Ultra-Low-Voltage High-Efficiency CMUTs with Piston-Structured Plates for Fill Level Sensing

Abstract

Capacitive micromachined ultrasonic transducers (CMUTs) provide manifold advantages over ultrasound transducers based on piezo-electric materials. However, a limited output pressure and a high voltage requirement are still drawbacks to address. We present a CMUT design with small geometrical measures for plate thickness, gap and insulation layer, in order to ensure low-voltage operation < 10 V. Three designs allow studying the bare piston effects for identical plate diameter. The piston-like deflection intends to overcome low output pressures. Moreover, we present an acoustic coupling approach to demonstrate through-wall fill level sensing through a conductive material. Displacement measurements in air via vibrometer are utilized to validate FEM simulations for the designs. The pull-in voltage is determined for the operational point for acoustic characterization in immersion as well as fill level sensing application. The piston design reveals an emitted surface sound pressure of 0.5 MPa and a transmit sensitivity of 24.8 kPa/V - an improvement over the bare plate design by a factor of 11.2 in sound pressure as well as 3.35 for transmit sensitivity, for a 100-ns-pulse with 80% of the pull-in voltage. With acoustic impedance matching via a silicone film stacked with epoxy material, we can improve the transmission into aluminum from 20% to 35%. By that, fill level measurements for 2.3 mm of water up to 610 mm through 4-mm-thick aluminum are demonstrated. The capabilities of an advanced CMUT design for applications other than medical imaging are shown, opening the field for diverse industrial and consumer applications of CMUTs.