Ultrasonic fingerprint sensor points to improved smartphone security
Fingerprint sensor technology currently used in smartphones like the iPhone 6 produces a two-dimensional image of a finger’s surface, which can be tricked fairly easily by using a printed image of the fingerprint. The new ultrasonic sensor eliminates that risk by imaging the ridges and valleys of the fingerprint’s surface, and the tissue beneath, in three dimensions.
“Using passwords for smartphones was a big security problem, so we anticipated that a biometric solution was ahead,” explained David A. Horsley, a professor of mechanical and aerospace engineering at the University of California, Davis who is a director of the Berkeley Sensor and Actuator Center, which is located on the campuses of UC Davis and the University of California, Berkeley and is co-directed by Professor Bernhard Boser at UC Berkeley.
“After Apple announced a fingerprint sensor in their new iPhone in 2013, it was inevitable that more would follow," said Horsley.
The technology is described in detail in the journal Applied Physics Letters.
The origins of the new technology began to come together in 2007, when the teams at the Berkeley Sensor and Actuator Center collaborated to initiate research into piezoelectric-micromachined ultrasonic transducers (PMUTs).
“We developed arrays of PMUTs, along with a custom application-specific integrated circuit (ASIC) and the supporting electronics,” explained Horsley. “Our work was so successful that we spun off Chirp Microsystems, in 2013, to commercialize it.” Shortly before then, in 2011, while exploring other uses for their PMUT technology, they quickly realized that fingerprint sensing was an ideal fit.
"Luckily, we recruited a group of exceptional students to realize our vision, as well as partners within the industry – our co-authors at InvenSense Inc. and a few other companies – who funded the work and fabricated our designs," said Horsley.
Next: Basic concepts
The basic concepts behind the researchers’ technology are akin to those of medical ultrasound imaging. The researchers created a tiny ultrasound imager, designed to observe only a shallow layer of tissue near the finger’s surface. “Ultrasound images are collected in the same way that medical ultrasound is conducted,” said Horsley.
“Transducers on the chip’s surface emit a pulse of ultrasound, and these same transducers receive echoes returning from the ridges and valleys of your fingerprint’s surface.”
The basis for the ultrasound sensor is an array of MEMS ultrasound devices with highly uniform characteristics, and therefore very similar frequency response characteristics.
To fabricate the imager, the group employed existing microelectromechanical systems (MEMS) technology, which smartphones rely on for such functions as microphones and directional orientation. The researchers used a modified version of the manufacturing process used to make the MEMS accelerometer and gyroscope found in the iPhone and
many other consumer electronics devices.
“Our chip is fabricated from two wafers – a MEMS wafer that contains the ultrasound transducers and a CMOS wafer that contains the signal processing circuitry,” explained Horsley. “These wafers are bonded together, then the MEMS wafer is ‘thinned’ to expose the ultrasound transducers.”
“Because we were able to use low-cost, high-volume manufacturing processes that produce hundreds of millions of MEMS sensors for consumer electronics each year, our ultrasound chips can be manufactured at an extremely low cost,” suggested Horsley.
Next: Charge pump required
The imager is powered by a 1.8-Volt power supply, using a power-efficient charge pump on their ASIC or application-specific integrated circuit. “Our ultrasound transducers have high sensitivity and the receiver electronics are located directly beneath the array, which results in low electrical parasitics,” noted Horsley.
“Using low-voltage integrated circuits will reduce the cost of our sensor and open up myriad new applications where the cost, size, and power consumption of existing ultrasound sensors are currently prohibitive.”
“Our ultrasonic fingerprint sensors have the ability to measure a three-dimensional, volumetric image of the finger surface and the tissues beneath the surface – making fingerprint sensors more robust and secure.”
Beyond biometrics and information security purposes, the new technology is expected to find many other applications, including “low-cost ultrasound as a medical diagnostic tool or for personal health monitoring,” added Horsley.
The group also made arrays of MEMS ultrasound devices with highly uniform characteristics, which allowed them to verify that the PMUTs have similar frequency response characteristics.
Reference
Ultrasonic Fingerprint Sensor Using a Piezoelectronic Micromachined Ultrasonic Transducer Array Integrated with CMOS Electronics," is authored by Y. Lu, H. Tang, S. Fung, Q. Wang, J.M. Tsai, M. Daneman, B.E. Boser and D.A. Horsley. Published in the journal Applied Physics Letters on June 29, 2015 (DOI: 10.1063/1. 4922915).
Related articles and links:
https://scitation.aip.org/content/aip/journal/apl/106/26/10.1063/1.4922915
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