Patent Publication Number: US-2015060930-A1

Title: Light Emitting Device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No.2013-180680, filed on Aug. 30, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a light emitting device. 
     BACKGROUND 
     For example, there is a light emitting device (Chip On Board) in which a semiconductor light emitting element is mounted on a substrate and which is sealed by resin. In such a light emitting device, it is preferable that reliability be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  to  FIG. 1C  are schematic views illustrating a light emitting device and a lighting device according to a first embodiment; 
         FIG. 2A  and  FIG. 2B  are schematic cross-sectional views illustrating the light emitting device according to the first embodiment; 
         FIG. 3A  and  FIG. 3B  are schematic cross-sectional views illustrating a light emitting device according to a second embodiment; 
         FIG. 4  is a schematic cross-sectional view illustrating the light emitting device according to the second embodiment; and 
         FIG. 5A  and  FIG. 5B  are schematic cross-sectional views illustrating a light emitting device according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments will be described hereinafter with reference to the accompanying drawings. 
     Moreover, the drawings are schematic or conceptual and a relationship between a thickness and a width of each portion, a ratio of a size between portions, or the like is not necessarily limited to the same as that in reality. Further, even if the same portions are indicated in drawings, dimensions or ratios thereof may be indicated differently from each other depending on the drawings. 
     Moreover, in the specification and each view of the application, the same reference numerals are given to similar elements described already with respect to the foregoing drawings and detailed description is appropriately omitted. 
     First Embodiment 
       FIG. 1A  to  FIG. 1C  are schematic views illustrating a light emitting device and a lighting device according to a first embodiment. 
       FIG. 1A  is a plan view.  FIG. 1B  is a cross-sectional view illustrating a part of a cross-section along line A 1 -A 2  of  FIG. 1A . 
     As shown in  FIG. 1A  and  FIG. 1B , a light emitting device  110  according to the embodiment includes a base member  71 , a grease layer  53 , a heat radiation plate  51 , a bonding layer  52 , a mounting substrate section  15  and a plurality of semiconductor light emitting elements  20 . The light emitting device  110  is, for example, utilized in a lighting device  210 . 
     A direction from the base member  71  to the mounting substrate section  15  is referred to as a laminating direction (a Z-axis direction). One direction perpendicular to the Z-axis direction is referred to as an X-axis direction. A direction perpendicular to the Z-axis direction and the X-axis direction is referred to as a Y-axis direction. 
     The grease layer  53 , the heat radiation plate  51 , the bonding layer  52 , the mounting substrate section  15  and the plurality of semiconductor light emitting elements  20  are disposed on the base member  71  in this order. 
     That is, the plurality of semiconductor light emitting elements  20  are separated from the base member  71  in the Z-axis direction. The mounting substrate section  15  includes a ceramic substrate  10 . The ceramic substrate  10  has an upper surface  10   ue . For the ceramic substrate  10 , for example, a member formed of ceramic, a composite ceramic of ceramic and resin or the like is used. For the ceramic, for example, aluminum oxide (Al 2 O 3 ), aluminum nitride (AIN), beryllium oxide (BeO), steatite (MgO.SiO 2 ), zircon (ZrSiO 4 ), silicon nitride (Si 3 N 4 ), or the like is used. The ceramic substrate  10  is provided between the base member  71  and the plurality of semiconductor light emitting elements  20 . The heat radiation plate  51  is provided between the base member  71  and the mounting substrate section  15 . 
     As illustrated in  FIG. 1B , the bonding layer  52  is provided between the mounting substrate section  15  and the heat radiation plate  51 . The bonding layer  52  bonds the mounting substrate section  15  to the heat radiation plate  51 . 
     The grease layer  53  is provided between the base member  71  and the heat radiation plate  51 . The grease layer  53  transmits heat of the heat radiation plate  51  to the base member  71 . 
     Hereinafter, an example of the light emitting device  110  (and the lighting device  210 ) shown in  FIG. 1A  and  FIG. 1B  is described. 
     A light emitting section  40  is provided in the light emitting device  110 . The light emitting section  40  is provided on the heat radiation plate  51 . The bonding layer  52  is provided between the heat radiation plate  51  and the light emitting section  40 . 
