The hottest CCD sensor and its application

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CCD sensor and its application research

1. Introduction

image sensor is a functional device that uses the optical electrical conversion function of photoelectric devices to convert the light image on its photosensitive surface into an electrical signal "image" that is proportional to the light image

solid state image sensor is an integrated and functional optoelectronic device composed of several photosensitive units and shift registers arranged on the same semiconductor substrate. Photosensitive units are called "pixels" or "image points" for short. They are independent of each other in space and electricity. The solid-state image sensor uses the photoelectric conversion function of the photosensitive unit to convert the optical image projected on the photosensitive unit into an electrical signal "image", that is, the spatial distribution of light intensity is converted into the spatial distribution of charge packets of different sizes proportional to the light intensity. Then, using the function of shift register, these charge packets are read and output under the control of clock pulse, forming a series of timing pulse sequences with different amplitudes

compared with ordinary image sensors, solid-state image sensors have the characteristics of small volume, small distortion, high sensitivity, anti vibration, moisture resistance and low cost. These characteristics determine that it can be widely used in automatic control and automatic measurement, especially in image recognition technology. Based on the analysis of the principle of solid-state image sensor, this paper focuses on its application in the fields of oil resistance, corrosion control and image recognition

II. Charge coupled devices and working principle

chargecoupledevices (CCD) is a sensitive device of solid-state image sensor. Like ordinary MOS, TTL and other circuits, it is an integrated circuit, but CCD has many unique functions, such as photoelectric conversion, signal storage, transfer (transmission), output, processing and electronic shutter

The basic principle of charge coupled device CCD is to apply an appropriate pulse voltage to a series of MOS capacitor metal electrodes to repel most carriers in the semiconductor substrate, forming a "potential well" movement, and then achieving the transfer of signal charge (minority carriers). If the transferred signal charge is generated by light image irradiation, CCD has the function of image sensor; If the transferred charge is obtained by external injection, the CCD can also have the functions of time delay, signal processing, data storage and logic operation

The basic principle of charge coupled device CCD is closely related to the physical mechanism of metal oxide silicon (MOS) capacitor. Therefore. Firstly, the principle of MOS capacitor is analyzed

Figure 1 shows a MOS capacitor formed by thermal oxidation of metal deposited on p-Si (p-Si) substrate. If a forward voltage is applied to its metal electrode at a certain time, most carriers (holes at this time) in p-Si will be repelled, and a depletion region will be formed on the Si surface. This depletion region, like the common PN junction, is also a space charge region composed of ionization acceptors. Moreover, under certain conditions, the larger, the deeper the depletion layer. At this time, the potential (i.e. surface potential V) of Si surface absorbing minority carriers (electrons at this time) The bigger. It is obvious that the MOS capacitor can hold a larger amount of minority carrier charge. According to this, we can just use the image metaphor of "surface potential well" (referred to as potential well) to explain that MOS capacitor is in V; The ability to store (signal) charges under the action of Ling. Traditionally, the potential well is thought of as a barrel, and a few carriers (signal charges) are thought of as a fluid at the bottom of the barrel. When analyzing solid-state devices, the potential in the semiconductor substrate is often taken as zero, so it is more convenient to take the positive increasing direction of the surface potential ring downward (Fig. 1 (b))

Figure 3 MOS capacitor and its surface potential well concept

surface potential V; Is a very important physical quantity. In the case shown in figure 1ca), if the added VC does not exceed a certain limit value, the surface potential is:

the above formula is obtained by solving the Poisson equation of the potential distribution in the semiconductor. Because XA is controlled by VG, V; It is also a function of VC

cd charge (minority carrier) can be generated in two ways: voltage signal injection and optical signal injection. As an image sensor, CCD receives optical signal, that is, optical signal injection method. When the optical signal irradiates the CCD silicon chip, it absorbs photons in the depletion region near the gate to produce electron hole pairs. At this time, under the action of the gate voltage, most carriers (holes) will flow into the substrate, while a few carriers (electrons) will be collected in the potential well to form a signal charge for storage

in this way, those photons higher than the semiconductor band gap can establish a stored charge proportional to the light intensity

CCD, which is composed of many MOS capacitors, generates the signal charge of photogenerated carriers under the illumination of light image, and then makes it have the self scanning function of transferring signal charge, that is, it constitutes a solid-state image sensor

Figure 2 is a comparison of the basic principles of the photoconductive camera tube and the solid-state image sensor. In figure 2ta), when the incident light image signal irradiates the surface of the intermediate electrode of the camera tube, a potential distribution proportional to the amount of light irradiated at each point will be generated. If the intermediate electrode is scanned with an electron beam, a variable discharge current will be generated on the load RL. The load current changes due to the different amount of light, which is exactly the required output electrical signal. The deflection or bunching of the electron beam used is controlled by the magnetic field or electric field

comparison of the basic principle between the photoconductive camera tube and the solid-state image sensor in Fig. 2

the output signal of the solid-state image sensor shown in Fig. 2 (b) does not need additional scanning electron beam, and it can be directly obtained by self scanning the pixels on the semiconductor substrate. Such an output electric signal corresponds to the position of its corresponding pixel, which is undoubtedly more accurate, and the distortion of the regenerated image is very small. Obviously, it is difficult to avoid the distortion of regenerated images caused by deflection distortion or focusing change of scanning electron beam in image sensors such as photoconductive camera tubes

solid state image sensor with minimal distortion is very suitable for testing technology and image recognition technology. In addition, compared with the camera tube, the solid-state image sensor has many advantages, such as small volume, light weight, durability, impact resistance, vibration resistance, strong anti electromagnetic interference ability and low power consumption, and the cost of the solid-state image sensor is also low

