Abstract:
A method for mounting a luminescent device having a mount layer on a substrate, comprising the steps of coating a metallic nano-particle paste on the substrate, disposing the mount layer of the luminescent device on the metallic nano-particle paste, and heating the mount layer and the metallic nano-particle paste to form an alloy, thereby bonding the luminescent device and the substrate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for mounting a luminescent device of mounting a luminescent device on a mounted board by a flip chip method or a face up method. 
     2. Description of the Related Art 
     A light emitting diode chip (which is hereinafter abbreviated as an LED chip) using a gallium nitride compound semiconductor (Al 1-X-Y In X Ga Y N, 0≦X≦1, 0≦Y≦1, 0≦X+Y≦1) capable of emitting blue light with a high light emission intensity is being subjected to practical use. Upon accumulating a gallium nitride compound semiconductor, epitaxial growth on an insulating growth substrate, such as a sapphire substrate, is often employed. Therefore, it is difficult to form an electrode on the side of the sapphire substrate, and both the anode and cathode are formed often on the side of the semiconductor layer. 
     An LED chip having the aforementioned structure is mounted on a mounting substrate mainly in one of the following two methods. 
     One of the methods is a flip chip mounting method, in which an LED chip is mounted on a mounting substrate with the semiconductor layer side thereof having the cathode and anode directed to the mounting substrate. In the mounting method, such a method for mounting an LED chip on a mounting substrate is subjected to practical use that: a solder layer is formed the anode and cathode of the LED chip; a solder paste is coated on the mounting substrate by screen printing; the LED chip is disposed on the mounting substrate with the semiconductor layer side thereof directed to the mounting substrate; and then the assembly is subjected to reflow (see, for example, JP-2007-266396). 
     The other mounting method is a face up mounting method, in which an LED chip is mounted on a mounting substrate with the growth substrate side thereof directed to the mounting substrate. In the mounting method, such a method for mounting an LED chip on a mounting substrate is subjected to practical use that: a metal having good wettability with solder, such as gold, is formed on the back surface of the growth substrate; a solder paste is coated on the mounting substrate by screen printing; the LED chip is disposed on the mounting substrate with the growth substrate side thereof directed to the mounting substrate; and then the assembly is subjected to reflow (see, for example, JP-2005-072148). 
     In the flip chip method and the face up method that use a solder paste, a flux, which is an organic activator for bonding, contained in the solder paste remains between the LED chip and the mounting substrate. The flux deteriorates the reliability of bonding between the LED chip and the mounting substrate, and therefore, the flux is necessarily removed by rinsing, which complicates the mounting process. 
     Furthermore, a solder paste that has been subjected to practical use contains only large microscopic solder particles having an average particle diameter of from 2 to 50 μm. Accordingly, the solder paste fails to pass favorably through a metallic mask upon screen printing, and precise transfer of the solder paste may not be expected. Consequently, the yield may be deteriorated in the flip chip method, in which failures due to short circuit between the anode and cathode are necessarily prevented. 
     SUMMARY OF THE INVENTION 
     The invention has been made in view of the problems associated with the conventional techniques, and an object of the invention is to simplify the mounting process of an LED chip on a mounting substrate, thereby providing a method for mounting a luminescent device, such as an LED chip, on a mounting substrate, capable of enhancing the bonding property between the LED chip and the mounting substrate. 
     In order to solve the above problem, this invention provides with a method for mounting a luminescent device having a mount layer on a substrate, comprising the steps of: coating a metallic nano-particle paste on the substrate; disposing the mount layer of the luminescent device on the metallic nano-particle paste; and heating the mount layer and the metallic nano-particle paste to form an alloy, thereby bonding the luminescent device and the substrate. 
     According to the method for bonding a luminescent device of the invention, the use of the metallic nano-particle paste having a small average particle diameter enables high-precise coating of the metallic nano-particle paste on a substrate. 
     Furthermore, the metallic nano-particles have a high activity upon bonding owing to the large specific surface area thereof. Accordingly, the metallic nano-particles can be bonded at a lower temperature as compared to microscopic particles, thereby bonding the luminescent device and the substrate without the use of a flux, which may remain as a residue. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are each a cross sectional view showing an example of an LED chip used in a method for mounting an LED chip according to the first embodiment of the invention. 
         FIGS. 2A to 2D  are cross sectional views showing an example of a method for mounting an LED chip according to the first embodiment of the invention. 
         FIG. 3  is a cross sectional view showing an example of an LED chip used in a method for mounting an LED chip according to the second embodiment of the invention. 
