CCD linear image sensor

From the working principles of the two photosensitive devices, it can be seen that the advantage of CCD lies in the good imaging quality, but the manufacturing cost remains high because of the complex manufacturing process, which only a few manufacturers can master, especially the large CCD, which is very expensive. At the same resolution, CMOS is cheaper than CCD, but the image quality produced by CMOS devices is lower than CCD. Most consumer-grade and high-end digital cameras on the market use CCD as sensors; CMOS sensors are used in some cameras as low-end products. If any camera manufacturer uses CCD sensors, the manufacturer will spare no effort to publicize them as a selling point, and even call them "digital cameras". For a time, whether there is a CCD sensor has become one of the standards for people to judge the grade of digital cameras.

CCD sensor-principle editor The CCD sensor in this paragraph is a new photoelectric conversion device, which can store the signal charge generated by light. When a pulse with a specific time sequence is applied to it, the stored signal charge can be transmitted directionally in CCD to realize self-scanning. It is mainly composed of photosensitive unit, input structure and output structure. It has the functions of photoelectric conversion, information storage and time delay, and has high integration and low power consumption. It has been widely used in three fields: camera shooting, signal processing and storage, especially in the application of image sensors. CCD can be divided into area array and linear array. Area array is a device that arranges CCD pixels in a plane. Linear array is a device that arranges CCD pixels in a straight line. Because the area CCD is mainly used in the military field, this paper mainly introduces the area CCD.

CCD sensor-type editing There are generally three types of CCD structures in this section. The first one is frame-transitive CCD. It consists of an upper part and a lower part. The upper part is the photosensitive area where pixels are concentrated, and the lower part is the storage area where vertical registers are concentrated by shading. Its advantages are simple structure and easy to increase the number of pixels, but its disadvantages are large CCD size and easy to produce vertical smear. The second is the transfer CCD between lines. It is the mainstream product of CCD at present. They are pixel groups and vertical registers on the same plane. Its characteristics are low price and easy to obtain good photographic characteristics on a single chip. The third is the transfer CCD between frames. It is a combination of the first and the second, with complex structure, but it can greatly reduce vertical smear and easily realize variable speed electronic shutter.

area CCD: allows the photographer to shoot moving objects with one exposure at any shutter speed.

linear CCD: scan a picture with a row of pixels and make three exposures-corresponding to red, green and blue filters respectively. As the name implies, a linear sensor captures one-dimensional images. Initially used in the advertising industry to shoot static images, linear arrays, and deal with high-resolution images, which are limited to non-moving objects with continuous illumination.

Three-line sensor CCD: In a three-line sensor, three rows of parallel pixels are covered with RGB filters respectively. When capturing a color picture, a complete color picture is composed of multiple rows of pixels. Three-line CCD sensors are mostly used in high-end digital cameras to produce high resolution and spectral gradation.

Interleaved transmission CCD: This sensor uses a separate array to capture images and convert electricity, allowing reading the current image when shooting the next image. Interleaved transmission CCD is usually used in low-end digital cameras, video cameras and broadcast cameras that shoot animation.

Full-format CCD: This kind of CCD has more power handling capacity, better dynamic range, low noise and transmission optical resolution. Full-format CCD allows instant shooting of full-color pictures. Full-format CCD consists of parallel floating-point register, serial floating-point register and signal output amplifier. Full-width CCD exposure is controlled by mechanical shutter or gate to save images, and parallel registers are used for metering and reading metering values. The image is projected onto a parallel array as a projection screen. This element receives the image information and divides it into discrete quantized elements determined by the number. These information flows will flow from parallel registers to serial registers. This process is repeated until all the information is transmitted. Then, the system carries out accurate image reorganization.

