Abstract:
An image sensor device formed in an integrated circuit (IC) with one or more shading structures configured to provide a predetermined shading pattern to control the locations at which light impinges on the photodiode areas of the pixels. The predetermined shading patterns preventing color cross-talk and providing the pixels with substantially symmetric angular responses to light to thereby eliminate or reduce the occurrence of artifacts in the output image produced by the image sensor device.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to image sensor devices. More particularly, the invention relates to preventing or reducing color cross-talk between adjacent pixels in an image sensor device. 
     BACKGROUND OF THE INVENTION 
     An image sensor device is an integrated circuit (IC) having an array of pixels and circuitry for sampling the pixels and processing the pixel sample values. Pixel dimensions in image sensor devices are continually decreasing. At the same time, efforts are continually being made to increase the photodiode area of the pixels. One way to increase photodiode area is to share transistors that perform the same function amongst multiple pixels. Multiplexing devices and techniques are used in image sensor devices to allow these same-function transistors to be shared amongst multiple pixels. This pixel multiplexing makes it possible to increase full-well capacity, fill-factor, and sensitivity of the pixels, and thus to beneficially increase photodiode area. Pixel multiplexing also makes it possible to reduce the number of metal interconnect routes that are needed, which also allows photodiode area to be increased. 
     In image sensors that use pixel multiplexing, the pixels are spatially arranged in the image sensor such that an intrinsic spatial asymmetry exists between adjacent pixels. An example of a bayer block of a known image sensor device is shown in  FIG. 1 . A bayer block is a 2-by-2 group of pixels that are covered by green, red, blue, and green color filters (not shown) and together can be used to reassemble the red, green and blue components of the white light illuminating the image sensor device. The bayer block includes a green pixel  2 , and red pixel  3 , a blue pixel  4 , and a green pixel  5 . The reset (RST) and source follower (SF) transistors  6  and  7  are shared amongst the pixels  2 - 5 , as is the floating diffusion node  8 . Each of the pixels  2 ,  3 ,  4 , and  5  has a transfer transistor  8 ,  9 ,  11 , and  12 , respectively. Thus, the bayer block shown in  FIG. 1  has a total six transistors. 
     The horizontal routes  14  of the bayer block are formed in the lowest metal layer, the metal-1 layer. The vertical routes  15 - 18  are formed in the next layer above the metal-1 layer, metal layer 2. The vertical routes  15  and  16  are part of the network of conductors that provide power from the power supply, PVDD, to the pixels  2 - 5 . The vertical route lines  17  and  18  are the even and odd bit columns, respectively. Multiplexing circuitry (not shown) is used to select (i.e., turn on) only one the transfer transistors  8 ,  9 ,  11  and  12  at any given time to sample the selected pixel. 
     While the pixels  2 - 5  have very good symmetry with regard to mirroring about a horizontal or vertical axis, the overlaying color filters follow translational symmetry, which produces an asymmetrical optical angular response for the combined structure of the bayer block. This asymmetrical optical angular response often results in color cross-talk between adjacent pixels, i.e., light of one color bleeding over into a pixel intended to receive light of a different color. This color cross-talk is problematic because it can lead to artifacts in the final output image produced by the image sensor device. 
     The manner in which color cross-talk occurs in the image sensor device shown in  FIG. 1  can be in seen in  FIG. 2 .  FIG. 2  illustrates a cross-sectional view of a portion of an image sensor device comprising two adjacent pixels  3  and  23 , a color filter device  37  and a microlens structure  38 . The pixel  3  on the left corresponds to the red pixel  3  shown in  FIG. 1 . The pixel  23  on the right of pixel  3  is an adjacent green pixel, which cannot be seen in  FIG. 1 . In the red pixel  3 , the bottom layer  21  is the substrate, which is typically polysilicon, and the layer  22  above it is the photodiode layer that contains the photosensitive area  35  of the photodiode. The blocks  8  and  9  correspond to transfer transistors  8  and  9 , respectively, shown in  FIG. 1 . Transfer transistor  9  is part of pixel  3 , whereas transfer gate  8  is part of the green pixel  2  shown in  FIG. 1 , which is not shown in  FIG. 2 . The blocks  16 ,  17  and  18  correspond to vertical routes  16 ,  17  and  18 , respectively, shown in  FIG. 1 , which are formed in the metal-2 layer. In the green pixel  23 , the bottom layer  31  is the polysilicon substrate, and the layer  32  above it contains the photosensitive area  36  is the photodiode itself. The blocks  24  and  25  are transfer transistors. The transfer transistor  24  is part of the green pixel  23 , whereas the transfer transistor  25  is part of the red pixel (not shown) to the right of green pixel  23 . The blocks  26  and  27  are vertical routes formed in the metal-2 layer. Of course, layers  21  and  22  and layers  31  and  32  correspond, respectively, to the same layers. 
