Fabrication technology of capacitive micromachined ultrasonic transducers (CMUTs): A review

Capacitive micromachined ultrasonic transducers (CMUTs) have become an important research topic in ultrasonic applications and are already used across many fields. Their development is inseparable from the support of advanced fabrication technologies. Through rigorous surveying, scrutiny, and perception, this review, for the first time, covers the significant contributions of CMUT fabrication technology over the past 30 years in a short yet comprehensive manner. It innovatively introduces the development history, milestone works, classification, detailed processes, design rules, precautions, and process solutions for special applications of CMUTs fabrication technology from different perspectives such as time, design, and application. In particular, this paper presents for the first time to readers the process solutions for parasitic capacitance, dielectric charging, process temperature and electronic integration of CMUTs. This paper discusses the advantages and limitations of the sacrificial-release process, the wafer-bonding process, and the PolyMUMPs process, and summarizes the methods for testing and evaluation to inspire any peers working on CMUTs. Moreover, major challenges and future predictions for CMUTs fabrication technology in large-scale commercialization are proposed. In short, this review can serve as a one-stop reference guide, filling the gap by providing a systematic and rapid overview of CMUTs fabrication technology for anyone interested in CMUTs.