Abstract: At present, CMOS image sensors mainly focus on high-resolution, high-dynamic range, high-sensitivity, ultra-miniature, digital, and multi-functional orientation.
I. INTRODUCTION At the beginning of the 1970s, the United States Bell Labs successfully developed the first electrical coupling device (CCD), after which the CCD technology developed rapidly. As a new type of photoelectric converter, CCD image sensor has been widely used in camera, image acquisition, scanner and industrial measurement. With the expansion of the CCD application, its shortcomings are gradually exposed. To this end, people have developed several other solid image sensor technologies. Among them, the most eye-catching and most promising potential is the CMOS image sensor, which can obtain similar image quality as the CCD product, and has achieved great breakthroughs in power consumption and integration.
Second, CCD structure and its characteristics CCD charge-coupled devices, is a high-performance miniature image sensor. This new type of photoelectric imaging device has a series of advantages such as high sensitivity, wide optical latency response, high integration, easy maintenance, and low cost, and is applied in such fields as defense, medical science, industry, medicine, biology, astronomy, geology, and aerospace science. The wide range of applications in all areas of technology is one of the most important image sensors in modern times. Charge-coupled device (CCD) is characterized by the fact that charge is the signal, unlike most other devices that use current or voltage as the signal. The basic unit constituting the CCD is a MOS (Metal-Oxide-Semiconductor) structure. The basic function of the CCD is the storage of charge and the transfer of charge. When working, it is necessary to apply a certain bias to the metal gate to form a potential well to accommodate the charge. The amount of charge is basically linear with the light intensity. When the charge is read out, it moves from one position to the next under the action of a shift pulse of a certain phase relationship until the CCD is removed and converted into an analog signal after charge-voltage conversion. Because of the limited ability of the potential well of each pixel of the CCD to accommodate the charge, if the illumination is too strong, the electrons will “overflow†once the charge in the potential well is filled. In addition, when the charge of the CCD is read out, it is a charge transfer process from one position to the next, and there is a problem of charge transfer efficiency and transfer loss. The structure and working principle of the CCD image sensor determine that these devices have the following advantages:
1CCD is a solid-state device, small size, light weight, high reliability, long life;
2 The image distortion is small and the size reproducibility is good;
3 has a high spatial resolution;
4 The geometrical accuracy of the distance between photosensitive elements is high, and high positioning accuracy and measurement accuracy can be obtained;
5 has a high photoelectric sensitivity and a large dynamic range.
Third, CMOS image sensor structure CMOS image sensor generally consists of photosensitive cell array, line strobe logic, column strobe logic, timing and control circuitry, and on-chip analog signal processor (ASP). More advanced CMOS image sensors also integrate on-chip analog-to-digital converters (ADCs). This type of device uses a single 5V power supply.
The row select logic and column select logic can be shift registers or decoders. Timing and control circuits limit the signal readout mode, set the integration time, and control the data output rate. The on-chip analog signal processor performs functions such as signal integration, amplification, sampling and hold, correlated double sampling, and double sampling. The on-chip analog/digital converter is necessary for on-chip digital imaging systems. The CMOS image sensor can be an entire imaging array with one ADC or several ADCs (one for each color), or it can be an imaging array with one for each column. The photosensitive unit of the ADC. photosensitive unit converts the optical signal into an electrical signal, and after processing in the chip signal processing circuit, outputs the signal in analog or digital form.
Fourth, CMOS and CCD comparison 1, the sensitivity of sensitivity comparison represents the sensor's photosensitive unit to collect photons to generate charge signals. CCD image sensor sensitivity is 30%~50% higher than CMOS image sensor. This is mainly due to the fact that the depletion region of the CCD image element can reach a depth of 10mm, and has a complete collection capability of the visible light and near-infrared spectral segments. CMOS image sensor adopts 0.18~0.5mm standard CMOS technology. Because low-resistance silicon wafers must maintain low operating voltage, the pixel depletion region depth is only 1~2mm, and the absorption cut-off wavelength is less than 650nm, resulting in pixel pairs of red light. It is difficult to absorb near-infrared light. 3.2 Electron-voltage conversion rate The electron-voltage conversion rate represents the magnitude of each signal electronically converted to a voltage signal. Because the CMOS image sensor uses a high-gain, low-power complementary amplifier structure in the picture element, its voltage conversion rate is slightly better than the CCD image sensor. To achieve the same voltage conversion rate, CCD image sensors need to pay a further increase in device power consumption.
