Abstract:
A solid-state imaging-device includes a base, frame-shaped ribs provided on the base and forming an internal space, a plurality of wiring members for electrically leading the internal space of a housing formed by the base and the ribs to an external portion, an imaging element fixed to the base inside the internal space, a transparent plate fixed to an upper surface of the ribs, and connecting members electrically connecting electrodes of the imaging element to the wiring members, wherein a plurality of protrusions are provided in a region of the base that faces the imaging element, and the imaging element is fixed by adhesive to the base while being supported by the protrusions. The protrusions enable the imaging element to avoid distortion caused by following the surface of the base, thereby suppressing the effect on electrical properties of the imaging element.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to solid-state imaging devices in which an imaging element, such as a CCD or the like, is mounted in a housing.  
       BACKGROUND OF THE INVENTION  
       [0002]     Solid-state imaging devices, which are widely used for video cameras and still cameras or the like, are provided in the form of a package, in which an imaging element, such as a CCD or the like, is mounted on a base made of an insulating material, with the photo-detecting region being covered by a transparent plate. In order to make the device more compact, the imaging element is mounted on the base as a bare chip.  FIG. 9  shows the solid-state imaging device disclosed in JP 2001-77277, which is a conventional example of such a solid-state imaging device.  
         [0003]     In  FIG. 9 , numeral  41  denotes a housing, which is made of a base  41   a  and frame-shaped ribs  41   b  formed in one piece by resin molding. An internal space  42  is formed on the upper side of the housing  41 . A die pad  43  positioned at the center of the base  41   a  and leads  44  positioned below the ribs  41   b  are embedded in the housing  41 . An imaging element chip  45  disposed at the center of the internal space  42  is fixed to the upper side of the die pad  43 . The leads  44  include internal terminal portions  44   a  that are exposed to the internal space  42  at the upper side of the base  41   a  on the inner side of the ribs  41   b  and external terminal portions  44   b  that are exposed at the bottom side of the base  41   a  below the ribs  41   b.  The internal terminal portions  44   a  and the bonding pads of the imaging element chip  45  are connected by bonding wires  46  made of metal. A transparent sealing glass plate  47  is fixed to the upper surface of the ribs  41   b,  thus forming a package for protection of the imaging element chip  45 .  
         [0004]     This solid-state imaging device is mounted on a circuit board with the sealing glass plate  47  facing upward, as shown in  FIG. 9 , and the external terminal portions  44   b  are used to connect it to the electrodes on the circuit board. Although not shown in the drawings, a lens barrel incorporating an imaging optical system is mounted on top of the sealing glass plate  47  so that a relative position with respect to the photo-detecting region formed in the imaging element chip  45  is adjusted with a predetermined precision. During the imaging operation, object light that has passed through the imaging optical system incorporated in the lens barrel is focused on the photo-detecting region and photoelectrically converted.  
         [0005]     Since a solid-state imaging device with such a configuration is connected at the external terminal portions  44   b  exposed from the bottom surface of the housing to electrodes on the circuit board, the height and the occupied surface area of the package are smaller than in configurations using a connecting structure with outer leads bent downward from the sides of the housing, thus making it suitable for high-density packaging.  
         [0006]     In the configuration of the above-described conventional solid-state imaging device, it is necessary that the base  41   a  of the housing  41  has a sufficient degree of flatness. When, as conventionally, the imaging element chip  45  is fixed to the surface of the base  41   a  with adhesive, and the adhesive hardens, a force is caused that tends to made the imaging element chip  45  follow the surface of the base  41   a.  Thus, if the degree of flatness of the base  41   a  is not favorable, warping and internal stress are generated in the imaging element chip  45 , and the electrical properties and the like of the imaging element chip  45  are adversely affected.  
         [0007]     However, when the base  41   a  is molded, it is difficult to avoid a certain degree of twisting or warping of the cross-directional shape, and the degree of flatness is less than desired.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a solid-state imaging device in which, even if the imaging element is fixed to a base with unfavorable flatness, generation of distortions caused by the imaging element following the surface of the base can be avoided, and the effect on electrical properties of the imaging element caused by the distortions can be suppressed.  
