What are the Different Types of Fingerprint Sensors?

Catalog

Ⅰ Introduction

In this modern era, it is increasingly difficult to secure physical location and digital data. The thief can pick the lock or enter by force. Hackers can steal passwords or perform various other intrusions.

Biometric data is gradually becoming a key that cannot be lost or a password that cannot be copied. Biometric technologies such as facial recognition and retinal patterns are good tools, but fingerprint recognition is the most common tool. Smartphones and other wearable devices use fingerprint sensors to improve the security of the devices, and fingerprint sensors are becoming more and more popular among people to achieve highly secure access to buildings or facilities.

A fingerprint sensor is a key device to realize automatic fingerprint collection. Fingerprint sensors are classified into optical fingerprint sensors, semiconductor capacitive sensors, semiconductor thermal sensors, semiconductor pressure-sensitive sensors, ultrasonic sensors, and radiofrequency RF sensors according to the sensing principle, that is, the principle and technology of fingerprint imaging. The manufacturing technology of fingerprint sensors is high-tech with strong comprehensiveness, high technical complexity, and a difficult manufacturing process. The earliest fingerprint recognition technology is an optical recognition system based on optical sensors. The recognition range is limited to the surface layer of the skin. It is usually called the first generation fingerprint recognition technology. The second-generation fingerprint recognition system uses capacitive sensor technology, realizing the conversion of the recognition range from the epidermis to the dermis, which greatly improves the accuracy of recognition and the security of the system. It is also the basis of most fingerprint recognition devices on the market.

Ⅱ Optical fingerprint sensor

The optical sensor is the earliest and most widely used fingerprint sensor. The key to its technology is the total reflection of light. The finger is placed on the filming table (usually hard plastic, different materials from different manufacturers). When it is irradiated on the glass surface with fingerprints, the reflected light passes through the charge-coupled device (CCD) is converted into the corresponding electrical signal and transmitted to the back end for further processing. Among them, the intensity of reflected light depends on two factors: the depth of the ridges and valleys of fingerprints pressed on the glass surface, and the oil and moisture between the skin and the glass. Since the light irradiates the valley area through the glass, it is totally reflected to the CCD at the interface between the glass and the air, and the light directed to the ridge is absorbed by the contact surface of the ridge and the glass or diffusely reflected to other places. So the CCD can be used to convert the fingerprint image composed of dark ridges and light valleys into a digital signal. Of course, in order to obtain a higher-quality fingerprint image, the brightness of the image must be adjusted automatically or manually.

Optical fingerprint collector

The optical fingerprint sensor mainly uses the principle of light refraction and reflection. The light is emitted from the bottom to the prism and is emitted through the prism. The angle of the reflected light on the uneven lines of the fingerprint on the finger surface and the reflected light will be different. The CMOS or CCD optical device will collect the image information of different brightness and darkness, and then complete the fingerprint collection.

The optical scanner takes a digital photo of the fingerprint in the scanner under strong light. By seeing the ridges and valleys of the fingerprint, a digital image is made from a photosensitive microchip, and the digital image data is converted into a binary code. The reference code on the scanner is compared with the code of the finger to determine whether it can be entered. However, digital images can be artificially copied and input into the sensor, although this rarely happens.

The advantages of the optical fingerprint sensor are mainly antistatic ability, good system stability, and long service life. It can also provide images with a resolution of 500dpi (dot per inch), especially for fingerprint image collection in a larger area. When the image capture area is larger, the required focal length is also longer, and the size of the capture device must be increased accordingly, otherwise, the edge line of the captured image will be distorted.

The limitations of optical fingerprint sensing are reflected in potential fingerprints (potential fingerprints are left after the finger is pressed on the platen), which will not only reduce the quality of the fingerprint image but in severe cases, it may also cause two fingerprints to overlap, which is difficult to meet the actual requirements.

The optical fingerprint recognition system cannot penetrate the surface of the skin (dead skin layer). It can only scan the surface of the finger skin, or scan the dead skin layer, but cannot penetrate deep into the dermis. In this case, the cleanliness of the finger surface directly affects the recognition effect. If a lot of dust is stuck on the user's finger, there may be an error in recognition. Moreover, people make a fingerprint hand model according to their fingers, or through the identification system, which is not very safe for users to use.

Low cost has always been regarded as the biggest advantage of optical sensors, but because of the consistency of its manufacturing process, the cost advantage of optical sensors is no longer obvious with the large-scale development of semiconductor sensors represented by capacitive sensors. Although most companies are still using optical sensors, the development trend is novel, high-quality capacitive fingerprint sensing.

Ⅲ Semiconductor fingerprint sensor

Semiconductor fingerprint sensors mainly use the principles of capacitance, electric field, temperature, and pressure to collect fingerprint images. It has the advantages of low price, small size, and high recognition rate, but it lacks a little stability. It is often used for mobile phones, computers, cars, or house security identification.

This type of sensor, whether it is capacitive or inductive, has a similar principle. On a "plate" integrated with thousands of semiconductor devices, a finger is attached to it to form the other side of the capacitance (inductance). The surface of the finger is uneven, the actual distance between the convex point and the concave point contacting the plate is different, and the formed capacitance/inductance value is also different. The device will collect the different values according to this principle, and the fingerprint collection is completed.

