New CMOS Sensor Enhances Moving Object Capabilities

    This is a translation-adaptation of an article published by Canon engineers in the Japanese Journal of Applied Physic, Japanese Journal of Applied Physics .

    The use of photosensitive matrices in photographic technology allowed us to move away from the use of a mechanical shutter and its variations. This gave a positive effect: the absence of vibrations at the time of shutter release and the ability to significantly increase the shooting speed without complicating the design. But the transition of photographic equipment to a new level has brought new problems that are associated with high-speed shooting.

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    To understand the essence of the difficulties, it is necessary to analyze the principle of operation of photosensitive matrices. Speaking of them in the plural, we mean matrices made using different technologies. There are similarities and fundamental differences in their work. Let's start with the general features. Any photosensitive matrix consists of a set of photodiodes that convert the light flux incident on them into an electrical signal. The difference lies in the method of accumulating and reading signals: the exposure time of the image is determined not by the time by which the shutter opens, but by the time between zeroing the matrix charge and the moment of reading information from it.

    In a CCD matrix, the signal is read out line by line, and such a shutter is called a traveling or rolling shutter. During line-by-line reading, a fast-moving object manages to change position, so distortion occurs in the picture. And the greater the speed of the object, the greater the distortion in the picture.

    This problem is partially solved in CMOS matrices, which have recently become an alternative to CCD matrices. Here, the signal can be read from any fragment of the matrix and in any order. This not only increases the speed of data exchange, but also allows you to get random access to individual pixels.

    In fact, the CMOS matrix is ​​an integrated circuit, where each pixel forms a separate cell and has its own strapping that converts the charge of the photodiode into voltage directly in the pixel itself. In general, a cell consists of:

    • photodiode;
    • electronic shutter;
    • a capacitor that collects charge from the photodiode;
    • signal amplifier;
    • line reading buses;
    • signal transmission buses to the processor;
    • reset signal lines.

    During shooting, the image is formed due to the synthesis of several frames. On the one hand, this gives the depth and saturation of the image, but on the other hand, when jittering or shooting moving objects, the image quality decreases. This translates into blur, a “double” image or the effect of a running shutter. The reason for this is the alternation of the processes of exposure and reading. Let us take conditionally for the exposure time t. Then at time t, the first frame is shot. In the period t + t, the data of this frame is read. Then, after resetting the matrix, the next frame is executed. Thus, the gap between frames is t. This situation is similar to the rolling shutter algorithm.

    One of the solutions to this problem was proposed by our developers, and it was as follows. In a regular cell of the CMOS matrix, a single capacitor with a strapping is used that performs the function of a memory element; therefore, at any point in time, the cell is either in the state of charge of this capacitor (exposure) or discharge (reading). In the cell of our development, two memory elements are used. Due to this, two processes can occur simultaneously. After shooting the first frame, while reading data from one memory element, the next frame is immediately displayed with the recording on the second memory element. This ensures continuous recording and image stability.

    However, the meaning of this invention is not limited only to the continuity of shooting. In fact, we got several different modes of operation of the CMOS sensor. It all depends on the procedure for reading pixels.

    • When reading at a high frame rate, pixel saturation can occur due to either multiple saturation of the photodiode, or a single saturation of the memory element. At the same time, image clarity is combined with saturation.
    • In shooting mode with high saturation, two storage elements are filled and read simultaneously. At the same time, the reading frequency is reduced, which as a bonus gives a decrease in the total power consumption.

    The possibility of multiple accumulation is used when performing series of exposures, for example, when alternating short and long. At the same time, storage elements alternate: on one, the signal of short exposures is accumulated, and on the other, long ones. When compared with a CMOS matrix with one storage element and a total shutter speed equal to a series of 5 short and 4 long exposures, the improvement in the dynamic range is about 42 dB.

    An increase in pixel strapping results in an increase in spurious noise. To reduce its influence, the cell elements are located diagonally symmetrically with respect to the photodiode. From the influence of light flux they are protected by a light screen. Only for the photodiode, an aperture of 1.3 μm was left. The light incident on the photodiode is focused using a double lens unit and a light guide. In the block between the lenses is a color filter in accordance with the Bayer template. A material with a high refractive index is used for the fiber. Due to this, the fiber in the shape of an inverted cone has a small height corresponding to three layers of copper wiring. The upper diameter of the fiber is 2.4 microns, and the lower is 1.1 microns.

    A single pixel of the matrix, according to the Bayer pattern, consists of a pair of pixels with double memory cells. A unit pixel block includes:

    • 2 photodiodes;
    • 4 storage elements (capacitor);
    • 13 transistors.

    The total size of the matrix is ​​2676 N × 2200 V, which is almost 5.9 megapixels.
    The comparative table gives the characteristics of the various read modes of the developed CMOS matrix with dual intra-pixel memory and a conventional matrix with comparable indicators.

    Read mode2 high frame rate CDMEM2 CDMEMs high saturation2 high DRM CDMEM1 CDMEM normal
    Process technologyFSI, 130 nm1P4M + LS CMOS
    Optical format2/3 inches
    Pixel pitchSquare 3.4, microns
    Quantity Eff. pixels2592 (V), ×, 2054 (V) = 5.3 M pixels
    Source of power3.3 V (analog), 1.2 V (digital)
    Maximum frame rate120 frames per second100 frames per second60 frames per second120 frames per second
    Power consumption480 mW400 mW480 mW450 mW
    Total well capacity9500 e -19 000 e -940,000 e - (equivalent)8 100 e - or 16 200 e -
    Sensitivity @ green30 000 e - / lk.s28 000 e - / l.s.
    Temporary noise2,81.8
    Dynamic range71 dB77 dB111 dB73 dB
    Pls cdmem−83 dB−89 dB

    In fact, the developed CMOS image sensor with a pixel pitch of 3.4 microns with dual intra-pixel memory has about 5.3 effective megapixels and a dynamic range of more than 110 dB when exposed in a single frame with alternate multiple accumulation. This mode is especially suitable for shooting moving objects and can be used in movie cameras, computer vision devices, automobiles, aerial photography and surveillance cameras.

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