Increasingly, smartphone developers are adding fingerprint sensors to boost security. Unfortunately, a majority of them rely on a two-dimensional image of a finger’s surface, which isn’t hack-proof and can be manipulated. Thanks to new technology that uses ultrasonic sensors, that may no longer be the case.
These sensors use imaging technology that captures the edges and valleys of the fingerprint's surface, in addition to the tissue underneath. This makes it virtually impossible to get past the security step with a 2D fingerprint image.
The new ultrasonic technology dates back to 2007, when a team at the Berkeley Sensor &
Actuator Center started to research piezoelectric-micromachined ultrasonic transducers (PMUTs). “We developed arrays of PMUTs, along with a custom application-specific integrated circuit and the supporting electronics,” said David A. Horsley, a mechanical and aerospace engineering professor at the University of California, Davis. “Our work was so successful that we spun off Chirp Microsystems, in 2013, to commercialize it.”
Horsley recruited students and teamed up with a company called InvenSense Inc. to push the research forward and fabricate the designs. The group used technology similar to medical ultrasound imaging. They crafted a small ultrasound imager to observe a shallow layer tissue that sits close to the surface of the finger. “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.”
Meanwhile, the ultrasound sensor uses a number of microelectromechanical systems (MEMS) with “highly uniform characteristics,” leading to similar frequency response characteristics. The group relied on existing MEMS technology to create its imager, but tweaked the manufacturing process slightly. The technology plays a key role in smartphone functions such as microphones and directional orientation.
“Our chip is fabricated from two wafers, a MEMS wafer that contains the ultrasound transducers and a CMOS [complementary metal-oxide semiconductor] 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.”
According to the team, ultrasound could be the next big thing in MEMS technology. “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,” said Horsley.
The imager uses a 1.8-volt power supply. “Our ultrasound transducers have high sensitivity and the receiver electronics are located directly beneath the array, which results in low electrical parasitics,” Horsley added. “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.”
In addition to smartphone security applications, Horsley said his technology could have implications for the healthcare industry, including “low-cost ultrasound as a medical diagnostic tool, or for personal health monitoring.”
The group recently published an article, “Ultrasonic Fingerprint Sensor Using a Piezoelectronic Micromachined Ultrasonic Transducer Array Integrated with CMOS Electronics,” in the journal Applied Physics Letters . For more information, visit AIP Publishing’s website .