Donghwan Kim

This article presents the fabrication and characterization of on-diaphragm Pt thermistors for temperaturecompensated calibration of piezoelectric pressure sensors. The micromachined pressure-sensitive diaphragms are 700 µm in diameter and employ AlN as the piezoelectric material. The thermistors reside on top of the diaphragms and are patterned into a 100-nm-thick sputtered Pt electrode layer. Experimental characterization up to 600 °C demonstrates the importance of annealing to realize hysteresis-free resistance vs. temperature characteristics. Dynamic frequency response measurements of the pressure-sensitive diaphragms vs. temperature demonstrate a marked shift in compliance with temperature, and therefore demonstrates the importance of temperature-compensated pressure calibration in high-temperature measurement environments.

A microelectromechanical-system piezoelectric pressure sensor targeting hypersonic flow applications is presented. The sensing diaphragms have 500-μm diameter and are comprised of a composite stack of Si, SiO ₂ , and AlN. This paper presents the fabrication and packaging details of the sensor and then presents test results from a Mach-6 hypersonic-flow facility, in which the prototype sensor data are compared against data from a mature commercial sensor present in the same tests. Salient hypersonic-flow features are observed in signals captured by the prototype. The package is comprised of an on-chip two-element array with 650-μm pitch, and signals from the elements captured during hypersonic-flow tests are highly correlated.

Acoustic particle velocity sensors for underwater applications typically work by measuring the motion of a small, neutrally buoyant body entrained in the acoustic field. Motivated by this, we have investigated the use of balloons for acoustic particle velocity measurements in air by exploring the behavior of an elastic balloon vibrating in response to an incident acoustic wave. In this presentation, we will present the results of experiments performed in an anechoic chamber, in which a pair of laser Doppler vibrometers simultaneously captured the velocities of the front and back surfaces of a Mylar balloon in an acoustic field. From phase measurements, the motion is described in terms of contributions from odd-order vibration modes (including bulk translation) and even-order vibration modes. The measured entrainment factors for the balloon at low frequencies are seen to be in good agreement with a physical model based on the scattering from an entrained, rigid sphere. This demonstrates the feasibility of using balloons for direct measurement of acoustic particle velocity in air.

The parasitoid fly Ormia ochracea has the remarkable ability to locate crickets using audible sound. This ability is, in fact, remarkable as the fly’s hearing mechanism spans only 1.5 mm, which is 50 times smaller than the wavelength of sound emitted by the cricket. The hearing mechanism is, for all practical purposes, a point in space with no significant interaural time or level differences to draw from. It has been discovered that the fly's hearing organ utilizes multiple vibration modes to amplify interaural time and level differences (see Miles et al., J. Acoust. Soc. Am. 98 (6), December 1995). Here, we present a fully integrated mimic of the Ormia’s hearing mechanism capable of replicating the remarkable sound localization ability of the special fly. A silicon-micromachined prototype is presented which uses multiple piezoelectric sensing ports to simultaneously transduce two orthogonal vibration modes of the sensing structure, thereby enabling simultaneous measurement of sound pressure and pressure gradient.

Pressure-differential microphones inspired by the hearing mechanism of a special parasitoid fly have been described previously. The designs employ a beam structure that rotates about two pivots over an enclosed back volume. The back volume is only partially enclosed due to open slits around the perimeter of the beam. The open slits enable incoming sound waves to affect the pressure profile in the microphone's back volume. The goal of this work is to study the net moment applied to pressure-differential microphones by an incoming sound wave, which in-turn requires modeling the acoustic pressure distribution within the back volume. A lumped-element distributed transmission-line model of the back volume is introduced for this purpose. It is discovered that the net applied moment follows a low-pass filter behavior such that, at frequencies below a corner frequency depending on geometrical parameters of the design, the applied moment is unaffected by the open slits. This is in contrast to the high-pass filter behavior introduced by barometric pressure vents in conventional omnidirectional microphones. The model accurately predicts observed curvature in the frequency response of a prototype pressure-differential microphone 2 mm × 1 mm × 0.5 mm in size and employing piezoelectric readout.

The microfabrication, modeling, and experimental evaluation of an unconventional acoustic sensor are described. The sensor is comprised of two vacuum-sealed capacitively transduced pistons coupled with each other by a pivoting beam. The use of a pivoting beam can, in principle, enable high rotational compliance to in-plane small-signal acoustic pressure gradients, while resisting piston collapse against large background atmospheric pressure. A design path toward vacuum-sealed surface-micromachined broadband microphones is a motivation to explore the sensor concept. Fabrication of surface-micromachined prototypes is presented, followed by finite element modeling and experimental confirmation of successful vacuum sealing. Dynamic frequency response measurements are obtained using broadband electrostatic actuation and confirm a first fundamental rocking mode near 250 kHz. Successful reception of airborne ultrasound in air at 130 kHz is also demonstrated, and followed by a discussion of design paths toward improved signal-to-noise ratio beyond that of the initial prototypes presented.