Abstract:
A light emitting diode (LED) structure ( 10 ) has semiconductor layers, including a p-type layer, an active layer, and an n-type layer. The p-type layer has a bottom surface, and the n-type layer has a top surface through which light is emitted. Portions of the p-type layer and active layer are etched away to expose the n-type layer. The surface of the LED is patterned with a photoresist, and copper is plated over the exposed surfaces to form p and n electrodes electrically contacting their respective semi-conductor layers. There is a gap between the n and p electrodes. To provide mechanical support of the semiconductor layers between the gap, a dielectric layer ( 34 ) is formed in the gap followed by filling the gap with a metal ( 42 ). The metal is patterned to form stud bumps ( 40, 42, 44 ) that substantially cover the bottom surface of the LED die, but do not short the electrodes. The substantially uniform coverage supports the semiconductor layer during subsequent process steps.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to light emitting diodes (LEDs) and, in particular, to a flip chip LED having a robust mechanical support structure and improved thermal resistance. 
       BACKGROUND 
       [0002]    Flip chip LEDs are desirable in many applications since they do not use wire bonding. Both electrodes are located on a bottom surface of the LED for direct bonding to metal pads on a submount. Bonding may be accomplished by ultrasonic bonding, solder, conductive adhesive, or other means. Light exits the surface of the LED opposite the electrodes. 
         [0003]    In a typical LED flip chip, the epitaxial p-type layer is the bottom layer and is contacted by the bottom anode electrode. A portion of the p-type layer and active layer must be etched away to expose the underside of the epitaxial n-type layer, which allows a connection to the bottom cathode electrode. This etching creates distributed vias through the p-type layer that expose the bottom surface of the n-type layer. The via openings are then insulated, and metal is deposited in the openings for contacting the n-type layer. 
         [0004]    Such topography is typically achieved by dry-etch of the semiconductor material (e.g., GaN) in a plasma environment. 
         [0005]    The metal contacting the n-type layer and the metal contacting the p-type layer are separated by gaps. Therefore, there is no mechanical support of the brittle semiconductor layers between the metal electrodes. 
         [0006]    At the end of wafer level processing, the growth substrates of the LED wafers are thinned and individual dies are formed by singulation. The LED electrodes are then bonded to metal pads on a submount tile, populated by many other LEDs. To prevent breakage of the semiconductor layers, it is known to inject a dielectric, organic based underfill material between the semiconductor layers and the submount. Such an injection process is time-consuming, since the submount tile may support hundreds of LEDs. 
         [0007]    To increase light extraction, after the LED electrodes are bonded to the submount tile and the underfill is injected, the growth substrate is removed and thin semiconductor layers, with a typical thickness about 5 microns, are exposed. Such LED structures are referred to as thin film flip chip (TFFC) LEDs. The semiconductor layers are very delicate and susceptible to breakage, and the thinning and the substrate removal process create stresses on the semiconductor layers. Thus, the underfill is required. The submount tile is then singulated, making the mounted devices ready for the next level of packaging. 
         [0008]    The underfill material, such as a silicone or epoxy-based composite material (e.g., a molding compound), inherently has some material mismatch with the semiconductor layers, such as coefficient of thermal expansion (CTE) mismatch and Young&#39;s modulus mismatch. This leads to delamination or other reliability problems during temperature cycling or other stress conditions. 
         [0009]    What is needed is a technique to form a robust TFFC without requiring an underfill for mechanical support. 
       SUMMARY 
       [0010]    In one embodiment of the invention, a flip chip LED is formed by growing n-type layers, an active layer, and p-type layers over a growth substrate. Portions of the p-type layers and active layer are then etched away to expose the n-type layer for electrical contact. Metal electrodes for the n-type layers and p-type layers are then formed, where the n and p electrodes are separated by gaps to avoid shorting. 
         [0011]    To provide mechanical support of the bottom surface of the LED between the electrodes, the sidewalls and bottom surface of the gap are insulated with a dielectric layer, and the gap is filled with a metal by electroplating. The metal filling the gap is electrically insulated from at least one of the electrodes to prevent shorting. When the LED electrodes are bonded to the pads of a submount, the metal filling the gap abuts one of the pads. Therefore, the entire bottom surface of the LED is substantially supported by the combination of the electrodes and the metal filling the gap after mounting the LED on a submount tile, thus obviating the need for an underfill. Therefore, the drawbacks of an underfill are avoided. The CTE and Young&#39;s modulus of the metal are much closer to those of the semiconductor layers than those of organic based underfill materials, thus greatly increasing the reliability of the LED during the thermal stresses incurred in operation. 