     The color filter device  37  and the microlens structure  38  are spatially arranged such light is received by them at angles that are non-normal with respect to the plane of the color filter device  37 . The spatial arrangement is intended to match the principle ray bundle angle resulting from the off-axis locations of the pixels. The principle ray bundle is represented by arrows  41 . Each ray bundle is represented by a red component  41 A, a green component  41 B and a blue component  41 C, which together form white light. The portion of the color filter device  37  shown in  FIG. 3  includes a red color filter  43  and a green color filter  44 . The red color filter  43  passes only the red component  41 A and filters out the green and blue components  41 B and  41 C. The green color filter  44  passes only the green component  41 B and filters out the red and blue components  41 A and  41 C. 
     The color filter device  37  and microlens structure  38  are spatially arranged as shown to ensure that the red component  41 A is only incident on the photosensitive area  35  of the red pixel  3  and the green component  41 B is only incident of the photosensitive area  36  of the green pixel  23 . However, because of the spatial asymmetry of the adjacent pixels  3  and  23 , and the angle of the light, some of the red components  41 A may be incident on, or bleed into, the photosensitive area  36  of the green pixel  23 , thereby resulting in color cross-talk. The optical asymmetry of adjacent pixels that results from the spatial asymmetry of the pixels is often phrased as the pixel having an asymmetrical angular response. It is also possible, but less likely because of the angle of the light, that green components  41 B will be incident on the photosensitive area  35  of the red pixel  3 . The situation is reversed on the opposite edge of the imaging array, where it is more likely that some green components will bleed onto the photosensitive area of the red pixel than it is that some red components will bleed onto the photosensitive area of the green pixel. 
     Color cross-talk between adjacent pixels can produce artifacts in the output image of the imaging device in the form of color variations across the imaging array of pixels where there should be color uniformity. For example, when imaging a target of uniform color (in particular, of uniform hue), the asymmetrical angular responses of the pixels may result in the output image having a displeasing greenish hue on one edge of the image and purple-ish hue on the other edge of the image. Furthermore, the asymmetrical angular response of pixels is even more pronounced in pixels located farther away from optical center of the imaging array, which can result in cross-talk amongst pixels closer to the optical center being unequal to cross-talk amongst pixels farther from the optical center. This unequal cross-talk typically results in more pronounced hue artifacts in the image. 
     Accordingly, a need exists for a way to eliminate or reduce color cross-talk between adjacent pixels in image sensor devices. 
     SUMMARY OF THE INVENTION 
     The invention provides an image sensor device formed in an integrated circuit (IC) and comprising a plurality of pixels, each of which has a photodiode area, and 
     one or more shading structures configured to provide a predetermined shading pattern to control locations at which light impinges on the photodiode areas of the pixels. The predetermined shading patterns provide the pixels with substantially symmetrical angular responses to light and eliminate or reduce the occurrence of artifacts in the output image produced by the image sensor device. 
     The method comprises designing an IC to include one or more shading structures, and fabricating an IC based on the design to produce an IC having said one or more shading patterns. Each shading structure provides a predetermining shading pattern to thereby control the locations at which light impinges on photodiode areas of the pixels. The predetermined shading patterns provide the pixels with substantially symmetrical angular responses to light, thereby preventing or reducing the occurrence of artifacts in the output image produced by the image sensor device. 
     These and other features and advantages of the invention will become apparent from the following description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a plan view of a bayer block of a known image sensor device. 
         FIG. 2  illustrates a cross-sectional view of a portion of a known image sensor device comprising two adjacent pixels, a color filter device and a microlens structure. 
         FIG. 3  illustrates a cross-sectional view of a portion of an image sensor device of the invention in accordance with an exemplary embodiment showing two adjacent pixels. 
         FIG. 4  illustrates a flowchart that represents the method of the invention in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the invention, one or more shading structures are used in the image sensor device to compensate for the non-normal angle of the incident light ray bundle, thereby making the angular response of the pixels more symmetrical. By making the angular response of the pixels more symmetrical, color cross-talk between adjacent pixels can be eliminated, or at least reduced. Making the angular responses of the pixels more symmetrical also eliminates or reduces the possibility of artifacts occurring in the output image of the image sensor device. 
