|CCD image sensors|
|Friday, 01 September 2006|
A charge coupled device (CCD) is an active device for transferring, under the control of a clock pulse, a signal charge corresponding to incident light incoming along a predetermined path. A charge coupled device has a structure in which a multiplicity of metal oxide semiconductor (MOS) structure electrodes are arranged on a surface of a semiconductor, and generally is used as a so-called solid-state imaging device in various types of electronic cameras, facsimiles, etc. A CCD typically contains several pixels, with each pixel holding an amount of charge proportionate to the intensity of incident light and the length of time the light is allowed to fall on the pixel. A charge coupled device transfers pulse charges in one direction by use of the potential difference within a semiconductor device induced by the potential difference applied to each transfer electrode. Charge coupled devices (CCDs) are often used to capture images received in the form of light. Charge coupled devices are used for accomplishing charge domain operations such as charge packet transfer, summation, and splitting. The charge coupled device has found a wide variety of application and is widely utilized for a solid state image sensing device.
The charge coupled device has typically been used in image processing devices, particularly as image sensors. In general, a charge coupled device is an image sensor which converts an image signal into an electrical signal. Charge coupled device (CCD) image sensors typically include a silicon or other semiconductor substrate, an optical-electrical conversion region, a plurality of vertical charge coupled devices (VCCD), a horizontal charge coupled device (HCCD), and a sensing amplifier. A CCD image sensor is generally formed by disposing a plurality of photodetectors and a plurality of CCDs as a photoscanner on a substrate of a semi-conductor material, such as silicon oxide. The CCD imager in general includes a number of light receiving sections arrayed in a two-dimensional matrix and signal charges for producing the image information are produced in these light receiving sections. The light receiving region is used for receiving light and generating a signal charge in response to the received light, and a signal charge transfer region for transferring the generated signal charge in a single direction. The VCCD is used for transferring a signal charge converted by the photodiode (PD) in a vertical direction, the HCCD is used for transferring the charge transferred from the VCCI) in a horizontal direction, and the sense amplifier is used for sensing the signal charge transferred from the HCCD.
Charge coupled devices are widely used in digital imaging devices such as optical scanners, fax machines, digital copiers, video cameras and digital cameras. In an image scanning apparatus, a charge-coupled device (CCD) is used to acquire optical signals representing images scanned, convert the optical signals into image signals, and output the image signals to an analog signal processing circuit for further image signal processing in the next stage. A CCD image sensor utilizing CCDs typically converts light energy through the photodetector into an electric signal, with the photoelectric efficiency being very low in a wavelength of a red color type, wherein the amount of light energy is small. A solid state image scanner is generally formed by disposing a plurality of photodetectors and a plurality of photoscanners on a substrate of a semi-conductor material. CCDs used in scanning type imaging devices have a series of small photosensor elements or pixels positioned in a closely spaced linear array. The imaging is typically done by a lens assembly having a substantial image reduction ratio, such that the image projected onto the linear photosensor array is much smaller. In a typical CCD imager, signal charge representative of incident radiation is accumulated in an array of pixels in an image area. Each pixel of the CCD generates an electronic data signal representative of the intensity of the light that is impinged thereon during a short period of time known as a sampling interval.
Solid image sensors utilize CCD's and are categorized according to the construction of a light-receiving element, which element converts incident light charge, and the type of CCD transfer element. Generally, the signal transfer of a charge coupled device (CCD) image sensor may be of three types, a frame transfer type, an interline transfer type, and a frame-interline transfer type. The interline transfer mode is divided into an EIA (Electronic Industries Association) and CCIR (Consultative Committee International Radio) modes according to a scanning mode. Interline CCDs are used primarily in image sensors used for capturing movie or video-type images. The CCD image sensor of the interline transfer type may be applied to a commercial system requiring a lower resolution rather than the broadcasting equipment and military equipment, and it may particularly be applied to a television set or a camcorder of an interlaced scanning type. Full frame CCD image sensors are able to capture more of the image, than interline CCD image sensors, but do not operate as quickly because they must perform the photocollection and charge transfer functions in sequence. The CCD image sensor of the frame transfer type may be applied to a system requiring a higher resolution such as broadcasting equipment. The charge coupled device used in the image sensor is categorized in a frame-transfer type. The CCD image sensor of the frame transfer type may also be applied to a commercial system requiring about one million pixels such as a high definition television (HDTV).
The interline transfer CCD (IT-CCD) image sensor is formed by integrating, onto a semiconductor substrate, a plurality of matrix arrayed photo diodes, which serve as image sensing cells, a plurality of columns of vertical CCD registers for reading out the signal charges from the photo diodes, and a horizontal CCD register for reading out the signal charges transferred by the vertical CCD registers. The interline sensor comprises a number of columns of photosensitive elements (pixels) in which the collected image can be converted during an integration period into charge packets, whose size corresponds to the local intensity of the collected image. The image sensor is established into an accumulating mode and a read out mode of operation, and an optical image is converted into electric charges in the accumulating mode of operation. These CCD imagers use a parallel array of CCD line registers shielded from or not photosensitive to image irradiation, for transporting lines of charge packets representative of image element samples. These charge packets are transferred into the CCD line registers from the photosensors at selected times. The advantage of interline CCD image sensors is the separation of the photocollection function from the charge transfer function. This separation allows each photosensor to form charge packets in proportion to the intensity of light exposed to the particular photosensor which can be quickly shifted out to the interline CCD for transfer allowing the photosensors to begin storing charge packets for the next image. Thus, images can be captured and transferred out in a rapid sequence, making interline CCD image sensors particularly useful for imaging apparatus for making movies or videos.
The CCD image sensor of the frame transfer type has a plurality of photodetectors formed on a plane thereof and a plurality of signal transfer regions formed under the photodetectors. In full frame CCD image sensors, the CCDs include a photosensitive layer which covers all or virtually all of the surface area which is to be exposed to the image to be captured. Unlike interline CCD image sensors, the photocollection function is not separated from the charge transfer function in these sensors. The image is projected onto the image sensor section and is converted into a pattern of charge packets, which after the integration period are rapidly transported to the storage section and are read out linewise by means of a horizontal read-out register. A frame transfer type CCD image sensor also includes a drive apparatus which drives a frame transfer type CCD image sensor. The drive apparatus further includes a timing control circuit which generates a timing clock signal in accordance with a system clock signal. The timing clock signal determines the timing of generating pulse signals which are output from the vertical and horizontal drivers. With full frame CCD image sensors, all or virtually all of an image exposed to the sensor can be scanned or captured.