ScAlN Thick-Film Ultrasonic Transducer in 40-80 MHz

• Published in: IEEE transactions on ultrasonics, ferroelectrics, and frequency control Vol. 65; no. 11; pp. 2097 - 2102
• Main Authors: Sano, Ko-hei, Karasawa, Rei, Yanagitani, Takahiko
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.11.2018; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustics; Coupling coefficients; Diagnostic systems; Film growth; Frequency control; Piezoelectric films; piezoelectric resonators; Piezoelectricity; Polyvinylidene fluorides; Residual stress; ScAlN; sputtering; Substrates; Thick films; thick piezoelectric film; Transducers; ultrasonic transducer; Ultrasonic transducers
• ISSN: 0885-3010; 1525-8955
• DOI: 10.1109/TUFFC.2018.2865791

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 <inline-formula> <tex-math notation="LaTeX">k_{t}^{2} </tex-math></inline-formula> 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-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula>-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- (<inline-formula> <tex-math notation="LaTeX">k_{t}^{2}= 18 </tex-math></inline-formula>.5%) and 43-MHz (<inline-formula> <tex-math notation="LaTeX">k_{t}^{2}= 15.2 </tex-math></inline-formula>%) ultrasonic generation by using the 43- and 90-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> 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.