Research on CCD Sensor and Its Application

I. Introduction

Image Sensor is a functional device that uses the light-to-electric 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. A solid-state image sensor refers to an integrated and functional photoelectric device composed of several photosensitive units and shift registers arranged on the same semiconductor substrate. Photosensitive units are referred to as "pixels" or "pixels", which are spatially and electrically independent of each other. Solid-state image sensors use the photoelectric conversion function of photosensitive units to convert the optical image projected on the photosensitive units into an electrical signal "image", that is, to convert the spatial distribution of light intensity into a spatial distribution of charge packets of varying sizes that are proportional to the light intensity. Then, the function of the shift register is used to read and output these charge packets under the control of clock pulses, forming a series of timing pulse sequences of varying amplitudes. Compared with ordinary image sensors, solid-state image sensors have the characteristics of small size, low distortion, high sensitivity, vibration resistance, 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. This paper starts with the analysis of the principle of solid-state image sensors, focusing on its analysis and discussion in the field of measurement and control and image recognition. 2. Charge coupling device and working principle Charge coupling device (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 coupling device CCD is to apply appropriate pulse voltage to a series of MOS capacitor metal electrodes to repel the majority carriers in the semiconductor substrate, forming a "potential well" movement, and then achieve 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, CCD can also have functions such as delay, signal processing, data storage and logical operation. The basic principle of charge coupling device CCD is closely related to the physical mechanism of metal oxide silicon (MOS) capacitor. Therefore. First analyze the principle of MOS capacitor. Figure 1 shows a MOS capacitor formed by depositing metal on a thermally oxidized P-type Si (p-Si) substrate. If a forward voltage is applied to its metal electrode at a certain moment, the majority carriers in the p-Si (holes at this time) will be repelled, so a depletion region will be formed on the Si surface. This depletion region is the same as the ordinary pn junction, and is also a space charge region composed of ionized acceptors. Moreover, under certain conditions, the larger the , the deeper the depletion layer. At this time, the potential (i.e., surface potential V) of the Si surface to absorb minority carriers (electrons at this time) is also greater. Obviously, the amount of minority carrier charge that the MOS capacitor can accommodate at this time is greater. Based on this, the image metaphor of "surface potential well" (abbreviated as potential well) can be used to illustrate the ability of MOS capacitors to store (signal) charges under the action of V (or in V). Conventionally, the potential well is imagined as a bucket, and the minority carriers (signal charges) are imagined as the fluid at the bottom of the bucket. When analyzing solid-state devices, the potential inside the semiconductor substrate is often taken as zero, so it is more convenient to take the positive value of the surface potential ring with the increasing direction pointing downward (Figure 1(b)).

Figure 3 MOS capacitor and its surface potential well concept

The 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 is also a function of Vc.

There are two ways to generate the charge (minority carrier) of the CCD: voltage signal injection and light signal injection. As an image sensor, CCD receives light signals, that is, light signal injection method. When the light signal is irradiated on the CCD silicon chip, the depletion region near the gate absorbs photons to generate electron-hole pairs. At this time, under the action of the gate voltage, the majority carriers (holes) will flow into the substrate, while the minority carriers (electrons) are collected in the potential well to form signal charges and store them.

In this way, those photons that are higher than the semiconductor bandgap width can establish a storage charge proportional to the light intensity.

The CCD, which is composed of many MOS capacitors, generates signal charges of photogenerated carriers under the irradiation of light images, and then enables it to have the self-scanning function of transferring signal charges, that is, to constitute a solid-state image sensor.

Figure 2 is a comparison of the basic principles of photoconductive camera tubes and solid-state image sensors. In Figure 2ta), when the incident light image signal irradiates the surface of the middle electrode of the camera tube, a potential distribution proportional to the amount of light irradiated at each point will be generated on it. If the middle electrode is scanned by an electron beam, a variable discharge current will be generated on the load R1. The load current changes due to the different light amounts, which is exactly the required output electrical signal. The deflection or focusing of the electron beam used is achieved by magnetic field or electric field control.

