Patent Document

CROSS REFERENCE  
       [0001]     The present Application is a Division of co-pending U.S. application Ser. No. 10/668,555 by the same inventors filed on Sep. 22, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a method for producing a light emitting diode and, particularly to a method for producing a light emitting diode with a permanent substrate plated beneath a mirror.  
         [0004]     2. Related Prior Arts  
         [0005]     Currently, light emitting diodes (LEDs) are one of the most important light sources. The conventional procedures for producing LEDs are primarily to epitaxy a layered light emitting structure with pn junction on a GaAs substrate. The wafer is then bonded to a transparent substrate or a substrate with a mirror at high temperature. For bonding to the transparent substrate, processing temperature above 500° C. is necessary, and therefore the epitaxial structure is easily damaged. Certainly, the yields and heat dissipation are not satisfied. As for bonding to the substrate with a mirror, the processing temperature is usually above 300° C., which also destroys the mirror and reduces reflectivity thereof.  
         [0006]     R.O.C. Patent Application No. 477,079 disclosed a method for producing a semiconductor device having a permanent metal substrate formed by means of plating or sputtering. In this patent, at least one electrode is formed after the permanent metal substrate is completed. Therefore, damage and crack of the metal substrate and the epitaxial structure occur due to obvious difference between their coefficients of thermal expansion. Moreover, a metal substrate is temporarily deposited or plated on a semiconductor structure, and then removed after the permanent substrate is formed. In practice the epitaxial structure is also damaged during removal of the temporary metal substrate. In other words, it&#39;s difficult to form electrodes on opposite sides of an LED with a metal substrate.  
         [0007]     Accordingly, it is desirable to provide an improved method for producing an LED with a plated substrate to mitigate and/or obviate the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0008]     The major object of the present invention is to provide a method for producing a light emitting diode with a plated substrate, whereby a lower cost is demanded and the product performs high brightness and better heat dissipation.  
         [0009]     The method of the present invention primarily first provides a substrate with an LED epitaxial structure thereon. The LED epitaxial structure includes a second cladding layer, an active layer, a first cladding layer, a window, and a metal contact layer sequentially formed on the substrate. This substrate can be made from GaAs, sapphire or InP. The LED epitaxial structure is preferably made from II-VI or III-V compounds with direct-bandgap.  
         [0010]     Next, the LED epitaxial structure is etched to expose the second cladding layer. A first electrode and a second electrode are then respectively formed on the metal contact layer and the exposed cladding layer. Between the LED epitaxial structure and the first electrode, a transparent conductive film can be further added to improve current spreading. After rapid thermal annealing is completed for ohmic contact of the electrodes, a temporary substrate is bonded to the LED epitaxial structure and the first electrode. Consequently, the substrate for epitaxing can be removed.  
         [0011]     To enhance brightness of the light emitting device, a mirror is formed beneath the LED epitaxial structure by means of evaporation, sputtering or ion beam sputtering. The mirror can be a metal capable of forming high bandgap with the LED epitaxial structure, or a composite of a metal with low refractivity and an insulating layer with high refractivity. The insulating layer is adjacent to the LED epitaxial structure.  
         [0012]     At last, a permanent substrate is plated beneath the mirror, and then the temporary substrate can be removed. Preferably, sawing streets of the wafer is retained without plating the substrate thereon.  
         [0013]     According to the above procedures, the light emitting diode with the plated substrate is obtained and exhibits high brightness.  
         [0014]     Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIGS. 1-6  show the procedures for producing the light emitting diode of the present invention.  
         [0016]      FIG. 7  shows the cross section of the second embodiment including a metal mirror.  
         [0017]      FIG. 8  shows the cross section of the third embodiment, in which the permanent substrate is partially plated beneath the mirror. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]      FIGS. 1-6  show the general procedures for producing the light emitting diode of the present invention. First, a GaAs substrate  19  with an LED epitaxial structure is provided. On the substrate  19 , a second cladding layer  11 , an active layer  12 , a first cladding layer  13 , a window  14 , and a metal contact layer  15  are sequentially epitaxed. According to the size of dice and position of electrodes, the metal contact layer  15 , the window  14 , the first cladding layer  13 , the active layer  12  and the upper portion of the second cladding layer  11  are partially etched to expose the second cladding layer  11 , as shown in  FIG. 1 .  
