Abstract:
A solid-state imaging device has a plurality of photoelectric conversion elements two dimensionally arrayed in an imaging area, a light shielding film that regulates the amount of external light incident on the photoelectric conversion elements by a wiring pattern, a wiring layer placed between the light shielding film and the photoelectric conversion elements, and a plurality of contacts electrically connecting the light shielding film with the wiring layer in a lamination direction. The shape of the light shielding film is defined by a plurality of first figures overlapping with a second figure, each first figure being placed over a different contact in plan view, and the second figure having a plurality of apertures each corresponding to a different photoelectric conversion element. The center of each aperture in the second figure is displaced further from the center of a corresponding photoelectric conversion element toward the middle of the imaging area in plan view, as distance from the middle of the imaging area increases. Furthermore, a positional relation of the first figures with the second figure differs depending on location in the imaging area.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on application no. 2005-050436 filed in Japan, the content of which is hereby incorporated by reference.  
       BACKGROUND OF INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a solid-state imaging device, and more particularly to technology for reducing sensitivity shading.  
         [0004]     2. Related Art  
         [0005]     Solid-state imaging devices, remarkably popularized in recent years, typically receive external light via an optical lens.  
         [0006]      FIG. 1  is a cross-sectional view of a digital camera illustrating the optical path of external light incident on a solid-state imaging device via an optical lens. As shown in  FIG. 1 , digital camera  1  is provided with an optical lens  101  and a solid-state imaging device  102 , and external light  111  to  113  is incident on solid-state imaging device  102  via optical lens  101 .  
         [0007]     External light  112  indicated by the solid line is incident substantially at the middle of the imaging area of solid-state imaging device  102 . In this case, the chief ray of external light  112  substantially coincides with the optical axis of optical lens  101 , and is incident substantially vertically on the imaging area. In other words, the angle of incidence of external light  112  is zero.  
         [0008]     On the other hand, external light  111  indicated by the broken line and external light  113  indicated by the dash-dotted line are, as shown in  FIG. 1 , incident at an angle at the periphery of the imaging area. In this case, the angle of incidence of the external light increases as the position of incidence moves away from the middle of the imaging area.  
         [0009]     This produces a difference in light reception efficiency between pixel cells in the middle of the imaging area and pixel cells at the periphery, resulting in sensitivity shading.  FIG. 2  is a cross-sectional view showing the structure of a typical pixel cell. As shown in  FIG. 2 , pixel cell  2  is provided with a semiconductor substrate  201 , insulating films  202 ,  205  and  208 , metal layers  203  and  206 , a contact  204 , a color filter  207 , a collecting lens  209 , and a photodiode  210  (see Japanese patent application publications No. 10-150182, and No. 2003-46865, for example).  
         [0010]     External light incident on pixel cell  2  is incident on photodiode  210  via color filter  207  after having been focused by collecting lens  209 . In this case, the external light must pass through metal layers  203  and  206  and the aperture of contact  204 . However, when the angle of incidence is large, the incident light is shaded by metal layers  203  and  206  and contact  204 . The incident light thus has difficulty reaching photodiode  210 , resulting in sensitivity shading.  
         [0011]     Metal oxide semiconductor (MOS) image sensors and charge coupled devices (CCDs) are known solid-state imaging devices. MOS image sensors are provided with a MOS-FET per pixel cell for amplifying the output charge of the photodiode, and thus require a plurality of metal wiring layers. Since this lengthens the optical path from collecting lens  209  to photodiode  210 , sensitivity shading is particularly marked.  
       SUMMARY OF INVENTION  
       [0012]     The present invention, arrived at in view of the above problem, aims to provide a solid-state imaging device that realizes high image quality by reducing sensitivity shading.  
