Patent Publication Number: US-7713776-B1

Title: Method of making a light emitting diode

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method of making a solid-state light emitting device, more particularly to a method of making a light emitting diode. 
     2. Description of the Related Art 
     Referring to  FIG. 1 , a conventional vertically structured light emitting diode (LED)  1  includes an epitaxial layer  11  that generates light when electricity is supplied, a reflective layer  12  that is formed on a bottom surface  116  of the epitaxial layer  11  to reflect light, an electrode  13  that is disposed on a top surface  115  of the epitaxial layer  11 , and a substrate  14  that is connected to the reflective layer  12 . 
     The epitaxial layer  11  is formed by epitaxying a gallium nitride series semiconductor material on an epitaxial substrate (not shown), and includes p-type and n-type cladding layers  111 , 112  that are formed via doping, and an active layer  113  that is formed between the p-type and n-type cladding layers  111 , 112 . A band gap exists between the p-type and n-type cladding layers  111 , 112 . When electricity is supplied to the epitaxial layer  11 , recombination of electron-hole pairs occurs in the active layer  113 , thereby releasing energy in a form of light. The top surface  115  of the epitaxial layer  11  is roughened so as to prevent full reflection of the light produced from the epitaxial layer  11 . As a result, the light can be directly emitted out of the LED  1  to the utmost through the top surface  115 , thereby enhancing light emitting efficiency of the LED  1 . 
     The reflective layer  12  is made of a material that has high reflectivity. Generally, the reflective layer  12  is made of a metal that has high reflection coefficient and is disposed in ohmic contact with the epitaxial layer  11  for electrical conduction. When the light generated from the epitaxial layer  11  is transmitted to the bottom surface  116 , the reflective layer  12  is capable of reflecting the light back to the top surface  115 , thereby emitting the light out of the LED  1  and improving the light emitting efficiency of the same. 
     The substrate  14  is made of a thermally and electrically conductive material that is normally an alloy or a metal, is able to support the reflective layer  12  and the epitaxial layer  11 , and serves as another electrode relative to the electrode  13 . Furthermore, when the electricity is supplied to the epitaxial layer  11 , the substrate  14  is capable of dissipating waste heat that is generated by the epitaxial layer  11  and that is subsequently transferred to the reflective layer  12 . Therefore, the epitaxial layer  11  in use is maintained at a temperature that is unable to influence radiative recombination efficiency. 
     When the electricity is supplied to the epitaxial layer  11  by virtue of the electrode  13  and the substrate  14 , current passes through the p-type and n-type cladding layers  111 , 112 , and the active layer  113 . Consequently, the light is generated due to the recombination of the electron-hole pairs. One portion of the light is directly transmitted to the top surface  115  and is able to pass through the top surface  115  with a higher chance due to roughness of the same. Another portion of the light is transmitted to the bottom surface  116 , is reflected to the top surface  115  via the reflective layer  12 , and can pass through the top surface  115  with a greater possibility on account of the roughness of the same as well. 
     During production of the LED  1 , the reflective layer  12  is coated on the bottom surface  116  of the epitaxial layer  11  that is still connected to the epitaxial substrate (not shown), and is bonded to the epitaxial layer  11  by virtue of a wafer bonding process. The substrate  14  is bonded to the reflective layer  12  through wafer bonding as well. Afterward, the epitaxial substrate (not shown) is removed from the n-type cladding layer  112  so as to expose and roughen the top surface  115  of the epitaxial layer  11 . 
     However, bonding between the epitaxial layer  11  and the reflective layer  12 , and between the reflective layer  12  and the substrate  14  is not strong enough since only wafer bonding is conducted, thereby lowering a production yield of the LED  1 . Furthermore, the reflective layer  12  is bonded to the epitaxial layer  11  and is disposed in ohmic contact with the same through the wafer bonding process performed at a temperature that ranges from 200° C. to 400° C. Thus, quality of the reflective layer  12  may be degraded on account of the high temperature (200° C. to 400° C.) such that reflectivity of the reflective layer  12  may be influenced. The light emitting efficiency of the LED  1  may be further lowered. 
     For increasing the production yield of the LED  1 , the bottom surface  116  of the epitaxial layer  11  is required to be sufficiently flat so as to be tightly bonded to the reflective layer  12 . Consequently, only one surface (i.e., the top surface  115 ) of the epitaxial layer  11  is roughened. External quantum efficiency of the LED  1  needs to be improved further. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method of making a light emitting diode for overcoming the aforesaid drawbacks of the prior art. 