     In the specification of the application, a state which is provided above also includes a state in which another element is inserted therebetween, in addition to the state which is provided above directly. 
     A direction from the heat radiation plate  51  to the light emitting section  40  corresponds to the laminating direction. In the specification of the application, a state of being laminated also includes a state in which another element is inserted and overlapped in addition to a state of being overlapped directly. 
     The heat radiation plate  51  is, for example, a plate shape. A main surface of the heat radiation plate  51  is, for example, substantially parallel to an X-Y plane. A planar shape of the heat radiation plate  51  is, for example, rectangular. The heat radiation plate  51  has, for example, first to fourth sides  55   a  to  55   d.  The second side  55   b  is separated from the first side  55   a . The third side  55   c  connects an end of the first side  55   a  and an end of the second side  55   b.  The fourth side  55   d  is separated from the third side  55   c  and connects the other end of the first side  55   a  and the other end of the second side  55   b.  A plate-shaped corner section of the heat radiation plate  51  may be a curved shape. The plate shape of the heat radiation plate  51  may not be rectangular and is optional. 
     For the heat radiation plate  51 , for example, a substrate made of a metal material of copper, aluminum or the like, a composite material of metal and ceramic, or the like is used. Another metal layer such as Ni plating may be formed on a surface of the heat radiation plate  51 , from the viewpoint of preventing oxidation of the member and improving wettability of a solder. 
     The light emitting section  40  emits light. At the same time, the light emitting section  40  generates heat. The bonding layer  52  efficiently conducts the heat generated in the light emitting section  40  to the heat radiation plate  51 . For the bonding layer  52 , for example, the solder or the like is used. That is, the bonding layer  52  includes the solder. For example, for the bonding layer  52 , the solder including at least one kind or more of gold, silver, copper, bismuth, nickel, indium, zinc, antimony, germanium and silicon in a base of tin can be used. For example, SnAgCu alloy or the like is used. 
     The light emitting section  40  includes the mounting substrate section  15  and a light emitting element section  35 . 
     The mounting substrate section  15  includes the ceramic substrate  10 , a first metal layer  11  and a second metal layer  12 . 
     The ceramic substrate  10  has a first main surface  10   a  and a second main surface  10   b.  The second main surface  10   b  is a surface on the side opposite the first main surface  10   a.  The heat radiation plate  51  faces the second main surface of the ceramic substrate  10 . In other words, the second main surface  10   b  is a surface on the side of the heat radiation plate  51 . That is, the second main surface  10   b  is a surface on the side of the bonding layer  52 . 
     In the specification of the application, a facing state also includes a state where another element is inserted therebetween, in addition to a state of facing directly. 
     The first main surface  10   a  includes a mounting region  16 . For example, the mounting region  16  is separated from an outer edge  10   r  of the first main surface  10   a.  In the example, the mounting region  16  is provided in a center portion of the first main surface  10   a.  The first main surface  10   a  further includes a peripheral region  17 . The peripheral region  17  is provided around the mounting region  16 . 
     The ceramic substrate  10  includes, for example, alumina. For the ceramic substrate  10 , for example, a ceramic mainly composed of alumina is used. High thermal conductivity and a high insulating property can be obtained. High reliability can be obtained. 
     The first metal layer  11  is provided on the first main surface  10   a.  The first metal layer  11  includes a plurality of mounting patterns  11   p.  The plurality of mounting patterns  11   p  are provided in the mounting region  16 . At least two or more of the plurality of mounting patterns  11   p  are separated from each other. For example, at least one of the plurality of mounting patterns  11   p  is an island shape. Two of the plurality of mounting patterns  11   p  are independent to each other. The plurality of mounting patterns  11   p  include, for example, a first mounting pattern  11   pa  and a second mounting pattern  11   pb  or the like. 
