High electromechanical coefficient k_t²=19% thick ScAlN piezoelectric films for ultrasonic transducer in low frequency of 80 MHz

Ultrasonic transducers in the frequency ranges of 20-100 MHz is not well-developed because of less applications or less suitable piezoelectric materials. PVDF are usually used for ultrasonic transducers in the 10-50 MHz ranges. However, their electromechanical coupling coefficient k_t² of 4% is not enough for the practical uses. In order to excite ultrasonic in the 20-100 MHz, 125 μm-25 μm thick piezoelectric film is required. It is difficult to grow such a thick piezoelectric film without a crack caused by the internal stress during the PVD deposition technique. We achieved stress free film growth by employing the unique hot cathode sputtering technique without heating substrate. We demonstrated high efficient 81 MHz (k_t²=18.5%) and 43 MHz (k_t²=11.9%) ultrasonic generation by using the 43 μm and 90 μm extremely thick ScAlN(Sc:39%) films, respectively.