         [0012]    With the elimination of a tile level underfill process, more LED packaging steps can be processed at wafer level, resulting in better production scalability and further manufacturing cost reduction. An example is that the LED wafers are bonded to a carrier wafer with corresponding electrode pads properly aligned, or the plated structure is sufficiently thick and mechanically stiff to form a wafer carrier. The LEDs on the carrier wafer are then processed at wafer level simultaneously, such as by removing the growth substrate, roughening the top semiconductor layer for increased light extraction, encapsulating the LEDs, and singulating for next level packaging. The metal virtually covering the bottom surface of the semiconductor layers provides good mechanical support for the semiconductor layers during the wafer level processing. 
         [0013]    Other embodiments of the methods and structures are also described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a simplified cross-sectional view of LED semiconductor layers grown on a growth substrate. The p-type layer, active layer, and n-type layer may each comprise multiple layers. 
           [0015]      FIG. 2  illustrates portions of the p-type layer and active layer etched away, to allow ohmic contact to the n-type layer to form a flip chip, and a dielectric layer and copper seed layer formed over the structure. 
           [0016]      FIG. 3  illustrates a simplified version of the LED semiconductor layers (the thickness of the p-type layer and active layer has been ignored for simplicity) having formed over the surface photoresist portions, followed by plating steps to form at least a layer of copper electrically contacting the n-type layer and the p-type layer. 
           [0017]      FIG. 4  illustrates the structure of  FIG. 3  after the photoresist portions have been stripped and after the exposed seed layer has been etched away. 
           [0018]      FIG. 5  illustrates a dielectric layer insulating the sidewalls and bottom surface of the gap between the metal electrodes. 
           [0019]      FIG. 6  illustrates a gold seed layer sputtered on the surface of the dielectric layer. Photoresist portions (not shown) are then formed to expose areas of the gold seed layer where gold is to be plated. 
           [0020]      FIG. 7  illustrates the structure after the exposed seed layer is plated with gold and after the seed layer is etched back. The gold fills the gaps between the copper electrodes and covers a portion of the n and p electrodes. 
           [0021]      FIG. 8  illustrates the LED chip mounted to a submount wafer for further processing. 
           [0022]      FIG. 9  illustrates a portion of an LED die having another electrode configuration, where electrical contact is made to both the n and p-type layers by the metal filling the gap. 
       
    
    
       [0023]    Elements labeled with the same numerals in the various figures may be the same or equivalent. 
       DETAILED DESCRIPTION 
       [0024]      FIGS. 1-7  illustrate cross-sections of a small portion of an LED wafer showing only a single LED, where the central portion of the single LED is greatly reduced laterally so as to show detail of the side edges. To simplify the description, only the periphery of the n-type layer for each LED is contacted by an electrode. In an actual device, the n-type layer may be contacted by distributed electrodes for improved current spreading. 
         [0025]      FIG. 1  illustrates conventional LED semiconductor GaN layers  10  epitaxially grown over a sapphire substrate  12  and represents, in the order of layers grown, a nucleation layer, stress relief layers, n-layers  14 , active layers  16  (emitting light), p-layers  18 , and any other semiconductor layers that are used to form LEDs. The LEDs formed on the wafer may be AlInGaN LEDs, depending on the desired peak wavelength desired. Alternatively, the LEDs need not be GaN based and may be any other type of LED using any type of growth substrate. The invention is applicable to forming any LED as a flip chip. 
         [0026]      FIG. 2  illustrates that the wafer has been masked and dry etched to remove the p-layers  18  and active layers  16  from the edges of the LED to expose the surface of the n-layers  16  around the periphery of the LED. This is performed for all the LEDs on the wafer. Such a process is conventional to form a flip chip. 
         [0027]      FIG. 2  also shows a dielectric layer  20 , such as SiN X , deposited over the surface of the wafer and then etched, using conventional techniques, at areas  21   a  to expose a portion of the surface of the p-layers  18  and at areas  21   b  and  21   c  to expose potions of a surface of n-layers  14 . The deposition may be by spray coating. Any suitable dielectric material may be used. The dielectric layer  20  covers the side walls of the opening in the p-layers  18  and active layers  16  and covers a portion of the surface of the p-layers  18 . 
