Abstract:
A light emitting diode having an adhesive layer and manufacturing method thereof is disclosed. An adhesive layer having a thickness of about 0.1 μm to 1 μm is used to adhere an LED stack and a high heat-dissipating substrate, wherein the substrate is of a thermal conductivity greater than or equal to 100 W/mk. The present invention enhances the heat-dissipating effect of the light emitting diode so as to improve the stability and the light-emitting efficiency of the light emitting diode.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a light emitting diode having an adhesive layer and manufacturing method thereof, and more particularly, to a high heat-dissipating light emitting diode and manufacturing method thereof.  
         [0003]     2. Description of the Related Art  
         [0004]     Light emitting diodes (LED) have been widely utilized. For example, LEDs are utilized in optical display devices, traffic signs, data storing devices, communication devices, lighting devices, and medical equipment. Increasing the luminance of LEDs has therefore become an important task in the field.  
         [0005]     Disclosed in U.S. Pat. No. 10/142,954 are an LED and manufacturing method thereof, wherein An LED epitaxial structure is formed on a first light absorbing substrate, and then a dielectric adhesive layer of polymer material is utilized to connect the LED epitaxial structure to a second high thermal conductivity substrate so as to enhance the heat dissipation of the chip and the light-emitting efficiency of the LED. As disclosed in the above-mentioned patent, the epitaxial stack is grown on the first light-absorbing substrate and then the adhesive layer is utilized to adhere the epitaxial stack and the second substrate. Next, the first light-absorbing substrate is removed to reduce the thermal resistance, to improve the heat dissipation and to enhance the light-emitting efficiency. The thermal resistance of an LED device is corresponding to its thickness and thermal conductivity of compositions thereof. The relationship between them is shown in the following equation: 
 
Thermal resistance R th   =L/kA    equation (1) 
 
         [0006]     In the above-mentioned patent, the thermal resistance of the whole LED structure is equal to the thermal resistance of the sum of the LED epitaxial structure, the dielectric adhesive layer, and the substrate. Here, the thermal resistance is calculated as shown in the following equation: 
 
Device thermal resistance=LED epitaxial structure thermal resistance+dielectric adhesive layer thermal resistance+substrate thermal resistance 
 
( L/kA ) device =( L/kA ) LED epitaxial structure +( L/kA ) dielectric adhesive layer +( L/kA ) second substrate    equation (2) 
 
         [0007]     Furthermore, the thermal resistance of the LED device originally formed on the first substrate is equal to the sum of the epitaxial structure thermal resistance and the substrate thermal resistance. The relationship among them can be described by the following equation: 
 
Original device thermal resistance=epitaxial structure thermal resistance+first substrate thermal resistance 
 
( L/kA ) original device =(L/kA) LED epitaxial structure (L/kA) first substrate    equation (3) 
 
         [0008]     As shown in equation (2) and equation (3), even though the high heat-dissipating substrate is utilized, when the sum of the adhesive layer thermal resistance and the high heat-dissipating substrate thermal resistance is larger than the original first substrate thermal resistance, the heat dissipation characteristic of the high thermal conductivity substrate of the LED device is not good enough and consequently the LED device has the disadvantage of poor heat-dissipation.  
       SUMMARY OF THE INVENTION  
       [0009]     To avoid the above-mentioned disadvantage,an object of the invention is to provide a high heat-dissipating light emitting diode and manufacturing method thereof.  
         [0010]     One aspect of the present invention is to utilize the heat-dissipating characteristic of the substrate having a high thermal conductivity. Another object of the present invention is to utilize a preferable thickness of the adhesive layer and the second substrate of high thermal conductivity so as to reduce the thermal resistance of the device and improve the heat-dissipating efficiency.  
         [0011]     A traditional LED structure of the prior art comprises a first substrate on which is formed an epitaxial stack, which can be made of the material selected from the group consisting of GaAs and Ge. Assuming that the device area is A, the thermal conductivity of the first substrate is k1 (W/mk), the thickness of the first substrate is x1 μm, the thermal conductivity of the epitaxial stack is k2 (W/mk), and the thickness of the epitaxial stack is x2 μm, then the thermal resistance of the device of the first substrate is calculated by the following equation: 
 
