Abstract:
A flip-chip light emitting diode and fabricating methods are disclosed. A soft transparent adhesive layer is utilized to past a transparent conductive substrate onto a light emitting diode epitaxy structure on a substrate, and the substrate is next removed entirely. Then, a mesa-etching process is performed to form a first top surface and a second top surface on the light emitting diode epitaxy structure for respectively exposing an n-type layer and a p-type layer in the light emitting diode epitaxy structure. Next, a metal reflective layer and a barrier layer are formed on the light emitting diode epitaxy structure in turn, and electrodes are finally fabricated on the barrier layer.

Description:
PRIORITY CLAIM 
   This Application claims priority of Taiwan Patent Application No. 094104049 filed on Feb. 5, 2005. 
   FIELD OF THE INVENTION 
   The present invention relates to a flip-chip light emitting diode (LED) and its fabricating method, and more particularly, to a flip-chip LED with high light output intensity and its fabricating methods. 
   BACKGROUND OF THE INVENTION 
   Because of the low cost, simple structure, low power consumption and small size, the light emitting diode (LED) is applied in display and illumination technologies. 
   For general LED fabrication, an LED epitaxy structure is formed on a substrate directly, while a cathode electrode and an anode electrode are respectively fabricated on different sides of the substrate. This conventional structure has a better current spreading efficiency, but an increased LED package area is necessary. Therefore, a flip-chip LED has been gradually developed over the last few years. 
   The p-type semiconductor layer and the n-type semiconductor layer of the flip-chip LED are exposed on the same side of the LED epitaxy structure in the flip-chip LED fabrication to allow the anode electrode and the cathode electrode to be on the same side of the LED, and the LED epitaxy structure with electrodes can thus be flipped onto a solder directly by flip-chip packaging technology. Thus, conventional wire bonding for the package is not necessary, and a smaller package size and higher device reliability are obtained. 
   However, the light emitted upward from the flip-chip LED is absorbed by the substrate and cannot pass through the substrate for complete outward output. Therefore, although the flip-chip LED is beneficial for device package, the flip-chip LED suffers a lowered LED light output intensity. 
   Further, the field of LED technology is highly focused on the development of LED with higher brightness. Unfortunately, only the light emitted upward from the LED counts as light output; the light emitted downward is partially absorbed by the material below the LED and cannot be another source for light output. More particularly, the light emitted downward from the flip-chip LED is easily blocked and scattered by electrodes. Thus, the light output of the LED only depends on the luminance properties of the LED itself, and the light output intensity is limited in improvement. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is to provide a flip-chip LED with the superiority of the flip-chip structure, and to efficiently increase the light output intensity of the LED, the product quality and brightness of the LED is thereby greatly improved. 
   A metal reflective layer of the LED of the present invention has a high reflectivity, results in the light from the LED epitaxy structure to the electrodes reflects to the internal medium of the LED, the brightness of the LED is thereby increased. 
   The interface of the LED epitaxy structure and the transparent substrate is rough, the total reflection phenomenon is thereby reduced and the light extraction efficiency of the LED is improved. Moreover, the transparent substrate has a rough surface, the total reflection phenomenon is thereby reduced and the light output intensity of the LED is improved. 
   Another aspect of the present invention is to provide a method for fabricating a flip-chip LED. The LED is supported by the transparent substrate instead of the traditional substrate, so that most light emitted from the LED is able to pass through the transparent substrate and be successfully output outwards. Thus, the superiority of the flip-chip LED can be hold for promoting the device reliability, and the light output intensity of the flip-chip LED is enhanced. 
   According to a fabricating method of the present invention, a second semiconductor layer (e.g., p-type semiconductor layer) is below the first semiconductor layer (e.g., n-type semiconductor layer) through a “twice reverse” process, the distance between the active layer and the transparent substrate thereby increases and the problem of total reflection phenomenon is reduced. The present invention makes use of the light emitted from the LED more effectively for improving the light output intensity greatly, and the brightness of LED is increased. 