     Each of the plurality of mounting patterns  11   p  includes, for example, a first mounting portion  11   a  and a second mounting portion  11   b.  In the example, the mounting pattern  11   p  further includes a third mounting portion  11   c.  The third mounting portion  11   c  is provided between the first mounting portion  11   a  and the second mounting portion  11   b,  and connects the first mounting portion  11   a  and the second mounting portion  11   b . Examples of the mounting portions are described below. 
     The first metal layer  11  may further include a connection section  44  connecting the plurality of mounting patterns  11   p  to each other. In the example, the first metal layer  11  further includes a first connector electrode section  45   e  and a second connector electrode section  46   e.  The first connector electrode section  45   e  is electrically connected to one of the plurality of mounting patterns  11   p.  The second connector electrode section  46   e  is electrically connected to another which is different from the one of the plurality of mounting patterns  11   p.  For example, the semiconductor light emitting element  20  is disposed on a part of one mounting pattern  11   p.  The first connector electrode section  45   e  is electrically connected to one of the mounting patterns  11   p  by the semiconductor light emitting element  20 . Further, the semiconductor light emitting element  20  is disposed on a part of another mounting pattern  11   p.  The second connector electrode section  46   e  is electrically connected to another mounting pattern  11   p  by the semiconductor light emitting element  20 . 
     In the example, the light emitting section  40  further includes a first connector  45  and a second connector  46  provided on the first main surface  10   a.  The first connector  45  is electrically connected to the first connector electrode section  45   e.  The second connector  46  is electrically connected to the second connector electrode section  46   e.  In the example, the first connector  45  is provided on the first connector electrode section  45   e.  The second connector  46  is provided on the second connector electrode section  46   e.  The light emitting element section  35  is disposed between the first connector  45  and the second connector  46 . Power is supplied to the light emitting section  40  through the connectors. 
     The second metal layer  12  is provided on the second main surface  10   b.  The second metal layer  12  is electrically insulated from the first metal layer  11 . At least of a part of the second metal layer  12  overlaps the mounting region  16  when projecting in the X-Y plane (a first plane parallel to the first main surface  10   a ). 
       FIG. 1C  is a perspective plan view illustrating a part of the light emitting device  110 . 
     The second metal layer  12  is separated from the outer edge  10   r.  A planar shape of the second metal layer  12  is, for example, rectangular. The second metal layer  12  has first to fourth sides  12   i  to  12   l . The second side  12   j  is separated from the first side  12   i.  The third side  12   k  connects an end of the first side  12   i  and an end of the second side  12   j.  The fourth side  12   l  is separated from the third side  12   k  and connects the other end of the first side  12   i  and the other end of the second side  12   j.  An intersecting point of each side, that is, a corner section may be a curved shape (a rounded shape). The planar shape of the second metal layer  12  may not be rectangular and is optional. 
     As described above, the first metal layer  11  is provided on the upper surface (the first main surface  10   a ) of the ceramic substrate  10  and the second metal layer  12  is provided on a lower surface (the second main surface  10   b ) of the ceramic substrate  10 . 
     The light emitting element section  35  is provided on the first main surface  10   a  of the ceramic substrate  10 . The light emitting element section  35  includes the plurality of semiconductor light emitting elements  20  and a wavelength conversion layer  31 . 
     In the example, the plurality of semiconductor light emitting elements  20  are disposed in an array shape. The semiconductor light emitting elements  20  are, for example, disposed in a substantially circular shape. For example, the semiconductor light emitting elements  20  are disposed in a substantially equal pitch. 
     The plurality of semiconductor light emitting elements  20  are provided on the first main surface  10   a.  Each of the plurality of semiconductor light emitting elements  20  emits the light. For example, the semiconductor light emitting element  20  includes a nitride semiconductor. The semiconductor light emitting element  20  includes, for example, In y Al z Ga 1-x-y N (0≦x≦1, 0≦y≦1, x+y≦1). However, in the embodiment, the semiconductor light emitting element  20  is not limited to this. 
     The plurality of semiconductor light emitting elements  20  include, for example, a first semiconductor light emitting element  20   a,  a second semiconductor light emitting element  20   b , or the like. 
     Each of the plurality of semiconductor light emitting elements  20  is electrically connected to one mounting pattern  11   p  in the plurality of mounting patterns  11   p  and to another which is adjacent to the one mounting pattern  11   p  in the plurality of mounting patterns  11   p.    
