Patent Publication Number: US-2007108462-A1

Title: Fabrication method of light emitting diode incorporating substrate surface treatment by laser and light emitting diode fabricated thereby

Description:
RELATED APPLICATION  
      The present application is a divisional of U.S. application Ser. No. 10/953,815 filed Sep. 30, 2004 which claims priority from, Korean Application Number 2004-64535, filed Aug. 17, 2004, the disclosures of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a Light Emitting Diode (LED), and in particular, to a fabrication method of LEDs incorporating a step of surface-treating a substrate by a laser and an LED fabricated by such a fabrication method. More particularly, the present invention can use a laser in order to implement finer surface treatment to an LED substrate over the prior art thereby improving the light extraction efficiency of an LED while protecting the substrate from chronic problems of the prior art such as stress or defects induced from chemical etching and/or physical polishing.  
      2. Description of the Related Art  
      In general, nitride semiconductors such as InAlGaN are widely used for Light Emitting Diodes (LEDs) for realizing blue or green light. The nitride semiconductors have a representative formula Al x In y Ga (1-X-y) N, wherein 0≦x≦1, 0≦y≦1, 0≦x+y≦1. A nitride semiconductor is fabricated by growing nitride epitaxial layers including an n-cladding layer, an active layer and a p-cladding layer on a substrate of for example sapphire via Metal Organic Chemical Vapor Deposition (MOCVD).  
      The light emitting efficiency of an LED is determined by an internal quantum efficiency, which represents light quantity generated by the LED with respect to external voltage, and an external quantum efficiency, which is measured outside the LED. Herein external quantum efficiency is expressed by the multiplication of internal quantum efficiency with light extraction efficiency. Therefore, it is essential to improve not only internal quantum efficiency but also external quantum efficiency in order to raise the light efficiency of the LED. In general, internal quantum efficiency is determined by active layer structure and epitaxy layer quality, and external quantum efficiency is determined by material refractivity and surface flatness.  
      Flip-chip LEDs have been increasing in their use. In a flip-chip LED, light generated from an active layer is emitted to the outside through a substrate of for example sapphire. Therefore, the external quantum efficiency of the flip-chip LED is determined by the interfacial state between a substrate and a buffer layer or an n-cladding layer and the outer surface state of the substrate.  
      Problems occurring in such a flip-chip LED will be described with reference to  FIG. 1 . As shown in  FIG. 1 , a fabrication process of a flip-chip LED  10  includes growing an n-GaN layer  14 , an active layer  16  and a p-GaN layer  18  in their order on a substrate  12  for example sapphire and then etching a resultant structure into a mesa structure to expose a partial area of the n-GaN layer  14 . Then, a p-electrode  20  is formed on the p-GaN layer  18 , and an n-electrode  22  is formed on the exposed partial area of the n-GaN layer  14 . Preferably, the p-electrode  20  is designed to cover the p-GaN layer  18  as large as possible so as to reflect light generated by the active layer  16  toward the sapphire substrate  12 . The p-electrode  20  is properly made of Ag or Al having high reflectivity, and more preferably, made of Ag. The completed LED  10  having the electrodes  20  and  22  is mounted on a board via solder bumps  24  and  26  made of conductive paste, and electrically connected with patterns of the board.  
      However, such a flip-chip LED  10  has following problems. As shown in  FIG. 1 , when a light beam L 1  is introduced into the sapphire substrate  12  directly from the active layer  16  or after reflecting from the p-electrode  20 , total internal reflection takes place to the light beam L 1  in a predetermined angle range owing to the refractivity difference between the n-GaN layer  14  and the sapphire substrate  12 . Then, the light beam L 1  is reflected several times between the sapphire substrate  12  and the reflective layer of p-electrode  20 . In this way, the light beam L 1  is absorbed and extinguished by the p- and n-GaN layers  14  and  18 . This as a result causes light loss thereby to lower the light extraction efficiency and thus the external quantum efficiency of the flip-chip LED  10 .  
