Abstract:
The present invention provides a method to fabricate a diode whose heat stability is improved. The diode has a layer of high reflective ohmic contact and an alloy metal is used in the layer. With the alloy metal used in the layer, the heat stability of the diode is improved.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to fabricating a diode; more particularly, relates to fabricating a high reflective ohmic contact alloy layer in the diode to improve a heat stability of the diode 
       DESCRIPTION OF THE RELATED ARTS 
       [0002]    A first prior art, “A novel light emitting diode (LED)”, is proclaimed in Taiwan, comprising a substrate, a base and a crystal grain, where the substrate has at least one through-hole and a plurality of contacts; the through-hole penetrates through the substrate; the base is located in the through-hole; the crystal grain is deposed on a surface of the base where the surface is plated with a reflective metal layer; and the metal layer is made of silver or tin. 
         [0003]    A second prior art is proclaimed in Taiwan, “A LED and a fabricating method thereof.” The second prior art comprises a substrate; a semiconductor layer deposed on the substrate; a plurality of electrodes deposed on the semiconductor layer; a protecting layer deposed on the semiconductor layer with a plurality of openings to expose the plurality of electrodes; a bottom layer of a plurality of metal balls deposed on the plurality of electrodes; and a light reflective layer deposed on the protecting layer and electrically insulated from the electrodes and the bottom layer, where the bottom layer is made of the same material as the light reflective layer; and the material is gold, silver or titanium/tungsten. 
         [0004]    Each of the above prior arts has a layer of a pure metal. But the metal will be aggregated after a thermal treatment so that the current in the diode is not distributed evenly and contact resistances differ very much so as to weaken the light emitted. Hence, the prior arts do not fulfill users′ requests on actual use. 
       SUMMARY OF THE INVENTION 
       [0005]    The main purpose of the present invention is to fabricate a diode where a heat stability of the diode is effectively improved by using a metal alloy in a high reflective ohmic contact layer of the diode. 
         [0006]    To achieve the above purpose, the present invention is a method for fabricating a diode having a reflective electrode of an alloy metal. A first embodiment comprises steps of: (a) obtaining a p-side up gallium nitride (GaN) wafer comprising a first substrate, a buffer layer and an epitaxy layer; (b) forming a p-GaN mesa through a lithography and an etching on the epitaxy layer of the p-side up GaN wafer; (c) plating an alloy metal layer on the p-GaN mesa and processing a thermal treatment to a surface of the alloy metal layer to form a high reflective ohmic contact alloy layer; (d) forming an n-pad through a lithography and a metal depositing process on the p-side up GaN wafer; (e) forming a p-pad through a lithography and a metal depositing process on the high reflective ohmic contact alloy layer; and (f) packaging a structure, obtained through step (a) to step (e), on a second substrate through a packaging process of flip chip with a metal material to connect the second substrate to the structure. 
         [0007]    A second embodiment comprises steps of: (a1) obtaining a p-side up GaN wafer comprising a first substrate, a buffer layer and an epitaxy layer; (b1) plating an alloy metal layer on the epitaxy layer of the p-side up GaN wafer and forming a high reflective ohmic contact alloy layer through a thermal treatment on a surface of the alloy metal layer; (c1) obtaining a conjoining layer to conjoin the high reflective ohmic contact alloy layer and a third substrate; (d1) processing a laser lift-off process to lift the first substrate and the buffer layer off the epitaxy layer of the p-side up GaN wafer; and (e1) forming an n-pad on the epitaxy layer through a lithography and a metal depositing process. 
         [0008]    A third embodiment comprises steps of: (a2) obtaining a p-side up GaN wafer comprising a first substrate, a buffer layer and an epitaxy layer; (b2) forming a p-GaN mesa through a lithography and an etching on the epitaxy layer of the p-side up GaN wafer; (c2) forming a transparency conductive layer (TCL layer) on the p-GaN mesa; (d2) forming an n-pad through a lithography and a metal depositing process on the p-side up GaN wafer; (e2) forming a p-pad through a lithography and a metal depositing process on the TCL layer; and (f2) plating an alloy metal layer on a bottom surface of the first substrate of the p-side up GaN wafer and forming a high reflective ohmic contact alloy layer through a thermal treatment on a surface of the alloy metal layer. 
