CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Taiwan Patent Application No. 94120796 entitled “Light Emitting Diode And Method Of The Same,” filed on Jun. 22, 2005, which is incorporated herein by reference and assigned to the assignee herein.  
       FIELD OF INVENTION  
       [0002]     The present invention generally relates to a light emitting diode and a method making the same, and more particularly, to a light emitting diode and method making the same with a thermal spreading layer.  
       BACKGROUND OF THE INVENTION  
       [0003]     Since the light emitting diodes (LEDs) are widely applied in various application fields, many different structures and manufacturing methods for making the light emitting diodes have been developed. For example, U.S. Pat. No. 6,462,358 issued to Lin et al., entitled “Light emitting diode and method for manufacturing the same” discloses a light emitting diode with a transparent substrate which is connected to an epitaxial structure by an adhesive layer. The light emitting diode of Lin et al. has better luminous efficiency due to the lower absorption of light, lower forward voltage and better current distribution of the transparent substrate. However, the light emitting diode may impose a local thermal concentration and further limit its performance due to the low thermal conductivity of the transparent substrate. U.S. Pat. No. 6,458,612 issued to Chen et al., entitled “Method of fabricating high efficiency light-emitting diode with a transparent substrate” also discloses a light emitting diode with a transparent substrate.  
         [0004]     For another example, U.S. Pat. No. 6,812,067, issued to Chen et al., entitled “Method for integrating compound semiconductor with substrate of high thermal conductivity” discloses a compound semiconductor structure with a nontransparent substrate, which employs a metal of low melting point to form two bonding layers on the substrate and the epitaxial structure respectively, and connects the substrate with the epitaxial structure by connecting two bonding layers. However, the light emitting diode in U.S. Pat. No. 6,812,067 adopts the high conductivity material for the nontransparent substrate restricting the structure of the light diode.  
         [0005]     Therefore, there is a need to provide a light emitting diode and a method making the same suitable for various structures of the light emitting diode to improve the thermal conductivity of the light emitting diode.  
       SUMMARY OF THE INVENTION  
       [0006]     In viewing the drawbacks of prior art light emitting diodes, the present invention provides a light emitting diode and a method making the same for improving the thermal conductivity of the light emitting diode.  
         [0007]     One aspect of the present invention is to provide a light emitting diode with a thermal spreading layer on a transparent substrate.  
         [0008]     Another aspect of the present invention is to provide a light emitting diode with a thermal spreading layer on a non-transparent substrate.  
         [0009]     Still another aspect of the present invention is to provide a method of fabricating a light emitting diode, which comprises the following steps: providing a first substrate; forming a thermal spreading layer on the first substrate; forming a connecting layer on the thermal spreading layer; providing a second substrate; forming an etching stop layer on the second substrate; forming an epitaxial structure on the etching stop layer; forming an ohmic contact epitaxial layer on the epitaxial structure; forming a first ohmic contact layer on the ohmic contact epitaxial layer; connecting the epitaxial structure with the thermal spreading layer by the connecting layer; removing the etching stop layer and the second substrate; removing a part of the epitaxial structure and parts of the ohmic contact epitaxial layer to expose a part of the ohmic contact epitaxial layer; etching the ohmic contact epitaxial layer to form a hole to expose parts of the first ohmic contact layer; forming a first metal adhesive layer on the exposed part of the ohmic contact epitaxial layer to electrically connect with the first ohmic contact layer through the hole; forming a second ohmic contact layer touching the epitaxial structure; and forming a second metal adhesive layer touching the second ohmic contact layer.  
         [0010]     A further aspect of the present invention is to provide a method of fabricating a light emitting diode, which comprises the following steps: providing a first substrate; forming a protective layer on the first substrate; forming a thermal spreading layer on the protective layer; forming a wetting layer on the thermal spreading layer; forming a barrier layer on the wetting layer; forming a first connecting layer on the barrier layer; providing a second substrate; forming an epitaxial structure on the second substrate; forming a second connecting layer on the epitaxial structure; connecting the epitaxial structure with the barrier layer by connecting the first connecting layer with the second connecting layer; and removing the second substrate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0012]     FIGS.  1  to  3  illustrate a cross-section view of a light emitting diode in accordance with an embodiment of the present invention; and  
         [0013]     FIGS.  4  to  8  illustrate a cross-section view of a light emitting diode in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The present invention provides a light emitting diode and method making the same. For better understanding, please read the following description in conjunction with the accompanying drawings.  
