Ultrasonic fingerprint sensor using a piezoelectric micromachined ultrasonic transducer array integrated with complementary metal oxide semiconductor electronics

This paper presents an ultrasonic fingerprint sensor based on a 24 x 8 array of 22MHz piezoelectric micromachined ultrasonic transducers (PMUTs) with 100 mu m pitch, fully integrated with 180 nm complementary metal oxide semiconductor (CMOS) circuitry through eutectic wafer bonding. Each PMUT is directly bonded to a dedicated CMOS receive amplifier, minimizing electrical parasitics and eliminating the need for through-silicon vias. The array frequency response and vibration mode-shape were characterized using laser Doppler vibrometry and verified via finite element method simulation. The array's acoustic output was measured using a hydrophone to be similar to 14 kPa with a 28V input, in reasonable agreement with predication from analytical calculation. Pulse-echo imaging of a 1D steel grating is demonstrated using electronic scanning of a 20 x 8 sub-array, resulting in 300mV maximum received amplitude and 5: 1 contrast ratio. Because the small size of this array limits the maximum image size, mechanical scanning was used to image a 2D polydimethylsiloxane fingerprint phantom (10mm x 8mm) at a 1.2mm distance from the array. (C) 2015 AIP Publishing LLC.