Abstract:
A solid-state imaging device includes: a first photodiode receiving light of a first color; a second photodiode that is arranged next to the first photodiode in a first direction and receives light of a second color; a third photodiode that is arranged next to the second photodiode in a second direction and receives light of the first color; a fourth photodiode that is arranged next to the third photodiode in the first direction and receives light of a third color; a first reset transistor for discharging a charge generated in the first photodiode and the second photodiode; and a second reset transistor for discharging a charge generated in the third photodiode and the fourth photodiode. The first photodiode and the third photodiode have a small difference in area.

Description:
BACKGROUND 
       [0001]    The entire disclosure of Japanese Patent Application No. 2010-275444, filed Dec. 10, 2010 is expressly incorporated by reference herein. 
         [0002]    1. Technical Field 
         [0003]    The present invention relates to solid-state imaging devices. 
         [0004]    2. Related Art 
         [0005]    It has been proposed to arrange a reset transistor for discharging electric charge generated at each transistor in a solid-state imaging device that is used as an imaging sensor (see, for example, Japanese Laid-open Patent Application 2008-210928). 
         [0006]    However, arrangement of a reset transistor at each of the photodiodes would encroach on the area where the light-detecting section of each of the photodiodes can be arranged, such that a sufficient area cannot be secured for each of the photodiodes, which may lower the sensitivity. On the other hand, when a reset transistor is shared by a plurality of photodiodes, it is possible to suppress such reduction in the sensitivity. However, the areas where the light-detecting sections of the photodiodes can be arranged become unequal among the plurality of photodiodes, which may cause differences in the areas of photodiodes that receive light of the same color. 
       SUMMARY 
       [0007]    The invention has been made in view of the technical difficulty described above. An aspect of embodiments of the invention relates to a solid-state imaging device having photodiodes, and pertains to an implementation of a reset transistor to be shared by two pixels, while suppressing the inconvenience associated with variations in the areas of the photodiodes in the solid-state imaging device. 
         [0008]    In accordance with an aspect of an embodiment of the invention, a solid-state imaging device includes: a first photodiode receiving light of a first color, a second photodiode arranged next to the first photodiode in a first direction and receiving light of a second color, a third photodiode arranged next to the second photodiode in a second direction and receiving light of the first color, a fourth photodiode arranged next to the third photodiode in a first direction and receiving light of a third color, a first reset transistor for discharging a charge generated in the first photodiode and the second photodiode, and a second reset transistor for discharging a charge generated in the third photodiode and the fourth photodiode. In an aspect, a difference between the area of a light-receiving section of the first photodiode and the area of a light-receiving section of the third photodiode is smaller than a difference between the area of the light-receiving section of the first photodiode and the area of a light-receiving section of the second photodiode, and smaller than a difference between the area of the light-receiving section of the third photodiode and the area of a light-receiving section of the fourth photodiode. According to the aspect described above, while reducing a difference in the area between the first and third photodiodes corresponding to pixels of the same color, and allowing a difference in the area between the aforementioned photodiodes and the second and fourth photodiodes corresponding to pixels of different colors, each of the reset transistors can be shared by two pixels. 
         [0009]    In the embodiment described above, the area of the light-receiving section of the first photodiode and the area of the light-receiving section of the third photodiode may preferably be the same. Accordingly, by forming the first and third photodiodes corresponding to the same color to have the same area, a difference in the characteristic of the photodiodes corresponding to the same color can be reduced. 
         [0010]    In the embodiment described above, the light-receiving section of the first photodiode and the light-receiving section of the third photodiode may preferably have the same shape. Accordingly, by forming the first and third photodiodes corresponding to the same color to have the same shape, a difference in the characteristic of the photodiodes corresponding to the same color can be reduced. 
         [0011]    In the embodiment described above, the area of the light-receiving section of the first photodiode may preferably be smaller than the area of the light-receiving section of the second photodiode, and the area of the light-receiving section of the third photodiode may preferably be smaller than the area of the light-receiving section of the fourth photodiode. Accordingly, the areas of the first and third photodiodes become relatively small, such that the gain to be applied to light detection signals from the second and fourth photodiodes can be made smaller. 
         [0012]    In the embodiment, the first direction may preferably be perpendicular to the second direction. Also, in the embodiment described above, the light of the second color and the light of the third color may preferably be light of the same color. 