         [0013]     To solve the above problem, a solid-state imaging device pertaining to the present invention includes a plurality of photoelectric conversion elements two dimensionally arrayed in an imaging area, a light shielding film that regulates the amount of external light incident on the photoelectric conversion elements by a wiring pattern, a wiring layer placed between the light shielding film and the photoelectric conversion elements in a lamination direction, and a plurality of contacts electrically connecting the light shielding film with the wiring layer. The shape of the light shielding film is defined by a plurality of first figures overlapping with a second figure, each first figure being placed over a different contact in plan view, and the second figure having a plurality of apertures each corresponding to a different photoelectric conversion element. The center of each aperture in the second figure is displaced further from the center of a corresponding photoelectric conversion element toward the middle of the imaging area in plan view, as distance from the middle of the imaging area increases, and a positional relation of the first figures with the second figure differs depending on location in the imaging area.  
         [0014]     This structure enables sensitivity shading to be reduced by ensuring a sufficient amount of received light even at the periphery of the imaging area, since the second figure portion can be significantly shifted toward the middle of the imaging area, while at the same time ensuring electrical connectivity of the light shielding film with the wiring layer in the first figure portion. Accordingly, high image quality can be realized.  
         [0015]     Furthermore, the displacement of each aperture center from the center of a corresponding photoelectric conversion element may increase in proportion to distance from the middle of the imaging area.  
         [0016]     This structure enables the distance between the aperture centers of the second figure and the centers of corresponding photoelectric conversion elements to be optimally adjusted according to the angle of incidence of external light, since the angle of incidence of external light on the imaging area increases as distance from the middle of the imaging area increases.  
         [0017]     Furthermore, the displacement of each aperture center from the center of a corresponding photoelectric conversion element may increase stepwise according to distance from the middle of the imaging area.  
         [0018]     This structure enables the trouble of designing the second figure to be eliminated, since the angles of incidence of external light are similar at positions close to one another in the imaging area, with the change in the angle of incidence being small particularly at the periphery of the imaging area. Accordingly, solid-state imaging devices with reduced sensitivity shading can be designed more cost effectively.  
         [0019]     Furthermore, the first figures may have substantially similar positional relations with corresponding contacts regardless of location in the imaging area, may be substantially similar in size regardless of location in the imaging area, and may be large enough to overlap with the second figure at a furthest location from the middle of the imaging area.  
         [0020]     This structure enables the trouble of designing the second figure to be eliminated, while at the same time ensuring electrical connectivity between the light shielding film and the wiring layer.  
         [0021]     Furthermore, the interval between adjacent first figures interposed with a photoelectric conversion element in plan view may increase as distance from the middle of the imaging area increases.  
         [0022]     This structure enables sensitivity shading to be reduced by reducing the amount of incident light shaded by the first figure portion of the light shielding film.  
         [0023]     In this case, by dividing the imaging area into a plurality of sub-areas, and making the interval between adjacent first figures in respective sub-areas constant, the trouble of designing such solid-state imaging devices can be lessened.  
         [0024]     Furthermore, each first figure may increase in size as distance from the middle of the imaging area increases.  
         [0025]     This structure enables light reception efficiency in the middle of the imaging area to be improved, since the first figures are made smaller as distance between the second figure and the contacts is reduced toward the middle of the imaging area.  
         [0026]     Furthermore, the pattern width of each first figure in the middle of the imaging area maybe equivalent to the pattern width of the second figure, in a pattern widthwise direction of the second figure near a contact corresponding to the first figure.  
         [0027]     This structure enables the amount of incident light shaded by the first figure portion of the light shielding film to be reduced, while at the same time maintaining a processing margin necessary for ensuring electrical connectivity between the first figures and the contacts.  
         [0028]     Furthermore, the imaging area may be rectangular, and each first figure may be placed adjacent to a closest photoelectric conversion element in the short direction of the imaging area.  
         [0029]     This structure enables sensitivity shading at the periphery of the imaging area to be reduced.  
         [0030]     According to the present invention, a high performance, high-reliability solid-state imaging device is obtained that can suppress sensitivity shading by reducing the amount of shading around pixel arrays, and provide excellent connectivity between lower and upper layer wiring.  
         [0031]     Furthermore, this solid-state imaging device can be manufactured using processes similar to a conventional structure, thereby removing the need for additional manufacturing processes, which is advantageous in terms of cost. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0032]     These and other objects, advantages, and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings, which illustrate specific embodiments of the present invention.  
