Electronic Design

Fingerprint Authentication

Sponsored by Fujitsu


How does fingerprint sensor (authentication) technology work?
Fingerprint authentication is one of many biometric forms of human identification. A fingerprint sensor captures a digital image of a fingerprint pattern, normally at 500dpi (dots per inch) resolution in gray-scale using 8 bits per dot.

First, one fingerprint image (or more) is registered on the device used to perform the authentication (e.g., a personal computer with integrated fingerprint scanner), and a template of the fingerprint is stored on a local database.

When users want to log in, they must have their registered finger scanned again, and a second template is generated. Next, a pattern analysis is performed to determine if there is a match and if the logon should be allowed.

The match is determined using points of interest (minutia) on the fingerprint, such as ridge bifurcations and ridge endings (Fig. 1). If enough of the minutia points and vectors match, the fingerprint is considered a match and the logon is permitted.

What are the basic types of fingerprint sensor technology in use today?
Fingerprint sensors can use optical or capacitive technology. Each of these options suits different implementations.

Like a digital camera, optical technology employs visible light to capture a digital image. It uses a light source to illuminate the finger's surface while a charge-coupled device (CCD) captures the image, which is then converted to a digital signal.

There are two types of capacitive technologies: passive and active. Passive capacitive technology uses a silicon-based structure containing an array of plates that are used to apply a small electrical current and measure the voltage discharge through the finger when it is placed over the sensor.

The plates act as a parallel array of capacitors when the finger is placed on the sensor, and the capacitance at each point (pixel) is measured. Fingerprint ridges have a substantially greater discharge potential than valleys containing air, which may have little or no discharge (Fig. 2a). In either case, the result is an 8-bit high-contrast gray-scale image (Fig. 2b).

How does capacitive fingerprint sensor technology compare to optical technology?
Optical sensor technology creates patterns and images using light rather than current. As such, if the touch surface has the slightest blemish, such as a scratch or stain, the captured image will be affected, potentially resulting in false mismatches. Also, the finger itself must be free of dirt, debris, and scratches. However, optical sensors are less susceptible to electrostatic discharge (ESD).

Because capacitive sensors are inherently CMOS-based, they can provide significant advantages over their optical counterparts, such as reduced cost, part, size, and power requirements. Also, the small form factor allows capacitive sensors to be integrated into some portable devices, whereas the optical solution would be much too large and less durable.

What is the difference between passive and active capacitive fingerprint sensing?
Active capacitive technology is similar to passive, but requires an initial excitation of the epidermal skin layer of the finger by applying a voltage. Next, the output capacitance is measured at each point (pixel), and this analog measurement is converted to digital by an analog-to-digital converter. The initial current draw required for the excitation voltage is typically around twice the current draw required by passive capacitor sensors. Active sensors are also more susceptible to dry or worn minutia that may fail to drive the output amplifier. Also note that passive sensors are capable of producing images regardless of the contact resistance and require significantly less power.

What precautions can be taken to overcome the potential damage of ESD inherent with capacitive fingerprint technology?
Sensors designed with this issue in mind can be made to tolerate ESD by including a robust silicon substrate, plate electronics, and a protective layer providing several kilovolts of ESD tolerance.

What else should designers consider when choosing a fingerprint scanning technology?
Used in a laptop system, integration with the boot BIOS and compatibility with several operating systems (OSs) is a major consideration. With BIOS integration, the authentication can be performed prior to the OS loading.

Since the BIOS is the very first "boot" point, a malicious user could insert a boot device and change the BIOS to look at that boot device first, enabling the user to bypass the OS security in place and potentially access or easily destroy the machine's data. It is also important to be compatible with a variety of OSs to ensure compatibility across several PC platforms and enable hard-drive locking features when appropriate.

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