Patent Publication Number: US-8975658-B2

Title: Shunting layer arrangement for LEDs

Description:
FIELD OF THE INVENTION 
     This invention relates to light emitting diodes (LEDs) and, in particular, to a patterned metal layer on the light emitting surface LED die that improves current distribution yet does not increase light blockage. 
     BACKGROUND 
     Prior art  FIG. 1  is a top down view of an LED die  10 , and  FIG. 2  is a simplified cross-sectional view of the LED  10  along line  2 - 2  in  FIG. 1 . In the example, the LED die  10  is GaN based and has its growth substrate removed. The structure is well known. A bottom metal anode electrode  12  is typically bonded directly to a submount pad or to a circuit board. A metal reflector  14  over the electrode  12  reflects light upward. The LED&#39;s epitaxially-grown semiconductor layers include a first p-type layer  16 , a p-type cladding layer  18 , an active layer  20 , an n-type cladding layer  22 , a first n-type layer  24 , and a second n-type layer  26 . The various p and n-type layers that interface between the cladding layers and the metal contacts may have different doping amounts and different compositions to achieve different functions such as lattice matching and current spreading. There may be many more layers. The semiconductor layers are transparent. 
     A transparent current spreading layer  28  is formed over the second n-type layer  26 , and a metal cathode electrode  30  is electrically connected to an edge of the current spreading layer  28 . A wire (not shown) is bonded to the cathode electrode  30 . The current-spreading layer material is selected for low optical loss, low resistivity, and good electrical contact. Suitable materials for the current-spreading layer  28  include are Indium Tin Oxide, Zinc Oxide, or other transparent conducting oxides. The current spreading layer  28  is only a few microns thick so has a low vertical resistance and a much higher lateral resistance. It is important that the current distribution over the p-type cladding layer  18  and n-type cladding layer  22  is fairly uniform to achieve uniform light generation across the active layer  20 . 
     To compensate for the relatively high lateral resistance of the current spreading layer  28 , a low-resistance metal shunting layer  32  is patterned to extend across the current spreading layer  28  yet block only a small amount of light. There is a tradeoff between minimizing current crowding and minimizing light blockage. The shunting pattern shown in  FIG. 1  is typical, with metal bus bars along the periphery of the die  10  and perpendicular metal bus bars connecting them. These shunting strips are formed very narrow to minimize light blockage. 
       FIG. 2  shows the current flow through the LED die  10  with thick arrows  36  and some photon trajectories with thin arrows  38 . A simplified emitted light pattern  39  is also shown. 
     The top surface of the LED die  10  is roughened to increase light extraction. 
     One problem with the conventional shunting designs is that the thin shunting strips exhibit a contact resistance at the interface of the strips and the current spreading layer  28 , where the contact resistance is directly related to the width of the strips. 
     For the particular case of a patterned shunting layer characterized by bus bars as shown in  FIG. 1 , the contact resistance of one of the three inner crossing bus bars may be expressed as, 
     
       
         
           
             
               
                 
                   
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     where resistance R s  is the sheet resistance (in Ω/□) of the current spreading layer  28 , L is the length of the bus bar section, w is the width of the bus bar, and L t  is the transfer length, expressed in unit length. The transfer length is defined as, 
     
       
         
           
             
               
                 
                   
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     where ρ c  is the contact resistivity of the metal-semiconductor interface, expressed in Ω·m 2 . 
     As is well known, lateral current between a conductive layer and a metal contact is not uniform across the contact. The voltage is highest near the edge of the contact and drops substantially exponentially with distance. The 1/e distance of the voltage curve is another way to determine transfer length. 
       FIG. 3  represents the above contact resistance expression normalized against R s  as function of the normalized quantity w/L t  for the case of L=L t . The curve indicates that for contact widths smaller than 2L t  the contact resistance increases inversely proportional to w, as 
               R     C   ⁡     (   ric   )         →         R   s     wL     ⁢       L   t   2     .             
On the other hand, for contact widths higher than 2L t  the contact resistance approaches the quantity
 
                 R   s       2   ⁢   L       ⁢     L   t           
as
 
             coth   ⁡     (     w     2   ⁢     L   t         )           
tends to 1.
 
