2F-4 High Frequency (>30 MHz) Flexible Broadband Transducers

Flexible ultrasonic transducer (UT) arrays consisting of a thin substrate, a bottom electrode if the substrate is an insulator, a piezoelectric composite ceramic layer and a top electrode with a total thickness less than 100 mum have been produced. A stainless steel foil, aluminum foil and polyimide film were used as a substrate. On top of the polyimide film a thin (<1 mum thick) nickel alloy layer was coated as a bottom electrode by an electroless plating method. A lead zirconate titanate (PZT) ceramic film of 30 mum thick was made by a sol-gel spray technique. The density and thickness of the PZT film were optimized so that high frequency ultrasound can be efficiently generated and received. An array of top electrodes was made by a colloidal silver spray. Each top electrode of the UT array had a size of 3 mm by 3 mm and this size may be varied with ease. Due to the porosities inside the PZT film and thin substrate, high flexibility and broad bandwidth of the developed UTs have been realized. Ultrasonic measurements with these three types of flexible UTs have shown that their frequency components could be higher than 30 MHz and the 6-dB bandwidth can reach 80%. It is also shown that the polyimide film could be a suitable substrate material of the flexible UTs for ultrasonic measurements in tissue and tissue-like materials due to its good acoustic coupling to such materials. The presented flexible UT fabrication technique allows a selection of the substrate material (either polymer or metal) with a thickness suitable for the specimen to be characterized.