All-polymer ultrasonic transducer design for an intravascular ultrasonography application

Intravascular ultrasonography (IVUS), a medical imaging modality, is used to obtain cross-sectional viewsof blood vessels from inside. In IVUS, transducers are brought to the proximity of the imaging targets so that highresolution images can be obtained at high frequency without much concern of signal attenuation. To eliminate mechanicalrotation rendered in conventional IVUS, it is proposed to manufacture a transducer array on a flexible substrate andwrap it around a cylindrical frame. The transducer of consideration is a capacitive micromachined ultrasonic transducer(CMUT). The whole device needs to be made out of polymers to be able to endure a high degree of bending (radius: 1mm). Bending of the devices leads to considerable changes in the device characteristics, including resonant frequency andpull-in voltage due to geometrical dimension changes and stress induced. The main purpose of this work is to understandthe effect of bending on the device characteristics by means of finite element analysis. Another objective of the work isto understand the relationships between such an effect and the device geometries. It is learned that the bending-inducedstress depends strongly on anchor width, membrane thickness, and substrate thickness. It is also learned that resonantfrequency and pull-in voltage become lower in most cases because of using a flexible substrate in comparison to thoseof the device on a rigid substrate. Bending-induced stress increases the spring constant and hence increases resonantfrequency and pull-in voltage, although this effect is relatively weaker. For most of the device geometries, pull-in voltageis too high for the polymer material to endure. This is the main drawback of the all-polymer CMUT. In order to meetthe design goal of 20 MHz resonant frequency, the membrane radius has to be smaller than 7.7 µm for a thickness of 3 µm.

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