Fujitsu shrinks fingerprint sensor for portable apps

Fujitsu Microelectronics America said it has developed a tiny fingerprint sensor that is about one-tenth the size and less than half the cost of conventional fingerprint sensors.

Users of the MBF300 SweepSensor are required to slide the tip of a finger across a narrow sensor region rather than press down on a finger sensor pad. Software then reconstructs an image of the fingerprint, identifies its unique markers and decides whether it matches the fingerprint templates stored in a system's memory.

The SweepSensor is intended for use in portable gear such as personal digital assistants and cellular phones. To make it small enough for portable applications, Fujitsu Microelectronics cut the sensor area down to 4.3-by-14mm, or about one-tenth the size of the company's current fingerprint sensors. The entire SweepSensor is 1.2mm high.

Advanced packaging and a flexible connector allow the sensor to be embedded onto the side of a PDA or into the base of a cellular phone. "This device can easily fit into most cell phone platforms," said Douglas McArthur, director of fingerprint sensor development at Fujitsu.

Fujitsu also aims to slash the cost of fingerprint sensors by offering samples for $13.50 and $10.50 apiece in quantities of 1,000, starting this month. Its larger fingerprint sensors, which capture the image of a fingerprint using a pressing rather than sweeping motion, are priced between $25 and $30.

"We're attacking the two big issues related to fingerprint sensor technology—cost and size," McArthur said.

To generate interest in fingerprint biometric sensors, Fujitsu is working with software vendors to make the technology compatible with various CPUs and operating systems. The company also hopes to create partnerships with some chip and software vendors that are defining PDA and cellular phone platforms, such as Intel, Microsoft, Motorola, Palm and Symbian, McArthur said.

Like its current line of fingerprint sensors, the MBF300 is based on sensor and software technology Fujitsu licensed from Bell Labs spin-off Veridicom. The CMOS-based sensor is made up of an array of capacitor cells that creates an image of a fingerprint by detecting differences in discharge rates as they encounter ridges and valleys on the skin.

The software captures the fingerprint image at several intervals and then reconstructs the image. In this case, there are 32 rows of capacitors to detect the finger at each interval as it sweeps the sensor, but McArthur said there is software available that can capture as few as eight rows at a time.

Keeping the number of rows to a minimum helps to keep the sensor's size small and reduce power consumption. The MBF300 consumes 100mW of operating current and 50?W during standby.

Embedded algorithms work to identify fingerprint minutiae, defined as points where the ridges terminate or split off. Typically, the software identifies 10 to 20 minutiae points that are compared against a database of "enrollment" sweeps the user made prior to usage. The sensor produces an image at 500dpi resolution, which meets FBI standards, McArthur said.

The sensor's typical false accept rate is once every 1,000 sweeps, which is considered medium-grade when compared with other competing biometric sensors. Iris scanners have lower false accept rates, but integrating them into portable applications is considered impractical. Voice recognition has started to appear as a way to identify users, but it is sensitive to ambient noise and changes in a person's voice due to illness, McArthur said.

The MBF300's sensitivity can be adjusted depending on the number of user "enrollments," or initial sweeps that serve as templates stored in memory. As user enrollments increase, the chance that a false accept will occur decreases, but it's more likely that the sensor will fail to identify a valid user. To minimize the number of false rejects, users must learn how to correctly sweep their finger across the sensor.

"We think the learning curve is simple enough that it will be adopted by users," McArthur said.

Fujitsu hopes to create new markets for these fingerprint sensors by making them smaller and cheaper, but it isn't the only company that has developed a sweep sensor.

Atmel also based here, has a CMOS-based device with a sensing area of 0.4-by-14mm that is based on a thermal sensing technique. Atmel claims its FingerChip sensor works well with poor-quality fingerprints that are dry and have shallow valley depths and are resistant to ESD.

Compared with thermal sensors, Fujitsu's capacitor-based MBF300 sensors don't have temperature limitations and use more advanced packages, McArthur said. The sensor is housed in either a 54-pin FPBGA or FLGA package.

Fujitsu will eventually migrate the technology to 0.25?m design rules, at which point it can integrate functions such as a CPU, matching engines and encrypted links to a host CPU, according to the company.

— Anthony Cataldo

EE Times