Introduction

The smartphone has become the equivalent of a personal assistant that now has a role in virtually every aspect of people’s lives, ranging from basic voice and data communications to sophisticated applications for banking, navigation and even entertainment. For most of these applications, users want the largest possible display to enjoy a more engaging experience. But given the dependence on their smartphones for personal affairs, users also want the best possible security. And these twin desires are creating some challenges for device manufacturers.

Maximizing the viewing area requires eliminating most of the hardware that is now located on the front of the smartphone. The result is a so-called “infinity display” that eliminates the bezels on the sides, and has only minimal bezels on the top and bottom. With no side bezels, the display is edge-to-edge and can even wrap around to the sides, making it possible to enhance many existing applications and implement some innovative new ones.

Gone will be the physical home button where fingerprint scanners are usually located in smartphones today. For a variety of reasons, fingerprints have become the primary means of biometric authentication, and users will expect that to remain the same for smartphones with infinity displays. It will therefore be necessary to provide fingerprint scanning somehow or somewhere else. The challenges involved and options available are discussed in the first section,指纹扫描技术“后面”Infinity.

One feature that will need to remain on the front of every smartphone is the camera, which is needed for taking selfie photos and conducting video calls. The same camera could also be used for user authentication via facial recognition or iris scanning, and it is expected that these will complement fingerprint scanning as additional biometric factors. Such multi-factor authentication is among the topics discussed in the second section,New Options for Biometric Authentication.

指纹扫描技术“后面”Infinity

Having a large, high-resolution display is essential to creating a quality user experience, but a satisfactory user experience also demands easy, convenient, high levels of security. And this constrains the fingerprint scanner to remain on the front of the device. The reason is: When the device is sitting on a desk with the display visible, only a front-facing scanner can enable the user to simply touch the phone to unlock it. Indeed, one of the biggest user drawbacks to phones with fingerprint scanners on the back is the need to pick up the device to unlock it.

With the top and bottom bezel areas on infinity displays shrinking, potentially to disappear almost entirely, the fingerprint scanner will need to be located within the display area, and there are some significant engineering challenges that will need to be overcome to make that possible. One challenge involves being able to scan reliably through the cover glass at the resolution required to discern the miniscule ridges and valleys in fingerprints. That limit for capacitive technology is around 300-400 microns (0.3-0.4 millimeters). For any greater thickness, capacitive sensing for fingerprint scanning will not work.

我s important to note, however, that while capacitive sensing may not be viable for fingerprint scanning in infinity displays, it is likely to remain the ideal technology for touchscreen navigation in all smartphones, including in those with infinity displays. Capacitive sensing has become the dominant technology for touchscreens because it works well and is inexpensive, especially at the economies of scale achieved via its dominance, and that is expected to remain the case for the foreseeable future.

The only two viable technology choices today for scanning fingerprints through a display’s typical 700 microns (0.7 mm) cover glass are ultrasonic and optical. Ultrasonic scanning can be made quite sensitive, but its high power consumption and high cost will likely make this technology viable only for specialty devices. Optical scanning technology has been used successfully and affordably in commercial and government applications for years, and recent advances have made the hardware small enough to be placed behind the display inside of smartphones. This new use also eliminates what has been a disadvantage for optical technology: the need to provide backlighting, which is obviously not an issue for a scanner located within a display.

我s reasonable to expect that optical fingerprint scanners will become commercially viable initially in a designated area of the display where the photo diode sensors will be able to take advantage of the display itself as the requisite light source. To encourage market acceptance, such a design could be made the equivalent of a virtual home button. Users are already familiar with and quite comfortable using home buttons, so having a virtual one appear on the screen will preserve that positive user experience.

For reasons explained in the next section, it will eventually be desirable to be able to scan fingerprints anywhere in the display. And that presents a far more difficult set of challenges. In fact, the engineering effort required to implement whole-display scanning will be significant. The optical scanning technology will need to interoperate with both the display driver and the touch controller, and such display integration is likely to mimic the evolution of techniques previously used to integrate display and touch functions in stack-up architectures.

