Large-Scale and High-Density pMUT Array Based on Isolated Sol-Gel PZT Membranes for Fingerprint Imaging

Abstract

In this work, we proposed a novel ultrasonic transducer array based on an array of 50 × 50 piezoelectric micromachined ultrasonic transducers (pMUTs). The structure was specially designed for fingerprint imaging application. The pMUTs array were fabricated with isolated piezoelectric lead zirconate titanate (PZT) cells to reduce the crosstalk between adjacent units, and released by deepsilicon etching from the back side. The cell size and pitch of pMUTs were 50 μm and 100 μm, respectively. Layer-by-layer annealing method was used instead of one-time annealing during the fabrication of sol-gel based PZT film. The resonance frequency of the pMUT was about 24.82 MHz which agreed well with simulated 25.02 MHz. Besides, the effective electro-mechanical coupling coefficient (k eff ) and mechanical quality factor (Q factor) of the transducer were 0.1293 and 198, respectively. The equivalent circuit of the transducer was established and analyzed. The fitted admittance circle agreed well with the experimental result. This demonstration of pMUTs array has profound potential for large-scale, high-density, and high-frequency fingerprint imaging. Optical, capacitive, piezoelectric and acoustic mechanisms based sensors have been developed to capture the electronic images of human fingerprints in the last twenty years.1 The capacitive fingerprint sensors are the most widely used in consumer electronics, duo to their high array density and complementary metal-oxide-semiconductor transistor (CMOS) compatibility.2-4 However, the image captured by capacitive fingerprint sensor is two-dimensional, which can be spoofed easily by a printed template. And capacitive fingerprint sensors are extremely sensitive to contamination and moisture on the finger, which may lead to false recognition. Ultrasonic transducer array provides a potential solution to these problems. The acoustic impedances of valleys and ridges on fingerprints are greatly different, and can be easily distinguished by ultrasonic transducer.5 Thus, the ultrasonic pulse-echo imaging technology can be applied to imaging the ridges, valleys of the fingerprints, and even the tissue beneath. The three-dimensional images captured by ultrasonic fingerprint sensor eliminates the risks of failure and spoof.6 Large-scale, high-density pMUTs array based on AlN were used in high resolution fingerprint imaging with small size, high frame-rate and low cost. 7,8 The lead zirconate titanate (PZT) has piezoelectric coefficient that is generally two-orders-of-magnitude higher than AlN, 9-11 which can be used to fabricate fingerprint sensor with better performance. A PZT based fingerprint sensor was reported with single-pixel and mechanicalscanning working mode, which was large size, low frame-rate, and high cost. 12In this report, we demonstrate an ultrasound transducers array for fingerprint imaging based on sol-gel PZT technique. We designed the structures and parameters of the transducer by simulation, and fabricated the arrays of pMUTs based on the sol-gel PZT techniques. 13 The isolat...

Development of a High-Density Piezoelectric Micromachined Ultrasonic Transducer Array Based on Patterned Aluminum Nitride Thin Film

“…The pMUT immersed in water exhibited a lower transmitting sensitivity and resonant frequency than in air due to the dampening effect of the water, which has a greater viscosity 1.0 × 10 −3 kg/m•s than air 1.983 × 10 −5 kg/m•s [42]. The acoustic pressure of the fabricated pMUT array with the high fill factor was estimated as 19.79 kPa, which is comparable to previously reported AlN pMUT sensors developed by Wang et al as 6 kPa [32], Jiang et al as 15 kPa [11], and Liu et al as 1 kPa [20] at their resonant frequency in water, even at relatively low resonant frequencies [9,18,30].…”

“…The five neighboring pMUTs surrounding the activated pMUT in closed packed array exhibit small mechanical amplitude around 2.82 nm (6% of mechanical displacement of the selected pMUT) caused by inevitable acoustic interaction. However, this amplitude level by the crosstalk effect does not seem severe due to the patterned AlN thin film [9]. The acoustic performance of the pMUT was evaluated based on the acoustic pressure of the pMUT array generated by the membrane oscillation, as follows [11]:…”

“…MUT-based systems integrated with complementary metal-oxide semiconductor (CMOS) circuits are suitable for portable devices. For instance, Lu et al developed an integrated MUT array with a CMOS using wafer-level conductive eutectic bonding for short-range pulse-echo imaging [9]. An ultrasound fingerprint sensor consisting of an MUT array and CMOS signal processing electronics was demonstrated by Jiang et al [10].…”

“…Capacitive type fingerprint sensor uses electric arrays to recognize the pattern of fingerprint detected by the changes of capacitance between that of the ridge and the valley with air gap from the conductive sensor surface. However, the capacitive fingerprint sensor hardly distinguishes between capacitance of the finger and water [8,9]. According to the operating principle of MUT, the MUT is divided into two types: A capacitive micromachined ultrasonic transducer (cMUT) and a piezoelectric micromachined ultrasonic transducer (pMUT).…”

Ultrasound Systems for Biometric Recognition

“…An example of PMUT based on PZT can be found in the literature as well [80]. The transducer is composed of an array of 50 × 50 PMUTs and is fabricated by using a sol-gel PZT technique.…”