     According to this invention, a method of making a light emitting diode comprises the steps of: (a) forming a plurality of electrically conductive members at intervals on a first surface of an epitaxial layer which generates light so that the electrically conductive members are in ohmic contact with the epitaxial layer; (b) forming a light incident layer on the first surface at regions where none of the electrically conductive members are formed; (c) forming a light reflecting layer on the light incident layer and the electrically conductive members; (d) providing a permanent substrate; (e) providing an adhesive; and (f) disposing the permanent substrate on the light reflecting layer, and bonding the permanent substrate to the light reflecting layer. 
     The light incident layer is higher than the electrically conductive members. The light reflecting layer has indented parts where the light reflecting layer overlies the electrically conductive members, and non-indented parts where the light reflecting layer overlies the light incident layer. The adhesive is provided in the indented parts of the light reflecting layer. The permanent substrate is bonded to the indented parts through the adhesive and to the non-indented parts through wafer-bonding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which; 
         FIG. 1  is a schematic view of a conventional vertically structured light emitting diode; 
         FIG. 2  is a flow chart of the first preferred embodiment of a method of making a light emitting diode according to this invention; 
         FIG. 3  is a schematic partly sectional view to illustrate the light emitting diode made according to the method of the first preferred embodiment; 
         FIG. 4  is a schematic view to illustrate a first surface of an epitaxial layer, which is roughened according to the method of the first preferred embodiment; 
         FIG. 5  is a schematic partly sectional view to illustrate electrically conductive members that are formed on the roughened first surface according to the method of the first preferred embodiment; 
         FIG. 6  is a schematic partly sectional view to illustrate a light incident layer that is formed on the roughened first surface according to the method of the first preferred embodiment; 
         FIG. 7  is a schematic partly sectional view to illustrate alight reflecting layer that is formed on the light incident layer and the electrically conductive members according to the method of the first preferred embodiment; 
         FIG. 8  is a schematic partly sectional view to illustrate an adhesive that is provided in indented parts of the light reflecting layer according to the method of the first preferred embodiment; 
         FIG. 9  is a schematic partly sectional view to illustrate a permanent substrate that is disposed on the light reflecting layer according to the method of the first preferred embodiment; 
         FIG. 10  is a flow chart of the second preferred embodiment of the method of making a light emitting diode according to this invention; 
         FIG. 11  is a schematic view to illustrate an epitaxial layer that is epitaxied on an epitaxial substrate according to the method of the second preferred embodiment; 
         FIG. 12  is a schematic partly sectional view to illustrate an electrode that is formed on a roughened second surface of the epitaxial layer according to the method of the second preferred embodiment; 
         FIG. 13  is a schematic partly sectional view to illustrate a temporary substrate that is connected detachably to the roughened second surface of the epitaxial layer through wax according to the method of the second preferred embodiment; and 
         FIG. 14  is a schematic partly sectional view to illustrate the epitaxial substrate that is removed from a first surface of the epitaxial layer according to the method of the second preferred embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the present invention is described in greater detail, it should be noted that the same reference numerals have been used to denote like elements throughout the specification. 
     Referring to  FIG. 2 , according to the present invention, the first preferred embodiment of a method of making a light emitting diode includes steps  201  to  208 . 
     Referring to  FIG. 3 , the first preferred embodiment of the method is conducted to produce a light emitting diode (LED)  3  that has a vertical structure and that includes an epitaxial layer  31 , an electrode  32 , a plurality of electrically conductive members  331 , a light incident layer  332 , a light reflecting layer  333 , an adhesive  34 , and an electrically conductive permanent substrate  35 . 
     The epitaxial layer  31  generates light when electricity is supplied, is made from a gallium nitride series semiconductor material, has a first surface  315  and a second surface  316  that is opposite to the first surface  315 , and includes a first cladding layer  311  that has the second surface  316 , a second cladding layer  312  that has the first surface  315 , an active layer  313  that is formed between the first and second cladding layers  311 , 312 , and a current diffusion layer  314  that is made of a transparent and electrically conductive material and that is formed on the first cladding layer  311 . The electrode  32  is disposed on the second surface  316  of the epitaxial layer  31  and in ohmic contact with the epitaxial layer  31 . 
