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Transducers and beam forming
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
Tony Whittingham, Kevin Martin
The two principal types of MUT devices are the capacitive micro-machined ultrasonic transducer (CMUT) and the piezoelectric micro-machined ultrasonic transducer (PMUT). Although the basic CMUT and PMUT units have some similarities in construction, their principles of operation are fundamentally different.
An alN-based piezoelectric micromachined ultrasonic transducer with a double-beam suspended structure
Published in Journal of the Chinese Institute of Engineers, 2022
Yihsiang Chiu, Li Wang, Dan Gong, Yang Yang, Weili Wang, Nan Li, Shenglin Ma, Yufeng Jin
A much lower electromechanical conversion efficiency for the conventional bulk piezoelectric transducer is caused by poor acoustic coupling (Oralkan et al. 2003). This issue can be overcome by miniaturizing the capacitive micromachined ultrasonic transducer (CMUT) using integrated circuit technology. On the other hand, a tiny capacitive air gap underneath transducer is required for sensible ultrasonic reception, restricting the displacement of membrane and hence leading to a lower propagatable sound pressure level (SPL). Therefore, high voltage (>100 V) and big air gap are needed for the CMUT to generate a large vibration amplitude. By contrast, piezoelectric micro-machined ultrasound transducers (PMUT), which make use of the piezoelectric effect independent of a direct current (DC) polarization voltage can be utilized in the same way. The process of fabrication is more straightforward than that of the CMUT. Meanwhile, the PMUT exhibits a displacement from elastic supporting layer and actuating membrane deflection due to the piezoelectric effect. Lead zirconate titanate (PZT) (Baborowski et al. 2003), zinc oxide (ZnO) (Li et al. 2017), and aluminum nitride (AlN) are serveral piezoelectric materials that are commonly used. In particular, PZT is frequently applied in medical imaging due to its extraordinary piezoelectric coefficient. Moreover, the integration of ZnO MEMS into circuits is complicated by the diffusion of Zn ions, which results in contamination of the CMOS circuit. Although it is possible to integrate ZnO MEMS devices into circuits, Zn ions usually diffuse, which easily induces CMOS circuits contamination. Furthermore, ZnO devices are associated with higher power dissipation attributed to their high conductivity. In contrast, AlN is processed at low temperature, which is beneficial for post-CMOS compatible fabrication with a lower residual stress, compared with other materials (Smyth, Sodini, and Kim 2017; Shelton et al. 2009).