CCD developers are further studying the structure of the new sense amplifier to improve the response rate.
2. Response speed response speed Since the CCD adopts the serial continuous scanning working mode, one-time reading of the entire row or entire column of pixel data must be performed. Because COMS adopts single-point signal transmission, through simple XY addressing technology, data can be read from the entire arrangement, part or even the unit, thereby increasing the addressing speed and realizing faster signal transmission.
3, noise comparison CCD is characterized by the fact that from the full retention of the signal in the transmission is not distorted (with exclusive channel design). Uniform processing through each pixel set to a single amplifier. Data integrity can be maintained. In contrast, each pixel in the CMOS design is directly connected to an ADC (amplification and analog/digital signal converter), and there is no channel design. Therefore, you must first enlarge and then integrate the data of each pixel. Therefore, the CMOS calculation will be more early than the CCD, which will affect the image quality.
4, the cost comparison Because the CMOS sensor uses the semiconductor circuit most commonly used CMOS technology, can be easily.
Peripheral circuits (such as AGC, DDS, clock, DSP, etc.) are integrated into the sensor chip, so the cost of the peripheral chip can be saved; while the CCD sensor uses charge transfer to transfer data. One of the pixels does not work. Will result in a whole row of data can not be transmitted. Controlling the yield of CCD sensors is much more difficult than CMOS sensors. Therefore, the cost of CCD sensors is higher than that of CMOS.
Therefore, in general, CCD and CMOS are compared. Although CCD sensors and CMOS sensors were developed in the 1970s, they have become the mainstream of image sensors because of their high sensitivity and low noise. Complementary Metal Oxide (CMOS) image sensors have not received attention and development due to process reasons, and have not been free from the disadvantages of low light sensitivity, noise reduction, and low image resolution.
CCD image sensors also make the CCD camera/camera bulky and consume large power due to the fact that the sensitive elements and signal processing circuits cannot be integrated on the same chip.
CMOS sensors have the advantages of high integration, low power consumption, and low cost. If CMOS image sensors can overcome the above shortcomings and maintain the original advantages, they have advantages over CCD sensors. Due to the improvement of integrated circuit design technology and the level of technology, the shortcomings of CMOS image sensors in the past can now be found to overcome the methods, and its inherent advantages are even CCD devices can not be compared, and thus it has become a research hotspot again. The CCD sensor requires a number of different voltages to make it work, and the CMOS sensor requires only a single voltage operation, which is another great advantage compared to the CCD sensor. The CCD sensor needs to be externally connected to an amplifier, an analog-to-digital converter, a sequential circuit, and the like, resulting in a large volume and a limited reading speed. The CMOS image sensor is rather a complete image system. A typical CMOS image sensor usually contains: an image sensor core, which is similar to a CCD image sensor, all sequential logic circuits, a single clock, and in-chip programmable functions such as gain adjustment, integration time, window, and analog-to-digital converter . Compared with the traditional CCD image system, integrating the entire image system on one chip not only reduces power consumption, but also has the advantages of lighter weight, less space, and lower overall price.
V. Current Status of Development Currently, CMOS image sensors are oriented toward high-resolution, high-dynamic range, high-sensitivity, ultra-miniature, digital, and multi-functional orientation. In 1996, a 2048×2048 array CMOS image sensor was developed using a 0.5m CMOS process. When the 0.25mCMOS process is used, it is believed that a sensor with a higher array will be produced. By adopting a new process and improving the related double sampling circuit, the fixed mode noise can be effectively reduced and the dark current can be reduced; the prism can be used to increase the fill factor by 70%; The use of a layer of doped layer below the pixel cell allows the fill factor to reach 100%. A step-gate reset gate voltage technology can increase the dynamic range of the APSCMOS image sensor by 90dB; the thin-film image sensor using ASIC technology can enhance the local pixel contrast and can achieve a dynamic range of 120dB. Consider the small size of the CMOS image sensor. , low power consumption, high integration, new USB computer interface and infrared interface technology these outstanding advantages, I believe a new era of digital imaging technology is approaching.
