Enhanced echographic images obtained improving the membrane structural layer of the cMUT probe

Abstract

The validity of cMUT technology for medical imag- ing applications has been proved by many authors. The large bandwidth obtainable with capacitive Micromachined Ultrasonic Transducers (cMUT) leads to improved image resolution if compared to their piezoelectric counterpart. However cMUTs actually show lower sensitivity than piezoelectric transducers, mainly due to their non optimized design and fabrication technol- ogy. Recently, we developed in our laboratories a dual-frequency plasma-enhanced chemical-vapour-deposition (DF-PECVD) pro- cess, improving the electrical and mechanical characteristics of the silicon nitride films that we employed to fabricate cMUTs. The DF-PECVD technique makes it possible to control, with extreme precision and over a wide range of values, the stress of the grown silicon nitride film. The result is an increased cohesion and an increased resistivity of the silicon nitride film. As a consequence, the porosity of the obtained DF-PECVD nitride film is extremely reduced thus contributing to effectively sealing the underlying cavities of the cMUT membranes. The improvement of both mechanical and electrical properties of the cMUT structural layer has led to higher transmission and reception sensitivity. Using this silicon nitride deposition technique, we have fabricated 64-elements, 5 MHz echographic probes featuring 100% bandwidth and higher sensitivity characteristics leading to a remarkable improvement in the quality of the echographic images.