Brief introduction of basic principle and application of CMOS image sensor

1 Introduction

Image sensor is a device that converts optical signals into electrical signals, which is widely used in digital TV and visual communication market. The new concept of solid State imaging and a one-dimensional CCD (charge-coupled device charge-coupled device) model were proposed in the late 60, when the charge was found to be transferred through a semiconductor potential well in the American Bell face room. By the early 90, CCD technology has been relatively hot, and has been widely used. However, with the expansion of CCD application scope, its shortcomings are gradually exposed. First of all, CCD technology chip technology complex, not compatible with the standard process. Second, the CCD technology chip requires a large voltage consumption, so the CCD technology chip is expensive and inconvenient to use.

At present, the most noticeable and most promising is the use of standard CMOS(complementary Metal Oxide Semiconductor complementary metal oxide FET) technology to produce image sensors, CMOS image sensor . CMOS image sensor chip adopts CMOS process, which can integrate image Acquisition unit and signal Processing unit on the same chip . Because of the above characteristics, it is suitable for large-scale mass production, suitable for small size, low price, camera quality without excessive requirements of the application, such as security with small, micro camera, mobile phone, computer network video conferencing system, wireless handheld video conferencing system, barcode scanner, fax machine, toys, bio-microscopic counting, Some of the car camera systems and other commercial areas. In the 1980s, the University of Edinburgh succeeded in making the world's first monolithic CMOS image sensor. At present, CMOS image sensor is widely used, and has a strong market competitiveness and broad prospects for development.

2 CMOS image sensor basic operating principle

First of all, the external light irradiation pixel array, the photoelectric effect, the corresponding charge is generated in the pixel cell. Row selection Logical unit selects the corresponding row pixel units as needed. The image signal in the row pixel cell is transmitted to the corresponding analog signal processing unit and A/D converter through the signal bus of the respective column, and converted into digital image signal output. The row selection logical unit can scan the pixel array row by line or interlaced. The row selection logical unit is used with the column selection logical unit to implement the window extraction function for the image. The main function of the analog signal Processing unit is to amplify the signal and improve the SNR. In addition, in order to obtain the quality of the practical camera, the chip must contain a variety of control circuits, such as exposure time control, automatic gain control. Multiple timing control signals must be used in order for each part of the chip to act on a specified beat. In order to facilitate the application of the camera, it is also required that the chip can output some timing signals, such as synchronization signal, line Start signal, field start signal and so on.

3 Pixel Array Working principle

Image sensor An intuitive performance indicator is the ability to reproduce the image. And the pixel array is the key function module directly related to this index. Depending on the pixel array cell structure, pixel cells can be divided into passive pixel unit PPS (passive pixel schematic), Active pixel unit APS (active pixel schematic) and logarithmic pixel units, Active Pixel unit APS can also be divided into photosensitive diode-type APS, grating-type APS.

Each of the various pixel array units has its own characteristics, but they have the same basic working principle . The following first introduces their basic working principle, and then introduces the characteristics of various pixel units. is a single pixel.

(1) First enter the "Reset state", at this time to open the door tube m. Capacitance is charged to V, the diode is in reverse state;

(2) then the person "sampling state". Then close the door tube m, in the light of the diode to generate light current, so that the capacitance storage charge discharge, after a fixed time interval, the capacitance C on the remaining charge amount is proportional to the illumination, then an image is ingested into the sensitive element array;

(3) Finally enter the "readout state". Then open the door tube m, reading each pixel on the capacitance C storage charge voltage.

The non-source pixel unit PPS appears earliest, since the appearance of the structure has not changed much. The non-active pixel unit PPS has a simple structure, high pixel filling rate, and high quantum efficiency, but it has two significant drawbacks. First, its readout noise is relatively large, its typical value is 20 electrons, while the commercial CCD-grade technology chip readout noise typical value of 20 electrons. Second, with the increase of the number of pixels, the readout rate is accelerated, so the readout noise becomes larger.

Photodiode-Type APS quantum efficiency is relatively high, due to the use of new noise cancellation technology, output graphics signal quality is much higher than before, the readout noise is generally 75~100 electronic, this kind of structure c3& suitable for middle and low applications.

In the grating type APS structure, the fixed figure noise is suppressed. The readout noise is 10~20 electrons. But its process is more complex, strictly speaking, it is not a complete CMOS process. Due to the introduction of polycrystalline silicon coating, the quantum efficiency is lower, especially for blue light. For now, its overall performance advantage is not very prominent.

4 main problems affecting the performance of CMOS sensors

4.1 Noise

This is the primary problem that affects the performance of CMOS sensors. This noise includes fixed graphics noise FPN (fixed pattern noise), dark current noise, thermal noise, and so on. Fixed graphics noise is caused by the fact that a beam of the same light that emits two different pixels produces an output signal that is not exactly the same. This is how noise is introduced. Dual sampling or correlated dual sampling techniques can be used to deal with fixed graphics noise. Specifically, it's a bit like introducing a differential pair to suppress common-mode noise when designing an analog amplifier. Double sampling is to read the light generated by the charge integral signal, staged and then the object element unit to reset, and then read the pixel unit output signal. The image signal is subtracted from the two. Both samples can effectively suppress the fixed graphics noise. In addition, the relevant dual sampling requires a temporary storage unit, and as the pixel increases, the storage unit also increases.