     As seen, the widths of the bus bars in  FIG. 1  cannot be made too small or else the contact resistance will be too high, yet narrow widths are desirable to block less light. 
     Therefore, it would be desirable to reduce the contact resistance between a metal shunting layer and the current spreading layer without adversely impacting the light extraction of the LED die. Conversely, it would be desirable to increase the light extraction of the LED die without reducing the contact resistance between a metal shunting layer and the current spreading layer. It is also desirable to improve the current distribution uniformity across the surface of the LED die. 
     SUMMARY 
     Various metal shunting patterns are disclosed herein that reduce contact resistance and improve current distribution uniformity without reducing light extraction. 
     In one embodiment, the shunting pattern comprises an array of metal circular dots having diameters that are wider than the widths of conventional bus bars and cross bars, but are on the order of 2L t -10L t  so as not to block a significant amount of light. In one embodiment, the radius of each dot is greater than 2L t  and less than 10L t , and preferably less than 5L t . The total dot area is less than the total area of the prior art bus bars and cross bars, so there is less blockage of light. Shapes other than circular dots can be used, such as polygons (e.g., squares and rectangles). All such shapes are referred to herein as dots. 
     In one embodiment, the widths of the dots (between 2L t  and 10L t ) are about 15 microns for the typical metals used and current spreading layer used, in order to ensure low contact resistance. Each dot represents a current injection area. Typically, there would be a density of 50-60 discrete injection areas per square millimeter for good current distribution. For a minimum 2L t  width and 50 square dots per mm 2 , the top surface area of an LED die will have about 1% of its surface covered by the dots. For a large die of 1 mm 2 , the total area of the dots will be about 0.01 mm 2 . In one embodiment, the top surface area of an LED die covered by the dots is preferably less than 5%. 
     To cause the current to be evenly distributed over the top surface of the LED, the dots are connected with a grid of very thin metal connectors, where the contact resistance between the metal connectors and the current spreading layer is relatively high, due the width of the connectors being much less than 2L t , but has little effect on current injection since the current is being injected by the dots. 
     As a result of the dot array, there is a lower overall contact resistance and less light blockage, thus improving the efficiency of the LED. 
     In one embodiment, a wire bond electrode is formed near the middle of the top surface of the LED to create a more uniform current distribution. 
     In one embodiment, in addition to the array of dots being interconnected by a grid of thin metal connectors, some dots are also connected to the wire bond electrode with radially extending thin metal connectors to cause the connection resistances between the dots and the wire bond electrode to be more uniform. 
     In one embodiment, the dots are formed larger as the dots are located further from the wire bond electrode to create more uniform current distribution over the entire surface of the LED. 
     In one embodiment, the dots are spaced closer and closer together as they extend away from the wire bond electrode to create more uniform current distribution. 
     In one embodiment, there is a dielectric between the wire bond electrode and the current spreading layer to reduce current crowding under and around the periphery of the wire bond electrode. 
     In an alternative embodiment, and in order to avoid the use of a dielectric layer between the wire bond electrode and the current spreading layer, a concentric shunting ring surrounding the wire bond electrode at a certain distance is used to reduce current crowding under and around the periphery of the wire bond electrode. 
     In an embodiment where there is a shunting bar that extends around the periphery of the top surface of the LED, the width of the bar is reduced near the corners to reduce or eliminate current crowding near the corners. 
     In one embodiment, an angled mirror structure is formed beneath each dot and connecting grid, The mirror below each dot and connector not only reflects light away from the absorbing underside of each dot/connector but also avoids any current crowding directly below each dot (and to a lesser extend below each connector) by causing the active layer below each dot to not generate light. In one embodiment, each mirror is formed in a trench that extends through the active layer below each dot and connector. 
     Other embodiments are described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top down view of a top surface of a prior art LED die showing a metal shunting pattern. 
         FIG. 2  is a cross-sectional view along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a graph of normalized width of a contact and the normalized contact resistance, showing that normalized widths less than 2L t  result in higher and higher contact resistances. 
         FIG. 4  is a top down view of the top surface of an LED showing a metal shunting pattern in accordance with one embodiment of the invention. 
         FIG. 5  shows the shunting pattern of  FIG. 4  with a relatively large wire bond electrode located near the middle of the LED die surface for substantially uniform current distribution. 
         FIG. 6  shows the shunting pattern of  FIG. 5  with additional radial connectors between the wire bond electrode and various dots. 
         FIG. 7  is a top down view of the top surface of an LED showing a metal shunting pattern having dots that increase in size as the dots are located further and further away from the wire bond electrode. 
         FIG. 8  is a top down view of the top surface of an LED showing a metal shunting pattern having dots that increase in density as the dots are located further and further away from the wire bond electrode. 
         FIG. 9  is a top down view of the top surface of an LED showing a metal shunting pattern having square dots and an enlarged center wire bond dot. 
         FIG. 10  is a cross-sectional view of the wire bond electrode area having an underlying dielectric layer to avoid current crowding under the electrode. 
         FIG. 11  is a top down view of the top surface of an LED showing a metal shunting ring pattern that surrounds the wire bond electrode to mitigate current crowding under and around the periphery of the wire bond electrode. 
         FIG. 12  is a top down view of the top surface of a prior art LED die showing a metal shunt that goes around the periphery of the top surface, similar to  FIG. 1 . 
         FIG. 13  is a magnified top down view of the corner of an LED die showing how current crowding near the corner can be avoided by reducing with width of the shunt at the corner. The same technique would be used at the corners of any crossing bus bars. 
         FIG. 14  is a cross-sectional view of an LED die in accordance with one embodiment of the invention, where angled mirrors are formed in trenches below each dot. 
     
