Micromachinable leaky wave air transducers

Abstract

In this paper, ultrasonic air transducers which use the lowest order antisymmetric (A/sub 0/) mode Lamb waves in a thin plate as a means of efficient coupling of ultrasonic energy to air are discussed. For a silicon plate of 1 /spl mu/m thickness, the energy leak rates can go up to 0.6 dB per wavelength. At MHz frequencies the plate thickness should be in the range of 1-10 /spl mu/m, which requires micromachined structures to be used. The radiation pattern of the transducers can be controlled by the geometry of the transducer, which can also be used for focusing. A theoretical model to calculate the efficiency and optimized transducer dimensions is presented. This model is applied to common micromachining materials such as silicon, silicon nitride and silicon dioxide. The analysis show that, with these transducers it is possible to achieve a conversion loss with a minimum of 8.7 dB and 78% fractional bandwidth. Experimental results on transmission imaging are also presented using an implementation of the transducer operating around 580 kHz.