Robust capacitive MEMS ultrasonics transducers for liquid immersion

Capacitive diaphragm MEMS ultrasonic transducers are of great interest because they offer wide bandwidth and ready integration into arrays. However, fragility of these transducers is a significant barrier to their application. In this talk, we report on robust transducers which have been fabricated using the MUMPS process. The transducer design has been optimized to minimize stray capacitance between the output node and the substrate. We report the use of a protective silicone layer which protects the transducers from liquid exposure and, to a degree, from mechanical damage. The silicone layer has been applied with high transducer yield without the need for prior closure of the etch holes, and coated transducers survive extended immersion in water. The thickness of the silicone layer must be carefully controlled, however, in order to prevent pulse distortion.