Abstract:
Based on the unique properties of the flip chip packaging process and GaN based LEDs with transparent substrates, new principles and methods for designing the layout of P contact pads and N contact pads are disclosed. The new designs of the present invention drastically increase the light extraction efficiency of LEDs by reducing the current crowding effect, increasing the uniformity of the spreading current in the active layer, and utilizing most of the available light emitting semiconductor material of the active layer. The present invention combined with the flip chip packaging process significantly improves the LEDs&#39; heat dissipation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to new P and N contact pads layout designs of GaN based Light Emitting Diodes (LEDs) with transparent substrates for flip chip packaging and a new method of manufacturing the same. This invention drastically increases light extraction efficiency of GaN based LEDs. This invention makes a major improvement on the LED&#39;s heat dissipation.  
         [0003]     2. Prior Art  
         [0004]     There are three major issues for the LED design and manufacture: the current crowding effect, the heat dissipation problem, and the problem of a large contact pad blocking the emitted light.  
         [0005]     Given the common LED die designs, the electrical current can&#39;t be evenly spread through the LED active layer or most of the current concentrates at a portion of the active layer (the current crowding effect). The current crowding effect is one of the primary limiting factors in LED die design and manufacture. It results in an unstable luminous flux output with drifting bright and dim spots on the LED chip and it prevents the effective usage of the available light emitting semiconductor material and the low quantum yield in term of the total active material. For high power LEDs, the current crowding effect limits the output luminous flux.  
         [0006]     One of the approaches to reduce the currently crowding effect is to widen the current path by applying a current spreading layer. The effectiveness of the active layer depends on the current spread layer&#39;s thickness.  
         [0007]     The flip chip packaging flips LED chips to face a submount with better thermal conductivity compared to the original substrate that the device is fabricated on. The flip chip packaging method completely eliminates the issue of large contact pads hindering the extraction of light and releases all the restrictions on the contact pad design that are related with the hindering effect. With the flip chip packaging method, the contact area of P and N contact pads can be designed very differently to minimize the current crowding effect and utilize the entire active layer.  
         [0008]     There are varieties of prior art discussing flip chip packaging technology for gallium nitride (GaN) based LEDs with transparent substrate, including U.S. Pat. No. 6,483,196 B1 by Wojnarowski et al. for flip chip, U.S. Pat. No. 6,455,878 B1 by Bhat et al. for a low refractive index under fill, and U.S. Pat. No. 6,649,437 by Yang et al. for a manufacturing method. However there lacks of prior art that discloses other alternative P and N contact pad layout design rather than the conventional ones for GaN based LEDs with flip chip packaging. The advantages of applying flip chip packaging for the LEDs are far from having been realized and utilized. The increasingly demands to manufacture high efficiency and high power LEDs cost effectively requires new designs of P and N contact pads layout of GaN based LEDs.  
       SUMMARY OF THE INVENTION  
       [0009]     In the present invention, new principles, methods, and embodiments of new designs of P and N contact pad layout of GaN based LEDs with transparent substrate for flip chip packaging are disclosed.  
         [0010]     The primary object and advantage of this invention is to provide new principles for designing P and N contact pad layout for flip chip packaging of GaN based LEDs with high extraction efficiency of emitted light.  
         [0011]     The second object and advantage is to provide new P and N contact pad layout designs for efficiently utilizing light emitting material of active layer.  
         [0012]     The third object and advantage is to provide new P and N contact pad layout designs for uniformly distributing the current and, thus increasing the current density.  
         [0013]     The fourth object and advantage is to provide new P and N contact pad layout designs for more uniform and bright surface emission.  
         [0014]     The fifth object and advantage is to provide new P and N contact pad layout designs for reducing current crowding effects.  
         [0015]     The sixth object and advantage is to provide new P and N contact pad layout designs for generating less heat and improving heat dissipation when LEDs are flip chip bonded to a substrate with better thermal conductivity.  
         [0016]     The seventh objective and advantage is to provide new P and N contact pad layout designs without employing current spreading layer.  
         [0017]     Further objects and advantages of the present invention will become apparent from a consideration of the following description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS  
       [0018]     The novel features believed characteristics of the present invention are set forth in the claims. The invention itself, as well as other features and advantages thereof will be best understood by referring to detailed descriptions that follow, when read in conjunction with the accompanying drawings.  