      Various approaches have been proposed to solve these problems related with the light loss of such flip-chip LED  10 . Representative examples may include Japan Patent Application Publication No. 2002-164296, (Unites States Patent Application Publication Nos. 2004-0038049 and 2004-0048471 both claiming the benefit of Japan Patent Application Publication No. 2002-164296) and Japan Patent Application Publication No. 2002-280611 (Unites States Patent Application Publication No. 2004-0113166 claiming the benefit of Japanese Patent Application Publication No. 2002-280611). These documents propose in common to roughen the interface between a substrate and an n-GaN layer in order to reduce light loss induced from the refractivity difference between the substrate and the n-GaN layer.  
      However, these approaches produce a roughened structure in common through chemical etching and thus disadvantageously have a difficulty in realizing a fine geometry. Since a substrate for example of sapphire is resistant to etching, harsh etching conditions are necessary. Under the harsh etching conditions, a photoresist having a pattern corresponding to a desired roughened geometry is also etched. As a result, those etching techniques using photoresists can hardly form fine surface geometries for example of pore or pillar size under 1 μm in substrates. Of course, it is much more difficult to uniformly produce a fine roughened geometry.  
      As another drawback of the above approaches, defects such as etching stress exist on the top of the roughened structure.  
      As a result, such etching-associated drawbacks cause nonuniform lighting to the flip-chip LED while degrading the efficiency thereof. In addition, substrates produced according to the above approaches need an additional process such as photolithography and dry etching to increase the entire process time thereby raising cost.  
      In the meantime, the flip-chip LED also has a following light loss problem, which will be described with reference to  FIG. 2  as follows.  
      A flip-chip LED  10  shown in  FIG. 2  has a structure substantially the same as shown in  FIG. 1 . When a light beam L 2  generated in the LED  10  is directed in a predetermined angle range toward a sapphire substrate  12  instead of a p-electrode  20  functioning as a reflecting surface, the light beam L 2  is reflected from the sapphire substrate  12  via total internal reflection owing to the refractivity difference between the sapphire substrate  12  and the air or external sealant such as silicone and resin. Then, the light beam L 2  is reflected several times between the sapphire substrate  12  and the p-electrode  20 . In this way, the light beam L 2  is absorbed and extinguished by the sapphire substrate  12  and the n- and p-Gan layers  14  and  18 . This as a result causes light loss to reduce the light extraction efficiency of the flip-chip LED  10  and thus the external quantum efficiency thereof.  
      Although it is desired to impart a roughened structure to the outer surface of the sapphire substrate  12  in order to overcome such a problem, a suitable approach has not been proposed up to the present. More specifically, a fabrication process of the LED  10  polishes the substrate  12  with a grinder containing for example diamond slurry to reduce the thickness thereof after forming the electrodes  20  and  22 , and thus the outer surface of the substrate  12  can be roughened after the polishing. However, the foregoing etching cannot be performed to form the roughened structure in the outer surface after the formation of the semiconductor layers  12  to  18  and the electrodes  20  and  22 .  
     SUMMARY OF THE INVENTION  
      The present invention has been made to solve the foregoing problems of the prior art and it is therefore an object of the present invention to provide a fabrication method of LEDs incorporating a step of surface-treating a substrate by a laser to implement finer surface treatment to an LED substrate over the prior art, thereby improving the light extraction efficiency of an LED while protecting the substrate from chronic problems of the prior art such as stress or defects induced from chemical etching and/or physical polishing.  
      It is another object of the present invention to provide an LED produced by the above fabrication method.  
      According to an aspect of the present invention for realizing the object, there is provided a fabrication method of Light Emitting Diodes (LEDs) comprising the following steps of: (a) preparing a sapphire substrate; (b) emitting a laser beam onto a first side of the sapphire substrate to form a fine roughened structure; (c) forming an n-doped semiconductor layer, an active layer and a p-semiconductor layer in their order on the roughened side of the sapphire substrate; (d) etching a resultant structure obtained in the step (c) into a mesa structure to expose a partial area of the n-doped semiconductor layer; and (e) forming a p-electrode on the p-doped semiconductor layer and an n-electrode on the exposed area of the n-doped semiconductor layer.  