 Rth   1   original device =( x 2/ k 2* A ) epitaxial stack +( x 1 /k 1 *A ) first substrate  
 
         [0012]     A dielectric adhesive layer of polymer materials is utilized to connect the LED epitaxial stack to the second substrate having high conductivity so as to replace the LED structure of the first substrate. This structure comprises a second substrate having a high thermal conductivity, an epitaxial stack, and a dielectric adhesive layer of polymer materials for connecting the epitaxial stack to the second substrate. Assuming that the device area is A, the thermal resistance factor of the epitaxial stack is k2 (W/mk), the thickness of the epitaxial stack is x2 μm, the thermal conductivity of the adhesive layer is k3 (W/mk), the thickness of the adhesive layer is x3 μm, the thermal conductivity of the second substrate is k4 (W/mk), and the thickness of the second substrate is x4 μm, then the thermal resistance of the device is calculated by the following equation: 
 
 Rth 2 device =( x 2 /k 2 *A ) epitaxial stack +( x 3 /k 3 *A ) adhesive layer +( x 4/ k 4 *A ) second substrate  
 
         [0013]     As mentioned above, utilizing the adhesive layer for connecting the epitaxial stack to the second substrate of high thermal conductivity to replace the first substrate reduces the thermal resistance and increases the heat-dissipating efficiency. It follows that Rth2 should be smaller than Rth1.  
         [0014]     According to an embodiment of the present invention, an LED structure comprises a second substrate having high thermal conductivity for replacing a first substrate, an epitaxial stack, and a BCB adhesive layer for connecting the epitaxial stack. Assume that the device area is A, the thermal conductivity of the epitaxial stack is 6 (W/mk), the thickness of the epitaxial stack is 3 μm, the thermal conductivity of the BCB adhesive layer is 0.2 (W/mk), the thickness of the adhesive layer is x2 μm, the thermal conductivity of the second substrate is k3 (W/mk), and the thickness of the second substrate is 170 μm. The first substrate is composed of GaAs, whose thermal conductivity is 50 (W/mk). Please note that thermal conductivities of normal materials and organic materials of LEDs can be seen in table 1 and table 2:  
                             TABLE 1                           a table of the thermal conductivities of normal materials of LEDs                Material   Thermal conductivity (W/mk)                       GaAs   44-58           Al 0.5 Ga 0.5 As   11           (Al 0.5 Ga 0.5 )In 0.5 P   6           Ga 0.5 In 0.5 P   5           GaP   75-100           Sapphire   35-40           GaN   a.130 b.170           Si   125-150           SiC   270           Copper   393           Silver   418           Gold   297           Aluminum   240           Au-Sn(80-20)   57           In   81.8-86           Aluminum Nitride   a.170-200 b.285           SiO 2     1.5           Glass   0.8           Al 2 O 3     10-35                      
 
         [0015]     References:  
         [0016]     (1) R. R. Tummala and E. J. Rymaszewski: “Microelectronics Packaging Handbook” (van Nostrand Reinhold, 1988)  
         [0017]     (2) G. Slack, J. Phys. Chem. Solids 34,321(1973)  
         [0018]     (3) P. D. Maycock, Thermal Conductivity of Silicon, Germanium, III-V Compounds and III-V Alloys, Solid-State Electronics, vol. 10. pp 161-168, 1967s  
         [0019]     (4) http://hyperphysics.phy-astr.gsu.edu/hbase/mecref.html#c1  
                                           TABLE 2                           a table of the thermal conductivities of organic materials of LEDs                Material   Thermal conductivity (W/mk)                            Epoxy-Kevlar(x-y) (60%)   0.2           Polymide-Quartz(z-axis)   0.35           Polymide   0.2           Fr-4(x-y plane)   0.2           Benzocyclobutene   0.2           Teflon( ™DuPont Co.)   0.1                      
 
