Abstract:
Provided is a light emitting device assembly. The light emitting device assembly includes a submount including bumps, each having a bonded surface and a lateral surface surrounding the bonded surface; and a light emitting device including pads, each having a bonded surface corresponding to a bonded surface of a corresponding bump. Herein, edges of the bonded surface of each of the bumps are spaced a predetermined distance inward from edges of a corresponding pad.

Description:
This application claims the priority of Korean Patent Application No. 2003-68991, filed on Oct. 4, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light emitting device assembly, and more particularly, to a flip-chip bonding type light emitting device assembly. 
     2. Description of the Related Art 
     Generally, a laser beam of a laser diode (LD), which has a narrow frequency bandwidth and a good property of going straight ahead, has been recently put to practical use in the fields of optical communications, multiple communications, and space communications. One of major applications of the LD, along with the optical communications, is a pickup for an optical disc. 
     A GaN semiconductor LD includes a plurality of compound semiconductor layers, which are grown by crystalline grain growth and stacked on a substrate formed of sapphire, GaN, or SiC. Specifically, such compound semiconductor layers as an n-type GaN contact layer, an n-type AlGaN/GaN clad layer, an n-type GaN wave guide layer, an InGaN active layer, a p-type GaN wave guide layer, a p-type AlGaN/GaN clad layer, and a p-type GaN contact layer are sequentially stacked on the substrate. In the GaN semiconductor LD having the above-described stack structure, a ridge for forming a p-type electrode is provided on the uppermost portion of the LD. A passivation layer is formed between the ridge and the p-type electrode to define a conducting path in the semiconductor stack structure. 
     The GaN semiconductor LD is flip-chip bonded to a submount to facilitate transfer of heat generated during drive. By the flip-chip bonding, a top surface of a light emitting device, such as the LD where the p-type electrode is formed, is fixed to the submount using a solder bump. 
     When the light emitting device is flip-chip bonded to the submount, since the light emitting device and the submount are directly connected to each other using the solder bump, thermal resistance and line resistance are reduced as compared with when wire bonding is used. 
       FIG. 1  is a schematic cross-sectional view of a light emitting device assembly in which a light emitting device is flip-chip bonded to a submount. 
     In  FIG. 1 , reference numeral  10  denotes the light emitting device, such as an LD, and  21  denotes the submount. The light emitting device  10  is turned over to be bonded to the submount  21 . The light emitting device  10  includes a compound semiconductor layer  12  and a substrate  11  on which the compound semiconductor layer  12  is grown. The compound semiconductor layer  12  includes an n-type compound semiconductor layer (not shown), a p-type compound semiconductor layer (not shown), and an active layer (not shown) disposed therebetween. A first pad layer  22   a  and a second pad layer  22   b  are formed on the submount  21 . The first pad layer  22   a  and the second pad layer  22   b  respectively face two stepped regions of the compound semiconductor layer  12  and are spaced apart from each other. The two stepped regions of the compound semiconductor layer  12  correspond to a region where an n-type electrode (not shown) is formed and a region where a p-type electrode (not shown) is formed, respectively. There is a step difference S between the two stepped regions. A pad  13   a  is formed in the region where the n-type electrode is formed, and a pad  13   b  is formed in the region where the p-type electrode is formed. The pads  13   a  and  13   b  are in contact with the n-type electrode and the p-type electrode, respectively. 
     A solder bump  30   a  is interposed between the pad  13   a  disposed on the compound semiconductor layer  12  and the corresponding first pad layer  22   a  disposed on the submount  21 , and a solder bump  30   b  is interposed between the pad  13   b  disposed on the compound semiconductor layer  12  and the corresponding second pad layer  22   b  disposed on the submount  21 . The solder bumps  30   a  and  30   b  each include stacked conductive materials. 