      The fabrication method of the invention may further comprise the steps of: (f) polishing a second side of the substrate to reduce the thickness of the substrate after the step (e); and (g) emitting a laser beam onto the second side of the substrate to form a fine roughened structure therein.  
      According to another aspect of the present invention for realizing the object, there is provided a fabrication method of Light Emitting Diodes (LEDs) comprising the following steps of: (a) preparing a sapphire substrate; (b) forming a fine roughened structure in a first side of the sapphire substrate; (c) forming an n-doped semiconductor layer, an active layer and a p-doped semiconductor layer in their order on the roughened sapphire substrate; (d) etching a resultant structure obtained in the step (c) into a mesa structure to expose a partial area of the n-doped semiconductor layer; (e) forming a p-electrode on the p-doped semiconductor layer and an n-electrode on the exposed area of the n-doped semiconductor layer; (f) polishing a second side of the substrate to reduce the thickness of the substrate; and (g) illuminating a laser beam onto the second side of the substrate to form a fine roughened structure therein.  
      In the fabrication method of the invention, the step (b) preferably comprises illuminating a laser beam onto the first side of the substrate to form the fine roughened structure therein.  
      In the foregoing fabrication methods of the invention, the step (b) of forming a fine roughened structure may comprise loading the substrate on a movable pedestal with the first side thereof facing upward, moving the movable pedestal at a predetermined rate, emitting the laser beam from a laser source, and opening a shutter on the path of the laser beam between the laser source and the substrate to form a groove at the movement of the pedestal to a predetermined pitch.  
      The step (b) of forming a fine roughened structure may comprise loading the substrate on a movable pedestal with the first side thereof facing upward, placing a mask having a number of holes matching the roughened structure above the substrate, and emitting the laser beam from a laser source above the mask onto the substrate so that the laser beam illuminates the first side of the substrate through the holes of the mask.  
      The step (b) of forming a fine roughened structure may comprise loading the substrate on a movable pedestal with the first side thereof facing upward, and moving the pedestal while emitting the laser beam onto the first side of the substrate, the laser beam having a diameter less than a groove in the roughened structure.  
      In addition, the step (b) of forming a fine roughened structure may comprise loading the substrate on a pedestal with the first side thereof facing upward and emitting the laser beam onto the first side of the substrate while moving a laser source, the laser beam having a diameter less than a groove formed in the roughened structure.  
      In the foregoing fabrication methods of the invention, the sapphire substrate may be replaced by one selected from the group consisting of a SiC substrate, an oxide substrate and a carbide substrate.  
      According to further another aspect of the present invention for realizing the object, there is provided an LED produced by the foregoing methods, comprising: a sapphire substrate having a fine roughened structure formed in a first side thereof, the fine roughened structure being formed via laser illumination; an n-doped semiconductor layer formed on the roughened first side of the substrate; an active layer and a p-doped semiconductor layer formed in their order on the n-doped semiconductor layer to expose a partial area of the n-doped semiconductor layer; a p-electrode formed on the p-doped semiconductor layer; and an n-electrode formed on the exposed area of the n-doped semiconductor layer.  
      In the LED of the invention, the sapphire substrate preferably has a second side polished to reduce the thickness of the sapphire substrate and a fine roughened structure formed in a second side via laser illumination.  
      According to yet another aspect of the present invention for realizing the object, there is provided an LED produced by the foregoing methods, comprising: a sapphire substrate having a first side having a fine roughened structure formed therein and a second side having a fine roughened structure formed therein via laser illumination; an n-doped semiconductor layer formed on the roughened first side of the sapphire substrate; an active layer and a p-doped semiconductor layer formed in their order on the n-doped semiconductor layer to expose a partial area of the n-doped semiconductor layer; a p-electrode formed on the p-electrode layer; and an n-electrode formed on the exposed area of the n-doped semiconductor layer.  
