Optical Measurements of Picovibrations on Capacitive Micromachined Ultrasonic Transducers

Abstract

A research group at the Norwegian University of Science and Technology plan to integrate Capacitive Micromachined Ultrasonic Transducers (CMUTs) in a probe which can be used to image human blood vessels from the inside. With CMUT technology still being in its youth, it is important to characterize different CMUT prototypes to learn which design is best suited for the probe. In this thesis, a network analyzer and a heterodyne interferometer are used to characterize two prototypes, both electrically and optically. The heterodyne interferometer was proven to be extremely sensitive and thus a valuable tool when investigating the behaviour of single CMUT cells. Measurements revealed that the cells within both the CMUT arrays had different resonance frequencies. This effectively broadens the band widths of the prototypes, but reduces their maximum acoustical output power, given that all the cells operate at the same frequency. It was found probable that the CMUT characteristics were influenced by how charges moved within the prototypes. These charging effects are quite complex and would in practice make it hard to predict where the centre frequency of the CMUT array lies from time to time. The vibration pattern of most the cells at their respective resonance frequencies looked healthy both in air and immersed in a liquid when measuring with the interferometer. Immersed, the resonant frequency range of the CMUT become wider, but the peak was located at a lower frequency than in air. The next CMUT prototype to be tested in the future does not fit into the experimental setup. A plan to adapt the setup has been presented in this thesis, making it more practical to handle different samples. This design would also make it easier to investigate crosstalk effects between neighbouring CMUT elements within the arrays. Knowledge aquired in this thesis will hopefully help researchers figure out which areas of the CMUT design have the potential to be improved.