Fingerprint sensors flourish as smartphone security tightens

A growing trend in mobile phones is the use of biometric sensors used to identify the user before certain features are unlocked - for security purposes. The most commonly used is the fingerprint sensor, which has been readily adopted by consumers as it’s quick and easy to use and much faster than typing in a passcode every time you want to unlock your phone. The market for fingerprint sensors is growing steadily as this technology begins to filter down from high end devices such as the Apple iPhone 5S, Samsung Galaxy S5 and S6 and Huawei Ascend Mate 7 to mid-level and entry level phones.

“Fingerprints should remain the main biometric embedded in smartphones during the next five years,” said Guillaume Girardin, Technology and Market Analyst for MEMS and Sensors at Yole Développement. Yole estimates that 856 million fingerprint sensors will be produced by 2020, reaching a $3.4bn market (hardware only).

Above: an example of the resolution of Qualcomm’s Sense ID ultrasonic fingerprint sensor.

Capacitive sensing

There are three main technology types for fingerprint sensors on the market today. The oldest is the optical fingerprint sensor, which persists in some applications but has largely been superseded. Optical fingerprint sensors use a CCD camera, effectively an array of light sensitive diodes, to capture an image of the fingerprint placed on the scanner. The finger is illuminated by light from a built-in LED, and the ridges of the fingerprint reflect more light back to the sensor than the valleys do. This creates an image of the fingerprint which can be compared to stored data to see if it’s a match. The trouble is, it’s fairly easy to fake a fingerprint if you have a decent quality photo of it, as the sensor is only looking for light or dark areas.

Capacitive sensors are the type widely used in mobile phones today. Their principle of operation is similar to the way capacitive touch screens work, just on a much smaller scale. A capacitive fingerprint sensor is a chip with an array of conductive electrodes on its surface, which form the pixels of the sensor. Above the pixels is an insulating layer that prevents the finger making direct electrical contact with the electrodes. Instead, proximity of the fingerprint ridges changes the capacitance of the electrodes below them, which can be detected – the valleys cause a different change in capacitance. Circuitry in the lower layers of the chip processes the signals from these electrodes and produces a map of the ridges and valleys that can be matched with stored data. Obviously, the electrodes are tiny as the sensor’s resolution has to be smaller than the features it’s trying to detect.

Above: a prototype smartphone with Qualcomm Sense ID fingerprint sensor built-in.

nlike the optical sensor, capacitive sensors can’t be fooled by pictures - you have to have something a lot more like an actual finger to beat the sensor by producing the required capacitance change. However, there are still many ways the sensors can be hacked, such as through the software. The main issue with these sensors is they don’t like wet fingers, as this has an effect on the finger’s capacitance, and substances such as oil on the fingers can also cause problems.

Being a highly integrated semiconductor IC, capacitive sensors can easily be produced in high volumes and in tiny packaging. For example, Synaptics’ latest fingerprint sensor, the Natural ID FS4304, comes in an ultra-slim format which is just 3.5mm wide. This sensor is intended to be fitted into the side of devices, perhaps even within buttons such as the volume rocker switch, which the company claim is a natural resting place for the user’s fingers anyway.

“Synaptics’ FS4300 family is market proven with over 50 million units shipped into mass production, and with this new ultra-slim format OEMs can differentiate by designing fingerprints into a variety of small spaces,” commented Ritu Favre, Senior Vice President and General Manager for the biometrics product division at Synaptics. “The rapid growth of secure fingerprint authentication across multiple markets, including China and mid to lower end smartphone models, is driving our leading portfolio of solutions.”

Ultrasonic technology

Another completely different technology, which is brand new to the market, uses ultrasonic to build a map of a fingerprint’s ridges and valleys. This type of fingerprint sensor, pioneered by Qualcomm, transmits a pulse of ultrasonic sound which is reflected back to the sensor by the fingerprint.

The data can be used to create a high resolution 3D image of the surface of the finger, which is detailed enough to show not only the location of the ridges and valleys but their relative depth, and even the location of sweat pores in the skin. This level of detail, combined with a secret ‘liveness detection’ feature which verifies that the finger is real and alive, means it’s practically impossible to fake a fingerprint, claim Qualcomm. In fact, Qualcomm’s ultrasonic fingerprint technology was previously used for US government level secure applications before being applied to consumer devices.

Aside from its high resolution, the Qualcomm Sense ID ultrasonic fingerprint sensor can work through plastic phone covers, and through all kinds of phone enclosures, including metal. The LeEco X2 and LeMax Pro smartphones, for example, have a Sense ID sensor behind their metal enclosures. In both cases, the sensor is positioned on the back of the device so it’s convenient to use for frequent authentication such as to unlock the phone. The Sense ID sensor can work through up to 400µm of metal or glass, thicker than for other technologies, offering more possibilities for fancy product design. It can also scan through common contaminants such as water, oil or hand cream.

The Sense ID sensor also features a Snapdragon processor on chip, which means the biometric data gathered doesn’t need to leave the sensor for authentication, so it’s harder to intercept or hack it.

“Mobile devices increasingly store our most valuable and sensitive information, while passwords alone do not provide the protection consumers deserve,” said Raj Talluri, Senior Vice president for product management at Qualcomm. “Snapdragon Sense ID 3D Fingerprint Technology’s unique use of ultrasonic technology revolutionises biometrics from 2D to 3D, allowing for greater accuracy, privacy and stronger authentication.”

As smartphones gain the ability to function as a mobile wallet, with greater than ever access to the user’s personal details, credit card details and more, the demand for device security will continue to rise. Biometric sensors like fingerprint sensors, provided they can be made to work reliably, can provide this higher level of security, while being unobtrusive and easy to use. Combining biometric sensing with another means of authentication such as a passcode in a two-step process, can provide an easy way to step up levels of security and will likely be used to maintain consumer trust in applications where the most security is required.