ScAlN Thick-Film Ultrasonic Transducer in 40–80 MHz

Abstract

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.

A Review of UltraHigh Frequency Ultrasonic Transducers

“…E Wistrela also mentioned that XRD and surface topography analysis (Wistrela et al, 2018) can as a straightforward approach to estimate the piezoelectric activity of Cr doped AlN thin films. K San reported a 40-80 MHz Cr-doped AlN ultrasonic transducer (Sano et al, 2018). BP Zhu reported a 200 MHz Cr-doped AlN ultra-high frequency ultrasonic transducer (Zhu et al, 2017), the preparation of the transducer is shown in the upper right of Figure 3, the pulseecho and resolution are shown in Figure 3 bottom left.…”

Piezoelectric MEMS—evolution from sensing technology to diversified applications in the 5G/Internet of Things (IoT) era

“…When an external force is applied to this type of material, an equal amount of positive and negative charges will be generated on the opposite surfaces of the material [64], as sketched in figure 1(b). On the contrary, by applying a voltage across the material, the material [58][59][60][61][62][63]. can be deformed accordingly, which is named the converse piezoelectric effect.…”

“…Commonly used piezoelectric materials are lead zirconate titanate (PZT), zinc oxide (ZnO), lithium niobate (LiNbO 3 ), and aluminum nitride (AlN). The properties of these materials are summarized in figure 1(c) [58][59][60][61][62][63], including piezoelectric coefficients, electromechanical coupling coefficient, and acoustic velocity. Among them, PZT has the highest piezoelectric coefficients, making it well-suited for use in microactuators [20,65].…”

Analysis of leaky surface acoustic wave characteristics propagating on high piezoelectric ScAlN film/high velocity quartz substrate

“…20,21) This high piezoelectric ScAlN film has attracted attention as a new piezoelectric layer for film bulk acoustic wave resonators [22][23][24][25][26][27][28] and ultrasound transducers. 29) The longitudinal and shear mode electromechanical coupling coefficient in ScAlN FBARs increase with increasing Sc concentration. The longitudinal mode k t 2 of 18.5% in c-axis oriented Sc 0.4 Al 0.6 N films is approximately 200% of that in a pure AlN single crystal.…”

“…28) The ultrasound intensity in ScAlN thick film transducers operating in the megahertz range is stronger than that of ZnO film or PVDF film transducers. 29) Also, several researchers have reported that the coupling factors of SAW resonators with ScAlN films/high velocity substrates are higher than those of AlN film SAW resonators. [30][31][32][33][34][35][36] Especially, Sezawa mode SAW resonators consisting of ScAlN films/high velocity SiC or diamond substrates have both high phase velocity and high coupling.…”