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CMOS sensor
Dec 15, 2017

CMOS sensor

CMOS (Complementary Metal-Oxide-Semiconductor), the Chinese name is complementary metal oxide semiconductor, which is an important chip in the computer system, which preserves the most basic information of system guidance. The manufacturing technology of CMOS and general computer chip have what difference, mainly using semiconductor silicon and germanium, these two elements made, the coexisting with N in CMOS (with electricity) and P (+ power level) semiconductor, current can produce these two complementary effects are processed chip record and interpretation of image. Later, it was found that CMOS can also be used as image sensor in digital photography. CMOS sensor can also be subdivided into passive pixel sensor (Passive Pixel Sensor CMOS) and active pixel sensor (Active Pixel Sensor CMOS).

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One of the prerequisites for the extensive application of the latest CMOS sensors is the higher sensitivity, shorter exposure time and the shrinking pixel size. A measure of pixel sensitivity is the product of the fill factor (the ratio of the photosensitive area to the whole pixel area) and the quantum efficiency (the number of electrons generated by the photons bombarded on the screen). CCD sensors have a large filling factor because of the inherent characteristics of their technology. In the CMOS image sensor, in order to achieve comparable CCD converter noise index and sensitivity level, people on the active pixel sensor for CMOS image sensor assembly (APS), and to reduce the fill factor, the reason is the pixel surface a considerable part of the area was occupied by the amplifier transistor, the available space for small photoelectric diode. Therefore, an important development goal of today's CMOS sensors is to expand the filling factor. Sepp Lars (FillFactory), through its patent licensing technology, can greatly improve the fill factor. This technology can transform the largest part of a standard CMOS silicon chip into a light-sensitive area. With the pixel size is smaller, the more difficult to improve the fill factor, the most popular technology is from the traditional photographic (FSI Front, Side Illumination) to back (BSI, Back Side type photosensitive Illumination), amplifier transistor and interconnect circuit is placed in front of the back, leaving all the photodiode, thus the fill factor of 100% (as shown in the right diagram).

In addition, for a typical industrial image sensor, because many scenes are under the condition of poor lighting, it will be very useful to have a large dynamic range. CMOS image sensor operation to achieve this goal through the multi slope conversion curve is composed of linear part of different inclination, which together form a nonlinear characteristic curve.  Therefore, the dark part of a scene may occupy a large part of the conversion range of the integrated analog-to-digital converter: the conversion characteristic curve is the steepest in here, so as to achieve high sensitivity and contrast. The flatness of the upper part of the feature curve will capture several orders of overexposure in the bright part of the image, and show them in a more detailed scale. The use of multiple slopes to operate the LUPA-4000 will match the optical dynamic range of up to 90dB with a 10 bit A/D conversion range.

The IM-001 series CMOS image sensors with VGA resolution are further developed on this basis; they are designed for automotive applications. Its pixels are composed of photodiodes and can provide an adaptive dynamic range of up to 120dB. ACM 100 camera module for automotive applications on the use of these sensors, the camera module is said to complete the first to market similar products in the form of camera solutions: the visual solution is seen as a key element for the driver protection, anti-collision, night vision support and tire tracking oriented future automotive safety system.

Besides, for the portable applications independent of the power grid, CMOS technology, known for its low power consumption, has obvious advantages: CMOS image sensor is designed for 5V and 3.3V supply voltage. The CCD chip requires about 12V of the power supply, so a voltage converter has to be used, which leads to an increase in power consumption. In terms of total power consumption, integrating control and system functions into CMOS sensors will bring another advantage: it removes all external connections with other semiconductor components. Its high power drive has been abandoned nowadays, because the energy consumed inside the chip is much lower than that achieved by PCB or the external implementation of the substrate.