         [0009]     The solid-state imaging device of the present invention comprises a base, frame-shaped ribs provided on the base and forming an internal space, a plurality of wiring members for electrically leading the internal space of a housing formed by the base and the ribs to an external portion, an imaging element fixed to the base inside the internal space, a transparent plate fixed to an upper surface of the ribs, and connecting members electrically connecting electrodes of the imaging element to the wiring members. A plurality of protrusions are provided in a region of the base that faces the imaging element, and the imaging element is fixed by adhesive to the base while being supported by the protrusions. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross-sectional view showing the configuration of a solid-state imaging device in accordance with an embodiment of the present invention.  
         [0011]      FIG. 2  is a bottom view of the solid-state imaging device of  FIG. 1 .  
         [0012]      FIG. 3  is a lateral view of the solid-state imaging device of  FIG. 1 .  
         [0013]      FIG. 4  is a top view of the solid-state imaging device in  FIG. 1  without a transparent plate.  
         [0014]      FIGS. 5A  to  5 F are cross-sectional views illustrating a method for manufacturing the solid-state imaging device in  FIG. 1 .  
         [0015]      FIG. 6  is a top view of the lead frame which is used in this manufacturing method.  
         [0016]      FIG. 7  is a top view of the molded resin product with the lead frame embedded in this manufacturing method.  
         [0017]      FIGS. 8A  to  8 C are a cross-sectional view showing the resin molding step of this manufacturing method in more detail.  
         [0018]      FIG. 9  is a cross-sectional view of a conventional solid-state imaging device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     In the configuration of the solid-state imaging device according to the present invention, a plurality of protrusions are provided in a region in which an imaging element is mounted to the upper side of base which forms a housing, and the imaging element is fixed by an adhesive to the base while being supported by the protrusions. Consequently, the effect of the imaging element following the surface of the base is suppressed. Thus, even if the imaging element is fixed to a surface of a base which does not have a favorable degree of flatness, adverse effects on electrical properties caused by distortions is lessened.  
         [0020]     In the configuration described above, it is preferable that the number of protrusions is not less than 3 and not greater than 5. Further, it is also preferable that the protrusions are substantially hemispherical. The configuration of the present invention is particularly effective with a configuration in which the base and the ribs are molded as one piece with a resin with the wire members embedded.  
         [0021]     An embodiment of a solid-state imaging device according to the present invention is explained in further detail below with reference to the drawings.  FIG. 1  is a cross-sectional view,  FIG. 2  is a bottom view, and  FIG. 3  is a lateral view of the solid-state imaging device.  
         [0022]     Numeral  1  denotes a housing made of a plastic resin, such as epoxy resin, having ribs  3  arranged as a rectangular frame on a planar base  2  and fabricated by molding in one piece. An imaging element  5  is fixed by an adhesive  6  on a base  2  facing an internal space  4  of the housing  1 . A transparent plate  7  made of glass, for example, is fixed with an adhesive  8  to the upper surface of the ribs  3 , thus sealing the internal space  4  of the housing  1  and forming a package.  
         [0023]     A plurality of hemispherical protrusions  2   a  are provided inside the region at the upper surface of the base  2  that faces the imaging element  5 . Thus, the imaging element  5  is fixed by the adhesive  6  to the base  2  while being supported by the hemispherical protrusions  2   a.  Because of this, the imaging element  5  is substantially supported at points, and does not follow the surface of the base  2 . As a result, the generation of distortions of the imaging element  5  caused by an unevenness of the base is suppressed. In practice, it is preferable that the number of hemispherical protrusions  2   a  is not less than 3 and not more than 5. It is preferable that the shape of the protrusions is substantially hemispherical, but other shapes are also applicable.  