         [0033]     In the drawings:  
         [0034]      FIG. 1  is a cross-sectional view of a digital camera illustrating the optical path of external light incident on a solid-state imaging device via an optical lens;  
         [0035]      FIG. 2  is a cross-sectional view showing the structure of a typical pixel cell;  
         [0036]      FIG. 3  is a circuit diagram showing a main structure of a MOS image sensor pertaining to an embodiment 1 of the present invention;  
         [0037]      FIGS. 4A  to  4 D show the structure of a pixel cell  311  pertaining to embodiment 1 of the present invention, FIGS.  4 A and  4 C being a plan view and a cross-sectional view of pixel cell  311  located in the middle of an imaging area  310 , and  FIGS. 4B and 4D  being a plan view and a cross-sectional view of pixel cell  311  located at the periphery of imaging area  310 ;  
         [0038]      FIGS. 5A and 5B  show the relation between the location of pixel cell  311  in imaging area  310  and the placement of metal layers  406 A and  406 B pertaining to embodiment 1 of the present invention,  FIG. 5A  being a graph showing the displacement between the centers of metal layer  406 A and a photodiode  410 , and  FIG. 5B  being a graph showing the pitch between a pair of metal layers  406 B sandwiching photodiode  410 ;  
         [0039]      FIGS. 6A and 6B  are schematic views showing an exemplary division of an imaging area pertaining to an embodiment 2 of the present invention,  FIG. 6A  showing an example of the imaging area divided by concentric circles centered on the middle of the imaging area, and  FIG. 6B  showing an example of the imaging area divided by rectangles centered on the middle of the imaging area and similar in shape to the external form of the imaging area;  
         [0040]      FIG. 7  is a graph illustrating the pitch of the metal layers per sub-area in the case of  FIG. 6A  pertaining to embodiment 2 of the present invention;  
         [0041]      FIGS. 8A  to  8 D show the structure of a pixel cell provided in a MOS image sensor pertaining to an embodiment 3 of the present invention,  FIGS. 8A and 8C  being a plan view and a cross-sectional view of the pixel cell located in the middle of an imaging area, and  FIGS. 8B and 8D  being a plan view and a cross-sectional view of the pixel cell located at the periphery of the imaging area;  
         [0042]      FIGS. 9A and 9B  show a pattern width d of a metal layer  806 B pertaining to embodiment 3 of the present invention,  FIG. 9A  being a plan view in the case of positional deviation, and  FIG. 9B  being a graph showing the relation between the pattern width d and a distance c from a central position of an imaging area; and  
         [0043]      FIG. 10  is a plan view showing the positional relation between photodiodes, contacts, and metal layers pertaining to an embodiment 4 of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]     Embodiments of a solid-state imaging device pertaining to the present invention are described below with reference to the drawings, taking a MOS image sensor as an example.  
       EMBODIMENT 1  
       [0045]     A MOS image sensor pertaining to an embodiment 1 of the present invention is characterized in that the position and shape of a metal layer in each pixel cell varies depending on the location of the pixel cell in an imaging area, so as to increase the amount of external light incident on a photodiode.  
         [0046]     (1) Structure of MOS Image Sensor  
         [0047]     Firstly, the structure of a MOS image sensor pertaining to the present embodiment is described.  FIG. 3  is a circuit diagram showing the main structure of a MOS image sensor pertaining to the present embodiment.  
         [0048]     As shown in  FIG. 3 , MOS image sensor  3  is provided with an imaging area  310 , a vertical shift register  321 , a horizontal shift register  322 , reset lines  323 , horizontal pixel selection lines  324 , vertical selection transistors  325 , a horizontal signal line  327 , and vertical voltage input transistors  328 .  
         [0049]     A large number of pixel cells are two dimensionally arrayed in imaging area  310 . Individual pixel cells  311  are provided with a photodiode  312 , a charge transfer transistor  313 , a reset transistor  314 , and an amplifying transistor  315 .  