         [0019]      FIG. 1   a  is a cross-sectional view of a GaN based LED of prior art.  
         [0020]      FIG. 1   b  is a cross-sectional view of flip chip packaging of the GaN based LED of  FIG. 1   a  bonded on a submount of prior art.  
         [0021]      FIG. 2   a  is a top view of a preferred embodiment of new designed layout of LEDs with a P contact pad at the center portion.  
         [0022]      FIG. 2   b  is a cross-sectional view of the LED of  FIG. 2   a.    
         [0023]      FIG. 2   c  is a top view of a submount for bonding the LED of  FIG. 2   a  to it.  
         [0024]      FIG. 2   d  is a cross-sectional view of the LED mounted on the submount of  FIG. 2   c.    
         [0025]      FIG. 2   e  is a top view of another submount with ball bumps for bonding the LED of  FIG. 2   a  to it.  
         [0026]      FIG. 2   f  is a cross-sectional view of the submount of  FIG. 2   e.    
         [0027]      FIG. 3   a  is a top view of a preferred embodiment of new designed layout of LEDs with a N contact pad at the center portion.  
         [0028]      FIG. 3   b  is a cross-sectional view of the LED of  FIG. 3   a.    
         [0029]      FIG. 4   a  is a top view of a preferred embodiment of new designed layout of LEDs with a plurality of N contact pads surrounded by a P contact pad.  
         [0030]      FIG. 4   b  is a top view of a preferred embodiment of new designed layout of LEDs with a plurality of P contact pad surrounded and separated by a cross-ring shaped N contact pad.  
         [0031]      FIG. 4   c  is a top view of a preferred embodiment of new designed layout of LEDs with a plurality of triangle-shaped N contact pads respectively embedded in multiple P contact pads that is surrounded and separated by a cross-ring shaped N contact pad.  
         [0032]      FIG. 4   d  is a top view of a preferred embodiment of new designed layout of LEDs with a plurality of P contact pads surrounded and separated by a N contact pad.  
         [0033]      FIG. 5   a  is a top view of a preferred embodiment of new designed layout of LEDs with a N contact pad at the center portion.  
         [0034]      FIG. 5   b  is a cross-sectional view of the LED of  FIG. 5   a.    
         [0035]      FIG. 6   a  is a top view of a preferred embodiment of new designed layout of LEDs with a P contact pad at the center portion.  
         [0036]      FIG. 6   b  is a cross-sectional view of the LED of  FIG. 6   a.    
         [0037]      FIG. 6   c  is a top view of a submount for bonding the LED of  FIG. 6   a  to it.  
         [0038]      FIG. 7   a  is a top view of a preferred embodiment of new designed layout of LEDs with fork-shaped N and P contact pads.  
         [0039]      FIG. 7   b  is a top view of a preferred embodiment of new designed layout of LEDs with fork-projection-shaped N contact pads.  
         [0040]      FIG. 7   c  is a top view of a preferred embodiment of new designed layout of LEDs with more complicated fork-projection-shaped N contact pads.  
         [0041]      FIG. 8   a  is a top view of a preferred embodiment of new designed layout of LEDs with a first P contact pad surrounded by a N contact pad which is surrounded by a second P contact pad.  
         [0042]      FIG. 8   b  is a cross-sectional view of the LED of  FIG. 8   a.    
         [0043]      FIG. 9   a  is a top view of a preferred embodiment of new designed layout of LEDs with a first N contact pad surrounded by a P contact pad which is surrounded by a second N contact pad.  
         [0044]      FIG. 9   b  is a cross-sectional view of the LED of  FIG. 9   a.    
         [0045]      FIG. 9   c  is a top view of a submount for bonding the LED of  FIG. 9   a  to it.  
         [0046]      FIG. 9   d  is a top view of a preferred embodiment of new designed layout LED with multiple P and N contact pads alternately surrounding each other. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]     With the application of the flip chip packaging process to LEDs layout design and manufacture, the conventional principles for P and N contact pad layout designs of GaN based LEDs need to be modified. The quantity, sizes, shapes, and positions of P and N contact pads all become useful variables for optimizing the contact pad layout designs. The designs of P and N contact pad layout of LEDs can be focused on certain issues such as the current crowding effect and the utilization of the light emitting semiconductor material of the active region.  
