Abstract:
The system and apparatus of the present invention are directed to a camera module in which an operational defect (generation of a ghost image) as a result of a reduction in thickness is eliminated. The camera module includes a substrate provided with a through-hole for light transmission, a light receiving portion provided on a first surface of an imaging element. The imaging element is flip chip mounted on a first side of the substrate such that the light receiving portion is exposed through the through-hole, and a shielding layer on a back surface of the imaging element wherein the back surface is opposite the first surface having the light receiving portion. A lens unit is mounted a second side of the substrate.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an imaging element and an imaging device using the same and, further, to a camera module provided with an imaging element and a camera system using the same and an optical module. 
   2. Description of the Related Art 
   Recently, a camera module using an imaging element is required to be mounted in a small information terminal, such as a personal computer or a mobile viewphone, as a camera system including a signal processing system. With this requirement, there is a stronger demand for a reduction in the size of a camera module. 
   A conventionally known camera module using an imaging element, such as a CCD imaging element or a CMOS imaging element, uses as a function device an imaging device of a QFP (Quad Flat Package) type which hermetically seals a chip-shaped imaging element in a hollow package. In a well-known camera module of this type, the above-mentioned package is mounted on a substrate such as a printed circuit board, and a lens unit for image formation is mounted on the upper portion of the package. 
   In the case of the camera module constructed as described above, the thickness of the entire module is the sum total of the thickness of the imaging element package, the mounting substrate and the lens unit constituting the same. Thus, to achieve a reduction in the thickness of the camera module, it is necessary to reduce the thickness of each component. 
   At present, however, there is a limitation to a reduction in the thickness of the imaging device package, the mounting substrate and the lens unit. Thus, it is very difficult to achieve a further reduction in the thickness of the camera module. 
   SUMMARY OF THE INVENTION 
   Accordingly, it is an object of the present invention to provide an ultra thin camera module. 
   A camera module according to the present invention comprises a substrate provided with a through-hole for light transmission, a light receiving portion, an imaging element flip chip mounted on one side of the substrate such that the light receiving portion is exposed through the through-hole, and a lens unit mounted on the other side of the substrate so as to cover the space over the light receiving portion of the imaging element. 
   In the camera module construction described above, a ghost image can appear in the image actually obtained through imaging. 
   After carefully examining the cause of the ghost image generation, the present inventor has reached the following conclusion. 
   Usually, the imaging element detects light from the surface of the element where the light receiving portion and the lens are provided and supplies a thereby obtained image signal to a signal processing circuit or the like to cause the image to be displayed on a screen such as display. In the above-described conventional camera module, the imaging device is constructed such that the imaging element is mounted at the bottom of the hollow package, with the light receiving portion facing upward (in a face up state). By contrast, in the camera module construction in which flip chip mounting (bare chip mounting) is effected, the imaging element is mounted such that the light receiving portion is exposed through the through-hole with respect to the substrate provided with the through-hole for light transmission. Thus, in the former case, the back side of the imaging element is shielded from the exterior by the package, whereas in the latter case, the back side of the imaging element is exposed to the exterior. 
   Due to this difference in construction, in the former case, as shown in  FIG. 1A , light impinging from the back surface  31  side is blocked by the package  32 , whereas, in the latter case, as shown in  FIG. 1B , light directly impinges upon the back surface  31  of the imaging element  30 . In this situation, the silicon substrate (silicon wafer or the like) often used as the base substrate of the imaging element  30  transmits a light having an optically large wavelength (large wavelength band from the infrared range of the visible light range). Thus, in the latter case, not only the incident light from the light receiving portion side (front surface side) of the imaging element  30  but also light impinging from the element back surface  31  is transmitted through the interior of the element, and this transmitted light reaches the light receiving portion and is detected, whereby the generation of a ghost image is caused. 
   In view of this, a camera module according to another aspect of the present invention adopts a construction comprising a substrate provided with a through-hole for light transmission, an imaging element having on one side a light receiving portion, flip chip mounted on one side of the substrate with the light receiving portion being exposed through the through-hole, and having a shielding layer on the element back surface on the opposite side of the light receiving portion, and a lens unit mounted on the other surface of the substrate. 
