Neal A. Hall

We characterize a 20-μm-thick bimorph X-cut lithium niobate (LN) piezoelectric micromachined ultrasonic transducer from room temperature to 1000 °C using an open-air measurement platform built around a fully packaged device—gold wire bonds on a ceramic printed circuit board. The platform sustains continuous operation at 800 °C, confirmed by frequency-response measurements at the start and end of a 1-h dwell. A sodium-silicate bondline that fills coefficient-of-thermal-expansion-driven substrate cracks as they form is central to sustained operation above 500 °C. The flexural resonance shifts downward with increasing temperature up to the onset of substrate cracking, and a coherent simple-harmonic-oscillator/modified Butterworth–Van Dyke fit extracts a temperature-dependent effective stiffness for LN up to ∼400 °C; above the 500–600 °C cracking onset, the resonance evolves non-monotonically as crack formation and bondline stabilization successively modify the modal boundary conditions. The measurement terminates near 1000 °C as the gold electrodes dewet and diffuse approaching their melting point (∼1064 °C), rather than at any LN material limit, indicating that packaging and metallization—not the piezoelectric film—set the practical ceiling for these devices. The platform is reusable for high-operating-temperature microelectromechanical systems beyond this device.

The deployment of active electronics in extreme environments—such as hypersonic combustion zones and deep supercritical geothermal wells—requires semiconductor devices capable of reliable operation at temperatures exceeding 500 °C. While III-Nitride (III-N) high electron mobility transistors (HEMTs) are theoretically suited for this regime, experimental validation in oxidizing atmospheres remains scarce, with most studies limited to vacuum or inert-gas environments. In this work, we report a new method for the electrical characterization of HEMTs up to 800 °C in an open-air environment. To overcome the challenges of thermal drift and measurement latency at these extremes, we introduce a rapid dual-frequency simultaneous extraction technique, which captures both transfer and output characteristics in a single sweep by modulating the gate and drain voltages at distinct frequencies. Our results demonstrate that the HEMT retains functional gate control, distinct saturation regions, and an on/off ratio exceeding 103 up to 700 °C, significantly outperforming commercial silicon depletion-mode FETs, which exhibited intrinsic carrier failure above 190 °C. Although permanent degradation was observed at 800 °C due to contact metallization failure, the demonstrated survivability up to 750 °C in air provides a critical existence proof for uncooled, non-hermetic electronics, directly supporting the objectives of the DARPA high operational temperature sensors program.

Piezoelectric micromachined ultrasonic transducers (PMUTs) are widely utilized in applications that demand mechanical resilience, thermal stability, and compact form factors. Recent efforts have sought to demonstrate that single-crystal lithium niobate (LN) is a promising PMUT material platform, offering high electromechanical coupling (k²) and bidi-rectional performance. In addition, advances in LN film transfer technology have enabled high quality periodically poled piezoelectric films (P3F), facilitating a bimorph piezoelectric stack without intermediate electrodes. In this work, we showcase a bimorph PMUT incorporating a mechanically robust, 20 μm thick P3F LN active layer. We establish the motivation for LN PMUTs through a material comparison, followed by extensive membrane geometry optimization and subsequent enhancement of the PMUT’s k². We demonstrate a 775 kHz flexural mode device with a quality factor (Q) of 200 and an extracted k² of 6.4%, yielding a high transmit efficiency of 65 nm/V with a mechanically robust active layer. We leverage the high performance to demonstrate extreme-temperature resilience, showcasing stable device operation up to 600 °C and survival up to 900 °C, highlighting LN’s potential as a resilient PMUT platform.

This study presents the design, modeling, and experimental characterization of a magnetostrictive hydrophone capable of operation up to 5 kHz, below its fundamental mechanical resonance. The transducer incorporates a Terfenol-D core within a compliant, magnetically permeable steel housing that provides both mechanical prestress and tunable compliance through Belleville washer assemblies. To enable calibration without reliance on large open-water facilities, a tank-based measurement method is developed using burst-tone excitation and post-excitation ringdown analysis, which effectively eliminates electromagnetic interference. The measured sensitivity exhibits the expected +6 dB/octave increase with frequency below resonance and agrees closely with finite-element simulations and a reduced-order network model. The minimum detectable pressure, estimated from the modeled self-noise and sensitivity, remains below the Sea State 0 noise floor and is projected to outperform a commercial reference hydrophone across the sub-resonant band. These results confirm the viability of magnetostrictive transduction for low-noise underwater acoustic sensing. To the best of our knowledge, this work reports the first realization and validation of a hydrophone based on giant magnetostrictive materials, extending their application beyond power projection to precision underwater sound detection.

