Fingerprint sensor technology currently used in smartphones like iPhone 6 produces a two-dimensional image of a finger's surface, which can be spoofed easily with a printed image of the fingerprint.
The ultrasonic sensor eliminates that risk by imaging the ridges and valleys of the fingerprint's surface, and the tissue beneath, in three-dimension.
"Using passwords for smartphones was a big security problem, so we anticipated that a biometric solution was ahead," said professor David A. Horsley from the University of California, Davis.
The basic concepts behind the researchers' technology are akin to those of medical ultrasound imaging. They 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."
"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."
To fabricate their imager, the group employed existing micro-electro-mechanical systems (MEMS) technology, which smartphones rely on for such functions as microphones and directional orientation.
They 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.
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", Horsley said.
The new technology was described in the journal Applied Physics Letters.