Originally intended solely for securing access to a device and its stored content, fingerprint sensors have evolved into a multi-function touch control capable of much more than just authentication.

Art Stewart, AuthenTec

Mobile Phone Growth Drives Need for Security, Enhanced User Functionality via Fingerprint Sensors
Figure 1. Already designed into millions of today's newest smart phones, AuthenTec's AES1710 provides convenient security and offers multi-function touch control that associates each finger with a unique function such as speed dial, application launch and password replacement.
More than 10 million cell phones today integrate tiny semiconductor-based fingerprint sensors to provide an additional level of security, safeguarding the phone from unauthorized use if the device is lost or stolen. This inexpensive biometric technology helps to secure the phone and its stored information — photos, contact lists, email, text messages, etc. — while also enabling secure mobile payments via Near Field Communication (NFC) technology at hundreds of thousands of retail locations worldwide.

But these high performance semiconductor-based sensors provide much more than just security. Today's sensors are being transformed into multi-function user touch controls that offer cell phone users easy device and file navigation and personalization features via the simple swipe of a finger across the face of the sensor.

Helping to lead this revolution in fingerprint sensor technology is AuthenTec, which is pushing the bounds of fingerprint sensor designs in nearly 9 million of fingerprint-enabled phones by improving fingerprint imaging performance, sensor durability and software that controls a host of user features.

The Evolution of Fingerprint Sensors
Fingerprint sensors started as touch-based sensors which typically used an optical technology to read a fingerprint image in a similar method to a digital camera or a scanner. These sensors were large, expensive, power hungry, and easily affected by finger surface conditions and contaminants. Since large, power intensive sensors were not appropriate for small portable devices, major advancements were made to develop small, low-cost, low power, semiconductor-based sensors designed for use in mobile phones.

According to research firm Frost & Sullivan, the two most prominent characteristics of a silicon-based fingerprint sensor are its small size and low power
Mobile Phone Growth Drives Need for Security, Enhanced User Functionality via Fingerprint Sensors
Figure 2. Fujitsu's new FOMA F906i ultra-thin world phones is the 16th Fujitsu cell phone model to integrate an AuthenTec fingerprint sensor and the first to offer "TrueNav" navigation capability, giving users a full 360º of browser, menu and document navigation via AuthenTec's AES1710 sensor.
consumption. Frost & Sullivan's World Silicon Fingerprint Chip Market report published in 2007 states, "… the small form factor makes the silicon chip fingerprint sensor almost the only way of integrating biometrics technology into electronics devices with space constraints. Similarly, the low power consumption of the silicon fingerprint units enables their integration with battery powered devices such as cell phones and PDAs. The addition of the fingerprint sensing technology does not affect the battery charging type fingerprint sensor (a user slides the finger across the face of the sensor) in a small form factor (5 mm × 12 mm in a typical ball grid array package) that consumes very little power (8 microamps in finger detect mode; 2 milliamps in navigation mode; 23 milliamps in imaging mode). These sensor chips have to be extremely durable to withstand the punishment of outside-the-case mounting on phones where they are subjected to abrasions from keys, coins, pens and other sharp objects found in purses and pockets.

The most advanced silicon-based fingerprint sensors today can also be used for a variety of other convenience, control, and personalization functions, further mitigating the cost of additional cell phone components including mechanical switches. They also offer increased functionality including touchpad input control, personalization options, and convenient security.

Originally intended solely for securing access to a device and its stored content, fingerprint sensors have evolved into a multi-function touch control capable of much more than just user authentication. Security, or more accurately, personal data privacy and convenient but secure access, are still fundamental and necessary functions of fingerprint sensors in mobile devices. However, the value of fingerprint sensors can be significantly increased with the addition of personalization and navigation capabilities.

Personalization options include associative functions such as speed dial, application launch, and website password replacement tied to individual fingers. This allows the device to be customized by the user so that they can access their preferred application with just one touch of a finger. The advanced imaging capabilities of fingerprint sensors can also be leveraged to allow use of the sensor as a touchpad for 360º cursor and menu control, turbo scrolling, or 4-way rocker switch emulation. By enabling touchpad control, the sensor can replace the primary navigation key of a mobile phone, thus adding functionality while also facilitating component minimization and reducing overall cost.

A Multi-function "User Input Control"
It is the sensor's capability to serve as an input control component that has garnered the most interest from handset designers. Among the contributing factors that have led to increased interest in using a biometric sensor as a multi-function touchpad — versatile navigation capabilities, biometric benefits, and component minimization.

Mobile Phone Growth Drives Need for Security, Enhanced User Functionality via Fingerprint Sensors
Figure 3. FOMA F906i with AuthenTec AES1710 sensor.
The range of functions and emulation capabilities offered by the most advanced fingerprint sensors gives handset designers more options for innovative user input control, an important factor in the post-iPhone handset market. A prime example of fingerprint sensors with versatile input control options are AuthenTec's sensors with integrated TrueNav technology. TrueNav leverages AuthenTec sensors' high definition pixel array and subsurface imaging to track small movements of a finger across the sensor and translate that motion into on-screen actions.

