Abstract:
This invention provides a light-emitting element and the manufacture method thereof. The light-emitting element is suitable for flip-chip bonding and comprises an electrode having a plurality of micro-bumps for direct bonding to a submount. Bonding within a relatively short distance between the light-emitting device and the submount can be formed so as to improve the heat dissipation efficiency of the light-emitting device.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application claims the right of priority based on Taiwan Application Serial Number 095132845, filed on Sep. 5, 2006, the disclosure of which incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a light-emitting element, and more particularly, to a light-emitting element comprising an electrode comprising a plurality of micro-bumps. 
     BACKGROUND 
     Employing light emitting diode (LED) in high power illumination, besides continuously promoting brightness, heat dissipation is another main problem that is indeed necessary to be solved. When light extraction efficiency is not good, the light, which cannot pierce a light-emitting device including LED and its encapsulant, transforms into heat. If the heat cannot be dissipated out of the light-emitting device effectively, the temperature is going to raise during operation. Thus, reliability problem comes out. Prior arts provide a lot of methods for solving the problem of heat dissipation of elements. Taking a light-emitting element having GaN series grown on a sapphire substrate as example, double transfer method is used to remove the sapphire substrate with worse heat dissipation by laser lift-off or chemical etching, then a silicone substrate with better heat dissipation is connected to the light-emitting element in order to improve the heat dissipation effect of the light-emitting element. Another improving method is to take flip-chip bonding to replace traditional wire bonding.  FIG. 1  shows that a known light-emitting device with flip-chip bonding comprises a LED  10  and a submount unit  20 . Solder bumps  24  are formed on a first bond pad  22  and a second bond pad  23 , for connecting the LED  10  to the submount unit  20  in a bonding process. The solder bumps  24  are formed on the first bond pad  22  and second bond pad  23  one by one through the gold stud bump method. Through thermosonic bonding method, ultrasonic wave is provided on the junction of the solder bumps  24  of the submount unit  20  and electrodes  15  and  16  of the LED  10  to make the junction quickly rub to produce high heat for melting and bonding. Generally, diameter of a gold stud is about 50 um. Dimension of every gold stud must be close to avoid that the short gold stud cannot tough the junction to influence the characteristics of products and bonding performance. In addition, because the gold stud is that the front of a gold wire is melted to become a spheroid and then is bonded on the submount through thermosonice bonding, dimension of the gold stud is restricted to that of the gold wire not to be further contracted. Thickness is still larger than 20 um after bonding so the thermal resistance between the LED  10  and a submount  21  cannot be reduced. Thus, the application of the LED  10  in the high power device is restricted. 
     Therefore, this invention provides a light-emitting element which is applicable to direct flip-chip bonding, without additional solder bump on the submount. This invention also comprises the advantages of wide bonding area, short bonding distance, high heat dissipation, great reliability, and low cost. 
     SUMMARY OF THE INVENTION 
     This invention provides a light-emitting element and the manufacture method thereof. The light-emitting element is suitable for flip-chip bonding and comprises an electrode having a plurality of micro-bumps for directly bonding to a submount. A bonding within a relatively short distance between the light-emitting device and the submount can be formed so as to improve the heat dissipation efficiency of the light-emitting device. 
     Another object of this invention is to provide a light-emitting element, comprising a transparent substrate, a first electricity semiconductor layer formed on the transparent substrate, an active layer formed on the first type semiconductor layer, a second electricity semiconductor layer formed on the active layer, a contact layer formed on the second type semiconductor layer, and an electrode having a plurality of micro-bumps, formed on the contact layer for directly bonding to a submount. The plurality of micro-bumps is formed on the electrodes out of one piece. 
     The other object is to provide a light-emitting element comprising an LED, a submount, and a plurality of micro-bumps. The LED comprises a transparent substrate and at least one electrode. The submount comprises at least one bond pad. The plurality of micro-bumps is located on the intermediate between the electrode and the bond pad, and is formed on the electrode or bond pad out of one piece. 