         [0015]     FIGS.  1  to  3  illustrate the cross-section views corresponding to the steps of fabricating a light emitting diode in accordance with an embodiment of the present invention. At first, referring to  FIG. 1 , an etching stop layer  124  is formed on a substrate  126 , and then a cladding layer  122 , an active layer  120 , an upper cladding layer  118  and an ohmic contact epitaxial layer  116  are formed successively. Then, an ohmic contact layer  128  is formed on the ohmic contact epitaxial layer  116 .  
         [0016]     According to the present invention, the substrate  126  is selected form Si, Ge, GaAs, GaP, InP or the like. The active layer  120  includes a material such as AlGaInP, InGaN or AlGaAs, and has a conventional structure, such as a homostructure, single heterostructure, double heterostructure (DH), or multiple quantum well (MQW).  
         [0017]     The ohmic contact epitaxial layer  116  has a material selected from AlGaAs, AlGaInP, GaPAs or other materials having a larger energy gap than the active layer  120  and a high carrier concentration for becoming the ohmic contact layer.  
         [0018]     The etching stop layer  124  is selected form a group of III-V compound semiconductor. Any material having lattice matched with that of the substrate  126  and having an etching rate lower than the etching rate of the substrate  126  is suitable for the etching stop layer  124 .  
         [0019]     The etching stop layer  124  is preferably made of InGap or AlGaAs. If the etching rate of the lower cladding layer  122  is lower than the etching rate of the substrate  126  and the lower cladding layer  122  is thick enough, the etching stop layer  124  is then an option.  
         [0020]     Then, as illustrated in  FIG. 2 , a thermal spreading layer  112  is formed on a transparent substrate (TS), and then a transparent adhesive layer  114  is formed on the thermal spreading layer  112 . The material of the thermal spreading layer  112  is selected from a material of high thermal conductivity, such as diamond or diamond-like material, with a thickness of 1˜100 micron formed by chemical vapor deposition. The transparent adhesive layer  114  is selected from BCB (B-staged bisbenzocyclobutene) formed by spin-coat method. However, the transparent adhesive layer  114  is not limited to the BCB, and other adhesive materials of transparent property, such as epoxy, is suitable for the present invention.  
         [0021]     The transparent substrate  110  is selected from materials which do not absorb the light from the active layer  120 , such as glass, sapphire, SiC, GaP, GaAsP, ZnSe, ZnS and ZnSSe. The transparent substrate  110  can be selected from polycrystal or amorphous substrate to lower the cost.  
         [0022]     Then, after the ohmic contact layer  128  has been formed, the light emitting diode in  FIG. 1  is connected to transparent substrate  110  in  FIG. 2  by pressuring the BCB adhesive layer  114  at high temperature of around 250° C. for a while.  
         [0023]     The nontransparent substrate  126  is etched away by a corrosive etchant, such as 5H 3 PO 4 :3H 2 O 2 :3H 2 O or 1NH 4 OH:35H 2 O 2 . If the etching stop layer  124  is made of light-absorption materials, such as InGaP or AlGaAs, the etching stop layer  124  must be removed by the same solution.  
         [0024]     Then, referring to  FIG. 3 , parts of the lower cladding  122 , the active layer  120 , the upper cladding layer  118  and ohmic contact epitaxial layer  116  are removed by a dry etching process, such as RIE, to expose the ohmic contact epitaxial layer  116 . Subsequently, the exposed ohmic contact epitaxial layer  116  is etched to form a hole  130  exposing the ohmic contact layer  128 . Then, an ohmic contact layer  134  is formed on the lower cladding layer  122 . Then, a metal adhesive layer  132  is formed on the ohmic contact epitaxial layer  116  and filled within the hole  130  to electrically connect with the p-type ohmic contact layer  128 , and a metal adhesive layer  136  is formed on the ohmic contact layer  134  as well. Consequently, the two metal adhesive layers  132  and  136  are on the same side with respect to the transparent substrate  110 , as shown in  FIG. 3 .  
