Effects of a nano-scale embossed surface on the acoustic emission of air-coupled piezoelectric ultrasonic transducers

An air-coupled piezoelectric ultrasonic transducer (PUT) was fabricated to assess the effects of a nano-scale embossed Al diaphragm on the acoustic emission characteristics. A 310-μm-thick multilayer piezoelectric film and a 300-μm-thick Al plate were used for the PUT. It was found that the flexural modulus of the Al plate was decreased slightly from 3.73 GPa to 3.41 GPa by the effect of the embossed surface morphology, having an array of Al bumps with dimensions of 800 nm × 800 nm and a height of 320 nm. The decreased flexural modulus had a strong effect on the decrement of the acoustic resonant frequency and on the enhancement of the sound power radiation for the PUT. In comparison to a sound pressure level (SPL) of 117.8 dB at 30 cm away from a flat PUT at 41.4 kHz, the embossed PUT could result in a significantly enhanced SPL of 121.7 dB at the secondary resonant frequency of 40.5 kHz for the implementation of a smaller transducer. This result can motivate an efficient design parameter for better performance of a PUT through modifications of the nm-scale embossed surface.