         [0020]     References:  
         [0021]     (1) R. R. Tummala and E. J. Rymaszewski: “Microelectronics Packaging Handbook” (van Nostrand Reinhold, 1988)  
         [0022]     Utilizing the second substrate of high thermal conductivity to replace the first substrate can increase the heat-dissipating efficiency only if the above-mentioned Rth2 is less than the original thermal resistance Rth1 of the GaAs first substrate. In view of this statement, the relationship between the Rth1 and Rth2 can be determined by the following equations: 
 
 Rth 1 original device =(3/6* A ) epitaxial stack +(170/50 *A ) first substrate  
 
 Rth 2 device =(3/6 *A ) epitaxial stack +( x 3/0.2 *A ) adhesive layer +(170/ k 4* A ) second substrate  
 
 Rth 2 device   −Rth 1 original device &lt;0 
 
         [0023]     Therefore, we can obtain the following relationship: 
 
x3/0.2+170/k4&lt;3.4 
 
         [0024]     Table 3 shows the optimum thickness of the adhesive layer of different types of substrates having high thermal conductivity to replace the GaAs substrate, wherein the adhesive layer is made of BCB.  
                                                 TABLE 3                                       Thermal                   conductivity (k4) of   Thickness (x3) of           Material of the   the second   BCB adhesive layer           second substrate   substrate (W/mk)   (μm)                                        Sapphire   35   &lt;0              GaP   100   &lt;0.34            GaN   130   &lt;0.418           Si   150   &lt;0.453           Aluminum (Al)   240   &lt;0.538           SiC   270   &lt;0.554           Gold   297   &lt;0.556           Copper (Cu)   393   &lt;0.593           Silver(Ag)   418   &lt;0.599                      
 
         [0025]     An LED structure utilizing an adhesive layer to connect an LED epitaxial stack to a second substrate in order to replace the first substrate is disclosed. The relationship between the thermal conductivity of the second substrate and the thickness of the adhesive layer is shown in  FIG. 1 . As shown in  FIG. 1 , the condition of increasing the heat-dissipating efficiency is that the thickness of the adhesive layer has to be less than 0.5 μm. But because the surface of the epitaxial stack is not so flat, the epitaxial stack has thickness differences in itself. Referring to  FIGS. 2, 3 , in the real implementation, the BCB adhesive layer is utilized to connect the epitaxial stack to the Si substrate of high thermal conductivity in order to replace the original GaAs substrate. Furthermore, if the thickness of the adhesive layer is less than or equal to 1 μm, the LED structure can have better heat-dissipating efficiency and light-emitting efficiency than that of the prior art LED structure having a GaAs substrate. But if the thickness of the adhesive layer is less than 0.1 μm, the connection yield is very low. Therefore, the optimum thickness of the adhesive layer is between 0.1 μm and 1 μm. Furthermore, a reaction layer can be formed between the adhesive layer and the second substrate or between the adhesive layer and the epitaxial stack to increase the adhesive force so that the connection yield can be raised.  
         [0026]     The above-mentioned LED structure is produced through the following steps. First, the adhesive layer is utilized to connect the LED epitaxial stack to the second substrate of high thermal conductivity. Secondly, the connected LED epitaxial stack, the adhesive layer, and the second substrate are placed between two graphite plates. Thirdly, the two graphite plates are heated and pressured to increase the adhesive force. The graphite characteristics of good heat radiation and soft quality can be well utilized to form an adhesive layer having an even thickness.  
         [0027]     The inventor of the present invention obtains the same result in an experiment. According to the experimental data, in this experiment, the thermal conductivity of the second substrate is larger than 100 W/mk, the thickness BCB adhesive layer is less than or equal to 1 μm, and indeed the heat-dissipating efficiency is better than the prior art GaAs substrate. Furthermore, in this experiment, the thickness of BCB is between 0.5 μm and 0.8 μm. The experiment result is shown in Table 4. In another experiment, the same Si substrate is utilized, but a BCB adhesive layer having different thickness is formed. The experiment result is shown in Table 5.  
                               TABLE 4                                       Thermal                   conductivity of the   12 mil LED               second substrate   saturation current           LED structure   (μm)   (mA)                           EPI/GaAs   44-58   160-180           EPI/BCB/Si   125-150   180-200           EPI/BCB/SiC   270   180/220           EPI/BCB/Sapphire   35-40   100-120           EPI/BCB/Glass   0.8   50-60                      
 