    
    
     Elements that are the same or equivalent are labeled with the same numeral. 
     DETAILED DESCRIPTION 
       FIG. 4  illustrates one embodiment of a metal shunting pattern  40  on the top surface of an LED die, in accordance with one embodiment of the invention. The LED die may have the same layers as the prior art LED die in  FIG. 2 . 
     According to equation 1 above, one way to control the location of current injection into the semiconductor along the bus bar is by means of properly adjusting the geometric parameter w. Circular contacts  42  (dots) are preferred due to their substantially uniform current pattern. The contact resistance of a circle contact of radius r c  can be expressed as follows, 
     
       
         
           
             
               
                 
                   
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     In equation 3, I0 and I1 are the modified Bessel functions of the first and second kind, respectively. Like in the case of the bus bar, the contact resistance of a circle contact increases dramatically for r c &lt;2L t . Therefore, in the preferred embodiment, the radius of each circular contact is between about 2L t  to 10L t . 
     Accordingly, a shunting layer pattern may consist of a number of geometric shapes whose characteristics allow to selectively control the locations of current injection but are limited in size to not adversely affect light output. This can be applied, for instance, to improve current uniformity through the active layer of the device with minimum metal-semiconductor contact area. 
     Narrow metal connectors  44  are arranged in a grid to connect the contacts  42  together. The connectors  44  have widths preferably less than 2L t  since they are not required to inject current into the LED, and wider connectors will increase the blockage of light. 
     The contacts  42  and connectors  44  preferably are a multilayer composition of metals that provide low resistance yet do not migrate into the semiconductor layers. 
       FIG. 5  shows the shunting pattern of  FIG. 4  with a relatively large wire bond electrode  46  located near the middle of the LED die surface for substantially uniform current distribution. The size of the electrode  46  is preferably a minimum size to achieve a good wire bond. 
       FIG. 6  shows the shunting pattern of  FIG. 5  with additional radial connectors  48  between the wire bond electrode  46  and various contacts  42 . These radial connectors  48  provide a parallel connector path to the outer contacts  42  for more uniform current distribution, since the combined resistances of the grid connector  44  paths increase from the wire bond electrode  46 . 
       FIG. 7  is a top down view of the top surface of an LED showing a metal shunting pattern having contacts  50  that increase in size (diameter) as the contacts  50  are located further and further away from the wire bond electrode  46 . The larger area contacts inherently reduce the space between contacts near the perimeter, this increasing the current injection near the perimeter to offset the increased resistance of the connectors  44  and  48  leading to the outer contacts  50 . 
       FIG. 8  is a top down view of the top surface of an LED showing a metal shunting pattern having contacts  54  that increase in density as the contacts  54  are located further and further away from the wire bond electrode  46  to achieve a more uniform current density. 
       FIG. 9  is a top down view of the top surface of an LED die  55  showing a metal shunting pattern having square dots  56 , an enlarged center wire bond dot  57 , and narrow connectors  58  connecting the dots. The arrangements and widths of the square dots may be similar to those of the circular dots described above. 
     In one embodiment, the widths of the dots (between 2L t  and 10L t ) are about 15 microns for the typical metals used and current spreading layer used, in order to ensure low contact resistance (based on a graph similar to  FIG. 3  for the particular materials used). Each dot represents a current injection area. Typically, there would be a density of 50-60 discrete injection areas per square millimeter for good current distribution. For a minimum 2L t  width and 50 square dots per mm 2 , the top surface area of an LED die will have about 1% of its surface covered by the dots. For a large die of 1 mm 2 , the total area of the dots will be about 0.01 mm 2 . Circular dots of the same width as square dots cover less area, so would block less light. In one embodiment, the top surface area of an LED die covered by the dots is preferably less than 5%, such as 2%. Widths of the dots less than 5L t  but slightly greater than 2L t  are preferred, since widths greater than 2L t  do not provide significantly reduced contact resistance, and light blockage should be minimized. 