     For example, the first semiconductor light emitting element  20   a  is electrically connected to the first mounting pattern  11   pa  and the second mounting pattern  11   pb  in the plurality of mounting patterns  11   p.  The second mounting pattern  11   pb  is equivalent to another mounting pattern  11   p  which is adjacent to the first mounting pattern  11   pa.    
     For example, each of the plurality of semiconductor light emitting elements  20  includes a first semiconductor layer  21  of a first conductive type, a second semiconductor layer  22  of a second conductive type and a light emitting layer  23 . For example, the first conductive type is an n-type and the second conductive type is a p-type. The first conductive type may be the p-type and the second conductive type may be the n-type. 
     The first semiconductor layer  21  includes a first portion (a first semiconductor portion  21   a ) and a second portion (a second semiconductor portion  21   b ). The second semiconductor portion  21   b  lines up with the first semiconductor portion  21   a  in a direction (for example, the X-axis direction) intersecting the laminating direction (the Z-axis direction that faces from the heat radiation plate  51  to the light emitting section  40 ). 
     The second semiconductor layer  22  is provided between the second semiconductor portion  21   b  and the mounting substrate section  15 . The light emitting layer  23  is provided between the second semiconductor portion  21   b  and the second semiconductor layer  22 . 
     The semiconductor light emitting element  20  is, for example, a flip-chip type LED. 
     For example, the first semiconductor portion  21   a  of the first semiconductor layer  21  faces the first mounting portion  11   a  of the mounting pattern  11   p.  The second semiconductor layer  22  faces the second mounting portion  11   b  of the mounting pattern  11   p.  The first semiconductor portion  21   a  of the first semiconductor layer  21  is electrically connected to the mounting pattern  11   p.  The second semiconductor layer  22  is electrically connected to another mounting pattern  11   p.  For the connection, for example, the solder, or a gold bump having high electric conductivity and thermal conductivity, or the like is used. The connection is, for example, performed by a metal melting solder bonding. Otherwise, for example, the connection is performed by an ultrasonic thermo-compression bonding method using the gold bump. 
     That is, for example, the light emitting element section  35  further includes a first bonding metal member  21   e  and a second bonding metal member  22   e.  The first bonding metal member  21   e  is provided between the first semiconductor portion  21   a  and one mounting pattern  11   p  (for example, the first mounting portion  11   a ). The second bonding metal member  22   e  is provided between the second semiconductor layer  22  and another mounting pattern  11   p  (for example, the second mounting pattern  11   pb ). At least one of the first bonding metal member  21   e  and the second bonding metal member  22   e  includes the solder or the gold bump. Therefore, each cross-sectional area (a cross-sectional area when cutting in the X-Y plane) of the first bonding metal member  21   e  and the second bonding metal member  22   e  can be increased. Therefore, heat can be efficiently transmitted to the mounting substrate section  15  and heat radiation is improved through the first bonding metal member  21   e  and the second bonding metal member  22   e.    
     For example, another metal layer may be provided between the semiconductor light emitting element  20  and the mounting substrate section  15 . Therefore, oxidation of the first metal layer can be suppressed or wettability of the solder can be enhanced. The metal layer is not electrically connected to the semiconductor light emitting element  20  and the mounting pattern  11   p.  The metal layer is not related to a circuit. 
     The wavelength conversion layer  31  covers at least a part of the plurality of semiconductor light emitting elements  20 . The wavelength conversion layer  31  absorbs at least a part of the light (for example, a first light) emitted from the plurality of semiconductor light emitting elements  20 , and emits a second light. A wavelength (for example, a peak wavelength) of the second light is different from a wavelength (for example, a peak wavelength) of the first light. For example, the wavelength conversion layer  31  includes a plurality of wavelength conversion particles such as fluorescent body and a light-transmitting resin in which the plurality of wavelength conversion particles are dispersed. The first light includes, for example, blue light. The second light includes light of which the wavelength is longer than that of the first light. For example, the second light includes at least one of yellow light and red light. 