   In the imaging element constructed as described above, due to the provision of the shielding layer on the back surface of the imaging element, when this imaging element is flip chip mounted on the substrate in a bare chip state, the light impinging upon the element back surface is blocked by the shielding layer, whereby the incident light from the element back surface is not transmitted through the interior of the element and sensed by the light receiving portion. Thus, in a camera module constructed by using such an imaging element, it is possible to prevent the generation of a ghost image attributable to the incident light from the element back surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are diagrams illustrating the problem to be solved; 
       FIG. 2  is a side schematic view showing the construction of a camera system according to the present invention; 
       FIGS. 3A and 3B  are diagrams illustrating the construction of a camera module according to an embodiment of the present invention; 
       FIG. 4  is a perspective view showing the substrate structure of a camera module according to an embodiment of the present invention; 
       FIGS. 5A ,  5 B and  5 C are production process diagrams of an imaging element according to an embodiment of the present invention; 
       FIGS. 6A ,  6 B,  6 C and  6 D are production process diagrams of a camera module according to an embodiment of the present invention; 
       FIGS. 7A and 7B  are diagrams illustrating an example of the bump forming method; 
       FIG. 8  is a diagram illustrating the advantage of the present invention; 
       FIGS. 9A and 9B  are diagrams illustrating another application example of the present invention; and 
       FIG. 10  is a diagram illustrating another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of the present invention will now be described with reference to the drawings. 
     FIG. 2  is a side schematic view showing the construction of a camera system according to the present invention. The camera system  1  shown comprises a camera module  2  and a system module  3 . The camera module  2  and the system module  3  are connected by a flexible wiring board  4 . The flexible wiring board  4  is drawn out from the camera module  2  side, and a wiring pattern portion at the draw-out end thereof is electrically connected to a wiring pattern of the system module  3  through a connector  5 . 
   On a wiring board  6  of the system module  3 , there are double-side mounted, together with the connector  5 , various electronic parts  7 A through  7 D and system ICs  8 A through  8 C. The system ICs  8 A through  8 C constitute a drive circuit for driving the camera module  2 , an image processing circuit for performing various image processings (for example, image compression processing) on image signals obtained by the camera module  2 , etc. Further, mounted on the wiring substrate  6  is a USB (Universal Serial Bus) connector  9  for connecting the camera system  1  including the system module  3  to an information terminal such as a personal computer. 
     FIGS. 3A and 3B  are diagrams illustrating the construction of a camera module according to an embodiment of the present invention.  FIG. 3A  is a schematic plan view, and  FIG. 3B  is a side sectional view thereof. The camera module  2  shown comprises a substrate  10 , an imaging element  11  and a lens unit  12 . 
   As shown in  FIG. 4 , the substrate  10  is formed by joining a metal plate  13  and the flexible wiring board  4  to each other by means of an adhesive (not shown) or the like. The metal plate  13  consists of a thin stainless steel plate having a thickness of, for example, approximately 0.5 mm, and a square or rectangular configuration having an outside dimension larger than that of the imaging element  11 . The flexible wiring board  4  consists of a base film formed, for example, of polyester or polyimide and a wiring pattern (not shown) formed of a conductive material such as copper, and has an elongated band-shaped structure having substantially the same width as the metal plate  13 . And, the metal plate  13  is glued to the end portion of the flexible wiring board  4 , and, due to this glued portion, a sufficient strength (rigidity) for the substrate  10  is secured. 
   Further, the substrate  10  is provided with a through-hole  14  for light transmission. This through-hole  14  is provided substantially at the center of the glued portion of the flexible wiring board  4  and the metal plate  13 . Further, the through-hole  14  is formed as a rectangular hole having a size corresponding to the light receiving portion of the imaging element  11  described below. By contrast, the end portion of the wiring pattern of the flexible wiring board  4  is arranged around the through-hole  14  in correspondence with the electrode position of the imaging element  11 . 
