Abstract:
A color photo sensing structure, includes an array of multiple color photo sensing elements. The photo sensing structure includes a first pixel located laterally with respect to a second pixel in a substrate of a first conductivity. The first pixel includes a first doped region of a second conductivity formed in the substrate and a second doped region of a first conductivity formed in the substrate above the first doped region. The second pixel includes two doped regions formed in the substrate having a first conductivity and a second conductivity, respectively. The color photo sensing structure further includes a controller for sequentially providing a first photocurrent value of the first doped region, a second photocurrent value of both the first and second doped regions and a third photocurrent value of the two doped regions of the second pixel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Imagers, including complimentary metal oxide semiconductor (CMOS) imagers and charge-coupled devices (CCD), may be used in digital imaging applications to capture scenes. An imager includes an array of pixels. Each pixel in the array includes at least a photosensitive element for outputting a signal having a magnitude proportional to the intensity of incident light contacting the photosensitive element. When exposed to incident light to capture a scene, each pixel in the array outputs a signal having a magnitude corresponding to an intensity of light at one point in the scene. The signals output from each photosensitive element may be processed to form an image representing the captured scene. 
         [0002]    To capture color images, the photo sensors should be able to separately detect photons of wavelengths of light associated with different colors. For example, a photo sensor may be designed to detect first, second, and third colors (e.g., red, green and blue photons.) In one imager design, each pixel cell may be sensitive to only one color or spectral band. For this, a color filter array may be placed over each pixel cell so that each pixel cell ideally measures only wavelengths of the color of the pixel&#39;s associated filter. A group of four pixels (2 green, 1 red and 1 blue) are typically used to capture three different colors of incident light. The groups of four may be repeated throughout an imager array to form an array of many rows and columns. 
         [0003]    In another imager design, one pixel may measure all three colors. This design takes advantage of the absorption properties of semiconductor materials. That is, in a typical semiconductor substrate, different wavelengths of light are absorbed at different depths in the substrate. For example, blue light is absorbed in a silicon substrate primarily at a depth of about 0.2 to 0.5 microns, green light is absorbed in a silicon substrate primarily at a depth of about 0.5 to 1.5 microns and red light is absorbed in the silicon substrate at a depth of about 1.5 to 3.0 microns. 
         [0004]    This pixel structure includes three stacked pixels formed from two levels of N diffusions and a P well that are diffused in a silicon substrate. This results in a structure having three p-n junctions forming three photodiodes at different depths in the substrate, each designed to primarily absorb a particular color of incident light. Typically, the blue photodiode will be closest to the incident light, the red photodiode will be farthest from the incident light and the green photodiode will be between the blue and red junctions, due to the absorption properties described above. In this way, only one vertically stacked pixel is needed to absorb three or more different colors of light. 
         [0005]    In operation, however, the spectral characteristics of the three colors in the vertically stacked pixel are poorly separated. That is, some photons may be absorbed by the wrong layer. Thus, extensive post-processing of the signals is necessary to arrive at the actual values for each of the red, green and blue photodiodes. 
         [0006]    In a related design, the vertically stacked photodiodes include vertically stacked color filter segments that incorporate non-silicon materials. These designs improve upon color separation properties of the vertically stacked layers, but also require more complex processing to fabricate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Included in the drawings are the following figures: 
           [0008]      FIG. 1A  is a diagram of a two pixel group including green and magenta pixels according to an embodiment of the present invention. 
           [0009]      FIGS. 1B and 1C  are diagrams of the magenta pixel and its equivalent circuit, according to the embodiment shown in  FIG. 1A . 
           [0010]      FIG. 2  is a graph showing the spectral response characteristics of an embodiment of a magenta filter which may disposed over the magenta pixel shown in  FIGS. 1A and 1B . 
           [0011]      FIG. 3  is a graph showing the spectral response characteristics of the magenta pixel according to the embodiment shown in  FIGS. 1A and 1B . 
           [0012]      FIG. 4A  is a diagram showing an embodiment of the electrical connections for the green pixel shown in  FIGS. 1A and 1B . 
           [0013]      FIG. 4B  is a structural view of the green pixel according to the embodiment of  FIG. 4A . 
           [0014]      FIG. 4C  is a side structural view of the embodiment of the green pixel shown in  FIG. 4B . 
           [0015]      FIG. 5A  is a diagram showing an embodiment of the electrical connections for the magenta pixel shown in  FIGS. 1A and 1B . 
           [0016]      FIG. 5B  is a structural view of the magenta pixel according to the embodiment of  FIG. 5A . 
           [0017]      FIG. 5C  is a side structural view of the embodiment of the magenta pixel shown in  FIG. 5B . 
           [0018]      FIG. 6  is a block diagram of an imager incorporating an array of groups of pixels according to the embodiments shown in  FIGS. 1A ,  1 B,  4 A,  4 B,  4 C,  5 A,  5 B and  5 C. 
           [0019]      FIG. 7  is a flow chart showing operation steps of the imager shown in  FIG. 6  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]    The example embodiments described below utilize groups of two pixels to absorb three separate colors of incident light. For example, a single photodiode pixel absorbs green photons and a vertically stacked magenta pixel absorbs red and blue photons. A magenta filter may be disposed over the magenta pixel to block green wavelengths from entering the pixel. 
         [0021]    This structure uses two pixels to absorb a full color spectrum. Thus, it improves pixel density 1.5 times relative to four pixel group structures. Further, the pixel provides better spectral separation between the two colors, with less overlap between the spectral responses, than a system that attempts to absorb all blue, green and red photons in one vertically stacked structure. Also, the structure may be used with standard 4T read out techniques. Finally, fabrication requires no special processing relative to standard CMOS imager chips. 
         [0022]      FIG. 1A  shows a group  200  of two pixels according to an embodiment of the present invention. As shown, one pixel  202  may measure one color of incident light. In this embodiment, pixel  202  may measure green light (G). Pixel  202  may include a filter designed to transmit green photons and block blue and red photons from entering the pixel. The second pixel  204  may measure two colors of incident light. This pixel (the “magenta pixel”) may measure red and blue photons. Thus, as shown, only two pixels are needed to measure a full color spectrum. 
         [0023]    An embodiment of a magenta pixel  204  is shown in  FIG. 1B . As shown, pixel  204  is made up of two stacked photodiodes having an equivalent circuit as shown in  FIG. 1C . The upper photodiode  206  absorbs blue photons (B) and the lower photodiode  208  absorbs red photons (R). This structure takes advantage of the absorption properties of the semiconductor substrate. That is, blue photons are absorbed at a shallower depth than red photons. Thus, the blue photodiode is disposed in the upper position in the stack (closer to the incident light) and the red photodiode is disposed in the lower position in the stack (farther from the incident light). 
         [0024]    A magenta filter  205  may be disposed over pixel  204  as shown in  FIG. 1A . The spectral response characteristics of an embodiment of the magenta filter are shown in  FIG. 2 . As shown in  FIG. 2 , the magenta filter transmits wavelengths between 400 nm and 480 nm (corresponding substantially to blue wavelengths) and transmits photons having wavelengths between at least 620 and 680 nm (corresponding substantially to red wavelengths). The magenta filter may transmit a relatively small percentage of photons having wavelengths between approximately 480 and 620 nm (corresponding substantially to green wavelengths). Use of the magenta filter improves color separation properties of the magenta pixel because the green wavelengths are substantially blocked and, therefore, cannot be inadvertently absorbed by the blue and red photodiodes. Use of the magenta filter may be combined with use of an IR blocking filter to block near IR wavelengths of greater than 680 nm. 
         [0025]      FIG. 3  shows the spectral response characteristics of the red and blue photodiodes for an embodiment of the magenta pixel. The blue wavelengths are plotted as T (transmission) and the red wavelengths are plotted as 1-T (absorption). As shown, the blue photodiode absorbs photons having wavelengths between 400 and 480 nm and the red photodiode absorbs photons having wavelengths between 620 and 680 nm. This assumes that the magenta filter substantially blocks out photons having wavelengths between 480 and 620 nm. The red photodiode may absorb as much as 30% of the 480 nm photons and the blue photodiode may absorb as much as 30% of the 620 nm photons. The effect of these overlaps may be reduced by post-processing of the pixel signals. One example method of post-processing is described below. 
         [0026]      FIGS. 4A and 5A  show the electrical connections for an embodiment of a 4T structure for the green and magenta pixels  300  and  400 , respectively. As shown, both pixels include a photodiode region, depicted as  302  and  304  for the green pixel and  402  and  404  for the magenta pixel. Both pixels also include a floating diffusion region  310  for the green pixel and  410  for the magenta pixel. The photodiode region and the floating diffusion region, for each pixel, is formed in substrate  306  and  406 , respectively. Both pixels further include CMOS circuitry including, for example, a reset transistor ( 312 ,  412 ), a source follower transistor ( 316 ,  416 ), a row select transistor ( 318 ,  418 ), a transfer transistor ( 308 ,  408 ), a column readout line ( 320 ,  420 ) and a pixel supply voltage VDD ( 314 ,  414 ). The magenta pixel  400  also includes a blue photodiode transistor  405 . 
