High Speed CMOS Image Sensor and Its Development Trend

key word:

CMOS, development trend, image sensor

The sensor architecture can be composed of two fractions, four fractions or a pixel array. The output can be a parallel analog output, or a 10 bit digital output or a digital serial LVDS output. Each output can be sampled up to 50 million times per second, which can achieve a throughput of 5.5 billion pixels per second. So far, the image sensor has the highest continuous pixel throughput. The image quality reaches at least 10 bit accuracy, so after the camera is digitized, the data throughput can be 55gbit per second. Such high-speed applications usually require 6 transistor snapshot pixels, and require high sensitivity and dynamic range. The sensitivity of image sensor largely depends on the pixel size, and large pixel size requires a large area of customized image sensor for specific applications. Internal multiplexing technology can support random windows with higher frame rate. If the window size is reduced to a smaller ROI (circle the target area), the frame rate of the fastest device can reach 170000 frames per second. Most sensors use a 0.25 process.

At present, CMOS is the preferred technology for high-speed imaging. In the current market, we can find that there are three development trends of high-speed image sensors: one is to develop towards extremely high speed, the other is to develop towards on-chip feature integration, and the third is to develop towards general-purpose high-speed image sensors.

The combination of resolution and frame rate plays an important role. At present, we can launch 1024 × 1024 pixel image sensor, working speed up to 5000 full frames per second. If the analog-to-digital conversion is 10 bits, this means that the total data rate on the camera can reach 55gbit per second. In order to achieve extremely high data rate and high image quality on the sensor, especially for this highly sensitive application, we should not only design the correct electronic circuit, but also ensure the good balance of the whole circuit layout. That is to say, the power lines should achieve excellent distribution, and all optical and stray light sensitivity of each line node in the layout should be well controlled. Low power module design is required to ensure that the overall power consumption requirements are met.

Another trend in the field of high-speed imaging is the on-chip integration of high-speed ADC, timing generator, LVDS transmitter and correction algorithm. This kind of image sensor is generally inferior to the above image sensor in speed and sensitivity, but it has advantages in ease of use and system integration function. At present, the third emerging image sensor in the market is the general high-speed image sensor. Old (simple) general-purpose image sensors with analog output or without timing generator function are being replaced by faster and more complex image sensors. This new image sensor enables us to design a general-purpose high-speed camera in a short time.   

6 transistor pixels

Most of cypress's high-speed image sensors use so-called 6 transistor (6T) pixels. One very important thing about this image sensor is its pipelined global shutter feature. In terms of global shutter, all pixels start and stop optical integration at the same time, which is very important for high-speed applications to control the motion image blur of all pixels. The global shutter technology enables the image sensor to fix the high-speed moving scene. The following figure shows a typical high-speed capture sequence (a small warhead hits a matchstick). The pipeline feature ensures that the pixel array can carry out the optical integration in the pixels of the next frame during reading, so as to ensure that the frame rate is not affected by the integration time.   

Typical high-speed capture sequence.

In order to achieve the highest possible sensitivity, the photodiode that collects photocharges and converts them into voltage should be designed as small as possible to reduce the parasitic capacitance of the photodiode as much as possible. In addition, the pixel fill factor (the ratio of pixel open area to photosensitive area) should be as large as possible. We can use the patented technology of n-well pixel and cooperate with the p-well opening around the photodiode to realize the miniaturization of photodiode and large filling factor. In addition to high sensitivity, we should also make the pixel storage capacitance, achieve good shielding with light, and reduce leakage to ensure that it will not cause any noise. The pixel architecture achieves good performance in signal storage during pixel reading. However, this pixel architecture still lacks the fixed pattern noise correction function of pixels, which must be completed on systems other than image sensors.

Faster speed

The speed of image sensor refers to the product of accuracy and frame rate, which reflects the pixel rate of the sensor. In the very high-end high-speed imaging market, people's demand for speed is endless. In the high-end market, customers want to achieve extremely high full frame rate through advanced cameras, such as car collision detection.