     The first and second cladding layers  311 , 312  are formed through doping. A band gap exists between the first and second cladding layers  311 , 312 . When the electricity is supplied to the epitaxial layer  31 , recombination of electron-hole pairs occurs in the active layer  313 , thereby releasing energy in a form of light. The current diffusion layer  314  is able to uniformly and transversely diffuse current so as to enhance internal quantum efficiency. The first and second surfaces  315 , 316  of the epitaxial layer  31  are roughened such that reflection of the light generated by the epitaxial layer  31  is prevented. Accordingly, possibility that the light can pass through the first and second surfaces  315 , 316  is enhanced. 
     The electrically conductive members  331  are formed on the first surface  315  of the epitaxial layer  31  in ohmic contact with the epitaxial layer  31 , are spaced apart from each other, and are substantially equal in height. 
     The light incident layer  332  is formed on the first surface  315  of the epitaxial layer  31  at regions where none of the electrically conductive members  331  are formed, and has a refractive index lower than a refractive index of the epitaxial layer  31 , and a height larger than a height of each of the electrically conductive members  331 . The light incident layer  332  can be made from silicon oxide. 
     The light reflecting layer  333  is made of a highly reflective and electrically conductive material such as aluminum, and is formed on the light incident layer  332  and the electrically conductive members  331 . The light reflecting layer  333  has a uniform thickness and includes indented parts  3331  where the light reflecting layer  333  overlies the electrically conductive members  331 , and non-indented parts  3332  where the light reflecting layer  333  overlies the light incident layer  332 . Preferably, the light reflecting layer  333  is a layer of a metal or an alloy in order to have higher reflectivity and to facilitate electrical connection with the permanent substrate  35  through direct contact with the permanent substrate  35 . 
     The adhesive  34  is disposed in the indented parts  3331  of the light reflecting layer  333 , is in liquid form at a temperature of a wafer bonding process, and has a bottom side that is flush with a bottom side of the non-indented parts  3332  of the light reflecting layer  333 , and that cooperates with the same to define a plane. The adhesive  34  may be made from a highly transparent heat-curable silicone die-bond material. The adhesive  34  in this embodiment is a product (Model No. KER-3000-M2) of Shin-Etsu Silicone Taiwan Co., Ltd. 
     The permanent substrate  35  is formed on the light reflecting layer  333  and the adhesive  34 , is bonded to the light reflecting layer  333  through the adhesive  34  and through wafer bonding, and includes a base layer  351  that is made from a thermally conductive material, and a top layer  352  that is formed on the base layer  351 , that is bonded to the light reflecting layer  333  and the adhesive  34 , and that is made from an electrically conductive material so as to serve as a second electrode relative to the electrode  32 . Specifically, the top layer  352  is bonded to the non-indented parts  3332  of the reflecting layer  333  by dint of wafer bonding and to the indented parts  3331  through adhesion of the adhesive  34 . Preferably, the base layer  351  is made from a material that is both electrically and thermally conductive. In this embodiment, the top layer  352  is made from a gold/tin alloy. 
     When electricity is supplied to the LED  3  through the electrode  32  and the permanent substrate  35 , current passes through the current diffusion layer  319  and is evenly spread. Afterward, the current flows through the first and second cladding layers  311 , 312 , and the active layer  313 . Light is generated due to the recombination of the electron-hole pairs. A first portion of the light is directly transmitted towards the second surface  316  and is able to pass through the second surface  316  with a great possibility due to roughness of the same. A second portion of the light is transmitted towards the first surface  315 . One part of the second portion of the light is reflected from the electrically conductive members  331 , and moves towards and passes through the second surface  316 . Another part of the second portion of the light enters the light incident layer  332  whose refractive index is lower than that of the epitaxial layer  31 , is reflected from the light reflecting layer  333  to the first surface  315 , enters the epitaxial layer  31  with a great chance due to roughness of the first surface  315 , and is able to pass through the second surface  316  with a large possibility on account of the roughness of the same. Therefore, the light emitting efficiency of the LED  3  is enhanced. 
     The method of the first preferred embodiment is described as follows. Referring to  FIGS. 2 and 4 , in step  200 , the epitaxial layer  31  is connected to a temporary substrate  51  so that the temporary substrate  51  is bonded to the second surface  316  of the epitaxial layer  31  opposite to the first surface  315 . Subsequently, the first surface  315  of the epitaxial layer  31  is roughened in step  201 . 
     Referring to  FIGS. 2 and 5 , in step  202 , the electrically conductive members  331  are formed at intervals on the roughened first surface  315  of the epitaxial layer  31  so that the electrically conductive members  331  are in ohmic contact with the epitaxial layer  31 . In this embodiment, the electrically conductive members  331  are made of a titanium/aluminum alloy. 
