Tapered acoustic matching layers

Typical ultrasonic transducers for immersion applications use one or more matching layers to increase their efficiency and bandwidth. In microwave circuits, it is common to use tapered impedance transformers to increase bandwidth. This has not been possible for acoustic transducers because of the difficulty of fabricating materials with a smoothly tapered acoustic impedance. We propose the use of silicon micromachining and vacuum impregnation to fabricate a composite material of silicon and a polymer filler. The acoustic properties of the composite are determined by the volume fraction of the silicon in the composite. By tapering the volume fraction of silicon in the direction of wave propagation, a tapered acoustic impedance results. We have fabricated a transducer operating at 5.7 MHz with a tapered acoustic matching layer and a silicon buffer layer. This transducer shows a 65% 3 dB fractional bandwidth