Fingerprint Sensor Integration - Challenges & Solutions

The incorporation of biometrics into devices is becoming a prerequisite especially in products using multi-user authentication or in devices that run confidential user information like mobile banking and payment. One of the most popular authentication methods is fingerprint authentication is and one that users associate with convenience. Fingerprint sensor technologies can be divided into two main categories: capacitive and optical, explains Dr. Rouzet Agaiby, who will be speaking at Sensor Expo 2018.

Capacitive and Optical Fingerprint Sensors

Historically, capacitive fingerprint sensors have been more prevalent, mostly in mobile phones where they take the form of a home button on the front screen or a button on the back of the phone. Such capacitive sensors are rigid and are made of silicon.

Optical sensors, on the other hand, require complex and bulky optics which makes their integration into thin, compact devices like mobile phones more difficult. They are common in border control units which allow for a bulkier construction.

Thanks to the improved signal-to-noise ratio in optical fingerprint sensors compared with capacitive sensors, there has been much innovation aimed at solving the challenge of integrating them into products. Two key technical challenges have been the focus of the industry:

1. making the optical fingerprint sensor module compact, thin and easy to integrate and
2. the ability to place it under displays (for example in smartphones) to enhance the user experience.

Challenges and Solutions

At first, these challenges were being solved using traditional CMOS optical fingerprint sensors, but new technologies have emerged which enable an optical fingerprint sensor to be made using thin film transistors (using a manufacturing process like that used for displays). Such technologies offer the ability to develop large area sensors similar in size to the display but without the significant increase in cost as is the case with CMOS-based sensors.

As the Internet of Things expands, devices will be expected not only to securely authenticate their users but also to have attractive designs. Many consumer electronics feature curved and non-rectangular surfaces - considered more natural and appealing – creating a need for biometric modules that can conform to and wrap around surfaces while still being thin enough not to significantly increase the device thickness. Flexible sensors make this possible.

As flexibility, thinness and the ability to scale to large areas cost-effectively become a necessity, the thin film transistor technologies suitable for optical fingerprint sensors become limited. One such technology is organic thin film transistors (OTFT) on plastic: by combining OTFT with organic photodiodes (OPD), an organic optical sensor is created suitable for fingerprint imaging. The thickness of such a sensor is only 300 microns, reduced to 175 microns by employing a thinner plastic substrate. Such a thin flexible sensor can bend around a car steering wheel, curve around the edge of a phone or tablet or can be placed under an OLED display in any application by employing the correct optical stack between the sensor and the display, without adding thickness to the full module.

The key innovation is then the development of the compact optical stack that maintains the signal-to-noise ratio of the sensor and allows seamless integration into the product. This is typically a pin-hole matrix structure engineered with the display and cover-glass thickness in mind, as well as the transparency of the OLED display and the light generated by the sensor. The end goal is a display-enabled device that can authenticate its users seamlessly and enhance their user experience.

This information has been sourced, reviewed and adapted from materials provided by Sensors Expo & Conference.

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