      In the LED of the invention, the fine roughened structure in the first side of the substrate is preferably formed via laser illumination.  
      In the foregoing LEDs of the invention, the sapphire substrate may be replaced by one selected from the group consisting of a SiC substrate, an oxide substrate and a carbide substrate.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIGS. 1 and 2  are cross-sectional views illustrating light loss generating in conventional flip-chip LEDs;  
       FIG. 3  is a flowchart of an LED fabrication method according to a first embodiment of the present invention;  
       FIG. 4  is a cross-sectional view of an LED produced by the LED fabrication method according to the first embodiment of the present invention;  
       FIG. 5  is a flowchart of an LED fabrication method according to a second embodiment of the present invention;  
       FIG. 6  is a cross-sectional view of an LED produced by the LED fabrication method according to the second embodiment of the present invention;  
       FIG. 7  is a flowchart of an LED fabrication method according to a third embodiment of the present invention;  
       FIG. 8  is a cross-sectional view of an LED produced by the LED fabrication method according to the third embodiment of the present invention; and  
      FIGS.  9  to  11  are perspective views of apparatuses for forming a fine roughened structure on a sapphire substrate of an LED according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
       FIG. 3  is a flowchart of an LED fabrication method according to a first embodiment of the present invention. Referring to  FIG. 3 , a substrate of for example sapphire is prepared in S 102 , and a laser beam is emitted onto one side of the sapphire substrate to form a fine roughened structure in S 104 . Then, semiconductor layers including an n-GaN layer, an active layer and a p-GaN layer are formed in succession on the roughened surface of the sapphire substrate in S 106 , S 108  and S 110 , and a resultant structure is etched into a mesa structure in S 112 . Then, a p-electrode is formed on the p-GaN layer and an n-electrode is formed on an exposed partial area of the n-GaN layer to complete an LED in S 114 .  
      In order to form a fine roughened structure in the substrate surface, the present invention can utilize various lasers including 308 nm, 428 nm and 198 nm Excimer lasers, an Nd:YAG laser (YAG is the short form of Yttrium Aluminum Garnet), a He—Ne laser and an Ar-ion laser. These lasers can easily form a micro-pattern by focusing a laser beam on a desired spot, in which the laser beam can produce a roughened structure more precisely in inverse proportional to its wavelength. Therefore, when the laser beam is illuminated onto one side of the substrate, the width and intensity of the laser beam is adjusted according to the physical property of the sapphire and the dimension of a wanted surface structure to be roughened so that grooves having a desired dimension can be formed in the surface of the substrate thereby to impart a fine roughened structure to the entire substrate surface.  
      The above process can adjust the width of the illuminating laser beam to uniformly form the roughened structure, in which grooves have a width of 1 μm or less. In addition, even though formed at such a fine dimension, the roughened structure or the grooves do not cause stress to a top portion of the roughened structure so that an LED of high quality can be produced.  
      In addition, the sapphire substrate can be replaced with a SiC substrate, an oxide substrate or a carbide substrate.  
       FIG. 4  is a cross-sectional view of an LED produced by the LED fabrication method according to the first embodiment of the present invention. Referring to  FIG. 4 , an LED of this embodiment includes a sapphire substrate  102  having a fine roughened structure  120 , which is formed on one side of the sapphire substrate  102  via a laser beam, an n-GaN layer  104  formed on the roughened surface of the sapphire substrate  102 , an active layer  106  formed on the n-GaN layer  104  to expose a partial area of the n-GaN layer  104 , a p-GaN layer  108  formed on the active layer  106 , a p-electrode  110  formed on the p-GaN layer  108  and an n-electrode  112  formed on the exposed area of the n-GaN layer  106 . Preferably, the p-electrode  110  is designed to cover the p-GaN layer  108  as large as possible so as to reflect light generated by the active layer  106  toward the sapphire substrate  102 . The p-electrode  110  is properly made of Ag or Al having high reflectivity, and more preferably, made of Ag. In addition, solder bumps  114  and  116  are formed on the p- and n-electrodes  110  and  112 , respectively, to mount the LED  100  on a board while electrically connecting the LED  100  to patterns of the board.  
