Spurred by the size and cost advantages of fingerprint sensors and a growing need for security, biometric fingerprint technology is being integrated into an increasing number of devices. With fingerprint sensors being integrated into millions of products that will be shipped globally, it’s important that the sensor system be able to function for any type of finger in any situation. For example, these sensors are found in rugged laptops used on construction sites, mobile phones deployed in humid locales, and door locks installed in cold Northern towns. This means that modern fingerprint sensors need to not only work with all skin types (dry, worn, calloused, or oily), but they also must be durable enough to survive in harsh outdoor environments.

To meet those challenges, AuthenTec Corp. developed TruePrint® technology, which reads below the surface layer of the skin where the fingerprint is formed. Advances in fingerprint sensor coatings help ensure that TruePrint technology can go where no sensor has gone before, allowing engineers to leverage biometric technology that is both easy to use and robust enough to be integrated into a wide range of new products that can function in diverse environmental conditions. 

One key benefit derived from reading below the surface layer of the skin is the ability for the technology to tolerate electrostatic discharge and permit much thicker protective coatings.

The AuthenTec fingerprint sensor is a robust component that can provide designers with flexibility regarding its placement on the product.

In contrast to traditional capacitive sensors, TruePrint sensors are RF-based devices that detect the ridges and valleys in the live layer of skin cells located just beneath the dead cells that make up the skin surface. The RF antenna array measures these subsurface features by generating and detecting linear electric field geometries. This is in contrast to the spherical field geometries generated by simple capacitive sensors that only fringe the very top surface of the skin. Because the TruePrint devices are RF-based, they are also able to penetrate and image through thick layers of protective coatings. The inability of capacitive sensors to do likewise requires them to use only thin coatings, or no coatings at all.

These fingerprint sensor ICs are manufactured by traditional production semiconductor photolithographic techniques. When the wafer fabrication is completed, a final silicon-dioxide layer is deposited on top of the wafer to form a protective coating. Typical protective layers generally range from 0.4 microns to 1 micron in thickness. This protective layer allows the fingerprint sensor chips to be assembled into the final package without damage to the delicate circuitry underneath. After traditional packaging, the sensor IC is fully protected by a thick layer of plastic on top of the chip.

Enlarge this picture A cross section image of the AuthenTec AES1510/AES1610 sensor taken with a Scanning Electron Microscope (SEM). This product is manufactured using a 0.35 micron triple metal process. The grids shown in the SEM photo represent 1 micron spacing in both X and Y dimensions. This cross section shows the 2.4 micron protective sandwich coating.

In the case of fingerprint sensors, the top surface of the package is open to allow physical contact between the finger and the sensing elements. The sensing elements are no longer covered with the package’s protective plastic top, but only with the final protective silicon-dioxide layer. This exposes the sensing array to a host of hazards, many of which can easily damage the sensor.

AuthenTec researchers recognized this potential for sensor damage even prior to the company’s formation, and in 1999, worked with fabrication partners to construct a thicker layer measuring 2.2 microns to 2.4 microns. This layer is formed by both the original protective layer of silicon dioxide and by a tough organic material commonly used to product high RPM ball bearings. This dual coating is three to six times thicker than on competing products and significantly more durable.

On AuthenTec’s newest fingerprint sensors, including the AES1710 for the wireless market, the organic coating is 5.5 times thicker than competing sensors, and the underlying oxide coating is nearly twice as thick. The improved AuthenTec coating thickness delivers demonstrable durability improvements to impact and scratch resistance, creating fingerprint sensors that are able to withstand more than 10 million rubs.

Enlarge this picture The illustration shows how the TruePrint RF-sensing technology penetrates the outer layer of skin and reads the live conductive layer below, allowing the sensor to read past cuts, scrapes, and oil that might otherwise interfere with accurate sensing. The technology also enables the use of tougher coatings to create more durable devices.

This enhanced level of durability gives OEM product designers the flexibility to mount TruePrint sensors on the outside of the device where they can be more easily accessed by the end-user. On a phone, for example, the sensor might be placed on the side, back, or open front surface.

The user interface is always an important consideration for integrating a fingerprint sensor into a product design. For example, the first question should be whether the finger or thumb is most likely to be used for authentication on a given device. When examining these placement issues, product designers should strive for a design that permits one-handed operation, which is usually easier for the end user. Space above and below the sensor must also be carefully planned to create finger guides that ensure an optimal finger swipe by the user.

On the electrical side of things, integration options are numerous. As with any other biometric access component, the fingerprint sensor will require a connection to the host processor, the secured memory, and the printed circuit board. With embedded applications, either an SPI or parallel interface will be required. Software design kits are available to simplify the implementation of biometrics into a device. Such tools have made it easier and faster to integrate biometrics into product design than in the past.

For more information email: info@authentec.com