       FIG. 10  is a cross-sectional view of the wire bond electrode  46  area having an underlying dielectric layer  64  to avoid current crowding under and around the periphery of the electrode  46 . The metal contacts the current spreading layer  28  with a ring having a width Wx. Wx is preferably 0.5L t &lt;Wx&lt;L t . Also shown is a wire  60  and bond metal  62 . 
       FIG. 11  shows a concentric shunting ring  65  surrounding the wire bond electrode  46  at a certain distance. The shunting ring  65  reduces current crowding under and around the periphery of the wire bond electrode  46 . The width (Wr) of the shunting ring  65  is proportional to L t , preferable higher than 0.1L t  and lower than L t , to provide an adequately low current resistance. The diameter (D) of the ring  65  is preferably at least 20% larger than the diameter of the wire bond electrode  46 . 
       FIG. 12  is a top down view of the top surface of a prior art LED die showing a metal shunt  66  that goes around the periphery of the top surface of the LED, similar to  FIG. 1 , with a wire bond electrode  68  near one corner. Due to the arms of the shunt  66  approaching each other at each corner, there will be current crowding near the corners, resulting in non-uniform light output and perhaps an over-current in those areas. To substantially prevent such current crowding in the corners, the metal shunt configuration of  FIG. 13  may be used. 
       FIG. 13  is a close up of one corner of an LED die, showing that the metal shunt  70  has a reduced width Wc in the corners to reduce the current injection from each arm near the corner. Wc is preferably less than L t  (e.g., 0.1L t ) to increase the contact resistance needed to create a substantially uniform current distribution near the corner. The width of the remaining part of the shunt is greater than L t . Unlike the inner contacts, in the edge contacts, current flows from only one side of the contact area and hence the 2L t  minimum width does not apply here. A wire bond electrode is preferably located midway along a shunt arm to avoid current crowding near a corner. Each corner will be similar to  FIG. 13 . 
     The same technique would be used at the corners of any crossing bus bars. 
     The contacts  42 ,  50 ,  56  in the middle area of the LED die are connected to the metal shunt  70  using the narrow connectors  44 ,  48  previously described. 
       FIG. 14  is a cross-sectional view of an LED die in accordance with one embodiment of the invention, where angled mirrors  76  are formed in trenches  78  below each circular contact  80  and grid connector. The mirrors  76  reduce the blockage of light by the overlying circular contacts  80  and connectors and prevent the creation of high current density regions below each circular contact  80  and to a lesser extent below the connectors. Details regarding the formation of such mirrors  76  are found in U.S. application Ser. No. 12/770,550, by Rafael Aldaz, filed on 30 Apr. 2010, incorporated herein by reference. 
     The geometric shapes of the mirrors  76  may be tailored to enhance light extraction efficiency. This is provided that the top contacts  80  (similar or identical to any of the contacts described previously) can be combined with the use of mirror walls located in the semiconductor underneath the contacts  80 , as depicted in  FIG. 14 . The illustration shows a case where the mirror  76  (usually a metal) penetrates into the semiconductor and crosses the active layer  20  in the regions underneath the contacts  80 . To prevent electrical shorts between layers, the mirror  76  is covered with a transparent dielectric  84 . The population within the chip of these mirror walls enhances light extraction in detriment of reducing active area where photons generate. Because of this trade-off, it is preferable that the width Ws of each contact  80  is minimized and hence the number of mirrors maximized. This, in turn, translates into the minimization of the distance between mirrors that maximizes light extraction. 
     Preferably, the pattern of the shunting layer should be designed to optimize the following performance related aspects:
         Uniform current injection into the semiconductor active layer (e.g., control distribution of contacts)   Minimization of voltage drop across the shunting layer (e.g., use thin metal connectors)   Maximization of light extraction (e.g., optimize size of contacts and form mirrors)   Maximization of active area (e.g., optimize size of mirrors)       

     While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.