         [0050]     (2) Structure of Pixel Cell  311   
         [0051]     Next, the structure of pixel cell  311  is described.  FIGS. 4A  to  4 D show the structure of pixel cell  311  pertaining to the present embodiment.  FIGS. 4A and 4C  are a plan view and a cross-sectional view of pixel cell  311  located in the middle of imaging area  310 , while  FIGS. 4B and 4D  are a plan view and a cross-sectional view of pixel cell  311  located at the periphery of imaging area  310 .  
         [0052]     As shown in  FIGS. 4C and 4D , each pixel cell  311  is provided with a semiconductor substrate  401 , insulating films  402 ,  405  and  408 , metal layers  403 ,  406 A and  406 B, a contact  404 , a color filter  407 , a collecting lens  409 , and a photodiode  410 .  
         [0053]     Note that although metal layers  406 A and  406 B are distinguished as different figures when designing MOS image sensor  3 , they form a single metal layer in MOS image sensor  3  after manufacture.  
         [0054]     Photodiode  410  is formed in semiconductor substrate  401 , and on top of this are sequentially formed insulating films  402  and  405 , metal layer  403 , contact  404 , insulating film  408 , metal layers  406 A and  406 B, color filter  407 , and collecting lens  409 . Contact  404  electrically connects metal layer  403  with metal layers  406 A and  406 B.  
         [0055]     Metal layers  403 ,  406 A and  406 B function as wiring that electrically connects elements or circuits within MOS image sensor  3 .  
         [0056]     (3) Reduction of Sensitivity Shading  
         [0057]     In the present embodiment, metal layer  406 A functions primarily as a light shielding film. The center of metal layer  406 A in the middle of imaging area  310  substantially coincides with the center of photodiode  410  in plan view ( FIG. 4A ). On the other hand, the center of metal layer  406 A at the periphery of imaging area  310  is displaced from the center of photodiode  410  toward the middle of imaging area  310  in plan view ( FIG. 4B ).  
         [0058]     This enables sensitivity shading to be reduced by preventing metal layer  406 A from shading external light.  
         [0059]     Note that the centers of color filter  407  and collecting lens  409  at the periphery of imaging area  310  are also displaced from the center of photodiode  410  toward the middle of imaging area  310  in plan view, so that an appropriate amount of external light is incident on photodiode  410 . The displacement from the center of photodiode  410  increases in order of metal layer  406 A, color filter  407  and collecting lens  409  ( FIG. 4D ).  
         [0060]     (4) Ensuring Electrical Connectivity  
         [0061]     Note that since the positional relation between photodiode  410  and metal layer  403  remains constant regardless of the location of pixel cell  311  in imaging area  310 , the positional deviation between metal layer  403  and metal layer  406 A increases as pixel cell  311  is located further from the middle of imaging area  310  toward the periphery, until finally the centers of metal layers  403  and  406 A fail to overlap at all in plan view, as shown in  FIG. 4B .  
         [0062]     In the present embodiment, metal layer  406 B is provided to ensure electrical connectivity between metal layer  403  and metal layer  406 A. Metal layer  406 B is placed so that the center of metal layer  406 B coincides with the center of contact  404  in plan view, regardless of the location of pixel cell  311  in imaging area  310 . Thus, the placement pitch of metal layers  403  and  406 B and contact  404  is equivalent to that of pixel cells  311  in imaging area  310 .  
         [0063]     The size and shape of metal layer  406 B is designed to ensure electrical connectivity with metal layer  403  even when the centers of metal layer  403  and metal layer  406 A are at maximum displacement.  
         [0064]     (5) Placement of Metal Layers  406 A and  406 B  
         [0065]     Next, the placement of metal layers  406 A and  406 B is further described in detail.  FIGS. 5A and 5B  show the relation between the location of pixel cell  311  in imaging area  310  and the placement of metal layers  406 A and  406 B.  FIG. 5A  is a graph showing the displacement between the centers of metal layer  406 A and photodiode  410 , while  FIG. 5B  is a graph showing the pitch between a pair of metal layers  406 B sandwiching photodiode  410 .  
