Abstract:
A method for fabricating a plurality of individual light emitting diode units includes forming a GaN epitaxial layer on a sapphire substrate, forming a plurality of exhaust trenches on the GaN epitaxial layer, wherein the exhaust trenches define a plurality of individual light emitting diode units, forming a reflective layer on the GaN epitaxial layer, attaching the reflective layer to a conductive substrate, removing the sapphire substrate from the GaN epitaxial layer via a laser lift-off process, wherein a gas produced during the laser lift-off process is exhausted via the exhaust trenches, and dicing the conductive substrate along the exhaust trenches to form the plurality of individual light emitting diode units.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a LED units fabrication method. 
     DESCRIPTION OF RELATED ART 
     Light emitting diodes (LEDs) are well-known semiconductor devices that convert current into light. The color of the light (wavelength) emitted by an LED depends on the semiconductor materials used. Gallium-Nitride (GaN) has gained much attention, because it is found that GaN can be combined with indium to produce InGaN/GaN semiconductor layers that can emit green, blue, and ultraviolet light. This wavelength controllability enables an LED semiconductor designer to tailor material characteristics to achieve beneficial device characteristics. Accordingly, GaN-based opto-electronic device technology has rapidly evolved since their commercial introduction in 1994. 
     Sapphire substrates are typically used as a base in the fabrication of GaN-based LEDs. Fabricating LEDs on the sapphire substrate is typically performed by growing a GaN epitaxial layer on the sapphire substrate. Then, a plurality of individual devices, such as GaN-based LEDs, is fabricated on the GaN epitaxial layer using normal semiconductor processing techniques. The individual devices are then detached from the sapphire substrate. However, because the sapphire substrate is hard, chemically resistant, and has no natural cleave angles, it is very difficult to separate the individual devices from the sapphire substrate. 
     Recently, a laser lift-off (LLO) process has been introduced to remove the sapphire substrate from the individual devices. During the process, the sapphire substrate is irradiated from a rear side by a laser to decompose the GaN epitaxial layer, such that the sapphire substrate is removed. However, the thin GaN epitaxial layer may buckle or crack during the LLO process, because it is usually not strong enough to withstand a high-energy laser shock wave. 
     Thus, a method for fabricating LED units which overcomes the described limitations is desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the views. 
         FIG. 1  is a flowchart of an embodiment of LED units fabrication method. 
         FIG. 2  through  FIG. 7  are schematic, partial cross-sections of at least a part of LED units fabricated using the method of  FIG. 1 , with each of  FIG. 2  through  FIG. 7  relating to at least one step of the method of  FIG. 1 . 
         FIG. 8  is a schematic, top down view of the GaN epitaxial layer showing formation of a plurality of exhaust trenches. 
         FIG. 9  is a schematic, top down view of the GaN epitaxial layer during the laser lift-off process. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an embodiment of a method for fabricating LED units is described as follows: 
     In step S 1 , a sapphire substrate  10 , which has a round flat surface, is provided. 
     In step S 2 , referring to  FIG. 2 , a GaN epitaxial layer  20  is formed on the flat surface of the sapphire substrate  10 . In one embodiment, the GaN epitaxial layer  20  is formed using metal organic chemical vapor deposition (MOCVD). The GaN epitaxial layer  20  includes a lamination of an n-GaN layer  21 , an active layer  23 , and a p-GaN layer  25 . In alternative embodiments, the GaN epitaxial layer  20  may also be formed with other appropriate epitaxial growth methods, such as molecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE). 
     In step S 3 , referring to  FIG. 3 , a plurality of exhaust trenches  27  is formed in the GaN epitaxial layer  20  using reactive ion etching, preferably inductively coupled plasma (ICP) reactive ion etching. The exhaust trenches  27  pass through the n-GaN layer  21 , the active layer  23  and the p-GaN layer  25  of the GaN epitaxial layer  20 . A width of each exhaust trench  27  is preferably in a range from about 10 microns (μm) to 500 μm. Referring also to  FIG. 8 , the plurality of exhaust trenches  27  is arranged substantially perpendicular to each other, and defines a preserve region  28  in a middle portion of the GaN epitaxial layer  20  (bounded by a broken line) and a sacrifice region  29  in a marginal portion of the GaN epitaxial layer  20  surrounding the preserve region  28 . In the preserve region  28 , the exhaust trenches  27  further define a plurality of individual LED units  281  having the same shape. Each of the individual LED units  281  is beneficially a square having a width in a range from about 100 μm to about 2000 μm. The individual LED units  281  are arranged in a matrix defined by the exhaust trenches  27 . Formation of the exhaust trenches  27  includes forming scribe lines and etching. Scribe lines are formed on the GaN epitaxial layer  20  using a photo-resist coating, which is beneficially fabricated from a relatively hard photo-resist material that withstands intense plasma. The photo-resist coating is then patterned to form the scribe lines. The GaN epitaxial layer  20  is then etched along the scribe lines using an ICP etcher to form the exhaust trenches  27 . In alternative embodiments, the exhaust trenches  27  may be formed using other etching methods such as chemical etching. 