         [0013]    In the embodiment described above, the solid-state imaging device may preferably further include a first modulation element that is provided adjacent to the first photodiode in a third direction, and outputs a modulation signal according to a charge generated in the first photodiode, a second modulation element that is provided adjacent to the second photodiode in the third direction, and outputs a modulation signal according to a charge generated in the second photodiode, a third modulation element that is provided adjacent to the third photodiode in the third direction, and outputs a modulation signal according to a charge generated in the third photodiode, and a fourth modulation element that is provided adjacent to the fourth photodiode in the third direction, and outputs a modulation signal according to a charge generated in the fourth photodiode. The first reset transistor may preferably be disposed between the first modulation element and the second modulation element, and the second reset transistor may preferably be disposed between the third modulation element and the fourth modulation element. Accordingly, the first reset transistor is disposed between the first modulation element and the second modulation element, and the second reset transistor is disposed between the third modulation element and the fourth modulation element, such that the positional relation of the first reset transistor with respect to the first photodiode and the positional relation of the second reset transistor with respect to the third photodiode can be matched each other. Accordingly, differences in the area and the shape between the first and third photodiodes corresponding to pixels of the same color can be reduced. 
         [0014]    In the embodiment described above, the first reset transistor may preferably include a first gate electrode adjacent to the first modulation element, a second gate electrode adjacent to the second modulation element, and a first common drain section disposed between the first gate electrode and the second gate electrode. The second reset transistor may preferably include a third gate electrode adjacent to the third modulation element, a fourth gate electrode adjacent to the fourth modulation element, and a second common drain section disposed between the third gate electrode and the fourth gate electrode. Accordingly, as the reset transistors have the common drain section, the occupying area of the reset transistors can be made smaller, and the light detection area of the photodiode can be made greater, such that the light detection sensitivity can be increased. 
         [0015]    In the embodiment described above, the solid-state imaging device may preferably further include a first light transmission section that transmits light of the first color toward the first photodiode, a second light transmission section that transmits light of the second color toward the second photodiode, a third light transmission section that transmits light of the first color toward the third photodiode, and a fourth light transmission section that transmits light of the third color toward the fourth photodiode, and the first through fourth light transmission sections may preferably have a Bayer arrangement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a plan view showing the structure of a solid-state imaging device in accordance with an embodiment of the invention. 
           [0017]      FIG. 2  is a cross-sectional view taken along a line II-II′. 
           [0018]      FIG. 3  is a plan view showing the structure of a solid-state imaging device in accordance with a first comparison example. 
           [0019]      FIG. 4  is a plan view showing the structure of a solid-state imaging device in accordance with a second comparison example. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0020]    An embodiment of the invention will be described in detail below. It is noted that the embodiment described below will not unduly limit the contents of the invention set forth in the scope of patent claims. Also, not all of the compositions described in the embodiment would necessarily be essential for the solution provided by the invention. Furthermore, the same components will be appended with the same reference numbers, and their description will not be repeated. 
       1. Structure of Embodiment 
       [0021]      FIG. 1  is a plan view showing the structure of a solid-state imaging device in accordance with an embodiment of the invention, and  FIG. 2  is a cross-sectional view taken along a line II-II′ of  FIG. 1 . As shown in  FIG. 1 , a solid-state imaging device  100  is configured with a plurality of solid-state imaging elements regularly arranged in a first direction (for example, in a rightward direction) and a second direction (for example, in a downward direction). 
         [0022]    Each one of the solid-state imaging elements  1  includes one photodiode ( 2 Gr,  2 R,  2 Gb,  2 B) and one modulation element ( 3 Gr,  3 R,  3 Gb,  3 B). Each of the modulation elements ( 3 Gr,  3 R,  3 Gb,  3 B) has a gate electrode  31  in a ring shape to be described below. The gate electrode  31  is disposed in a manner not to overlap the photodiode ( 2 Gr,  2 R,  2 Gb,  2 B), and to allow the photodiode ( 2 Gr,  2 R,  2 Gb,  2 B) to be able to secure an area as large as possible. In  FIG. 1 , each of the gate electrodes  31  is disposed adjacent to each of the photodiodes ( 2 Gr,  2 R,  2 Gb,  2 B) in a third direction (for example, a diagonally leftward and downward direction), respectively. 