     The structures that are used to make the angular response of the pixels more symmetrical include, for example, shading structures that are introduced into, or already existing in, the image sensor device that provide shading of light to control the locations at which light impinges on the photodiode area. The shading structures may have any number of shapes, including, but not limited to, slabs, stubs, square rings, circular rings, oval rings, elliptical rings, etc., and may be in any layer of the image sensor device or in any layer available in the fabrication process. In addition, the locations of the shading structures may vary depending on whether pixels that are being shadowed are located nearer to or farther away from the optical center of the imaging array. 
       FIG. 3  illustrates a cross-sectional view of a portion of an image sensor device  50  showing two adjacent pixels  51  and  61 . For exemplary purposes, it will be assumed that the structure of the image sensor device  50  is identical to the portion of the image sensor device shown in  FIG. 2 , and uses shared transistors and pixel multiplexing. It should be understood, however, that the invention is not limited to any particular type of image sensor device, but applies equally to any types of image sensor devices in which it is desirable to prevent color cross-talk. The invention also is not limited with respect to the type of process that is used to create the image sensor device. The invention is particularly well suited for use in image sensor devices having imaging arrays in which adjacent pixels are intended to respond to light of different colors. Such image sensor devices typically include color filters and microlens devices such as those shown in  FIG. 2 , although this is not a requirement of the invention. 
     With reference again to  FIG. 3 , pixel  51  has a substrate layer  53 , a photodiode layer  54 , and a transfer transistor  55 . The transfer transistor  56  is part of an adjacent pixel (not shown) to the left of pixel  51 . Vertical routes  57 - 59  are formed in the metal-2 layer of the image sensor device. Pixel  61  has a substrate layer  63 , a photodiode layer  54 , and a transfer transistor  65 . The transfer transistor  66  is part of an adjacent pixel (not shown) to the right of pixel  61 . Vertical routes  67  and  68  are formed in the metal-2 layer of the image sensor device. 
     As stated above, in accordance with the invention, one or more shading structures are included in the image sensor device to provide primary shading of light onto the photosensitive areas  71  and  81  of the pixels  51  and  61 , respectively. In accordance with this exemplary embodiment, shading structures  60 ,  70  and  80  are included in the image sensor device  50 . The bracket  72  is used to indicate the beam of light that is partially blocked by shading structures  60  and  70  to ensure that the light impinges on photodiode area  71  of pixel  51 , but does not impinge on photodiode are  81  of pixel  61 . Likewise, shading structures  70  and  80  partially block light beam  82  to ensure that the light impinges on photodiode area  81  of pixel  61 , but does not impinge on photodiode are  71  of pixel  51 . Thus, color cross-talk is prevented or at least significantly reduced. In addition, the shading structures  60 ,  70  and  80  ensure that the pixels  51  and  61  will have symmetrical angular responses to light, which prevents artifacts from occurring in the final output image. 
     In the exemplary embodiment represented by  FIG. 3 , the shading structures  60 ,  70  and  80  are formed by extending metal-2 layer. The structures could instead have been formed in a different metal layer, such as the metal-3 layer, for example. As stated above, the invention is not limited to where in the image sensor device the shading structures are formed, or with respect to the materials that are used to create the shading structures. 
       FIG. 4  illustrates a flowchart that represents the method of the invention in accordance with an exemplary embodiment for putting the shading structures in the image sensor device. The shading structures preferably are put in the image sensor device at the wafer level of the fabrication process that is used to create the image sensor ICs. Thus, the shading structures have been designed prior to fabrication to achieve the best possible results (i.e., to control locations of impingement of light and provide pixels with symmetrical angular responses) taking into account other IC design considerations. Therefore, the first step in the process is to design an image sensor device to include one or more shading structures that are configured to control the locations at which light impinges on the photodiode areas of the pixels. This step is represented by block  101 . After the image sensor device has been designed to include one or more shading structures, the image sensor device having one or more shading structures is fabricated, as indicated by block  103 . 
     It should be noted that the invention has been described with reference to particular embodiments for the purpose of demonstrating the principles and concepts of the invention. The invention, however, is not limited to these embodiments. Those skilled in the art will understand, in view of the description provided herein, the manner in which modifications can be made to the embodiments described herein, and the manner in which the principles and concepts of the invention can be extended to cover other embodiments. All such modifications and extensions are within the scope of the invention.