Optimization of backside structures with wideband reflectivity reduction for a CMUT

When capacitive micromachined ultrasonic transducers (CMUTs) receive echo signals from test objects, some parts of ultrasound energy are converted into electric energy. However, the rest of the signals propagate to the backside of the CMUT cells through the rims. For medical use, those might cause an unnecessary acoustic reverberation, which degrades the quality of diagnostic ultrasound images. In this study, to reduce such reflected ultrasound, optimized CMUT models have been investigated by one-dimensional transmission line simulation and finite element method (FEM) simulation. The simulation results show that, to keep the broadband character of the CMUT, the reflectivity should be reduced over a wide band for a range of practical uses. Especially, below the CMUT resonance frequency, the thickness of the substrate and the acoustic impedance of the backing material are important factors which dominate the reflectivity. On the basis of this result, a suitable backing material based on carbon fiber reinforced plastic (CFRP) is investigated. This material has low acoustic impedance and a low linear expansion coefficient. Resultantly, the effect of reflectivity reduction was experimentally confirmed under optimum assembly conditions, and the experimental results agreed very well with the simulation results.