deflection effect from the water pressure. To accurately model the displacement profile of the CMUT under the water pressure, Von Karman equations and the perturbation method are employed to calculate the membrane deformation from a uniform pressure. The equations for an annular-ring plate model are first applied to calculate the displacement profile of the uniform CMUT membrane. The lateral force due to the membrane elongation is considered in the proposed model, which is used to calculate the displacement profiles for both conventional and collapse mode CMUTs under different external pressures. When compared with finite-element method results, the proposed model can predict the displacement profiles of the conventional-mode CMUT under water pressure ranging from 0.8 to 4 MPa with an error of <1%. It can also estimate CMUT membrane that operates in collapse mode with an error in the deflection profile for <4.7% from 5 to 14 MPa. In addition, it is worth to mention that the proposed model can cover the small deflection scenarios but with relatively larger error under collapse mode.

Output pressure enhancement of CMUTs by using multiple Helmholtz resonance apertures

Multiple Helmholtz resonance apertures are proposed to enhance the output pressure of air-coupled capacitive micromachined ultrasonic transducers (CMUTs) for non-contact ultrasound imaging applications. The methodologies of defining the design parameters of CMUTs and the resonant apertures on the membrane of CMUTs are discussed. In comparing certain configurations of resonant apertures with conventional CMUTs, simulation results show that a prospective improvement of output pressure (up to 32.1%) can be achieved.

High-power CMUTs: design and experimental verification

Capacitive micromachined ultrasonic transducers (CMUTs) have great potential to compete with piezoelectric transducers in high-power applications. As the output pressures increase, nonlinearity of CMUT must be reconsidered and optimization is required to reduce harmonic distortions. In this paper, we describe a design approach in which uncollapsed CMUT array elements are sized so as to operate at the maximum radiation impedance and have gap heights such that the generated electrostatic force can sustain a plate displacement with full swing at the given drive amplitude. The proposed design enables high output pressures and low harmonic distortions at the output. An equivalent circuit model of the array is used that accurately simulates the uncollapsed mode of operation. The model facilities the design of CMUT parameters for high-pressure output, without the intensive need for computationally involved FEM tools. The optimized design requires a relatively thick plate compared with a conventional CMUT plate. Thus, we used a silicon wafer as the CMUT plate. The fabrication process involves an anodic bonding process for bonding the silicon plate with the glass substrate. To eliminate the bias voltage, which may cause charging problems, the CMUT array is driven with large continuous wave signals at half of the resonant frequency. The fabricated arrays are tested in an oil tank by applying a 125-V peak 5-cycle burst sinusoidal signal at 1.44 MHz. The applied voltage is increased until the plate is about to touch the bottom electrode to get the maximum peak displacement. The observed pressure is about 1.8 MPa with -28 dBc second harmonic at the surface of the array.

CMUT With Substrate-Embedded Springs For Non-Flexural Plate Movement.

A conventional capacitive micromachined ultrasonic transducer (CMUT) is composed of many cells connected in parallel. Since the plate in each CMUT cell is anchored at its perimeter, the average displacement is several times smaller than the displacement of an equivalent ideal piston transducer. In addition, the post areas, where the plates are anchored to, are non-active and, thus, do not contribute to the transduction. We propose a CMUT structure that resembles an ideal capacitive piston transducer, where the movable top plate only undergoes translation rather than deflection. Our proposed CMUT structure is composed of a rigid plate connected to a substrate using relatively long and narrow posts, providing the spring constant for the movement of the plate. Rather than the flexure of the plate as in a conventional CMUT, this device operates based on the compression of the compliant posts. For a capacitive transducer, a thin electrostatic gap is provided under the top plate. We used finite element analysis (FEA) to design and verify the structure's functionality. The simulation results show a fractional bandwidth of over 100% in immersion for all the designs. They also confirm that the average displacement of the top plate is above 90% of its peak displacement. We fabricated the first prototype based on this idea, which only requires a simple 3-mask fabrication process. In addition to 128-element 1-D arrays, we fabricated a variety of 240 μm × 240 μm, single-element transducers with different post configurations. We successfully measured the electrical input impedance of the fabricated devices and confirmed their resonant behavior in air. Further, we measured the acoustic pressure using a calibrated hydrophone at a known distance. Using this measurement, we calculated a peak-to-peak pressure of 1.5 MPa at the face of the transducer. Our results show that it is possible to fabricate CMUTs that exhibit ideal piston-like plate movement. Because of the substrate-embedded spring elements, the plate does not need to be operated in flexural mode, as in a conventional CMUT, resulting in a significantly improved fill-factor, and, thus, a more efficient device.

