Aluminum Nitride Ultrasonic Air-Coupled Actuator

A piezoelectric micromachined ultrasonic radiator was developed using aluminum nitride (AlN) for air-coupled applications. A commercially proven CMOS-compatible fabrication process was leveraged to form the devices. The transducer design consists of a radially nonuniform circular composite diaphragm with an integrated layer of AlN between two annular electrodes. Included in the overall system design is a tunable package with back cavity depth control. A multiphysics model was developed that integrates the electrical, mechanical, and acoustic energy domains of the complete system using composite plate mechanics, lumped-element modeling, and linear acoustic theory. Acoustical, mechanical, and electrical characterization of the transducers was conducted, including acoustic far field, laser vibrometry, and electrical impedance measurements. The device characterization results showed poor quantitative match with the system-level model due to large uncertainties in film stress. However, a qualitative comparison between the device behavior and the system model with varying back cavity depth showed promising results. The overall device performance is comparable to those in previous works that utilized alternate piezoelectric films.

I Introduction

The development of a piezoelectric micromachined ultrasonic radiator is presented. The design objective of this work was to maximize the output sound pressure level (SPL) of the transducer using a commercially available AlN piezoelectric film. Applications for air-coupled ultrasonic transducers include industrial automation, object identification, collision avoidance, parametric array sources, etc. The transducers are used as receivers, radiators, echolocation sensors, or propagation path detectors. The desired characteristics for these transducers are large output SPL, matched device characteristics in terms of resonant frequency and phase, minimal vibration distortion, small footprint, and reasonable cost. Other possible transducer characteristics of interest include directivity and far-field acoustic response, although these properties are adaptable by incorporating the transducers in array configurations. Table I

Piezoelectric Film Properties

Material Property	AlN [1]	PZT [2], [3]	ZnO [4]
(GPa)	283	96	98.6
(Mg/m3)	3.26	7.7	5.7
(pm/V)	−2.6	−130	−5.5
(pm/V)	5.5	290	10.3
	10.7	1300	8.5