Reliability issue related to dielectric charging in capacitive micromachined ultrasonic transducers: A review

The long-term reliability of MEMS devices related to the dielectric charging phenomenon is one of the main hurdles in the commercialization of these devices. This paper presents a comprehensive review of the dielectric charging and its associated reliability issues in capacitive micromachined ultrasonic transducers (CMUTs). Due to the ease of versatile fabrication and large bandwidth in immersion, CMUTs are one of the promising technologies for ultrasonic applications that require miniaturized transducers, but due to electrostatic actuation, CMUTs suffer from inherent dielectric charging issues. In this review, the effects of dielectric charging and discharging on the CMUT performance, modeling of dielectric charging, and the methods to mitigate the reliability issue due to dielectric charging are extensively discussed. The mechanisms of dielectric charging are presented in detail to demonstrate the effects of dielectric charging on the drift. Structural, operational, and material modifications suggested in the literature to improve the long-term reliability of CMUTs are also reviewed. It is concluded that these methods have improved the reliability issues to great extent but there is still a room for improvements such as through exploration of different dielectric materials.