         [0024]     A plurality of metal lead pieces  9  are embedded in the housing  1  by insert molding. The metal lead pieces  9  are members for providing electrical leads from the internal space  4  of the housing  1  to the outside, and include an internal terminal portion  9   a  exposed at the surface of the base  2  on the side of the internal space  4 , an external terminal portion  9   b  exposed at a position corresponding to the internal terminal portion  9   a  at the rear surface of the base  2 , and a lateral electrode portion  9   c  exposed at the outer lateral surface of the base  2 . The pad electrodes  5   a  of the imaging element  5  and the internal terminal portions  9   a  of the metal lead pieces  9  are connected by thin metal wires  10 . The thickness of the overall package is set to not more than 2.0 mm, for example.  FIG. 4  is a top view of the planar shape of the solid-state imaging device in  FIG. 1  without the transparent plate  7 .  
         [0025]     As shown in  FIG. 1 , the rear side of the metal lead pieces  9  functions as the external terminal portions  9   b,  which correspond to the positions of the internal terminal portions  9   a.  Furthermore, at these portions the metal lead pieces  9  have substantially the same thickness as the base  2 . Those portions of the metal lead pieces  9  that are located below the ribs  3  are formed thin by half-etching, and their bottom surface is covered by resin.  
         [0026]     As shown in  FIGS. 1 and 3 , the outer lateral surface of the housing  1 , that is to say the outer peripheral surface of the ribs  3 , forms a planar surface that is substantially perpendicular to the surface of the base  2 . Moreover, the end surface of the transparent plate  7  and the surface of the lateral electrode portions  9   c  are substantially flush with the outer lateral surface of the housing  1 . Such a flush shape can be formed with a favorable flatness by cutting the ribs  3  and the transparent plate  7  together during the manufacturing process.  
         [0027]     A method for manufacturing a solid-state imaging devices configured as above is explained with reference to  FIGS. 5A  to  5 F,  FIG. 6  and  FIG. 7 .  
         [0028]     First, as shown in  FIG. 5A , a lead frame  21  is prepared. The lead frame  21  is composed of a plurality of lead portions  22  to be used for forming the metal lead pieces  9  shown in  FIG. 1 , which are linked as shown in the top view in  FIG. 6 . The thickness of the lead portions  22  at the positions corresponding to the internal terminals  9   a  is adjusted to be substantially the same as the thickness of the base  2 . The lead portions  22  have recessed portions  23  formed in their bottom surface by half-etching, and the shape of the metal lead pieces  9  shown in  FIG. 1  is attained by cutting at this portion in a later process step.  
         [0029]     Next, the lead frame  21  is embedded, as shown in  FIG. 5B , and molded in one piece with a resin, thus fabricating together a plurality of housings  26  made of a base  24  and rib forming members  25 . A plurality (the drawing shows  4  protrusions inside each housing) of the hemispherical protrusions  24   a  are formed on the upper surface of the base  24 .  FIG. 7  shows the planar shape after molding. The embedding is performed such that the upper and lower surfaces of the lead portions  22  are exposed at the upper and lower surface of the bases  24 , forming the internal terminal portions  9   a  and the external terminal portions  9   b,  respectively. The rib forming members  25  are formed such that those for adjacent housings  26  are joined into one piece.  
         [0030]     Next, as shown in  FIG. 5C , the adhesive  6  is applied to the region of the housings  26  where the hemispherical protrusions  24   a  are formed and the imaging elements  5  are to be mounted. Thus, the imaging elements  5  are mounted such that they are supported on the hemispherical protrusions  24   a  and fixed by the adhesive  26 . Furthermore, the pad electrodes  5   a  of the imaging element  5  and the internal terminal electrodes  9   a  are connected by the thin metal wires  10 .  
         [0031]     Next, as shown in  FIG. 5D , an adhesive  28  is applied to the upper surface of the rib forming members  25 , and a transparent plate  27  is placed thereon and fixed.  
