Fabrication and Integration of CMUT Sensors

Capacitive micromachined ultrasound transducers (CMUTs) are a promising replacement technology for piezoelectric composite transducers, as they offer a more flexible architecture, wider signal bandwidth, and improved impedance matching between the sensor and the sensing medium. While CMUTs have demonstrated these advantages they have yet to surpass piezoelectric based sensors due to their low fabrication yield and long term reliability issues. This thesis, entitled ”Fabrication and Integration of CMUT Sensors,” outlines detailed process flows for fabricating novel CMUT sensors using both sacrificial release and wafer bonded architectures. Working with the sacrificial release structure we show that adapting a new electrode etching process, modifying the basic CMUT cell structure, and tuning the fabrication parameters produces very reliable process flows with the ability to build larger 2D arrays with high yield. These large 2D arrays are critical to enabling 3D ultrasound imaging with high resolution and a large field of view. Working with the wafer bonded architecture we introduce a new fabrication process for building 2D arrays without requiring aligned wafer bonding equipment. This process features isolated isolation post structures within each CMUT cell that have the potential for long term reliable operation of these sensors without any negative dielectric charging effects. While we detail the process for fabricating these structures there are still issues with low dielectric strength of the silicon dioxide, and a large number of particulate defects present in the devices, which lower the fabrication yield. A modification to the isolated isolation post architecture is proposed, combined with a few changes in the process flow that could yield successful results in the future. High yield production of robust CMUT sensors would have significant market appeal, and large 2D arrays, potentially occupying a whole wafer, could enable whole organ ultrasound imaging. Such arrays could be the ultrasound-equivalent of x-ray flat panel detector arrays and could help to replace operator intervention in ultrasound scanning with automated electronic scanning.