         [0048]     The P contact pad can be designed with larger area and different shapes. The larger contact area will reduce the contact resistance and therefore the heat generation, because the contact resistance is inversely proportional to the contact area. Multiple P and N contact pads can be integrated into one LED die.  
         [0049]     While embodiments of the present invention will be described below, those skilled in the art will recognize that other designs and methods are capable of implementing the principles and scope of the present invention. Thus the following description is illustrative only and not limiting.  
         [0050]     Note the followings that apply to all of new designed P and N contact pad layout of the present invention: 
        (1) The dimensions of all of drawings are not to scale;     (2) The P and N contact pads in each figure may have different shapes other than what shown in the figures.     (3) P contact pads and N contact pads may be interchanged and the current flow reversed and, then the LEDs are still function.     (4) Quantity of P and N contact pads of LEDs of the present invention may vary depending on the sizes of the LEDs and P and N contact pads. The area of N contact pad(s) is much smaller than that of P contact pad(s). Although separations between P and N contact pads are not shown in  FIGS. 2   a ,  3   a ,  4   a ,  4   b ,  4   c ,  4   d ,  5   a ,  6   a ,  7   a ,  7   b ,  7   c ,  8   a ,  9   a , and  9   d,  P contact pads and N contact pads in all of LEDs of the present invention are separated electrically by mesa edges. Mesa(s) is formed by a mesa etch process. Mesa is only showed in  FIGS. 2   b  and  3   b,  since the space limitation. The current spreading layer on top of P confinement layer is no longer necessary, because P contact pads may be made as large as needed up to cover a large portion of or even the entire top surface of mesa(s).     (5) All of embodiments of LEDs of the present invention shown in  FIG. 2  to  FIG. 9  have the same epitaxial structure, i.e., an epitaxial layer is grown on a transparent substrate. The epitaxial layer comprises the P and the N confinement layers and an active region (or layer) sandwiched in between.     (6) There is a reflective layer between the P contact pad and the P confinement layer, which reflects the emitted light towards to substrate, although the reflective layer are not shown in some of FIGS.     (7) The design principles of the present invention may apply to other LEDs with different epitaxial structures as long as either the substrate is transparent or the non-transparent substrate is removed after flip chip bonding.     (8) A N contact pad is disposed on the N confinement layer and its elevation may be either lower than or equal to that of P contact pad.     (9) Although the P confinement layer is shown on top of the N confinement layer in all of cross-sectional views of preferred embodiments, their positions may be reversed for other preferred embodiments of the present invention.     (10) The P contact pads in new P and N contact pad layout designs of LEDs of the present invention are much larger than that of conventional LEDs, so that the LEDs have much better thermal performance.     (11) Four embodiments of submounts of the present invention are shown in  FIGS. 2   c,    2   e,    6   c,  and  9   c  for the LEDs. However, following the same principles, submounts for all of LEDs with new P and N contact pad layout designs of the present invention may be designed without difficulty. The principles for design a submount are that positions and shapes of N bumps of the submount should match up with that of the corresponding N contact pads of LEDs and that N bumps are electrically connected to each other, although N pads of LEDs may not be electrically connected. It is similar design principle for the P bumps of a submount.     (12) The P and the N bumps on submounts may have different forms, the ball shape bump and the flat bonding surface bump. The present invention utilizes the major advantages of the flat bonding surface bumps over the ball bumps: (1) having significantly larger contact area (especially for the P bump); and (2) capable to integrate multiple P and N contact pads on one LED. The larger contact area of the bump and pad yields higher heat transfer rate, which is critical for the high power LEDs including the white LEDs. Depositing multiple P and N flat bonding surface bumps on a submount and making multiple P and N contact pads on one LED result in a better uniformity of the current distribution and spreading.        
 
         [0063]     For a simple P and N contact pad layout design of LEDs, such as  FIG. 2   a , ball bumps on submount may be used to bond LEDs to submounts. For complex patterns of P and N contact pad layout designs, such as in  FIG. 9   d,  the flat bonding surface bumps have to be used due to the limited surface area of LEDs and the minimum size of ball bump. 