     Specifically, the solder bump  30   a  includes a first gold layer  31   a  that contacts the pad  13   a  of the light emitting device  10 , a first platinum layer  33   a  that contacts the first pad layer  22   a , and an AuSn solder  32   a  disposed therebetween. Likewise, the solder bump  30   b  includes a second gold layer  31   b  that contacts the pad  13   b  of the light emitting device  10 , a second platinum layer  33   b  that contacts the second pad layer  22   b , and an AuSn solder  32   b  disposed therebetween. 
     The light emitting device  10  is turned over and the solder bumps  30   a  and  30   b  are closely bonded to the pads  13   a  and  13   b , respectively, due to a predetermined pressure. In this state, the resultant structure is heated for several seconds to a temperature of about 280° C. or higher. As a result, the light emitting device  10  is fixed to the submount  21  by the solder bumps  30   a  and  30   b . However, in this conventional light emitting device  10 , the pads  13   a  and  13   b  are unreliably bonded to the solder bumps  30   a  and  30   b , respectively, because of the following reasons. 
     Typically, the pads  13   a  and  13   b  of the light emitting device  10  are patterned using lift-off. When lift-off is used, as shown in a left view of  FIG. 2 , which illustrates a conventional light emitting device assembly before bonding, a flare type fence  14  is formed along an edge of each of the pads  13   a  and  13   b . Accordingly, as shown in a right view of  FIG. 2 , which illustrates the conventional light emitting device assembly after bonding, when the pads  13   a  and  13   b  come close to the bumps  30   a  and  30   b , the pads  13   a  and  13   b  become the first portions that contact the top surfaces of the bumps  30   a  and  30   b . After the pads  13   a  and  13   b  are closely bonded to the bumps  30   a  and  30   b , a gap is formed between the pads  13   a  and  13   b  and the bumps  30   a  and  30   b  due to the protruding fences  4 . 
       FIG. 3A  is a microscopic photograph of the pads  13   a  and  13   b  of the light emitting device  10 ,  FIG. 3B  is an exploded scanning electronic microscope (SEM) photograph of a portion illustrated with a left circle shown in  FIG. 3A , and  FIG. 3C  is an exploded SEM photograph of a portion illustrated with a right circle shown in  FIG. 3A . 
     As shown in  FIG. 3A , pads are disposed on both outer portions of the light emitting device  10 . The pads are formed using a typical method, i.e., lift-off. Thus, as described above, fences are formed along edges of the pads, respectively. It can be seen from  FIGS. 3B and 3C  that metal fences are formed during the lift-off. The metal fences are produced when a metal material is torn off during the lift-off, and each of them has a greater height than adjacent portions. 
       FIG. 4A  is a photograph of the conventional light emitting device assembly in which the light emitting device is fixed to the submount, and  FIG. 4B  is a photograph of a plan view of a solder bump of the submount after the light emitting device is separated from the submount. In the light emitting device assembly bonded as shown in  FIG. 4A , a gap occurs between the solder bump and the pad of the light emitting device and thus, the solder bump is badly bonded to the pad. This poor bonding state not only precludes effective heat emission, but also increases contact resistance between the pad and the bump, thus resulting in an elevation of driving voltage. 
     SUMMARY OF THE INVENTION 
     The present invention provides a light emitting device assembly, which effectively prevents a gap between a light emitting device and a bump. Thus, a pad of the light emitting device is reliably bonded to the bump, thus enabling effective heat emission and a reduction of driving voltage. 
     According to an aspect of the present invention, there is provided a light emitting device assembly including a submount including bumps, each having a bonded surface and a lateral surface surrounding the bonded surface; and a light emitting device including pads, each having a bonded surface corresponding to a bonded surface of a corresponding bump. Herein, edges of the bonded surface of each of the bumps are spaced a predetermined distance inward from edges of a corresponding pad. 
     The lateral surface of each of the bumps may be inclined at a predetermined angle with respect to the bonded surface of each of the bumps. 
