Zhenhao Li

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.

Fabrication of a Curved Row–Column Addressed Capacitive Micromachined Ultrasonic Transducer Array

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 μm thick with a 12.8-mm × 12.8-mm aperture. The final flexible device exhibits a 4.5-MHz center frequency with a -3-dB fractional bandwidth of 82%.

A row-column addressed micromachined ultrasonic transducer array for surface scanning applications.

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 5.3MHz. The CMUT array was used to perform C-scans on test objects with holes that have diameters of 1.0mm and 0.5mm. The array transducer has an aperture size of 4.8mm by 4.8mm, and it was used to scan an area of 4.0mm by 4.0mm. 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 N(2)) 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.

PMMA/MWCNT nanocomposite for proton radiation shielding applications

Radiation shielding in space missions is critical in order to protect astronauts, spacecraft and payloads from radiation damage. Low atomic-number materials are efficient in shielding particle-radiation, but they have relatively weak material properties compared to alloys that are widely used in space applications as structural materials. However, the issues related to weight and the secondary radiation generation make alloys not suitable for space radiation shielding. Polymers, on the other hand, can be filled with different filler materials for reinforcement of material properties, while at the same time provide sufficient radiation shielding function with lower weight and less secondary radiation generation. In this study, poly(methyl-methacrylate)/multi-walled carbon nanotube (PMMA/MWCNT) nanocomposite was fabricated. The role of MWCNTs embedded in PMMA matrix, in terms of radiation shielding effectiveness, was experimentally evaluated by comparing the proton transmission properties and secondary neutron generation of the PMMA/MWCNT nanocomposite with pure PMMA and aluminum. The results showed that the addition of MWCNTs in PMMA matrix can further reduce the secondary neutron generation of the pure polymer, while no obvious change was found in the proton transmission property. On the other hand, both the pure PMMA and the nanocomposite were 18%-19% lighter in weight than aluminum for stopping the protons with the same energy and generated up to 5% fewer secondary neutrons. Furthermore, the use of MWCNTs showed enhanced thermal stability over the pure polymer, and thus the overall reinforcement effects make MWCNT an effective filler material for applications in the space industry.

Fabrication of capacitive micromachined ultrasonic transducers based on adhesive wafer bonding technique

This paper reports the fabrication process of wafer bonded capacitive micromachined ultrasonic transducers (CMUTs) using photosensitive benzocyclobutene as a polymer adhesive. Compared with direct bonding and anodic bonding, polymer adhesive bonding provides good tolerance to wafer surface defects and contamination. In addition, the low process temperature of 250 °C is compatible with standard CMOS processes. Single-element CMUTs consisting of cells with a diameter of 46 µm and a cavity depth of 323 nm were fabricated. In-air and immersion acoustic characterizations were performed on the fabricated CMUTs, demonstrating their capability for transmitting and receiving ultrasound signals. An in-air resonance frequency of 5.47 MHz was measured by a vibrometer under a bias voltage of 300 V.

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

HighlightsWe presented a lumped element model of air‐coupled annular‐cell CMUTs.Explicit resonance frequency and static displacement of a clamped annular plate were developed.The proposed lumped model was verified through both simulation and experimental studies.Detailed analysis of the static, frequency, and transient response was given.The proposed methodology can also be employed to study CMUTs of other cell geometries. Abstract 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.

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]

A CMUT array based on annular cell geometry for air-coupled applications

An annular cell has been reported with improved transmit sensitivity over a conventional circular cell for air-coupled CMUTs. The particular geometry of an annular cell also allows multi of them to be arranged in a concentric layout such that high-intensity ultrasound can be focused in the depth direction. This paper presents a 9.3-mm-diameter CMUT array consisting of four concentric annular cells. The wafer-bonded CMUT has a plate thickness of 18.3 μm and cell width of 930 μm. Under atmospheric pressure, the ratio of average-to-maximum displacement (RAMD) of the annular cells was measured to be 0.53 using a profilometer. At 200-V DC bias and 20-Vpp AC actuation voltage, the frequency response of the maximum displacement was measured using a laser Doppler vibrometer, and a resonance frequency of 182.5 kHz was observed. At the same DC bias, the resonance frequency of the array was also measured to be 182.5 kHz using a vector network analyzer. Based on the frequency response, the acoustic filed of the CMUT array was simulated to have an 11.4-mm natural focal depth with -3-dB beam width of 3.14 mm. This study demonstrates the potential of using concentric annular-cell CMUT array to generate focused ultrasound in air.

Fabrication of polymer-based wafer-bonded capacitive micromachined ultrasonic transducers

The fabrication process and characterization results of a single element capacitive micromachined ultrasonic transducer (CMUT) are presented in this paper. The fabrication process is based on polymer adhesive wafer bonding of silicon and low pressure chemical vapor deposited silicon nitride wafers. The fabricated transducer has an average resonant frequency of 5.5 MHz in air. In immersion, the transmitted ultrasound signal from the transducer was measured to show a centre frequency of 4.9 MHz with a 3dB bandwidth of 114%. The capability of receiving ultrasound signal was also demonstrated using an off-the-shelf transducer. Initial characterization results suggest that the proposed technique can be an attractive alternative for CMUT fabrications.

An Optimization and Comparative Study of Air-Coupled CMUT Cells With Circular and Annular Geometries

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.