Abstract:
The present invention relates to a method for forming LED. In the present invention, LED dies are defined by photolithography and etching processes to replace a cutting step, and a metal substrate of the LED is formed by chemical or physical method.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to a method for forming LED (light emitting diode), and more particularly to a method for forming LED by a chemical method or a physical method.  
       BACKGROUND  
       [0002]     Basically, the principle of LED is to employ a characteristic of semiconductor to emit light. This is different from the conventional lighting apparatuses that lighting by discharging or heating, so the LED is called “cold light.” Compared with the conventional the light bulb or the fluorescent tube, the LED has advantages of high durability, long life, light, low power consumption, and mercury-free.  
         [0003]     The basic structure of LED is a PN diode structure that comprises a P-type epitaxial layer, an N-type epitaxial layer, and an active layer there between. In general, a current is input by two pads on the epitaxial layers of the LED. The pads may be formed on the same side or opposite sides according to the material selection of substrate and epitaxial layer, as shown in  FIG. 1A  and  FIG. 1B . For example of GaN-based LED, the common substrate is an unconductive sapphire (Al 2 O 3 ) or a conductive SiC.  FIG. 1A  shows a structure of a GaN LED with a sapphire substrate  40 . An N-type epitaxial layer  30 , an active layer  20 , and a P-type epitaxial layer  10  are on the substrate  40  in sequence. Pads  25  and  15  are formed on an exposed region of the N-type epitaxial layer  30  and the P-type epitaxial layer  10 , respectively.  FIG. 1B  shows a structure of a GaN LED with a SiC substrate  50 . An N-type epitaxial layer  30 , an active layer  20 , and a P-type epitaxial layer  10  are on the substrate  50  in sequence. Pads  15  and  25  are formed on an upper surface of the P-type epitaxial layer  10  and a lower surface of the substrate  50 , respectively.  
         [0004]     Due to the light-absorbing and light-covering problems of the pads  15  and  25 , some manufacturers produce LEDs by Flip-Chip technology for improving the illumination of LEDs. As shown in  FIG. 1C , the LED structure is a reverse structure of that shown in  FIG. 1A . The structure has a P-type epitaxial layer  10 , an active layer  20 , an N-type epitaxial layer  30 , and a substrate  40  in sequence from bottom to top, and light is emitted upwards from the side of substrate  40 . The substrate  40  is transparent, so it has no problem of light-absorbing or light-covering. The flip-chip LED is generally adhered to a submount  60 , and a reflecting layer is formed to reflect the downward light emitted by the LED upwards. Furthermore, the submount  60  may be made of a material (as metal) having good cooling effect to diffuse the heat of LED, so it is proper to operate under high current. Hence, the LED having flip-chip structure improves the illumination and cooling effect.  
         [0005]     Although the illumination of flip-chip LED is substantially increased and the flip-chip LED is proper to operate under high current, the product yield thereof is not well. That is resulted from the conventional Flip-Chip technology is to connect the pads of LED with the submount by bonding, and the bonding yield is not well. Hence, the yield of flip-chip LED is not efficiently increased, and the product cost is higher.  
       SUMMARY  
       [0006]     In those conventional arts, the Flip-Chip technology for LED has some drawbacks and problems. Therefore, one of objectives of the present invention is to provide a method for forming LED to increase the LED yield.  
         [0007]     Another objective of present invention is to form a substrate of LED by chemical or physical method for increasing the conductivity and cooling effect.  
         [0008]     Still another objective of present invention is to form a reflecting layer for increasing the illumination of LED.  
         [0009]     As aforementioned, the present invention provides a method for forming LED comprising the following steps. First, an LED epitaxial layer is formed on a provisional substrate. Then, the LED epitaxial layer is etched to form LED chips by means of photolithography. A reflecting layer formed on the LED chips. Then, a metal layer formed on the reflecting layer by means of a chemical method or a physical method as permanent substrate. The provisional substrate is removed to expose surfaces of the LED chips. Pads are formed on the surfaces of the LED chips. Finally, the metal layer is separated to form individual LED chips by means of mechanical force.  
         [0010]     The present invention also provides a method for forming LED comprising the following steps. First, an LED epitaxial layer is formed on a provisional substrate. Then, a reflecting layer is formed on the LED epitaxial layer. A metal layer is formed on the reflecting layer by means of a chemical method or a physical method as permanent substrate. Next, the LED epitaxial layer, the reflecting layer, and the metal layer are etched to form LED chips by means of photolithography. The provisional substrate is removed to expose surfaces of the LED chips. Finally, pads are formed on the surfaces of the LED chips.  
         [0011]     As aforementioned, in the present invention, the conductivity and cooling effect of the substrate is increased by replacing the conventional Al 2 O 3  or SiC substrate with a metal substrate according to the present invention. Furthermore, the pads are formed on opposite sides, so only one pad need to bond in package process and the bonding yield is increased. The LED is proper to operate with high current due to the well cooling effect of the metal layer, so the present invention can make high power LED. Moreover, a reflecting layer is formed on LED chip to guide the light of LED to emit outwards at the same direction, and the illumination of the LED is raised. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1A  to  FIG. 1C  are schematic diagrams of LED structure in conventional arts;  
         [0013]      FIG. 2A  to  FIG. 2D  are schematic diagrams of the method for forming LED of a preferred embodiment in the present invention; and  
         [0014]      FIG. 3A  to  FIG. 3D  are schematic diagrams of the method for forming LED of another preferred embodiment in the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Some sample embodiments of the invention will now be described in greater detail. Nevertheless, it should be recognized that present invention can be practiced in a wide range of other embodiments besides those explicitly described, and the scope of the present invention is expressly not limited expect as specified in the accompanying claims.  