     Referring to  FIGS. 2 and 6 , in step  203 , the light incident layer  332  is formed on the roughened first surface  315  at regions where none of the electrically conductive members  331  are formed. In this embodiment, the light incident layer  332  is made from silicon oxide. 
     Referring to  FIGS. 2 and 7 , in step  204 , the light reflecting layer  333  is formed on the light incident layer  332  and the electrically conductive members  331 . Meanwhile, in step  205 , the permanent substrate  35  (see  FIG. 9 ) is provided. In this embodiment, the light reflecting layer  333  is composed of one layer of aluminum and one layer of a gold/tin alloy. 
     Referring to  FIGS. 2 and 8 , in step  206 , the adhesive  34  in liquid form is provided on the light reflecting layer  333  during the temperature of the wafer bonding process, which normally ranges from 180° C. to 200° C. The adhesive  34  is electrically conductive and is a thermosetting adhesive that is in liquid form at a temperature ranging from 180° C. to 200° C. 
     Referring to  FIGS. 2 and 9 , the permanent substrate  35  is provided in step  205 . In step  207 , the top layer  352  of the permanent substrate  35  is disposed on the light reflecting layer  333  at the temperature (180° C. to 200° C.) of the wafer bonding process. As the adhesive  34  between the top layer  352  and the light reflecting layer  333  is partially squeezed out during the wafer bonding process, the adhesive  34  is almost not left on the non-indented parts  3332  of the light reflecting layer  333 . Therefore, the top layer  352  is bonded to the non-indented parts  3332  via wafer-bonding and to the indented parts  3331  via the adhesive  34 . After the wafer bonding process, a cooling process is performed to achieve a room temperature. In step  208 , the temporary substrate  51  is removed from the epitaxial layer  31  so that the second surface  316  of the epitaxial layer  31  is exposed. 
     As described above, the permanent substrate  35  which is used as the electrode is bonded to the light reflecting layer  333  by employing an adhesive bonding in addition to the wafer-bonding, thereby providing a higher bonding strength between the permanent substrate  35  and the light reflecting layer  333 . Therefore, the present invention eliminates the problem of insufficient bonding and poor electrical connection encountered in the prior art in which the substrate  14  is connected to the reflective layer  12  only through wafer bonding. 
     In addition, the light reflecting layer  333  is not in ohmic contact with the epitaxial layer  31  according to the present invention. Because the electrically conductive members  331  are formed in ohmic contact with the epitaxial layer  31  by virtue of a high temperature process that employs a temperature higher than the temperature (180° C. to 200° C.) of the wafer bonding process before the light incident layer  332  and the reflecting layer  333  are formed, and because the high temperature process for ohmic contact is not needed when the light incident layer  332  and the light reflecting layer  333  are formed, the quality degradation problem suffered by the reflective layer  12  in the prior art due to the high temperature process can be eliminated. 
     The second preferred embodiment of the method according to the present invention is conducted to make the LED  3 . 
     Referring to  FIG. 10 , the method of the second preferred embodiment includes steps ( 41  to  45 ) in place of step  200  (see  FIG. 2 ) of the first preferred embodiment. Only steps  41  to  45  are shown in  FIG. 10 . 
     Referring to  FIGS. 10 and 11 , in step  41 , an epitaxial substrate  54 , which is a sapphire substrate suitable for the gallium nitride series semiconductor material, is used so that the epitaxial layer  31  is grown on the epitaxial substrate  54 . Thus, the first surface  315  of the epitaxial layer  31  is connected to the epitaxial substrate  54 , and the second surface  316  of the epitaxial layer  31  is exposed. After step  41 , the second surface  316  is roughened in step  42 . 
     Referring to  FIGS. 10 and 12 , in step  43 , the electrode  32  is formed in ohmic contact with the second surface  316  of the epitaxial layer  31 . 
     Referring to  FIGS. 10 and 13 , in step  44 , the temporary substrate  51  is connected detachably to the second surface  316  by disposing wax  53  between the temporary substrate  51  and the second surface  316 . 
     Referring to  FIGS. 10 and 14 , in step  45 , the epitaxial substrate  54  is removed from the first surface  315  of the epitaxial layer  31  until the first surface  315  is exposed. Subsequently, steps  201  to  208  (see  FIG. 2 ) are performed. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.