Seamless integration of fingerprint sensors into mobile phones
This is a guest post by Dr. Rouzet Agaiby, Senior Business Development Engineer at FlexEnable.
The adoption of biometrics in smartphones started with the introduction of fingerprint recognition technology in Apple’s iPhone 5 in 2013. Soon after many other smartphone OEMs started integrating fingerprint sensors, and more recently facial recognition, either as multi-modal or standalone user authentication solutions.
Today, fingerprint sensors remain the most commonly used biometric authentication technology in mobile phones, and their integration continues to be a hot topic within the industry because of the need to meet users’ expectations while solving technical challenges. On one hand, smartphone users have preference for larger and bezel-less displays. On the other hand, there is a need for implementing better security methods that are also convenient for the user.
Since the introduction of fingerprint sensors to smartphones they have been sharing space with the display on the front side of the phone. This integration requires the protective cover glass of the display, which is usually 600-700 microns thick, to have holes engineered into it to accommodate the sensor, which adds cost and complexity. The hole is needed because capacitive fingerprint sensors cannot operate effectively under a cover glass thickness of more than 400 microns.
Trends for fingerprint sensor integration
With new phone designs, consumers expect decreasing display bezel size (more display area), which in turn causes a reduction of the front-surface sensor size or totally relegating the sensor to the back or side of the phone. Reducing the sensor size may lead to a lower quality of the fingerprint captured and may compromise the user authentication. Smartphone OEMs are therefore exploring ways to break this constraint by integrating the fingerprint sensor into the display. Some of the concepts under development include adding an optical or ultrasonic sensor behind the display component, placing a semi-transparent sensor in front of the display or embedding the sensor pixels between the display pixels. Each of these integration methods has its own challenges and therefore has not been implemented into products yet.
Design challenges of fingerprint sensor integration into displays
While the integration of fingerprint sensors into displays could enable more elegant mobile phone designs, it makes the design more complex and hence more expensive. This implementation will be more challenging as manufacturers need to assemble more components without increasing the thickness and weight of the phone. Moreover, it may lead to compromises with the sensor performance such as reduced speed of fingerprint detection, lower image quality (if the sensor is embedded under various layers of components) or increased power consumption needed to process lower quality images. In some cases there will be trade-offs between the sensor resolution and the display resolution. It might also not be intuitive for a user to not see a fingerprint sensor yet use the display for authentication.
New ways of fingerprint sensor integration
There are other ways of integrating a large area fingerprint sensor in a mobile phone without compromising the display area or adding complexity to the design. The back cover or side of the phone are considered dead areas on the phone. A thin flexible sensor can be integrated on the back cover or the side of the phone thereby activating that surface and making it more intuitive for the user to authenticate themselves with a single hand while holding the phone or wrapping their fingers around it. In such cases, the sensor is a standalone component that can be optimized to work with the back cover or side of the phone. Additionally, the size of the sensor can be defined by the designer to accommodate the camera and the buttons. Such implementation will also not be dependent on the display technology used.
Flexible, wrappable sensors have the potential to solve integration design challenges, enabling brands to develop smartphone designs that are attractive to users while also being technologically viable, secure and user-friendly.
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