I. INTRODUCTION At the beginning of the 1970s, the United States Bell Labs successfully developed the first electrical coupling device (CCD), after which the CCD technology developed rapidly. As a new type of photoelectric converter, CCD image sensor has been widely used in camera, image acquisition, scanner and industrial measurement. With the expansion of the CCD application, its shortcomings are gradually exposed. To this end, people have developed several other solid image sensor technologies. Among them, the most eye-catching and most promising potential is the CMOS image sensor, which can obtain similar image quality as the CCD product, and has achieved great breakthroughs in power consumption and integration.
Second, CCD structure and its characteristics CCD charge-coupled devices, is a high-performance miniature image sensor. This new type of photoelectric imaging device has a series of advantages such as high sensitivity, wide optical latency response, high integration, easy maintenance, and low cost, and is applied in such fields as defense, medical science, industry, medicine, biology, astronomy, geology, and aerospace science. The wide range of applications in all areas of technology is one of the most important image sensors in modern times. Charge-coupled device (CCD) is characterized by the fact that charge is the signal, unlike most other devices that use current or voltage as the signal. The basic unit constituting the CCD is a MOS (Metal-Oxide-Semiconductor) structure. The basic function of the CCD is the storage of charge and the transfer of charge. When working, it is necessary to apply a certain bias to the metal gate to form a potential well to accommodate the charge. The amount of charge is basically linear with the light intensity. When the charge is read out, it moves from one position to the next under the action of a shift pulse of a certain phase relationship until the CCD is removed and converted into an analog signal after charge-voltage conversion. Because of the limited ability of the potential well of each pixel of the CCD to accommodate the charge, if the illumination is too strong, the electrons will “overflow†once the charge in the potential well is filled. In addition, when the charge of the CCD is read out, it is a charge transfer process from one position to the next, and there is a problem of charge transfer efficiency and transfer loss. The structure and working principle of the CCD image sensor determine that these devices have the following advantages:
1CCD is a solid-state device, small size, light weight, high reliability, long life;
2 The image distortion is small and the size reproducibility is good;
3 has a high spatial resolution;
4 The geometrical accuracy of the distance between photosensitive elements is high, and high positioning accuracy and measurement accuracy can be obtained;
5 has a high photoelectric sensitivity and a large dynamic range.
Third, CMOS image sensor structure CMOS image sensor generally consists of photosensitive cell array, line strobe logic, column strobe logic, timing and control circuitry, and on-chip analog signal processor (ASP). More advanced CMOS image sensors also integrate on-chip analog-to-digital converters (ADCs). This type of device uses a single 5V power supply.
The row select logic and column select logic can be shift registers or decoders. Timing and control circuits limit the signal readout mode, set the integration time, and control the data output rate. The on-chip analog signal processor performs functions such as signal integration, amplification, sampling and hold, correlated double sampling, and double sampling. The on-chip analog/digital converter is necessary for on-chip digital imaging systems. The CMOS image sensor can be an entire imaging array with one ADC or several ADCs (one for each color), or it can be an imaging array with one for each column. The photosensitive unit of the ADC. photosensitive unit converts the optical signal into an electrical signal, and after processing in the chip signal processing circuit, outputs the signal in analog or digital form.
Fourth, CMOS and CCD comparison 1, the sensitivity of sensitivity comparison represents the sensor's photosensitive unit to collect photons to generate charge signals. CCD image sensor sensitivity is 30%~50% higher than CMOS image sensor. This is mainly due to the fact that the depletion region of the CCD image element can reach a depth of 10mm, and has a complete collection capability of the visible light and near-infrared spectral segments. CMOS image sensor adopts 0.18~0.5mm standard CMOS technology. Because low-resistance silicon wafers must maintain low operating voltage, the pixel depletion region depth is only 1~2mm, and the absorption cut-off wavelength is less than 650nm, resulting in pixel pairs of red light. It is difficult to absorb near-infrared light. 3.2 Electron-voltage conversion rate The electron-voltage conversion rate represents the magnitude of each signal electronically converted to a voltage signal. Because the CMOS image sensor uses a high-gain, low-power complementary amplifier structure in the picture element, its voltage conversion rate is slightly better than the CCD image sensor. To achieve the same voltage conversion rate, CCD image sensors need to pay a further increase in device power consumption.