         FIGS. 4A to 4D  are cross sectional views showing an example of a method for mounting an LED chip according to the second embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments for carrying out the invention will be described below with reference to the attached drawings. 
     First Embodiment 
       FIGS. 1A and 1B  are each a cross sectional view showing an example of an LED chip used in a method for mounting an LED chip according to the first embodiment of the invention. The LED chip used in the embodiment is an LED chip to be mounted on a mounting substrate by the flip chip method. 
     An LED chip  10  shown in  FIG. 1A  will be described. 
     The LED chip  10  shown in  FIG. 1A  contains a growth substrate  11  formed of sapphire, a buffer layer (which is not shown in the figure) provided on the growth substrate  11 , and a semiconductor laminated body formed of a gallium nitride compound semiconductor (Al 1-X-Y In X Ga Y N, 0≦X≦1, 0≦Y≦1, 0≦X+Y≦1) containing an n-type layer  12 , a luminescent layer  13  and a p-type layer  14  laminated in this order from the side of the growth substrate  11 . A p-side ohmic electrode  16   a  having light transmissibility formed of ITO (indium tin oxide) forming ohmic bonding to the p-type layer  14  is provided on the surface of the p-type layer  14 . The prescribed area of the LED chip  10  is subjected to dry etching that exposes from the p-type layer  14  to the n-type layer  12 , and an n-side contact electrode  17   a  formed of V/AI is provided on the exposed bottom surface. 
     An insulating layer  18  having light transmissibility formed of SiO 2  is formed on the exposed surfaces of the semiconductor laminated body, the p-side ohmic electrode  16   a  and the n-side contact electrode  17   a . The insulating layer  18  has openings formed to expose partially the p-side ohmic electrode  16   a  and the n-side contact electrode  17   a , and a p-side barrier electrode  16   b  and an n-side barrier electrode  17   b  each formed of Ti/Ni are provided to be in contact with the p-side ohmic electrode  16   a  and the n-side contact electrode  17   a , respectively, through the openings. A p-side mount electrode  16   c  and an n-side mount electrode  17   c  each formed of AuSn are formed on the surfaces of the p-side barrier electrode  16   b  and the n-side barrier electrode  17   b , respectively. 
     The insulating layer  18  has a reflective layer  19  buried therein excluding the positions of the openings, and light emitted from the luminescent layer  13  passing through the p-side ohmic electrode  16   a  and the insulating layer  18  is reflected by the reflective layer  19  and then radiated outside the LED chip  10 . 
     The layers of the semiconductor laminated body each may be formed, for example, by the metal organic chemical vapor deposition (MOCVD), the molecular beam epitaxy (MBE), the halide vapor phase epitaxy (HVPE) or the like. 
     The growth substrate is not limited to sapphire, and may be spinel, gallium nitride, silicon carbide, gallium oxide or the like instead. 
     The p-side ohmic electrode  16   a , the n-side contact electrode  17   a , the p-side barrier electrode  16   b , the n-side barrier electrode  17   b , the p-side mount electrode  16   c  and the n-side mount electrode  17   c  each may be formed by the sputtering method, the vacuum vapor deposition method or the like. The p-side ohmic electrode  16   a  is not limited to ITO, and may be other transparent electroconductive oxides, such as ICO (indium cerium oxide) and TNO (titanium niobium oxide), or a metallic thin film, such as Co/Au and Ni/Au, instead, and the n-side contact electrode  17   a  is not limited to V/Al, and may be Ti/Al or the like instead. The p-side barrier electrode  16   b  and the n-side barrier electrode  17   b  each are not limited to Ti/Ni, and may be a single substance, such as Ni, Ti and Pt, or a laminated structure containing a combination thereof instead. The p-side mount electrode  16   c  and the n-side mount electrode  17   c  each are not limited to AuSn, and may be other eutectic alloys, such as AuSb and AuGe, instead. 
     The insulating layer  18  may be formed by the plasma CVD method or the like. The insulating layer  18  is not limited to SiO 2 , and may be a transparent layer, such as Si 3 N 4 , TiO 2  and Al 2 O 3 , instead. 
     The reflective layer  19  may be formed by patterning through photolithography into a desired shape in the course of the formation of the insulating layer  18 , and further forming the insulating layer  18 , thereby burying the reflective layer  19  inside the insulating layer  18 . 
     An LED chip  20  shown in  FIG. 1B  will be described. 