     In the example, the light emitting element section  35  further includes a reflecting layer  32 . The reflecting layer  32  surrounds the wavelength conversion layer  31  inside the X-Y plane. The reflecting layer  32  includes, for example, a plurality of particles such as a metal oxide and a light transmitting resin in which the particles are dispersed. The particles such as the metal oxide have light reflective properties. For the metal oxide or the like, for example, at least one of TiO 2  and Al 2 O 3  can be used. The light emitted from the semiconductor light emitting element  20  can be efficiently emitted along a direction (for example, an upward direction) along the laminating direction by providing the reflecting layer  32 . 
     The light emitting section  40  is, for example, a chip-on board (COB) type LED module. 
     In the embodiment, a luminous emittance of light emitted from the light emitting element section  35  (the plurality of semiconductor light emitting elements  20 ) is 10 lm/mm 2  (lumens/square millimeter) or more and 100 lm/mm 2  or less. Preferably, the luminous emittance is 20 lm/mm 2  or more. That is, in the embodiment, a ratio (the luminous emittance) for the light emitted from the light emitting element section  35  with respect to a light-emitting area is very high. In the specification of the application, the light-emitting area substantially corresponds to an area of the mounting region  16 . 
     For example, the light emitting device  110  according to the embodiment is used in the lighting device  210  such as a projector. 
     For the grease layer  53 , lubricant (grease) of liquid or solid, or the like is used. For the grease layer  53 , for example, lubricant (insulating grease) having an insulating property, lubricant (conductive grease) having conductivity or the like is also used. The insulating grease includes, for example, silicone and ceramic particles which are dispersed in the silicone. The conductive grease includes, for example, silicone and metal particles which are dispersed in the silicone. In the conductive grease, for example, the thermal conductivity that is higher than that of the insulating grease is obtained. For example, heat of the light emitting element section  35  is transmitted and radiated to the base member  71  by the grease layer  53 . 
     In the light emitting device  110  according to the embodiment, for example, the heat radiation plate  51  has an area of 5 times or more an area of the mounting region  16  when the heat radiation plate  51  is projected in the X-Y plane. That is, in the embodiment, the area of the heat radiation plate  51  is set to be a lot greater than that of the mounting region  16 . Therefore, the heat generated in the light emitting element section  35  provided on the mounting region  16  spreads in an in-plane direction (an in-plane direction of the X-Y plane) by the heat radiation plate  51  having a large area. Then, the heat spread in the in-plane direction is transmitted, for example, toward the base member  71  and is efficiently radiated. 
       FIG. 2A  and  FIG. 2B  are schematic views illustrating the light emitting device  110  according to the first embodiment.  FIG. 2A  is a schematic cross-sectional view illustrating the vicinity of a side surface of the second metal layer  12  of the light emitting device  110  according to the first embodiment. 
       FIG. 2A  illustrates the ceramic substrate  10  provided on the heat radiation plate  51 . 
     As illustrated in  FIG. 2A , the second metal layer  12  is provided on the lower surface (the second main surface  10   b ) of the ceramic substrate  10 . The second metal layer  12  comes into contact with the ceramic substrate  10 . The bonding layer  52  is provided between the ceramic substrate  10  and the heat radiation plate  51 . The bonding layer  52  bonds the second metal layer  12  and the heat radiation plate  51 . 
     The second metal layer  12  includes a first bonding surface  12   a,  a second bonding surface  12   b  (a first surface) and a third bonding surface  12   c.  The first bonding surface  12   a  comes into contact with the lower surface (the second main surface  10   b ) of the ceramic substrate  10 . In order to improve adhesion of the first bonding surface  12   a  and the second main surface  10   b,  a seed layer may be provided therebetween. The second bonding surface  12   b  is a surface on the side opposite the first bonding surface  12   a.  The heat radiation plate  51  faces the second bonding surface  12   b.  That is, the second bonding surface  12   b  is a surface on the side of the heat radiation plate  51 . The third bonding surface  12   c  intersects a surface (for example, the X-Y plane) perpendicular to a direction (for example, the Z-axis direction) facing from the first bonding surface  12   a  to the second bonding surface  12   b.  For example, the third bonding surface  12   c  is the side surface of the second metal layer  12 . 
