Patent Publication Number: US-2005141396-A1

Title: Package for light emitting element and process for fabricating same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a package for mounting a light emitting element thereon, a light emitting device including the package, and a process for fabricating the package.  
      2. Description of Related Art  
      A package for a light emitting element heretofore comprises a base substrate  7  and a frame body  71  each made of ceramic and which are integrally bonded together, as shown in  FIG. 10 . A cavity for housing the light emitting element  2  therein is formed inside the frame body  71 . The frame body  71  has an inner peripheral surface formed with a metallic layer  50  defining an entire periphery of the cavity. Furthermore, the base substrate  7  has an upper surface formed with a pair of land layers  37 ,  47  to be connected, respectively, to a pair of terminals (a power supply terminal and a grounding terminal) of the light emitting element  2 . The land layers  37 ,  47  are connected, respectively, to a pair of external electrodes  30 ,  40  arranged on a rear surface of the base substrate  7 , by via holes  38 ,  48  (JP-A No. 232017/2002).  
      The light emitting element  2  is mounted on a surface of one land layer  47  of the pair of land layers. One terminal of the pair of terminals is connected to the land layer  47  while the other terminal is connected to the other land layer  37  via a wire  21 .  
      With a light emitting device as described, the light emitting element  2  emanates light in all directions. The light emanating forward (upward direction in  FIG. 10 ) keeps traveling forward as it is while the light emanating sideways is reflected off a surface of the metallic layer  50  to change its traveling direction thereby traveling forward. Further, the land layers  37 ,  47  each serves a function as a reflecting layer, and reflects forward the light emanating from the light emitting element  2 .  
      The light emitting device shown in  FIG. 10  has, however, a gap G for electrical insulation and provided between the land layers  37 ,  47  or between one of the land layers  37 ,  47  and the metallic layer  50 , so that the light emanating from the light emitting element  2  is passed through the gap G to leak into the inside of the base substrate  7 . This entails the problem of a low light emitting efficiency.  
      We have developed a light emitting device comprising a metallic layer  51  defining a cavity  80  of a package  8 , a pair of land layers  81 ,  82  exposed on a bottom surface of the cavity  80 , which land layers  81 ,  82  are further enlarged to surroundings thereof, to provide the light emitting device having no gap between one of the land layers  81 ,  82  and the metallic layer  51  on the bottom surface of the cavity  80 , as shown in  FIG. 11  and  FIG. 12 . The light emitting device has the pair of land layers  81 ,  82  connected to a pair of external electrodes  84 ,  86  by via holes  83 ,  85 , respectively.  
      In the case where the pair of land layers  81 ,  82  are in contact with the metallic layer  51 , a short circuit is provided between the pair of land layers  81 ,  82 . The metallic layer  51  is therefore formed in depth dimension such that its lower end does not reach the land layers  81 ,  82 .  
      With the light emitting device described, almost all the light emanating from the light emitting element  2  to the bottom surface of the cavity is reflected off the surfaces of the land layer  81 ,  82  to travel forward. This makes the light emitting device higher in light emitting efficiency than conventionally.  
      However, the metallic layer  51  and the land layers  81 ,  82  are away from each other, as described above, so that all of the light emanating sideways from the light emitting element  2  cannot be reflected by the metallic layer  51 . Furthermore, the gap between the land layers  81 ,  82  is indispensable, so that the light emanating from the light emitting element  2  to the gap area cannot be reflected forward. This entails the problem of still failing to obtain a sufficiently high light emitting efficiency.  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to provide a package for a light emitting element for obtaining a sufficiently higher light emitting efficiency than conventionally, and a process for fabricating the same.  
      The present invention provides a package for a light emitting element comprising a base substrate  11  made of insulation material, a frame body  14  made of insulation material and mounted on an upper surface of the base substrate  11 . A cavity  10  for housing the light emitting element therein is formed inside the frame body  14 . The frame body  14  has an inner peripheral surface formed with a reflecting layer  5  defining an entire periphery of the cavity  10 . The base substrate  11  has an upper surface formed with a pair of land layers  31 ,  41 , each to be connected to a pair of terminals of the light emitting element. The land layers  31 ,  41  have surfaces exposed on a bottom surface of the cavity  10 , and are connected to a pair of external electrodes  3 ,  4  arranged on the base substrate  11 , respectively.  
