Grooved Backing Structure for CMUTs

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) manufactured on silicon substrates need an acoustic backing to suppress substrate ringing when such transducers are in operation. The acoustic backing most often used for ultrasound transducers is a composite of epoxy and tungsten powder. To absorb the acoustic energy, the backing of a CMUT should have an acoustic impedance that matches that of the silicon substrate and it should be lossy. If the backing is thick enough, it will absorb the acoustic wave in the backing without reflecting it back to the transducer, and thus will not create any trailing echoes. However, if we intend to use the transducer in applications in which there is no room for a thick backing, for example in intravascular ultrasound (IVUS), a grooved backing structure might be used. The grooves at the bottom of the backing provide extra attenuation by scattering the waves in different directions so that a thinner backing is sufficient. The scattering removes power from the specular reflection from the back surface which otherwise degrades the image quality. It has been shown that this type of structure reduces the specular reflection for a range of frequencies. When CMUTs are used in practical applications, the propagation of waves from a fluid medium into the backing or vice versa is blocked to some degree by total reflection, except for a range of steering angles around broadside. This is due to the difference in acoustic velocities of silicon and the fluid medium. This blocking is accompanied by the generation of surface waves in the silicon substrate, which also may impact the imaging and therefore must be controlled. In this paper, we investigate the acoustic signal transmitted into the backing relative to the signal transmitted into the fluid medium when CMUT arrays on top of the silicon substrate are excited. Furthermore, the performance of the grooved backing structure is studied for the waves traveling in normal as well as in oblique directions to the bottom surface of the backing.