         [0066]     The displacement between the centers of metal layer  406 A and photodiode  410  increases stepwise as the location of pixel cell  311  moves further from the middle of imaging area  310  toward the periphery.  
         [0067]     Similarly increasing the displacement between pixel cells  311  stepwise according to distance from the middle of imaging area  310  has the advantage of facilitating design. However, the variation in brightness between adjacent pixel cells  311  increases when the displacement between adjacent pixel cells  311  fluctuates greatly. In view of this, the ideal is to optimize the displacement per pixel cell according to distance from the middle of imaging area  310 , or in other words, to change the displacement continuously.  
         [0068]     On the other hand, the pitch of metal layers  406 A remains constant regardless of the location of pixel cell  311  in imaging area  310 , given the necessity of ensuring electrical connectivity with contact  404 .  
         [0069]     (6) Size of Metal Layers  406 A and  406 B  
         [0070]     Next, the size of metal layers  406 A and  406 B is described in detail. As shown in  FIGS. 4A  to  4 D, metal layer  406 A is lattice-shaped in plan view, while metal layers  406 B and contacts  404  are all square-shaped in plan view.  
         [0071]     In this case, a pattern width d of metal layer  406 B is calculated using the following equation, where g is the pattern width of metal layer  406 A, c max  is the maximum displacement between the centers of metal layer  406 A and photodiode  410 , w is the pattern width of contact  404 . 
 
 d=w+c   max −( g/ 2)  (1) 
 
         [0072]     For example, if g is set from 1.0 μm to 3.0 μm, c max  is set from 0.4 μm to 1.0 μm, and w is set from 0.1 μm to 0.4 μm, d will be approximately 0.5 μm to 2.0 μm.  
         [0073]     If the centers of metal layer  406 A and photodiode  410  are aligned regardless of the location of pixel cell  311  in imaging area  310 , sensitivity shading occurs whereby light detected at the periphery drops to 60% or less of that detected in the middle, when originally the same amount should be detected regardless of location in imaging area  310 . In contrast, the present embodiment enables the amount of light detected at the periphery to be improved to 70% to 90% of that detected in the middle.  
       EMBODIMENT 2  
       [0074]     A MOS image sensor pertaining to the present embodiment is provided with generally the same structure as a MOS image sensor pertaining to embodiment 1, except that the pitch of the metal layers for ensuring electrical connectivity (equivalent to metal layers  406 B in embodiment 1) differs depending on location in the imaging area. The present embodiment is described below, focusing exclusively on this difference.  
         [0075]     (1) Division of Imaging Area  
         [0076]     In the present embodiment, the imaging area is divided into a plurality of sub-areas, with the pitch of the metal layers for ensuring electrical connectivity being the same within respective sub-areas. This pitch becomes smaller the further the sub-area is from the middle of the imaging area.  
         [0077]      FIGS. 6A and 6B  are schematic views showing an exemplary division of the imaging area.  FIG. 6A  shows an example of the imaging area divided by concentric circles centered on the middle of the imaging area, while  FIG. 6B  shows an example of the imaging area divided by rectangles centered on the middle of the imaging area and similar in shape to the external form of the imaging area.  
         [0078]      FIG. 7  is a graph illustrating the pitch of the metal layers for ensuring electrical connectivity per sub-area in the case of  FIG. 6A . The pitch decreases gradually from sub-area  1  to sub-area  3 , as shown in  FIG. 7 . Also, the values of P 1 , P 2  and P 3  in respective sub-areas  1 ,  2  and  3  are uniform within any one sub-area.  
         [0079]     The variation in the angle of incidence of external light is greater the closer the pixel cell is to the middle of the imaging area, and decreases toward the periphery. Given that the metal layers for ensuring electrical connectivity also contribute to the shielding of external light, increasing the pitch in the middle of the imaging area enables the shielding of external light by these metal layers to be reduced, even if the angle of incidence varies greatly.  
         [0080]     On the other hand, since the variation in the angle of incidence is small at the periphery of the imaging area, the incidence of external light can be ensured even if the pitch of the metal layers is small.  
