Abstract:
Color filter arrays (CFA) and image sensors using same are provided. A color filter array includes a two-dimensional array including a plurality of first color filters, a plurality of second color filters, and a plurality of third color filters, wherein the first, second and third color filters are periodically arranged, and at least the first, second and third color filters formed in a first region of the two-dimensional array and the first, second and third color filters formed in a second region of the two-dimensional array are symmetrically mirrored.

Description:
BACKGROUND 
       [0001]    The invention relates to image sensors, and more particularly to a color filter array for improving color symmetry in different regions of an image sensor. 
         [0002]    Image sensors are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional image sensors include both charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. 
         [0003]    An image sensor typically includes a plane array of pixel cells, each pixel cell comprising a photogate, photoconductor or a photodiode having a doped region for accumulating photo-generated charge. 
         [0004]    A periodic pattern of dyes of different colors is superimposed over the plane array of pixel cells. This pattern is known as a color filter array (CFA). A plurality of microlenses is superimposed over the color filter array (CFA). A square, or circular microlens is utilized to focus light onto one initial charge accumulation region of each of pixel cells. By collecting light from a large light collecting area and focusing it on a small photosensitive area of the image sensor microlenses may significantly improve the image sensor photosensitivity. 
         [0005]      FIGS. 1 and 2  are schematic diagrams showing an image sensor  10  disclosed in U.S. Pat. No. 6,995,800 issued to Takahashi et al.  FIG. 1  shows a plan view of a pixel group and  FIG. 2  shows a cross sectional view of the pixels in the first, third and fifth columns of the pixel group shown in  FIG. 1 . As shown in  FIGS. 1 and 2 , reference numeral  1  represents a pixel having a photodiode or photoelectric conversion element  5  formed in the surface layer of a silicon substrate (Si substrate)  18 . Reference numeral  2  represents a light-shielding layer having a light-shielding area for shielding the area of the pixel  1  excepting the photodiode  5 . Reference numeral  3  represents an opening area formed through the light-shielding layer  2  through which light is incident upon the photodiode  5 . Reference numeral  4  represents a microlens for converging light on the photodiode  5 . Reference numeral  6  represents a color filter layer of red, green, blue or other colors. Although only a 5×5 pixels is shown in  FIG. 1  for the purposes of simplicity, there may in practice be several hundred thousands to several millions of pixels disposed two-dimensionally. 
         [0006]      FIGS. 1 and 2  show the pixel  1  disposed nearer the peripheral area than the center of the pixel group has a center of gravity of the light reception area of the photodiode  5  positioned nearer to the peripheral area than the centers of gravity of the microlens  4  and opening area  3 . The optical axis of light converged by the microlens  4  thus becomes coincident with the center of gravity of the light reception area of the photodiode  5 . 
         [0007]    More specifically, the pixel  1  of the first column has the centers of gravity of the microlens  4  and opening area  3  positioned to the right as viewed in  FIG. 1  relative to the center of gravity of the light reception area of the photodiode  5 . The pixel  1  of the third column has the centers of gravity of the microlens  4  and opening area  3  positioned at the center of gravity of the light reception area of the photodiode  5 . The pixel  1  of the fifth column has the centers of gravity of the microlens  4  and opening area  3  positioned to the left as viewed in  FIG. 2  relative to the center of gravity of the light reception area of the photodiode  5 . 
         [0008]    As described above, the pixel  1  disposed nearer to the peripheral area than the center of the pixel group has a center of gravity of the photodiode  5  positioned nearer to the peripheral area than the centers of gravity of the microlens  4  and opening area  3 . Thus, light passing through the microlens  4  and being incident upon the photodiode  5  is not intercepted by the light-shielding area of the light-shielding layer  2 . A difference between the maximum and minimum values of output signals of the pixels at different locations in the image sensor illustrated in  FIGS. 1 and 2  can thus be reduced to a level less than 10% of the average of the output signals. This is because light converged on the photodiode  5  is not intercepted by the light-shielding layer  2  and a variation in light reception sensitivities can be reduced. 
