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CMOS sensor introduction

2011-10-21

Today's CMOS image conversion technology not only serves "traditional" industrial image processing, but is also being accepted by a growing number of new consumer applications for its superior performance and flexibility. In addition, it ensures high safety and comfort when driving a car. Initially, CMOS image sensors were used in industrial image processing; it remains a vital part of new automation solutions designed to increase productivity, quality, and economics of production processes.

According to the forecast of market research company IMS Research , in the next few years, the annual growth rate of the European industrial image processing market will reach 6% . Among them, the market share of intelligent solutions integrating software functions in cameras will continue to expand. . In Germany, according to data provided by its national machine tool supplier association VDMA , the image processing market growth rate in 2004 reached 14% . The market research company In-Stat / MDR also pointed out that, as far as the secondary market of image sensors is concerned, its annual growth rate will be as high as 30% , and this situation will continue until 2008 . Most importantly: The growth rate of CMOS sensors will reach seven times that of CCD sensors. The rapid popularization of camera phones and digital cameras is the main driving factor for this demand.

Obviously, people are so optimistic about the growth prospects of CMOS image converters based on the fact that, compared with the CCD technology that has monopolized this field for more than 30 years , it can better meet the needs of users for new image sensors in various applications. Improved quality requirements, such as more flexible image capture, higher sensitivity, wider dynamic range, higher resolution, lower power consumption, and better system integration. In addition, CMOS image converters have created novel applications that have not yet been economically implemented. In addition, there are several "soft" standards that are beneficial for CMOS sensors, including: application support, radiation resistance, shutter types, windowing, and spectral coverage. However, this distinction is somewhat arbitrary because the importance of these standards will vary depending on the application ( consumer, industry, or automotive ) .

Difficulties in detailed performance

As we know from analog photography, taking a picture of a complete scene is quite common, and so is a camera phone. However, for industrial or automotive applications, the situation is quite different: there are occasions when a high full frame data rate is not required. For example, in surveillance cameras, as long as a change in a scene can be found (because this change may indicate a suspicious situation), a lower resolution is completely acceptable. On this basis, it is necessary to use the full resolution to collect more detailed information. The actions that follow will only be played in a certain part of the camera's field of view, and in the captured scene, only this part is the focus of the monitoring staff.

For a CCD image sensor that only provides a full frame image , only a separate evaluation circuit can provide two observation angles, which means an increase in processing time and cost. However, the working principle of CMOS image sensor is similar to that of RAM . All memory bits can be read separately. Although the sub-sampling of the CMOS sensor provides a lower resolution, the frame rate is higher; the windowing allows random selection of a region of interest.

CMOS sensor has advantages of high sensitivity, wide dynamic range and low power consumption

A prerequisite for the widespread use of the latest CMOS sensors is their higher sensitivity, shorter exposure times, and shrinking pixel sizes. One measure of pixel sensitivity is the product of the fill factor ( the ratio of light-sensitive area to the total pixel area ) and the quantum efficiency ( the number of electrons generated by the photons that bombard the screen ) . CCD sensors have a large fill factor due to the inherent characteristics of their technology. In the CMOS image sensor, in order to achieve a noise index and sensitivity level comparable to that of a CCD converter, people have equipped the CMOS image sensor with an active pixel sensor (APS) , and the fill factor has been reduced because the pixel surface is equivalent. A large area is occupied by the amplifier transistors, leaving less space available for the photodiode. Therefore, an important development goal of today's CMOS sensors is to expand the fill factor. Cypress (FillFactory) through its patented technology can greatly increase the fill factor, this technology can turn the largest part of the area of a standard CMOS silicon chip into a photosensitive area.