Measurement of the added specular reflection attenuation by using a grooved bottom surface in the backing of CMUTs

The backing structure mostly used in Capacitive Micro machined Ultrasonic Transducers (CMUTs) is a composite of epoxy and tungsten powder. To be able to absorb the acoustic signals, it should have high acoustic impedance that matches with the silicon substrate (on which CMUTs are manufactured) and it should be lossy. If we are able to make this structure thick enough, then it will damp out the signal in the backing so that it would not reflect back to the transducer. But if we intend to use the transducer in applications where there is no room for the thick backing, for example in IVUS (Intra vascular Ultrasound), a groove structured backing could be used. The grooves give extra attenuation by scattering the waves in other directions so that a thinner backing would be enough. The scattering removes power from the specular reflection from the back surface. This reflection is otherwise harmful for the imaging. In this paper, we will present how to make such a structure. Moreover, we will present some experimental results to show that this type of structure reduces the specular reflection and compare the obtained results with theoretical calculations. It is shown that the desired reflection level for a medical imaging application, estimated to be about 20 dB for CMUTs at the top of the backing, is obtained over a wide range of frequencies around 20 MHz using an absorber thickness of 200 μm.