Smart structures and microelectromechanical systems (MEMS)

Abstract

Capacitor transducers have been around for as long as piezoelectric transducers. They have not presented much competition for piezoelectric transducers as transmitters and receivers because the method of their manufacture did not optimize and highlight their performance. With the advent of silicon micromachining, it is now possible to make capacitors with very thin gaps that sustain electric fields of the order of 109 V/m or more. At these levels of electric field, the transformer coupling between the electrical and mechanical parts of the capacitor transducer, and thus its performance, become comparable to that of piezoelectric transducers. Advantages such as practically infinite bandwidth, ease of manufacture, and the ability to integrate electronic circuitry make this type of transducer a very important candidate for many ultrasonic applications. We will review the design and performance of capacitor transducers for both immersion and airborne ultrasound applications. Single-element, 1-D, and 2-D arrays of transducers will be presented along with imaging results. The overall dynamic range of systems with these transducers will be shown to be over 140 dB/V/vHz. We will also present a design for making surface-wave and Lamb-wave transducers using the same capacitor concept. [This work was sponsored by the Office of Naval Research, the Defense Advanced Research Projects Agency, and the Air Force Office of Scientific Research.]