Kapasitif mikroişlenmiş ultrasonik çeviricilerin radyasyon empedansı

Capacitive micromachined ultrasonic transducers (cMUTs) are used to transmitand receive ultrasonic signals. The device is constructed from circular membranesfabricated with surface micromachining technology. They have wider bandwidthwith lower transmit power and lower receive sensitivity compared to the piezo-electric transducers, which dominate the ultrasonic transducer market. In orderto be commercialized, they must overcome these drawbacks or ¯nd new applica-tion areas, where piezoelectric transducers perform poorly or cannot work. In thisthesis, the latter approach, ¯nding a new application area, is followed to designwide band and highly e±cient airborne transducers with high output power bymaximizing the radiation resistance of the transducer.The radiation impedance describes the interaction of the transducer with thesurrounding medium. The real part, radiation resistance, is a measure of theamount of the power radiated to the medium; whereas the imaginary part, ra-diation reactance, shows the wobbled medium near the transducer surface. Theradiation impedance of cMUTs are currently not well-known. As a ¯rst step,the radiation impedance of a cMUT with a circular membrane is calculated ana-lytically using its velocity pro¯le up to its parallel resonance frequency for boththe immersion and the airborne applications. The results are veri¯ed by ¯niteelement simulations. The work is extended to calculate the radiation impedanceof an array of cMUT cells positioned in a hexagonal pattern. The radiationimpedance is determined to be a strong function of the cell spacing. It is shownthat excitation of nonsymmetric modes is possible in immersion applications.A higher radiation resistance improves the bandwidth as well as the e±ciencyand the transmit power of the cMUT. It is shown that a center-to-center cell spac-ing of 1.25 wavelength maximizes the radiation resistance for the most compactarrangement, if the membranes are not too thin. For the airborne applications,the bandwidth can be further increased by using smaller device dimensions, whichdecreases the impedance mismatch between the cMUT and the air. On the otherhand, this choice leads to degradation in both e±ciency and transmit power dueto lowered radiation resistance. It is shown that by properly choosing the ar-rangement of the thin membranes within an array, it is possible to optimize theradiation resistance. To make a fair analysis, same size arrays are compared. Theoperating frequency and the collapse voltage of the devices are kept constant. Theimprovement in the bandwidth and the transmit power can be as high as threeand one and a half times, respectively. This method may also improve the noise¯gure when cMUTs are used as receivers. A further improvement in the noise¯gure is possible when the cells are clustered and connected to separate receivers.The results are presented as normalized graphs to be used for arbitrary devicedimensions and material properties.