TFTs Used as Fingerprint Sensors for Smart Cards

Features Nikkei Electronics Asia -- June 2009
TFTs Used as Fingerprint Sensors for Smart Cards
Jun 17, 2009 16:30

Nikkei Electronics Asia

An extremely thin and flexible fingerprint sensor, only 0.2mm thick, has been developed by e-Smart Technologies Inc of the US (Fig 1). Estimates indicate that the volume production price will be no more than US$2.00/unit. The firm commented that conventional thin fingerprint sensors run between US$8.00 and US$20.00 apiece. The new sensors can be mounted on non-contact smart cards with functions including employee identification, credit cards and electronic money, providing a method for implementing user authentication. This would minimize the risk of unauthorized use in the event that a card is lost. The flexible structure also means it could be mounted in soft materials such as passports, opening up a wide range of other possible applications.


New LCD Application
The developed fingerprint sensor consists of a sheet of pressure-sensitive material on top of a thin-film transistor (TFT) made of amorphous or poly-crystalline silicon, with a thin stainless steel substrate. The design detects the ridges in the fingerprint using pressure-sensing technology. e-Smart chief technical officer (CTO) Tamio Saito explains, "It's an application of LCD panel technology. With amorphous silicon TFTs we can volume-produce for no more than US$2.00 apiece."

The dimensions of the sensing elements, equivalent to the pixels in the liquid crystal display (LCD), are 75um square. e-Smart's Saito continues, "We used the largest size we could and still recognize fingerprints, to keep costs down. We hope to reduce size down to about 57um square to be able to recognize children's fingerprints, too." The company is also developing a sensor to judge paper quality, using the same technology.

e-Smart has already commercialized a non-contact smart card with a different type of fingerprint sensor. It has been used in the Ubiquitous Security Service Project currently being trialed in Gwangju and other Korean cities, providing non-contact smart cards for use as resident registration cards*, electronic money, etc. The card uses a capacitance* fingerprint sensor on a glass substrate, manufactured by sensor manufacturer BMF Corp of Japan.

Even in that volume, though, Saito admits that the capacitance sensor still costs between US$8.00 and US$20.00. In search of a thinner, cheaper design, e-Smart and BMF jointly developed this new fingerprint sensor on a flexible substrate. They are now developing a design using a plastic substrate instead of stainless steel.


Wireless Authentication
e-Smart has introduced a number of proprietary technologies in the non-contact smart card mounting the sensor, too (Fig 2; Note 1).

For example, one technology has eliminated the need to mount a battery on the smart card to drive the sensor. Sensor power is instead transmitted via radio during communication between the non-contact smart card and the card reader.

This was not a simple task, explains Saito, because "Under the ISO14443* communications standard for non-contact smart cards, the card can only be supplied with a maximum of 50mW. This is much less than the 120mW needed to drive the fingerprint sensor." They resolved the problem by adopting a high-Q antenna in the card, boosting received power. He says, "We made use of magnetic resonance to draw the card reader magnetic field to the smart card."

The data used to authenticate identity is stored in the card, which also handles comparison processing. "The reference fingerprint data could be stolen if stored in the card reader, but because authentication is handled inside the card itself, this isn't a worry. The data requires only a small amount of memory to store," adds Saito.

The company revealed that a Japanese hospital is very close to adopting the cards as patient identification cards, but Saito believes that electronic money is the most promising application. "This fingerprint sensor makes a safe electronic money card possible, because even if the user drops it, it can't be used by anyone else," he explains. e-Smart is already marketing the idea as electronic money under the name Global Market Exchange (GME).

by Tetsuo Nozawa

* Resident registration card: All citizens are required to carry an ID card in Korea at all times. The owner's fingerprint is clearly visible on the card

* Capacitance method: Capacitors are fabricated in each TFT cell, etc, making it possible to measure fingerprint ridges and valleys based on the strength of external magnetic fields.

Note 1: The card is also provided with wired communication functionality, utilized for reference fingerprint data registration and other tasks.

* ISO14443: A standard for RFID communication, with a working range of about 2cm. Two versions are defined: Type A (equivalent to the MIFARE technology developed by NXP Semiconductors of the Netherlands), and Type B (developed by Motorola of the US, and utilized in Japan's "taspo" authentication card required to purchase cigarettes from a vending machine, etc). e-Smart says their technology will work with both, just by swapping the chip.

* Resident registration card: All citizens are required to carry an ID card in Korea at all times. The owner's fingerprint is clearly visible on the card.

* Capacitance method: Capacitors are fabricated in each TFT cell, etc, making it possible to measure fingerprint ridges and valleys based on the strength of external magnetic fields.

Note 1: The card is also provided with wired communication functionality, utilized for reference fingerprint data registration and other tasks.

* ISO14443: A standard for RFID communication, with a working range of about 2cm. Two versions are defined: Type A (equivalent to the MIFARE technology developed by NXP Semiconductors of the Netherlands), and Type B (developed by Motorola of the US, and utilized in Japan's "taspo" authentication card required to purchase cigarettes from a vending machine, etc). e-Smart says their technology will work with both, just by swapping the chip.

http://techon.nikkeibp.co.jp/article/HONSHI/20090527/170835/