         [0027]    An example structural layout of the green pixel is shown in  FIGS. 4B and 4C  and an example structural layout of the magenta pixel is shown in  FIGS. 5B and 5C . It should first be noted that the photodiodes in both the green and magenta pixels do not include N or P wells. Instead, the P-type substrate ( 306 ,  406 ) is doped with different doping levels at different depths into the substrate. This is achieved using implants of different types of doping materials. The specific doping materials may be selected according to a TSMC process such as, for example, the TSMC 0.25 μm CIS option or the TSMC 0.18 μm CIS option. The different implants are generally depicted as BGP and BGN in  FIGS. 4A-C  and  5 A-C. The BGP implant and the BGN implant are selected to adjust the threshold of the transfer transistor and set the doping level in the photodiodes to optimize photocurrent. Both pixels include an upper photodiode formed at the junction between the BGP ( 302 ,  402 ) and BGN ( 304 ,  404 ) implants and a lower photodiode formed at the junction between the BGN implant ( 304 ,  404 ) and the P-type substrate ( 306 , 406 ). 
         [0028]    As shown in  FIGS. 4B and 4C , for the green pixel, the BGP implant  302  overlaps the BGN implant  304 . The top photodiode formed at the junction between BGP implant  302  and BGN implant  304  is shorted to P well  303 . P well  303 , together with oxide isolation region  307 , is used to electrically isolate adjacent pixels. The top photodiode may be shorted to P well  303 . 
         [0029]    As shown in  FIGS. 5B and 5C , for the magenta pixel, the BGP implant  402  is completely contained within the BGN implant  404 . This is different from the green pixel in that the upper photodiode formed at the junction between the BGP implant  402  and the BGN implant  404  is not shorted to P well  403 . This allows the upper photodiode to be separately read using, for example, contact  409  shown in  FIG. 5B  and transistor  405  shown in  FIG. 5A . 
         [0030]    As described above, the two pixel group may be repeated to form an array of lines and columns of the example green and magenta pixels shown in, for example,  FIGS. 4A ,  4 B,  4 C,  5 A,  5 B and  5 C. An example array  30 , including associated imager processing electronics, is shown in  FIG. 6 . An operation of the pixel array  30  is described below with reference to  FIGS. 4A ,  4 B,  4 C,  5 A,  5 B,  5 C and  6 . 
         [0031]    For pixel array  30 , all pixels in the same row may be sampled, for example, by applying row select signal RS to row select transistors  318  and  418  of the selected row. Alternatively, green pixels in a row may be independently selected by applying RS only to row select transistor  318  and magenta pixels in a row may be independently selected by applying RS only to row select transistor  418 . Specific pixels in each column may be selectively output by respective column select lines (e.g.,. lines  320  and  420  shown in  FIGS. 4A and 5A , respectively). A plurality of row and column lines (not shown) may be provided for the entire array  30 . The row lines may be selectively activated in a sequence by row driver  20  in response to row address decoder  10 . Similarly, the column select lines may be selectively activated in a sequence for each row activation by column driver  50  in response to column address decoder  60 . 
         [0032]    As shown in  FIG. 6 , the example CMOS imager is operated by timing and control circuit  40 , which controls address decoders  10  and  60  to select appropriate row and column lines for pixel readout and controls row and column driver circuitry  20  and  50  to apply driving voltages to the drive transistors (not shown) of the selected row and column lines. 
         [0033]    An example sequence for operating the two pixel group described in the above embodiments is shown in the flow chart of  FIG. 7 . The sequence begins at step  500 . At step  500 , blue photodiode transistor  405  is opened (Tpon opens or closes transistor  405  by way of its gate). Green and magenta pixels  300  and  400  are integrated over an integration period. At the end of the integration period, floating diffusion  410  of magenta pixel  400  is reset by step  502 . The level of floating diffusion  410  is read out through source follower transistor  416  onto column line  420 . The level read from the floating diffusion is placed on a first sample and hold capacitor. 
         [0034]    At step  504 , transfer transistor  408  of the magenta pixel is closed by applying signal Tx to the gate of transistor  408 . The level of the lower photodiode is thereby transferred to floating diffusion  410  and read out through source follower transistor  416  onto column line  420 . The level read from the lower photodiode is placed on a second sample and hold capacitor. It should be noted that because the blue photodiode transistor is open during this processing, the above-described operation for the magenta pixel is only carried out for lower photodiode  404 . However, the values read out and stored are primarily for the red pixel with a small amount of the blue pixel (due to the possibility of some spectral overlap, as described above). 