        (13) The elevations of P bump, N bump, P contact pad, and N contact pad are determined such that both P and N contact pads of LEDs can be bonded simultaneously to their corresponding bump of a submount when a LED flip chip bonded to the submount. The top surface of N contact pad may be either in the same elevation as P contact pad or lower.        
 
         [0065]      FIG. 1   a  is a cross-sectional view of a GaN base LED of prior art. N confinement layer  11  is grown on substrate  10  and etched at one side for depositing N contact pad  12 . P contact pad  14  is grown on P confinement layer  13 . There is current spreading layer  15  deposited on P confinement layer  13 . When powered up the LED, current  16  and current  17  flow respectively from P contact pad  14  and current spreading layer  15  to N contact pad  12 . P contact pad  14  sizing about  100  micrometer blocks light emitted from the active region. Current spreading layer  15  is not fully transparent and, therefore, it blocks the emitted light too.  
         [0066]      FIG. 1   b  shows a flip chip packaging of the LED of  FIG. 1 a  on submount  19 . Bump bonding pad  196  and  195  connect to P bonding pad  193  and N bonding pad  194  respectively. Ball bump  191  bonds bump bonding pad  196  to P contact pad  14 . Ball bump  192  bonds bump bonding pad  195  to N contact pad  12 . The minimum size of ball bumps can introduce restrictions on P and N contact pad layout design.  
         [0067]      FIG. 2   a  is a top view of a LED of the present invention with P contact pad  22  at the center portion of the LED. N contact pad  21  surrounds P contact pad  22 . Note that the P and N contact pads in this layout design may have other shapes, such as circular. Dotted current flow line  24  shows the direction of current flow.  
         [0068]      FIG. 2   b  shows a cross-sectional view of the LED in  FIG. 2   a.  N confinement layer  20  disposes on substrate  27 . P confinement layer  26  grows on active region  23  that disposes on N confinement layer  20 . P contact pad  22  disposes on reflective layer  25  that is on P confinement layer  26 . N contact pad  21  contacts N confinement layer  20  by etching down to N confinement layer  20  first. The etching process generates mesa  200 . The edge of the mesa  200  separates P contact pad  22  from N contact pad  21 . The current  24  flows from P contact pad  22  to N contact pad  21  through active region  23 .  
         [0069]     Note that the elevation of N contact pad  21  is the same as that of P contact pad in  FIG. 2   b,  however, it can be lower.  
         [0070]      FIG. 2   c  shows a top view of an embodiment of submount  251  for the LED of  FIG. 2   a  to be bonded on. N bonding pad  28  and P bonding pad  29  are disposed on submount  251 . N bonding pad  28  connects electrically with N flat bonding surface bump  211  and is for wire bonding to external power source. P bonding pad  29  connects electrically with P flat bonding surface bump  221  and is for wire bonding to external power source. N and P bonding pad  28  and  29  are separated electrically. N flat bonding surface bump  211  is electrically separated from P flat bonding surface bump  221 .  
         [0071]      FIG. 2   d  is a cross-sectional view of the LED of  FIG. 2   a  mounted on submount  251 . The LED of  FIG. 2   a  is flip chip bonded to submount  251 . P and N contact pad  22  and  21  are respectively bonded to P flat bonding surface bump  221  and N flat bonding surface bump  211 . It is more efficiency way to transfer the heat from the LED to the submount, because a larger area of the LED is bonded to the submount by the soldering metal instead of the underfill organic polymer. The elevations of both N flat bonding surface bump  211  and P flat bonding surface bump  221  are higher than that of N and P bonding pad  28  and  29 .  
         [0072]     Note that the elevations of P flat bonding surface bump  221 , N flat bonding surface bump  211 , P contact pad  22 , and N contact pad  21  are so determined that P flat bonding surface bump  221  and N flat bonding surface bump  211  are respectively bonded to P contact pad  22  and N contact pad  21 .  
         [0073]      FIG. 2   e  shows a top view of another embodiment of submount for the LED of  FIG. 2   a  to be bonded on. N bonding pad  28  and P bonding pad  29  dispose on submount  252 . N bonding pad  28  connects electrically with N ball bump  261  and is for wire bonding to external power source. P bonding pad  29  connects electrically with P ball bump  271  and is for wire bonding.  