     An additional object is to provide a manufacture method of a light-emitting element comprising providing an LED comprising a transparent substrate and at least one electrode, and forming a plurality of micro-bumps on the electrode. The plurality of micro-bumps is formed on the electrode out of one piece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic diagram of a light-emitting element according to a prior art. 
         FIG. 2  shows a schematic diagram of one embodiment of a light-emitting element according to this invention. 
         FIG. 3  shows a schematic diagram of another embodiment of the light-emitting element according to this invention. 
         FIG. 4  shows a schematic diagram of another embodiment of the light-emitting element according to this invention. 
         FIG. 5  shows a schematic diagram of another embodiment of the light-emitting element according to this invention. 
         FIG. 6  shows a top plan view of an electrode according to this invention. 
         FIG. 7  shows a top plan view of another electrode according to this invention. 
         FIG. 8  shows a diagram of efficiency of current and flux measured from this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 2 , a light-emitting element comprises an LED  30  and a submount unit  40 . The LED  30  comprises a transparent substrate  31 , a light-emitting stack  32 , a first contact layer  32 , a second contact layer  33 , a first electrode  35 , and a second electrode  36 . The transparent substrate  31  comprises at least one material selected from the group consisting of Al 2 O 3 , GaN, Glass, Gap, Sic, and CVD diamond. The light-emitting stack  32  is formed on the transparent substrate  31  and emits light when receiving a forward voltage. Color of the light depends on the material of the light-emitting stack  32 . For example, the material of (Al z Ga 1-z ) 0.5 In 0.5 P series can emit red light, yellow light, or green light in accordance with the value of z. The material of Al x In y Ga (1-x-y) N series can emit blue light or purple light in accordance with the combination of the value of x and y. The light-emitting stack  32  comprises a first electricity semiconductor layer  321 , an active layer  322 , and a second electricity semiconductor  323 . The first electricity semiconductor layer  321  can be n type or p type semiconductor layer, and the electricity thereof is opposite to that of the second electricity semiconductor layer  323 . Portions of the second electricity semiconductor layer  323  and the active layer  322  are removed to expose a portion of the first electricity semiconductor layer  321  by etching. The first contact layer  33  is formed on the exposed portion of the first electricity semiconductor layer  321 , and the second contact layer  34  is formed on the second electricity semiconductor layer  323 . The first contact layer  32  and the second contact layer  34  are used to form ohmic contact with the first electricity semiconductor  321  and the second electricity semiconductor layer  323  respectively. The material of the first contact layer  33  or the second contact layer  34  is selected form the group consisting of BeAu, ZnAu, SiAu, and GeAu. The first electrode  35  is formed on the first contact layer  33  and the second electrode  36  is formed on the second contact layer  34 . In one embodiment, the LED  30  further comprises an electrode pad layer  37  formed on the intermediate between the second contact layer  34  and the second electrode  36 . Thus, the upper surfaces of the second electrode  36  and the first electrode  35  are substantially located on the same level. The first electrode  35  or the second electrode  36  comprises at least one material selected from the group consisting of Au, Ag, Al, and Cu. The material of the electrode pad layer  37  can be the same as that of the second electrode  36 , the second contact layer  34 , or other metal materials. There is a plurality of micro-bumps formed on the surfaces of the first electrode  35  and the second electrode  36 . When the plurality of micro-bumps is directly bonded to a submount unit  40 , taking the printed circuit board (PCB) as example, current conducts to the submount via the plurality of micro-bumps functioning as a plurality of current channels. The method of direct bonding can be thermosonic bonding to directly bond the plurality of micro-bumps to the submount unit  40 . The submount unit  40  comprises a submount  41 , and a first bond pad  42  and a second bond pad  43  formed on the submount  41  to be bonded to the first electrode  35  and the second electrode  36  respectively. The submount  41  comprises a substance with good heat dissipation, comprising at least one material selected from the group consisting of Si, SiC, AlN, CuW, Cu, and CVD diamond. The first bond pad  42  or the second bond pad  43  comprises a metal or an alloy material, for example, alloy of Au, Ag, Al, or Sn. In one embodiment, through known lithography and etching, the plurality of micro-bumps can be patterned and a portion of depth of the electrode layer can be etched to form the plurality of micro-bumps. Thickness of the plurality of micro-bumps is between 0.3 um and 20 um, and the shape thereof can be a polygon, a circle, or a rectangle. Dimension of the plurality of micro-bumps, for example, length of the shortest side of the polygon, diameter of the circle, or length of short sides of the rectangle is between 1 um and 250 um. In a preferred embodiment, thickness of the smallest micro-bump is 0.3 um. After bonding, the plurality of micro-bumps can offer short bonding distance to reduce the thermal resistance between the LED  30  and the submount element  40 , and can own acceptable shear strength to maintain great bonding. In another embodiment,  FIG. 3  shows that electrodes  55  and  56  are the plurality of micro-bumps separate among each other. They also can be formed by known lithography and etching, or alternatively by electroplate or film growth. 