Figure 2 Comparison of the basic principles of a photoconductive camera tube and a solid-state image sensor

The output signal of the solid-state image sensor shown in Figure 2(b) does not require an external scanning electron beam. It can be directly obtained by scanning the pixels on the semiconductor substrate. Such an output electrical signal corresponds to the position of its corresponding pixel, which is undoubtedly more accurate, and the distortion of the regenerated image is extremely small. Obviously, it is often difficult to avoid the distortion of the regenerated image caused by the deflection distortion or focus change of the scanning electron beam in image sensors such as photoconductive camera tubes.

Solid-state image sensors with extremely low distortion are very suitable for testing technology and image recognition technology. In addition, compared with camera tubes, solid-state image sensors have many advantages such as small size, light weight, durability, impact resistance, vibration resistance, strong anti-electromagnetic interference ability and low power consumption, and the cost of solid-state image sensors is also low.

3. Classification, structure and characteristics of solid-state sensors

From the perspective of use, solid-state image sensors can be divided into two categories: linear and planar solid-state image sensors. According to the different sensitive devices used, it can be divided into CCD, MOS linear sensors and CCD, MOS surface sensors. Linear solid-state image sensors are mainly used in testing, fax and optical character recognition technology. The development direction of surface solid-state image sensors is mainly used as small cameras for tape recording. This article mainly introduces the structure of linear solid-state image sensors commonly used in engineering testing.

Figure 3 shows the structure of linear solid-state image sensors. Its photosensitive part is a linear array of photosensitive diodes. 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 the odd-numbered PD is transferred to the transfer register on the lower side; the even-numbered PD is transferred to the transfer register on the upper side. The CCD transfer register is driven by another signal, and the signal charge is read out from the photosensitive diode PD in sequence through the common output terminal.

Figure 3 Structure of a linear solid-state image sensor

Figure 4 Cross-sectional structure of high-sensitivity linear sensor

Usually, the photosensitive diode of the photosensitive part is made into MOS form, and the electrode is made of polysilicon. Although the polysilicon film can transmit light images. However, it has a strong absorption effect on blue light, especially when fluorescent lamps are used as light sources, the blue light spectrum response of the sensor will become extremely poor. In order to improve this situation, a light window can be opened on the polysilicon electrode. Since the photogenerated signal charge of the sensor of this structure is generated and accumulated in the MOS capacitor, the capacity is increased, and the dynamic range is greatly expanded. Figure 5 is its spectral response characteristics. The dotted line in the figure represents the sensor characteristics of the CCD using only polysilicon electrodes without opening a light window; the solid line represents the PD formed by opening a light window, and the CCD sensor characteristics of the signal charge accumulated in the MOS container. Obviously, the blue spectrum response characteristics of the latter are significantly improved and 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 sensors are:

①. Modulation transfer function MTF characteristics: Solid-state image sensors are composed of pixel matrices and corresponding transfer parts. Although solid-state pixels have been made very small and their intervals are also very small, this is still the main obstacle to identifying tiny images or reproducing subtle parts of images.

②. Output saturation characteristics: When a strong light image above the saturation exposure is irradiated onto the image sensor, the output voltage of the sensor will be saturated. This phenomenon is called output saturation characteristics. The fundamental reason for the output saturation phenomenon is that photodiodes or MOS capacitors can only generate and accumulate photogenerated signal charges within a certain limit.

③. Dark output characteristics: Dark output is also called no-light output, which refers to the characteristic that the sensor still has a tiny output when there is no light image signal irradiated. The output comes from the dark (no-light) current. ④

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

⑥. Smearing: The overbright light image above the saturation exposure will generate and accumulate oversaturated signal charge in the pixel. At this time, the oversaturated charge will diffuse from the potential well of one pixel through the substrate to the potential well of the adjacent pixel. In this way, the place where a certain brightness should not be shown on the regenerated image will show brightness instead. This situation is called the dispersion phenomenon.

⑥. Afterimage: After scanning a certain pixel and reading its signal charge, the phenomenon that the read signal after the next scan is still affected by the last remaining signal charge is called afterimage.

⑦. Equivalent noise exposure: The exposure that produces the same value as the dark output (voltage) is called the equivalent noise exposure of the sensor. IV

. Application of solid-state image sensors

1. Automatic measurement

Figure 6 is a basic principle diagram of measuring the size of an object using a linear solid-state image sensor.