   The metal plate  13  serves to mechanically reinforce the mounting portion and secure the positioning accuracy of the lens unit  2  in the optical axis direction when mounting the imaging element  11  and the lens unit  12  on the substrate  10  as described below. Thus, when a sufficient strength (rigidity) can be obtained by increasing the thickness of the flexible wiring board  4 , there is no need to provide the metal plate  13 . Further, as the substrate material, all or a part of the substrate  10  may be formed of a polyimide type organic material, a glass epoxy type organic material, or a ceramic type material. Whichever substrate material may be used, it is necessary to provide wiring pattern for electrical connection with the imaging element  11 . 
   The imaging element consists, for example, of a CCD imaging element, a CMOS imaging element or the like, and has on its main surface a light receiving portion  15  consisting of a large number of reading pixels arranged two-dimensionally. In the periphery of the imaging element  11 , there are formed a plurality of electrode portions (not shown) consisting, for example, of aluminum pads, in such a way as to surround the light receiving portion  15 . This imaging element  11  is mounted (flip chip mounted) on one surface (the lower surface of the flexible wiring board  4 ) of the substrate  10  in a bare chip state through the intermediation of bumps  16 , whereby the electrode portions (not shown) of the imaging element  11  and the wiring pattern of the flexible wiring board  4  are electrically connected through the intermediation of the bumps  16 . Further, in this mounting state, the front surface (main surface) of the imaging element  11  is opposed to the substrate  10  and the light receiving portion  15  of the imaging element  11  is exposed through the through-hole  14  of the substrate  10 . 
   On the other hand, a shielding layer  22  is provided on the surface (hereinafter referred to as the element back surface) of the imaging element  10  which is on the opposite side of the surface (main surface) on which the light receiving portion  15  of the imaging element  11  is formed. This shielding layer  22  consists, for example, of a metal layer such as aluminum, and is formed over the entire back surface of the imaging element  11 . As the method of forming the shielding layer  22  consisting of a metal layer, it is possible to adopt vacuum evaporation, sputtering or the like (described in detail below). 
   Further, to the entire periphery of the imaging element  11 , a sealing resin  17  is applied. This resin  17  serves to enhance the mechanical strength of the electrical connection portion (bump joint portion) of the imaging element  11  and the substrate  10  and to prevent intrusion of dust through the gap therebetween. As the sealing resin  17 , it is desirable to adopt a resin material which involves as little gas generation as possible, for example, an epoxy resin or the like. This is because when gas generated from the sealing resin  17  adheres to the lens, the lens surface is fogged to thereby adversely affect the imaging performance. 
   The lens unit  12  comprises a holder  18 , a lens barrel  19 , an optical filter  20  and a lens  21 . The holder  18  is of a cylindrical structure and the lens barrel  19  is fitted in the inner periphery thereof. In the inner peripheral surface of the holder  18  and the outer peripheral surface of the lens barrel  19 , screw threads are formed as needed. By forming these screw threads and threadedly engaging the holder  18  with the lens barrel  19 , these components can be relatively moved in the central axis direction (optical axis direction) to effect focusing. The forward end portion of the lens barrel  19  is bent at substantially right angles toward the central axis, whereby a diaphragm portion  19 A for restricting the incident light is integrally formed. 
   The optical filter  20  serves as a so-called infrared cutting filter to cut off the infrared portion, for example, of the incident light impinging through the diaphragm  19 A. This optical filter  20  is fixed to a position near the forward end of the lens barrel  19  in close vicinity of the diaphragm portion  19 A. The lens  21  serves to effect image formation at the light receiving portion  15  of the imaging element  11  from the light impinging through the diaphragm portion  19 A and the optical filter  20 . This lens  21  is mounted to the interior of the lens barrel  19  together with the optical filter  20  with positioning being effected using the diaphragm portion  19 A as a reference. 