      Furthermore, one land layer  41  of the pair of land layers  31 ,  41  formed on the upper surface of the base substrate  11  extends along the upper surface of the base substrate  11 , and has an outer peripheral portion connected to a lower end portion of the reflecting layer  5  thereof. The other land layer  31  comprises an exposure portion  33  exposed on the upper surface of the base substrate  11 , a buried portion  34  extending from the exposure portion  33  to its surrounding and buried inside the base substrate  11 . The buried portion  34  has an outer peripheral portion extending to a position below a lower end portion of the reflecting layer  5  or extending outwardly beyond the position thereof. The base substrate  11  and the frame body  14  are each made of ceramic or synthetic resin.  
      With the light emitting device having the light emitting element mounted on the package for the light emitting element, the light emanating sideways from the light emitting element  2  is reflected off a surface of the reflecting layer  5  to travel forward. Incidentally, the reflecting layer  5  is formed on an entire periphery defining the cavity  10  of the package  1 , and is formed to a depth dimension wherein a lower end portion of the reflecting layer  5  is in contact with the bottom surface of the cavity  10 , so that all the light emanating toward an inner wall of the cavity  10  is reflected off thereof.  
      The light emanating from the light emitting element  2  to the bottom surface of the cavity  10  is reflected off the surfaces of the land layers  31 ,  41  to travel forward. In this case, the one land layer  41  extends along the upper surface of the base substrate  11 , and has an outer peripheral portion connected to a lower end portion of the reflecting layer  5  thereof, so that there is no gap between the land layer  41  and the reflecting layer  5  and no light therefore passes through the gap.  
      The other land layer  31  has the buried portion  34  extending from the exposure portion  33  to its surrounding and buried inside the base substrate  11 . The buried portion  34  has an outer peripheral portion extending to a position below a lower end portion of the reflecting layer  5  or extending outwardly beyond the position thereof. Therefore the light incident on an area between the exposure portion  33  of the land layer  31  and an end portion of the reflecting layer  5  is reflected forward by the buried portion  34 .  
      Incidentally interposed between the land layer  31  and the lower end portion of the reflecting layer  5  is an insulation layer formed by a surface layer portion of the base substrate  11 , so that the land layer  31  is electrically insulated from the reflecting layer  5  to prevent the occurrence of short circuit between the land layers  31 ,  41 .  
      The package for the light emitting element embodying the present invention has no gap between the reflecting layer and each of the land layers for passing the light therethrough, to thereby obtain a sufficiently higher light emitting efficiency than conventionally.  
      The present invention provides a process for fabricating the package for the light emitting element, which process has the steps of: 
          preparing one or more layers of first ceramic green sheets  15 ,  16  to serve as the base substrate  11 ,     preparing one or more layers of second ceramic green sheets  17  to serve as the frame body  14 , and     bonding the second ceramic green sheet  17  to an upper surface of the first ceramic green sheets  15 ,  16  to fire the ceramic green sheets.        

      The step of preparing the first ceramic green sheets  15 ,  16  comprises: 
          a first step of forming a pair of metallic layers  36 ,  46  on an upper surface of a ceramic green sheet  16   a  to serve as the pair of land layers,     a second step of forming an insulation layer  9  covering a part of the upper surface of the ceramic green sheet  16   a , including an area of one metallic layer  36  of the pair of metallic layers  36 ,  46  and bonded to the second ceramic green sheet, and     a third step of compressing into a thickness direction the ceramic green sheet  16   a  formed with the insulation layer  9  to flatten the upper surface of the ceramic green sheet  16   a.          