   A flip-chip LED of the present invention includes a transparent substrate, an LED epitaxy structure, a metal reflective layer, a first electrode and a second electrode. The LED epitaxy structure is on the transparent substrate. The LED epitaxy structure includes a first upper surface and a second upper surface. The first upper surface and the second upper surface are different doped types. The metal reflective layer is positioned on the first upper surface and the second upper surface of the LED epitaxy structure. The first electrode is positioned on the metal reflective layer and the first upper surface is underneath the first electrode. The second electrode is positioned on the metal reflective layer and the second upper surface is underneath the second electrode. 
   A manufacturing method of the flip-chip LED of the present invention includes: forming an LED epitaxy structure on a substrate; bonding a transparent substrate to the LED epitaxy structure by utilizing a soft transparent adhesive layer; removing the substrate to expose a lower surface of the LED epitaxy structure; bonding a transparent substrate to the lower surface; removing the temporary substrate; etching the LED epitaxy structure to partially remove the LED epitaxy structure to a depth, so that the LED epitaxy structure includes a first upper surface and a second upper surface, the first upper surface and the second upper surface are of different doped types; forming a metal reflective layer on the first upper surface and the second upper surface of the LED epitaxy structure; and finally, fabricating a first electrode and a second electrode on the metal reflective layer respectively, in which the first upper surface of the LED epitaxy structure is underneath the first electrode and the second upper surface of the LED epitaxy structure is underneath the second electrode respectively. 
   Another manufacturing method of the flip-chip LED of the present invention includes: forming an LED epitaxy structure on a substrate; bonding a temporary substrate to the LED epitaxy structure; removing the substrate to expose a lower surface of the LED epitaxy structure; bonding a transparent substrate to the lower surface; removing the temporary substrate; etching the LED epitaxy structure to partially remove the LED epitaxy structure to a depth, therefore, the LED epitaxy structure includes a first upper surface and a second upper surface, the first upper surface and the second upper surface are of different doped types; forming a metal reflective layer on the first upper surface and the second upper surface of the LED epitaxy structure; and fabricating a first electrode and a second electrode on the metal reflective layer respectively, wherein the first upper surface of the LED epitaxy structure is underneath the first electrode and the second upper surface of the LED epitaxy structure is underneath the second electrode respectively. 
   Another manufacturing method of the flip-chip LED of the present invention includes: forming an LED die on a substrate; bonding a temporary substrate to the LED die; removing the substrate to exposure a lower surface of the LED die; bonding a transparent substrate to the lower surface; and removing the temporary substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A–1D  are cross-sectional, schematic diagrams showing the process for forming the flip-chip LED in accordance with the preferred embodiment of the present invention; 
       FIG. 2  is a cross-sectional, schematic diagram showing the structure of another flip-chip LED in accordance with the preferred embodiment of the present invention; 
       FIGS. 3A–3D  are cross-sectional, schematic diagrams showing the process for forming the flip-chip LED in accordance with the preferred embodiment of the present invention; and 
       FIGS. 4A–4D  are cross-sectional, schematic diagrams showing the process for forming the flip-chip LED in accordance with the preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiment 1 
   A flip-chip LED  10  is disclosed in accordance with the present invention.  FIGS. 1A–1D  are cross-sectional, schematic diagrams showing the process for forming the flip-chip LED  10  in accordance with the preferred embodiment of the present invention. 
   In  FIG. 1A , an LED epitaxy structure  101  is first formed on a substrate  100 . The LED epitaxy structure  101  mentioned herein includes an AlGaInP LED epitaxy structure and an AlGaInN LED epitaxy structure. To obtain an epitaxy structure of high quality, the selected material of the substrate  100  corresponding to the AlGaInP LED epitaxy structure is Ge, GaAs or InP. As for the AlGaInN LED epitaxy structure, the preferred material of the substrate  100  is sapphire, SiC, Si, LiAlO 2 , ZnO or GaN. The LED epitaxy structure  101  is fabricated by forming an n-type semiconductor layer  102 , an active layer  104 , and a p-type semiconductor layer  106  on the substrate  100 . Further, the surface of the p-type semiconductor layer  106  is next roughened to form a rough surface; for example, a photolithography and etching process is utilized to micro-etch the surface of the p-type semiconductor layer  106  for producing surface roughness of the p-type semiconductor layer  106 . 