Unlike mechanical switches, AuthenTec sensors with TrueNav can be setup to emulate a variety of input control screens via manufacturer-selectable emulation modes. TrueNav has the ability to emulate full 360º cursor control, such as what is available via touch pads or optical mice, but can also be set to mimic limited mechanical movements to act as a 2-, 4-, or 8-way rocker switch.

TrueNav is a combination of silicon navigation engine and application software used for navigation which enables rapid scrolling through long contact lists and emails; touchpad emulation for 360º of cursor control; and single or double tapping to select menu items. This enhanced silicon and software solution reduces the computational burden to the host processor and eliminates the need for navigation buttons or mechanical switches on cell phones, PDAs, MP3 players and other devices where space is extremely limited and touch-based user interfaces are a hot market trend.

In addition to touchpad or rocker switch emulation, AuthenTec sensors with TrueNav also offer a unique turbo-scrolling feature. Turbo scroll allows a user to rapidly scroll through a long list of contacts, lengthy emails, or music play lists without repetitive finger movements. To activate turbo scroll, a user simply slides their finger over the sensor in the desired direction and then holds their finger on the sensor after making that movement. For example, sliding a finger from the top of the sensor to the bottom and then maintaining contact with the sensor will cause a continuous and accelerating scroll through a list of contacts until the user lifts their finger from the sensor.

Saving Money, Enhancing the User Experience
There are multiple advantages of TrueNav to both handset manufacturers and end users. A handset manufacturer will realize cost savings from replacing multiple components with just one sensor while also enabling a more engaging experience for the customer. End users will benefit from a sensor that can be used to easily navigate phone menus, customized to launch specific applications tied to individual fingers, and leveraged to protect their private messages and data through biometric authentication.

From a technical perspective, TrueNav works by using the sensor's imaging pixel array to track a finger as it moves across the sensor's surface, then interprets that finger movement via proprietary ballistic algorithms into an onscreen action that aligns with the sensor's navigation setting (touchpad, rocker switch, turbo scroll, etc.). The sensor is not imaging the full finger or matching a finger during navigation, so it steps into a lower power setting so as not to impact the phone's normal battery recharging cycle.

Other Design Considerations
Handset designers and component engineers are tasked with evaluating components not just on functionality, but also on the technical specifications associated with integration such as connectivity, OS support, and space requirements. As a hardware component, fingerprint sensors must also meet the component engineers' demanding requirements of small size, low power, and hardware compatibility. To meet these needs, the most advanced type of fingerprint sensors today use a narrow imaging array of only a few pixels in height and read the fingerprint as the finger is 'slid' across the sensor. By utilizing a slide format, the sensors can be made much smaller, use less power, and offer significant cost advantages over larger touch-based sensors.

Slide sensors can be as narrow as 12 mm in width and as thin as a single millimeter. These same small form factor sensors also offer very low power consumption with the sensor operating at less than 8µA during its most common state (finger detection mode). As an imaging device, most fingerprint sensors offer both SPI and Parallel connectivity to the host processor and will support commonly used operating systems such as Symbian, Windows Mobile, REX, Linux, and others.

The Next Evolution: Sensor Packaging
In the competitive mobile handset market, product improvements and innovation are requirements for sustainable success. This is especially true for input components such as fingerprint sensors which are featured as a highly visible external component of the handset. As an external sensor, these small semiconductor chips must be designed for factors not required by any other chip or sensor in the phone — customizable aesthetics and resilience due to outside-of-the-case mounting.

Due to the importance of aesthetics and durability, significant amounts of R&D have been dedicated to new and innovative fingerprint sensor packaging. On the leading edge of this innovation is AuthenTec, which recently announced a new packaging technology called TouchStone. Most semiconductor-based fingerprint sensors require an integrated circuit die to be embedded in a rigid plastic package and connected to a PCB via a Ball Grid Array (BGA) package.

TouchStone packaging embeds the silicon die into a flat piece of flexible tape, providing an ultra thin profile and an extra durable surface coating that is uniform while offering various aesthetic options. The end of the tape package, or tail, includes a ZIF connector or other sealed epoxy alternative for interfacing with the PCB. This tail can be several centimeters long to allow for integration into device locations that would not be possible with standard BGA/PCB configurations.

Additional innovations to fingerprint sensors are expected in the future. As with all semiconductor-based components, reductions in size, power and cost will continue, but the most exciting advancements will come from revolutionary new packaging options and significant improvements in overall sensor functionality such as advanced input control and biometric personalization.

DD Art Stewart is the vice president of Wireless Products for AuthenTec, to which he brings 25 years of experience in the semiconductor and communication industries.