         [0035]    At step  506 , floating diffusion  310  of the green pixel is reset. The level of floating diffusion  310  is read out through source follower transistor  316  onto column line  320 . The level read from the floating diffusion is placed on a third sample and hold capacitor. At step  508 , transfer transistor  308  of the green pixel is closed by applying signal Tx to the gate of transistor  308 . The level of the photodiode is thereby transferred to floating diffusion  310  and read out through source follower transistor  316  onto column line  320 . The level read from the photodiode is placed on a fourth sample and hold capacitor. 
         [0036]    At step  510 , blue photodiode transistor  405  is closed by applying signal Tpon to transistor  405 . The magenta pixel is integrated again over an integration period. At step  512 , floating diffusion  410  of the magenta pixel is reset. The level of floating diffusion  410  is read out through source follower transistor  416  onto column line  420 . The level read from the floating diffusion is placed on a fifth sample and hold capacitor. 
         [0037]    At step  512 , transfer transistor  408  of the magenta pixel is closed by applying signal Tx to the gate of transistor  408 . The level of the photodiode is thereby transferred to floating diffusion  410  and read out through source follower transistor  416  onto column line  420 . The level read from the photodiode is placed on a sixth sample and hold capacitor. Here, the levels read out and stored are for the sum of the red and blue pixels together. 
         [0038]    Referring back to  FIG. 6 , the first, second, third, fourth, fifth and sixth sample and hold capacitors are represented by S/H block  70 . In step  516  of  FIG. 7 , column readout of the stored levels is carried out for each set of levels (first and second, third and fourth, fifth and sixth). In  FIG. 6 , the read floating diffusion values are represented by Vrst and the read photodiode values are represented by Vsig. The respective Vrst and Vsig values may be provided to the same differential amplifier  80  or a plurality of different differential amplifiers  80 . In either case, Vrst is subtracted from Vsig to obtain an analog differential output signal for each set of signals. The analog output signals are converted into digital signals by analog to digital converter  90  and then transferred to image processor  100  for additional processing. Such processing may include, for example, the post-processing calculations described below. The calculations are performed at step  518  in  FIG. 7 . 
         [0039]    It should be noted that the above sequence is just one example. Depending on the circuitry used, the sequence may be performed differently. For example, all green pixels in a column may be connected to an output line that is used for reading out green pixels and all magenta pixels in a column may be connected to another output line that is used for reading out magenta pixels. In this example, simultaneous integrations and readouts may be performed for the green and magenta pixels. 
         [0040]    By way of another example, all green and magenta pixels in a column may be connected to the same readout line. Here, the magenta pixels may be integrated and read. Then, the green pixels may be integrated and read. The possibility of different sequences may, therefore, depend on how the pixels are connected to the column lines going to the sample and hold capacitors. 
         [0041]    The post-processing calculations performed at step  518  are for obtaining a desired red value (R) and a desired blue value (B) from the two read out and differentially amplified digital signal values for the magenta pixel described above (represented by U for the signal value from the upper photodiode and L for the combined signal value from the lower photodiode). The U and L signal values may be represented by the following equations: 
         [0000]        U=fb*B+fr*R    (1) 
         [0000]        L=B+R    (2) 
         [0042]    In equations (1) and (2), fb and fr represent the fraction of blue photons read by the blue photodiode and the fraction of red photons read by the blue photodiode, respectively. From equations (1) and (2), the desired values R and B may be determined according to the following equations: 
         [0000]        R =( fb*L−U )/( fb−fr )   (3) 
         [0000]        B =( U−fr*L )/( fb−fr )   (4) 
         [0043]    When there is no spectral overlap, fb=1 and fr=0. In this scenario, R=L−U and B=U, as expected. That is, with no spectral overlap, the desired value for red is the combined signal minus the blue signal. Similarly, the desired value for blue is simply the blue signal. 
         [0044]    In the example described above, fr=0.3 and fb=0.7. In this scenario, R and B may be determined according to the following equations: 
         [0000]        R= 1.75 L− 2.5 U    (5) 
         [0000]        B= 2.5 U− 0.75 L    (6) 
         [0045]    This example may cause some amount of noise increase in both the red and blue signals. It is more likely, however, that the fr and fb values would be closer to the ideal values (no spectral overlap) using a more realistic spectral distribution of an image input into the imager array. 
         [0046]    Accordingly, using these calculations, any spectral overlap may be compensated for by selecting appropriate values for fr and fb. 
         [0047]    While example embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the scope of the invention.