Modelling the operating frequency of thin film piezoelectric transducers

We have investigated the operating frequency of aluminium nitride thin film transducers up to 350 MHz. A sample consisting of 44 different sized devices (0.25-9 mm ) was fabricated. Each device consisted of a gold electrode (90 nm thick), aluminium nitride active layer (4 mum thick) on a glass substrate (1mm thick). Time domain and frequency domain results in pulse echo mode were also recorded from each device. It was found that using a JSR Ultrasonics DPR500 pulser receiver with 500MHz bandwidth, echoes could be received with frequency content between 60 MHz and 450 MHz. However this result was limited by the bandwidth of the digital scope being 350MHz. Using the DPR300 model from the same company with 50MHz bandwidth echoes were received between 5MHz and 65MHz. These results illustrated the effect of the harmonics in the driving pulse on the operating frequency of the thin film transducer. This investigation confirmed the results of a nondestructive testing experiment in which A-Scans were successfully obtained from a 65 mm thick ferritic steel steam pipe at a frequency of 5MHz using a thin film transducer.