   The optical filter  20  is not restricted to a infrared cutting filter but it is also possible to use various filters according to the imaging use (for example, an optical band pass filter). Further, it is possible to use a material having an infrared cutting function as the material (glass material) of the lens  21 , or cause such a material to the surface of the lens  21  by coating, evaporation or the like to thereby provide the lens  21  itself with an infrared cutting function. In that case, there is no need to use an infrared cutting filter for the optical filter  20 . Further, it is also possible to form the lens unit  12  without using the holder  18 . 
   The lens unit  12 , constructed as described above, is mounted on the other surface of the substrate  10  (the upper surface of the metal plate  13 ). In this mounting state, the imaging element  11  and the lens unit  12  are mounted on either side of the substrate  10  with the substrate  10  ( 13 ,  4 ) being therebetween. Further, the light receiving portion  15  of the imaging element  11  and the lens  21  of the lens unit  12  are opposed to each other in the same axis (optical axis) through the through-hole  14  of the substrate  10 , and the space above the light receiving portion  15  of the imaging element  11  is covered with the lens unit  12 . 
   In this camera module  2 , the light receiving portion  15  of the imaging element  11  is exposed through the through-hole  14  of the substrate  10 , so that when imaging is actually effected, the light impinging from the diaphragm portion  19 A of the lens unit  12  through the optical filter  20  undergoes image formation at the light receiving portion  15  of the imaging element  11  by the refracting effect of the lens  21 . Further, the image signal obtained through photoelectric conversion at the light receiving portion  15  of the imaging element  11 , where the light is received, is transmitted to the system module  3  (See  FIG. 2 ) through the wiring pattern of the substrate  10  (flexible wiring board  4 ). 
   Next, a camera module producing method according to an embodiment of the present invention will be described. 
   First, in the production process for the imaging element  11 , as shown in  FIG. 5A , an element forming layer  24  including the light receiving portion  15  is formed on a wafer  23  consisting of a silicon substrate or the like, and then, as shown in  FIG. 5B , the back surface of the wafer  23  is ground by a back surface grinder so that the wafer  23  may have a predetermined thickness (for example, 400 μm). 
   Subsequently, aluminum is deposited by vacuum evaporation on the back surface of the wafer  23  ground in the above step, whereby a metal layer  25  consisting of aluminum is formed on the back surface of the wafer  23 . This vacuum evaporation using aluminum as the evaporation material (layer forming material) is conducted, for example, under the following conditions: degree of vacuum: 1 mTorr, evaporation material heating method: crucible heating, substrate (wafer) temperature: 100° C., evaporation material growth layer thickness: 3 μm. After this, dicing of the wafer  23  is effected along a predetermined cutting line, whereby a plurality of imaging elements  11  each having on its back surface a shielding layer  22  (metal layer  25 ) are obtained. 
   While in the above example vacuum evaporation is adopted as the method for forming the metal layer (aluminum layer)  25 , a similar metal layer  25  can also be formed by sputtering. The conditions for sputtering may, for example, be as follows: target material: pure aluminum, base degree of vacuum: 1.0 −8  Torr, degree of vacuum at the time of sputtering: 5 mTorr, gas introduced: argon gas, substrate temperature: 100° C., growth layer thickness: 3 μm. Further, it is also possible to use other metals such as gold, silver, tungsten, and molybdenum as the material for the metal layer  25  constituting the shielding layer  22 . 
   On the other hand, in the process for producing the substrate  10 , as shown in  FIG. 4 , the metal plate  13  and the flexible wiring board  4  are glued to each other by using an adhesive or the like, and then the through-hole  14  is formed, for example, by stamping, substantially at the center of the glued portion. It is also possible to form the through-hole  14  beforehand in both the metal plate  13  and the flexible wiring board  4  before they are glued together. 