     In  FIG. 4 , another embodiment is that a light-emitting element comprises an LED  60  and a submount unit  40 . The LED  60  comprises a transparent substrate  31 , a light-emitting stack  62 , a first contact layer  63 , a second contact layer  64 , a first electrode  65 , and a second electrode  66 . The transparent substrate  31  comprises a first region a, a second region b, and a trench c. The light-emitting stack  62  formed on the first region a and second region b of the transparent substrate  31  comprises a first electricity semiconductor layer  621 , an active layer  622 , and a second electricity semiconductor layer  623  in order. The light-emitting stack  62  located on the first region a and the second region b is connected to the transparent substrate  31  by the first electricity semiconductor layer  621 . The first contact layer  63  is formed on the second electricity semiconductor layer  623  of the first region a, and the second contact layer is formed on the second semiconductor layer  623  of the second region b. The first electrode  65  is formed on the first contact layer  63 , and the second electrode  66  is formed on the second contact layer  64 . Because the light-emitting stack  62  under the first electrode  65  is not removed, the first electrode  65  and the second electrode  66  can maintain on the same level. The first electrode  65  and the second electrode  66  comprise a plurality of micro-bumps for providing a plurality of current channels to conduct the current from the plurality of micro-bumps to the submount when directly bonded to the submount unit  40 . The direct bonding method can utilize thermosonic bonding to directly bond the plurality of micro-bumps to the submount unit  40 . Through known lithography and etching, the plurality of micro-bumps can be patterned and a portion of depth of the electrode layer can be etched to form the plurality of micro-bumps. Thickness of the plurality of micro-bumps is between 0.3 um and 20 um, and the shape thereof can be a polygon, a circle, or a rectangle. Dimension of the plurality of micro-bumps, for example, length of the shortest side of the polygon, diameter of the circle, or length of short sides of the rectangle is between 1 um and 250 um. In a preferred embodiment, thickness of the smallest micro-bump is 0.3 um. After bonding, the plurality of micro-bumps can offer short bonding distance to reduce the thermal resistance between the LED  30  and the submount element  40 , and can own acceptable shear strength to maintain great bonding. In another embodiment, the first electrodes  65  and the second electrode  56  are the plurality of micro-bumps separate among each other. They also can be formed by known lithography or etching, or alternatively by electroplate or film growth. 
     In another embodiment,  FIG. 5  shows that an LED  70  comprises a transparent substrate  31 , a light-emitting stack  72 , a first contact layer  73 , a second contact layer  74 , a first electrode  75 , and a second electrode  76 . The transparent substrate  31  comprises a first region a, a second region b, and a trench c. The light-emitting stack  72  formed on the first region a and the second region b of the transparent substrate  31  comprises a first electricity semiconductor layer  721 , an active layer  722 , and a second electricity semiconductor layer  723  in order, wherein the surface shape of the second semiconductor layer  723 , is uneven. Moreover, the light-emitting stack  72  located on the first region a and the second region b is connected to the transparent substrate  31  by the first electricity semiconductor layer  721 . Conforming to the surface shape of the second semiconductor layer  723 , the first contact layer  73  is formed on the second electricity semiconductor layer  723  of the first region a, and the second contact layer  74  is formed on the second semiconductor layer  723  of the second region b respectively. Conforming to the surface shapes of the first contact layer  73  and the second contact layer  74 , the first electrode  75  is formed on the first contact layer  73 , and the second electrode  76  is formed on the second contact layer  74  respectively. Thus, there is a plurality of micro-bumps formed on the surfaces of the first electrode  75  and the second electrode  76  for providing a plurality of current channels to conduct the current from the plurality