     The lateral surface of each of the bumps may be perpendicular to the bonded surface of each of the bumps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a schematic cross-sectional view of a conventional light emitting device assembly; 
         FIG. 2  is a view for explaining a gap between bonded surfaces due to fences formed at pads of a light emitting device of the conventional light emitting device assembly; 
         FIG. 3A  is a microscopic photograph of the pads of the conventional light emitting device, which are formed by lift-off; 
         FIG. 3B  is a scanning electronic microscope (SEM) photograph of a fence formed along an edge of a pad by lift-off, which corresponds to a portion illustrated with a left circle shown in  FIG. 3A ; 
         FIG. 3C  is an SEM photograph of a fence formed along an edge of another pad by lift-off, which corresponds to a portion illustrated with a right circle shown in  FIG. 3A ; 
         FIG. 4A  is a photograph of the conventional light emitting device assembly in which the light emitting device is fixed to a submount; 
         FIG. 4B  is a photograph of a plan view of a solder bump of the submount after the light emitting device is separated from the submount; 
         FIG. 5A  is a schematic cross-sectional view of a light emitting device assembly according to an embodiment of the present invention; 
         FIG. 5B  is a plan view of the light emitting device assembly shown in  FIG. 5A  to explain a relationship between pads  130   a  and  130   b  and bumps  300   a  and  300   b;  and 
         FIG. 6  is a schematic cross-sectional view of a light emitting device assembly according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. In the drawings, the thicknesses, sizes, and arrangements of layers and regions are exaggerated for clarity of explanation. 
       FIG. 5A  is a schematic cross-sectional view of a light emitting device assembly according to an embodiment of the present invention. The light emitting device assembly may be, for example, a GaN compound semiconductor laser diode (LD) assembly or a light emitting diode (LED) assembly.  FIG. 5B  is a plan view of the light emitting device assembly shown in  FIG. 5A  to explain a relationship between pads  130   a  and  130   b  and bumps  300   a  and  300   b.    
     Referring to  FIGS. 5A and 5B , a light emitting device  100 , such as an LED or LD, includes a substrate  101  and a compound semiconductor layer  102 , which is stacked on a bottom surface of the substrate  101 . The substrate  101  may be a highly resistive substrate, such as a sapphire substrate, or a transparent substrate that transmits rays emitted from the compound semiconductor layer  102 . The compound semiconductor layer  102  includes an n-type compound semiconductor layer (not shown), a p-type compound semiconductor layer (not shown), and an active layer (not shown) disposed therebetween. When the light emitting device  100  is an LD, an n-type clad layer may be further interposed between the active layer and the n-type compound semiconductor layer, and a p-type clad layer may be further interposed between the active layer and the p-type compound semiconductor layer. A first pad layer  220   a  and a second pad layer  220   b  are formed on a submount  21 . The first and second pad layers  220   a  and  220   b  respectively face two stepped regions of the compound semiconductor layer  102  and are spaced apart from each other. The two stepped regions of the compound semiconductor layer  102  correspond to a region where an n-type electrode (not shown) is formed and a region where a p-type electrode (not shown) is formed, respectively. Also, a pad  130   a  is formed in the region where the n-type electrode is formed, and a pad  130   b  is formed in the region where the p-type electrode is formed. The pads  130   a  and  130   b  are in contact with the n-type electrode and the p-type electrode, respectively. 
     A solder bump  300   a  is interposed between the pad  130   a  disposed on the compound semiconductor layer  102  and the corresponding first pad layer  220   a  disposed on the submount  210 . Likewise, a solder bump  300   b  is interposed between the pad  130   b  disposed on the compound semiconductor layer  102  and the corresponding second pad layer  220   b  disposed on the submount  210 . 
     The solder bump  300   a  includes a first gold layer  310   a  that contacts the pad  130   a  of the light emitting layer  100 , a first platinum layer  330   a  that contacts the first pad layer  220   a , and an AuSn solder  320   a  disposed therebetween. Also, the solder bump  300   b  includes a second gold layer  310   b  that contacts the pad  130   b  of the light emitting layer  100 , a second platinum layer  330   b  that contacts the first pad layer  220   b , and an AuSn solder  320   b  disposed therebetween. 
