Effect of Acoustic Properties of Lens Materials on Performance of Capacitive Micromachined Ultrasonic Transducers

The acoustic lens in capacitive micromachined ultrasonic transducers (cMUTs) is pivotal not only for elevation focusing but also for insulation from high DC bias voltages. The selection of the lens material is crucial for realizing the full potential of a cMUT; in particular, the acoustic impedance, frequency-dependent attenuation, and shear velocity of the lens materials should be closely considered during the material selection process. By using the finite element method, we determined the effects of these acoustic properties on cMUT performance in terms of signal strength, center frequency, and spectral bandwidth. From the simulation results, it was found that acoustic impedance does not considerably influence cMUT performance if the acoustic impedance value ranges from 1.3 to 1.6 MRayl, which is similar to that of human tissue. However, both frequency-dependent attenuation and shear velocity cause a downshift in center frequency, a reduction in bandwidth, and a decrease in signal strength, which are the main factors leading to deterioration in the spatial resolution and signal-to-noise ratio of an ultrasound image.