         [0028]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                   
               
               
                   
                 Thickness of the 
                 12 mil LED 
               
               
                   
                 BCB adhesive 
                 saturation 
               
               
                 LED structure 
                 layer (W/mk) 
                 current (mA) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 EPI/BCB/Si 
                 10 
                 60 ˜ 80 
               
               
                   
                 5 
                 About 120 
               
               
                   
                 3 
                 About 140 
               
               
                   
                 1 
                 About 200 
               
               
                   
                 0.5 
                 200 ˜ 210 
               
               
                   
               
             
          
         
       
     
         [0029]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a diagram of a relationship between thermal conductivity of a substrate and thickness of an adhesive layer.  
         [0031]      FIG. 2  and  FIG. 3  are three-dimensional diagrams illustrating a flat degree of an epitaxial stack surface.  
         [0032]      FIG. 4  is a diagram of a preferred embodiment of an LED structure according to the present invention.  
         [0033]      FIG. 5  is a diagram of a first layer structure before connection when producing the LED structure shown in  FIG. 4 .  
         [0034]      FIG. 6  is a diagram illustrating production of the adhesive layer having 0.1 μm-1 μm thickness when producing the LED structure shown in  FIG. 4 .  
         [0035]      FIG. 7  is a diagram of a second layer structure after connecting the first layer to the second substrate but before removing the first substrate.  
         [0036]      FIG. 8  is a diagram of a third layer structure after removing the first substrate when producing the LED structure shown in  FIG. 4 .  
         [0037]      FIG. 9  is a diagram of another embodiment of an LED structure according to the present invention.  
         [0038]      FIG. 10  is a diagram of a fourth layer structure before connection when producing the LED structure shown in  FIG. 9 .  
         [0039]      FIG. 11  is a diagram of a fifth layer structure before connection when producing the LED structure shown in  FIG. 9 .  
         [0040]      FIG. 12  is a diagram of a sixth layer structure after connecting the fourth layer and the fifth layer but before removing the first substrate when producing the LED structure shown in  FIG. 9 .  
         [0041]      FIG. 13  is a diagram of a seventh layer structure after removing the first substrate when producing the LED structure shown in  FIG. 9 .  
         [0042]      FIG. 14  is a diagram of the other embodiment of an LED structure according to the present invention. 
     
