Giant Pressure Output Efficiency of Capacitive Micromachined Ultrasonic Transducers Using Nano-Silicon-Springs

Background

Conventional ultrasound probes rely on piezoelectric ceramics, which require labor-intensive dicing of each channel, thick matching and backing layers, and lead-based materials for high-quality imaging. CMUTs are lead-free, compatible with CMOS electronics and need no matching or backing layers, which suits miniaturization and wearable devices. Their low output pressure and high bias voltage, however, lead to narrow imaging depth and high driving voltage and have hindered their use as commercial imaging probes.

Design and approach

The design places nano-silicon posts under the moving plate to act as springs, so that the plate moves in parallel as a piston rather than flexing, which increases the average displacement. The design draws on the large elastic strain of monocrystalline silicon at the nanoscale, with silicon nanopillars reported to reach elastic strain of up to 16%. The device was made by fusion bonding two silicon-on-insulator (SOI) wafers with a device-layer resistivity below 0.005 ohm-cm, which formed a 180 nm vacuum gap. Deep reactive ion etching defined the elements and the lateral dimensions of the springs, the handle layer of the top wafer was ground and polished, micro-bump structures and the top plate were patterned, and 500 nm aluminum contact pads were deposited.

How it was evaluated

The fabricated arrays were examined by scanning electron microscopy, including a focused ion beam cross-section of a spring. Electrical input impedance of one element was measured with an impedance analyzer under DC bias in air. Plate displacement was measured with a laser Doppler vibrometer scanned over 28 steps of 10 μm, and acoustic output was measured with a hydrophone, both with the device immersed in corn oil.

Key results

One element showed an average open-circuit resonant frequency of 6.7 MHz at a bias of 40 V in air. The maximum peak-to-peak displacement efficiency in corn oil was 5.37 nm/V under a 50 V DC bias with a continuous 1 VPp AC signal.

The hydrophone measured an acoustic pressure of 37.5 kPa at a distance of 3.5 mm, from which the maximum surface pressure was calculated as 1.1 MPa for 5-cycle pulses at 4.7 MHz and 5 VPP. The resulting pressure output efficiency of 220 kPa/V compares with 13.4 kPa/V for a commercial PZT probe, 6.30 kPa/V for a conventional CMUT, 21.5 kPa/V for a CMUT with embedded micro-silicon posts and 61.2 kPa/V for a CMUT with electrode posts; the authors state that it is 10 times and 3.6 times higher than the last two.

Open questions

The authors identify the implementation of a portable and wearable ultrasonic imaging device with this CMUT as future work.

How it connects to other research

Khuri-Yakub and Oralkan reviewed CMUT structures, fabrication and circuit integration for medical imaging and therapy; this paper changes how the plate is supported, using embedded nano-silicon springs to obtain piston-like plate motion.