A Miniature Capacitive Ultrasonic Imager Array

This paper describes the development of a miniature capacitive micromachined ultrasonic transducer (CMUT) array suitable for minimally invasive medical imaging and diagnosis. In contrast to conventional laboratory-scale CMUT platforms, which are generally integrated on a silicon substrate thicker than 550 mum, this imager array is integrated on a probe shaped silicon substrate with a typical shank dimension of 60 mum(width) times 40 mum(thickness) times 4-10 mm(length) for 1-D arrays, and 0.4-2.3 mm(width) times 100 mum(thickness) times 6-12 mm(length) for 2-D arrays. Such miniature CMUT arrays are suitable for implantation into tissue through a fine incision or by being placed inside an organ for close-range imaging. In a close-range diagnosis made possible by using such miniature CMUT arrays, ultrasound of a higher frequency can be used and the conflict associated with the penetration depth and image resolution can be resolved. This imager array was fabricated using a two-layer polysilicon surface micromachining process followed by a double-sided deep silicon etching for substrate shaping. The total mask count was eight. The central frequency of ultrasound transmitted by a circular 46 mum-diameter transducer was 3.8 MHz, while its fractional bandwidth was 116% in water. A simple transducer-fluid model was used to predict the acoustic characteristics of this device in water. Preliminary B-mode imaging using a 21-element 1-D array was demonstrated.

Capacitive micromachined ultrasonic transducers (CMUTs) with isolation posts

In this paper, an improved design of a capacitive micromachined ultrasonic transducer (CMUT) is presented. The design improvement aims to address the reliability issues of a CMUT and to extend the device operation beyond the contact (collapse) voltage. The major design novelty is the isolation posts in the vacuum cavities of the CMUT cells instead of full-coverage insulation layers in conventional CMUTs. This eliminates the contact voltage drifting due to charging caused by the insulation layer, and enables repeatable CMUT operation in the post-contact regime. Ultrasonic tests of the CMUTs with isolation posts (PostCMUTs) in air (electrical input impedance and capacitance vs. bias voltage) and immersion (transmission and reception) indicate acoustic performance similar to that obtained from conventional CMUTs while no undesired side effects of this new design is observed.

CMUTS with dual electrode structure for improved transmit and receive performance

In this paper, we introduce capacitive micro-machined ultrasonic transducers (CMUTs) with electrically isolated multiple electrodes embedded in the same silicon nitride CMUT membrane. Some of the advantages of this structure are demonstrated using a dual-electrode CMUT with separate transmit and receive electrodes as an example. By locating the transmit electrodes near the edges of a rectangular CMUT membrane, the stable displacement range, hence the maximum pressure amplitude during transmit mode is increased without collapsing the membrane when operated within static collapse voltage range. In the receive mode, the center receive electrode is brought closer to the substrate by biasing the side electrodes, and a higher electromechanical transformer ratio is obtained at low direct current (DC) bias. Therefore, dual-electrode CMUT has an effectively larger gap as compared to conventional CMUT during transmit, and it has an effectively smaller gap during receive. Demonstrative experiments are performed on dual-electrode CMUTs with rectangular membranes with different side and center electrode sizes for transmit and receive measurements. By using the two 4-microm wide side electrodes and an 8-microm wide center electrode on a 20-microm wide membrane, a 6.8 dB increase in maximum output pressure is obtained with side electrode excitation as compared to conventional center electrode. Similarly, the receive performance improvement was demonstrated while reducing the DC bias requirements. Simple finite-element and equivalent circuit-based models were developed to successfully model the behavior of dual-electrode CMUTs. Simulations show that, with simple modifications, more than 10 dB overall sensitivity improvement is feasible with dual-electrode CMUTs with rectangular membranes.

A solution to the charging problems in capacitive micromachined ultrasonic transducers

We report on a capacitive micromachined ultrasonic transducer (CMUT) featuring isolation posts (PostCMUT) as a solution to the charging problems caused by device fabrication and operation. This design improves the device reliability. The PostCMUTs were fabricated using a newly developed process based on the wafer-bonding technique. Paired tests showed the superior reliability characteristics of the PostCMUT design compared to those of conventional CMUT designs. No deleterious effect of the new design was seen in preliminary ultrasonic tests or in process yield. PostCMUTs, a design that serves as a solution to the aforementioned reliability problem, constitutes a major contribution to CMUT commercialization.