         [0074]      FIG. 2   f  is a cross-sectional view of submount  252 . P and N ball bump  261  and  271  are ball shape like the ball bumps for conventional flip chip packaging.  
         [0075]     The elevations of the top surface of both N ball bump  261  and P ball bump  271  may be different, depending on the elevations of N contact pad  21  and P contact pad  22  of the LED of  FIG. 2   a.    
         [0076]      FIG. 3   a  is a top view of a LED. N contact pad  31  is at the center portion of the LED and surrounded by P contact pad  32 . P and N contact pad  32  and  31  may be in different shapes respectively. Dotted current flow line  35  indicates the direction of current flow.  
         [0077]      FIG. 3   b  is a cross-sectional view of the LED of  FIG. 3a . Current  35  flows from P contact pad  32  to N contact pad  31  through active layer  33 . N contact pad  31  is disposed on N confinement layer  39 . Reflective layer  34  is sandwiched between P contact pad  32  and P confinement layer  38 .  
         [0078]     In  FIG. 3   b,  only a portion of N contact pad  31  is shown, since space is need to place symbol “{” for indicating mesa  100 .  
         [0079]     Quantity of each of P and N contact pads in  FIG. 2   a  and  FIG. 3   a  may be more than one as long as P and N contact pads are separated and alternately surrounded by each other.  
         [0080]      FIG. 4   a  shows a new designed layout for a LED of the present invention. A plurality of N contact pad  42  are separated and surrounded by P contact pad  41 . N contact pads  42  may be in different shapes, such as rectangular. Quantity of N contact pads may be either more or less than  4 . Dotted current flow line  40  in  FIG. 4   a  to  FIG. 4   d  indicates the direction of current flow.  
         [0081]      FIG. 4   b  shows a new designed layout of a LED of the present invention. There are four of triangle-shaped P contact pad  43  separated by cross-ring shaped N contact pad  44 . P contact pad  43  may be in different shape, such as circular. Quantity of P contact pad  43  may be either more or less than 4.  
         [0082]      FIG. 4   c  shows a new designed layout of a LED with four of triangle-shaped N contact pad  46  embedded in four of triangle-shaped P contact pad  45  respectively. Multiple P contact pad  45  are separated by cross-ring shaped N contact pad  44 .  
         [0083]      FIG. 4   d  shows a new design for LEDs. Four of rectangular-shaped P contact pad  48  are separated and surrounded by cross-ring shaped N contact pad  47 . Quantity of P contact pads may be more or less than 4. P contact pad  48  may be in different shape, such as circular. Submounts may be designed for the LED of  FIG. 4   a  to  4   d.  Quantity and positions of N and P flat bonding surface bumps on the submount need to match that of multiple N and P contact pads respectively. All the N and P flat bonding surface bumps need to be electrically connected respectively.  
         [0084]      FIG. 5   a  shows a top view of a LED with stripe-shaped N contact pad  50 ,  53 ,  54 , and P contact pad  51  and  52 . N contact pad  50 ,  53 , and  54  are separated by P contact pad  51  and  52  respectively. Dotted current flow line  55 ,  57 ,  58 , and  59  indicate the direction of current flow.  
         [0085]      FIG. 5   b  is a cross-sectional view of the LED of  FIG. 5   a.  Current  55  flows from P contact pad  51  to N contact pad  50  through active region  56 . Current  57  flows from P contact pad  51  to N contact pad  53  through active region  56 . Current  58  flows from P contact pad  52  to N contact pad  53  through active region  56 . Current  59  flows from P contact pad  52  to N contact pad  54  through active region  56 . Reflective layer  503  is sandwiched between P confinement layer  502  and both of P contact pad  51  and  52 . Active layer  56  disposes between P confinement  502  and N confinement  501  that is grown on substrate  500 .  
         [0086]     While N contact pad  53  is at the center portion of a LED in  FIG. 5   a ,  FIG. 6   a  shows a LED with P contact pad  63  at the center portion. P contact pad  61 ,  63 , and  65  are separated by N contact pad  62  and  64  respectively. Dotted current flow line  691 ,  692 ,  693 , and  694  indicate the direction of current flow.  
