When Capacitive Micromachined Ultrasonic Transducers (CMUTs) are coupled with water, they show high front-face acoustic reflectivity, due to the impedance mismatch between the transducer substrate material, typically based on silicon, and the propagation medium. During pulse-echo operation, surface reflectivity is responsible for multiple reflections of the received acoustic signals, which result in a set of unwanted echoes. In ultrasound imaging applications, this signal reverberation creates artifacts and reduces the image contrast. In this paper, a method to reduce front-face reflectivity is proposed, and a Reverberation Level (RL) index is introduced in order to quantify the unwanted reverberation of the signal returned to the transducer surface. The proposed method combines the increase of the bias voltage, the application of an optimized resistive load and the addition of a low-impedance acoustic backing to CMUTs realized by Reverse Fabrication Process (RFP). In this way, the mechanical energy conversion and transmission to the backing, as well as the electrical energy dissipation, are improved, thus reducing the energy reflection into the medium. The proposed method is analyzed by means of Finite Element simulations and is experimentally validated by characterizing single-element RFP-CMUTs, provided with different backing materials and electrical loads. In the analyzed prototypes, a RL reduction of 8.6dB is obtained.
Keywords: CMUT; Reflectivity; Reverberation; Ultrasound imaging.
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