Michael L. Kuntzman

Sound source localization inspired by the ears of the Ormia ochracea

The parasitoid fly Ormia ochracea has the remarkable ability to locate crickets using audible sound. This ability is, in fact, remarkable as the flys hearing mechanism spans only 1.5 mm which is 50× 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 evolution has empowered the fly with a hearing mechanism that utilizes multiple vibration modes to amplify interaural time and level differences. Here, we present a fully integrated, man-made mimic of the Ormias 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.

Micromachined piezoelectric microphones with in-plane directivity

Micromachined piezoelectric microphones with in-plane directivity are introduced. A beam rotates about center torsional pivots and is attached to piezoelectrically active end-springs. Rotation of the beam in response to sound pressure gradients produces spring deflections, which, in turn, produce an open-circuit voltage at the piezoelectric films. Prototypes are presented that contain a 20-μm-thick silicon beam and end-springs with 900-nm-thick chemical solution deposited lead zirconate titanate atop the surface of the end-springs. Acoustic directivity measurements are presented that confirm device functionality.

Micromachined In-Plane Pressure-Gradient Piezoelectric Microphones

This paper presents the fabrication, modeling, and characterization of a micromachined piezoelectric directional microphone. The microphone structure consists of a 20-μm-thick semirigid beam structure that rotates about torsional pivots in response to in-plane pressure gradients across the length of the beam. The motion of the beam structure is transduced by piezoelectric cantilevers, which deflect when the structure rotates. While the structure has been introduced in prior publications, this is the first paper summarizing rigorous acoustic characterization of first generation prototypes. An analytical model and multimode, multiport network model utilizing finite-element analysis for parameter extraction are presented and compared with acoustic sensitivity measurements. Directivity measurements are interpreted in terms of the multimode model. A noise model for the sensor and readout electronics is presented and compared with measurements.

Performance and Modeling of a Fully Packaged Micromachined Optical Microphone

A microelectromechanical systems (MEMS) optical microphone that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm is described ( JASA, v. 122, no. 4, 2007). In the present embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. Advantages of this optical detection technique have been demonstrated with both omnidirectional microphones and biologically inspired directional microphones. In efforts to commercialize this technology for hearing aids and other applications, a goal has been set to achieve a microphone contained in a small surface-mount package (occupying 2 × 2 mm × 1 mm volume), with ultralow noise (20 dBA) and a broad frequency response (20 Hz-20 kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today but would have noise performance characteristics of professional-audio microphones significantly larger in size and more expensive to produce. This paper will present several unique challenges in our effort to develop the first surface-mount packaged optical MEMS microphone. The package must accommodate both optical and acoustical design considerations. Dynamic models used for simulating frequency response and noise spectra of fully packaged microphones are presented and compared with measurements performed on prototypes.

Microfabrication and Experimental Evaluation of a Rotational Capacitive Micromachined Ultrasonic Transducer

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.

Piezoelectric micromachined microphones with out-of-plane directivity

Piezoelectric microphones with out-of-plane directivity are introduced. Structures are comprised of circular diaphragms suspended on compliant circumferential springs and open to ambient at front and back sides. The springs contain thin piezoelectric films for integrated piezoelectric readout. Prototypes are presented in which diaphragm and springs are etched into a 10-μm-thick epitaxial Si layer with 800-nm-thick lead-zirconate-titanate films on the spring surface. Directivity and frequency response measurement confirm anticipated device functionality. A discussion of signal-to-noise ratio (SNR) merits of the approach is presented, concluding that up to 20-dB SNR improvements may be possible beyond what is achievable with present state-of-the-art commercial microphones.

Rotational Capacitive Micromachined Ultrasonic Transducers (cMUTs)

A rotational capacitive micromachined ultrasonic transducer is introduced. Two pressure-sensitive diaphragms atop vacuum-sealed cavities are mechanically coupled via a rocking beam structure. The rocking structure is designed to reduce deflection of the vacuum-sealed diaphragms under atmospheric pressure while introducing ideally zero rotational stiffness to differential diaphragm pressure. The rocking structure responds to in-plane pressure gradients and is therefore anticipated to have dipole-type directivity to ultrasound. Prototypes employing 200 μm diameter polysilicon diaphragms are presented with a fundamental resonance frequency of 250 kHz.

Micromachined Piezoelectric Accelerometers via Epitaxial Silicon Cantilevers and Bulk Silicon Proof Masses

Deep reactive ion etch (DRIE) processes are performed on both sides of a silicon-on-insulator (SOI) wafer to realize piezoelectric accelerometers consisting of 20- μm-thick epitaxial silicon cantilevers with bulk silicon masses at the tip. Sol-gel deposition of lead-zirconate-titanate (PZT) is used to realize an 800-nm-thick film on the top surface of the beams. Prior to accelerometer characterization, a system identification procedure based on laser Doppler vibrometry is performed, providing a complete description of the devices and predicted sensitivities. Frequency response measurements confirm device sensitivities in the 3.4-50 pC/g range in the flat-band (depending on device geometry), and resonance frequencies in the 60-Hz-1.5-kHz range. The self-noise for the longest beam of 8.5 mm is measured as 1.7 μg/√{Hz} at 30 Hz and is limited by dielectric loss of the PZT film which is estimated to have a tanδ of 0.02. Scaling relationships for this particular device geometry are presented to provide insight into possible future design directions.

Electrical Admittance Spectroscopy for Piezoelectric MEMS

A simple measurement architecture based on a transimpedance amplifier is demonstrated for the electrical admittance spectroscopy of small-scale transducers. The simplicity and low cost of the measurement system as compared to dedicated network analyzers may make spectroscopy measurements more widely accessible. The approach is adaptable to cover a broad frequency range and can be used for transducers with very small capacitance and high impedance. Measurements are demonstrated on piezoelectric micromachined transducers fabricated on silicon-on-insulator wafers and composed of 20-μm -thick epitaxial beams with 800-nm-thick lead zirconate titanate films along the surface. A complete system identification (ID) procedure based purely on measured phase spectra is also summarized. A unique perspective to the system ID procedure is provided based on poles and zeros of the admittance transfer function and geometry in the complex plane. A rigorous procedure for simultaneously extracting the transducer coupling ratio, undamped natural frequency, and damping ratio is summarized which is particularly useful for lightly coupled sensors, in which case fitting parameters by inspection is challenging and can lead to errors. This system identification approach is summarized on beams with coupling coefficients as small as 0.35%. The system ID procedure further consists of extracting effective e31 coefficients, which, for the sensors in this work, are in the range 8.7-9.3 C/m2.

A broadband, capacitive, surface-micromachined, omnidirectional microphone with more than 200 kHz bandwidth

A surface micromachined microphone is presented with 230 kHz bandwidth. The structure uses a 2.25 μm thick, 315 μm radius polysilicon diaphragm suspended above an 11 μm gap to form a variable parallel-plate capacitance. The back cavity of the microphone consists of the 11 μm thick air volume immediately behind the moving diaphragm and also an extended lateral cavity with a radius of 504 μm. The dynamic frequency response of the sensor in response to electrostatic signals is presented using laser Doppler vibrometry and indicates a system compliance of 0.4 nm/Pa in the flat-band of the response. The sensor is configured for acoustic signal detection using a charge amplifier, and signal-to-noise ratio measurements and simulations are presented. A resolution of 0.80 mPa/√Hz (32 dB sound pressure level in a 1 Hz bin) is achieved in the flat-band portion of the response extending from 10 kHz to 230 kHz. The proposed sensor design is motivated by defense and intelligence gathering applications that require broadband, airborne signal detection.