Piezoelectric Micromachined Ultrasonic Transducers for Fingerprint Sensing

Fingerprint identification is the most prevalent biometric technology due to its uniqueness, universality and convenience. Over the past two decades, a variety of physical mechanisms have been exploited to capture an electronic image of a human fingerprint. Among these, capacitive fingerprint sensors are the ones most widely used in consumer electronics because they are fabricated using conventional complementary metal oxide semiconductor (CMOS) integrated circuit technology. However, capacitive fingerprint sensors are extremely sensitive to finger contamination and moisture. This thesis will introduce an ultrasonic fingerprint sensor using a PMUT array, which offers a potential solution to this problem. In addition, it has the potential to increase security, as it allows images to be collected at various depths beneath the epidermis, providing images of the sub-surface dermis layer and blood vessels.
Firstly, PMUTs are analyzed using FEM and analytical method to obtain maximized PMUT sensitivity by optimizing the layer stack and electrode design, and the increased coupling coefficient via series transduction. Moreover, a broadband PMUT with 97% fractional bandwidth is achieved by utilizing a thinner structure excited at two adjacent mechanical vibration modes with overlapping bandwidth. In addition, we proposed waveguide PMUTs, which function to direct acoustic waves, confine acoustic energy, and provide mechanical protection for the PMUT array. 
Furthermore, PMUT arrays were fabricated with different processes to form the membrane, including front-side etching with a patterned sacrificial layer, front-side etching with additional anchor, cavity SOI wafers and eutectic bonding. Additionally, eutectic bonding allows the PMUT to be integrated with CMOS circuits. PMUTs were characterized in the mechanical, electrical and acoustic domains. Using transmit beam forming, a narrow acoustic beam was achieved, and high-resolution (sub-100 μm) and short-range (~1 mm) pulse-echo ultrasonic imaging was demonstrated using a steel phantom.
Finally, a novel ultrasonic fingerprint sensor was demonstrated using a 24×8 array of 22 MHz PMUTs with 100 μm pitch, fully integrated with 180 nm 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. Pulse-echo imaging of a 1D steel grating is demonstrated using electronic scanning of a 20×8 sub-array, resulting in 300 mV 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 PDMS fingerprint phantom (10 mm by 8 mm) at a 1.2 mm distance from the array.
Nesrin Sarigul-Klijn
Tingrui Pan
Publication date: 
June 30, 2015
Publication type: 
Ph.D. Dissertation
Lu, Y. (2015). Piezoelectric Micromachined Ultrasonic Transducers for Fingerprint Sensing. United States: University of California, Davis.

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