Most people are familiar with the biometric sensors used in offices and other places for checking fingerprints. So far, these fingerprint sensors were flat and sensed only 2D images of the surface of your finger. Now, researchers at the University of California, Davis and Berkeley have an improved ultrasonic 3D fingerprint sensor that measures not only a volumetric image of the ridges on fingers, but also measures the tissues beneath the finger’s skin. That makes it almost impossible to spoof.

Most smartphones now sport a fingerprint sensor to verify the authenticity of its user. Apple first introduced this technology in 2013, when it incorporated the fingerprint scanner in its iPhone 5s. Unless you have just come back from swimming, the sensor was accurate enough, and now, many other smartphones use it.

However, most of these sensors are of the capacitive type, and subject to serious security leaks. For example, you can easily fool it by placing a printed image of your fingerprint on top of the sensor. This is because the sensing is only in two dimensions. That is why the 3D fingerprint sensor is assuming such importance.

Using low-depth ultrasound, Professor David Horsley and his team has now overcome this issue. Ultrasound images the valleys and ridges of the finger’s surface and a part of the tissue under it in three dimensions. The main reasons why portable gadgets manufacturers are interested in this technology is its ultra-compact size and the capability to operate with a supply of only 1.8V.

Inspired by sophisticated medical equipment, the technology for the low-depth ultrasound technology for measuring fingerprints started to come together in 2007. This was when the researchers were working with PMUTs or Piezoelectric Micro-machined Ultrasonic Transducer arrays. Later on, they found this array to be a good fit for sensing fingerprints.

The group built their imager by embedding the PMUT arrays within a chip and integrating it. This technology is similar to the MEMS or micro-electromechanical systems that today’s smartphones already use. Using MEMS is very effective for accelerometers, gyroscopes and microphones.

According to Prof. Horsley, the chip is made from two wafers. One of the wafers contains the ultrasound parts, while the other carries the second circuit to take care of the signal processing. After bonding the two wafers, the MEMs wafer part is shaved off partially to expose the ultrasonic transducers.

The researchers explain that collection of the ultrasonic images follows the same method as that of medical ultrasound. From the chip’s surface, the transducers first emit a pulse of ultrasound and then process the echoes returning from the valleys and ridges on the surface of the finger.

Scanning a finger in 3D makes the mechanism more secure and increases the challenge several folds for those trying to get around it. As the world moves towards mobile payments, such secure systems will assume increasing importance.

When manufactured in high volumes and with modern manufacturing techniques, OEMs can expect the cost of the sensor to dip to very low levels. Apart from making better fingerprint scanners, this technology is likely to find use in personal health monitoring and low-cost ultrasonic medical diagnostics as well.