Fabrication of silicon-polymer composite acoustic matching layers for high frequency transducers

We have used micromachining methods taken from the MEMS industry to fabricate acoustic matching layers for high frequency transducers. The matching layers are made as silicon-polymer composites, with acoustic impedance determined by the ratio between the two materials. Two different fabrication methods were used, anisotropic wet etch and deep reactive-ion etch (DRIE), and the composites were fabricated as both type 1-3 and 2-2 connectivity. These methods were used to fabricate structures suitable for a 15 MHz transducer. The resulting 2-2 composite has silicon width 4.5 μm and polymer width 18 μm, whereas the 1-3 composite has 7 μm wide posts and 9 μm spacing between posts. Both composites have 20% volume fraction of silicon, giving acoustic impedance 7 Mrayl. The acoustic behavior of the stack was investigated by FEM simulations, with emphasis on the composite layer. The simulations of pulse shape and bandwidth show that the stack should perform well as acoustic matching layers for high frequency transducers.