   When the imaging element  11  and the substrate  10  have been thus prepared, a bump  16  is formed on each electrode portion of the imaging element  11 , as shown in  FIG. 6A . The bump  16  can be formed, for example, as follows, as shown in  FIG. 7A . A ball is formed at the forward end of a gold wire drawn out from the forward end of a capillary  22 , and this is crimped to the electrode portion (aluminum pad)  11 A of the imaging element  11 . Then, as shown in  FIG. 7B , the gold wire  23  is cut at the ball portion, without drawing the gold wire  23  out of the capillary  22 . This bump forming method is called a ball bump method (or stud bump method). Apart from this, it is also possible to adopt a bump forming method such as electroless plating, transfer bump method or soldering technique. 
   Next, as shown in  FIG. 6B , the imaging element  11  is mounted (flip chip mounted) on one surface of the substrate  10  through the intermediation of the bumps  16 . In this mounting process, the substrate  10  is placed on a base (not shown), and the imaging element  11  held by a bonding tool (not shown). And, in the condition in which positioning is effected between the substrate  10  on the base and the imaging element  11  held by the bonding tool, the bumps  16  formed on the electrode portions of the imaging element  11  are electrically and mechanically connected to the wiring pattern of the substrate  10  (flexible wiring board  4 ) by ultrasonic connection. 
   The alignment of the substrate  10  and the imaging element  11  is conducted under the condition in which in a direction perpendicular to the direction in which pressurizing is effected by the bonding tool (generally speaking, in the horizontal direction), the positions of the through-hole  14  of the substrate  10  and the light receiving portion  15  of the imaging element  11  and the position of the wiring pattern of the substrate  10  and the electrode portions of the imaging element corresponding thereto coincide with each other. The ultrasonic connection is effected, for example, under the following conditions: frequency: 50 KHz, tool temperature 100° C., base temperature: 100° C., connection time: 0.5 sec, tool pressurizing force: 100 g per bump, amplitude 2.5 μm. 
   It is desirable that the heating temperature at the time of ultrasonic connection be set not higher than 170° C. so that a micro lens formed on the main surface (light receiving portion  15 ) of the imaging element  11  may not thermally damaged. In ultrasonic connection, processing can be effected at a temperature of approximately 130° C., so that there is no fear of the micro lens being thermally damaged. However, as the connection method for mounting the imaging element  11  on the substrate  10 , it is also possible to adopt a connection method other than ultrasonic connection as long as low temperature connection satisfying the above temperature condition (not higher than 170° C.) is realized. Specifically, it might possible to adopt connection using silver paste or connection using indium, or a connection method using an anisotropic conductive material. 
   Next, as shown in  FIG. 6C , a sealing resin (underfill material)  17  is applied to the periphery of the imaging element  11  by using a dispenser or the like. At this time, a resin  17  having an appropriate viscosity is used, whereby the resin  17  applied by the dispenser or the like is prevented from flowing to the light receiving portion  15  of the imaging element  11 . After the application of the resin  17 , it is cured by air drying or heat processing. As the material of the resin  17 , a phenol-novolak type epoxy resin, for example, is used. The curing condition in the above heat treatment, is 2 hours at 120° C. 
   Subsequently, as shown in  FIG. 6D , the lens unit  12  which has been assembled beforehand is mounted on the other surface of the substrate  10 . In this mounting process, an epoxy type adhesive (not shown), for example, is applied to the end surface of the holder  18  of the lens unit  12  or the other surface of the substrate  10  corresponding to the mounting position of the lens unit  12 . After this, with the lens unit  12  and the imaging element  11  being aligned, the lens unit  12  is pressed against the other surface of the substrate  10 , whereby the lens unit  12  is fastened to the substrate  10  through the intermediation of the above adhesive. In this way, the camera module  2  shown in  FIGS. 3A and 3B  is obtained. 
   When forming the shielding layer  22  consisting of a metal layer on the back surface of the imaging element  11 , apart from forming it by vacuum evaporation or the like in the production process for the imaging element  11  as described above, it is also possible to form the shielding layer  22  consisting of a metal layer on the back surface of the element by mounting the imaging element  11  obtained by dicing the wafer  23  on the substrate  10  or mounting the lens unit  12  on the substrate  10 , and then applying a metal paste (silver paste or the like) to, for example, the back surface of the imaging element. 
