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Capacitive micromachined ultrasonic transducers (CMUTs) have shown great potential to make a strong impact on ultrasound imaging technologies over the years. However, a critical weakness of CMUTs is their relatively low output pressure at transmission. To address this, we present a novel dual-element ultrasonic transducer that incorporates two transducer technologies by using a circular commercial piezoelectric element for ultrasound transmission and an annular CMUT element for reception. The hybrid transducer combines the broad bandwidth and high receive sensitivity of the CMUT and the high output pressure of the piezoelectric transducer to improve the overall sensitivity and axial resolution. The annular CMUT is designed, fabricated, and concentrically aligned with the piezoelectric probe via a custom housing. A through-wafer etching scheme based on deep reactive-ion etching is developed to create the CMUT annulus. Immersion characterization reveals that the CMUT has a center frequency of 2.63 MHz, a −6-dB fractional bandwidth of 131%, and a receive sensitivity of 22.57 mV/kPa, compared with 2.34 MHz, 42%, and 4.28 mV/kPa of the piezoelectric probe. The round-trip measurements show that the hybrid dual-element transducer improves the axial resolution by 25.58% and the signal-to-noise ratio by 8.55 dB over the commercial piezoelectric probe. [2017-0305] [ABSTRACT FROM AUTHOR]

One shortcoming of capacitive micromachined ultrasonic transducers (CMUTs) for commercial use is the weak output power relative to traditional ultrasonic transducers. Recently, we reported an annular CMUT cell with an improved transmit efficiency over a conventional circular cell in air. Extending this paper, we designed and fabricated a nine-element concentric CMUT array to enhance the transmit power and offer depth focusing. The proposed 200-kHz array has an aperture diameter of 2.13 cm and a fill factor of 81%. A pillar-free etching process was developed to create the deep large-area cavities of the proposed wafer-bonded CMUT. The fabricated device was characterized for the static and dynamic performance using a profilometer and a laser Doppler vibrometer, respectively. The average maximum plate dynamic displacements of the CMUT driven by 20-Vpp ac excitation voltage at dc biases of 100 and 150 V were measured to be 1.28 and 1.97 μm, respectively. Accordingly, the surface output power densities were calculated to be 0.40 and 0.96 KW/m2. We investigated the crosstalk between neighboring cells and the plate-cracking phenomenon, and provided relative suggestions for improvement. This paper demonstrates the feasibility of the concentric annular-cell CMUT array design for air-coupled applications. [ABSTRACT FROM PUBLISHER]

Air-coupled capacitive micromachined ultrasonic transducers (CMUTs) with annular cell geometry have recently been reported to have a promising transmit sensitivity. This paper reports three optimization schemes, which further improve the transmit sensitivity and also help achieve a reasonable comparison between the novel annular and conventional circular cells. Lumped element models of both cell types with laminate plate structures are presented. Based on these models, a design optimization flowchart was constructed to facilitate analytical optimization on the three schemes. Circular and annular CMUTs with a common 97-kHz natural resonance frequency were fabricated and characterized to verify the efficacy of the optimization principle. Using the optimization flowchart, annular and circular cells with frequencies ranging from 100 to 300 kHz were analytically optimized and then compared. The comparison results demonstrate that, given the same dc bias and ac excitation voltage, the output power density at the plate surface of the optimized annular cell is double that of the optimized circular cell. Additionally, when generating the same surface power density, an optimized annular cell requires either half the dc bias or half the ac excitation voltage of an optimized circular cell. This paper provides a practical optimization framework for CMUT cell design and demonstrates the superiority of annular cells for air-coupled applications. [ABSTRACT FROM AUTHOR]

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] [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM AUTHOR]

We study the feasibility of fabricating piezoelectric micromachined ultrasonic transducers (PMUTs) using a commercially available multi-user MEMS process (MUMPs). MUMPs provide a low cost and easily accessible option for MEMS researchers, but the resulting device performance is usually sacrificed. In this work, we investigate the potential limitations of MUMPs PMUTs using an experimental approach. The square-shaped PMUT structure that we fabricated has a dimension of 500 μm by 500 μm. It consists mainly of a 10 μm silicon layer and a 0.5 μm aluminum nitride piezoelectric layer. Vibrometer measurements revealed that the PMUTs resonate at 600 kHz in air. Acoustic experiments showed a resonance frequency of 1.2 MHz in immersion, which was caused by a higher order vibration mode. Transmit and receive experiments using a hydrophone and an off-the-shelf transducer demonstrated the functionality of PMUTs in immersion, with measured transmit and receive sensitivity at 0.95 kPa/V and 1.35 μV/Pa, respectively, for a 1.8 mm by 1.8 mm element. Limitations of the multi-user process that restrict the operating frequency and output pressure are discussed. More specifically, the maximum fundamental modal frequency is limited to about 2 MHz in immersion by the process design rule, and the maximum output pressure is restricted by a maximum driving voltage of 25 V pp because of potential dielectric breakdown of the piezoelectirc layer. Acoustic experiments also showed ringing that affects the axial resolution of the devices. The ringing limits the usefulness of the devices in pulse-echo applications. We conclude that MUMPs PMUTs are more suitable for applications that rely mainly on the device receive performance. [ABSTRACT FROM AUTHOR]

