Date and Time: 11th December 2013 (Wednesday), 11:00 AM

Venue: N204, University of Macau



Capacitive micromachined ultrasound transducers (CMUTs) consist of a thin vibrating membrane with an embedded electrode that is separated from the substrate with a small sacrificial gap that vibrates in order to transmit or receive ultrasonic waves. CMUTs are now used as good alternatives to state-of-the-art piezoelectric transducers. The main drawbacks of bulk piezoelectric transducers are the high cost, the need for a precise acoustic matching layer, the lack of integration capability with front-end electronics, and non-biocompatibility. On the other hand, by proper design and taking advantage of the micro-machining techniques for fabricating optimized structures, CMUTs can provide wide bandwidth and high coupling efficiency. The additional advantages of CMUTs are full integration capability, low power consumption, elimination of the need for a matching layer, and low cost.
In addition to conventional CMUTs, flexible CMUTs are emerging with the advantage of being able to conform to the body part being imaged, which is beneficial for medical imaging. However, to date, no flexible CMUT with high reliability and efficiency has been demonstrated. From current research, the flexibility is mainly provided either from a whole polymer substrate or trenched silicon substrate connected by polymer material. The former cannot give high resonant frequency due to the softness of the polymer substrate, and the latter has the problem of bonding strength between the polymer and silicon.

This study focuses on new fabrication techniques for a flexible CMUT with high reliability and efficiency. Firstly, a high frequency CMUT array using silicon nitride membranes and nickel bottom electrodes was investigated. Then a flexible CMUT array using an all nickel electrodes and membranes, with a polymer substrate is implemented. Recently, we have attempted to make 1-D CMUT array using polymer membranes with nickel bottom electrodes for ultrasonic imaging. The theoretical calculation results showed that the concave bottom made from reflowed photoresist has a better membrane profile and larger deflection compared to the flat bottom electrode. Therefore the fabricated CMUTs potentially have much better performance in terms of capacitance, sensitivity and output pressure compared to the CMUTs with flat bottom electrodes in referred to contemporary research.


Dr. Ching-Hsiang Cheng obtained his B.S. degree in Mechanical Engineering from National Taiwan University in 1993, three Master’s degrees in both Electrical and Mechanical Engineering at Cornell University in 1998, and Ph.D. degree in Electrical Engineering at Stanford University in 2005. After completing his postdoctoral research in Stanford, he joined Industrial Technology Research Institute (ITRI) in Taiwan as a researcher and became a project leader. In 2006, he has been appointed as an Assistant Professor and also a Research Engineer in the Research Institute of Innovative Products and Technologies (RIIPT) of The Hong Kong Polytechnic University (PolyU). In 2010, he joined Department of Industrial and Systems Engineering in PolyU as an Assistant Professor. His research interests include capacitive micromachined ultrasonic transducers (CMUTs), shear and normal force sensors, large strain and tactile sensors, solar cells, microvalves and micropumps, gas sensors, electrical through-wafer interconnects, nanotechnology, and microelectromechanical systems (MEMS) for healthcare applications.