         [0009]    Accordingly, a novel method for fabricating a diode having a reflective electrode of an alloy metal is obtained. 
     
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0010]    The present invention will be better understood from the following detailed descriptions of the preferred embodiments according to the present invention, taken in conjunction with the accompanying drawings, in which 
           [0011]      FIG. 1  is the view showing the flow chart of the first preferred embodiment according to the present invention; 
           [0012]      FIG. 2A  is the structural view of the first preferred embodiment; 
           [0013]      FIG. 2B  is the structural view of the p-side up GaN wafer; 
           [0014]      FIG. 3  is the flow-chart view of the second preferred embodiment; 
           [0015]      FIG. 4  is the structural view of the second preferred embodiment; 
           [0016]      FIG. 5  is the flow-chart view of the third preferred embodiment; and 
           [0017]      FIG. 6  is the structural view of the third preferred embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    The following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention. 
         [0019]    Please refer to  FIG. 1 ,  FIG. 2A  and  FIG. 2B , which are a view showing a flow chart of a first preferred embodiment according to the present invention, a structural view of the first preferred embodiment and a structural view of a p-side up gallium nitride (GaN) wafer. As shown in the figures, the present invention provides a method for fabricating a diode having a reflective electrode of an alloy metal. A first embodiment comprises the following steps: 
         [0020]    (a) Obtaining a p-side up GaN wafer  11 : A p-side up GaN wafer  21  is obtained at first. The p-side up GaN wafer  21  comprises a first substrate  211 , a buffer layer  212  and an epitaxy layer (epi-layer)  213 , where the first substrate  211  is a transparent substrate of sapphire, silicon carbide (SiC), gallium arsenide (GaAs), lithium gallium oxide (LiGaO 3 ) or aluminum nitride (AlN); the epi-layer  213  comprises an n-GaN  2131  and a p-GaN  2132 ; and the epi-layer  213  is made of GaAs, aluminum gallium nitride (AlGaN), AlN, gallium indium nitride (GaInN), aluminum gallium indium nitride (AlGaInN), indium nitride (InN), gallium indium arsenic nitride (GaInAsN) or gallium indium phosphorus nitride (GaInPN). 
         [0021]    (b) Forming a p-GaN mesa  12 : The epi-layer  213  of the p-side up GaN wafer  21  is processed through a lithography and an etching to form a p-GaN mesa  22  while exposing a part of the n-GaN layer  2131  of the epi-layer  213 . 
         [0022]    (c) Forming a high reflective ohmic contact alloy layer  13 : An alloy metal layer is plated on the p-GaN mesa  22  and a thermal treatment is processed to a surface of the alloy metal layer so that a high reflective ohmic contact alloy layer  23  is formed. Therein, the alloy metal layer is made of nickel(Ni)/silver(Ag); and a trace amount of aluminum (Al) is further added into the alloy metal layer to improve a thermal stability of the high reflective ohmic contact alloy layer  23  without lowering a reflection rate of the high reflective ohmic contact alloy layer  23 . 
         [0023]    (d) Forming an n-pad  14 : The exposed part of the n-GaN layer  2131  of the p-side up GaN wafer  21  is processed through a lithography and a metal depositing process so that an n-pad  24  is formed. 
         [0024]    (e) Forming a p-pad  15 : The high reflective ohmic contact alloy layer  23  is processed through a lithography and a metal depositing process so that a p-pad  25  is formed. 
         [0025]    (f) Packaging a structure through a packaging process of flip chip  16 : A structure formed through step (a) to step (e) is packaged on a second substrate  26  through a packaging process of flip chip with a metal material  261  to connect the second substrate  26  to the structure, where the metal material  261  is gold. 
         [0026]    Thus the first embodiment of a novel method for fabricating a diode having a reflective electrode of an alloy metal is obtained. 
         [0027]    Please refer to  FIG. 3  and  FIG. 4 , which are a flow-chart view and a structural view of a second preferred embodiment. As shown in the figures, a second embodiment of the present invention comprises the following steps: 
         [0028]    (a1) Obtaining a p-side up GaN wafer  31 : A p-side up GaN wafer  21  (as referred to  FIG. 2B ) is obtained. The p-side up GaN wafer  21  comprises a first substrate  211 , a buffer layer  212  and an epi-layer  213 , where the first substrate  211  is a transparent substrate of sapphire, SiC, GaAs, LiGaO 3  or AlN; the epi-layer  213  comprises an n-GaN  2131  and a p-GaN  2132 ; and the epi-layer  213  is made of GaAs, AlGaN, AlN, GaInN, AlGaInN, InN, GaInAsN or GaInPN. 