     As shown in  FIG. 2A , in the example, the bonding layer  52  comes into contact with the second bonding surface  12   b  of the second metal layer  12 . The bonding layer  52  comes into contact with the third bonding surface  12   c  of the second metal layer  12 . The bonding layer  52  covers the second bonding surface  12   b  and the third bonding surface  12   c    
     The bonding layer  52  covers the lower surface (the second bonding surface  12   b ) and the side surface (the third bonding surface  12   c ) of the second metal layer  12 . The bonding layer  52  bonds the heat radiation plate  51  and the second metal layer  12 . As illustrated in  FIG. 2A , a part of the bonding layer  52  comes into contact with the second main surface  10   b  of the ceramic substrate  10 . 
     For example, a thickness t12 of the second metal layer  12  is not less than 20 μm and not more than 300 μm. For example, in a cross-section (the Z-Y plane) perpendicular to the X-axis, a contact distance L52 between the bonding layer  52  and the second main surface  10   b  is, for example, not less than 3 μm and not more than 100 μm. 
       FIG. 2B  is a schematic cross-sectional view illustrating the vicinity of the side surface of the second metal layer  12  of another light emitting device according to the first embodiment. 
     As shown in  FIG. 2B , the bonding layer  52  may be provided so as to come into contact with a part of the side surface of the second metal layer  12 . In the example, the bonding layer  52  covers the lower surface of the second metal layer  12 . 
     The semiconductor light emitting element  20  emits the light by electrification. For example, if the semiconductor light emitting element  20  is electrified, a part of power which is consumed turns into heat. If power consumed by the semiconductor light emitting element  20  is great, the semiconductor light emitting element  20  may excessively generate heat. A light emitting efficiency of the semiconductor light emitting element  20  may be decreased by the heat generated when the semiconductor light emitting element  20  emits the light. For example, if the light emitting device using the semiconductor light emitting element  20  is mounted on the lighting device, the mounting method is devised. Therefore, heat radiation is performed. 
     In the light emitting device  110  according to the embodiment, the bonding layer  52  is provided between the lower surface (the second bonding surface  12   b ) of the second metal layer  12  and the heat radiation plate  51 . Therefore, for example, the second metal layer  12  is electrically connected to the heat radiation plate  51 . Therefore, the heat generated when the semiconductor light emitting element  20  emits the light is radiated. For the bonding layer  52 , for example, the solder is used. 
     The solder may include, for example, a metal filler having high thermal conductivity. The metal filler is, for example, Ni, Cu, Al, Zn, Ag, Mg, W, Ir, Be or Mo. The solder including the metal filler may be partially used. For example, the solder including the metal filler may be used in the bonding layer  52  which is positioned below the semiconductor light emitting element  20 . 
     As a bonding method of the heat radiation plate  51  and the second metal layer  12 , for example, there is a method where grease is provided between the second metal layer  12  and the heat radiation plate  51 . Grease has, for example, fluidity. When the light emitting device is lighted on and off repeatedly, the member expands and contracts repeatedly and grease may be discharged (pumped out) to the outside by the heat generated due to the light emission. If operating environments of the light emitting device are changed, pump-out occurs and reliability may decrease. In the light emitting device  110  according to the embodiment, grease is not used between the heat radiation plate  51  and the second metal layer  12 . Therefore, phenomenon such as pump-out does not occur. 
     For example, a thermal expansion coefficient of the ceramic substrate  10  and a thermal expansion coefficient of the heat radiation plate  51  are different from each other. Therefore, cracks occur in the bonding layer  52  and reliability may be decreased. The cracks occur, for example, from an end of the bonding surface of the bonding layer  52  and the second metal layer  12  and expand in the bonding layer  52 . In the light emitting device  110  according to the embodiment, the lower surface (the second bonding surface  12   b ) and the side surface (the third bonding surface  12   c ) of the second metal layer  12  are covered by the bonding layer  52 . Therefore, for example, the expansion of the cracks can be delayed. Further, if a corner section of the second metal layer  12  is a rounded shape, it is further effective against the cracks. 
