Abstract:
A CMOS image sensor obtains color through the use of two or three superposed layers. Each pixel in the image sensor includes a plurality of superposed photosensitive p-n junctions with individual charge integration regions. The combination of each of the superposed layers provides increased sensitivity and resolution of a single chip color imager.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. application Ser. No. 09/522,286, filed Mar. 9, 2000, which claims the benefit of the U.S. provisional application Ser. No. 60/124,084, filed Mar. 9, 1999. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to image sensors, and more particularly to active pixel sensors having superposed regions.  
       BACKGROUND  
       [0003]     CMOS image sensors have a significant advantage of allowing lower power consumption. An active pixel sensor (APS) is one example of a low power consumption image sensor which has photoreceptors, and buffer circuitry, and processing circuitry, all on one substrate.  
         [0004]     Many different things can be done using the CMOS technology. For example, many of the applications by Photobit, Inc. of Pasadena, Calif. have enabled various operations to be carried out on the same substrate as the image sensor.  
         [0005]     Certain resolutions are desired for different operations. For example, for a still camera, one often wants very high resolution, e.g. similar to the resolution that one could get from a photograph. This could require more than 1½ megapixels. However, people are accustomed to obtaining less resolution in a video environment, which shows a progression of information, e.g., 30 to 60 frames per second.  
         [0006]     Another consideration is the way in which one obtains color from a color sensor. Each pixel value includes an indication of values for red, green and blue at the location of that pixel. However, in actuality, the system obtains red values from one pixel area, green from another, and blue from yet another. The three values are neighboring values, so the actually-obtained information is interpolated to obtain postulated magnitudes of colors at other locations.  
         [0007]     Another way in which this can be done is by putting small prisms at each pixel. A lot of adjustment can be required.  
       SUMMARY  
       [0008]     The present invention obtains color in a CMOS image sensor with the use of two or three superposed layers. Each pixel in the image sensor includes a plurality of superposed photosensitive p-n junctions with individual charge integration regions. The combination of each of the superposed layers provides increased sensitivity and resolution of a single chip color imager.  
         [0009]     One aspect of the invention includes a photosensor comprising a first charge collection region having a first absorption length and a second charge collection region having a second absorption length. The first charge collection region and the second charge collection region are superposed. The photosensor further comprises a third charge collection region having a third absorption length. The third charge collection region is superposed with the first and second charge collection region.  
         [0010]     Another aspect of the invention is a method of generating color in an active pixel sensor comprising generating light of a first color in a first charge collection region and generating light of a second color in a second charge collection region. The method further superposes the light of the first color with the light of the second color. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0011]     These and other features and advantages of the invention will become more apparent upon reading the following detailed description and upon reference to th accompanying drawings.  
         [0012]      FIG. 1  illustrates a color sensor with three superposed charge collection regions.  
         [0013]      FIG. 2  illustrates a pixel layout according to the present invention using a 4:2:2 sampling standard.  
         [0014]      FIG. 3  is a cross-section and schematic diagram for three superposed p-n junction color APS according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     During video signal processing, numerous data formats are used to represent image information associated with each pixel of a video field so that an original image can be faithfully reproduced. For example, one common color format represents a color using red, green, and blue color components. With this color format, the color of each pixel is represented by quantities of red (R), green (G) and blue (B) color components detected in the original.  
         [0016]      FIG. 1  illustrates a color sensor  100  with three superposed charge collection regions  105 . The charge collection regions  105  are superposed to provide the color sensor  100  with increased color sensitivity and resolution. The charge collection regions  105  include a first p-n junction  110 , a second p-n junction  115 , and a third p-n junction  120 . Because the absorption length for incident photons in silicon is wavelength dependent, the charge collection regions  105  are highly sensitive to light of different color. The first p-n junction  110  is sensitive to blue light, the second p-n junction  115  is sensitive to green light, and the third p-n junction  120  is sensitive to red light. The spectral response of the charge collection regions  105  is dependent upon the thickness and location of the layers.  
         [0017]     The superposed charge collection regions  105  may be used with a pixel having a 4:2:2 sampling mode. The 4:2:2 mode is a ratio of sampling frequencies used to digitize the luminance (Y) and color difference components (R-Y and B-Y). For example, the first color difference component may represent the difference between the red image information and the luminance image information (R-Y) and the second color difference component may represent the difference between the blue image information and the luminance image information (B-Y). The term 4:2:2 denotes that for every four samples of Y, there are 2 samples each of R-Y and B-Y, giving more chrominance bandwidth in relation to luminance compared to standard 4:1:1 sampling.  
         [0018]      FIG. 2  illustrates a pixel  200  layout according to the present invention using a 4:2:2 sampling mode. Applying color separation in an APS through the use of superposed regions is possible through development of pinned and buried photodiodes, advances in color processing and the continuous scaling down of the CMOS features. The pixel  200  includes a green component  205 , a blue component  210 , and a red component  215 . The combination of the color components  205 ,  210 ,  215  provides for increased color sensitivity in the pixel  200 .  
         [0019]      FIG. 3  is a cross-section and schematic diagram for three superposed p-n junction color APS  300  according to the present invention. The APS  300  comprises a plurality of N+ floating diffusion regions  305 , a plurality of P+ floating diffusion regions  310 , a P—buried region  315 , a N—surface region  320 , a fully depleted N—well  325 , NMOS reset transistors  330 ,  335 , a PMOS reset transistor  340 , a red output transistor  345 , a blue output transistor  350 , and a green output transistor  355 . Each of the floating diffusion regions  305 ,  310  is connected to a reset transistor  330 ,  335 ,  340  and an output transistor  340 ,  350 ,  355 . The P+ diffusion region  310  is connected to the PMOS reset transistor  340  and the N+ diffusion region  305  is connected to the NMOS reset transistor  330 . The fully depleted N—well  325  overlaps the N+ diffusion region  330 . The N—well  325  also surrounds the P− buried region  315  and the N—surfaced region  320 . The N—surfaced region  320  is proximate the P+ diffusion region  310  and the P−buried region  315 . Each of the floating diffusion regions  305 ,  310  preferably have different integration periods that allow each spectral selection to have flexible saturation exposure.  
         [0020]     The color components of the pixel  300  are provided by the output transistors  340 ,  350 ,  355 . In one embodiment, the output transistor  340  outputs the red component, the output transistor  350  outputs the blue component, and the output transistor  355  outputs the green component. If only two output transistors are desired, the green component may be omitted.  
         [0021]     The color APS  300  is capable of performing 4:4:4 sampling mode. In the 4:4:4 sampling mode, there are always an equal number of samples of luminance (Y) and color difference components (R-Y and B-Y). The 4:4:4 sampling mode provides for more data to form the images, and thus the potential of images having a higher resolution and clarity. To perform 4:4:4 sampling, the color APS  300  preferably has at least two separate reset control lines. The number of reset control line is dependent upon the number of implemented superposed layers, with each layer having a separate reset control line.  
         [0022]     Numerous variations and modifications of the invention will become readily apparent to those skilled in the art. Accordingly, the invention may be embodied in other specific forms without departing from its spirit or essential characteristics.