         [0019]     The LED epitaxial structure is made from II-VI or III-V compounds with direct-bandgap, for example, Ga x Al y In 1-x-y N, (Al x Ga 1-x ) y In 1-y P, In x Ga 1-x As, and ZnS x Se 1-x ; wherein 0≦x≦1, 0≦y≦1. In the preferred embodiment of the present invention, the active layer  12  is undoped (Al x Ga 1-x ) y In 1-y P with quantum well structure, the first cladding layer  13  is p-(Al x Ga 1-x ) y In 1-y P or p-GaP, and the second cladding layer  11  is n-(Al x Ga 1-x ) y In 1-y P.  
         [0020]     The first electrode  31  and the second electrode  32  are respectively formed on the metal contact layer  15  and the exposed second cladding layer  11 . The metal contact layer  15  can be further etched to remain only the portion beneath the first electrode  31 , so that the emitted light absorbed by the metal contact layer can be decreased.  
         [0021]      FIG. 2  shows the cross section of the LED in accordance with the present invention, in which a glass substrate  29  is bonded to the epitaxial layer. The glass substrate  29  is previously coated with epoxy or wax, and then attached to the wafer at 70-150° C. As this bonding procedure is performed at a low temperature, damage to the chip is prevented. Consequently, the GaAs substrate  19  is useless and can be removed by etching, as shown in  FIG. 3 .  
         [0022]     In order to further promote brightness of the LED, a mirror  25  is formed beneath the second cladding layer  11  by means of physical film deposition, as shown in  FIG. 4 . The mirror  25  in this embodiment is composed of a metal layer  251  with low refractivity and an insulating layer  252  with high refractivity. The metal layer  251  and the insulating layer  252  are respectively made from Al and Al 2 O 3 . In addition to Al/Al 2 O 3 , other composites such as Al/SiO 2 , Al/MgF 2 , Pt/Al 2 O 3 , Pt/SiO 2 , Pt/MgF 2 , Al/Al 2 O 3 , Al/SiO 2 , Al/MgF 2 , Au/Al 2 O 3 , Au/SiO 2 , Au/MgF 2 , Ag/Al 2 O 3 , Ag/SiO 2 , Ag/MgF 2  can be applied, too. As shown in  FIG. 4 , the insulating layer  252  is adjacent to the LED epitaxial structure.  
         [0023]     Next, the wafer with the mirror  25  is immersed in an electrolyte containing Cu +2   to plate a copper substrate  21  beneath the metal layer  251  through a redox reaction. The copper substrate  21  is a permanent substrate and about 30 μm thick, as shown in  FIG. 5 . Optionally, a film of catalyst such as Pd, can be coated beneath the metal layer  251  to accelerate the reaction, that is electroless copper. In the present invention, the electrolyte is not restricted, and preferably not to corrode the semiconductor device, for example, copper cyanide. After completing the metal substrate, the glass substrate  29  can be easily removed at low temperature, and the high brightness LED of the present invention is obtained.  
         [0024]     Furthermore, in order to meliorate current crowding effect and the opaque center of conventional LEDs, a transparent conductive film (not shown in drawings) such as an ITO film, can be added between the first electrode  31  and the metal contact layer  15 .  
         [0025]      FIG. 7  shows the cross section of the second embodiment, in which the composite mirror  25  is replaced with a silver mirror  26 . Alternatively, other metals or alloys such as Pt, Au, Au/Zn, Au/Be, Au/Ge, Au/Ge/Ni, In, Sn, Al, Zn, Ge, Ni can be applied, too.  
         [0026]      FIG. 8  shows the cross section of the third embodiment in accordance with the present invention. The substrate  21  is selectively plated beneath the mirror  26 . That is, sawing streets for dicing are temporarily covered without plating copper thereon.  
         [0027]     By plating the metal substrate, manufacture cost can be effectively reduced, and the production yield is promoted. Particularly, bonding at high temperature is not necessary, and reflectivity of the mirror can be reserved. For conventional procedures, the epitaxial structure is easily damaged during rapid thermal annealing due to difference between their coefficients of thermal expansion. In the present invention, the electrodes are completed before plating the metal substrate, which significantly prevents the above problem. Furthermore, the plated copper substrate also facilitates heat dissipation.

Technology Category: 5