Micromachinable ultrasonic leaky wave air transducers

Abstract

Ultrasonic air transducers using leaky waves on thin membranes are analyzed using perturbation and normal mode approaches. The transducers utilize the efficient coupling of ultrasonic energy to air through radiation of these leaky wave modes when their phase velocity is close to the sound speed in air. Theoretical results on optimum transducer dimensions and bandwidth estimation show that a minimum conversion loss of 8.7 dB with a 78% fractional bandwidth is possible. Common micromachining materials are shown to be suitable transducer materials and result in feasible devices. This is demonstrated by fabricating a 580 kHz transducer using a silicon membrane bonded to a ring of PZT-5H. With this configuration the transducer is self line focusing. Results of through transmission experiments on silicon and transmission images on paper are reported. © 1998 American Institute of Physics. ͓S0003-6951͑98͒00232-0͔Due to the large impedance mismatch between common piezoelectric materials and air, conventional piezoelectric transducers are not very efficient sources of ultrasound in air.1 The efficiency can be increased by matching layers at the expense of bandwidth. Recently, several micromachined capacitive transducers have been developed. The devices made by stretching metallic membranes over micromachined grooves in silicon provide wide bandwidth, however, their efficiency is low.2 Efficient ultrasonic transmission up to 11 MHz is demonstrated using a resonant thin silicon nitride membrane over a sub-micron thick surface micromachined air gap.3,4 For wideband operation with these devices, dynamic range should be sacrificed. Ultrasonic wave transmission in air at 1 MHz using leaky waves has been demonstrated. 5 In this letter, a simple theoretical model for these transducers to predict the optimum dimensions and the bandwidth is presented. The use of micromachining for the fabrication of efficient transducers in the MHz range is proposed and its feasibility is demonstrated using a selffocusing transducer made of silicon.Any ultrasonic wave propagating on a solid surface radiates energy to the surrounding fluid medium if the phase velocity is larger than the velocity of sound in the fluid. Various calculation methods predict that the rate of this energy transfer is maximized when the phase velocity of the mode approaches to that of the fluid. 5,6 In Fig. 1 the phase velocity as well as the attenuation due to energy leakage of the lowest order antisymmetric Lamb waves (A 0 mode͒ in free plates of common silicon micromachining materials are plotted as a function of the frequency thickness product ͑fd͒. It is observed that the phase velocity of the A 0 mode can be adjusted so that it is close to the sound speed in air ͑330 m/s͒ by changing the thickness of the membrane at a fixed frequency. For example, at 7 MHz, a 1 m thick silicon plate is required to match the sound speed in air. These thin films can be fabricated by micromachining, and have already been used for this purpose. 7 The phase velocity matching can be achieve...