         [0009]    The color filter layer  6  used in the image sensor  10  illustrated in  FIGS. 1 and 2  can be a two-dimensional color filter array (CFA) including a periodic pattern of dyes of different primary colors such as red (R), green (G) and blue (B) filters, as illustrated in  FIG. 3 . The periodic pattern illustrated in  FIG. 3  is a so-called Bayer pattern, including a red (R) filter, a blue (B) filter and a pair of green (G) filters. Moreover, the color filter layer  6  illustrated in  FIGS. 1 and 2  can also be a two-dimensional color filter array including a periodic pattern of a cyan (Cy) filter, a magenta (Mg) filter and a pair of yellow (Ye) filters, as illustrated in  FIG. 4 . 
         [0010]    Although the output signal variation of pixels at different locations in an image sensor can be improved by the relative arrangement illustrated in  FIG. 1 . Some issues such as “color separation” may occur in an image sensor with structures similar to that illustrated in  FIGS. 1 and 2 . Color separation may be discovered when an image sensor is subjected to a chip probe (CP) test in which an image sensor is examined under a collimated white light. Color separation may occur in the photodiode or photoelectric conversion element  5  when the image sensor is formed in an irregular pattern rather a radially symmetrical pattern due to line routing or other device design requirements. An image sensor suffering from color separation may cause “image shading” in an optoelectronic device using the image sensor, thus, optoelectronic device may display abnormal images. 
         [0011]    Attachments  1  and  2  are simulated images of an image sensor (not shown) having a structure similar to that illustrated in  FIG. 1 , wherein the image sensor incorporates a CFA arranged in the Bayer pattern illustrated in  FIG. 3 . The simulated images were obtained by a CP test using a collimated white light, wherein attachment  1  shows a simulated image obtained by the image sensor having photodiodes with only one-axial asymmetrical, such as with x-axial asymmetrical, and attachment  2  shows a simulated image obtained by an image sensor having photodiodes with at least two-axial asymmetries, such as x-axial and y-axial asymmetries. 
         [0012]    Referring to attachment  1 , the simulated image shows an uneven image profile presenting a red-deflected color at an upper portion of the image sensor and a blue-deflected color at a lower portion of the image sensor. In attachment  2 , the illustrated image shows an uneven image profile presenting a red-deflected color at an upper-left portion of the image sensor, a blue-deflected color at a lower-right portion of the image sensor, and a green-deflected color at an upper-right portion and a lower-left portion of the image sensor. These uneven image profiles illustrated in attachments  1  and  2  are the above described “color separation” and are not desirable in an image sensor because “image shading” may occur in an optoelectronic device employing said image sensor. 
       SUMMARY 
       [0013]    The invention provides color filter arrays (CFA) and image sensors using the same for reducing or preventing color separation. 
         [0014]    An exemplary embodiment of a color filter array comprises a two-dimensional array including a plurality of first color filters, a plurality of second color filters, and a plurality of third color filters, wherein the first, second and third color filters are periodically arranged, and at least the first, second and third color filters formed in a first region of the two-dimensional array and the first, second and third color filters formed in a second region of the two-dimensional array are symmetrically mirrored. 
         [0015]    An exemplary embodiment of an image sensor comprises a semiconductor substrate with a plurality of photoelectric conversion elements formed therein. A light-shielding layer comprising a plurality of opening areas, each exposing a part of the photodiode, is formed over the semiconductor substrate. A color filter array is superimposed over the light-shielding layer, wherein the color filer array comprises a two-dimensional array comprising a plurality of first color filters, a plurality of second color filters, and a plurality of third color filters. A plurality of microlens superimposes over the color filter array, each covering the opening area of the underlying light-shielding layer. The first, second and third color filters are periodically arranged, and at least the first, second and third color filters formed in a first region of the two-dimensional array and the first, second and third color filters formed in a second region of the two-dimensional array are symmetrically mirrored. 
         [0016]    A detailed description is given in the following embodiments with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the office upon request and payment of the necessary fee. 
