At present, there are four main types of fingerprint image acquisition technologies used in the market: optical scanning equipment, temperature-sensitive fingerprint sensor, semiconductor fingerprint sensor and ultrasonic fingerprint scanning.
I. Optical identification technology
Fingerprint collection by optical technology is the oldest and most widely used technology. Put a finger on an optical lens, and project it on a charge coupled device (CCD) with a prism under the illumination of a built-in light source, so as to form a digital multi-gray fingerprint image with ridges (lines with a certain width and direction in the fingerprint image) in black and valleys (concave parts between lines) in white, which can be processed by a fingerprint device algorithm.
Optical fingerprint collection technology has obvious advantages: it has been tested for a long time, adapted to the variation of temperature to a certain extent, and can reach a higher resolution of 500DPI, and most importantly, it is low in price. There are also obvious disadvantages: because the optical path is required to be long enough, the size is required to be large enough, and excessively dry and greasy fingers will also make the effect of optical fingerprint products worse.
The limitation of optical fingerprint sensing lies in the potential fingerprint (the potential fingerprint is left after the finger is pressed on the platen), which will not only reduce the quality of fingerprint image, but also lead to the overlap of two fingerprints in serious cases, which obviously cannot meet the needs of practical application. In addition, the coating on the platen and the CCD array will wear out over time, which may lead to the degradation of the quality of the collected fingerprint image. However, it has some disadvantages, such as unable to identify living fingerprints and poor applicability to wet and dry fingers.
Optical fingerprint identification system can only scan the surface of finger skin or the dead skin layer, but it can’t penetrate into the dermis layer. In this case, the cleanliness of the finger surface directly affects the recognition effect. If there is a lot of dust on the user’s finger, there may be a recognition error. Moreover, if people make a fingerprint hand model according to their fingers, it may also pass through the recognition system, which is not very safe and stable for users to use.
Second, the temperature difference induction identification technology
The temperature difference induction recognition technology is based on the principle of temperature induction. Each pixel is equivalent to a miniaturized charge sensor, which is used to sense the temperature difference at a certain point between the finger and the chip image area and generate an electrical signal representing the image information.
Its advantage is that the fingerprint image can be obtained within 0.1s, and the sensor has the smallest volume and area, that is, the sliding fingerprint reader usually uses this technology at present. Disadvantages: Limited by temperature, the finger and the chip will be at the same temperature after a long time.
Three, semiconductor silicon induction technology (capacitive identification technology)
In the late 1990s, the technology based on the capacitance effect of semiconductor silicon became mature. The silicon sensor becomes one plate of the capacitor, and the finger is the other plate. By using the capacitance difference between the ridges and valleys of the fingerprint line relative to the smooth silicon sensor, an 8-8bit gray image is formed.
The capacitance sensor sends out an electronic signal, which will pass through the surface of the finger and the dead skin layer, and directly reach the living layer (dermis layer) of the finger skin, so as to directly read the fingerprint pattern. Because it goes deep into the dermis, the sensor can capture more real data, and it is not easily affected by the dust on the finger surface, which improves the identification accuracy and effectively prevents identification errors. Semiconductor fingerprint sensor includes semiconductor pressure sensor, semiconductor temperature sensor, etc. Among them, semiconductor capacitive fingerprint sensor is the most widely used.
The semiconductor capacitance sensor judges which position is the ridge and which position is the valley according to the different capacitance values formed by the ridges and valleys of the fingerprint and the semiconductor capacitance sensing particles. Its working process is to pre-charge the capacitive sensing particles on each pixel to a certain reference voltage.
When the finger touches the fingerprint of semiconductor capacitor, because the ridge is convex and the valley is concave, different capacitance values will be formed at the ridge and valley according to the relationship between capacitance value and distance. And then discharge by using a discharge current. Because the capacitance values corresponding to ridges and valleys are different, the discharge speed is also different.
The pixel under the ridge (high capacitance) discharges slowly, while the pixel under the valley (low capacitance) discharges quickly. According to the different discharge rates, the positions of ridges and valleys can be detected, thus forming fingerprint image data.
Different from optical devices which mostly use manual adjustment to improve the image quality, capacitive sensors use automatic control technology to adjust the sensitivity of fingerprint image pixels and local range of fingerprints, and combine feedback information to generate high-quality images in different environments. Due to the ability of local adjustment, even images with poor contrast (such as areas where fingers are pressed lightly) can be effectively detected, and the sensitivity of these pixels can be improved at the moment of capture to generate high-quality fingerprint images.
Semiconductor capacitive fingerprint sensor has the advantages of good image quality, generally no distortion, small size and easy integration in various devices. The electronic signal will pass through the surface of the finger and the dead skin layer, reach the living layer (dermis layer) of the finger skin, and directly read the fingerprint pattern, thus greatly improving the security of the system.
