Charge CoupLED Image Sensor CCD (Charge CoupLED DevICe), which is made of a high-sensitivity semiconductor material, can convert light into electric charge, convert it into digital signal through analog-to-digital converter chip, and digital signal is compressed by The camera's internal flash memory or internal hard disk card saves data, making it easy to transfer data to a computer and use the computer's processing to modify the image as needed and imagined.
principle
The CCD sensor is a new type of photoelectric conversion device that stores signal charges generated by light. When a pulse of a specific timing is applied to it, the stored signal charge can be self-scanned by directional transmission in the CCD. It is mainly composed of a photosensitive unit, an input structure, and an output structure. It has the functions of photoelectric conversion, information storage and delay, and has high integration and low power consumption. It has been widely used in three fields of camera, signal processing and storage, especially in the application of image sensor. People's attention to development. The CCD has an area array and a line array, and the area array is a device in which CCD pixels are arranged in one plane; and a line array is a device in which CCD pixels are arranged in a straight line. Since the CCD is mainly used in the military field, the area array CCD is mainly introduced here.
species
Area array CCD
Area CCD: Allows the photographer to shoot moving objects at one exposure at any shutter speed.
There are generally three types of area array CCD structures. The first is a frame-turning CCD. It consists of two parts, the upper part is the photosensitive area where the pixels are concentrated, and the lower part is the storage area where the vertical registers are shaded and concentrated. The advantage is that the structure is relatively simple and the number of pixels is easily increased. The disadvantage is that the CCD has a large size and is prone to vertical smear. The second type is an interline transfer CCD. It is currently the mainstream product of CCD. They are pixel groups and vertical registers on the same plane. They are characterized by a low price on a single chip and easy to obtain good photographic characteristics. The third type is a frame interline transfer CCD. It is a composite type of the first type and the second type, and has a complicated structure, but can greatly reduce the vertical smear and easily realize the advantages of the variable speed electronic shutter.
Linear array CCD
Line CCD: Scans the picture with a row of pixels and makes three exposures—the three colors of red, green, and blue, respectively. As the name suggests, the linear sensor captures a one-dimensional image. Initially used in the advertising industry to capture still images, linear arrays, when dealing with high-resolution images, subject to non-moving continuous illumination of objects.
Three-wire sensor CCD
Three-line sensor CCD: In the three-line sensor, three rows of parallel pixels cover the RGB filter respectively. When capturing a color picture, the 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 color gradation.
Interlaced Transport CCD: This sensor utilizes a separate array to capture images and power conversion, allowing the current image to be read while the next image is being taken. Interlaced transmission CCDs are commonly used in low-end digital cameras, video cameras, and broadcast cameras that shoot movies.
Full-size CCD
Full-format CCD: This CCD has more power handling capability, better dynamic range, low noise and transmission optical resolution, and the full-format CCD allows instant shooting of full-color images. The full-frame CCD consists of a parallel floating-point register, a serial floating-point register, and a signal output amplifier. Full-format CCD exposure is controlled by a mechanical shutter or gate to save the image, and parallel registers are used for metering and reading the metering value. The images are projected onto a parallel array of projection screens. This element receives the image information and it is divided into discrete elements that are quantized by the number. These streams of information flow from the parallel registers to the serial registers. This process is repeated until all the information has been transferred. The system then performs an accurate image reorganization.
structure
The CCD is composed of a number of photosensitive pixels arranged in a regular pattern. Each pixel is a MOS capacitor (mostly a photodiode) which is oxidized to form a layer of SiO2 having a thickness of about 1000A to 15 00A on the surface of a P-type Si substrate, and then vapor-depositing a metal on the surface of SiO2. In the layer (polysilicon), a bias voltage is applied between the substrate and the metal electrode to form one MOS capacitor. When one beam of light is projected onto the MOS capacitor, the photon passes through the transparent electrode and the oxide layer and enters the P-type Si substrate. The electrons in the valence band in the substrate absorb the energy of the photon and leapt into the conduction band. The electron transition generated when a photon enters the substrate forms an electron-hole pair, and the electron-hole pair moves to both ends of the electrode under the action of an applied electric field, which is a signal charge. These signal charges are stored in a "potential well" formed by the electrodes.
The charge storage capacity of the MOS capacitor can be obtained by the following formula:
QS=Ci×VG×A
Where: QS is the charge storage amount;
Ci is the capacitance of the oxide layer per unit area; VG is the applied bias voltage;
A is the area of ​​the MOS capacitor gate.
It can be seen that the larger the area of ​​the photosensitive element, the higher the photoelectric sensitivity. A 3-phase drive works the process of charge transfer in a CCD.
(a) an initial state; (b) a charge is transferred from the 1 electrode to the 2 electrode; (c) the charge is uniformly distributed under the electrodes 1 and 2;
(d) The charge continues to be transferred from the 1 electrode to the 2 electrode; (e) the charge is completely transferred to the 2 electrode; (f) the 3 phase overlap pulse.
Assuming that the charge is initially stored in the potential well below electrode 1 (with a voltage of 10V), as shown in Figure 2(a), the voltage applied to all the electrodes of the CCD is usually kept above a certain threshold voltage Vth. Vth is called CCD threshold voltage, and Vth=2V. So there is a potential well below the 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 10V, then the potential wells under the two electrodes of 1 and 2 have the same depth and are merged together and originally stored in the electrode. The charge below 1 is evenly distributed under the two electrodes, as shown in (b) and (c), and then gradually lower the voltage under the electrode to 2V, causing the potential depth to decrease, (d) and (e) As shown, then all of the charge is transferred to the potential well below the electrode 2, which is the transfer of charge from the electrode 1 to the electrode 2. 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..., plus a certain timing of the drive pulse, the charge can be completed. The process of moving from left to right. A CCD driven by a 3-phase clock is called a 3-phase CCD.
characteristic
1 Modulation transfer function MTF characteristics: The solid-state image sensor is composed of a pixel matrix and a corresponding transfer portion. Although solid-state pixels have been made small and their spacing is small, this is still a major obstacle to identifying tiny images or reproducing subtle parts of an image.
2 Output saturation characteristics: When a strong light image above the saturated exposure is irradiated onto the image sensor, the output voltage of the sensor will be saturated. This phenomenon is called output saturation. The root cause of the output saturation phenomenon is that the photodiode or MOS capacitor can only generate and accumulate a certain limit of the photogenerated signal charge.
3 dark output characteristics: dark output, also known as unlit output, refers to no light image
When the CCD sensor signal is illuminated, the sensor still has a small output characteristic, and the output is derived from a dark (unlit) 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: An over-bright light image above the saturated exposure will generate and accumulate a supersaturated signal charge in the pixel. At this time, the super-saturated charge will diffuse from the potential well of one pixel through the substrate to the potential well of the adjacent pixel. . Thus, the portion of the reproduced image that should not exhibit a certain brightness exhibits brightness instead, which is called a dispersion phenomenon.
6 afterimage: After scanning a certain pixel and reading its signal charge, the phenomenon that the read signal is still affected by the last remaining signal charge after the next scan is called afterimage.
7 Equivalent noise exposure: The amount of exposure when the equivalent value to the dark output (voltage) is generated is called the equivalent noise exposure of the sensor.
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