      The fine roughened structure  120  formed in one side of the sapphire substrate  102  has a merit as follows: When the LED  100  of this embodiment generates light, even though a light beam L is introduced toward the sapphire substrate  102  directly from the active layer  106  or upon reflecting from the p-electrode  110  in the range of total internal reflection angle, the roughened structure  120  allows the light beam L to propagate from the n-GaN layer  104  into the sapphire substrate  102  without reflection and then radiate to the outside.  
      In addition, although it has been described that the foregoing semiconductor layer is made of GaN, the semiconductor layer also may be made of ZnSe.  
       FIG. 5  is a flowchart of an LED fabrication method according to a second embodiment of the present invention. Referring to  FIG. 5 , a substrate of for example sapphire is prepared in S 202 , one side of the sapphire substrate is pre-treated in S 204 , a plurality of semiconductor layers including n-GaN, active and p-GaN layers are formed in their order on the treated side of the sapphire substrate in S 206 , S 208  and S 210 , and a resultant structure is mesa-etched to expose a partial area of the n-GaN layer in S 212 . A p-electrode is formed on the p-GaN layer and an n-GaN layer is formed on the exposed area of the n-GaN layer in S 214 . Then, the sapphire substrate is polished at the other side with a grinder containing for example diamond slurry to reduce the thickness of the sapphire substrate and then a laser beam is illuminated onto the other side of the sapphire substrate to form a fine roughened structure in S 216 .  
      In order to form a fine roughened structure in the substrate surface, this embodiment can utilize various lasers including 308 nm, 428 nm and 198 nm Excimer lasers, an Nd:YAG laser, a He—Ne laser and an Ar-ion laser. These lasers can easily form a micro-pattern by focusing a laser beam on a desired spot, in which the laser beam can produce a roughened structure more precisely in inverse proportional to its wavelength. Therefore, when the laser beam is illuminated onto one side of the substrate, the width and intensity of the laser beam is adjusted according to the physical property of the sapphire and the dimension of a wanted surface structure to be roughened so that grooves having a desired dimension can be formed in the surface of the substrate thereby to impart a fine roughened structure to the entire substrate surface.  
      The above process can adjust the width of the illuminating laser beam to uniformly form the roughened structure with grooves having a fine width preferably of 1 μm or less. Therefore, laser beam illumination can be performed after the polishing of the sapphire substrate so that the roughened structure can be easily formed in the other side or outer surface of the sapphire substrate unlike the prior art. As a result, this can solve those problems of the prior art in which light loss takes place to lower the light extraction efficiency of an LED and thus the external quantum efficiency thereof.  
      In addition, the sapphire substrate can be replaced with a SiC substrate, an oxide substrate or a carbide substrate.  
       FIG. 6  is a cross-sectional view of an LED produced by the LED fabrication method according to the second embodiment of the present invention. Referring to  FIG. 6 , the LED  200  of this embodiment includes a sapphire substrate  202  having a fine roughened structure  222 , which is formed in an outer surface by a laser beam, an n-GaN layer  204  formed on an inner surface of the sapphire substrate  202 , an active layer  206  formed on the n-GaN layer  204  to expose a partial area of the n-GaN layer  204 , a p-GaN layer  208  formed on the active layer  206 , a p-electrode  210  formed on the p-GaN layer  208  and an n-electrode  212  formed on the exposed area of the n-GaN layer  204 . Preferably, the p-electrode  210  is designed to cover the p-GaN layer  208  as large as possible so as to reflect light generated by the active layer  206  toward the sapphire substrate  202 . The p-electrode  210  is properly made of Ag or Al having high reflectivity, and more preferably, made of Ag. In addition, solder bumps  214  and  216  made of conductive paste are provided on the p- and n-electrodes  210  and  212 , respectively, to mount the LED  200  on a board as well as electrically connecting the LED  200  to patterns of the board.  