         [0023]    As shown in  FIG. 2 , a color filter  16  and a micro lens array  17  are formed above various signal lines, such as, signal output lines  35  and the like to be described below. The micro lens array  17  includes regularly arranged convex lens. Each of the convex lenses is disposed opposite each of the photodiodes  2 Gr,  2 R,  2 Gb, and  2 B, respectively, and collects incident light onto each of the photodiodes  2 Gr,  2 R,  2 Gb, and  2 B, respectively. 
         [0024]    The color filter  16  includes light-transmission sections of three colors, for example, as shown in  FIG. 1 , red (R), green (Gr or Gb) and blue (B), regularly arranged therein, and each of the light-transmission sections is disposed opposite each of the photodiodes ( 2 Gr,  2 R,  2 Gb,  2 B), respectively, and selects wavelengths of light incident on each of the photodiodes ( 2 Gr,  2 R,  2 Gb,  2 B), respectively. In the embodiment shown in  FIG. 1 , a red (R) pixel is disposed adjacent to and on the right of a green (Gr) pixel, a green (Gb) pixel is disposed adjacent to and below the red (R) pixel, and a blue (B) pixel is disposed adjacent to and on the right of the green (Gb) pixel. Such an arrangement of colors is called a Bayer arrangement. Here, Gr indicates a green pixel that is disposed in the same row as that of the red (R) pixel (on the right), and Gb indicates a green pixel disposed in the same row as that of the blue (B) (on the right). Also, when one pixel is disposed adjacent to another pixel, something other than pixels, such as, a pixel isolation region may be provided between the adjacent pixels. 
       1-1. Photodiode 
       [0025]    As shown in  FIG. 2 , the photodiode  2 R is formed in a semiconductor substrate  110 , and the modulation element  3 R is formed on the surface of the semiconductor substrate  110 . The semiconductor substrate  110  is a substrate of a first conductivity type (for example, P type). It is noted that the substrate itself is not limited to the first conductivity type, but the substrate may be a semiconductor substrate of a second conductivity type (for example, N type), and a well of the first conductivity type may be formed at the surface side of the semiconductor substrate. 
         [0026]    A semiconductor region  12  of the second conductivity type (for example, N type) is formed on the surface side of the semiconductor substrate  110  within the semiconductor substrate  110 . Within the semiconductor region  12  of the second conductivity type, a semiconductor region  11  of the first conductivity type is formed on the surface side of the semiconductor substrate  110 . The semiconductor region  11  of the first conductivity type and the semiconductor region  12  of the second conductivity type form the photodiode  2 R. 
         [0027]    A pinning layer  15  of the second conductivity type is formed at the topmost surface of the semiconductor substrate  110 . The pinning layer  15  prevents generation of dark current at the substrate surface. It is noted that the description of the photodiode  2 R above similarly applies to the photodiodes  2 Gr,  2 Gb and  2 B. 
       1-2. Modulation Element 
       [0028]    The modulation element  3 R is a transistor including the gate electrode  31  formed through a dielectric film on the semiconductor substrate  110 . The gate electrode  31  is formed in a ring shape as viewed in a plan view (see  FIG. 1 ). The gate electrode  31  is connected to a modulation control line  33  through a gate contact  32 . 
         [0029]    On the surface of the semiconductor substrate  110 , a source section is formed in a region surrounded by the gate electrode  31  as viewed in a plan view. The source section is connected to a signal output line  35  through a source contact  34  on the semiconductor substrate  110 . Also, on the surface of the semiconductor substrate  110 , a drain section is formed outside the gate electrode  31  as viewed in a plan view. The drain section is connected to a drain line  37  through a drain contact  36  (see  FIG. 1 ) on the semiconductor substrate  110 . 
         [0030]    The semiconductor region  11  of the first conductivity type and the semiconductor region  12  of the second conductivity type of the photodiode  2 R extend into a region immediately below the modulation element  3 R of the semiconductor substrate  110 . Charge (for example, holes) generated in the photodiode  2 R that has received light is stored in a carrier pocket  13  that is a high concentration first conductivity semiconductor region formed in a region immediately below the modulation element  3 R of the semiconductor region  11  of the first conductivity type. Due to the charge accumulation, a substrate bias is applied to the modulation element  3 R, causing the threshold voltage of the modulation element  3 R to change. The amount of the change in the threshold voltage is detected as the amount of incident light. It is noted that the description of the modulation element  3 R made above similarly applies to the modulation elements  3 Gr,  3 Gb and  3 B. 