     In the light emitting device assembly having the above-described structure, the solder bumps  300   a  and  300   b  respectively have smaller bonded surfaces  305   a  and  305   b  than those of the corresponding pads  130   a  and  130   b  of the light emitting device  100 . Thus, when the bumps  300   a  and  300   b  are bonded to the pads  130   a  and  130   b , respectively, edges of the bumps  300   a  and  300   b  are spaced a predetermined distance “t” apart from edges of the pads  130   a  and  130   b , respectively. Accordingly, the bonded surfaces  305   a  and  305   b  of the bumps  300   a  and  300   b  are not affected by fences  140  produced along the edges of the pads  130   a  and  130   b  of the light emitting device  100 . As a result, when the bonded surfaces  305   a  and  305   b  of the bumps  300   a  and  300   b  are completely bonded to the pads  130   a  and  130   b , respectively, of the light emitting device  100 , there is no gap (refer to the right view shown in  FIG. 2 ) therebetween. 
     To remove the gap between the bumps  300   a  and  300   b  and the pads  130   a  and  130   b  as described above, the areas of the bonded surfaces  305   a  and  305   b  of the bumps  300   a  and  300   b  are appropriately reduced so that the bonded surfaces  305   a  and  305   b  of the bumps  300   a  and  300   b  are not in contact with the fences  140  formed along the edges of the pads  130   a  and  130   b , respectively, of the light emitting device  100 . In the present embodiment, the areas of the bonded surfaces  305   a  and  305   b  are reduced so as not to contact the fences  140  by uniformly reducing horizontal sectional areas of the bumps  300   a  and  300   b . Thus, lateral surfaces  301   a  and  301   b  of the bumps  300   a  and  300   b  are perpendicular to the submount  210  and the substrate  101  of the light emitting device  100 . 
       FIG. 6  is a schematic cross-sectional view of a light emitting device assembly according to another embodiment of the present invention. In  FIG. 6 , lateral surfaces  302   a  and  302   b  of bumps  300   a  and  300   b  are inclined with respect to a submount  210  and a substrate  101  of a light emitting device  100 . 
     Referring to  FIG. 6 , in the light emitting device assembly of the present embodiment, the lateral surfaces  302   a  and  302   b  of the bumps  300   a  and  300   b  are inclined such that the bumps  300   a  and  300   b  each have a trapezoidal sectional shape. These bumps  300   a  and  300   b , each having the trapezoidal sectional shape, are more stable against an external force applied to bonded surfaces  305   a  and  305   b  as compared with the bumps of the previous embodiment. 
     The light emitting device assembly, which is described in detail with reference to the drawings, is not limited to specific kinds of light emitting devices or specific stack structures of bumps. That is, the light emitting device assembly of the present invention can include any bumps, which have smaller bonded areas than pads of a light emitting device and of which edges are spaced a predetermined distance from edges of the pad. Here, all of four edges of a bump may not be located inside the edges of a pad. For example, two long edges of the four edges of the bump may be located inside the edges of the pad, while two short edges thereof may be located outside the edges of the pad. In the conventional case, all of four edges of the bump come into contact with the fence formed at the edges of the pad and thus, a gap is inevitably caused between the bump and the fence. However, in the present invention, as the bump is not in contact with the fence on at least two edges, the gap between the bump and the fence is greatly reduced. Nevertheless, it is preferable that all of four edges of the bump be spaced apart from the fence of the pad. 
     As explained thus far, according to the present invention, flip-chip bonding is enabled to provide good bonded surfaces between bumps and pads for a light emitting device. As a result, contact resistance between the bumps and the pads can be reduced, the light emitting device can operate at low voltage, and the reliability of the light emitting device assembly can be enhanced. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.