Fingerprint sensors secure portable systems

The growing need to keep data secure inside laptop computers, personal digital assistants and smart phones has piqued an interest in small, reliable biometric fingerprint sensors. The smaller the portable system, the smaller and less power-hungry the sensor has to be. That has sparked a market for swipe-type sensors, which use a simple linear array of sensor elements that capture fingerprint metrics as the fingertip slides across it. In contrast, larger sensors require that the entire fingertip contact the array, which demands more physical area and thus more power.

Most swipe sensors, like the latest release from Fujitsu Microelectronics, are implemented on silicon and use capacitive sensing. Avoiding the need for silicon, Synaptics Inc. has worked with Validity Sensors Inc. to create a swipe touchpad solution that uses a low-cost Kapton touch sensor and separate ASIC to process the signals.

Fujitsu's MBF320PBT-GE1 delivers a highly integrated system solution that combines eight rows of 256pixels, a full-speed USB 2.0 interface to simplify the host-to-sensor connection, and automatic finger-detect circuits that bring the chip out of a sub-200A standby mode to its active mode, which draws about 12mA from a 3-3.6V supply. But the sensor alone is not enough, said Philip Hopkins, product manager for the Embedded Product Solutions Business Group at Fujitsu.

"This sensor was developed specifically to support the Trusted Computing Platform model for PC, notebook and tablet PC systems," Hopkins said. "Sophisticated algorithms are included that generate unique minutiae templates that correspond to specific fingerprint features."

The sensor captures an image with a single swipe at a resolution of 500dpi. The template generated from the captured data is compared against a database of other templates with low false-acceptance and false-rejection rates. "We have also worked in close collaboration with Phoenix Technologies, which provided its BioTrust ID enrollment application software and the TrustedCore Pre-Boot Authentication (PBA) software development kit," said Hopkins.

Synaptics, the company known for the technology behind the touchwheel control on the initial Apple iPods, has taken a different route with the Validity sensor. Rather than use capacitive sensing on the surface of the finger, the Validity approach uses an RF scheme that measures subsurface ridge-and-valley information. The company claims this scheme is less susceptible to spoofing and thus more reliable than direct capacitive-sensing techniques. The SecurePad is also fully compatible with the leading biometric software applications, such as Softex Inc.'s OmniPass and Phoenix Technologies' Biotrust Windows application with TrustedCore PBA software.

The low-cost Kapton-based sensor in the SecurePad eliminates potential ESD problems that can often damage silicon-based sensors, the company said, further improving overall system reliability. Additionally, the surface of the Validity sensor provides superior resistance to general chemicals, liquids, abrasion and impact—critical factors for notebook use in real-world environments.

Synaptics avoids need for silicon sensor with low-cost Kapton swipe sensor.

Eight rows of 256pixels form Fujitsu's swipe sensor.

The sensor-and-ASIC combination consumes about 65mA when active and about 1mA in shutdown mode. The touchpad consumes about 4mA when generating the RF field. Like the Fujitsu sensor, the touchpad delivers a resolution of about 500dpi and the ASIC provides a full-speed USB 2.0 interface to the host.

However, according to John Peddie, president of John Peddie Research Inc., a market research company, any new technology such as the Kapton-based sensor will have to pass muster and overcome the skepticism of data center managers. That may not be easy, since previous-generation sensors have delivered less-than-stellar results. "Proof of performance will be key to getting design wins," Peddie said.

- Dave Bursky
EE Times