     At least a part of an entire region of the side surface of the second metal layer  12  in the light emitting device  110  may be a state as illustrated in  FIG. 2A . However, in order to further improve the heat radiation properties, it is preferable that the portion be the most (80% or more of the entire region) or the entire region. 
     According to the embodiment, the expansion of the cracks is suppressed in the bonding layer  52  and the light emitting device having high reliability is provided. 
     Second Embodiment 
       FIG. 3A  and  FIG. 3B  are schematic cross-sectional views illustrating a light emitting device  110   b  according to a second embodiment. 
     As shown in  FIG. 3A , the bonding layer  52  includes a first bonding metal film  52   a,  a second bonding metal film  52   b  and a third bonding metal film  52   c.  The second bonding metal film  52   b  is provided between the first bonding metal film  52   a  and the second metal layer  12 . The third bonding metal film  52   c  is provided between the first bonding metal film  52   a  and the second bonding metal film  52   b.    
     A thickness t52 b of the second bonding metal film  52   b  is, for example, not less than 3 μm and not more than 100 μm. For the second metal layer  12 , for example, Cu is used. For the bonding layer  52 , for example, the solder is used. The first bonding metal film  52   a  includes, for example, Sn. The second bonding metal film  52   b  is formed of, for example, electroless plating and includes Ni, Pd, Au or the like. After the second bonding metal film  52   b  is formed by the electroless plating, the first bonding metal film  52   a  is formed by the solder and the third bonding metal film  52   c  is formed when the second metal layer  12  and the heat radiation plate  51  are bonded. The second bonding metal film  52   b  is, for example, a film including Ni, Pd and Au. The third bonding metal film  52   c  includes a compound of elements included in the first bonding metal film  52   a  and the second bonding metal film  52   b,  for example, a compound of Ni and Sn. 
     Therefore, reaction of Sn included in the first bonding metal film  52   a  and Cu included in the second metal layer  12  is suppressed. For example, growth of an intermetallic compound of CuSn is suppressed. Growth of the intermetallic compound of Cu and Sn is fast. The intermetallic compound is weak. If the intermetallic compound grows, breakdown is likely to occur and the cracks may occur. According to the light emitting device  110   b  of the embodiment, formation of the intermetallic compound of Cu and Sn is suppressed and the light emitting device having high reliability is provided by providing the second bonding metal film  52   b  having main elements that are less reactive compared to Cu. 
     The first bonding metal film  52   a  is, for example, the solder including Sn. The second bonding metal film  52   b  is a plating layer including Ni. The third bonding metal film  52   c  includes, for example, a NiSn alloy. For example, NiSn has slower growing speed compared to CuSn. That is, since a weak region can be reduced in a compound layer formed of a NiSn alloy layer having Sn compared to a CuSn alloy layer, the cracks are unlikely to occur. For example, a thickness t52c of the third bonding metal film  52   c  is 1 μm to 100 μm. 
       FIG. 3B  is a schematic cross-sectional view of the light emitting device  110   b  in another Z-Y plane. In the example, the third bonding metal film  52   c  includes a NiSn alloy section. For example, after a plating layer including Ni is formed as the second bonding metal film  52   b,  the third bonding metal film  52   c  is formed when the first bonding metal film  52   a  is formed by the solder. 
     As shown in  FIG. 3B , in the bonding layer  52  coming into contact with the third bonding surface  12   c,  the third bonding metal film  52   c  is formed in a direction from the second bonding metal film  52   b  to the first bonding metal film  52   a.  That is, in the example, the NiSn alloy section extends in a direction intersecting the side surface (the third bonding surface  12   c ) of the second metal layer  12 . Therefore, for example, the expansion of the cracks can be suppressed. 
     For example, a thickness t52cv of the third bonding metal film in the side surface of the second metal layer  12  is thicker than a thickness t52ch of the third bonding metal film in the side surface of the second metal layer  12 . 