CCD sensor-structure editor The CCD in this section is composed of many photosensitive pixels arranged according to certain rules. Each pixel is a MOS capacitor (mostly a photodiode). It forms a MOS capacitor by oxidizing a layer of SiO _ 2 with a thickness of about 1 ~ 15 A on the surface of a P-type Si substrate, then evaporating a metal layer (polysilicon) on the surface of SiO _ 2, and applying a bias voltage between the substrate and the metal electrode. When a beam of light is projected on the MOS capacitor, the photon passes through the transparent electrode and the oxide layer and enters the P-type Si substrate, and the electrons in the valence band in the substrate will absorb the energy of the photon and jump into the conduction band. When photons enter the substrate, electron transitions form electron-hole pairs, which move to both ends of the electrode under the action of external electric field, which is the signal charge. These signal charges are stored in "potential wells" formed by electrodes.

the charge storage capacity of p>MOS capacitor can be obtained by the following formula:

QS=Ci×VG×A

where: QS is the charge storage capacity;

Ci is the capacitance of the oxide layer per unit area;

VG is the applied bias voltage;

A is the area of MOS capacitor gate.

thus, the larger the area of the photosensitive element, the higher its photoelectric sensitivity. The process of charge transfer in a 3-phase CCD.

(a) initial state; (b) the charge is transferred from ① electrode to ② electrode; (c) The charges are uniformly distributed under the electrodes ① and ②;

(d) the charge continues to transfer from ① electrode to ② electrode; (e) the charge is completely transferred to the electrode ②; (f)3-phase overlapping pulses.

suppose that the charge is initially stored in the potential well below the electrode ① (with a voltage of 1V), as shown in fig. 2(a), the voltage applied to all the electrodes of the CCD is usually kept higher than a certain threshold voltage Vth, which is called CCD threshold voltage, and Vth = 2v. So there is a potential well with a certain depth under each electrode. Obviously, the potential well below the electrode ① is the deepest. If the voltage of the electrode ② is gradually increased from 2V to 1V, the potential wells below the electrodes ① and ② will have the same depth and be merged together, and the charges originally stored under the electrode ① will be evenly distributed under the two electrodes, as shown in (b) and (c), and then the voltage below the electrode will be gradually reduced to 2V, so that the depth of the potential well will be reduced. (d) If there are many electrodes, the electrodes can be connected together in the order of 1, 4, 7…, 2, 5, 8… and 3, 6, 9… respectively, and a driving pulse with a certain time sequence can be added to complete the process of charge transfer from left to right. A CCD driven by a three-phase clock is called a three-phase CCD.

CCD sensor-characteristic editing This paragraph ① MTF characteristics of modulation transfer function: The solid-state image sensor is composed of pixel matrix and corresponding transfer parts. Although solid pixels have been made very small and their intervals are very small, it is still the main obstacle to identify tiny images or reproduce tiny parts of images.

② output saturation characteristic: when a strong light image with a saturated exposure is irradiated on the image sensor, the output voltage of the sensor will be saturated, which is called output saturation characteristic. The fundamental reason of output saturation is that photodiode or MOS capacitor can only generate and accumulate a certain limit of photogenerated signal charge.

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

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

⑥ dispersion: Overbright images with saturated exposure will generate and accumulate supersaturated signal charges in pixels. At this time, the supersaturated charges will diffuse from the potential well of one pixel to the potential well of adjacent pixels through the substrate. In this way, the place where a certain brightness should not appear on the reproduced image shows brightness instead, which is called diffusion phenomenon.

⑥ afterimage: after scanning a pixel and reading its signal charge, the phenomenon that the signal read after the next scanning is still affected by the signal charge left over from the last time is called afterimage.

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

CCD sensor-different from CMOS. The main advantage of CMOS for CCD in this paragraph is that it is very power-saving. Unlike CCD composed of diodes, CMOS circuits have almost no static power consumption, and only when the circuit is connected, there is power consumption. This makes the power consumption of CMOS only about 1/3 of that of ordinary CCD, which helps to improve people's bad impression that digital cameras are "electric tigers". The main problem of CMOS is that it is overheated due to too frequent current changes when dealing with fast-changing images. If the dark current is well suppressed, it is not a big problem, and if it is not well suppressed, it is very easy to appear clutter.

in addition, there are great differences between CMOS and CCD image data scanning methods. For example, if the resolution is 3 million pixels, the CCD sensor can scan 3 million charges continuously. The scanning method is very simple, just like passing a bucket from one person to another, and the signal can only be amplified after the last data scanning is completed. Each pixel of CMOS sensor has an amplifier that converts charge into electronic signal. Therefore, CMOS sensor can amplify the signal on a pixel-by-pixel basis, which can save any invalid transmission operation, so it can scan the data quickly with little energy consumption and reduce the noise. This is Canon's in-pixel charge transfer technology.