Abstract:
An image sensor includes an active pixel area for image capture; one or more black pixel areas disposed in a pre-determined, significant spaced apart distance from the active pixel area; and a light shield to prevent light from illuminating the black pixel areas.

Description:
This application claims the benefit of Provisional Application No. 60/267,849, filed Feb. 9, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to image sensors and, more particularly, to such image sensors having black reference pixels located in a non-intrusive and more efficient position. 
     BACKGROUND OF THE INVENTION 
     Currently used imaging sensors have black reference pixels located in the area immediately surrounding the active pixels. As is well known in the art, such black reference pixels are used for calibration of the imaging processing chain. These black reference pixels may become contaminated by lateral diffusion in which red light generated electrons from the active pixel area contaminate the first few rows and columns of the black reference pixels. This results in inaccurate reproduction of the desired image. 
     Currently used CCD imaging sensors compensate for lateral diffusion by adding additional rows and columns of pixels in which these buffer pixel rows and columns absorb the lateral diffused energy. This solution requires the use of a secondary light shield that prevents illumination of the buffer pixel rows and columns. An alternate solution is to increase the number of black pixels beyond the amount normally needed. The extra black pixels are used to absorb the energy from lateral diffusion. In this case, a secondary light shield is not necessary. 
     In CMOS image sensors, several layers of an inter-leaved material (usually metal) are used to shield light from the black reference pixels. These inter-leaved layers, however, allow light to leak therethrough and illuminate the black pixels, resulting in black pixels that are light sensitive. Obviously, this is an undesirable phenomenon. 
     Although the currently known and utilized methods and apparatus for lateral diffusion compensation are satisfactory, they include drawbacks. Adding buffer pixel rows and columns uses significant areas of the silicon. Similarly, additional rows and columns of black reference pixels also uses significant areas of the silicon, which is not cost-efficient. 
     In CMOS imagers the inter-leaved layers of material, used for black shielding, are not totally opaque to light. The solution to this problem is to add a secondary light shield, which in turn increases the cost. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in an image sensor comprising: (a) an active pixel area for image capture; (b) one or more black pixel areas disposed in a pre-determined, significant spaced apart distance from the active pixel area; and (c) a light shield to prevent light from illuminating the black pixel areas. 
     These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The present invention has the advantage of improved performance black reference pixels in electronic imagers. This is accomplished through the use of a coverglass secondary light shield and geographically moving the black pixels a significant distance from the imaging pixels. This invention can be applied to all solid state imagers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a drawing of a coverglass of the present invention; 
     FIG. 2 is a floor plan representation of a CMOS imager embodiment of the present invention; and 
     FIG. 3 is an alternative embodiment of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a coverglass  10  of the present invention. The coverglass  10  includes a clear central portion  20  in which light is transmitted therethrough and a light-shielded outer portion  30  in which light is not transmitted therethrough. The secondary light-shield outer portion  30  is formed by opaque material such as epoxy, paints, glues; metal; or oxides. This opaque material serves two purposes. The first purpose is to attach the coverglass  10  to the imager package and the second purpose is to shield the black pixels from the incoming light. It is instructive to note that the coverglass  10  and glue are one assembly. It is possible to accomplish the same result by using a separate secondary light shield and coverglass as is well known in the art. The secondary light shield  30  may also be formed from an opaque material which is coated directly onto a surface of the image sensor. 
     Referring to FIG. 2, there is shown a CMOS image sensor  40  of the present invention. The image sensor  40  includes an array of active picture pixels  50  arranged in a rectangular shape. As is well-known in the art, the active pixels  50  receive incident light which is converted into electrons and eventually a voltage for permitting capture of a digital representation of an image. 
     There is also shown a plurality of black reference pixels  60  and corresponding output multiplexing circuitry sections  70  both located at a predetermined significant distance from the active pixel area. The magnitude of the significant distance is established by the cone angle of the light of the focused image. The black pixels  60  are placed “far enough” under the secondary light shield such that image light can not illuminate the black pixels. Black reference pixels  60  are created by using active picture pixels with typically one or more metal layers deposited over them. The one or more metal layers are considered the primary light shield for the black pixels  60 . Positioning the black reference pixels  60  at a significant distance from the active pixel area permits the opaque glue  30 , on the coverglass  10 , shown in FIG. 1, to shield light from the black pixels. 
     The magnitude of the significant distance is determined by the optical design such that the black pixel signals aren&#39;t affected by either lateral diffusion or light sensitivity due to inter-leaved light shielding layers. This is accomplished by physically separating the black pixels  60  from the active image sensitive pixels  50  so no light illuminates the black pixels  60 . Additional distance can be added such that the overall design is insensitive to inaccuracy of coverglass  10  positioning. 
     To make this solution a low cost, production friendly, solution it can be loosely constrained. That is, it is desired not to have precise mechanical position requirements. The distance from the black pixel area(s)  60  to the active image area  50  is made great enough to satisfy these requirements, as those skilled in the art will be able to determine. The coverglass  10  can be positioned with “loose” tolerances due to the significant distance being chosen greater than the minimum required by the optical design. 
     In contrast, currently known and utilized imagers locate their black pixel area(s) adjacent to their active picture pixels. 
     It is instructed to know that one or more of the black reference pixel sections  60  and corresponding output multiplexing circuitry sections may be used depending on the design. Multiple black reference pixel sections  60  provide additional black pixel information. For example, due to processing variation and temperature differences across the imager die, the dark reference signal can vary across the area of the imager. This is compensated for by the having multiple black reference pixel areas  60  located at various locations on the imager die. In summary, these multiple black reference pixel areas  60  allow more accurate measurement of the black signal for the total image. It is also instructive to note that additional black reference pixel areas  60  may be used, as the illustrated locations are for example only. 
     The digital control circuitry  80 , analog output circuitry  90  and analog to digital converter  100  are shown for reference purposes as is typical for the current state of the art. Such components are well known and will not be discussed in detail herein. 
     Referring to FIG. 3, there is shown an alternative embodiment of the present invention for either CCD or CMOS imagers. In this embodiment, the black reference pixel sections  60  are located at a plurality of positions. One location for the black reference pixel section  60  is between wire bond pads  110 , and another location is on either side of the bond pads  110 . These locations are in general located on any or all sides of the image area  50 , separated a significant distance from the image area  50 . 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 coverglass 
               
               
                 20 
                 central portion 
               
               
                 30 
                 outer portion 
               
               
                 40 
                 sensor 
               
               
                 50 
                 pixels 
               
               
                 60 
                 black reference pixels 
               
               
                 70 
                 circuitry sections 
               
               
                 80 
                 digital control circuitry 
               
               
                 90 
                 output circuitry 
               
               
                 100 
                 A/D converter 
               
               
                 110 
                 bond pads