         [0028]    A copper seed layer  22  is formed over the surface of the wafer, which makes ohmic contact to the n and p layers through the openings in the dielectric layer  20  at areas  21   a - 21   c.  A bather layer, such as containing nickel, tungsten, chromium, vanadium and/or titanium, may be formed between the copper seed layer  22  and the semiconductor layers to avoid migration of Cu atoms. The copper seed layer  22  and barrier layer may be deposited over the entire wafer using any of a number of well known techniques, such as CVD, sputtering, etc. 
         [0029]    In  FIGS. 3-8 , the GaN layers  10  will be referred to hereinafter as a single semiconductor GaN layer  10 , and the growth substrate is ignored, for simplicity. The thickness of the p-layers  18  and active layers  16  is only a few microns, such as on the order of 5 microns, which is essentially planar relative to the much thicker plated electrodes (e.g., on the order of 50-100 microns) described below. Therefore, the height of the semiconductor mesa (layers  16  and  18 ) shown in  FIG. 2  is ignored for simplicity. The thicknesses of the various layers in the figures are not to scale. 
         [0030]    In  FIG. 3 , photoresist portions  26  are deposited and patterned by conventional lithographic techniques to expose only those portions of the seed layer  22  that are to be plated with copper. These exposed areas include the areas where the copper seed layer  22  electrically contacts the semiconductor layers at areas  21   a - 21   c  in  FIG. 2 . Other dielectric materials, such as an oxide or nitride, may be used as a mask instead of the photoresist. 
         [0031]    The exposed portions of the seed layer  22  are then plated with copper  28  to a desired thickness. Various well known electroplating techniques can be used, where the seed layer  22  is coupled to a potential, and the wafer is immersed in an electrolyte for transporting copper atoms from an electrode. Electroless plating may also be used. The copper  28  is advantageous for heat spreading and current spreading over the LED surface. Other metals and deposition techniques may be used. 
         [0032]    A thin nickel layer  30  and gold layer  32  are then plated over the copper  28  for providing a good bonding interface to submount pads. 
         [0033]    In  FIG. 4 , the photoresist portions  26  are stripped in a solution, leaving gaps  29 , and the exposed seed layer  22  is then etched away using conventional techniques. The seed layer below the copper  28  will no longer be separately identified. 
         [0034]    The copper  28  electrode electrically contacting the p-layers is isolated from the copper  28  electrode electrically contacting the n-layers by the gaps  29 . 
         [0035]    In  FIG. 5 , a dielectric layer  34  of, for example, SiN x , is then deposited over the wafer and patterned using conventional techniques. The deposition may be by spray coating or other suitable method. Any suitable low-K (dielectric constant) material may be used. The dielectric layer  34  is patterned to cover the sidewall and bottom surfaces in the gap  29  between adjacent copper  28  plated electrodes. The patterned dielectric layer  34  also covers a small area over the top surface of the gold layer  32  to ensure no sides of the plated electrodes are exposed and to provide a dielectric surface for supporting a metal layer, described below. 
         [0036]    In  FIG. 6 , a thin gold seed layer  36  is sputtered over the wafer surface. 
         [0037]    A photoresist (not shown) is then patterned over the seed layer  36  to expose only those areas that are to be plated with gold. 
         [0038]    As shown in  FIG. 7 , the exposed seed layer  36  is then electroplated with gold in a single electroplating step to fill the gaps  29  ( FIG. 6 ) with a conformal growth and form stud bumps simultaneously for subsequent die attach application. After photoresist removal, the exposed seed layer  36  is then etched back to form the following groups of gold stud bumps: 1) gold stud bumps  40  electrically contacting the n-type layers via the gold layer  32 ; 2) gold stud bumps  42  electrically contacting the p-type layers via the gold layer  32 ; and 3) gold stud bumps  44  over the dielectric layer  34  which are electrically insulated from both the n-type layers and the p-type layers. Note that the gold stud bumps  44  are formed overlying the dielectric layer  34  on the copper  28  electrode for the n-type layers. The gold stud bumps  44  act as isolation buffers between the closely spaced n and p electrodes and provide mechanical support for the surface next to the gap. 