      In the step of preparing the first ceramic green sheets  15 ,  16  of the process for fabricating the package for the light emitting element of the present invention, the ceramic green sheet  16   a  is compressed into a thickness direction in the third step to flatten the upper surface of the sheet  16   a , to thereby pressing into the surface of the green sheet  16   a  the metallic layers  36 ,  46  formed in the first step and the insulation layer  9  formed in the second step. In this case, the area of the metallic layer  36  covered with the insulation layer  9  is pressed by the insulation layer  9 , is thereby pressed more deeply by thickness of the insulation layer  9  than the area not covered with the insulation layer  9 , and is buried inside the green sheet  16   a . The second ceramic green sheet  17  is bonded to the upper surface of the first ceramic green sheets  15 ,  16  thus obtained to fire these ceramic green sheets, whereby the package for the light emitting element of the present invention is completed.  
      According to the process described, the package for the light emitting element embodying the present invention can be fabricated with the simple steps without a great change to the conventional production steps.  
      The present invention provides another package for a light emitting element comprising a base substrate  11  made of insulation material, a frame body  14  made of insulation material and mounted on an upper surface of the base substrate  11 . A cavity  10  for housing the light emitting element therein is formed inside the frame body  14 . The frame body  14  has an inner peripheral surface formed with a first reflecting layer  5  defining an entire periphery of the cavity  10 . The base substrate  11  has an upper surface formed with a pair of land layers  31 ,  41 , each to be connected to a pair of terminals of the light emitting element and which layers are away from each other. The land layers  31 ,  41  are connected to a pair of external electrodes  3 ,  4  arranged on the base substrate  11 , respectively. The base substrate  11  has a second reflecting layer  6  formed below an area which is not formed with the land layers  31 ,  41 , of areas at least exposed on the bottom surface of the cavity  10 . The base substrate  11  and the frame body  14  are each made of ceramic or synthetic resin.  
      With the light emitting device having a light emitting element mounted on a package for the light emitting element, the light emanating from the light emitting element  2  to a gap area between the land layers  31 ,  41  passes through the surface layer portion of the base substrate  11 , and is, however, reflected off a surface of the second reflecting layer  6  to travel forward. Incidentally the second reflecting layer  6  extends to the area which is not formed with the land layers  31 ,  41 , of the areas at least exposed on the bottom surface of the cavity  10 , so that all the light incident on the gap area between the land layers  31 ,  41  is reflected off without leakage. Further, the second reflecting layer  6  is away from the land layers  31 ,  41 . This prevents the occurrence of short circuit between the land layers  31 ,  41 .  
      With the package for the light emitting element embodying the present invention, all the light emanating from the light emitting element  2  to the bottom surface of the cavity  10  of the package  1  is reflected off, whereby a sufficiently higher light emitting efficiency than conventionally can be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a light emitting device embodying the present invention;  
       FIG. 2  is a view in cross section of the light emitting device;  
       FIG. 3  is an enlarged view in cross section of an A portion in  FIG. 2 ;  
       FIG. 4  is an enlarged view in cross section on a line B-B in  FIG. 2 ;  
       FIG. 5  is a perspective view of a package;  
       FIG. 6  is a plan view of the package;  
       FIG. 7  is an exploded perspective view of the package;  
       FIG. 8 ( a ) to  FIG. 8 ( c ) are diagrams showing a process for fabricating the package;  
       FIG. 9 ( a ) to  FIG. 9 ( c ) are diagrams showing a process for fabricating a green sheet to serve as an upper layer portion of a base substrate;  
       FIG. 10  is a sectional view of the conventional light emitting device;  
       FIG. 11  is a sectional view of another conventional light emitting device;  
       FIG. 12  is a plan view of a package of the light emitting device. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      With reference to the drawings, the embodiment of the present invention will be specifically described below.  