CCD developers are further studying the structure of the new sense amplifier to improve the response rate.
2. Response speed response speed Since the CCD adopts the serial continuous scanning working mode, one-time reading of the entire row or entire column of pixel data must be performed. Because COMS adopts single-point signal transmission, through simple XY addressing technology, data can be read from the entire arrangement, part or even the unit, thereby increasing the addressing speed and realizing faster signal transmission.
3, noise comparison CCD is characterized by the fact that from the full retention of the signal in the transmission is not distorted (with exclusive channel design). Uniform processing through each pixel set to a single amplifier. Data integrity can be maintained. In contrast, each pixel in the CMOS design is directly connected to an ADC (amplification and analog/digital signal converter), and there is no channel design. Therefore, you must first enlarge and then integrate the data of each pixel. Therefore, the CMOS calculation will be more early than the CCD, which will affect the image quality.
4, the cost comparison Because the CMOS sensor uses the semiconductor circuit most commonly used CMOS technology, can be easily.
Peripheral circuits (such as AGC, DDS, clock, DSP, etc.) are integrated into the sensor chip, so the cost of the peripheral chip can be saved; while the CCD sensor uses charge transfer to transfer data. One of the pixels does not work. Will result in a whole row of data can not be transmitted. Controlling the yield of CCD sensors is much more difficult than CMOS sensors. Therefore, the cost of CCD sensors is higher than that of CMOS.
Therefore, in general, CCD and CMOS are compared. Although CCD sensors and CMOS sensors were developed in the 1970s, they have become the mainstream of image sensors because of their high sensitivity and low noise. Complementary Metal Oxide (CMOS) image sensors have not received attention and development due to process reasons, and have not been free from the disadvantages of low light sensitivity, noise reduction, and low image resolution.
CCD image sensors also make the CCD camera/camera bulky and consume large power due to the fact that the sensitive elements and signal processing circuits cannot be integrated on the same chip.
CMOS sensors have the advantages of high integration, low power consumption, and low cost. If CMOS image sensors can overcome the above shortcomings and maintain the original advantages, they have advantages over CCD sensors. Due to the improvement of integrated circuit design technology and the level of technology, the shortcomings of CMOS image sensors in the past can now be found to overcome the methods, and its inherent advantages are even CCD devices can not be compared, and thus it has become a research hotspot again. The CCD sensor requires a number of different voltages to make it work, and the CMOS sensor requires only a single voltage operation, which is another great advantage compared to the CCD sensor. The CCD sensor needs to be externally connected to an amplifier, an analog-to-digital converter, a sequential circuit, and the like, resulting in a large volume and a limited reading speed. The CMOS image sensor is rather a complete image system. A typical CMOS image sensor usually contains: an image sensor core, which is similar to a CCD image sensor, all sequential logic circuits, a single clock, and in-chip programmable functions such as gain adjustment, integration time, window, and analog-to-digital converter . Compared with the traditional CCD image system, integrating the entire image system on one chip not only reduces power consumption, but also has the advantages of lighter weight, less space, and lower overall price.
V. Current Status of Development Currently, CMOS image sensors are oriented toward high-resolution, high-dynamic range, high-sensitivity, ultra-miniature, digital, and multi-functional orientation. In 1996, a 2048×2048 array CMOS image sensor was developed using a 0.5m CMOS process. When the 0.25mCMOS process is used, it is believed that a sensor with a higher array will be produced. By adopting a new process and improving the related double sampling circuit, the fixed mode noise can be effectively reduced and the dark current can be reduced; the prism can be used to increase the fill factor by 70%; The use of a layer of doped layer below the pixel cell allows the fill factor to reach 100%. A step-gate reset gate voltage technology can increase the dynamic range of the APSCMOS image sensor by 90dB; the thin-film image sensor using ASIC technology can enhance the local pixel contrast and can achieve a dynamic range of 120dB. Consider the small size of the CMOS image sensor. , low power consumption, high integration, new USB computer interface and infrared interface technology these outstanding advantages, I believe a new era of digital imaging technology is approaching.
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