This work presents a novel MEMS microphone inspired by the mechanically coupled tympana of the parasitoid fly Ormia ochracea, capable of simultaneously measuring omnidirectional pressure and two orthogonal in-plane pressure gradients. The device features a circular lead zirconate titanate (PZT)-on-silicon diaphragm supported by circumferential springs, with integrated piezoelectric sensing elements operating in the 3–1 mode. Using a laser Doppler vibrometer (LDV), we identified and imaged three distinct vibration modes at 1490, 2055, and 4255 Hz, each selectively responsive to either omnidirectional pressure or pressure gradients along orthogonal axes. The microphone was integrated with commercial low-noise integrated circuit amplifiers (ICAs) and packaged within a custom Faraday cage for electromagnetic shielding. Acoustic characterization in an anechoic chamber demonstrated the ability to decouple and independently measure the three pressure components, with sensitivity and directivity modeled and validated against experimental results. To our knowledge, this is the first MEMS microphone to achieve simultaneous measurement of pressure and its gradients along two orthogonal axes, enabling advanced source localization and improved performance in noisy environments.

Integrated photonics has emerged as a popular platform for performing a wide range of measurements with high sensitivity. Here an optical accelerometer in silicon nitride photonics platform was designed, and the fabricated device’s performance was measured. Light is first split into a reference and sensing arm. The sensing arm is a waveguide loop on top of a cantilever beam. When the cantilever is subjected to acceleration, a strain field is generated within the sensing arm that leads to a proportional optical phase change. Light passing through a reference arm is frequency shifted using an acousto-optic modulator. Light from the sensing arm, which undergoes phase modulation due to cantilever vibrations, is combined with the frequency-shifted light from the reference arm. After passing the combined beams through a photodiode, demodulation of the voltage waveform into the in phase and quadrature signals enables the phase from the sensing arm to be measured over many optical fringes. This approach extends the intrinsic dynamic range (IDR) of the optical interferometric readout beyond the limits of conventional interferometric sensors with an improved dynamic range of more than 40dB compared with conventional method with no active feedback. A calculated MDS of 190ng/√Hz was achieved while a further improvement was discussed. The prototype devices are fabricated using low loss silicon nitride photonics platform, all on an SOI wafer with Si device layer. The process enables two post-optical-waveguide fabrication processes; front side and backside deep silicon etch (DSE). The buried oxide of the SOI serves as a controlled etch stop for both top and backside silicon etch processes. We use this process to form thin and flexible cantilevers comprising only 50 μm thick silicon.

The remarkable directional hearing capabilities of the fly Ormia ochracea have inspired advancements in acoustic sensing technologies. Despite the small size of the fly’s ear relative to the wavelength of audible sound, it can precisely locate crickets by detecting their chirps. This article presents the design, fabrication, testing, andmodeling of a biologically inspired MEMS microphone that leverages the fly’s unique hearing mechanism. The device consists of a two-sided cantilever beam that rotates about torsional pivots, producing two primary vibration modes: (1) a rocking mode, which responds to pressure gradients with out-of-phase motion for sound localization, and (2) a bending mode, which reacts to omnidirectional sound pressure with in-phase motion. The microphone is fabricated using a silicon-on-insulator wafer with a 2-μm-thick device layer and incorporates a 500-nm-thick aluminum nitride piezoelectric film for transduction. Multipoint scans validate the sensing modes, which are accurately modeled using finite element analysis. Signal readout is achieved through an integrated circuit amplifier with high bias resistance. Acoustic measurements in air are performed across audible and ultrasonic frequencies, with sensitivity results compared to simulations. The initial prototypes demonstrate optimal performance at ultrasonic frequencies near 20 kHz. Simulations further explore potential designs for broadband applications spanning the audio frequency range of 20 Hz to 20 kHz.