      The fine roughened structure  220  formed in the outer surface of the sapphire substrate  202  has a merit as follows: When the LED  200  of this embodiment generates light, even though a light beam L propagates to the outside from the outer surface of the sapphire substrate  202  directly from the active layer  206  or upon reflecting from the p-electrode  210  in the range of total internal reflection angle, the roughened structure  222  allows the light beam to radiate from the sapphire substrate  202  to an ambient air layer or a sealant such as silicone or resin without total internal reflection. As a result, this LED structure can prevent the light loss of the prior art in which light reflects from the outer surface of the sapphire substrate  202  into the LED  200  owing to the refractivity difference between the outer surface of the sapphire substrate  202  and the foreign material (e.g., the air and sealant).  
       FIG. 7  is a flowchart of an LED fabrication method according to a third embodiment of the present invention. Referring to  FIG. 7 , a substrate of for example sapphire is prepared in S 302 , a laser beam is emitted onto one side of the sapphire substrate to form a fine roughened structure in S 304 , an n-GaN layer, an active layer and a p-GaN layer are formed in their order on the roughened side of the sapphire substrate in S 306 , S 308  and S 310 , and a resultant structure is mesa-etched to expose a partial area of the n-GaN layer in S 312 . A p-electrode is formed on the p-GaN layer and an n-electrode is formed on the exposed area of the n-GaN layer in S 314 . Then, the sapphire substrate is polished at the other side with a grinder containing for example diamond slurry to reduce the thickness of the sapphire substrate and then a laser beam is emitted onto the other side of the sapphire substrate to form a fine roughened structure in S 316 .  
      In this embodiment, the process steps of forming the fine roughened structures in both sides, that is, the inner and outer surfaces of the sapphire substrate are substantially the same as those in the foregoing first and second embodiments, and thus they will not be described further.  
       FIG. 8  is a cross-sectional view of an LED produced by the LED fabrication method according to the third embodiment of the present invention. Referring to  FIG. 8 , an LED  300  of this embodiment has technical features discriminated from the LEDs  100  and  200  of the first and second embodiments in that fine roughened structures  320  and  322  are formed in both sides, that is, inner and outer surfaces of a sapphire substrate  320  via laser beams.  
      When formed via the laser beams in one side of the sapphire substrate  302  on which the n-GaN layer  302  is grown, the fine roughened structure  320  can prevent total internal reflection between the substrate  302  and the n-GaN layer  302  to reduce light loss while reducing defects such as stress therein as previously described in the first embodiment in conjunction with  FIGS. 3 and 4 . In addition, the fine roughened structure  322  formed via the laser beams in the outer surface of the sapphire substrate  302  in contact with foreign material (e.g., the air and sealant) can prevent the loss of light L in the LED  300  by substantially removing the total internal reflection between the sapphire substrate  302  and the foreign material. As a result, the fine roughened structures  320  and  322  formed in the inner and outer surfaces of the substrate  302  can reduce light loss and therefore further improve light extraction efficiency.  
      Hereinafter several examples for forming a fine roughened structure in a sapphire substrate of an LED according to the present invention will be discussed with reference to FIGS.  9  to  11 .  
      A laser system shown in  FIG. 9  includes a laser source  400 , a shutter  402  and a movable pedestal  404 . The movable pedestal  404  serves to load a sapphire substrate  102 , and is designed to move at a predetermined rate in the direction of an arrow A and/or other directions.  