Only using parallel analog output can achieve such a high speed, but parallel analog output (up to 128 outputs) will also challenge the integration of camera system. The architecture of this type of image sensor is relatively simple: it includes a 6T pixel array that can be divided into several image limits, several parallel high-speed analog buses and parallel output amplifiers for driving output.

6T transistors can achieve very high speed.

The above chip does not use ADC, timing generator or other on-chip image processing technology. The analog bus on the whole chip ensures that all parallel outputs can be used, regardless of the width of the read image size. This can speed up the frame rate when reading part of the image. An important problem with this very high-speed image sensor is the ghosting phenomenon in the X direction, which is caused by the relatively large RC constant of the analog bus on the whole chip. Since it takes some time for the signal on the bus to stabilize to within 10 bit accuracy, part of the information of the previous pixel still exists on the current pixel. In the image, this causes ghosting in the X direction. This ghosting is difficult to correct in the image post-processing stage. We have developed a special technology for this image sensor, which precharges the bus in a short time before each new signal, which will eliminate the information of all previous pixels. This technology requires the generation of a short precharge pulse to short circuit the analog bus to ground. Most of these image sensors are customized products according to customer requirements.

At present, such extremely high-speed image sensors will not appear as general products. Over the years, cypress has launched several very high-speed image sensors for several top high-speed camera manufacturers in the market, with resolutions ranging from VGA to 10 million pixels, while the speed supports 500 to 10000 frames per second. In recent years, the speed and sensitivity have been continuously improved. Now the data throughput of the fastest product has reached 5.5G pixels per second. In a typical very high-speed image sensor architecture, two parts of the parallel readout graph provide 64 parallel analog outputs, so we can get 128 high-speed parallel analog outputs.

More applications

In contradiction to the (large) extremely complex camera system built around sensors above, the market has more and more demand for miniaturized high-speed image sensors, and hopes to improve their ease of use. High speed image sensors have begun to appear in similar consumer electronics applications such as scanners, vision systems and holographic data storage.

The above applications require the image sensor to integrate many system functions on the board. Therefore, ADC, timing generator, image processing and other output stages are integrated on the chip. For this kind of image sensor, on-board function integration is as important as sensitivity and speed. Most of these image sensors are customized according to customer needs to provide given features, which help to simplify the design of customized high-speed cameras. Such high-speed image sensors usually have only one clock input, a few power supplies and some synchronization pins. All other signals read and exposed by the image sensor are generated on the chip.

Universal high speed image sensor

The third type of high-speed image sensor we see in the market (launched last year) is the general high-speed image sensor. Its applications include machine vision camera, traffic monitoring, scientific motion capture and collision detection. The first universal high-speed image sensor includes only parallel analog output and does not provide on-board logic (much like today's extremely high-speed image sensor). However, now, the chip itself has integrated many features, making the image sensor suitable for a variety of different applications (multi slope exposure, sub sampling, grading, mirroring, gain, offset, etc.). Cypress currently provides a variety of high-speed global shutter image sensors with specifications ranging from VGA, 250 frames per second (lupa300) to 1.3m pixels and 1000 frames per second (under development). All these image sensors support pipeline snapshot function and multi slope exposure function. Simultaneous interpreting of board characteristics of different sensors.

Conclusion

This paper briefly introduces several different types of high-speed image sensors, which correspond to different market needs. We divide the products in the market into three categories:

Very high speed imaging: pure analog image sensor with extremely high frame rate and data throughput - > complex camera design - > mainly used for customized design.   

High speed imaging with board level characteristics: this image sensor has many specific board level characteristics, which is helpful to carry out the internal design of image sensor and meet the needs of high-speed cameras in similar consumer electronic applications. Relevant features are provided according to customer needs, mainly customized design.   

Universal high-speed image sensor: this image sensor combines the most common characteristics of the above image sensor with the universal image sensor, which can be suitable for the needs of cameras in a variety of applications.   

The market development trend proves that the requirements for board level characteristics, data rate and accuracy will continue to rise. The real challenge is how to effectively combine the image sensor with high data rate with many board level features such as LVDS and image processing.

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