Monolithic MEMS-CMOS ultrasonic rangefinder based on dual-electrode PMUTs

A monolithic MEMS-CMOS rangefinder using piezoelectric micromachined ultrasonic transducers (PMUTs) is presented. A dual-electrode PMUT design is used where the center electrode is driven externally to transmit an ultrasound pulse and the outer ring electrode is used to sense the returning echoes. We designed, fabricated, and characterized devices at two nominal Si thicknesses, 2 μm and 5 μm, designed to operate nominally at 200 kHz and 400 kHz respectively. The devices were fabricated by wafer-scale eutectic bonding of a 0.18 μm CMOS signal processing ASIC to the PMUT array. The ASIC provides each PMUT with a dedicated receiver, resulting in low parasitics and noise. Using the sensor for pulse-echo range finding, the maximum range of a single PMUT was found to be 98 cm round trip at 411 kHz.