      A light emitting device embodying the present invention comprises a ceramic package  1 , and a light emitting element  2  including LED and mounted on the package  1 , as seen in  FIG. 1 . The package  1  has a cavity  10  for housing the light emitting element  2  therein. The cavity  10  has a bottom surface formed with a first land layer  31  and a second land layer  41 , which are made of conductive material, such as silver, and which are separated from each other. A power supply terminal (not shown) provided on an upper surface of the light emitting element  2  is connected to the first land layer  31  by a wire  21 . A grounding terminal (not shown) provided on a rear surface of the light emitting element  2  is directly connected to the second land layer  41 . Furthermore, the package  1  has a pair of external electrodes  3 ,  4  arranged thereon. Referring to  FIG. 2 , the aforementioned first and the second land layers  31 ,  41  are connected to the pair of external electrodes  3 ,  4  by via holes  32 ,  42 , respectively.  
      With the light emitting device shown in  FIG. 1 , the external electrodes  3 ,  4  are mounted, as laid horizontally and facing downward, on a surface of a circuit board. The external electrodes  3 ,  4  are soldered to a pair of lands on the circuit board. Supplying power from the circuit board to the light emitting element  2  operates the light emitting element  2 , to emanate light in all directions.  
       FIG. 7  illustrates a laminate structure of the package  1 . As illustrated, a frame body  14  provided with a central hole  10   a  to serve as the cavity  10  is mounted on an upper surface of a base substrate  11  in the form of a flat plate and including a lower layer portion  12  and an upper layer portion  13 , to provide an integrated package  1  shown in  FIG. 5  and  FIG. 6 . Incidentally, for convenience sake,  FIG. 6  and  FIG. 7  show also by hatching areas in which the metallic layer such as land layers  31 ,  41  and the external electrodes  3 ,  4 , etc., are exposed.  
      With reference to  FIG. 7 , the frame body  14  constituting the package  1  has a first reflecting layer  5  made of metal like silver, etc., and formed on an inner peripheral surface of the central hole  10   a . The lower layer portion  12  of the base substrate  11  has the pair of external electrodes  3 ,  4  arranged on the side surface thereof. Formed on the upper surface of the lower layer portion  12  are electrode layers  39 ,  49  connected to the external electrodes  3 ,  4 , respectively. Furthermore, formed between the electrode layers  39 ,  49  is a second reflecting layer  6  made of metal like silver, etc.  
      Furthermore, the upper layer portion  13  of the base substrate  11  has an upper surface formed with the aforementioned first land layer  31  and the second land layer  41 , and also has provided with via holes  32 ,  42  for connecting to each other the land layers  31 ,  41  and the electrode layers  39 ,  49  on the upper surface of the lower layer portion  12 . The first land layer  31  formed on the upper layer portion  13  of the base substrate  11  comprises an exposure portion  33  which is exposed on the upper surface of the upper layer portion  13 , and a buried portion  34  extended from the exposure portion  33  to its surrounding and buried inside the upper layer portion  13 . On the other hand, the entire second land layer  41  formed on the upper layer portion  13  extends along an upper surface of the upper layer portion  13 . Formed between the first land layer  31  and the second land layer  41  is a gap required for insulation between the two land layers  31 ,  41 .  
      With reference to  FIG. 5  and  FIG. 6 , the exposure portion  33  of the first land layer  31  is exposed on a bottom surface of the cavity  10  of the package  1 , and is away from the first reflecting layer  5  and the second land layer  41  in its surrounding. The buried portion  34  of the first land layer  31  extends outwardly from a lower end portion of the first reflecting layer  5 . On the other hand, the second land layer  41  has an outer periphery connected to a lower end portion of the first reflecting layer  5 .  
       FIG. 3  shows an enlarged view of an A portion in  FIG. 2 .  FIG. 4  shows a sectional view on a line B-B in  FIG. 2 . As shown in  FIG. 3  and  FIG. 4 , the buried portion  34  of the first land layer  31  extends outwardly from a lower end portion of the first reflecting layer  5  with a slight gap S (10 to 30 μm) thereof from the lower end portion of the first reflecting layer  5 .  
      The second reflecting layer  6  is formed with an area involving a clearance portion formed between the two land layers  31 ,  41 , as shown in  FIG. 6 , and is buried inside the base substrate  11  as shown in  FIG. 2 . An insulation layer formed by a surface layer portion of the base substrate  11  is interposed between the second reflecting layer  6  and each of the land layers  31 ,  41 .  