         [0025]     FIGS.  4  to  8  illustrate the cross-section views corresponding to another embodiment of the present invention for fabricating a light emitting diode. As shown in  FIG. 4 , an epitaxial structure  222  is formed on a compound semiconductor substrate  220 . The compound semiconductor substrate  220  includes a material such as GaAs, InP, GaP, sapphire or SiC. Then, a first connecting layer  224  is formed on the epitaxial structure  222 .  
         [0026]     Referring to  FIG. 5 , a high thermal conductivity substrate  226  is provided, which has a thermal conductivity greater than that of the compound semiconductor substrate  220  and is selected form Si, Al, Cu, Au, Mo, Aluminum nitride, or the like. Then, a thermal spreading layer  228  and a wetting layer  230  are formed sequentially on the high thermal conductivity substrate  226 . The thermal spreading layer  228  is configured to improve the thermal conductivity of the light emitting diode, and is made of diamond and diamond-like material. The wetting layer  230  is configured to enhance the adhesion between layers, and includes a material such as Chromium, Titanium, Nickel or the like.  
         [0027]     Then, a barrier layer  232  is formed on the wetting layer  230 . The barrier layer  232  serves the purpose of preventing the internal diffusion of the material of subsequently formed second bonding layer  234  to the wetting layer  230  or the high thermal conductivity substrate  226 . The barrier layer  232  includes a material selected from a group consisting of Mo, Pt, W, Indium Oxide, Tin Oxide, Indium Tin Oxide, Zinc Oxide and Magnesium Oxide.  
         [0028]     Then, a second bonding layer  234  is formed on the barrier layer  232 . One of the first and the second bonding layer,  224  and  234 , is a metal layer, such as an Indium layer, having a melting point in the range between about 160° C. and 400° C. The other bonding layer ( 224  or  234 ) is a layer of any material, such as Gold, which forms an alloy-bonding layer with the metal layer. In addition, the method of forming the first and the second bonding layers,  224  and  234 , includes conventional processes of deposition, evaporation, or sputtering.  
         [0029]     Referring to  FIG. 6 , the first bonding layer  224  and the second bonding layer  234  are positioned face-to-face and pressed at a temperature to form an alloy layer  236  (as shown in  FIG. 7 ). The process temperature depends on the melting point of selected material of the bonding layers.  
         [0030]     Then, the compound semiconductor substrate  220  is removed by a conventional process, such as a wet chemical etching or dry etching process, and then a compound semiconductor is integrated with the high thermal conductivity substrate  226 , as shown in  FIG. 7 .  
         [0031]     Since the high thermal conductivity substrate  226  is usually highly active to chemical reactants, a protective layer  238  is optionally formed on the substrate  226  to prevent the high thermal conductivity substrate  226  from reacting with chemical reactant. In other words, the method further includes the step of forming a protective layer  238  over the surface of the substrate  226  by conventional electroplate technique prior to the formation of the thermal spreading layer  228 . The protective layer  238  includes a material of Ni, Au, Ag, Cr, or the like, and the thickness of the protective layer  238  is relatively small compared with the thickness of the substrate  226 .  FIG. 8  shows a cross-sectional view of the light emitting diode having the protective layer  238 .  
         [0032]     The light emitting diodes of the present invention have excellent heat dissipation, and therefore have better performance and can be operated at high current. Besides, the process method of the present invention is simple and suitable for various structures of the light emitting diode.  
         [0033]     By means of the detailed descriptions of what is presently considered to be the most practical and preferred embodiments of the subject invention, it is expected that the features and the gist thereof be clearly described. Nevertheless, these embodiments are not intended to be construed in a limiting sense. Instead, it will be well understood that any analogous variations and equivalent arrangements will fall within the spirit and scope of the invention.
A light emitting diode and the method of the same are provided. The light emitting diode includes a substrate, a thermal spreading layer, a connecting layer and an epitaxial structure. The substrate is selected from a transparent substrate or a non-transparent substrate, which corresponds to different materials of the connecting layers respectively. The thermal spreading layer, configured to improve the thermal conduction of the light emitting diode, is selected from diamond, impurity-doped diamond or diamond-like materials.