           [0018]    The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
           [0019]      FIG. 1  is a schematic top view showing a conventional image sensor; 
           [0020]      FIG. 2  is a cross sectional view of pixels in the first, third and fifth columns of the image sensor shown in  FIG. 1 ; 
           [0021]      FIG. 3  shows a conventional color filter array including a pattern of red (R), green (G) and blue (B) filters; 
           [0022]      FIG. 4  shows a conventional color filter array including a pattern of cyan (Cy), magenta (Mg), yellow (Ye) filters; 
           [0023]      FIG. 5  is a schematic top view showing an image sensor according to an embodiment of the invention; 
           [0024]      FIG. 6  is a cross sectional view of pixels in the first, fourth and eighth columns of the image sensor shown in  FIG. 5 ; 
           [0025]      FIG. 7  shows a color filter array including a pattern of red (R), green (G) and blue (B) filters according to an embodiment of the invention; 
           [0026]      FIG. 8  shows an arrangement of a color filter array including a pattern of red (R), green (G) and blue (B) filters according to another embodiment of the invention; and 
           [0027]      FIG. 9  shows an arrangement of a color filter array including a pattern of red (R), green (G) and blue (B) filters according to yet another embodiment of the invention. 
       
    
    
     DESCRIPTION 
       [0028]    The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
         [0029]      FIGS. 5 and 6  are schematic diagrams showing an image sensor  100  according to an exemplary embodiment of the invention. 
         [0030]      FIG. 5  shows a plan view of a pixel group of the image sensor  100  and  FIG. 6  shows a cross sectional view of pixels in the first, third/fourth and eighth columns of the pixel group shown in  FIG. 5 . 
         [0031]    As shown in  FIGS. 5 and 6 , reference numeral  30  represents a pixel having a photodiode or photoelectric conversion element  35  formed in the surface layer of a semiconductor substrate (e.g. a Si substrate)  38 . Reference numeral  32  represents a light-shielding layer having a light-shielding area for shielding the area of the pixel  30  excepting the photodiode  35 . Reference numeral  33  represents an opening area formed through the light-shielding layer  32  through which light is incident upon the photodiode  35 . Reference numeral  34  represents a microlens for converging light on the photodiode  35 . Reference numeral  36  represents a color filter layer of red, green, blue, or other colors. Although, for the simplicity, an area of only 8×8 pixels is shown in  FIG. 5 , there are, in practice, several hundred thousands to several millions of pixels disposed two-dimensionally. 
         [0032]    In  FIGS. 5 and 6 , the pixel  30  disposed nearer the peripheral area than the center of the pixel group has a center of gravity of the light reception area of the photodiode  35  positioned nearer the peripheral area than the centers of gravity of the microlens  34  and opening area  33 . The optical axis of light converged by the microlens  34  thus becomes coincident with the center of gravity of the light reception area of the photodiode  35 . 
         [0033]    Similar to the prior art image conventional image sensor illustrated in  FIG. 1 , the pixel  30  of the first column has the centers of gravity of the microlens  34  and opening area  33  positioned to the right as shown in  FIG. 5  relative to the center of gravity of the light reception area of the photodiode  35 . The pixel  30  of the fourth and fifth columns have the centers of gravity of the microlens  34  and opening area  33  positioned at the center of gravity of the light reception area of the photodiode  35 . The pixel  30  of the eighth column has the centers of gravity of the microlens  34  and opening area  33  positioned to the left as viewed in  FIG. 6  relative to the center of gravity of the light reception area of the photodiode  35 . The center of gravity of the opening area  33  is the center of gravity of an optional mass disposed in the opening area  33 . 
         [0034]    As described above, the pixel  30  disposed nearer to the peripheral area nearer the peripheral area than the center of the pixel group center of gravity of the photodiode  35  positioned nearer to the peripheral area than the centers of gravity of the microlens  34  and opening area  33 . Thus, the light-shielding area of the light-shielding layer  32  does not intercept light passing through the microlens  34  to be incident on the photodiode  35 . A difference between the maximum and minimum values of output signals of the pixels in the image sensor  100  at different locations in the image sensor of  FIGS. 5 and 6  can be reduced to a level less than 10% of the average of output signals but formed with color separation issues. The patterns of the color filter layer  36  of the image sensor  100  illustrated in  FIGS. 5 and 6  are modified as described in the following to further reduce or prevent color separation. 