       FIG. 8 ( a ) to  FIG. 8 ( c ) illustrate production process of the package  1 . First prepared are a ceramic green sheet  17  having a through-hole  17   a  to serve as the frame body  14  as shown in  FIG. 8 ( a ), a ceramic green sheet  16  including the first and the second land layers  31 ,  41  to serve as an upper layer portion  13  of the base substrate  11  as shown in  FIG. 8 ( b ), and a ceramic green sheet  15  including the external electrodes  3 ,  4  and the second reflecting layer  6  to serve as a lower layer portion  12  of the base substrate  11 , respectively. Subsequently the three green sheets  15 ,  16 ,  17  are bonded together and are fired, whereby an integrally formed package  1  is obtained shown in  FIG. 5  and  FIG. 6 .  
       FIG. 9 ( a ) to  FIG. 9 ( c ) illustrate production process of the ceramic green sheet  16  shown in  FIG. 8 ( b ). Referring to  FIG. 9 ( a ), a pair of metallic layers  36 ,  46  are formed on an upper surface of a ceramic green sheet  16   a  formed with via holes  35 ,  45 . Subsequently an insulation layer  9  made of overcoat glass is formed to cover an end portion of one metallic layer  36  therewith as shown in  FIG. 9 ( b ), and the ceramic green sheet  16  is thereafter compressed in a depth direction, with the result that the metallic layers  36 ,  46  and the insulation layer  9  are pressed into a surface of the ceramic green sheet  16   a  to flatten the surface of the green sheet  16   a  as shown in  FIG. 9 ( c ). Incidentally, by being pressed by the insulation layer  9 , the area in the metallic layer  36  and covered with the insulation layer  9  is pressed more deeply by thickness of the insulation layer  9  than the area not covered with the insulation layer  9 , and is thereby buried inside the ceramic green sheet  16   a , to obtain the ceramic green sheet  16  shown in  FIG. 8 ( b ). In this case the insulation layer  9  becomes a part of the surface layer portion of the base substrate  11  by firing.  
      A light emitting element  2  is mounted on the package  1  prepared according to the production steps described as shown in  FIG. 1  and  FIG. 2 , whereby the light emitting device of the present invention is completed.  
      With the light emitting device, among the light emanating from the light emitting element  2  in all directions, the light incident on the surface of the reflecting layer  5  is reflected off the surface to travel forward (an upward direction in  FIG. 1  and  FIG. 2 ). In this case, the reflecting layer  5  has a depth dimension extending to the bottom surface of the cavity  10 , so that all the light emanating sideways from the light emitting element  2  is reflected forward.  
      Further with the light emitting device, among the light emanating from the light emitting element  2  to the bottom surface of the cavity  10 , the light incident on the surfaces of the exposure portion  33  of the first land layer  31  and of the second land layer  41  is reflected off the surfaces to travel forward. Furthermore, the light incident on an area between the lower end portion of the reflecting layer  5  and the exposure portion  33  of the first land layer  31  passes through the surface layer portion of the base substrate  11 , and is, however, reflected off the surface of the buried portion  34  of the first land layer  31  to travel forward.  
      Furthermore the light incident on a gap area between the first land layer  31  and the second land layer  41  passes through the upper layer portion  13  of the base substrate  11 , and is, however, reflected off the surface of the second reflecting layer  6  to travel forward.  
      With the light emitting device of the present invention, all the light incident from the light emitting element  2  travels forward as described above, to obtain a sufficiently higher light emitting efficiency than conventionally.  
      Further, the process for producing the package for the light emitting element embodying the present invention makes it possible to produce the package for the light emitting element of the invention with ease merely by improving a part of the production steps of the green sheets as shown in  FIG. 9 ( a ) to  FIG. 9 ( c ).  
      The device of the invention is not limited to the foregoing embodiments in construction but can be modified variously by one skilled in the art without departing from the spirit of the invention as set forth in the appended claims. For example, the package  1  is not limitedly made of ceramic, but can be made of synthetic resin.