Figure 6 Basic principle of measuring object size using linear solid-state image sensor

Using the knowledge of geometric optics, it is easy to deduce the relationship between the length L of the measured object and the system parameters:

Because the light intensity of the light image perceived by the solid-state image sensor is the difference between the light intensity of the object being measured and the background light intensity. Therefore, in terms of specific measurement technology, 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, a sensor with more pixels should be selected and the field of view should be shortened as much as possible.

Figure 7 is an example of dimensional measurement, and the object being measured is the width of a hot-rolled plate. Because the two CCD linear sensors only measure a part of the plate end each, this is equivalent to shortening the field of view. When higher measurement accuracy is required, multiple sensors can be used simultaneously to take the average value, or according to the change in the measured plate width. Make d adjustable.

Figure 7 Principle of automatic measurement of hot rolled plate width

The CCD sensor shown in Figure 7 is used to capture the reflected light image of the laser on the plate, and its output signal is used to compensate for the measurement error caused by the change in plate thickness. The entire system is controlled by a microprocessor, so that the width of the hot-rolled plate can be detected online in real time. For a 2m wide hot-rolled plate, the final measurement accuracy can reach 10.025%. The principle of testing the workpiece scratches and surface dirt is basically the same as the dimension measurement method.

2. Image recognition

(1) Fax technology

Compared with the commonly used mechanical scanning or electric tube type, the use of linear solid-state image sensors as the input link of the fax device has many advantages, such as fewer mechanical rotating parts, good reliability, fast speed, small size and light weight.

Figure 8 is a schematic diagram of the input link of the fax device. The light source is a fluorescent lamp. In order to make the amount of incident light adjustable, a movable cover window can be set.

(2) Optical character recognition device

The solid-state image sensor can also be used as the "reading head" of the optical character recognition device. The light source of the optical character recognition device (OCR) can be a halogen lamp. An infrared filter is set between the light source and the lens to eliminate the influence of infrared light. Each scan time is 300Ns, so high-speed text recognition can be achieved. Figure 9 is the principle diagram of OCR. After the binary signal after A/D conversion passes through a special filter, the text becomes clearer. The next step is to cut out the text one by one. The above processing is called "pre-processing". After pre-processing, feature extraction is performed on each text in a fixed manner.

Figure 9 OCR principle diagram

Finally, the extracted features are compared with the pre-set text features to judge and identify the input text.

3. Online inspection, identification and control

The photoelectric detection capability of CCD photoelectric sensors combined with the signal processing capability of microprocessors (NPs) can greatly expand the application prospects of CCDs, such as for online part graphics inspection and identification, thereby improving the level of production automation and product quality. Figure 10 is an example of a linear CCD photoelectric sensor performing graphic recognition on mechanical parts.

Figure 10 Working principle of graphic inspection

The object to be measured is a shaft-like part, which moves at a constant speed on the transmission line. Under the illumination of the light source, its shadow sweeps across the photoelectric array in sequence, causing the sensor to output a signal corresponding to the shadow. The signal output by CCD and the movement speed information of the transmission line are simultaneously input into the microcomputer (NC), processed and compiled according to the input signal, and then compared with the standard graphic information in the memory of "C", the deviation information can be calculated, and the NC makes a judgment based on the deviation size, and issues instructions to receive or reject the parts. The cooperation of CCD photoelectric sensor and NC has been used to identify the solder joint pattern of large-scale integrated circuit (LSI) circuit, which not only improves the degree of automation, but also greatly improves the yield rate of LSI circuit.

V. Conclusion

This paper analyzes the working principle, structure, characteristics and principle of solid-state image sensor in industrial detection, so as to explain the current application status of solid-state image sensor in industry. Since solid-state image sensor is a high-precision detection sensor, its application has penetrated into various departments of industrial production, especially in fine processing, robot technology, industrial automation field, and has a wide range of applications, which has played a major role in the development of China's national economy. It is believed that with the improvement of solid-state image sensor manufacturing technology and the further development of image processing software, the application prospect of solid-state image sensor will be broader.