   The active layer  104  may be a homo-structure, single hetero-structure, double hetero-structure, or multi-quantum well structure (MQW). The rough surface of the p-type semiconductor layer  106  is composed of bar-shaped, triangular, or round salient figures. 
   A transparent substrate  110  having a surface coated with a soft transparent adhesive layer  108  is then bonded to the LED epitaxy structure  101 , in which the transparent substrate  110  is pasted down on the p-type semiconductor layer  106  by the adhesion property of the soft transparent adhesive layer  108 . After combing the transparent substrate  110  and the LED epitaxy structure  101 , the substrate  100  is removed to form a structure as shown in  FIG. 1B , and the LED epitaxy structure  101  is completely transferred from the substrate  100  to the transparent substrate  110 . 
   The selected materials of the transparent substrate  110  are transparent. In one preferred embodiment, the light emitted from the LED epitaxy structure  101  is able to pass through the transparent substrate  110  without being absorbed or aborted less than 50% by the transparent substrate  110 . For example, the transparent substrate  110  may be made of sapphire, glass, GaP, or SiC. The soft transparent adhesive layer  108  may be made of bisbenzocyclobutene (BCB) resin for tightly bonding the transparent substrate  110  and the p-type semiconductor layer  106 . 
   Next, referring to  FIGS. 1B and 1C , an LED die manufacturing process is performed. First, a mesa-etching process is performed on the n-type semiconductor layer  102 , the active layer  104 , and the partial p-type semiconductor layer  106 . The etching process is start with the surface of the n-type semiconductor layer  102 , proceeds with the vertical direction, and then a portion of the LED epitaxy structure  101  is removed to a depth, so that a upper surface  1022  of the n-type semiconductor layer  102  is formed and a second upper surface  1062  of the p-type semiconductor layer  106  is exposed. Then, a metal reflective layer  112  and a barrier layer  114  are formed on the n-type semiconductor layer  102  and the p-type semiconductor layer  106 . Finally, a cathode electrode (first electrode)  116  and an anode electrode (second electrode)  118  are fabricated on the barrier layer  114 . The cathode electrode  116  and the anode electrode  118  are made from metal materials with good conductivity, such as Au or Al. 
   The metal reflective layer  112  is made from a metal material with high reflectivity for light; for example, Au, Al, Ag, or Ag alloy. The metal reflective layer  112  has a great capacity for reflecting the light emitted from the LED epitaxy structure  101 . Further, the barrier layer  114  is utilized to prevent the cathode electrode  116  and the anode electrode  118  from metal diffusing into the metal reflective layer  112  and maintain the reflectivity of the metal reflective layer  112 . The preferable material of the barrier layer  114  is Ni, W, TiN, WN, Pt, ZnO, or ITO. 
   A solder  120  is formed above the barrier layer  114  to directly reverse onto a submount substrate  122  (as shown in  FIG. 1D ) for the following package process. 
   The embodiment utilizes the metal reflective layer  112  to reflect the light emitted downward from the LED epitaxy structure  101 , as shown in  FIG. 1D , and the reflected light becomes additional source of the light output. 
