Application Analysis of New Fingerprint Identification Sensor

One-touch sensors and scratch-type sensors are two new types of solid-state fingerprint sensors that collect information by changing the capacitance during the touch process. This paper analyzes the working principle and characteristics of the two types of sensors in detail, and introduces the application in Internet security certification, car keyless entry system.

There are currently two types of solid-state fingerprint sensors on the market: the first is a single-touch sensor that requires a finger to make a reliable touch in the fingerprint collection area; the second requires a finger to rub on the sensor surface, and the sensor collects a specific set. The data is then quickly analyzed and certified. These two types of fingerprint sensors will be used more and more widely.

The above two types of sensors work as follows: when the convex portion of the fingerprint is placed on the sensing capacitor pixel electrode, the capacitance is increased, and the data is collected by detecting the increased capacitance. The pixel in the sensor is 45 square microns with a 50 micron spacing and the capacitive pixel array has a resolution slightly higher than 500 dpi. These sensors are based on a standard single-polysilicon three-layer metal CMOS process and are designed using a 0.5 micron process.

The third layer of metal interconnects forms a capacitive pixel layer made of titanium nitride and covered with a layer of silicon nitride to a thickness of only 7,000 angstroms. The sensor formed by the combination of the hard metal electrode and the anti-wear coating layer is very durable and has a service life of many years.

First, fingerprint detection

The human fingerprint consists of closely adjacent embossed lines, and the texture of the fingerprint can be detected by using a standard reference discharge current for each pixel. Each pixel is precharged to a certain reference voltage and then discharged by the reference current. The rate of change of the voltage across the capacitor anode is proportional to the capacitance above it: Iref=C×dv/dt

The pixels under the bumps of the fingerprint (high capacitance) discharge slowly, while the pixels under the recess of the fingerprint (low capacitance) discharge faster. This different discharge rate can be detected by a sample-and-hold (S/H) circuit and converted into an 8-bit output. This detection method is highly sensitive to fingerprint bumps and depressions and can form very good originals. Fingerprint image.

Fingerprint recognition can be performed using sophisticated software algorithms. The software collects the original fingerprint image, digitizes the image information and extracts the detail template, and then tests to determine if the extracted detail template matches the reference template.

Second, the comparison process

The size and cost of a one-touch sensor and a wipe-on sensor are different. The contact sensor is large, usually with an effective contact surface of 15×15mm, which can quickly capture the largest fingerprint or thumb fingerprint. This sensor is easy to use and can transfer the entire fingerprint image quickly with an accuracy of 500 dpi (automatic fingerprint recognition standard).

These sensors are currently designed and used for fingerprinting by US government agencies and police stations. In the near future, it will gradually be used in automotive one-touch keyless entry systems and in emerging national security applications.

This sensor consists of 256 (column) x 300 (row) miniature metal electrodes, each connected to a pair of S/H circuits. The fingerprint images are sequentially acquired row by row, each metal electrode acts as one pole of the capacitor, and the finger in contact with it is the other pole of the capacitor. There is a passivation layer on the surface of the device as a dielectric layer between the two capacitor poles. When a finger is placed on the sensor, the bumps and depressions on the fingerprint produce different capacitance values ​​on the array and constitute a whole image for authentication.

The wiper sensor is a new type of fingerprint acquisition device that requires the user to swipe his finger over the device. The advantage of the wipe type sensor is that it is small in size (such as Fujitsu's MBF300 size is only 3.6 × 13.3 mm2) and low cost. These devices are primarily used in embedded security identification applications for mobile devices such as cell phones and PDAs. Sophisticated image reconstruction software quickly captures multiple images from the sensor at nearly 2,000 frames per second and organizes the data details of each frame together.

Third, information and certification

There is no doubt that portable low-cost fingerprint recognition technology has far-reaching implications for our lives. For example, in the future, the police can intercept a suspect in a high-risk area and ask for fingerprints instead of ID cards or car licenses. The person places the first, second or third finger of his right hand on a sensor connected to the wireless PDA, which quickly confirms the suspect's previous criminal record.

This identification technology is also good for stolen mobile phone users. When the phone is turned on, the user is required to pass a quick authentication process, and the user passes his finger across the sensor, and if authenticated, the user is authorized to use various functions of the mobile phone. If it is not an authorized user, the phone will remain locked. If the authentication fails for several consecutive times, the phone deletes the key information in the memory and then shuts down.

In the voice mail application, when a voice mail number is dialed, the user only needs to swipe his finger across the sensor to make the system recognize. With fingerprint recognition, you don't need to use a mailbox password or personal identification number.
In future automotive applications, users can enter fingerprint samples of family members and can be driven by authentication. The registration process is simple: each authorized driver puts his or her finger on the sensor and sets the car's various parameters to their personal preferences, and then saves these settings in the car's computer memory.

When the driver enters the car, he/she places his finger on the sensor and initiates the identification process. In less than a second, the computer compares the detected fingerprint template with the stored template and establishes a setting that matches the driver. The fingerprint template and matching software are stored in an embedded module inside the car. When the fingerprint match is successful, the car controls the rearview mirror, car seat, wireless base station, and interior air environment according to the programmed internal parameters. In addition, you can control the driving speed. If the driver is only a child of ten years old, the speed is limited to 55 kilometers per hour. The implementation of these features has a lot of usefulness.

Fourth, make mobile Internet access more secure

With the development of semiconductor and software technologies, mobile phones will gradually become a mobile terminal that can access personal and company data anytime and anywhere, so it is necessary to ensure the security of user access to prevent unauthorized access. The fingerprint identification method used by law enforcement agencies only stores data at specific points on the fingerprint instead of the entire image. In contrast, the biological fingerprint scanning system is more effective and reliable.

All the processing of this type of testing is divided into the following steps: first, the acquisition phase, the device collects the finger biological sample; then uses the pre-established mathematical formula or algorithm to extract its unique data from the sample and convert it into A template; the registration authentication program extracts at least seven feature matching points from 30 to 40 feature points of the fingerprint for verification, including texture bifurcation points and termination points constituting a certain fingerprint detail, and is defined as a distance between the feature points .

When registering, the information code is stored as a reference template for future user authentication. When the user enters the system, he/she will swipe his finger across the sensor area and the acquired live scan template is compared to the reference template. The entire process is completed in 1 or 2 seconds.

By comparison, the system determines whether the live scan template contains enough biometric data to match the reference template and determines if the two match. If it does not match, the authentication fails and waits for the next recognition.

The fingerprint detection system has high performance, and the probability of making an incorrect judgment on the effective fingerprint is less than 1%, and the possibility of misjudgement of the invalid fingerprint as a valid fingerprint is almost non-existent, and the probability is less than 0.01%.

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Label: Application Analysis of New Fingerprint Identification Sensors

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