Capacitive micromachined ultrasonic transducers (CMUTs) exhibit wide-bandwidth and can be made into tiny 2-D arrays that are integrable with electronics. A simple implementation of such 2-D array is the row–column addressed CMUT array. However, one critical limitation of a row–column array is its narrow field of view, which can be improved by curving the array. This paper reports a novel fabrication method that realizes a flexible 32 + 32 elements row–column addressed CMUT array using a two stage fabrication process. First, a functional row–column CMUT array is fabricated with a double silicon-on-insulator wafer bonding process. The second stage involves encapsulating the device with a polymer coating, yet leaving the bottom silicon substrate exposed for wet etching. The bottom buried oxide layer will also act as an etch stop layer. Pitch-catch experiments are presented to verify the functionality. Our final device, excluding the encapsulation layer, is less than 5~\mu \textm thick with a 12.8-mm $\times \,\, 12.8$ -mm aperture. The final flexible device exhibits a 4.5-MHz center frequency with a −3-dB fractional bandwidth of 82%. [2015-0317] [ABSTRACT FROM PUBLISHER]

Row-column addressed arrays for ultrasonic non-destructive testing (NDT) applications are analyzed and demonstrated in this paper. Simulation and experimental results of a row-column addressed 32 by 32 capacitive micromachined ultrasonic transducer (CMUT) array are presented. The CMUT array, which was designed for medical imaging applications, has a center frequency of 5.3 MHz. The CMUT array was used to perform C-scans on test objects with holes that have diameters of 1.0 mm and 0.5 mm. The array transducer has an aperture size of 4.8 mm by 4.8 mm, and it was used to scan an area of 4.0 mm by 4.0 mm. Compared to an N by N fully addressed 2-D array, a row-column addressed array of the same number of elements requires fewer (N instead of N²) pairs of interconnection and supporting electronic components such as pulsers and amplifiers. Even though the resulting field of view is limit by the aperture size, row-column addressed arrays and the row-column addressing scheme can be an alternative option of 2-D arrays for NDT applications. [ABSTRACT FROM AUTHOR]

Capacitive micromachined ultrasonic transducers (CMUTs) are an alternative to the conventional method of generating ultrasound that increases bandwidths, simplifies fabrication, and facilitates the integration with necessary electronics. We report the fabrication, characterization, and initial-phased array imaging results of a 64-element array CMUT fabricated using a fusion bonding process where both the membrane and insulation layers are user deposited silicon nitride. Individual cells have a diameter of 25 μm and a membrane thickness of 500 nm. The center frequency in immersion is 6.6 MHz with a −6 dB fractional bandwidth of 123%. A 90° phased array sector scan is made of a four-wire target using a 32-element subset of the array. Pressures in excess of 2 MPa are measured. An axial resolution of 130 μm and a lateral resolution of 0.03 rad are obtained from a wire target 15 mm away from the transducer. [ABSTRACT FROM PUBLISHER]

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 94.5 kHz at 170-V DC bias. The transmit sensitivity was measured to be 33.83 Pa/V and 25.85 Pa/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 7.7 mV/Pa for a 20-cycle burst, and 15.0 mV/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. [ABSTRACT FROM AUTHOR]

Highlights • We optimized the adhesive wafer bonding process for the fabrication of CMUTs. • A void-free and deformation-free adhesive wafer bonding process was demonstrated. • CMUTs with in-air resonance frequency of 6.44 MHz was fabricated and characterized. • An inverse CMUT structure is demonstrated enabling arbitrary cavity height control. Abstract It has been demonstrated that the adhesive wafer bonding technique benefits the fabrication of capacitive micromachined ultrasonic transducers (CMUTs) in the aspects of structural flexibility, CMOS process compatibility, and fabrication yield. Recently, the feasibility of fabricating CMUTs using benzocyclobutene (BCB) based polymer as both the adhesive and structural material was reported. However, the issues of voids at the wafer bonding interface, material deformation and electrode distance control need to be addressed. In this paper, we present an improved fabrication process that allows a void-free device with reduced polymer deformation. The chemical reactions during the BCB curing process were revealed using Fourier transform infrared spectroscopy. Microscopy and cross-sectional SEM images were employed to illustrate the structure improvement in terms of voids and polymer deformation. In addition, an inverse structure that allows an accurate control of the effective electrode distance was presented. The cross-sectional SEM images and the impedance measurement showing the dynamic response of the CMUT demonstrated the feasibility of this inverse structure. The differences between the inverse structure and the conventional structure were also discussed and compared. [ABSTRACT FROM AUTHOR]