   Moreover, since there&#39;s a large difference in refractive index between the semiconductor materials in the LED epitaxy structure  101  (e.g. the index of reflection of GaN is about 2.4) and the external medium of the LED  10  (e.g. the index of reflection of air is about 1.5), and the light emitted upward from the LED epitaxy structure  101  is easily reflected back into the LED  10  making lower the light output efficiency. The transparent substrate  110  has its index of reflection between that of the external medium of the LED  10  (e.g. air) and that of the semiconductor layer (e.g. GaN). Therefore, the light upward from the LED epitaxy structure  101  is able to pass through the transparent substrate  110  and output outwards without being blocked or absorbed by the transparent substrate  110 . The rough surface of the p-type semiconductor layer  106  is utilized to form a rough interface of the transparent adhesive layer  108  and the LED epitaxy structure  101 . The light scattering or refraction thus occurs at the interface reduces the total reflection phenomenon inside the LED  10  when the emitted light enters into the interface of the p-type semiconductor layer  106  and the transparent adhesive layer  108 . The light extraction efficiency of the LED  10  is further improved. 
   Embodiment 2 
     FIG. 2  shows a resulting structure of flip-chip LED  20  in accordance with the preferred embodiment of the present invention. With reference to  FIG. 2  in which like reference numerals refer to like feature in  FIGS. 1A–1D , and at a subsequent fabrication, the upper surface of the transparent substrate  210  is roughened to form a rough surface for improving the light output intensity, and the brightness of LED  20  is increased. 
   Moreover, not only the interface of the transparent substrate  210  and the soft transparent adhesive layer  208 , but also the lower surface of the transparent substrate  210  is formed with roughness property. 
   The upper and lower surface of the transparent substrate  210  are either roughened before the soft transparent adhesive layer  208  is coated or by micro-etching the surface of transparent substrate  210  after the transparent substrate  210  is adhered to the p-type semiconductor layer  206 . 
   Embodiment 3 
     FIGS. 3A–3D  are cross-sectional, schematic diagrams showing the process for forming the flip-chip LED  30  in accordance with the preferred embodiment of the present invention. 
   The LED epitaxy structure  301  shown in  FIG. 3A  is fabricated by forming an n-type semiconductor layer  302 , an active layer  304 , and a p-type semiconductor layer  306  on the substrate  300  in accordance with the manufacturing steps of Embodiment 1. The active layer  304  of this embodiment (Embodiment 3) is similar to the active layer  104  of Embodiment 1, i.e. the active layer  304  may be a homo-structure, single hetero-structure, double hetero-structure, or multi-quantum well structure (MQW). 
   A temporary substrate  310  having a surface coated with a soft transparent adhesive layer  308  is then bonded to the LED epitaxy structure  301 , in which the temporary substrate  310  is pasted down on the p-type semiconductor layer  306  by the adhesion property of the soft transparent adhesive layer  308 . The temporary substrate  310  is then removed. Further, the surface of the n-type semiconductor layer  302  is next roughened to form a rough surface  3022 ; for example, a photolithography and etching process is utilized to micro-etch the surface of the n-type semiconductor layer  302  for producing surface roughness of the n-type semiconductor layer  302 , i.e. the rough surface  3022 , as shown in  FIG. 3B . The “temporary substrate”  310  is used to support the LED epitaxy structure  301 , therefore the selected material of the temporary substrate  310  should have good adhesion to the semiconductor layer but have to be easily removed. For example, the temporary substrate  310  may be made of glass, silicon, ceramic, and Al 2 O 3 . The selected material of the soft transparent adhesive layer  308  should also be easily removed. For example, the soft transparent adhesive layer  308  may be made of polyimide, glass and bisbenzocyclobutene (BCB) resin. 
   A transparent substrate  314  is then pasted down on the lower surface  3022  of the n-type semiconductor layer  302  by the adhesion property of the soft transparent adhesive layer  312 . The material of the soft transparent adhesive layer  312  may be the same as that of the soft transparent adhesive layer  308 ; for example, BCB, polyimide, glass or epoxy, which results in good bonding of the transparent substrate  314  and n-type semiconductor layer  302 . The material of the transparent substrate  314  is selected from sapphire, glass, GaP or SiC having the transparent property. The temporary substrate  310  and the soft transparent adhesive layer  308  are then removed to form a structure shown in  FIG. 3C . 
