1–3 microfabricated composite acoustic matching layers for high frequency transducers

This paper presents the fabrication and characterization of a 1–3 silicon-polymer composite matching layer made by Deep Reactive Ion Etch (DRIE) method. A well-defined composite layer thickness of 83 µm was obtained by using Silicon-on-Insulator (SOI) wafers as substrate. The resulting composite has 7 µm size posts and 9 µm spacing between posts. A slight tapering of the posts was observed after the DRIE process, causing the posts to be narrower in the bottom than at the top. The composite was used as acoustic matching layer in an air-backed 15 MHz transducer and characterized by electrical impedance measurements in air. The effective acoustic properties of the composite, speed of sound and acoustic impedance, deduced from measured results, were found to be lower than those predicted from iso-strain model. This deviation can be explained by tapering of the trench walls and the dispersion caused by the finite dimensions of the bi-phase material, an explanation that was verified by FEM simulations.