| This dissertation introduces a new fabrication process developed for the capacitive micromachined ultrasonic transducer (CMUT) using wafer-bonding technology and presents CMUT designs and design optimizations for device performance and reliability improvement.; Ultrasound is used in many applications including medical diagnostics and therapy, underwater imaging, and nondestructive evaluation. Compared to the widely used piezoelectric (PZT) ultrasound transducer, the CMUT has advantages in device fabrication method, bandwidth and operation temperature. It also demonstrates a comparable dynamic performance to PZT transducers. The CMUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducer.; The new CMUT fabrication process based on wafer bonding technology offers advantages over traditional surface micromachining. First, the number of process steps involved in making a CMUT is greatly reduced by simplifying the cavity forming process, thus improving device yield. Second, this process enables a more accurate control over device parameters and material properties, which improves device reliability. Third, since the membrane and the cavity can be fabricated separately, it provides horizontal and vertical design flexibility, which offers more degrees of freedom for device optimization. All these advantages provide repeatable CMUT fabrication featuring predictable center frequency, bandwidth, collapse voltage and transduction. Using this new technique, CMUTs with operating frequency spanning from 50 kHz to 30 MHz have been fabricated and characterized for both airborne and immersion applications. Another major effort of the dissertation is focused on optimization of one kind of CMUTs that can be used for medical imaging, a promising application area. The pros and cons of conventional (pre-collapse) and non-conventional (collapse) mode operation of CMUTs have been investigated, as well as membrane configuration optimization. A CMUT featuring isolation posts has been invented to solve the device reliability issue. Finally, a CMUT featuring piston shaped membranes demonstrates the comparable output pressure and much better bandwidth, compared with currently used transducers. |