       EMBODIMENT 3  
       [0081]     Next, an embodiment 3 of the present invention is described. A MOS image sensor pertaining to the present embodiment is provided with generally the same structure as a MOS image sensor pertaining to embodiment 1, except that the pitch of the metal layers for ensuring electrical connectivity (equivalent to metal layers  406 B in embodiment 1) differs depending on location in the imaging area. The present embodiment is described below, focusing exclusively on this difference.  
         [0082]     (1) Structure of Pixel Cells  
         [0083]     Firstly, the structure of pixel cells provided in a MOS image sensor pertaining to the present embodiment is described.  
         [0084]      FIGS. 8A  to  8 D show the structure of a pixel cell provided in a MOS image sensor pertaining to the present embodiment.  FIGS. 8A and 8C  are a plan view and a cross-sectional view of the pixel cell located in the middle of the imaging area, while  FIGS. 8B and 8D  are a plan view and a cross-sectional view of the pixel cell located at the periphery of the imaging area.  
         [0085]     As shown in  FIGS. 8C and 8D , the pixel cell is provided with a semiconductor substrate  801 , insulating films  802 ,  805  and  808 , metal layers  803 ,  806 A and  806 B, a contact  804 , a color filter  807 , a collecting lens  809 , and a photodiode  810 .  
         [0086]     In the middle of the imaging area, the centers of photodiode  810 , metal layer  806 A, color filter  807  and collecting lens  809  coincide in plan view ( FIG. 8C ).  
         [0087]     On the other hand, at the periphery of the imaging area, the center of metal layer  806 A is displaced from the center of photodiode  810  by a distance c, the center of color filter  807  is displaced from the center of photodiode  810  by a distance b, and the center of collecting lens  809  is displaced from the center of photodiode  810  by a distance a ( FIG. 8D ).  
         [0088]     As shown in  FIGS. 8A and 8B , the size of metal layer  806 B (pattern width d) is small in the middle of the imaging area and large at the periphery. On the other hand, the size of contact  804  (pattern width w) is the same regardless of location in the imaging area.  
         [0089]     Furthermore, since the positional relation of photodiode  810  and contact  804  also remains the same regardless of location in the imaging area, the distance from the center of photodiode  810  to the center of metal layer  806 A is equal to the distance from the center of contact  804  to the center of metal layer  806 A, this being the distance c.  
         [0090]     (2) Pattern Width d of Metal Layer  806 B  
         [0091]     In the present embodiment, the pattern width d of metal layer  806 B for ensuring electrical connectivity varies depending on location in the imaging area. The pattern width d is designed using the following equation. 
 
 d=w+c− ( g/ 2)+ a   (2) 
 
 Here, w is the pattern width of contact  804 , c is the distance from the center of photodiode  810  to the center of metal layer  806 A, and g is the pattern width of metal layer  406 A. 
 
         [0092]     With equation 1 pertaining to embodiment 1, a fixed value c max  is used to derive the pattern width d. In contrast, since the distance c used in the present embodiment varies depending on location in the imaging area, the pattern width d also varies depending on location in the imaging area.  
         [0093]     The pattern width d in embodiment 1 is thus unnecessarily large the closer the pixel cell is to the middle of the imaging area, whereas in the present embodiment, the shading of incident light by metal layer  406 B can be minimized because the pattern width d can be kept to the minimum necessary value per pixel cell.  
         [0094]     Accordingly, the present embodiment enables image quality to be further improved.  
         [0095]     In equation 2, the value of a is determined such that the pattern width d is greater than or equal to the pattern width g of metal layer  806 A, even when the centers of photodiode  810  and metal layer  806 A coincide (c=0).  
         [0096]     This is due to the risk of positional deviation arising between contact  804  and metal layers  806 A and  806 B, given that the contact and metal layers are formed in separate processes during manufacturing. In other words, a processing margin is necessary to ensure electrical connectivity between contact  804  and metal layer  806 B regardless of positional deviation.  
         [0097]      FIGS. 9A and 9B  show the pattern width d of metal layer  806 B,  FIG. 9A  being a plan view in the case of positional deviation, and  FIG. 9B  being a graph showing the relation between the pattern width d and the distance c from a central position of the imaging area.  
