Experimental evaluation of CMUTs with vented cavities under varying pressure

We propose venting the cavities of CMUTs for environments with extreme pressure variations. The CMUT have zero differential pressure across the plate at any ambient pressure, thus ensuring a stable operating point and preventing mechanical failure. The venting vias are etched through the substrate. We observe two resonances from the vented CMUTs - the mechanical resonance of the plate and an acoustic Helmholtz resonance associated with the cavity and the venting vias. We fabricated a variety of CMUTs varying the plate radius, thickness, gap height and via arrangement to study these two resonances. A pair of CMUTs was characterized in a pitch-catch setup under varying ambient pressure. The CMUTs were successfully able to transmit and receive ultrasound under an ambient pressure of up to 20 bar. As the pressure increases, the plate resonance dominated mode becomes weaker while the Helmholtz resonance dominated mode becomes stronger. The Helmholtz resonance dominated mode maintains its frequency and bandwidth under varying ambient pressure.