Thickness design, fabrication, and evaluation of 100-MHz polyurea ultrasonic transducer

In this paper, we present a polyurea transducer that works at 100 MHz under water. The transducer was designed using an equivalent circuit model so that an aluminum (top)-polyurea-aluminum (bottom)-polyimide layer had a resonant frequency of 100 MHz and output sound pressure became maximum at that frequency. The thicknesses of the top aluminum electrode, polyurea, and bottom aluminum electrode were determined to be 3.3, 3.5, and 1.7 μm, respectively. A 100-MHz polyurea transducer with the designed thickness was fabricated using deposition equipment. To evaluate the performance of the designed and fabricated polyurea transducer, transmission-reception experiments with pulsed and burst waves were carried out. The results show that transmitting and receiving ultrasounds at a frequency of 100 MHz are possible as expected with the thickness design. To evaluate actual use, B-mode imaging of an onion was also performed using the transducer, which was formed into a line-focused shape. The result shows that the outer layer of the onion, of 0.1 to 0.2 mm thickness, was successfully imaged.