    
     DETAILED DESCRIPTION  
     Embodiment 1  
       [0043]     Referring to  FIG. 4 , the LED  1   a  comprises a second substrate  10  having high thermal conductivity, an adhesive layer  11  having a 0.1 μm-1 μm thickness formed on the second substrate  10 , a first protection layer  12  formed on the adhesive layer  11 , a reflection layer  13  formed on the first protection layer  12 , a second protection layer  14  formed on the reflection layer  13 , a first contact layer  15  formed on the second protection layer  14 , wherein the upper surface of the first contact layer  15  comprises a first surface area and a second surface area. The LED  1  a further comprises a first cladding layer  16  formed on the first surface area, a light-emitting layer  17  formed on the first cladding layer  16 ,a second cladding layer  18  formed on the light-emitting layer  17 , a second contact layer  19  formed on the second cladding layer  19 , a first wiring electrode  9  formed on the second cladding layer  19 , and a second wiring electrode  9  formed on the second surface area.  
         [0044]     Referring to  FIGS. 4, 5 , and  6 , the LED  1  a is produced by the following steps:  
         [0045]     Step  100 : Sequentially form an etching termination layer  20 , a second contacting layer  19 , a second cladding layer  18 , a light-emitting layer  17 , a first cladding layer  16 , a first contact layer  15 , a second protection layer  14 , a reflection layer  13 , and a first protection layer  12  on a first substrate  21  to form a first stacking layer  2   a  as shown in  FIG. 5 .  
         [0046]     Step  110 : Select an adhesive layer  11 , and utilize the adhesive layer to connect the protection layer  12  of the first stacking layer  2   a  to a first surface of the second substrate  10  having high thermal conductivity.  
         [0047]     Step  120 : Place a first graphite plate  5  on a second surface of the second substrate  10 , and place a second graphite plate  6  on the first substrate  21  of the first stacking layer  2   a  as shown in  FIG. 6 .  
         [0048]     Step  130 : Heat and pressure the first graphite plate  5  and the second graphite plate  6  for a specific time to form an adhesive  11  having even thickness 0.1 μm-1 μm and form a second stacking layer  3   a  as shown in  FIG. 7 .  
         [0049]     Step  140 : Remove the first substrate  21  and the etching termination layer  20  to form a third stacking layer  4   a  as shown in  FIG. 8 .  
         [0050]     Step  150 : Appropriately etching the third laminated layer  4   a  until the first contacting layer  15  forms an exposed surface of the first contact layer  15 , and respectively form the first wire electrode  9  and the second wire electrode  8  on the exposed surface of the first contact layer  15 .  
       Embodiment  2   
       [0051]     Referring to  FIG. 9 , the LED  5   a  comprises a second substrate  110  having high thermal conductivity, a reflection layer  111  formed on the second substrate  110 , a first reaction layer  112  formed on the reflection layer  111 , an adhesive layer  113  whose thickness is between 0.1 μm-1 μm formed on the first reaction layer  112 , a second reaction layer  114  formed on the adhesive layer  113 , a transparent conductive layer  115  formed on the second layer  114 , wherein the upper surface of the transparent conductive layer  115  comprises a first surface area and a second surface area.  
         [0052]     The LED  5   a  further comprises a first contact layer  116  formed on the first surface area, a first cladding layer  117  formed on the first contact layer  116 , a light-emitting layer  118  formed on the first cladding layer  117 , a second cladding layer  119  formed on the light-emitting layer  118 , a second contact layer  120  formed on the second cladding layer  119 , a first wire electrode  9  formed on the second contact layer  120 , and a second wire electrode  8  formed on the second surface area.  
         [0053]     Referring to  FIGS. 9,10 , and  11 , the LED  5   a  is produced through the following steps:  
         [0054]     Step  200 : Sequentially form a second contact layer  120 , a second cladding layer  119 , a light-emitting layer  118 , a first cladding layer  117 , a first contact layer  116 , a transparent layer  115 , and a second reaction layer  114  on a first substrate  121  to form a fourth stacking layer  6   a  as shown in  FIG. 10 .  