The manufacturing processes of semiconductor fingerprint sensors are complex and there are many sensing units on a unit area. More than 10,000 semiconductor sensor units are integrated on the surface of a wafer less than 0.5 square centimeters. It also includes an automatic gain circuit and logic control chip, as well as serial, parallel, USB, and other interface circuits. The sensitivity of the semiconductor fingerprint sensor is high, and the resolution has reached 500dpi or above. Its function has broken through the single sensing capability, and with software cooperation, it can be used as an omnidirectional navigator. Semiconductor fingerprint sensors are developing towards miniaturization.

1 Capacitive fingerprint sensor

Capacitive semiconductor fingerprint sensor

Most smartphones rely on capacitive fingerprint sensor technology. Just like the capacitive touch screen, the capacitive fingerprint sensor generates an electrostatic field by measuring the conductivity of the finger and generates a digital image based on the electrostatic field.

It is composed of an array of capacitors and contains approximately 10,000 miniaturized capacitors. When the user places his finger on the front, the skin forms a plate of the capacitor array, and the back of the capacitor array is an insulating plate. Since the distances between the ridges and valleys of the fingerprints in different regions are not equal, the capacitance of each unit changes accordingly, thereby obtaining a fingerprint image.

The tiny capacitor array circuit in the capacitive fingerprint sensor helps to track the details of the fingerprint. The charge stored in the capacitor is changed by a conductive plate using fingerprint ridges. The valley bottom of the fingerprint is filled with non-conductive air, which determines the conductivity of the finger in the sensor. The change is tracked by the operational amplifier integrator, and then the digital data is recorded and analyzed with an analog-to-digital converter.

Schematic diagram of two fingerprint sensor

The advantages of capacitive fingerprint sensors are good image quality, no distortion, small size, and easy to integrate into various devices. The electronic signal sent out will pass through the surface of the finger and the dead skin layer, and reach the living body layer (dermis layer) of the finger skin, and directly read the fingerprint pattern. With the continuous development of fingerprint recognition technology, capacitive sensors with high quality, low power consumption, and small size, as extremely important fingerprint image acquisition methods for portable products, are increasingly widely used.

2 Thermosensitive fingerprint sensor

The temperature difference sensitive fingerprint sensor is made based on the principle of temperature sensing. Each unit sensor represents a pixel, and the entire integrated fingerprint sensor is placed under constant temperature control (the temperature is slightly lower than the body temperature). The temperature of the fingerprint sensor is controlled below +33℃, the temperature of the ridge point on the fingerprint represents the body temperature, and the temperature of the valley point on the fingerprint is the ambient temperature. When the finger is placed on the fingerprint sensor, the temperature difference between the ridge point and the sensor is not equal to the temperature difference between the valley point and the sensor, and the fingerprint image can be obtained by scanning. The scanning rate of this kind of sensor is very fast, and the fingerprint image must be obtained in a short time (generally less than 0.1s). Because of a long time, the finger and the chip are at the same temperature.

Ⅳ Ultrasonic fingerprint sensor

The working principle of ultrasonic fingerprint technology is that the fingerprint recognition chip emits ultrasonic pulses to sample the fingerprint, and the sensor obtains the three-dimensional characteristics of the fingerprint by analyzing the reflected pulse. Ultrasonic pulses can penetrate glass and sapphire screens for scanning, so the Home button of traditional fingerprint recognition can be directly integrated into the mobile phone screen, which will further improve the mobile phone interaction design space.

Mobile phone ultrasonic fingerprint sensor

Ultrasonic fingerprint recognition sensor can not only scan the surface of the fingerprint but also scan the depth of the gully and the underlying tissue, which greatly improves the security.

Ultrasonic fingerprint collection is a new type of technology. Its principle is to use the ability of ultrasonic waves to penetrate materials and generate echoes of different sizes depending on the material (when ultrasonic waves reach the surface of different materials, the degree of absorption, penetration, and reflection is different). Specifically, the transducer on the surface of the fingerprint recognition chip can emit an ultrasonic pulse, which can conduct deeper analysis and sampling of the fingerprint and can even penetrate under the skin surface to identify the unique 3D characteristics of the fingerprint. The surface of the finger will be reflected, and the sensor can use the reflected pulse to obtain the three-dimensional information of the fingerprint. Therefore, by using the difference in acoustic impedance between the skin and the air, the fingerprint ridge and the ridge can be distinguished.

The ultrasonic frequency used by the ultrasonic technology is 1×104Hz-1×109Hz, and the energy is controlled to the extent that it does not damage the human body (the same intensity as the medical diagnosis).

Ultrasonic technology products can achieve the best accuracy. They have low requirements for the cleanliness of fingers and planes, but their collection time will be significantly longer than the aforementioned two types of products, and they are expensive and cannot achieve live fingerprint recognition.

The ultrasonic sensor realizes the mutual conversion of sound energy and electric energy, which refers to the energy conversion that converts the sound signal of a certain vibration frequency in the ultrasonic range into a detectable electrical signal of the corresponding frequency. Or IT converts the voltage that determines the alternating frequency into the sound of the corresponding frequency. Traditionally, ultrasonic sensors made of piezoelectric materials mainly use the piezoelectric effect of materials to achieve energy conversion.

Pressing the capacitive sensor produces a two-dimensional fingerprint image, while the ultrasonic scanning can perform a deeper analysis and sampling of the fingerprint, and can even penetrate under the skin surface to identify the fingerprint's unique 3D features, including fingerprint ridges and sweat pores. This can produce fingerprint surface maps that are rich in details and difficult to imitate.