         [0016]     Then, the components of the different elements are not shown to scale. Some dimensions of the related components are exaggerated and meaningless portions are not drawn to provide a more clear description and comprehension of the present invention.  
         [0017]     The essence of the present invention is to form a metal layer by a chemical method and a physical method, and the metal layer is employed as a permanent substrate to replace the Al 2 O 3  or SiC substrate in the conventional arts. Accordingly, the conductivity and the cooling effect of substrate can be efficiently increased, and further the pads are formed on opposite side, so only one pad need to bond in package process. Moreover, the LED having the metal substrate is a high power LED due to the metal substrate has well cooling effect and the LED can operated with high current. The chemical method and physical method mentioned above comprise electroplating, electroless plating, CVD (chemical vapor deposition), (PVD) physical vapor deposition (as evaporation or sputtering deposition), and so on. These methods can control the thickness of the metal substrate and do not need the polishing and cutting process, and hence the complexity of LED process is also reduced. Furthermore, a reflecting layer is formed between the metal layer and the LED chip (or the LED epitaxial layer) to efficiently guide the light emitted by the LED chip to the same (outward) direction for increasing the illumination of the LED.  
         [0018]     According to the aforementioned essence, the present invention discloses preferred embodiments for illustrating the method of forming the LED having the advantages mentioned above.  
         [0019]      FIG. 2A  to  FIG. 2D  are schematic diagrams of the method for forming LED of a preferred embodiment in the present invention. As shown in  FIG. 2A , an LED epitaxial layer  105  is formed on a provisional substrate  100 . Then, LED chips  110  is formed by etching the LED epitaxial layer with photolithography, as shown in  FIG. 2B . The preferable etching method is dry etching. Next, a reflecting layer  120  and a metal layer  130  are formed on the LED chips in sequence, wherein the metal layer  130  is formed by electroplating. The thickness of the metal layer between the every two LED chips is 5-30 μm for benefiting to separate the metal layer  130 , as shown in  FIG. 2C .  
         [0020]     After that, the provisional substrate is removing by polishing, etching, or laser ablation, and pads  140  are formed on the exposed surfaces of the LED chips  110 , as shown in  FIG. 2D . Next, the metal layer is separated to form individual LED chips by means of mechanical force.  
         [0021]      FIG. 3A  to  FIG. 3D  are schematic diagrams of the method for forming LED of another preferred embodiment in the present invention. An LED epitaxial layer  105 , a reflecting layer  120  and a metal layer  130  are formed on a provisional substrate  100  in sequence, wherein the metal layer  130  is formed by electroplating, as shown in  FIG. 3A . Then, LED chips  110  is formed by etching the aforementioned structure with photolithography, as shown in  FIG. 3B .  
         [0022]     Next, the aforementioned LED structure is adhered on a film  130 , and the provisional substrate is removing, as shown in  FIG. 3C . Finally, pads  140  are formed on the exposed surfaces of the LED chips  110 , as shown in  FIG. 3D .  
         [0023]     In the present invention, the reflecting layer reflects the light of LED to increase the outward illumination of LED. The preferable material of the reflecting layer is Ag, Al, Rh, Pt, Pd, Ni, Ti, Co, Au, and so on. If the LED is blue LED (as GaN LED), the material of the reflecting layer is not recommended to use Au due to Au may absorb the blue light.  
         [0024]     In addition, the preferable material of the metal layer is Cu or other metal having well cooling effect, such as Al, Ni, Mo, W, Ag, Au, Ti, Co, Pd, Pt, or Fe. Therefore, the effect of diffusing heat is increased and the stability and life are also increased. The preferable thickness of the metal layer is  30 - 100 ,pm.  
         [0025]     As aforementioned, the present invention provides a method for forming LED comprising the following steps. First, an LED epitaxial layer is formed on a provisional substrate. Then, the LED epitaxial layer is etched to form LED chips by means of photolithography. A reflecting layer formed on the LED chips. Then, a metal layer formed on the reflecting layer by means of a chemical method or a physical method. The provisional substrate is removed to expose surfaces of the LED chips. Pads are formed on the surfaces of the LED chips. Finally, the metal layer is separated to form individual LED chips by means of mechanical force.  
         [0026]     The present invention also provides a method for forming LED comprising the following steps. First, an LED epitaxial layer is formed on a provisional substrate. Then, a reflecting layer is formed on the LED epitaxial layer. A metal layer is formed on the reflecting layer by means of a chemical method or a physical method. Next, the LED epitaxial layer, the reflecting layer, and the metal layer are etched to form LED chips by means of photolithography. The provisional substrate is removed to expose surfaces of the LED chips. Finally, pads are formed on the surfaces of the LED chips.  
         [0027]     As aforementioned, in the present invention, the conductivity and cooling effect of the substrate is increased by replacing the conventional Al 2 O 3  or SiC substrate with a metal substrate according to the present invention. Furthermore, the pads are formed on opposite sides, so only one pad need to bond in package process and the bonding yield is increased. The LED is proper to operate with high current due to the well cooling effect of the metal layer, so the present invention can make high power LED. Moreover, a reflecting layer is formed on LED chip to guide the light of LED to emit outwards at the same direction, and the illumination of the LED is raised.  
         [0028]     Although specific embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from what is intended to be limited solely by the appended claims.