With the advent of the full-screen era, the technical application of under-screen fingerprints has become a hot spot in the industry. Due to the limitations of capacitive solutions, optical and ultrasonic solutions have become the mainstream of development.
Among them, the ultrasonic solution uses the echo strength to identify fingerprints, which has the advantages of oil and water resistance and strong penetration: the ultrasonic solution uses the ultrasonic waves of a specific frequency sent by the fingerprint module to scan the finger, because the ultrasonic waves are absorbed and penetrated when they reach the surface of different materials. Unlike the degree of reflection, the difference in acoustic impedance between skin and air or different skin layers can be used to identify the location of the ridge and ridge of the fingerprint. The advantage of the ultrasound solution is that it has stronger penetrability, can perform deep subcutaneous fingerprint recognition and can identify living bodies, so the security of the solution is higher; in addition, the ultrasonic solution is not susceptible to the interference of oil and water stains and strong light, thus unlocking More stable and reliable, has become an important direction for the development of fingerprint identification solutions.
A few days ago, Shanghai Sili Microelectronics has made breakthrough progress in the research and development of MEMS ultrasound technology which lasted for two years. Its self-developed ultrasonic transducer has passed a series of performance tests, with a conversion efficiency of up to 1.5% at 10MHz, showing excellent performance, and has been applied to the development of next-generation ultrasonic fingerprint recognition chips.
The piezoelectric ultrasonic transducer (PMUT) used by Silimicro uses the piezoelectric effect of aluminum nitride to convert between electrical and mechanical energy to detect the valley ridge information of the finger's epidermis and dermis to make accurate judgments .
Aluminum nitride is a very stable piezoelectric material with two important characteristics: inverse piezoelectric effect and piezoelectric effect. The inverse piezoelectric effect means that when a voltage is applied to both ends of the piezoelectric material, the internal deformation of the piezoelectric material will be generated. The deformation is proportional to the voltage. This is the process of converting electrical energy into mechanical energy; the piezoelectric effect refers to the piezoelectric material When deformation occurs under the action of force, the center of positive and negative charges inside the piezoelectric material is relatively displaced, so that the two ends of the piezoelectric material produce bound charges with opposite signs. The amount of charge is proportional to the pressure. This is the process of converting mechanical energy into electrical energy. .
Different from other SOI multi-silicon wafer processes, Silimicro’s piezoelectric ultrasonic transducer creates a single wafer etching structure and process, which is mainly composed of suspended transducer films, including bottom electrode, piezoelectric layer, top electrode and Elastic layer. Using the inverse piezoelectric effect of the piezoelectric material, as long as a fixed frequency voltage is applied to the bottom and top electrodes on the upper and lower sides of the piezoelectric material film, the film will vibrate and generate sound waves. Conversely, when sound waves reach the transducing film, the film is deformed, and the two ends of the piezoelectric layer will generate positive and negative auxiliary charges, and the peripheral circuit can collect the generated electrical signals through the top electrode and the bottom electrode.
When performing fingerprint recognition applications, an AC voltage is applied to the ultrasonic transducer, the ultrasonic transducer generates vibration, and the vibration is transmitted upward, that is, the ultrasonic wave is transmitted upward, passing through different media layers (screen, glass, etc.) to reach the valley or ridge of the finger. When the sound wave encounters the surface of the ridge, it is partially reflected and partially transmitted. Because the acoustic impedance of the air in the valley is much higher than that of the ridge, the sound wave is almost totally reflected when it encounters the valley. When the different sound wave energy reflected from the valleys and ridges is transmitted to the corresponding ultrasonic transducer surface, the corresponding ultrasonic transducer will generate different electrical signals (amplitude, frequency, phase, etc.).
Compared with other fingerprint recognition technology solutions, ultrasonic technology can realize 3D fingerprint recognition and has higher security. Acoustic focusing is used to focus the sound wave on the surface of the finger. If it encounters the ridge of the finger, part of the sound wave is reflected back, and the rest of the sound wave penetrates into the skin. This part of the sound wave is reflected back again after encountering the dermis; After that, two signals with different time will be reflected in the epidermis and dermis; and in the valley, the sound wave is only reflected once, and it is a total reflection signal. The ultrasonic fingerprint recognition program uses this method to collect the fingerprint signals of the epidermis and dermis to obtain 3D fingerprint information.
The PMUT transducer successfully developed by Silimicro adopts AIN material, based on CMOS process, and original MEMS structure, which is simple in structure and convenient for process realization. Silimicro has already carried out the research and development of under-screen ultrasonic fingerprint recognition based on this PMUT transducer, and it is expected to reach the mass production level in early 2019.
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