CMUTs with improved electrical safety & minimal dielectric surface charging

Capacitive micromachined ultrasound transducers (CMUTs) offer many potential advantages over piezoelectric transducers, but have not yet seen widespread implementation. Possible reasons for this may include key issues of (1) long-term device reliability and (2) electrical safety issues associated with relatively high voltage electrodes on device surfaces which could present an electrical safety hazard to patients. A double SOI CMUT design which addresses both these issues is presented. A 1-D model of dielectric surface charging, which suggests that minimal surface roughness of the dielectric layer can minimize surface charge accumulation is also proposed. Fabricated devices are engineered to minimize dielectric surface roughness. To provide maximum electrical safety to future patients, CMUT devices were engineered with the top membrane serving as a ground electrode. Bottom electrodes are individually-addressable. Our devices were modeled using a finite-element package. The experiment results show excellent agreement with modeled performance. Charge effects were explored by studying deflection hysteresis during snapdown and snapback cycles in the limit of long snapdown durations to simulate maximal dielectric charging conditions.