Initial implementations of a dedicated scanning area (the virtual home button) will likely have a discrete design with the optical fingerprint sensor being located above or below the display stack-up, which is analogous to discrete touch or on-cell touch technology. Next-generation designs that support scanning anywhere in the display are likely to integrate the sensor directly into the display layers, which is analogous to in-cell touch technology. (More information about on-cell and in-cell display stack-up architectures can be found in a Synaptics white paper about theLatest Advances in Touch and Display Integration for Smartphones and Tablets.)

As the integration evolves in the display stack-up, it will also evolve in the supporting circuitry. The state-of-the-art for today’s smartphones is Touch and Display Driver Integration (TDDI), which is a single IC that handles both touch and display functions for a touchscreen. For infinity displays, TDDI can be expected to coexist initially with a separate fingerprint scanner IC, much like it does today in devices where the scanner is in the home button. Over time, all three functions—touch, display and fingerprint—will likely be integrated into a single IC. Leveraging common display pixel wires in the sensor matrix and integrating all three essential functions in a single IC will reduce costs substantially by simplifying both engineering and manufacturing.

New Options for Biometric Authentication

The use of biometrics for user authentication has become increasingly accepted and popular for both unlocking devices and authorizing transactions. Scanning a finger is far more convenient than creating and remembering “strong” passwords, and has other advantages that enhance the security of mobile devices. Users have learned to like and trust the fingerprint scanners that are now available in virtually all high-end and mid-range smartphones, and expect to be able to continue using this proven method of authentication in any new device they buy.

The ability to scan fingerprints anywhere in the display will enhance security while also enhancing the user experience. No longer constrained by the size of a home button, security will be enhanced with larger and fuller fingerprint images that are more difficult to fake or spoof and, therefore, result in a more reliable match with a lower false reject rate. The larger fingerprint images will also reduce the match latency and the number of touches that may be needed during enrollment. In addition, the ability to scan anywhere will make it possible to perform authentication automatically and continuously as the user interacts with the display throughout a session or during a transaction.

Such background or persistent authentication is what will enhance the user experience. For example, a swipe to unlock the device could automatically identify and authenticate the user. Or a transaction could be authenticated (multiple times) as the user types in the amount. Alternatively, for transactions where it is important to make the authentication process more obvious and, therefore, fully intentional, a virtual button could appear on the screen instructing the user to “Touch Here to Approve.”

Security can be further enhanced by combining multiple forms of biometric identity using a technology like MultiMatch™ from Synaptics. This multi-factor fusion engine combines scores from both fingerprint and facial scans to make device- and application-level authentication virtually foolproof while maintaining a positive user experience. Anti-spoofing and artificial intelligence technologies are used to detect attempted uses of fake fingers and/or photographs intended to fool the biometric matching processes. These foundational technologies are all extensible, which will enable future use of other forms of biometric identity, such as voice recognition and even DNA.

Conclusion

Based on their ability to deliver a more immersive user experience (as evidenced by the excitement around the introduction of the Samsung Galaxy S8), extra long (XL) displays are becoming a key competitive feature in new models of high-end smartphones. And as the technologies involved achieve economies of scale, even basic models can be expected to have infinity displays.

But as also evidenced by user complaints about the Galaxy S8’s fingerprint scanner being on the back, future models with infinity displays can be expected to have the scanner return to the front of the device. This will enhance both the user experience and security by enabling persistent and multi-factor authentication.

While the technology challenges involved are daunting, they are not insurmountable. So just as touch and display integration evolved in stages, expect the additional integration of fingerprint scanning within the infinity touchscreen display to evolve in stages—initially in a designated location and eventually anywhere under the glass.

No vendor is better prepared to make that happen than Synaptics. As a pioneering leader in the human interface for smartphones, tablets and PCs, Synaptics has both the core competencies and intellectual property needed to integrate fingerprint scanning with touchscreen navigation in devices with infinity displays.