         [0035]      FIG. 7  shows a color filter array including a periodic pattern of red (R), green (G) and blue (B) filters according to an embodiment of the invention. As shown in  FIG. 7 , an exemplary color filter array  300  for the color filter layer  36  is provided for the purpose of reducing even preventing color separation issues of an image sensor, especially while the photodiodes  35  used in the image sensor  100  are x-axial asymmetrical patterns (not shown). As shown in  FIG. 7 , the color filter array  300  is illustrated as a two-dimensional color filter array substantially divided into two regions  302  and  304  along a center of the color filter array  300  in an x-direction, each including a periodic pattern of dyes of different primary colors such as red (R), green (G) and blue (B) filters. 
         [0036]    As shown in  FIG. 7 , the region  302  of the color filter array  300  is illustrated as an upper region of the color filter array  300  and the region  304  color filter array  300  is illustrated as a lower region of the color filter array  300  in  FIG. 7 . In the region  302 , the periodic pattern of red (R), green (G) and blue (B) filters of the color filter array  300  are arranged in a Bayer pattern  350  which includes one R filter, one B filter, and a pair of G filters. 
         [0037]    The patterns of the color filter array  300  in the region  304  are arranged as a periodic pattern of dyes of different colors such as red (R), green (G) and blue (B) filters but with a modified Bayer pattern configuration different from the Bayer pattern configuration  350  in the region  302 . The modified Bayer pattern configuration  350 ′ in the region  304  and the Bayer pattern configuration  350  in the region  302  are mirror symmetrical. Accordingly, the periodic pattern of dyes the R, G and B filters in the region  302  and  304  are symmetrically mirrored against an x-direction. 
         [0038]    Attachment  3  shows a simulated image of an image sensor incorporating the CFA  300  illustrated in  FIG. 7  having photodiodes in asymmetrical x-axial patterns. The simulated image is obtained in a CP test using a collimated white light. As shown in attachment  3 , the simulated image shows an even image profile presenting no red-deflected color in an upper portion (i.e. the region  302 ) of the image sensor and no blue-deflected color in a lower portion (i.e. the region  304 ) of the image sensor. A symmetrical image profile with even color uniformity and color symmetry is obtained and the conventional color separation issue is thus reduced or even prevented. An image with better white balance can be also provided. 
         [0039]      FIG. 8  shows an arrangement of a color filter array including a pattern of red (R), green (G) and blue (B) filters according to another embodiment of the invention. 
         [0040]    In this embodiment, the color filter array  300  illustrated in  FIG. 7  is modified while the photodiodes  35  used in the image sensors are of asymmetrical y-axial patterns (not shown), as shown in  FIG. 8 . In  FIG. 8 , the color filter array  300  is substantially divided into two regions  302  and  304  at a center of the color filter array  300  along a y-direction. The region  302  of the color filter array  300  is illustrated as a right region of the color filter array  300  and the region  304  of the color filter array  300  is illustrated as a left region of the color filter array  300 . 
         [0041]    In the region  302 , the periodic pattern of red (R), green (G) and blue (B) filters of the color filter array  300  are arranged in a Bayer pattern configuration  350  which including one R filter, one B filter, and a pair of G filters. The patterns of the color filter array  300  in the region  304  are arranged as a periodic pattern of dyes of different colors such as red (R), green (G) and blue (B) filters but with a modified Bayer pattern configuration different from the Bayer pattern configuration  350  in the region  302 . 
         [0042]    The modified Bayer pattern configuration  350 ′ in the region  304  and the Bayer pattern configuration  350  in the region  302  are symmetrically mirrored. Accordingly, the periodic pattern of dyes the R, G and B filters in the region  302  and  304  are symmetrically mirrored against an x-direction. Accordingly, a symmetrical image profile can be obtained but, for simplicity, are not shown here, and the conventional color separation is thus reduced or even prevented. An image with better white balance can be also provided. 