         [0032]     Then, as shown in  FIG. 5E , the transparent plate  27 , the rib forming members  25 , the lead portions  22  and the base  24  are cut with a dicing blade  29 , and separated into pieces forming individual solid-state imaging devices, as shown in  FIG. 5F . As shown in  FIG. 5E , the cutting is performed in a direction perpendicular to the base  24 , such that the width of the rib forming members  25  in the planar shape is cut in half. As a result, the transparent plate  27 , rib forming members  25 , lead portions  22  and bases  24 , which are thus cut, form a transparent plate  7 , metal lead pieces  9  and housing  1  made of a base  2  and ribs  3  for constituting one solid-state imaging device. Moreover, the lateral electrode portions  9   c  of the metal lead pieces  9  are exposed.  
         [0033]     According to this manufacturing method, the width of the one rib forming member  25  in which the two ribs for adjacent housing-equivalent portions  26  have been formed as one piece can be set to a smaller width than when each of the rib forming members  25  is molded individually. Consequently, when this one rib forming member  25  is cut in half as shown in  FIG. 5E , the width of the ribs  3  of each individual solid-state imaging device as shown in  FIG. 5F  becomes smaller than when each of the rib forming members  25  is formed individually, and the surface area of the solid-state imaging device can be reduced accordingly.  
         [0034]     Moreover, when cutting the rib forming members  25  into two pieces in the width direction, the cutting plane is perpendicular to the base  24 , and the transparent plate  27 , the rib forming members  25  and the lead portions  22  are cut together with the same dicing blade  29 . Therefore the lateral face of the package formed by the end face of the transparent plate  27 , the lateral face of the housing  1  and the end face of the metal lead pieces  9 , is substantially flush, so that a favorable flatness can be achieved. Consequently, when mounting a lens barrel accommodating the optical system above the imaging device, positioning of the optical system with respect to the photo-detecting portion of the imaging element  5  can be performed with high precision utilizing the lateral face of the package.  
         [0035]     Referring to  FIG. 8 , the following is a more specific explanation of the process step for molding the housing with a resin, as shown in  FIG. 5B  of the above-described manufacturing process.  
         [0036]     First, as shown in  FIG. 8A , a lead frame is arranged between the upper die  30  and the lower die  31 , and the upper and lower surfaces of the lead portions  22  are clamped by the upper die  30  and the lower die  31 . The upper surface of the lower die  31  is flat, but a recessed portion  32  is formed at the lower surface of the upper die  30 . By interposing the lead portions  22 , the empty portion  33  formed between the upper die  30  and the lower die  31 , the empty portion of the recessed portion  32  of the upper die  30 , and the empty portion of the recessed portion  23  of the lead portions  22  form the cavities for the resin molding. Hemispherical indentations  30   a  are formed on the surface of the upper mold facing the empty portion  33  for molding the base  24 , at a position (see  FIG. 5B ) corresponding to the hemispherical protrusions  24   a  on the base  24 .  
         [0037]     Next, as shown in  FIG. 8B , a resin is filled into the cavity, and the base  24  and the rib forming members  25  are molded. Hemispherical protrusions  24   a  are formed at the upper surface of the base  24 . After that, the dies are opened as shown in  FIG. 8C , and a molded product of linked housings as shown in  FIG. 5B  is retrieved.  
         [0038]     According to this molding step, it is possible to form easily the hemispherical protrusions  24   a  without the addition of steps other than usual molding for forming the housing, and with only minor alterations to the upper die  30 .  
         [0039]     It should be noted that in this molding step, the upper and lower surfaces of the lead portions  22  are clamped by the upper die  30  and the lower die  31 , ensuring that the die surfaces and the upper and lower surfaces of the lead portions  22  are consistently in close contact. Moreover, the border between the upper die  30  and the recessed portion  32  is located on the upper surface of the lead portions  22 . As a result, the creation of resin burrs caused by the molding can be effectively suppressed.  
         [0040]     Moreover, if a resin sheet for the prevention of resin flash burrs can be arranged between the dies and the lead frame  21  when resin molding the housing, then the creation of burrs can be suppressed even more effectively,  
         [0041]     The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.