         [0039]    By providing gold stud bumps, rather than a larger layer of gold, the gold is more easily melded in the submount gold pads when ultrasonically bonding the LED electrodes to the submount pads. 
         [0040]    The resulting LED wafer can then be singulated for die attach, or can be bound to a carrier wafer for further processing at the wafer level. Alternatively, the structure of the copper  28  layer can be sufficiently thick and mechanically stiff so as to act as a carrier wafer for continued wafer level packaging processing. 
         [0041]    In one embodiment, shown in  FIG. 8 , each individual LED die is then mounted on a submount wafer  50  having, for each LED die, a central gold pad  52  for the p-contact and a peripheral gold pad  54  for the n-contact. The contact pad and electrode configuration may be much more complex than shown in  FIG. 8 . For example, the n-electrodes for the LED die may be distributed over the surface of the LED die by vias through the p-layers and active layers, and the pads on the submount wafer would correspond to the locations of the electrodes on the LED die. The body  56  of the submount wafer  50  may be a ceramic or other suitable thermally conductive material. 
         [0042]    The polarities of the gold stud bumps on the LED die are designated as p, n, and d (for no polarity). The spacing between the gold stud bumps  40 ,  42 ,  44  may be very small since the spacing is determined by the masking for the plating, which can be made very precise. Although, the gold stud bump  42  at least partially filling the gap may not be planar with the other gold stud bumps  40  and  44 , the gold stud bump  42  provides mechanical support of the gap area. Also, due to the relatively malleable characteristics of gold, the ultrasonic bonding of the LED electrodes to the submount pads will somewhat flatten out any high points, providing substantially uniform contact over the entire bottom surface of the LED die. Therefore, substantially the entire bottom surface of the LED die is substantially uniformly supported by gold stud bumps, providing good mechanical support for the semiconductor layers during subsequent processing. 
         [0043]    The pads  52  and  54  on the submount wafer  50  may be formed close together without undue tolerance requirements for the placement of the LED die, since the gold stud bumps  44  are electrically isolated and will not short if some of the bumps  44  contact a p-metal pad  52  and some contact an adjacent n-metal pad  54  due to misalignment. 
         [0044]    In addition to the gold stud bumps  42  providing mechanical support by filling the gap between the copper  28 , they also increase the conductivity of the submount pad  52  to the p-type layers due to the added electrode area. 
         [0045]    In one embodiment, the LED dies on the submount wafer  50  are then subjected to a substrate laser lift-off process, where the sapphire growth substrate is lifted off after the 
         [0046]    LED die is subjected to a laser pulse. This creates a high downward pressure  55  on the semiconductor layers. The semiconductor layers are prevented from breaking due to the metal support of the gold stud bumps over virtually the entire back surface of the LED die. 
         [0047]    The LED dies are then subject to a thinning process, which may use chemical-mechanical polishing (CMP) or other technique, which thins the semiconductor layers to only a few microns. The exposed top surface is then roughened using an etching process to increase light extraction. 
         [0048]    The LED dies may then be encapsulated, such as by molding lenses over all the dies. 
         [0049]    The submount wafer  50  is then singulated (e.g., sawed) to form individual LEDs. 
         [0050]      FIG. 9  illustrates another embodiment of the electrode configuration on the LED die. No stud bumps are formed. After the copper  28  is plated over the semiconductor layers to electrical contact the n and p layers, the dielectric layer  34  is deposited and patterned to expose portions of the n and p copper  28  electrodes. A copper seed layer (not shown) is then deposited over the surface and masked with a photoresist to expose only those portions to be plated. A layer of copper  70  is then electroplated over the exposed seed layer to fill the gap between the copper  28  electrodes. The copper  70  is then plated with a nickel layer  72  and a gold layer  74 . A conventional solder mask material  80  is then patterned over the surface, and solder paste  78  is applied to the exposed gold layer  74  for die attach to pads of a submount. Upon heating, the solder paste  78  bonds to the submount pads. 
         [0051]    Other electrode configurations are also envisioned. 
         [0052]    When the LEDs are energized, light is emitted through the n-type layers overlying the p-layers and active layers. The electrode metal (e.g., the gold or nickel barrier layer) reflects light back up through the LED. 
         [0053]    Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.