What is a ToF sensor?

TOF is short for Time of flight, literally translated as Time of Flight, and is one of the mainstream 3D imaging technologies (the other two are structured light and binocular ranging). According to technical routes, the ToF products currently on the market can be divided into two categories: dToF (direct-to-flight, direct-ToF) and iToF (indirect-to-flight, indirect-ToF).

dToF: By directly emitting light pulses to the measuring object, and measuring the time interval between the reflected light pulse and the emitted light pulse, the flight time of the light is obtained, so as to directly calculate the depth of the object to be measured. The depth algorithm is relatively simple and low power consumption , Good anti-interference and other advantages, but the difficulty lies in the realization of SPAD with higher accuracy.

iToF: It detects the phase difference between the reflected modulated light and the emitted modulated light by emitting modulated light of a specific frequency, and measures the time of flight. It has the advantages of relatively simple sensor circuit, low cost, and high resolution, but from a principle point of view, The higher the modulation frequency, the better the ranging accuracy. High modulation frequency means that the corresponding ranging distance cannot be too large, and ambient light will interfere with the circuit. Therefore, the realization of ranging accuracy limits the maximum ranging distance to a certain extent, and the supporting depth algorithm is the main point and difficulty in solving this series of problems.

Where is the competitive focus of ToF sensors?

In the next 1.5 billion US dollars market, who can get the cake? In addition to depending on the market strategy, the most important thing is to have hard goods. Then what is hard goods? Careful friends may have found one or two in the IHS report. That's right, the number of ranging targets, accuracy, and responsiveness are the core indicators for measuring ToF sensors. Space is limited. Today we will discuss with dToF as an example.

As we all know, SPAD is a difficult point in dToF technology, which is also mentioned in the previous part of the article, so why should we introduce SPAD into dToF? This is because dToF needs to detect nanosecond or even picosecond light pulse signals, so the sensitivity to light will be very high. Therefore, at the receiving end, we usually choose SPAD (Single Photon Avalanche Diode) or APD (Avalanche Photodiode) ) This type of sensor is implemented, and the integration of these two sensors is weaker than that of ordinary CMOS image sensors, and their pixel size is generally greater than 10μm, so the resolution is usually poor and the cost is higher. In addition, the circuit design of dToF itself is more complicated and needs to occupy a larger on-chip size. As the core of dTOF technology, SPAD is not only technically difficult, but also complex in production process. Therefore, currently only a few manufacturers in the world have mass production capabilities. Including STMicroelectronics, Sony, Infineon and Sony.

Although STMicroelectronics, Sony, Infineon and Sony, and even some companies, including Samsung, are deploying the ToF industry, in terms of coverage, STMicroelectronics takes the lead. This is also reflected in the shipment volume. Lin Guozhi, Technical Marketing Manager of the Imaging Business Unit of Semiconductor Greater China and South Asia, told Netflix, "ST’s ToF products have shipped 1 billion units at the end of last year. Among them, more than 200 mobile phones are sold in the market. This model uses ST’s ToF solution. Behind this achievement, on the one hand, the accuracy of its products is not affected by the target color and reflectivity, and on the other hand, it benefits from ST’s FlightSense™ product series. Security and richness can meet the requirements of different customers and different applications. For example, the single-zone proximity detection sensor VL53L3CX, which is miniaturized to the extreme, is based on the full-frame FOV field of view, and the distance measurement capability can reach up to 8 meters. Zone or multi-zone detection sensor VL53L1CB, for example, support 15 frames per second 64 zone ranging, or 60 frames per second 4 x 4/16 zone ranging output, FoV field of view can be set in the long-range distance sensor VL53L5CX ."

Write at the end

The rise of a company is inseparable from the needs of the market and customers, and the rise of an industry is inseparable from a mature ecological chain. Focusing on the future, STMicroelectronics, a company that has been doing imaging products since 1986, believes that they have enough ability to meet changes in market demand, such as a hundred-point, a thousand-point, or even ten thousand-point dToF, such as 1 million. Pixel high-resolution iToF. As for the ToF industry, some people think that the AR/VR industry is the energy storage pool of ToF sensors. Once the AR/VR industry is implemented, the ToF sensor industry is bound to explode.

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