       1-3. Reset Transistor 
       [0031]    As shown in  FIG. 1 , a reset transistor  4 r is disposed between the modulation element  3 Gr and the modulation element  3 R, and a reset transistor  4   b  is disposed between the modulation element  3 Gb and the modulation element  3 B. 
         [0032]    As shown in  FIG. 2 , the reset transistor  4   r  is a transistor that includes two gate electrodes  41 Gr and  41 R formed through a dielectric film on the semiconductor region  12  of the second conductivity type of the semiconductor substrate  110 . The gate electrodes  41 Gr and  41 R extend, at both ends of the reset transistor  4   r , over the gate electrodes  31  of the modulation elements  3 Gr and  3 R through a dielectric film, respectively. The gate electrodes  41 Gr and  41 R are connected to a common reset control line  43  (see  FIG. 1 ) through the gate contacts  42  (see  FIG. 1 ). 
         [0033]    A common drain section  14  that is a high concentration first conductivity type semiconductor region is formed in the surface of the semiconductor substrate  110  between the gate electrodes  41 Gr and  41 R. The common drain section  14  is connected to a drain line  47  through a common drain contact  46   r  on the semiconductor substrate  110 . After the amount of incident light has been detected by the modulation elements  3 Gr and  3 R, a voltage greater than the threshold value is applied to the gate electrodes  41 Gr and  41 R, whereby channels are formed in regions immediately below the gate electrodes  41 Gr and  41 R of the semiconductor substrate  110 , and the charge stored in the carrier pocket  13  is discharged to the drain section  14 . It is noted that the description of the reset transistor  4   r  made above similarly applies to the reset transistor  4   b.    
       1-4. Arrangement of Reset Transistor 
       [0034]    In accordance with the present embodiment, the drain section  14  of the reset transistor  4   r  and the drain contact  46   r  are shared by two solid-state imaging elements  1  that are arranged adjacent to each other in the first direction (for example, the rightward direction), and the reset transistor  4   r  is positioned between the two modulation elements  3 Gr and  3 R that are arranged adjacent to each other in the first direction, Accordingly, in the present embodiment, the occupying area of the reset transistor  4   r  is reduced, compared to the case where the reset transistor  4   r  is provided for each one of the modulation elements, such that the light receiving areas of the photodiodes  2 Gr,  2 R,  2 Gb and  2 B can be increased, and thus the light detecting sensitivity can be improved. 
         [0035]    Similarly, the drain section  14  of the reset transistor  4   b  and the drain contact  46   b  are shared by two solid-state imaging elements  1  that are arranged adjacent to each other in the first direction (for example, the rightward direction), and the reset transistor  4   b  is positioned between the two modulation elements  3 Gb and  3 B that are arranged adjacent to each other in the first direction. Accordingly, the light receiving areas of the photodiodes  2 Gr,  2 R,  2 Gb and  2 B can be increased, and thus the light detecting sensitivity can be improved. 
         [0036]    Further, in accordance with the present embodiment, the reset transistors  4   r  and  4   b  are arranged at an interval of one photodiode in the second direction (for example, the downward direction) at positions shifted by one photodiode in the first direction (for example, the rightward direction). In the example shown in  FIG. 1 , each of the reset transistors  4   r  and  4   b  is always provided adjacent to a green (Gr or Gb) pixel downward in the second direction as viewed from the green pixel (on the opposite side to the second direction as viewed from a red (R) or a blue (B) pixel). According to the above, the photodiodes  2 Gr and  2 Gb corresponding to green (Gr or Gb) pixels can be formed in the same area and the same shape. In this manner, by reducing differences in the area and the shape between the photodiodes  2 Gr and  2 Gb corresponding to pixels of the same color, characteristic differences among pixels of the same color can be made smaller. 
         [0037]    In accordance with the present embodiment, the area and the shape of the photodiodes  2 R and  2 B corresponding to red (R) and blue (B) pixels are different from the area and the shape of the photodiodes  2 Gr and  2 Gb corresponding to green (Gr or Gb) pixels. However, for different colors, characteristic differences can be corrected (their spectral sensitivity can be adjusted) through signal processing in a succeeding stage. 