         [0029]    (b1) Forming a high reflective ohmic contact alloy layer  32 : The epi-layer  213  of the p-side up GaN wafer  21  is plated with an alloy metal layer and a high reflective ohmic contact alloy layer  41  is formed through a thermal treatment on a surface of the alloy metal layer, where the alloy metal layer is made of Ni/A g; and a trace amount of Al is further added into the alloy metal layer to improve a thermal stability of the high reflective ohmic contact alloy layer  41  without reducing a reflection rate of the high reflective ohmic contact alloy layer  41 . 
         [0030]    (c1) Obtaining a con joining layer  33 : A conjoining layer  42  is obtained to con join the high reflective ohmic contact alloy layer  41  and a third substrate  43  having a good heat-dissipation. 
         [0031]    (d1) Lifting the first substrate and the buffer layer off the epi-layer  34 : A laser lift-off process is processed to lift the first substrate  211  and the buffer layer  212  off the epi-layer  213 . 
         [0032]    (e1) Forming an n-pad  35 : The epi-layer  213  is processed through a lithography and a metal depositing process so that an n-pad  44  is formed. 
         [0033]    Thus the second embodiment of the novel method for fabricating a diode having a reflective electrode of an alloy metal is obtained. 
         [0034]    Please refer to  FIG. 5  and  FIG. 6 , which are a flow-chart view and a structural view of a third preferred embodiment. As shown in the figures, a third embodiment of the present invention comprises the following steps: 
         [0035]    (a2) Obtaining a p-side up GaN wafer  51 : A p-side up GaN wafer  21  (as referred to  FIG. 2B ) is obtained. The p-side up GaN wafer  21  comprises a first substrate  211 , a buffer layer  212  and an epi-layer  213 , where the first substrate  211  is a transparent substrate of sapphire, SiC, GaAs, LiGaO 3  or AlN; the epi-layer  213  comprises an n-GaN  2131  and a p-GaN  2132 ; and the epi-layer  213  is made of GaAs, AlGaN, AlN, GaInN, AlGaInN, InN, GaInAsN or GaInPN. 
         [0036]    (b2) Forming a p-GaN mesa  52 : A p-GaN mesa  61  is formed through a lithography and an etching on the epi-layer  213  of the p-side up GaN wafer  21  while a part of the n-GaN layer  2131  of the epi-layer  213  is exposed out. 
         [0037]    (c2) Forming a transparency conductive layer (TCL layer)  53 : A TCL layer  62  is formed on the p-GaN mesa  61 . 
         [0038]    (d2) Forming an n-pad  54 : An n-pad  63  is formed on the exposed part of the n-GaN layer  2131  of the p-side up GaN wafer  21 . 
         [0039]    (e2) Forming a p-pad  55 : A p-pad  64  is formed through a lithography and a metal depositing process on the TCL layer  62 . 
         [0040]    (f2) Forming a high reflective ohmic contact alloy layer  56 : An alloy metal layer is plated on a bottom surface of the first substrate  211  of the p-side up GaN wafer  21  and a high reflective ohmic contact alloy layer  65  is formed through a thermal treatment on a surface of the alloy metal layer, where the alloy metal layer is made of Ni/Ag; and a trace amount of Al is further added into the alloy metal layer to improve a thermal stability of the high reflective ohmic contact alloy layer  65  without lowering a reflection rate of the high reflective ohmic contact alloy layer  65 . 
         [0041]    Thus the third embodiment of the novel method for fabricating a diode having a reflective electrode of an alloy metal is obtained. 
         [0042]    To sum up, the present invention is a method for fabricating a diode having a reflective electrode of an alloy metal, where a high reflective ohmic contact alloy layer fabricated in the present invention effectively improves a heat stability of the diode of the present invention by keeping from a metal aggregation on a pure metal layer after a thermal treatment. 
         [0043]    The preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.