A Wearable Ultrasonic Neurostimulator-Part II: A 2D CMUT Phased Array System With a Flip-Chip Bonded ASIC.

A 2D ultrasonic array is the ultimate form of a focused ultrasonic system, which enables electronically focusing beams in a 3D space. A 2D array is also a versatile tool for various applications such as 3D imaging, high-intensity focused ultrasound, particle manipulation, and pattern generation. However, building a 2D system involves complicated technologies: fabricating a 2D transducer array, developing a pitch-matched ASIC, and interconnecting the transducer and the ASIC. Previously, we successfully demonstrated 2D capacitive micromachined ultrasonic transducer (CMUT) arrays using various fabrication technologies. In this paper, we present a 2D ultrasonic transmit phased array based on a 32 × 32 CMUT array flip-chip bonded to a pitch-matched pulser ASIC for ultrasonic neuromodulation. The ASIC consists of 32 × 32 unipolar high-voltage (HV) pulsers, each of which occupies an area of 250 µm × 250 µm. The phase of each pulser output is individually programmable with a resolution of 1/fC/16, where fC is less than 10 MHz. This enables the fine granular control of a focus. The ASIC was fabricated in the TSMC 0.18- µm HV BCD process within an area of 9.8 mm × 9.8 mm, followed by a wafer-level solder bumping process. After flip-chip bonding an ASIC and a CMUT array, we identified shorted elements in the CMUT array using the built-in test function in the ASIC, which took approximately 9 minutes to scan the entire 32 × 32 array. A compact-form-factor wireless neural stimulator system-only requiring a connected 15-V DC power supply-was also developed, integrating a power management unit, a clock generator, and a Bluetooth Low-Energy enabled microcontroller. The focusing and steering capability of the system in a 3D space is demonstrated, while achieving a spatial-peak pulse-average intensity ( ISPPA) of 12.4 and 33.1 W/ cm2; and a 3-dB focal volume of 0.2 and 0.05 mm3-at a depth of 5 mm-at 2 and 3.4 MHz, respectively. We also characterized transmission of ultrasound through a mouse skull and compensated the phase distortion due to the skull by using the programmable phase-delay function in the ASIC, achieving 10% improvement in pressure and a tighter focus. Finally, we demonstrated a ultrasonic arbitrary pattern generation on a 5 mm × 5 mm plane at a depth of 5 mm.

Design and Fabrication of Wideband Air-Coupled Capacitive Micromachined Ultrasonic Transducers With Varying Width Annular-Ring and Spiral Cell Structures.

Air-coupled transducers with broad bandwidth are desired for many airborne applications, such as obstacle detection, haptic feedback, and flow metering. In this article, we present a design strategy and demonstrate a fabrication process for developing improved concentric annular- and novel spiral-shaped capacitive micromachined ultrasonic transducers (CMUTs) that can generate high output pressure and provide wide bandwidth in air. We explore the ability to implement complex geometries by photolithographic definition to improve the bandwidth of air-coupled CMUTs. The ring widths in the annular design were varied so that the device can be improved in terms of bandwidth when these rings resonate in parallel. Using the same ring width parameters for the spiral-shaped design but with a smoother transition between the ring widths along the spiral, the bandwidth of the spiral-shaped device is improved. With the reduced process complexity associated with the anodic-bonding-based fabrication process, a 25- [Formula: see text] vibrating silicon plate was bonded to a borosilicate glass wafer with up to 15- [Formula: see text] deep cavities. The fabricated devices show an atmospheric deflection profile that is in agreement with the FEM results to verify the vacuum sealing of the devices. The devices show a 3-dB fractional bandwidth (FBW) of 12% and 15% for spiral- and annular-shaped CMUTs, respectively. We measured a 127-dB sound pressure level at the surface of the transducers. The angular response of the fabricated CMUTs was also characterized. The results demonstrated in this article show the possibility of improving the bandwidth of air-coupled devices by exploring the flexibility in the design process associated with CMUT technology.

An Improved CMUT Structure Enabling Release and Collapse of the Plate in the Same Tx/Rx Cycle for Dual-Frequency Acoustic Angiography.

