Albert I.H. Chen

A 32 x 32 element row-column addressed capacitive micromachined ultrasonic transducer

This paper presents characterization and initial imaging results of a 32 × 32 element two-dimensional capacitive micromachined ultrasonic transducer array. The devices are fabricated using a wafer bonding process in which both the insulation layer and the membrane are user-deposited silicon nitride. The transducers use a row-column addressing scheme to simplify the fabrication process and beamformer. By adjusting the number of rows and columns that are biased, the effective aperture of the transducer can be adjusted. This is significant because it permits imaging in the near-field of the transducer without the use of a lens. The effect on the transmit beam profile is demonstrated. The transducer has a center frequency of 5.9 MHz and a relative bandwidth of 110%. Images of horizontal and vertical wires are taken to demonstrate image resolution. A three-dimensional image of four pin heads is also demonstrated.

A CMUT-based real-time volumetric ultrasound imaging system with row-column addressing

Creating real-time three dimensional imaging with two dimensional transducers arrays is difficult to achieve due to the large number of interconnections and processing data. One method to greatly reduce the interconnections and also processing time is by introducing the row-column addressing scheme where the top and bottom electrodes are connected by rows on one side and columns on the other. As a result, only 2×N number of connection needs to be made instead of N×N. Another important advantage of using row-column method is that the data acquisition rate for a three dimensional image can be acquired at the same rate as a two-dimensional image using phased-array method. This paper presents an imaging system that uses a row-column addressing-based capacitive micromachined ultrasound transducers array to perform real-time ultrasound imaging.

An FPGA-based ultrasound imaging system using capacitive micromachined ultrasonic transducers

We report the design and experimental results of a field-programmable gate array (FPGA)-based real-time ultrasound imaging system that uses a 16-element phased-array capacitive micromachined ultrasonic transducer fabricated using a fusion bonding process. The imaging system consists of the transducer, discrete analog components situated on a custom-made circuit board, the FPGA, and a monitor. The FPGA program consists of five functional blocks: a main counter, transmit and receive beamformer, receive signal pre-processing, envelope detection, and display. No dedicated digital signal processor or personal computer is required for the imaging system. An experiment is carried out to obtain the sector B-scan of a 4-wire target. The ultrasound imaging system demonstrates the possibility of an integrated system-in-a-package solution.

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.

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.

Compensated Row-Column Ultrasound Imaging System Using Fisher Tippett Multilayered Conditional Random Field Model

3-D ultrasound imaging offers unique opportunities in the field of non destructive testing that cannot be easily found in A-mode and B-mode images. To acquire a 3-D ultrasound image without a mechanically moving transducer, a 2-D array can be used. The row column technique is preferred over a fully addressed 2-D array as it requires a significantly lower number of interconnections. Recent advances in 3-D row-column ultrasound imaging systems were largely focused on sensor design. However, these imaging systems face three intrinsic challenges that cannot be addressed by improving sensor design alone: speckle noise, sparsity of data in the imaged volume, and the spatially dependent point spread function of the imaging system. In this paper, we propose a compensated row-column ultrasound image reconstruction system using Fisher-Tippett multilayered conditional random field model. Tests carried out on both simulated and real row-column ultrasound images show the effectiveness of our proposed system as opposed to other published systems. Visual assessment of the results show our proposed system’s potential at preserving detail and reducing speckle. Quantitative analysis shows that our proposed system outperforms previously published systems when evaluated with metrics such as Peak Signal to Noise Ratio, Coefficient of Correlation, and Effective Number of Looks. These results show the potential of our proposed system as an effective tool for enhancing 3-D row-column imaging.

A 2D MEMS mirror with sidewall electrodes applied for confocal MACROscope imaging

This paper presents microelectromechanical system micromirrors with sidewall electrodes applied for use as a Confocal MACROscope for biomedical imaging. The MACROscope is a fluorescence and brightfield confocal laser scanning microscope with a very large field of view. In this paper, a microelectromechanical system mirror with sidewall electrodes replaces the galvo‐scanner and XYZ‐stage to improve the confocal MACROscope design and obtain an image. Two micromirror‐based optical configurations are developed and tested to optimize the optical design through scanning angle, field of view and numerical aperture improvement. Meanwhile, the scanning frequency and control waveform of the micromirror are tested. Analysing the scan frequency and waveform becomes a key factor to optimize the micromirror‐based confocal MACROscope. When the micromirror is integrated into the MACROscope and works at 40 Hz, the micromirror with open‐loop control possesses good repeatability, so that the synchronization among the scanner, XYZ‐stage and image acquisition can be realized. A laser scanning microscope system based on the micromirror with 2 μm width torsion bars was built and a 2D image was obtained as well. This work forms the experimental basis for building a practical confocal MACROscope.

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.