     In step S 4 , referring to  FIG. 4 , a reflective layer  30  is formed on the GaN epitaxial layer  20  using plasma-enhanced chemical vapor deposition (PECVD). The reflective layer  30  may be a Prague reflective layer, or a metal reflective layer comprising Ag, Ni, Al, Cu, or Au. The reflective layer  30  is configured to reflect light generated in the active layer  33  to direct the light towards the n-GaN layer  21 . In alternative embodiments, the reflective layer  30  may be formed using physical vapor deposition (PVD), sputtering, electroplating, or other suitable means. It should be noted that, during the formation, a photo-resist or a mask can be used to prevent the reflective layer  30  from forming in the exhaust trenches  27 . 
     In step S 5 , referring to  FIG. 5 , the reflective layer  30  is attached to a conductive substrate  40  using wafer boding or electroplating. In this embodiment, a nickel layer is electroplated on the reflective layer  30  to form the conductive substrate  40 . 
     In step S 6 , referring to  FIG. 6 , the sapphire substrate  10  is removed form the GaN epitaxial layer  20  using an LLO process. In this embodiment, a laser beam  50  emitted by an excimer laser irradiates the sapphire substrate  10  from a side thereof without forming the GaN epitaxial layer  20 . The laser beam  50  having a wavelength of about  300  nm almost completely passes through the sapphire substrate  10 , and is then absorbed almost completely in the GaN epitaxial layer  20 , whereby a temperature of an interface of the sapphire substrate  10  and the GaN epitaxial layer  20  rapidly rises. This result in decomposition of GaN into gallium and nitrogen and formation of a gap  11  between the sapphire substrate  10  and the GaN epitaxial layer  20 . Nitrogen generated at this time can exhaust from the exhaust trenches  27 , such that cracks on the GaN epitaxial layer  20  can be effectively reduced with no pressure applied to the individual LED units  281 . After the removal of the sapphire substrate  10 , excessive gallium is drained by acid etching or the like. 
     Referring also to  FIG. 9 , in order to increase the efficiency of the laser, the laser beam  50  projects a light spot  51  on the GaN epitaxial layer  20 . The light spot  51  is substantially square, and of the same size as that of the individual LED unit  281 . The LLO process further includes: 
     Projecting an initial light spot  51  on a marginal portion of the sacrifice region  29  aligned with a first row of the individual LED units  281 . The initial light spot  51  intercrosses a rim of the GaN epitaxial layer  20 , in other words, part of the initial light spot  51  extends beyond the GaN epitaxial layer  20 . Accordingly, the gap  11  formed by the initial light sport  51  directly communicates with the environment, and nitrogen generated at this time can also exhaust from the gap  11 . 
     Moving the initial light spot  51  along the exhaust trench  27  downward into the preserve region  28 , thus scanning one individual LED unit  281  of the first row. Because the initial light spot  51  is aligned with the first row, and the initial light spot  51  has the same shape and size of the individual LED unit  281 , the initial light spot  51  can entirely cover the individual LED unit  281 , such that the whole of the individual LED unit  281  separates the sapphire substrate  10  immediately. 
     Continuing moving the initial light spot  51  along the exhaust trench  27  and scanning each individual LED unit  281  in the first row. 
     When the initial light spot  51  passes through the preserve region  28  and reaches another sacrifice region  29 , stopping emission of the laser beam  50 . 
     Adjusting the excimer laser such that a new light spot  51  is projected on the original sacrifice region  29  aligned with a second row of the individual LED units  281 . The second row is adjacent to the first row. 
     Repeating movement of the initial light spot  51  until each row of the individual LED units  281  is scanned. 
     In step S 7 , referring to  FIG. 7 , the conductive substrate  40  is cut along the exhaust trenches  27  to be separated into chip-shaped pieces by dicing or the like. The GaN epitaxial layer  20  in the sacrifice region  29  is discarded, and a plurality of individual vertical topology GaN LED units  60  are then obtained by the individual LED units  281  in the preserve region  28 . 
     The present method for fabricating LED units  60  includes forming a plurality of exhaust trenches  27  on the GaN epitaxial layer  20 , such that nitrogen generated during the LLO process can be exhausted from the exhaust trenches  27 . Thus, cracking caused by high-energy laser beam irradiation on the GaN epitaxial layer  20  is reduced, and fabrication yield of the LED units  60  can be increased. 
     It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages.