|dc.description.abstract||In the last two decades, capacitive micromachined ultrasonic transducers (CMUTs) have proven themselves to be promising for various ultrasound imaging and chemical sensing applications. Although holding many benefits for ultrasound imaging, CMUTs have certain weaknesses such as the relatively low output pressure at transmission, which hinder their development in the diagnostic imaging application. In the sensing area, CMUTs have shown attractive features such as high mass sensitivity, miniaturized array configuration, and ease of functionalization. However, their potential for humidity sensing is less explored. The objectives of this thesis lie in two aspects. One is to offer a solution to overcome the limitation of low output pressure, and the other is to develop CMUTs as resonant gravimetric humidity sensors. The major efforts are made on the second task.
For the first objective, a novel dual-element ultrasonic transducer is proposed. It incorporates two transducer technologies by using a circular piezoelectric element for ultrasound transmission and an annular CMUT element for reception. The hybrid transducer combines the broad bandwidth and high receive sensitivity of the CMUT and the high output power of the piezoelectric transducer to improve the overall sensitivity and axial resolution. The annular CMUT is designed, fabricated, and concentrically aligned with the piezoelectric probe via a custom housing. Immersion measurements show that the hybrid dual-element transducer improves the axial resolution by 25.58% and the signal-to-noise ratio by 8.55 dB over the commercial piezoelectric probe.
For the second objective, a CMUT-based resonant humidity sensor is first developed with the direct wafer bonding technique. Graphene oxide (GO) is employed as the sensing material. Due to combination of the mass-sensitive CMUT and the moisture-sensitive GO, the sensor exhibits rapid response/recovery, good repeatability, and higher sensitivity than most of its competitors. The second generation of CMUT-based humidity sensors aims to further improve the relative humidity (RH) sensing performance by adopting the nitride-to-oxide wafer bonding technology for CMUT fabrication. In contrast to conventional wafer bonding CMUT processes that use expensive silicon-on-insulator (SOI) wafers to produce resonating membranes, the new process employs low-pressure chemical vapor deposition (LPCVD) silicon nitride as the membrane material. It provides thinner and lighter membranes, and thus more sensitive CMUT resonators. Additional benefits of the nitride-to-oxide wafer bonding technique are the reduced fabrication complexity and more controllable membrane thickness. Finally, a dual-frequency (10/14 MHz) CMUT is developed using this fabrication technique. It generates two RH response curves and can provide more accurate RH sensing. Due to the independence of the two resonance frequencies, the dual-frequency CMUT also shows great potential for identification of different chemicals. This thesis demonstrates that CMUT sensors can be strong candidates for miniaturized, highly sensitive, and reliable humidity sensors.||en