This study demonstrates, in detail, the potential of using capacitive micromachined ultrasonic transducers (CMUTs) for acoustic angiography of the microvasculature. It is known that when ultrasound contrast agents (microbubbles) are excited with moderate acoustic pressure around their resonance (2-4 MHz), they produce higher order harmonics (greater than third harmonic) due to their nonlinear behavior. To date, the fundamental challenge has been the availability of a transducer that can generate the transmit signals to excite the microbubbles at low frequencies and, in the same cycle, confocally detect harmonics in the higher frequencies. We present a novel device structure and dual-mode operation of a CMUT that operates with a center frequency of 4.3 MHz and 150% bandwidth in the conventional mode for transmitting and a center frequency of 9.8 MHz and a 125.5% bandwidth in collapse mode for receiving. Output pressure of 1.7 MPapp is achieved on the surface of a single unfocused transducer. The mechanical index at the transducer surface is 0.56. FEM simulations are performed first to show the functionality of the proposed device, and then, the device fabrication is described in detail. Finally, we experimentally demonstrate the ability to detect the microbubble signals with good contrast, and the background reflection is adequately suppressed, indicating the feasibility of the presented approach for acoustic angiography.

A Fast-Switching (1.35-µs) Low-Control-Voltage (2.5-V) MEMS T/R Switch Monolithically Integrated With a Capacitive Micromachined Ultrasonic Transducer (CMUT).

This paper describes the design and fabrication of an electrostatic MEMS switch that can be co-fabricated on the same substrate with a capacitive micromachined ultrasonic transducer (CMUT) as a transmit/receive (T/R) switch. The structure of the switch is modified from a single CMUT cell. An interrupted transmission line is defined across the center of the cell with control electrodes on both sides to pull a movable plate down. The plate has an insulation layer underneath and a metal bump is formed on the insulation layer and aligned to the transmission line gap, so that the switch could be turned on by pulling down the plate with electrostatic force and making the metal bump close the gap in the transmission line. The switch was designed using a finite-element model (FEM) and fabricated on a glass substrate using anodic bonding. A static characterization was first performed on a switch test structure, which shows the dc switching voltage was 68 V and the on-resistance was 50 Ω. The RF in and RF out isolation was measured as approximately 66 dB and insertion loss was approximately 4.5 dB for frequency range commonly used for medical ultrasound imaging. Then we performed the dynamic characterization in immersion. By setting the dc bias at 67 V, we found that the switch could be operated with a control-voltage as low as 2.5 V. The switching and release times are related to the rise time and the fall time of the control signal, respectively. The minimum switching time was measured as 1.34 µs with a control signal rise time of 300 ns, and the minimum release time was measured as 80 ns with a control signal fall time of 20 ns. We further demonstrated that a 1-kHz control signal with the optimized rise and fall times can be used to conduct and block a sinusoidal signal with 1-MHz frequency and 300-mV pp amplitude, as well as unipolar pulses with 5-V pp amplitude, 500-ns pulse width, and 2-kHz repetition rate. The presented MEMS switch could potentially eliminate the high-voltage process requirement for the on-chip front-end electronics of a CMUT-based ultrasound imaging system and thus improve the overall system efficiency.

Backward-Mode Photoacoustic Imaging Using Illumination Through a CMUT With Improved Transparency.

In this paper, we describe a capacitive micromachined ultrasonic transducer (CMUT) with improved transparency for photoacoustic imaging (PAI) with backside illumination. The CMUT was fabricated on a glass substrate with indium-tin oxide bottom electrodes. The plate was a 1.5- silicon layer formed over the glass cavities by anodic bonding, with a 1- silicon nitride passivation layer on top. The fabricated device shows approximately 30%-40% transmission in the wavelength range from 700 to 800 nm and approximately 40%-60% transmission in the wavelength range from 800 to 900 nm, which correspond to the wavelength range commonly used for in vivo PAI. The center frequency of the CMUT was 3.62 MHz in air and 1.4 MHz in immersion. Two preliminary PAI experiments were performed to demonstrate the imaging capability of the fabricated device. The first imaging target was a 0.7-mm diameter pencil lead in vegetable oil as a line target with a subwavelength cross section. A 2-mm-diameter single CMUT element with an optical fiber bundle attached to its backside was linearly scanned to reconstruct a 2-D cross-sectional PA image of the pencil lead. We investigated the spurious signals caused by the light absorption in the 1.5- silicon plate. For pencil lead as a strong absorber and also a strong reflector, the received echo signal due to the acoustic excitation generated by the absorption in silicon is approximately 30 dB lower than the received PA signal generated by the absorption in pencil lead at the wavelength of 830 nm. The second imaging target was a "loop-shape" polyethylene tube filled with indocyanine green solution ( ) suspended using fishing lines in a tissue-mimicking material. We formed a 3-D volumetric image of the phantom by scanning the transducer in the - and -directions. The two experimental imaging results demonstrated that CMUTs with the proposed structure are promising for PAI with backside illumination.