Ultrasonic fingerprint sensor based on a PMUT array bonded to CMOS circuitry

This paper describes a single-chip 591×438 DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500 DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT design and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56 PMUT array, composed of rectangular PMUTs achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 75 micron lateral resolution and 150 micron axial resolution over a 4.6 mm × 3.2 mm image.