         [0038]    Rather, the area of each of the photodiodes  2 R and  2 B corresponding to red (R) and blue (B) pixels may preferably be greater than the area of each of the photodiodes  2 Gr and  2 Gb corresponding to green (Gr or Gb) pixels. If these photodiodes have the same area, a specified amount of gain needs to be applied to light detection signals of the solid-state imaging elements corresponding to red (R) and blue (B) pixels to correct them in order to make their spectral sensitivity characteristics uniform in the visible light range. When the areas of the photodiodes  2 R and  2 B corresponding to red (R) and blue (B) pixels are made greater, such a gain can be smaller. Accordingly, making the area of each of the photodiodes  2 R and  2 B corresponding to red (R) and blue (B) pixels larger, the degree of freedom in correction can be made greater. In the example shown in  FIG. 1 , the area of each of the photodiodes  2 R and  2 B corresponding to red (R) and blue (B) pixels is slightly larger than the area of each of the photodiodes  2 Gr and  2 Gb corresponding to green (Gr or Gb) pixels. 
       1-5, Other Embodiments 
       [0039]    In the embodiment described above, an example in which the first direction and the second direction traverse each other at the right angle is shown. However, the first direction and the second direction may traverse each other at an angle other than the right angle. However, if the first direction and the second direction are in parallel with each other, photodiodes cannot be disposed on a plane. Therefore, the first direction and the second direction may preferably extend in directions that traverse each other and, more preferably, may extend in directions that traverse each other in the range between 50 degrees and 130 degrees. 
         [0040]    In the embodiment described above, the first direction and the third direction may traverse each other at 45 degrees, as an example. However, the first direction and the third direction may traverse each other at an angle other than 45 degrees. However, when the first direction and the third direction are in parallel with each other, restrictions in layout increase when a reset transistor is shared by two adjacent photodiodes arranged in the first direction. Therefore, the first direction and the third direction may preferably extend in directions that traverse each other and, more preferably, may extend in direction that traverse in the range between 30 degrees and 150 degrees. 
         [0041]    Also, the solid-state imaging element in accordance with the embodiment described above is a so-called substrate modulation type solid-state imaging element, which detects changes in the threshold voltages of the modulation elements  3 Gr,  3 R,  3 Gb and  3 B based on charges accumulated in the semiconductor substrate  110 , but may use other modulation methods or detection methods, and may be, for example, a CMOS (Complementary Metal Oxide Semiconductor) image sensor. 
         [0042]    Also, in the example shown in the above-described embodiment, the color filter  16  and the micro lens array  17  are formed on the front surface side of the semiconductor substrate  110 . However, without any particular limitation to the above, the color filter  16  and the micro lens array  17  may be formed on the rear surface side of the semiconductor substrate  110 , so as to receive light at the rear surface side. 
       2. Comparison Example 
       [0043]      FIG. 3  is a plan view showing the structure of a solid-state imaging device in accordance with a first comparison example. As shown in  FIG. 3 , a reset transistor  4  is disposed at each set of one photodiode  2  and one modulation element  3 . Such an arrangement would make the area of the light detection section of the photodiode  2  in the solid-state imaging device smaller, and therefore its sensitivity would be lowered. 
         [0044]      FIG. 4  is a plan view showing the structure of a solid-state imaging device in accordance with a second comparison example. As shown in  FIG. 4 , a reset transistor  4  is disposed at each set of two photodiodes  2  and two modulation elements  3 , and each one reset transistor  4  is shared by two solid-state imaging elements. According to such an arrangement, compared to the structure shown in  FIG. 3 , the area of the photodiode  2  corresponding to a blue (B) pixel and the area of the photodiode  2  corresponding to a green (Or) pixel arranged in the same row of red (R) pixels become greater. 
         [0045]    However, the area and the shape of the photodiode  2  corresponding to a green (Gb) pixel arranged in the same row of blue (B) pixels are different from the area and the shape of the photodiode  2  corresponding to a green (Gr) pixel arranged in the same row of red (R) pixels. According to such a structure, characteristic differences are generated among the photodiodes of the same color (green), such that defects, such as, streaks appearing in an image would occur. 
         [0046]    In contrast, according to the embodiment of the invention described with reference to  FIG. 1  and  FIG. 2 , the reset transistors  4   r  and  4   b  are arranged at an interval of one photodiode in the second direction (for example, the downward direction) at positions shifted by one photodiode in the first direction (for example, the rightward direction). Therefore, in accordance with the present embodiment of the invention, the photodiodes  2 Gr and  2 Gb corresponding to green (Gr or Gb) pixels can be formed in the same area and the same shape.