title: Capacitive micromachined ultrasonic transducers with piston-shaped membranes: fabrication and experimental characterization.
abstract: Capacitive micromachined ultrasonic transducers (CMUTs) featuring piston-shaped membranes (piston CMUTs) were developed to improve device performance in terms of transmission efficiency, reception sensitivity, and fractional 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 uniformly thick membranes (classical CMUT). Piston CMUTs with a more uniform surface displacement profile can achieve high output pressure with a relatively small electrode separation. 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 (transmission 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.
authors.affiliation: Kolo Technologies Inc, San Jose, CA, USA.
authors.forename: Yongli
authors.initials: Y
authors.lastname: Huang
authors.forename: Xuefeng
authors.initials: X
authors.lastname: Zhuang
authors.forename: Edward O
authors.initials: EO
authors.lastname: Haeggstrom
authors.forename: A Sanli
authors.initials: AS
authors.lastname: Ergun
authors.forename: Ching-Hsiang
authors.initials: CH
authors.lastname: Cheng
authors.forename: Butrus T
authors.initials: BT
authors.lastname: Khuri-Yakub
mesh_terms: D003198:Computer Simulation; D025802:Electric Capacitance; D004867:Equipment Design; D008567:Membranes, Artificial; D055617:Micro-Electrical-Mechanical Systems / Q000295:instrumentation*; D012680:Sensitivity and Specificity; D012825:Silicon / Q000737:chemistry; D014159:Transducers; D014465:Ultrasonics
chemical_list: D008567:Membranes, Artificial; D012825:Silicon