"Using passwords for smartphones was a big security problem, so we anticipated that a biometric solution was ahead," said Professor David Horsley at the University of California, Davis.
The device's origins can be tracked back to 2007, when teams at the Berkeley Sensor and Actuator Center started research into piezoelectric-micromachined ultrasonic transducers (PMUTs).
"We developed arrays of PMUTs, along with a custom ASIC and the supporting electronics," Prof Horsley said. "Our work was so successful that we spun off Chirp Microsystems in 2013 to commercialise it."
However, in 2011, while exploring other uses for PMUTs, it was realised that fingerprint sensing was an ideal fit.
The basic concepts are said to be akin to those of medical ultrasound imaging. The team created a tiny ultrasound imager, designed to observe only a shallow layer of tissue near the finger's surface. Prof Horsley said: "Transducers on the chip's surface emit a pulse of ultrasound and receive echoes returning from the ridges and valleys of your fingerprint's surface."
The chip is fabricated from two wafers – a MEMS wafer that contains the ultrasound transducers and a CMOS wafer that contains the signal processing circuitry. "These wafers are bonded together, then the MEMS wafer is 'thinned' to expose the ultrasound transducers."
The imager is powered by a 1.8V power supply, using a charge pump on the ASIC. "Our ultrasound transducers have high sensitivity and the receiver electronics are located directly beneath the array, which results in low electrical parasitics," Prof Horsley noted. "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."