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4.2 Dark Current

Physical devices can not be ideal, as the sub-threshold effect, due to impurities, heat and other factors, even if there is no light to the pixel, the pixel unit will also generate electrical charges, these charges generated dark current. It is difficult to distinguish between dark currents and the charge generated by light. Dark currents are not exactly the same everywhere in the pixel array, which can cause fixed graphics noise. For pixel units with integral functions, the fixed graphics noise caused by dark currents is proportional to the integration time. The generation of dark currents is also a stochastic process, which is a source of stray noise. Therefore, the dark current size produced by the thermal noise element is equal to the square root of the dark current electron number in the pixel element. When a long time integration unit is adopted, this type of noise becomes the main factor affecting the quality of the image signal, for dim objects, long-time integration is necessary, and the capacitance of the pixel unit is limited, so the accumulation of dark current electrons limit the maximum time of the integral.

In order to reduce the effect of dark current on image signal, we can take the cooling method first. However, it is not enough to cool the chip only, the fixed figure noise produced by the dark current cannot be overcome by double sampling completely. The effective method used now is to subtract the reference dark current signal from the obtained image signal.

Saturation and overflow blur of 4.3 pixels

Similar to the amplifier because of the limited range of linear region and there is an input limit, for the CMOS image sensor chip, it also has an input limit. If the input light signal exceeds this limit, the pixel unit will saturate without photoelectric conversion. For pixel units with integral functions, this upper limit is determined by the capacity size of the Photoelectron Integrator unit: For pixel elements that do not include integral functions, the upper limit is determined by the maximum current flowing through the photodiode or transistor. When the input light signal is saturated, the overflow blur occurs. The overflow blur is due to the photoelectron saturation of the pixel element and then outflow to the neighboring pixel cell. The overflow blur reflects a particularly bright area on the image. This is somewhat similar to overexposure in photos. Overflow blur can be overcome by adding an automatic vent tube within the pixel unit, which effectively discharges the excess charge. However, this only limits the overflow, but does not allow the pixel to actually restore the image.

5 market situation of CMOS image sensor

According to the market research company Cahners In-stat Group predicts that in the next few years, the image products based on CMOS image sensor will reach more than 50%, that is to say, CMOS image sensor will replace the CCD and become the mainstream of the market. It can be seen that the market prospect of CMOS camera is very broad.

In the next few years, global CMOS image sensor sales will rapidly increase, and will be in many digital image applications to the traditional CCD launch shock. This is because CMOS image sensor components have two advantages: first, the price is lower than the CCD device 15%~25%, the second is that the chip structure can be easily integrated with other silicon-based components, which can effectively reduce the cost of the entire system. Although the image quality of CMOS image sensor is lower than that of CCD in the past, it has been rapidly improved and has been approaching the technical level of CCD, which has been widely used in camera structure of digital camera, electronic toy, video conference and security system with lower resolution requirement.

Japan Nintendo Co., Ltd. launched the use of CMOS image sensor low-resolution digital camera, the first two months in the market, sales reached 1 million units. Mitsubishi, Motorola, Hewlett-Packard, Toshiba and Intel also listed the products shortly thereafter.

Application of 6 CMOS image sensor components

6.1 Digital Camera

People have used film cameras for hundreds of years, since the 1980s, people have used high-tech, the development of film-free CCD digital camera. has made a fundamental difference in the traditional film camera. The advent of electronically writable, inexpensive flash ROMs, and low-power, low-priced CMOS cameras. A new situation has been opened for digital cameras, and the functional block diagram of the digital camera is shown on the right.

It can be seen that the internal device of the digital camera has been completely different from the traditional camera, the color CMOS camera under the control of the electronic shutter, take a picture stored in DRAM, and then go to the Flash ROM storage. Depending on the capacity of the Flash ROM and the compression level of the image data, you can determine the number of images that can be stored. If the ROM is replaced by a PCMCIA card, you can increase the capacity of the digital camera by changing the card, which is like changing the film, the digital image information of digital camera is stored in the hard disk of the PC, which greatly facilitates the storage, retrieval, processing, editing and transmitting of the photo.

6.2 CMOS Digital Camera

A USB camera consisting of a OV7610 CMOS color digital image chip and a OV511 Advanced camera and USB interface chip is available at Omni Vison Inc., which has a resolution of up to 640 x 480 and is suitable for video systems transmitted over a universal serial bus. The introduction of OV511-type Advanced camera can make the PC function to obtain a large amount of video information in a more real-time way, the compression ratio of the compression chip can reach 7:1, thus guaranteeing the fast image transmission of the image sensor to the PC. For CIF image formats, the OV511 model supports up to 30 frames per second in transmission rates, reducing the image bounce that typically occurs in low bandwidth applications. The OV511 is a high-performance USB interface controller with sufficient flexibility for applications including video conferencing, video e-mail, computer multimedia, and security monitoring.

6.3 Other areas of application

CMOS image sensor is a multifunctional sensor, because it combines the performance of CCD image sensor, so it can enter the application field of CCD, but it has its own unique advantages, so open up a lot of new application fields. In addition to the main applications described above, CMOS image sensors can also be used in digital static cameras and medical small cameras. For example, a heart surgeon can install a small "silicon eye" on the patient's chest to monitor the effect after surgery, which is difficult to achieve with the CCD.

7 Summary

From the comparison with the CCD, this paper introduces the principle, performance, advantages, shortcomings and countermeasures of the physical level of CMOS image sensor device. Then, the market situation of CMOS image sensor and some application fields are discussed. It can be seen that as a new semiconductor device, CMOS has its own characteristics show great advantages and potential, this potential will be further developed in the near future.

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Brief introduction of basic principle and application of CMOS image sensor