III. classification, structure and characteristics of solid-state sensors

from the point of view of use, solid-state image sensors can be divided into linear and surface solid-state image sensors. According to the different sensitive devices used, it can be divided into CCD, MOS linear sensor, CCD, MOS surface type sensor and other linear solid-state image sensors, which are mainly used for testing, fax and optical character recognition technology. The development direction of surface type solid-state image sensors is mainly used as small cameras for magnetic tape video recording. This paper mainly introduces the structure of linear solid-state image sensor commonly used in engineering testing

Figure 3 shows the structure of the linear solid-state image sensor. The photosensitive part is a photodiode line array, 1728 PDs are located in the center of the sensor as photosensitive pixels, and CCD conversion registers are set on both sides. The register is covered with a light shield. The signal charge of PD with odd number bits moves to the transfer register on the lower side; The even number bits are moved to the upper transfer register. The CCD transfer register is driven by another signal, and the signal charge is read out from the photodiode PD through the common output

Figure 3 structure of linear solid-state image sensor

Figure 4 cross section structure of high sensitivity linear sensor

usually the photodiode of the photosensitive part is made into MOS form, and the electrode is made of polysilicon, although the polysilicon film can pass through the light image. However, it has a strong absorption of blue light, especially when the fluorescent lamp is used as the light source, the blue spectral response of the sensor will become very poor. In order to improve the situation, a light window can be set on the polysilicon electrode. Since the photo generated signal charge of the sensor with this structure is generated and accumulated in the MOS capacitor, the capacity is increased and the dynamic range is greatly extended. Figure 5 shows its spectral response characteristics. The dotted line in the figure indicates the sensor characteristics of CCD with only polysilicon electrode but no light window; The solid line indicates the characteristics of the PD formed by opening the light window and the CCD sensor whose signal charge is accumulated in the MOS container. Obviously, the blue spectral response characteristics of the latter have been significantly improved, so the latter is called a high-sensitivity linear solid-state image sensor

Figure 5 high sensitivity sensor

the main characteristics of solid-state image sensor are:

① MTF characteristics of modulation transfer function: the solid-state image sensor is composed of pixel matrix and corresponding transfer part. Although solid-state pixels have been made very small and their spacing is also very small, it is still the main obstacle to recognize small images or reproduce small parts of images

②. Output saturation characteristic: when a strong light image with a saturation exposure above irradiates the image sensor, the output voltage of the sensor will be saturated. This phenomenon is called output saturation characteristic. The root cause of output saturation is that photodiodes or MOS capacitors can only generate and store a certain limit of photogenerated signal charges

③. Dark output characteristic: dark output, also known as no light output, refers to the characteristic that the sensor still has a small output when there is no light image signal, and the output comes from the dark (no light) current

④. Sensitivity: the output photocurrent generated by the unit irradiance represents the sensitivity of the solid-state image sensor, which is mainly related to the pixel size of the solid-state image sensor

⑥. Dispersion: Evonik's innovative "interface and performance" business line with saturation exposure above will provide scientific and technological support and business experience for the utilization and processing process of the project. Over bright images will generate and accumulate oversaturated signal charges in pixels. At this time, oversaturated charges will diffuse from the potential well of one pixel to the potential well of adjacent pixels through the substrate. In this way, the areas on the regenerated image that should not show some brightness instead show brightness. This situation is called dispersion phenomenon

⑥. Residual image: after a pixel is scanned and its signal charge is read out, the readout signal after the next scan is still affected by the last residual signal charge, which is called residual image

⑦. Equivalent noise exposure: the exposure equivalent to the dark output (voltage) is called the equivalent noise exposure of the sensor

IV. application of solid-state image sensor

1. Automatic measurement

Figure 6 is the basic schematic diagram of measuring object size with linear solid-state image sensor

Figure 6 basic principle of measuring object size with linear solid-state image sensor

it is easy to deduce the relationship between the measured object length L and system parameters by using geometric optics knowledge:

because the light intensity of the light image sensed by the solid-state image sensor is the difference between the measured object and the background light intensity. Therefore, as far as the specific measurement technology is concerned, the measurement accuracy is related to the selection of the comparison reference value between the two, and depends on the ratio of the number of sensor pixels to the lens field of view. In order to improve the measurement accuracy, the sensor with many pixels should be selected and the field of view should be shortened as far as possible

Figure 7 is an example of dimension measurement, and the measured object is the width of hot rolled plate. Because the two CCD linear sensors only measure part of the plate end, this is equivalent to shortening the field of view. When higher measurement accuracy is required, the average value can be taken by multiple sensors at the same time, or according to the change of the measured plate width. Make d into an adjustable form

Figure 7 hot rolled plate width from

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