0885–3010/$25.00 © 2009 IEEE 136 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, . 56, . 1, JANUARY 2009 Abstract—Capacitive micromachined ultrasonic transduc- ers (CMUTs) featuring piston-shaped membranes (piston CMUTs) were developed to improve device performance in terms of transmission efficiency, reception sensitivity, and frac- tional bandwidth (FBW). A piston CMUT has a relatively flat active moving surface whose membrane motion is closer to ideal piston-type motion compared with a CMUT with uni- formly thick membranes (classical CMUT). Piston CMUTs with a more uniform surface displacement profile can achieve high output pressure with a relatively small electrode separa- tion. The improved device capacitance and gap uniformity also enhance detection sensitivity. By adding a center mass to the membrane, a large ratio of second-order resonant frequency to first-order resonant frequency was achieved. This improved the FBW. Piston CMUTs featuring membranes of different geometric shapes were designed and fabricated using wafer bonding. Fabricating piston CMUTs is a more complex process than fabricating CMUTs with uniformly thick membranes. However, no yield loss was observed. These devices achieved ~100% improvement in transduction performance (transmis- sion and reception) over classical CMUTs. For CMUTs with square and rectangular membranes, the FBW increased from ~110% to ~150% and from ~140% to ~175%, respectively, compared with classical CMUTs. The new devices produced a maximum output pressure exceeding 1 MPa at the transducer surface. Performance optimization using geometric membrane shape configurations was the same in both piston CMUTs and classical CMUTs. I. I C  micromachined ultrasonic transducers (CMUTs) have emerged as a promising technology for applications in medical ultrasound. Compared with piezoelectric transducers in medical applications, CMUTs reported so far possess broader fractional bandwidth (FBW) but poorer transmission (TX) and reception (RX) efficiencies [1]. In its simplest form, a CMUT is made of a mass with a flat surface and a spring actuated by an elec- trostatic force, as shown in Fig. 1(a). Ideally the membrane displacement is uniform across the device area, maximiz- ing the volume displacement and hence also transduction (TX and RX) efficiencies for a specific vacuum cavity height. Nearly all fabricated CMUTs reported in the lit- erature [2]–[6] featured membranes of uniform thickness. We reported on methods to improve CMUT performance by optimizing membrane geometry in the lateral mem- brane directions for more pistonlike membrane motion [7]. There is still a performance trade-off between transduc- tion efficiency and FBW when designing CMUTs with uniformly thick membranes (classical CMUTs). The room for improvement of such a CMUT is constrained because the mass and the spring constant are closely coupled. It is difficult to change one without substantially affecting the other, thus limiting design flexibility. In this paper, we report on results for CMUTs featur- ing piston-shaped membranes (piston CMUT) as shown in Fig. 1(c). The piston CMUT is a CMUT with a relatively flat moving surface. Consequently, the membrane motion is closer to ideal piston-type motion compared with a clas- sical CMUT with uniform membrane thickness. A center mass that could be made of silicon is added to each mem- brane, thus altering the membrane geometry in the verti- cal direction to have more ideal piston membrane motion. The motivation for this approach is to improve the device performance without substantially increasing the fabrica- tion process complexity or reducing device yield. The con- cept of CMUTs with piston-shaped membranes has been reported in a U.S. patent [8], and as a conference publica- tion in 2005 [9]. Other researchers have also reported on work based on similar concepts for CMUTs [10]–[12] and other types of MEMS devices [13]. The piston CMUT offers advantages when compared with a classical CMUT. Most noticeably, because the cen- ter mass can be controlled independently of the membrane size or thickness, there is more design flexibility. Specifi- cally, the spring constant, which is determined by both membrane and piston, and the membrane mass, which is mostly due to the center piston, are largely decoupled Capacitive Micromachined Ultrasonic Transducers with Piston-Shaped Membranes: Fabrication and Experimental Characterization Yongli Huang, Xuefeng Zhuang, Student Member, IEEE, Edward O. Hæggstrom, A. Sanli Ergun, Ching-Hsiang Cheng, and Butrus T. Khuri-Yakub, Fellow, IEEE Manuscript received March 21, 2007; accepted June 18, 2008. The authors would like to acknowledge the financial support of the Office of Naval Research and the National Institutes of Health. Xuefeng Zhuang was supported by a Weiland Family Stanford Graduate Fellowship. Dr. Hæggström acknowledges the Wihuri-Foundation and the Academy of Finland for financial support. Y. Huang is with Kolo Technologies Inc., San Jose, CA. X. Zhuang and B. T. Khuri-Yakub are with Edward L. Ginzton Laboratory, Stanford University, Stanford, CA (e-mail: xzhuang@ stanford.edu). E. O. Hæggstrom is with the Electronics Research Unit, University of Helsinki, Helsinki, Finland. A. S. Ergun is with Siemens Corporate Research, Mountain View, CA. C.-H. Cheng is with the Research Institute of Innovative Products and Technologies, Hong Kong Polytechnic University, Hong Kong, China. Digital Object Identifier 10.1109/TUFFC.2009.1013