         [0098]     As shown in  FIG. 9A , electrical connectivity between contact  804  and metal layer  806 B can be ensured through making the pattern width d a given size even when the distance c is small.  
         [0099]     In the present embodiment, the minimum value of the pattern width d is set to the pattern width g of metal layer  806 A. Setting the pattern width d to less than the pattern width g does not allow for a significant increase in the amount of received light, and instead makes it more difficult to ensure electrical connectivity. Also, when the distance c exceeds g/2, electrical connectivity cannot be ensured unless the pattern width d is increased.  
         [0100]     Thus, the relation between the distance c and the pattern width d is as shown in the  FIG. 9B  graph. In other words, the pattern width d is equal to the pattern width g when the distance c is g/2 or less, and increases linearly when the distance c is greater than g/2 to give the pattern width d obtained by equation 1 pertaining to embodiment 1.  
         [0101]     Note that the distance c in a typical MOS image sensor preferably is approximately 1.0 μm. Also, the positional deviation between contact  804  and metal layer  806 B is at most approximately 0.1 μm according to the current standards of typical processing technology.  
       EMBODIMENT 4  
       [0102]     Next, a MOS image sensor pertaining to an embodiment 4 of the present invention is described. A MOS image sensor pertaining to the present embodiment is provided with generally the same structure as a MOS image sensor pertaining to embodiment 1, except for a difference in the positioning of the contact. The present embodiment is described below, focusing on this difference.  
         [0103]     (1) Structure of MOS Image Sensor  
         [0104]     Firstly, the structure of the MOS image sensor is described.  FIG. 10  is a plan view showing the positional relation between photodiodes, contacts, and metal layers. The imaging area of a MOS image sensor pertaining to the present embodiment has a horizontal to vertical pixel ratio of 3:4. In  FIG. 10 , the “H direction” and “V direction” denote the horizontal and vertical directions, respectively. The “D direction” (not depicted) denotes the diagonal direction.  
         [0105]     As shown in  FIG. 10A , contacts  1004  and metal layers  1006 B are placed at positions sandwiched by adjacent photodiodes  1010  in the V direction.  
         [0106]     (2) Effects of Present Embodiment  
         [0107]     The angle of incidence of external light on the pixel cells is equal for pixel cells equally distant from the middle of the imaging area, regardless of whether in the vertical or horizontal direction. The angle of incidence is thus maximized at the outer edge in the direction with the greater width, in the case where the widths of the imaging area differ in the vertical and horizontal directions.  
         [0108]     To ensure the amount of received light, the distance between the centers of the metal layer and the photodiode needs to be increased the greater the angle of incidence, which consequently means that the pattern width of the metal layer for ensuring electrical connectivity is also increased.  
         [0109]     Increasing the pattern width of the metal layer for ensuring electrical connectivity impedes external light incident on the photodiode, which is contrary to the object of ensuring the amount of received light.  
         [0110]     In contrast, if the contact and the metal layer for ensuring electrical connectivity are placed between adjacent photodiodes in the direction with the smaller imaging area width (V direction in the present embodiment), the maximum pattern width of this metal layer can be suppressed. Accordingly, image quality can be improved by ensuring the amount of received external light to the photodiode. Higher pixelization can thus be achieved by making the pixel cells even smaller.  
         [0111]     Modifications  
         [0112]     The present invention, while having been described above based on the preferred embodiments, is of course not limited to these embodiments, and the following modifications may be implemented.  
         [0113]     (1) While not particularly addressed in the above embodiments, the metal layer that acts as a light shielding film is not particularly limited in terms of material or the like, provided it functions as a light shielding film and wiring.  
         [0114]     Furthermore, the metal layer connected via a contact to the metal layer that acts as a light shielding film may be metal wiring whose primary material is aluminum, tungsten or the like, or may be a transistor gate electrode or a diffusion layer.  
         [0115]     Furthermore, provided the contact is also a conductor, a material other than metal may be used, such as a low resistance polysilicon plug, for example.  
         [0116]     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.