CCD and CMOS main technical analysis

CCD and CMOS are the two main imaging technologies currently available. They are produced in different manufacturing process backgrounds, and they still have their own advantages and disadvantages in terms of current technology. The choice of CCD or CMOS camera should be based on the applicable environment and requirements, suitable for CCD or CMOS technology, can achieve the desired effect of image monitoring. In addition, it can be seen that COMS has a more powerful advantage than CCD as a development technology with great potential. This article will briefly analyze the main technologies of CCD and CMOS, and make a choice judgment.

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Whether it's traffic capture or high-definition video surveillance, as long as it is applied to visual imaging technology, it will involve photosensitive sensors - CCD or CMOS imaging technology.

Comparison of main technical characteristics of CCD and CMOS

For traffic capture and video surveillance, the technical features of the front-end imaging sensors CCD and CMOS are mainly concerned with the following points.

Electronic Shutter Electronic Shutter

The electronic shutter is used to control the charge integration time from the beginning to the end of the chip. Since the CCD chip is exposed to light, there is charge accumulation even if the charge is transferred. Therefore, if the target is measured, it will often produce the Smear phenomenon. The CCD solves the charge accumulation problem by means of interline transfer (ILT). Each pixel is divided into a photosensitive region and a charge transfer region, and the charge transfer region is not photosensitive, so that the charge is transferred to the transfer region at the end of the exposure. And read again, so that there is no charge integral in the reading process, and there is no Smear phenomenon caused by the target motion. However, it is clear that the ILT method reduces the photosensitive area of ​​the pixel and reduces the sensitivity. At this time, a microlens is usually added to the pixel to collect more charge.

The charge on the CMOS chip is read on each pixel. There is no problem with the CCD chip. Its electronic shutter is divided into Rolling shutter and Global shutter. The Rolling shutter usually adopts a 3T pixel structure, which can only be used at a time. Exposure control is performed on a row of pixels (as shown in Figure 1-1), that is, after one line of exposure, the next line is exposed, and the situation shown in Figure 1-2 appears. The Global shutter chip needs to have a 5T structure, so that all pixels of the entire image start and end exposure at the same time. Figure 1-3 shows the imaging effect of the Global shutter. However, the structure of the five photodiodes also reduces the photosensitive area, which can also be compensated by adding microlenses.

Figure 1 CCD and CMOS main technical analysis

Frame rate: Frame Rate

Another important consideration is the frame rate. For CCD photoreceptors, the capture and monitoring speed is mainly controlled by the readout speed of the charge. The readout clock determines the speed of the charge readout. The higher the resolution, the slower the readout speed of the CCD chip. In fact, the upper limit of the read clock depends on the bandwidth of the photo-electrically converted sense amplifier, and the higher read rate requires a wider bandwidth; on the other hand, the larger the bandwidth, the more noise. At the same time, the power of the high-speed and high-bandwidth sense amplifier will also increase. Therefore, for CCD photoreceptors, high speed is a balance between pixel resolution, noise, and power consumption. Multi-channel can solve the problem of readout speed to a certain extent, divide the image into multiple regions, read them separately with sense amplifiers, and then combine them. Multi-channel circuits are not suitable for all applications because they make the camera larger and consume more power.

For CMOS chips, the charge is converted to voltage in units of a single pixel, and the sense amplifier eliminates the need to increase the speed to support higher frame rates. Therefore, CMOS chips are more likely to achieve higher frame rates. At the same time, unlike CCDs, image data obtained by CMOS can be cleared without being read. This solves the problem that the machine vision system only images the region of interest in the image and only needs to read part of the image information. When it is only necessary to read out the region of interest, the CMOS chip can support a higher frame rate without increasing the pixel frequency.

Low light imaging (low illumination imaging) Low-light Operation

The CCD and CMOS photoreceptors use different techniques when imaging in low light. In low-light conditions, the sense amplifier is very important. The CCD uses a uniform amplifier to read out. Correspondingly, the consistency is better than CMOS. The low light condition means that the magnitude of the signal and noise are close, and the noise has a great influence on the quality of the image. The sense amplifiers on each CMOS photoreceptor pixel are low-bandwidth amplifiers that are less noisy than the high-bandwidth amplifiers used in CCD photoreceptors, so a better signal-to-noise ratio can be achieved by increasing the signal gain. Usually, the CCD is higher than the CMOS fill factor, and the number of charges collected under the same conditions will be more. At the same time, the CCD can pass the charge multiplication technique, and multiply the charge multiplication before reading, and increase the small amplitude gain each time to obtain a higher signal-to-noise ratio. In addition, the pixel combination function (Binning) can also improve the sensitivity of the CCD, and Binning for N pixels can increase the signal-to-noise ratio by N times. CMOS can also perform similar Binning, which is often used to sample and superimpose adjacent pixel voltage signals. Since sampling also introduces certain random noise, the signal-to-noise ratio of Binning for N pixels in CMOS can only be improved. Times. Non-visible light imaging Other Wavelenghs