     The LED chip  20  shown in  FIG. 1B  contains a growth substrate  21  formed of sapphire, a buffer layer (which is not shown in the figure) provided on the growth substrate  21 , and a semiconductor laminated body formed of a gallium nitride compound semiconductor containing an n-type layer  22 , a luminescent layer  23  and a p-type layer  24  laminated in this order from the side of the growth substrate  21 . A p-side ohmic electrode  26   a  having a high reflectance to visible light formed of Rh forming ohmic bonding to the p-type layer  24  is provided on the surface of the p-type layer  24 . The prescribed area of the LED chip  20  is subjected to dry etching that exposes from the p-type layer  24  to the n-type layer  22 , and an n-side contact electrode  27   a  formed of V/AI is provided on the exposed bottom surface. 
     An insulating layer  28  formed of SiO 2  is formed on the exposed surfaces of the semiconductor laminated body, the p-side ohmic electrode  26   a  and the n-side contact electrode  27   a . The insulating layer  28  has openings formed to expose partially the p-side ohmic electrode  26   a  and the n-side contact electrode  27   a , and a p-side barrier electrode  26   b  and an n-side barrier electrode  27   b  each formed of Ti/Ni are provided to be in contact with the p-side ohmic electrode  26   a  and the n-side contact electrode  27   a , respectively, through the openings. A p-side mount electrode  26   c  and an n-side mount electrode  27   c  each formed of AuSn are formed on the surfaces of the p-side barrier electrode  26   b  and the n-side barrier electrode  27   b , respectively. 
     Light emitted from the luminescent layer  23  is reflected by the p-side ohmic electrode  16   a  and then radiated outside the LED chip  20 . 
     The p-side ohmic electrode  26   a  is not limited to Rh, and may be Ag or an alloy containing Ag as a major component instead. The other elements of the LED chip  20  each may be substituted by the constitutional components described for the LED chip  10  shown in  FIG. 1A . 
     The method for mounting an LED chip on a mounting substrate by the flip chip method will be described with reference to the LED chip  10  shown in  FIG. 1A  as an example. 
     The metallic nano-particle paste used for mounting the LED chip will be described. The nano-particles are formed of a noble metal, such as Au, Ag and Cu, which is chemically stable, and have an average particle diameter (D50) of from 1 to 80 nm, and preferably from 2 to 40 nm. The metallic nano-particles are formed into a paste by mixing with a volatile binder formed of an alcohol, such as tetradecanol. 
     The method for mounting an LED chip using the metallic nano-particle paste will be described.  FIGS. 2A to 2D  are cross sectional views showing an example of the method for mounting an LED chip according to the first embodiment of the invention. 
     As shown in  FIG. 2A , firstly, on a surface of a mounting substrate  60  having a mother material  60   a  formed of ceramics or a glass-epoxy composite having formed thereon a wiring pattern  60   b , a metallic mask  70  having through holes provided at positions where the mount electrodes of the LED chip  10  and the wiring pattern  60   b  are to be connected is disposed. A metallic nano-particle paste  71  containing Ag is disposed on one end of the metallic mask  70 , and a squeegee  72  is moved from the one end of the metallic mask  70  to the other end thereof, thereby screen-printing the metallic nano-particle paste  71 . 
     The LED chip  10  is then disposed in such a manner that the metallic nano-particle paste  71  thus coated as shown in  FIG. 2B  corresponds to the mount electrodes of the LED chip  10 . 
     Thereafter, the assembly is heated to approximately 300° C. in a reflow furnace. Thus, the binder of the metallic nano-particle paste  71  as shown in  FIG. 2C  is evaporated, and AuSn constituting the mount electrodes is melted and formed into an alloy with Ag constituting the metallic nano-particle paste, thereby forming mount parts  75 . According to the procedures, the LED chip  10  and the mounting substrate  60  are connected to each other to complete the mounting operation of the LED chip  10 . 
     In the aforementioned method for mounting the LED chip  10 , the use of the metallic nano-particle paste  71  having a small average particle diameter is enhanced in the passing through the metallic mask  70  upon screen-printing, thereby enabling precise transfer. Accordingly, failures due to short circuit between the anode and cathode are prevented to improve the yield. 
     Furthermore, the metallic nano-particles have a high activity upon bonding owing to the large specific surface area thereof. Accordingly, the metallic nano-particles can bond the mount electrodes of the LED chip  10  and the wiring pattern  60   b  of the mounting substrate  60  without the use of a flux. Consequently, no binder remains as a residue, and high bonding reliability is obtained. 
     Second Embodiment 
       FIG. 3  is a cross sectional view showing an example of an LED chip used in a method for mounting an LED chip according to the second embodiment of the invention. The LED chip used in the embodiment is an LED chip to be mounted on a mounting substrate by the face up method. 
