A medical ultrasound diagnostic system and an ultrasonic microscope are generally used in the frequency range of 1-20 MHz and 100 MHz-2 GHz, respectively. Ultrasonic transducers in the frequency range of 20-100 MHz are, therefore, not well developed because of less application into ultrasonic imaging or suitable piezoelectric materials with this frequency range. Polyvinylidene difluoride (PVDF) is usually used for ultrasonic transducers in the 10-50-MHz ranges. However, their electromechanical coupling coefficient k 2 t of 4% is not enough for the practical uses. In order to excite the ultrasonic wave in the 20-100 MHz range, a 125-25-µm-thick piezoelectric film is required when the longitudinal velocity of the material is assumed to be 5000 m/s. However, it is difficult to grow such a thick piezoelectric film without a crack being caused by the internal stress during the dry deposition technique. We achieved a stress-free film growth by employing the unique hot target sputtering technique without heating the substrate. High-efficient 81-(k 2 t = 18.5%) and 43-MHz (k 2 t = 15.2%) ultrasonic generation by using the 43-and 90-µm extremely thick ScAlN (Sc: 39%) films were demonstrated, respectively. We discussed the advantage of ScAlN thick-film transducers by comparing them with the conventional PVDF transducer for the water medium.

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