   In the camera module  2  obtained in this way, the imaging element  11  is directly mounted by flip chip mounting to one side of the substrate  10  having the through-hole  14 , and the lens unit  12  is mounted on the opposite side, i.e., to the other side of the substrate  10 , so that, compared to the conventional module construction, it is possible to reduce the package thickness to hermetically seal the imaging element, and it is possible to arrange the substrate  10 , the imaging element  11  and the lens unit  12  more densely in the module thickness direction, whereby it is possible to provide an ultra thin camera module  2 . Further, in the camera system  1  using this camera module  2 , due to the reduction in the thickness of the camera module  2 , it is possible to incorporate it in an information terminal utilizing a smaller mounting space. 
   Further, since the shielding layer  22  is formed on the back surface of the imaging element  11 , even if the mounting is effected such that the back surface of the element is exposed to the exterior, it is possible, as shown in  FIG. 8 , to cut off light impinging from the back side of the imaging element  11  by the shielding layer  22 , whereby the incident light from the back side of the element (long wavelength light or the like) is not allowed to be transmitted through the interior of the imaging element  11  and sensed by the light receiving portion  15 . Thus, even when image taking is actually effected by the camera module  2 , it is possible to prevent the generation of a ghost image or noise attributable to the incident light from the element back side, making it possible to obtain a satisfactory image. 
   Further, in this embodiment, the imaging element  11  is connected to the flexible wiring board  4 , so that it is possible to freely change the orientation of the camera module  2  utilizing the flexibility of the flexible wiring board  4 , whereby, when incorporating the camera module  2  in an information terminal product, it is possible to arbitrarily adjust the mounting angle of the camera module  2 , thereby substantially enhancing the degree of freedom at the time of mounting. 
   Further, in the production of this camera module  2 , there is no need to perform a package process for hermetically sealing the imaging element  11 , so that it is possible to realize a reduction in cost through improvement in productivity. 
   While the above-described embodiment is applied to the camera module  2  comprising the substrate  10 , the imaging element  11  and the lens unit  12 , this should not be construed restrictively. For example, the present invention is also applicable to an imaging device in which, as shown in  FIG. 9A , instead of the lens unit  12 , the through-hole  14  of the substrate  10  is closed by a light transmitting plate-like member  26  such as a seal glass or an optical filter substrate, or an imaging device in which, as shown in  FIG. 9B , the imaging element  11  is flip chip mounted in a bare chip state to a transparent glass substrate (or an optical filter substrate or the like)  27 . 
   Further, while in the above-described embodiment the shielding layer  22  consisting of a metal layer is formed by vacuum evaporation on the back surface of the imaging element, this should not be construed restrictively. Apart from this, for example, it is possible to apply a black resin (epoxy resin, silicone resin or the like) to the back surface of the imaging element  11  to make it black or form a transparent resin layer on the back surface of the element and then apply a material containing black ink to the surface of the resin layer to make it black, thereby forming a resin shielding layer  22  on the back surface of the element. 
   When thus forming the shielding layer  22  using a resin, there is no need to form the shielding layer  22  in the process for producing the imaging element  11 . By applying a black resin (sealing resin)  17  so as to cover the back and side surfaces of the imaging element in the processing step shown in  FIG. 6C , it is possible to form the shielding layer  22  with a part (portion corresponding to the back surface of the element)  17 A of the resin  17 , as shown in  FIG. 10 , and enhance the joint strength between the imaging element  11  and the substrate  10  with the other portion (portion corresponding to the side surface of the element)  17 B to prevent separation, etc. 
   As described above, in accordance with the present invention, the imaging element is constructed such that a shielding layer is provided on the back surface of the element on the opposite side of the light receiving portion, so that when realizing a thin imaging device or camera module using this imaging element, light impinging from the back side of the element is cut off by the shielding layer, and it is possible to prevent the generation of a ghost image of the like attributable to the incident light, whereby it is possible to provide an imaging device or camera module which is very thin and has high imaging performance.