         [0087]      FIG. 6   b  is a cross-sectional view of a LED of  FIG. 6   a.  Current  691  flows from P contact pad  61  to N contact pad  62  through active region  66 . Current  692  flows from P contact pad  63  to N contact pad  62  through active region  66 . Current  693  flows from P contact pad  63  to N contact pad  64  through active region  66 . Current  694  flows from P contact pad  65  to N contact pad  64  through active region  66 . Reflective layer  695  is sandwiched between P confinement layer  69  and three of P contact layer  61 ,  63 , and  65 . Active layer  66  is between P confinement layer  69  and N confinement layer  67  that is grown on substrate  68 .  
         [0088]     Note that quantity of N pads and P pads may be either more or less than what shown in  FIG. 5  and  FIG. 6  respectively, depending on the sizes of P and N contact pads and LEDs.  
         [0089]     The elevations of N contact pad  62  and  64  are lower than that of P contact pad  61 ,  63 , and  65 . However the elevations of N contact pads may be the same as that of P contact pads.  
         [0090]     With either narrowed sizes of P and N contact pads or a LED with larger surface area (this is the case of high power LED), more P and N contact pads may be disposed on the LED as long as they are separated by each other. Therefore, uniformed current distribution and spreading can be achieved.  
         [0091]      FIG. 6   c  is a top view of a submount for the LED of  FIG. 6   a  to bond on. P flat bonding surface bump  611 ,  631 , and  651  disposed on the submount are electrically connected to P bonding pad  612  and will be bonded to P contact pad  61 ,  63 , and  65  respectively. N flat bonding surface bump  621  and  641  disposed on the submount are electrically connected to N bonding pad  622  and will be bonded to N contact pad  62  and  64  respectively. P and N bonding pads  612  and  622  are for wire bonding to external power source.  
         [0092]      FIG. 7   a  shows a new designed LED that comprises fork-shaped P contact pad  70  and fork-shaped N contact pad  71 . Fork-shaped P contact pad  70  has three legs, P leg  701 ,  702 , and  703 . Fork-shaped N contact pad  71  has two legs, N leg  711  and  712 . P leg and N leg point to opposite directions. At least portions of N leg  711  and  712  are interspersed with and separated from portions of P leg  701 ,  702 , and  703 . P leg  701 ,  702 , and  703  are electrically connected. N leg  711  and  712  are electrically connected. Current flows from P leg  701  and  702  to N leg  711 . Current flows from P leg  702  and  703  to N leg  712 . Dotted current flow line  700  indicates the direction of current flow.  
         [0093]      FIG. 7   b  is a layout of a fork-projection-shaped LED that comprising P and N fork  72  and  73 . P fork  72  have P leg  721 ,  722 , and  723 . N fork  73  has N leg  731  and  732 . P leg  721 ,  722 , and  723  are separated by N leg  731  and  732  respectively. Projection  791  and  792  of N leg  732  extend into opposite directions and into P leg  723  and  722  respectively. Projection  781  and  782  of N leg  731  extend into opposite directions and into P leg  721  and  722  respectively. Dotted current flow line  700  show the direction of current flow. Current flows from P leg  721  and  722  to N leg  731  and its projections. Current flows from P leg  722  and  723  to N leg  732  and its projections.  
         [0094]     Note each of the P and N forks may have different number of P and N legs. N legs may have either more or less projections.  
         [0095]      FIG. 7   c  is a modification of LED layout design of  FIG. 7   b.  P fork  74  has P leg  741 ,  742 , and  743 . N fork  75  has N leg  751  and  752 . P leg  741 ,  742 , and  743  are separated by N leg  751  and  752  respectively. Projection  763  and  762  of N leg  752  extend into opposite directions and into P leg  743  and  742  respectively. Projection  761  and  764  of N leg  751  extend into opposite directions and into P leg  742  and  741  respectively.  
         [0096]     A portion of Projection  762  of N leg  752  of N contact pad  75  is disposed between and spaced apart from respective portion of two of projection  761  of N leg  751 . Other projections are disposed in the same way. Dotted current flow line  700  show the direction of current flow. In this layout, the current distribution and spreading are more uniform.  
