Lukas Baumgartel

Resonance-Enhanced Piezoelectric Microphone Array for Broadband or Prefiltered Acoustic Sensing

We report an array of piezoelectric monocrystalline silicon microphones for audio-range acoustic sensing. Thirteen cantilever-type diaphragm transducers make up the array, each having a closely spaced and precisely controlled resonant frequency. These overlapping resonances serve to greatly boost the sensitivity of the array when the signals are added; if the signals are individually taken, the array acts as a physical filter bank with a quality factor over 40. Such filtering would enhance the performance and the efficiency of speech-recognition systems. In the “summing mode,” the array demonstrates high response over a large bandwidth, with unamplified sensitivity greater than 2.5 mV/Pa from 240 to 6.5 kHz. Both modes of operation rely on the precise control of resonant frequencies, often a challenge with large compliant microelectromechanical-system (MEMS) structures, where residual stress causes deformation. We mitigate these ill effects through the use of stress-compensating layer thicknesses and a stress-free monocrystalline diaphragm. For determining device geometry, we develop a simple analytical method that yields excellent agreement between designed and measured resonant frequency; all devices are within 4.5%, and four are within 0.5% (just several hertz). The technique could be useful not only for microphones but also for other low-frequency MEMS transducers designed for resonance operation at a specific frequency.

Microelectromagnetic energy harvester with integrated magnets

This paper presents a microelectromagnetic power generator with integrated magnets that can be fabricated on silicon wafers in a batch process. The generator is fabricated by MEMS technologies and characterized at different vibration frequencies, amplitudes and load resistances. This compact generator occupies a volume of 15×13×0.4mm3, as no external permanent magnets are needed. Experimental results show that with a 20-turn spiral coil, the device can generate an induced electromotive force (EMF) of 0.27mV at a vibration frequency and amplitude of 350Hz and 10µm, respectively.

Edge-released, piezoelectric MEMS acoustic transducers in array configuration

A sensitive, broad-bandwidth piezoelectric microelectromechanical systems (MEMS) transducer based on frequency interleaving of resonant transducers was designed and fabricated. A sputter-deposited piezoelectric zinc oxide (ZnO) thin film on the diaphragm is used to sense and generate acoustic pressure. A high compliance cantilever and spiral-beam-supported diaphragms are designed and built on the edge-released MEMS structure to release initial residual stress and to avoid in-plane tension when bent. Stress compensation has been achieved by adjusting the thickness of each layer of the cantilever and by compensating for the ZnO films compressive stress with the bimorph structure of the spiral-beam. For a given pressure level and diaphragm size, the maximum strain on the spiral-beam-supported diaphragm is about an order of magnitude larger than that of a rectangular cantilever diaphragm. Also, the acoustic transducer built on the spiral-beam-supported diaphragm has a much higher sensitivity (but with less tolerance on the fabrication process variation and at the cost of lower usable bandwidth) than the one built on a rectangular cantilever diaphragm. By connecting many transducers in parallel, both the sensitivity and acoustic output were improved about 30 times. The interleaving of the transducers increased not only the sensitivity, but also broadened the useable bandwidth.

Experimental optimization of electrodes for high Q, high frequency HBAR

In this paper we present experimental work towards the fabrication of a high-overtone bulk acoustic resonator (HBAR) that demonstrates large values of quality factor (Q) at high frequency (≫7GHz). We have optimized electrode design and fabrication technique with respect to deposition conditions, material, active-area size, and thickness. We fabricated devices with gold, aluminum, and molybdenum electrodes. Once the molybdenum electrodes were determined to be superior, we fabricated devices with 11 different active-area sizes, ranging from 5,300 µm² to 23,600 µm². Finally, we made HBARs with electrode thickness ranging, in 0.02 µm increments, from 0.06 µm to 0.14 µm. The best device utilized 0.14 µm thick molybdenum electrodes, an active-area of 6,500 µm², and had Q = 6501 at 7.520 GHz, yielding fQ = 4.49 × 10¹³.

A resonant piezoelectric microphone array for detection of acoustic signatures in noisy environments

This paper reports a MEMS acoustic resonator array that improves Automatic Speech Recognition (ASR) and signature detection characteristics in environments with high levels of acoustic interference. The experiments with the acoustic resonator array show an increase of 62.7 percentage points in ASR transcription accuracy for signals buried under noise with -15 dB Signal-to-Noise Ratio (SNR). The results of this study support the development of resonant acoustic sensors for a variety of pattern recognition applications.

Use of compressively-stressed zinc oxide to increase microspeaker response

A micromachined piezoelectric speaker was fabricated on a 5 × 5‐mm2, 1‐μm‐thick silicon nitride diaphragm. A 4 × 4‐mm2 zinc oxide (ZnO) piezoelectric transducer sits in the middle of the diaphragm, providing actuation. Two variations were fabricated: one with the compressively stressed ZnO covering the region between the transducer and diaphragm perimeter—causing wrinkling—and another with the ZnO removed in this region. In both variations, the stress gradient causes curvature in the active area, raising the resonant frequency to above 4 kHz. The displacement response is therefore approximately flat from 40 Hz to 4 kHz. The speakers are driven with a sinusoidal voltage, and the response is measured with a laser interferometer. The wrinkled device exhibits 11 times larger response and can be actuated by much smaller voltage, achieving lower THD while still having a larger deflection. The wrinkled device is driven at 2 V0.t.p. from 40 Hz to 4 kHz, demonstrating a response of 55 nm/V0.t.p. and an average THD...

MEMS ultrasonic transducers for highly sensitive Doppler velocity sensor for low velocity measurement

This paper describes a novel, highly-sensitive ultrasonic Doppler velocity sensing system for low velocity measurement in portable navigation systems. It is a compact velocity sensing system, in which MEMS ultrasonic transducers are incorporated with phase-locked-loop (PLL) circuitry for frequency detection and signal processing. The achieved voltage-velocity sensitivity is 0.22 V/(mm/s) and the minimum detectable velocity is 0.67 mm/s, corresponding to 0.11 Hz in Doppler frequency. To the best of our knowledge, the minimum detectable velocity is the best reported in literature. Also, the output of the PLL is a DC voltage linearly related to the velocity, and there is no need to convert the frequency shift to analog voltage.

Whispering gallery mode resonators as optical reference cavities

Highly stabilized lasers are an increasingly valuable tool for metrology. For many applications, however, existing Fabry Pérot systems are too bulky and cumbersome. We are investigating the use of miniature monolithic whispering gallery mode resonators as reference cavities for laser stabilization. We seek to exploit the benefit of small size and vibration resistance by suppressing thermally induced frequency fluctuations. We have theoretically investigated the viability of using a thin-film coating to achieve temperature compensation. We have experimentally investigated an active temperature stabilization scheme based on birefringence in a crystalline resonator. We also report progress of laser locking to the resonators.