According to MEMS Consulting, from the initial provision of ambient light and proximity sensors for smartphones and tablets, to the development of heart rate sensors for wearable devices, to the introduction of the "semi-customized" innovative business model of optical sensors, Epticore Microelectronics (Jiangsu) Co., Ltd. (hereinafter referred to as "Epticore") has always maintained the original intention of optical sensor solutions and a keen sense of the consumer electronics market since its establishment. It has become the first optical sensor company in the Chinese Mainland by virtue of its super core competitiveness and increasingly rich product lines. In the "24th" Meaningful "Seminar: 3D Visual Technology and Application" hosted by MEMS Consulting, Bizhou Lo, CEO of Epticore, brought a wonderful sharing about the miniaturization scheme of ToF products.
Epticore is one of the earliest ToF enterprises in China. Since 2015, it has started the research and development of ToF sensors in the collision sparks with its partners. Mr. Bizhou Li introduced at the beginning, "Currently, most of the module products are tens of millimetres in size, which is very limited for the end product design. It is a mainstream development trend to significantly reduce the size of ToF module products. We have successfully completed the millimetre-level ToF miniaturization program to provide the smallest ToF module products in the market at present. "
Epticore 's "two-part strategy" to achieve millimeter-level ToF miniaturization solutions
Step 1: Complete miniaturization of the ToF sensor chip
Mr. Bizhou Li introduced "ToF sensor adopts the principle of active light-emitting detection, which is consistent with the principle of proximity sensor we have made before. However, the linearity of the ToF sensor is very good, and it is not affected by the colour, so it has more performance advantages than ordinary proximity sensors. The current priority is to reduce the size of the ToF chip."
In the opinion of Mr. Bizhou Li, reducing the size of ToF chip can be done from various aspects. One of the important directions is to reduce the size of the emitter side, that is, the VCSEL chip. However, it is difficult to make the transmitter and receiver into a very small module. After comparison and verification, Epticore came up with the following two unique solutions:
"CCD + CMOS" array ToF technology solution:
In the ToF technology solution, the problem of large and complex off-chip power supply system can be achieved through the use of on-chip circuitry, and there are a number of solutions for this step. Mr. Bizhou Li analyzed that "some companies will adopt CCD specific process solutions, but the overall cost is high, and there are fewer foundries can process. CMOS is relatively low-cost, and OEM resources are rich. We decided to adopt "CCD + CMOS" array ToF technology solutions. It can effectively solve the triple problem of cost, size and sensitivity. In other words, the CMOS process is used to complete the ADC/PLL/Interface/DSP/Memory and other functions, combined with a customized CCD ToF back-end process to achieve a high-sensitivity pixel design, and the precise depth (distance) information is obtained through tens of thousands of integrations. This is equivalent to integrating two different ICs into one chip, and is the first step we take to achieve ToF miniaturization starting with the chip first."
Reduced pixel design.
Next is the pixel problem, Epticore has made in-depth research on this problem. After more than three years of research and development, it has reduced the pixel size to 17.5μm. It will also develop 10μm or even 5μm ToF pixel in the future. The selection of packaging technology is also very knowledgeable. Mr. Bizhou Li explained: "TSV package can effectively avoid the problem of wire bonding area, which is very competitive in the industry."
Step 2: Realize the miniaturization of ToF module
In Mr. Bizhou Li's view, to achieve ToF module miniaturization, on the one hand, we must focus on the packaging technology to solve the problems of heat dissipation, electrical interference, optical circuit design calibration and detection. And on the other hand, we must cooperate with efficient algorithms to finally win the battle of 3D ToF module miniaturization. “The core part is the lens. The precision of the material used for one-time injection moulding is not as high as that of glass. It will be very difficult to process. The smaller the size, the higher the requirement for accuracy. We have other way to reduce the size of the ToF module. The higher the resolution, the larger the chip size will be. Our current approach is to combine the 3D ToF sensor with the 2D Camera next to it to achieve accurate live detection through algorithms. We have also made a lot of efforts in adopting algorithm-assisted miniaturization, with the goal of reducing the size, which can eventually be applied to mobile phones." Mr. Li Bizhou also showed the audience that the Epticore product can capture the face image formed by 1000 ToF depth pixels through efficient algorithms (see the figure below), which can be completely applied to live detection.
When talking about the role of VCSEL in the miniaturization of ToF products, Mr. Bizhou Li thinks that "The current VCSEL still uses wire bonding to connect with the ToF sensor. The larger the transmission power of the VCSEL, the larger the size. In order to realize the miniaturization of the ToF module, VCSEL manufacturers still have a lot to do. First, improve the luminous efficiency. Second, solve the heat dissipation and efficiency problems of the drive circuit located under the VCSEL. These are the core issues related to the quality of the ToF module. I believe that many companies have made efforts in this regard. The VCSEL suppliers we work with, and today's speakers will also work in this direction. "
The opportunity and limit of ToF miniaturization coexist, and mass production is in sight
After sharing the results and experience of ToF miniaturization, Mr. Li concluded, "OEMs of consumer products such as mobile phones, and AR glasses often talk about the size of ToF modules directly. ToF modules can only be used on smartphones if they are small enough and the cost is acceptable. The ToF module size offered by Epticore is already the basis for a wide range of applications in mobile phones. At present, Sony has already made the first breakthrough in high-resolution 3D ToF modules, and ST's single-dot ToF modules have been widely commercialized. We are trying to build a medium-scale resolution, the world's smallest 3D ToF module, which is also an adventurous attempt for us. But we have done it and achieved results. It is estimated that mass production will be reached in 2019. "