of micro-bumps to the submount when directly bonded to the submount unit  40 . The direct bonding method can utilize thermosonic bonding to directly bond the plurality of micro-bumps to the submount unit  40 . In one embodiment, the uneven shape of the second electricity semiconductor layer  723  is formed by known lithography. The first electrode  75  and the second electrode  76  are formed on the second electricity semiconductor layer  723  by depositing an electrode layer of the contact layer conforming to the second electricity semiconductor layer  723 , then the known method, like lithography or lift-off, is utilized to remove the contact layer and the electrode layer in the trench. Thickness of the plurality of micro-bumps of the first electrode  75  and the second electrode  76  is between 0.3 um and 20 um, and the shape thereof can be a polygon, a circle, or a rectangle. Dimension of the plurality of micro-bumps, for example, length of the shortest side of the polygon, diameter of the circle, or length of short sides of the rectangle is between 1 um and 250 um. In a preferred embodiment, thickness of the smallest micro-bump is 0.3 um. After bonding, the plurality of micro-bumps can offer short bonding distance to reduce the thermal resistance between the LED  30  and the submount unit  40 , and can own acceptable shear strength to maintain great bonding. In another embodiment, the second electricity semiconductor layer  723  comprises a plain surface and the contact layer comprises the uneven surface. Conforming to the surface shape of the contact layer, the following electrode layer covers the contact layers to make the surface of the electrode have the shape of the micro-bump. 
     In another embodiment, utilizing the inventive theory disclosed in  FIG. 2  and  FIG. 3 , it also can achieve the same bonding effect that a plurality of micro-bumps are formed on the first bond pad  42  and the second bond pad  43  of the submount unit  40 . 
       FIG. 6  and  FIG. 7  are embodiments of the electrodes disclosed in this invention.  FIG. 6  and  FIG. 7  show that a plurality of micro-bumps of the first electrode  85  and  95  or the second electrode  86  and  96  are circular or rectangular and an array. For obtaining preferred bonding effect, dimension and arrangement of the plurality of micro-bumps can be adjusted, or the bonding area, summation of the surface areas of the plurality of micro-bumps, can be increased. With the increasing of the bonding area, the requirement of smoothness of the electrodes raises to avoid bad bonding of a portion of the hollow area resulting from variety of the plurality of micro-bumps&#39; thickness or bending resulting from chip stress. The ratio of the bonding area to the area surrounded by the electrodes is between 5% and 50%. 
       FIG. 8  shows that comparison of the effective curve of the traditional gold stud bond method and the direct bonding method of this invention. In this embodiment, the plurality of micro-bumps of the electrode is the circle with 10 um diameter and arrayed. It occupies 5% of the electrodes&#39; total area, equivalent to the contact area of the gold stud. In accordance with the curve in the diagram, the flux directly proportionates to the passing current. However, with increasing of the current, the flux can become saturating gradually. Applying a light-emitting element with a traditional gold stud bonding structure, the saturating current is about 0.7 A and the greatest flux is about 501 m. Nevertheless, the saturating current measured from the light-emitting element with direct bonding structure is about 1.2 A and the greatest flux is about 751 m, raising about 50%. The following table is the comparison of the measured value of thermal resistance. According to the following table, the thermal resistance between the light-emitting element and the submount unit of this invention is far lower than that between the traditional light-emitting element and the traditional submount unit. In summary, this invention has the improved effect of low thermal resistance and high luminance. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                 Thermal Resistance 
               
               
                 Bonding Method 
                 V f [V] 
                 I f [A] 
                 ΔT[K] 
                 [° C./W] 
               
               
                   
               
             
             
               
                 Gold Ball Bonding 
                 0.35 
                 2.25 
                 30.4 
                 38.6 
               
               
                 Au—Au Direct 
                 0.35 
                 2.26 
                 15.6 
                 19.7 
               
               
                 Bonding 
               
               
                   
               
             
          
         
       
     
     It should be noted that the proposed various embodiments are not for the purpose to limit the scope of the invention. Any possible modifications without departing from the spirit of the invention may be made and should be covered by the invention.