         [0097]     Note that depending on the sizes of LEDs, P and N legs, and projections, especially for high power LED with larger die size, fork-shaped P and N contact pads may have more P legs and N legs in order to have current distribution and spreading uniformly. P and N legs may have more projections. The quantity of projections of legs and legs of contact pads are not limited to what shown in  FIG. 7   b  and  7 c. Projections may also extend from P legs of P contact pad into N legs.  
         [0098]      FIG. 8   a  shows a new designed LED. There is first P contact pad  82  surrounded by N contact pad  81  that is surrounded by second P contact pad  80 . Dotted current flow line  800 ,  84 ,  85 ,  86 , and  87  indicate the direction of current flow.  
         [0099]      FIG. 8   b  is the cross-sectional view of the LED of  FIG. 8a . Current  84  and  85  respectively flow from P contact pad  80  and  82  to N contact pad  81  through active region  83 . Current  86  and  87  respectively flow from P contact pad  80  and  82  to N contact pad  81  through active region  83 . The reflective layer disposed between P contact pad  82  and  80  and P confinement layer  891  is not shown in  FIG. 8b . N confinement layer  89  is disposed on substrate  88 .  
         [0100]      FIG. 9   a  shows a new designed LED. There is first N contact pad  91  surrounded by P contact pad  92  which is surrounded by second N contact pad  90 . Dotted current flow line  900  indicates the direction of current flow.  
         [0101]      FIG. 9   b  is the cross-sectional view of the LED of  FIG. 9a . N confinement layer  97  is disposed on substrate  99 . Active layer  94  is sandwiched between P and N confinement layer  910  and  97 . P contact pad  92  is disposed on P confinement layer  910 . A reflective layer (not shown in  FIG. 9b ) is disposed between P contact pad  92  and P confinement layer  910 . Current  93  and  95  flow from P contact pad  92  to N contact pad  90  and  91  respectively through active region  94 . Current  96  and  98  flow from P contact pad  92  to N contact pad  91  and  90  respectively through active region  94 .  
         [0102]      FIG. 9   c  shows a top view of submount  955  for the LED of  FIG. 9   a  to be bonded on. N bonding pad  950  and P bonding pad  954  are disposed on submount  955  respectively and not electrically connected. N flat bonding surface bump  951 , Circle  952 , and bridge  956  are electrically contacted to N bonding pad  950 . P flat bonding surface bump  953  is electrically contacted to P bonding pad  954 . The elevations of N flat bonding surface bump  951 , circle  952 , bridge  956  and P flat bonding surface bump  953  are the same and higher than that of both P and N bonding pad  950  and  954 .  
         [0103]     When flip chip bonding the LED of  FIG. 9   a  to substrate  955 , N contact pad  91  and  90  are bonded to circle  952  and N flat bonding surface bump  951  respectively. P contact pad  92  is bonded to P flat bonding surface bump  953 .  FIG. 9   d  shows an embodiment of the present invention. A LED has a plurality of P contact pad  961 ,  963 ,  965 , and  967 , and a plurality of N contact pad  962 ,  964 , and  966 . Multiple P contact pads and multiple N contact pads are alternately surrounding each other. Note that P and N contact pads may have different shapes respectively. Quantity of P and N contact pads may be either more or less than what are showed in  FIG. 9d . Positions of P and N contact pads are interchangeable. Dimensions of P and N contact pads are not to scale.  
         [0104]     For the LEDs with larger surface area, especially for high power LEDs, there may be more P and N contact pads surrounding alternately each other so that the current distributes and spreads more uniformly.  
         [0105]     Note that combinations of P and N contact pad layout designs of  FIG. 2  to  FIG. 9  are equivalent to the new P and N contact pad layout designs of the present invention.  
         [0106]     It should be emphasized that although the description above contains many specifications, these should not be constructed as limiting the scope of the present invention. They just provide the illustrations of some of the presently preferred embodiments of the present invention.  
         [0107]     Variations and modifications may be made to the above-described embodiments of the present invention without departing from the principles of the invention. All of such modifications and variations are included within the scope of the present invention and protected by the following claims.  
         [0108]     Therefore the scope of the present invention should be determined by the claims and their legal equivalents.