The most important advantage of semiconductor silicon sensing technology is that it can achieve living fingerprint recognition. Better image quality than optical technology can also be obtained on a smaller surface, and the resolution of 200-300 lines can be obtained on a surface of 1cm×1.5cm (smaller surface also leads to cost reduction and can be integrated into smaller equipment). Because of its small size, low cost, high imaging accuracy and low power consumption, it is very suitable for security and high-end consumer electronic products, and is called the second generation fingerprint identification technology after optics.
Fourth, ultrasonic identification technology
Ultrasonic fingerprint collection is a new technology, and its principle is that ultrasonic wave has the ability to penetrate materials, and echoes with different sizes are generated with different materials (when ultrasonic waves reach the surfaces of different materials, they are absorbed, penetrated and reflected to different degrees). Therefore, the location of fingerprint ridges and valleys can be distinguished by the difference of impedance between skin and air to sound waves.
The ultrasonic frequency used in ultrasonic technology is 1×104Hz-1×109Hz, and the energy is controlled to the extent that it is harmless to human body (the same as the intensity of medical diagnosis). Ultrasonic technology products can achieve the best accuracy, which requires less cleanliness of fingers and planes, but its acquisition time will be significantly longer than the above two types of products, and it is expensive, and it can not achieve living fingerprint recognition, so it is rarely used at present.
Micro-optical fingerprint identification technology
At present, both the mainstream-capacitive fingerprint sensor and the emerging-ultrasonic fingerprint sensor need additional fingerprint sensors, but the former needs to open holes on the screen and the latter can be hidden under the screen.
In 2017, Apple and Huiding Technology introduced fingerprint identification technology based on micro-optical sensing technology, which skillfully integrates the display screen and fingerprint sensor, and supports fingerprint identification in multiple places on the whole screen.
The working principle of fingerprint identification sensor "begins" with optical type, "fills" with capacitive type, and "continues" with ultrasonic type and micro-optical type. Turn a circle, and now back to the path of optical sensing, the difference is that today’s micro-optical sensing technology is advanced enough to cause a full "face" innovation of smart phones. What is the principle of micro optical fingerprint sensor? Huiding Technology is different from the technology adopted by Apple. Huiding Technology has not disclosed the details yet, but we can learn about the MicroLED technology adopted by Apple.
Before 2012, MicroLED technology was still in the laboratory research and development stage. With Apple’s acquisition of LuxVue, which owns many patented technologies of MicroLED in May 2014, the market’s attention to this technology has reached an unprecedented height. MicroLED technology, namely LED miniaturization and matrix technology. It refers to a high-density micro-sized LED array integrated on a chip. Each pixel of the display screen can be addressed and individually driven to light, which reduces the pixel distance from millimeter to micron.
MicroLED display is to thin, miniaturize and array the LED structure design, and its size is only about 1~10μm, and then transfer the micro-LEDs to the circuit substrate in batches, which can be a hard and soft transparent and opaque substrate; Then the physical deposition process is used to complete the protective layer and the upper electrode, and the upper substrate can be packaged to complete the MicroLED display with simple structure.
According to Apple’s US Patent No.9570002 "Interactive Display Panel with Integrated Infrared Diodes" published by the US Patent and Trademark Office on February 14th, 2017, the touch display screen with MicroLED sensing technology and how to realize the fingerprint identification technology function are displayed.
Specifically, a single infrared emitting tube is connected with a sensing diode as a driver, and a selection circuit is used to create a sub-pixel circuit. Because of their small size, these infrared diodes can be embedded in the display substrate with RGB LED, or installed on a microchip and then integrated into the substrate.
During the fingerprint identification operation, a certain area or a few lines of the screen integrated with interactive pixels (so-called "interactive pixels", which can integrate red, green, blue, infrared light emitting diodes, infrared detectors and other color arrays on the panel with very high resolution) can scan the fingerprint information of the user.
When the distance reaches enough to be sensed, a bitmap will be generated and the approximate positioning data will be notified to the system. In some cases, bitmaps include incident light intensity information, allowing deep analysis of objects and their surfaces. For example, by checking the dark points and bright points of the bitmap, the sample system can detect the corresponding ridges and wrinkles in the user’s fingerprint. So as to realize the function of fingerprint identification.
Fingerprint identification technology is the mainstream technology of biometrics and identity authentication, and it has been widely used in all walks of life. At present, devices with fingerprint function must have a physical key for you to scan a fingerprint, but with the development of touch and display chips, it is highly integrated, and the future development trend is that we can identify fingerprints on the screen at any position. Embedded fingerprint identification appears at the core, so any restrictions do not affect the industrial design of equipment manufacturers. With the integration of fingerprint identification and touch display technology becoming more and more mature, it will be more and more widely used, such as fast payment, identification, personalized preference customization, etc., and it is likely that passwords will be replaced by fingerprint identification technology.