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High sensitivity and wideband design for impedance matching layer between protection Metal and PZT

Ultrasonic transducers for industrial applications often require front protection layer which have to be chemically inert material such as stainless steel (SS). This is particularly important for process control of semiconductor device production so as not to contaminate processing agent. It is known when metallic layer is added at front of ultrasonic transducers, boundary reflections reduce wave transmission and sensitivity is lowered. To overcome this problem, typically known method is to use very thin bonding layer between PZT and use λ/2 metallic protection layer. However, difference of thermal expansions in these materials limits operation temperature range, or otherwise causes delamination at an extreme temperature. The purpose of this work is to design optimum structure of the front protection layers and bonding layer for higher sensitivity and wider bandwidth.