     For example, the thickness t52ch of the third bonding metal film in the lower surface of the second metal layer  12  is not less than 1 μm and not more than 100 μm. The thickness t52cv of the third bonding metal film in the side surface of the second metal layer  12  is equal to or thicker than the thickness of the lower surface of the second metal layer  12 . 
     According to the embodiment, the expansion of the cracks is suppressed in the bonding layer  52  and the light emitting device having high reliability is provided. 
       FIG. 4  is a schematic cross-sectional view illustrating the light emitting device  110   b  according to the second embodiment. 
     As shown in  FIG. 4 , the light emitting device  110   b  according to the embodiment includes the first metal layer  11 . The first metal layer  11  is provided between the ceramic substrate  10  and the semiconductor light emitting element  20 . The first metal layer  11  includes a copper layer  13   a  and a plating layer  13   e.  At least a part of the plating layer  13   e  is provided between the copper layer  13   a  and the semiconductor light emitting element  20 . The plating layer  13   e  is also formed on a side surface of the copper layer  13   a.  The plating layer  13   e  is formed of, for example, the electroless plating, similar to the second bonding metal film  52   b.  The plating layer  13   e  includes, for example, Ni and Au. Ni suppresses generation of a CuSn alloy. Au suppresses absorption of the light from the semiconductor light emitting element  20  by suppressing oxidation of Ni. Since the metal layer including Ni and Au is also formed in the side surface in addition to the mounting surface of the copper layer  13   a  of the semiconductor light emitting element  20 , the reliability of connection of the solder can be increased while increasing the light emitting efficiency. The thickness of the Au layer, that is, an amount which is used is reduced and then costs thereof can be reduced by further containing Pd in the plating layer  13   e.    
     The plating layer  13   e  formed in the copper layer  13   a  and the second bonding metal film  52   b  formed in the second metal layer  12  are preferably formed in the same material and the same configuration. That is, the metal layers including Ni and Au are preferably formed on the surface and the side surface of both the copper layer  13   a  and the second metal layer  12 . Thus, structures (manufacturing methods) and materials thereof can be made common. Therefore, the costs can be further reduced. 
     Third Embodiment 
       FIG. 5A  and  FIG. 5B  are schematic cross-sectional views illustrating a light emitting device according to a third embodiment. 
     As shown in  FIG. 5A , the thickness of the second metal layer  12  of a light emitting device  110   c  according to the embodiment may not be uniform. In the example, a thickness t12c of the center section of the second metal layer  12  is thicker than a thickness t12e of the outer periphery section of the second metal layer  12 . Therefore, for example, the heat radiation properties of the center section can be improved and the light emitting device having high reliability is provided. 
     As shown in  FIG. 5B , the thickness of the heat radiation plate  51  of a light emitting device  110   d  may not be uniform. In the example, a thickness t15c of the center section of the heat radiation plate  51  is thicker than a thickness t15e of the outer periphery section of the heat radiation plate  51 . Therefore, for example, the heat radiation properties of the center section can be improved and the light emitting device having high reliability is provided. 
     According to the embodiment, the light emitting device having high reliability is provided. 
     Moreover, in the specification of the application, terms of “perpendicular” and “parallel” are not only strictly perpendicular and strictly parallel but also are intended to include variation thereof or the like, for example, in a manufacturing process, and may be substantially perpendicular and substantially parallel. 
     Above, the embodiments are described with reference to specific examples. However, the exemplary embodiment is not limited to the specific examples. For example, specific configurations of each element such as the semiconductor light emitting element, the mounting substrate section, the heat radiation plate, the ceramic substrate, the first metal layer, the second metal layer, and the bonding layer are included within the scope of the exemplary embodiment as long as the configurations can be executed similar to the exemplary embodiment and the same effects can be obtained by those skilled in the art. 
     Further, it is included in the scope of the exemplary embodiment that two or more elements of each specific example are combined in a technically possible range as long as they are included within the gist of the exemplary embodiment. 
     In addition, in the scope of the spirit of the exemplary embodiment, those skilled in the art can conceive various modification examples and alteration examples, and it is understood that the modification examples and the alteration examples belong within the scope of the exemplary embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.