      The laser source  400  is designed to emit a fine laser beam L for forming a fine roughened structure  120  in the sapphire substrate  102  to a predetermined width and depth. The shutter  402  is placed on the path of a laser beam L between the laser source  400  and the sapphire substrate  102  (as represented with a dotted line), and moves to a position drawn with a solid line in the direction of an arrow B whenever the movable pedestal  404  loaded with the sapphire substrate  102  moves to a predetermined pitch or a length so that the laser beam L can form grooves of the fine roughened structure  102  in the sapphire substrate  102 . As a result, this operation forms alternating grooves and protrusions in the sapphire substrate  102  at a predetermined gap. That is to say, every pair of groove and protrusion are formed corresponding to a single pitch. Repeating this process can form a number of grooves and protrusions and therefore embody the fine roughened structure  120  to the sapphire substrate  102 .  
      In this case, available examples of the laser source may include 308 nm, 428 nm and 198 nm Excimer lasers, an Nd:YAG laser, a He—Ne laser and an Ar-ion laser. In addition, the sapphire substrate can be replaced with a SiC substrate, an oxide substrate or a carbide substrate.  
      A laser system shown in  FIG. 10  includes a laser source  500  capable of emitting a large area laser beam L 1 , a mask  502  having a number of fine holes  504  of a desired diameter or width and a desired interval and a pedestal  506  for loading a sapphire substrate  102  thereon.  
      The laser source  500  is designed to emit the laser beam L 1  capable of illuminating the entire are of the sapphire substrate  102 , and the holes  502  of the mask  502  are arrayed in an outline matching the upper surface of the sapphire substrate to covert the laser beam L 1  into a number of fine laser beams L 2  of a desired diameter or width and a desired interval. Then, the fine laser beams L 2  are emitted onto the upper surface of the sapphire substrate  102  to impart a fine roughened structure  120  including a number of grooves of a desired diameter or width and a desired interval to the upper surface of the sapphire substrate  102 .  
      While the laser source applicable to this system is substantially the same as previously described in conjunction with  FIG. 9 , the laser beam L 1  is not necessarily intended to illuminate the entire area of the substrate  102  by one time. That is, the laser beam can be adapted to illuminate a partial area of the substrate  102 . The laser beam is converted into a number of fine laser beams to impart a fine roughened structure to the partial area of the substrate  102 , and then the pedestal  506  or the laser source  500  is moved to repeat the process of forming the fine roughened structure  120  to other areas of the substrate  102 .  
      As described above, available examples of the laser source may include 308 nm, 428 nm and 198 nm Excimer lasers, an Nd:YAG laser, a He—Ne laser and an Ar-ion laser. In addition, the sapphire substrate can be replaced with a SiC substrate, an oxide substrate or a carbide substrate.  
      A laser system shown in  FIG. 11  includes a laser source  600  and a movable pedestal  602 . The movable pedestal  602  is loaded with a sapphire substrate  102  and designed to move in the direction of an arrow C at a predetermined rate. Of course, the pedestal  602  is designed movable in various directions such as a direction crossing the arrow C or a diagonal direction.  
      As shown in  FIG. 11 , emitting a laser beam L onto the substrate  102  from laser source  600  while moving the movable pedestal  602  loaded with the sapphire substrate  102  in the direction of the arrow C forms slits  120 A in the sapphire substrate  102 . Of course, moving the pedestal  602  in a direction perpendicular to the arrow C forms the slits  120 B, in which only one slit is shown for the sake of brevity. In this way, the slits  120  and  120  can be formed in one direction or to cross each other in order to form the fine roughened structure desired in the present invention in the surface of the substrate  102 .  
      Alternatively, the slits  120 A and  120 B can be formed by moving the laser source  600  with respect to the fixed pedestal  602 .  
      As previously described, available examples of the laser source may include 308 nm, 428 nm and 198 nm Excimer lasers, an Nd:YAG laser, a He—Ne laser and an Ar-ion laser. In addition, the sapphire substrate can be replaced with a SiC substrate, an oxide substrate or a carbide substrate.  
      As described hereinbefore, the present invention can use the laser to implement more fine surface treatment to an LED substrate so as to improve the light extraction efficiency of an LED. In addition, the present invention can protect the substrate from chronic problems of the prior art such as stress and defects induced from chemical etching and/or physical polishing.  
      While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the present invention as defined by the appended claims.