   Next, an LED die manufacturing process is performed which is similar to the process described for  FIGS. 1B and 1C . First, a mesa-etching process is performed on the p-type semiconductor layer  306 , the active layer  304 , and the portions of n-type semiconductor layer  302  to expose a first upper surface  3022  of the n-type semiconductor layer  302  and form a second upper surface  3062  of the p-type semiconductor layer  306 . Then, a metal reflective layer  316  and a barrier layer  318  are formed. Finally, an anode electrode (e.g., first electrode)  320  and a cathode electrode (e.g., second electrode)  322  are fabricated on the barrier layer  318  to form a flip-chip LED  30  of the present invention, as shown in  FIG. 3D . The materials of the metal reflective layer  316 , barrier layer  318 , anode electrode  320  or the cathode electrode  322  are respectively identical to those mentioned in Embodiment 1. It&#39;s noted that in Embodiment 1, the p-type semiconductor  106  is positioned near to the transparent substrate  110 , but in Embodiment 3, the n-type semiconductor  302  is positioned near to the transparent substrate  314  by a “twice reverse” process. That is, in Embodiment 3, the distance between the active layer  304  and the transparent substrate  314  increases through the process of adhering two different substrates, i.e., temporary substrate  310  and transparent substrate  314 . The light emitted from the active layer  304  to the external medium of the LED  30  thereby increases. 
   Embodiment 4 
   The resulting structure of a flip-chip LED of Embodiment 4 as the same to that of Embodiment 3 (shown in  FIG. 3D ), but their manufacturing methods are different as recited in the followings. 
   An LED die is first fabricated by traditional methods. An n-type semiconductor layer  402 , an active layer  404 , and a p-type semiconductor layer  406  are formed on the substrate  400  to form an LED epitaxy structure  401 . Next, a photolithography and etching process is utilized to form a first upper surface  4022  of the n-type semiconductor layer  402  and a second upper surface  4062  of the p-type semiconductor layer  406 . A metal layer  408 , a barrier layer  410 , an anode electrode  414  and a cathode  412  are formed in accordance with Embodiment 1 to form an LED die shown in  FIG. 4A . 
   The temporary substrate  420  having a surface coated with a soft transparent adhesive layer  418  is then adhered to the cathode electrode  412  and a solder  416 . The substrate  400  is then removed to form a structure shown in  FIG. 4B . 
   Referring to  FIG. 4C , an exposed and lower surface of the n-type semiconductor layer  402  is next roughened. The lower surface of the n-type semiconductor layer  402  is mesa-etched by utilizing a photolithography and etching process to form a rough lower surface  4022 . A transparent substrate  424  having a surface coated with a soft transparent adhesive layer  422  is then bonded to lower surface  4022  of the n-type semiconductor layer  402  by the adhesion property of the soft transparent adhesive layer  422 . The temporary substrate  420  is removed to form the flip-chip LED. Referring to  FIG. 4D , the flip-chip LED is directly flipped onto the submount substrate  426  for the following package process, wherein a region numbered  428  is an isolated area. Through the “two reverse” process, the n-type semiconductor layer  402  is near the transparent substrate  424  and positioned in the upper layer, and the p-type semiconductor  406  is positioned in the lower layer, the distance between the active layer  404  and the transparent substrate  424  thereby increases. The light emitted from the active layer  404  to the external medium of the LED increases. The surface of the transparent substrate  424  may be also roughened by micro-etching to improving the light extraction efficiency of the LED. 
   By means of the detailed descriptions of what is presently considered to be the most practical and preferred embodiments of the subject invention, it is the expectation that the features and the gist thereof are plainly revealed. Nevertheless, these above-mentioned illustrations are not intended to be construed in a limiting sense. Instead, it should be well understood that any analogous variation and equivalent arrangement is supposed to be covered within the spirit and scope to be protected and that the interpretation of the scope of the subject invention would therefore as much broadly as it could apply.