         [0043]      FIG. 9  shows an arrangement of a color filter array including a pattern of red (R), green (G) and blue (B) filters according to yet another embodiment of the invention. 
         [0044]    As shown in  FIG. 9 , a color filter array  400  is a two-dimensional color filter array substantially divided into four regions  402 ,  404 ,  406  and  408  clockwise along a center of the color filter array  300  along both x and y directions, each including a periodic pattern of dyes of different colors such as red (R), green (G) and blue (B) filters. 
         [0045]    As shown in  FIG. 9 , the region  402  of the color filter array  400  is illustrated as an upper-right region of the color filter array  400 , the region  404  of the color filter array  400  is illustrated as a lower-right region of the color filter array  300 , the region  406  of the color filter array  400  is illustrated as an lower-left region of the color filter array  400 , and the region  404  of the color filter array  400  is illustrated as an upper-left region of the color filter array  400 . 
         [0046]    In the region  402 , the periodic pattern of red (R), green (G) and blue (B) filters of the color filter array  400  is arranged as Bayer pattern  450  which includes one R filter, one B filter, and a pair of G filters. The patterns of the color filter array  400  in the regions  404 ,  406  and  408  are arranged as a periodic pattern of dyes of different colors such as red (R), green (G) and blue (B) filters but with a modified Bayer pattern configuration different from the Bayer pattern configuration  450  in the region  402 . 
         [0047]    The modified Bayer pattern configuration  450 ′ in the regions  404  and the Bayer pattern configuration  450  in the region  402  are symmetrically mirrored corresponding to the x-axis. The modified Bayer pattern configuration  450 ″ in the regions  408  and the Bayer pattern configuration  450  in the region  402  are symmetrically mirrored corresponding to the y-axis. The modified Bayer pattern configuration  450 ″ in the regions  406  is symmetrically mirrored corresponding to the y-axis with the modified Bayer pattern configurations  450 ′ in the region  404  and is symmetrically mirrored corresponding to the x-axis with the modified Bayer pattern configurations  450 ″ in the region  408 . The modified Bayer pattern configuration  450 ″ in region  406  is also symmetrically radial corresponding to the Bayer pattern configuration  450  in the region  402 . 
         [0048]    Attachment  4  shows a simulated image of an image sensor incorporating the CFA  400  illustrated in  FIG. 9  and having photodiodes of both x-axial and y-axial asymmetrical patterns. The simulated image is obtained in a CP test using a collimated white light. 
         [0049]    As shown in attachment  4 , the illustrated image shows an even image profile presenting no red-deviated color at an upper-left portion (i.e. the region  408 ) of the image sensor, no blue-deviated color at a lower-right portion (i.e. the region  404 ) of the image sensor, and no green-deviated color at an upper-right and a lower-left and portions (i.e. the region  402  and  406 , respectively) of the image sensor. A symmetrical image profile is obtained and the conventional color separation is thus reduced or even prevented. An image with better white balance can be also provided. 
         [0050]    The color filter arrays  300  and  400  illustrated in  FIGS. 7 ,  8  and  9  are illustrated as two-dimensional color filter array including a periodic pattern of red (R), green (G) and blue (B) filters and are not limited thereto. The color filter arrays  300  and  400  illustrated in  FIGS. 7 ,  8  and  9  can additionally be a two-dimensional color filter array including a periodic pattern of different colors of cyan (Cy), magenta (Mg), yellow (Ye) filters. The red pattern can be substituted by the cyan pattern, the green pattern can be substituted by the yellow pattern and the blue pattern can be substituted by the magenta pattern. Note that while optoelectronic devices such as digital cameras, cellular phones, and toys incorporate the described image sensors illustrated in  FIG. 1  with a CFA illustrated in  FIG. 7 ,  8  or  9 , the signal output of the photodiode or photoelectric conversion elements in the region covered by such modified CFA patterns illustrated in, for example,  FIG. 7 ,  8  or  9  is to be suitably adjusted by wire routing or by software to conform to image protocols for correctly presenting an image. 
         [0051]    While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.