Thermoelastic Ultrasonic Actuator With Piezoresistive Sensing and Integrated Through-Silicon Vias

This paper presents technology development toward the goal of a micromachined acoustic proximity sensor for real-time cavity monitoring of underwater high-speed super-cavitating vehicles. Low-resistance polysilicon-based through-silicon vias (TSVs) have been integrated with the device to enable backside contacts for drive and sense circuitry. The sensor and vias were fabricated in a complementary-metal-oxide-semiconductor compatible process using deep reactive ion etching, producing a 1-mm-diameter composite diaphragm and 20-μm-diameter high-aspect-ratio TSVs on a silicon-on-insulator wafer. The diaphragm incorporates a central resistive heater for thermoelastic actuation and diffused piezoresistors for sensing acoustic pressure perturbations. Electrical, mechanical, and acoustic characterizations of the device indicate a transmitter source level of 50 dB (ref 20 μPa) at an operating frequency of 60 kHz, a flat-bandwidth receiving sensitivity of 0.98 μV/(V · Pa), a flat frequency response over the measured range of 1-20 kHz, a linear response from 60 to 140 dB, negligible leakage current for the junction-isolated diffused piezoresistors (<; 14 pA at -10 V), low interconnect resistance of 14 Ω, and a minimum detectable pressure of 31.9 dB for a 1-Hz bin centered at 60 kHz, at a bias of 9 V.