     An LED chip  30  shown in  FIG. 3  will be described. 
     The LED chip  30  shown in  FIG. 3  contains a growth substrate  31  formed of sapphire, a buffer layer (which is not shown in the figure) provided on the surface of the growth substrate  31 , and a semiconductor laminated body formed of a gallium nitride compound semiconductor containing an n-type layer  32 , a luminescent layer  33  and a p-type layer  34  laminated in this order from the side of the growth substrate  31 . A p-side ohmic electrode  36   a  formed of ITO (indium tin oxide) forming ohmic bonding to the p-type layer  34  is provided on the surface of the p-type layer  34 , and a p-side pad electrode  36   b  formed of Ni/Au is provided on a part of the surface of the p-side ohmic electrode  36   a . The prescribed area of the LED chip  30  is subjected to dry etching that exposes from the p-type layer  34  to the n-type layer  32 , and an n-side contact electrode  37  formed of V/Al is provided on the exposed bottom surface. 
     A barrier layer  38  formed of Ti/Ni and a mount layer  39  formed of AuSn are provided on the back surface of the sapphire substrate  31 . 
     The barrier layer  38  is not limited to Ti/Ni, and may be a single substance, such as Ni, Ti and Pt, or a laminated structure containing a combination thereof instead. The mount layer  39  is not limited to AuSn, and may be other eutectic alloys, such as AuSb and AuGe, instead. The other elements of the LED chip  30  each may be substituted by the constitutional components described for the LED chip  10  shown in  FIG. 1A . 
     The method for mounting an LED chip  30  using the metallic nano-particle paste by the face up method will be described.  FIGS. 4A to 4D  are cross sectional views showing an example of the method for mounting an LED chip according to the second embodiment of the invention. 
     As shown in  FIG. 4A , firstly, on a surface of a mounting substrate  80  having a mother material  80   a  formed of ceramics or a glass-epoxy composite having formed thereon a wiring pattern  80   b , a metallic mask  90  having through holes provided at positions where the mount electrodes of the LED chip  30  and the wiring pattern  30   b  are to be connected is disposed. A metallic nano-particle paste  91  containing Ag is disposed on one end of the metallic mask  90 , and a squeegee  92  is moved from the one end of the metallic mask  90  to the other end thereof, thereby screen-printing the metallic nano-particle paste  91 . 
     The LED chip  30  is then disposed in such a manner that the metallic nano-particle paste  91  thus coated as shown in  FIG. 4B  corresponds to the mount layer  39  of the LED chip  30 . 
     Thereafter, the assembly is heated to approximately 300° C. in a reflow furnace. Thus, the binder of the metallic nano-particle paste  91  as shown in  FIG. 4C  is evaporated, and AuSn constituting the mount layer  39  is melted and formed into an alloy with Ag constituting the metallic nano-particle paste, thereby forming mount parts  95 . According to the procedures, the LED chip  30  and the mounting substrate  80  are connected to each other. 
     Finally, the p-side pad electrode  36   b  and the n-side contact electrode  37  of the LED chip  30  are connected to the wiring pattern  80   b  with gold wires, thereby completing the mounting of the LED chip  30  on the mounting substrate  80 . 
     In the aforementioned method for mounting the LED chip  30 , the metallic nano-particles have a high activity upon bonding owing to the large specific surface area thereof as similar to the first embodiment, and thus the mount layer  39  of the LED chip  30  can be bonded to the wiring pattern  80   b  of the mounting substrate  80  without the use of a flux. Consequently, no binder remains as a residue, and high bonding reliability is obtained. 
     Other Embodiments 
     In the aforementioned embodiments, LED chips formed of a gallium nitride compound semiconductor are described as examples, but the invention may be applied to LED chips formed of other compound semiconductors, such as an indium phosphorus compound semiconductor (Al 1-X-Y Ga X In Y P, 0≦X≦1, 0≦Y≦1, 0≦X+Y≦1) and the like. In the aforementioned embodiments, LED chips are described as examples, but the invention may be applied to other luminescent devices, such as a laser diode and a light emitting thyristor. In the aforementioned embodiments, screen printing is employed as a method of coating the metallic nano-particle paste on the mounting substrate, but other coating methods, such as dispensing and stamping, may be employed. 
     The method for mounting a luminescent device according to the invention provides high reliability upon mounting on a mounting substrate, and thus is favorably employed for an LED lamp having an LED chip and COB (chip-on-board) having an LED chip.