CCD and CMOS photoreceptors are also very different in spectral imaging other than visible light. For example, infrared light (IR) will hit deeper than visible light when it hits the sensor chip. Therefore, in order to fully collect these charges, it is necessary to The silicon substrate is made thicker. This is easier for the CCD chip in the process. For CMOS, there are still some problems with the process. Making the photosensitive portion of the silicon substrate thicker means that other photodiodes are made to be equally thick, which affects the performance of these control gates, amplifiers, and the like.

For ultraviolet light (UV), it is impossible to pass through most of the integrated circuit electrode layers, or the circuit motor layer does not respond to ultraviolet light at all. This leads to a weak response caused by ultraviolet light if the front view is used. To solve this effect problem, it can be achieved by removing the base layer and back-illuminating. The thinning technology of CCD photoreceptors has been very mature, and the CMOS thinning technology has also made great breakthroughs.

CCD or CMOS? Make a choice

From the different effects of CCD and CMOS photoreceptors on different technologies of electronic shutter, frame rate, low-light imaging and non-visible imaging, it is easy to see that the choice of CCD or CMOS sensor should depend on the specific application (Figure 2). . For applications that require low-light or non-visible imaging, the advantages of CCD technology are obvious; for applications that require high frame rates and low power consumption, or applications that require imaging of some areas of interest, CMOS is a better choice; Electronic shutters have special needs, and both technologies have their own advantages and disadvantages.

Figure 2 CCD internal structure diagram

It can be seen that for traffic capture, the sensor must be Global shutter, in order to avoid the deformation of the license plate (Figure 3); at the same time, if the night effect is relatively high, it is best to choose a sensor with high sensitivity (CCD or high-end) CMOS chip); in order to avoid the smear to block the license plate, CMOS should be chosen to effectively compensate for this defect; for video surveillance, the real-time and smoothness of the image is more important, so the CMOS with more frame rate advantage Be the best choice.

For traffic capture and high-definition video surveillance, there are now vendors offering a total solution that integrates all major mature technologies into high-end CMOS sensors.

CMOS aftertaste

The main difference between CCD and CMOS is the difference in manufacturing process due to the different structure of the photosensitive unit and the readout circuit. After the photoelectric conversion of the CCD photosensitive unit is performed, it is stored in a charge manner and sequentially outputted in a charge transfer manner, and a special process is required. The CMOS image sensing unit is a photodiode that can be implemented in a general-purpose CMOS integrated circuit process, in addition to integrating image processing circuits for higher integration and lower power consumption.

Because CCD technology appeared earlier and relatively mature, it occupied most of the high-end market. Compared with CCD, early CMOS only has power consumption and cost advantages. Now with the continuous advancement of CMOS technology, performance has been continuously improved, and CCD technology has limited space for improvement and slow progress.

PACING: normal; BACKGROUND-COLOR: rgb(255,255,255); orphans: 2; widows: 2; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px"> Currently, image sensor technology trends It is a high-speed development, and CMOS is a technology favored by high-speed imaging. According to some sources, high-speed image sensors have three development trends, one is to ultra-high speed, the other is to multi-function integration on a single chip, and the third is to a general-purpose high-speed image sensor. Direction development.

Now, CMOS not only occupies almost all of the portable products and some high-end DSC (Digital Still Camera) market, but also has an impact on the traditional market of CCD, the surveillance market.

Conclusion

CMOS as a rising sensor technology, although there are still some performances that are not comparable to CCD, the main difference between CCD and CMOS manufacturing is that CCD is integrated on semiconductor single crystal material, and CMOS is integrated in metal oxide. On the semiconductor material of the object, there is no essential difference in the working principle, and the CCD manufacturing process is complicated. At present, the gap between CCD and CMOS actual effects has been greatly reduced. Compared with CCD, CMOS is small in size, low in power consumption, and low in cost. CMOS is a standard process manufacturing, which can utilize existing semiconductor devices, and the quality can be improved with semiconductor technology. Progress; CMOS sensors have a high degree of system integration, ie the functionality required for image sensors can be integrated on a single wafer.

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