Minimizing the bottom reflection in Ultrasonic CMUT Transducer backing using low profile structuring

Capacitive Micro-machined Ultrasonic Transducer (CMUT) transducers need an acoustic backing to ensure that any acoustic signal which propagates from the transducer into the substrate is absorbed in the backing. The backing should be made such that it does not give a false echo in the signal received by or transmitted from the transducer. Ideally, this backing material should provide high attenuation and it should match the acoustic impedance of the CMUT supporting structure (usually silicon). To avoid the echoes described above, a thick backing layer is required. But in many cases, there is little space available under the transducer so that it is difficult to accommodate a sufficiently thick layer of material with realistic propagation losses, to ensure that no signal is reflected back to the transducer. In this paper, we discuss irregular structures at the bottom surface that are used to scatter the waves. The proposed structure scatters the waves into waves with significantly changed propagation directions, giving long propagation paths back to the transducer. The structure is analyzed using FEM simulations for a simple 2D case. Different ways of implementing the structure is also discussed.