Dynamic mechanism of micromachined electrostatic airborne ultrasonic transducers

Abstract

Micromachining technology allows electrostatic (or capacitive) airborne ultrasonic transducers to be made with highly reproducible characteristics, thus providing the possibility to model their behaviors reliably. The behavior of a transducer depends on its geometric structure and the materials to be used. Its mechanical stiffness results from the compressibility of air in cavities, the bending stiffness of the diaphragm, and the plane tension applied to the diaphragm, if any. It is clarified that there are two main types of dynamic mechanisms according to different structures. In one type in which the air in cavities is enclosed, the diaphragm can be treated as a thin plate supported by an air spring or a plate founded on an air cushion, thus adopting the plate-on-air–spring model or short-tube model, such as those transducers with V-grooved, U-grooved, or pit-array-texture backplates [L.-F. Ge, Chin. Sci. Bull. Acad. Sin. 10 (1997)]. In a second type, air is not enclosed, so that the effect of air spring can be omitted, thereby taking the membrane-under-tension model, such as conventional condenser microphones and ultrasonic transducers with acoustic holes. Resonant frequencies of a transducer can be determined by the vanishing of the reactance of its inversive impedance [L.-F. Ge, J. Acoust. Soc. Am. 96, 3318 (1994)]. [Work supported by the 211 Engineering Foundation of Anhui University.]