         [0055]     Step  210 : A reflection layer  111  is formed on a second substrate  110  having high thermal conductivity, and a first reaction layer  112  is formed on the reflection layer  111  to form a fifth stacking layer  7   a  as shown in  FIG. 11 .  
         [0056]     Step  220 : An adhesive layer  113  is utilized to connect the first reaction layer  114  of the fourth stacking layer  6   a  to the first reaction layer  112  of the fifth layer  7   a.    
         [0057]     Step  230 : The procedure of the adhesive connection is the same as that of the above-mentioned embodiment 1. As shown in  FIG. 6 , a stacking layer  6   a  replaces the stacking layer  2   a,  an adhesive layer  113  with a thickness between 0.1 μm-1 μm replace the adhesive  11 , and a stacking layer  7   a  replace the second substrate  10 . Then, a sixth stacking layer  8   a  is formed. Next, the first substrate  121  is removed to form a seventh stacking layer  9   a,  as shown in  FIG. 13 . Step  230 : Appropriately etch the seventh stacking layer  9   a  and stop at the transparent conductive layer  115  to form an exposed surface area of the transparent conductive layer  115 . Respectively form a first wire electrode  9  and a second wire electrode  8  on the exposed surface area of the transparent conductive layer  115 .  
       Embodiment  3   
       [0058]     Referring to  FIG. 14 , which shows the other embodiment of an LED  10   a  having an adhesive layer according to the present invention, the structure and the production procedure are similar to the LED  5   a  of the embodiment 2. The difference of the present embodiment is that a transparent conductive layer  122  is formed on the second contact layer  120  in order to improve the current distribution efficiency.  
         [0059]     The above-mentioned first substrate is made of the material selected from the group consisting of GaAs, Ge, and Sapphire.  
         [0060]     The above-mentioned second substrate whose thermal conductivity is larger than 100 W/mk is made of the material selected from the group consisting of GaP, Si chip, SiC, Cu chip, Al chip and other replaceable materials.  
         [0061]     The above-mentioned adhesive layer whose thickness is between 0.1 μm-1 μm is made of the material selected from the group consisting of Pl, BCB, and PFCB.  
         [0062]     The above-mentioned first contacting layer is made of the material selected from the group consisting of GaP, GaAs, GaAsP, InGaP, AlGaInP, AlGaAs, GaN, InGaN, and AlGaN.  
         [0063]     The above-mentioned cladding layer is made of the material selected from the group consisting of AlGaInP, AlInP, AlN, GaN, AlGaN, InGaN, and AlGaInN.  
         [0064]     The above-mentioned light-emitting layer is made of the material selected from the group consisting of AlGaInP, InGaP, GaN, AlGaN, InGaN, and AlGaInN.  
         [0065]     The above-mentioned second cladding layer is made of the material selected from the group consisting of AlGaInP, AlInP, AlN, GaN, AlGaN, InGaN, and AlGaInN.  
         [0066]     The above-mentioned second contacting layer is made of the material selected from the group consisting of GaP, GaAs, GaAsP, InGaP, AlGaInP, AlGaAs, GaN, InGaN, and AlGaN.  
         [0067]     The above-mentioned reflection layer is made of the material selected from the group consisting of In, Sn, Al, Au, Pt, Zn, Ag, Ti, Pb, Pd, Ge, Cu, AuBe, AuGe, Ni, PbSn, and AuZn.  
         [0068]     The above-mentioned first protecting layer is made of the material selected from the group consisting of Silicon Nitride, Silicon Dioxide, Aluminum Oxide, Magnesium Oxide, Zinc Oxide, Tin Oxide, Indium Oxide, and Tin Indium Oxide.  
         [0069]     The above-mentioned second protecting layer is made of the material selected from the group consisting of Silicon Nitride, Silicon Dioxide, Aluminum Oxide, Magnesium Oxide, Zinc Oxide, Tin Oxide, Indium Oxide, and Tin Indium Oxide.  
         [0070]     The above-mentioned first reaction layer is made of the material selected from the group consisting of SiNx, Ti, and Cr.  
         [0071]     The above-mentioned second reaction layer is made of the material selected from the group consisting of SiNx, Ti, and Cr.  
         [0072]     And the above-mentioned transparent conductive layer is made of the material selected from the group consisting of Tin Indium Oxide, tin Cadmium Oxide, Tin antimony Oxide, Zinc Oxide, and Tin Zinc Oxide.  
         [0073]     Those skilled in the art can readily understand that numerous modifications and alterations of the device and method in accordance with the invention may be made within the spirit and claims of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.