Abstract:
An underfill formation technique for LEDs molds a reflective underfill material to encapsulate LED dies mounted on a submount wafer while forming a reflective layer of the underfill material over the submount wafer. The underfill material is then hardened, such as by curing. The cured underfill material over the top of the LED dies is removed using microbead blasting while leaving the reflective layer over the submount surface. The exposed growth substrate is then removed from all the LED dies, and a phosphor layer is molded over the exposed LED surface. A lens is then molded over the LEDs and over a portion of the reflective layer. The submount wafer is then singulated. The reflective layer increases the efficiency of the LED device by reducing light absorption by the submount without any additional processing steps.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to light emitting diodes (LEDs) and, in particular, to providing a reflective layer on a mounting surface. 
       BACKGROUND 
       [0002]    LEDs are typically mounted on a submount wafer that is later diced to separate out the individual LEDs/submounts. Each submount portion of the wafer has top electrodes that are bonded to electrodes on the LED, such as by ultrasonic bonding. An underfill material, such as epoxy or silicone, is then injected under the LED to provide mechanical support and protect the LED from contaminants. Any underfill material substantially outside of the LED footprint (e.g., outside of 20 microns) is removed so that the submount surface is clean. One reason to remove the underfill material that extends beyond the LED die footprint is that, if the underfill is epoxy (starts off yellow) and is exposed to UV light, the epoxy turns black and absorbs light. 
         [0003]    The submount also has a set of more robust electrodes, electrically connected by a metal pattern to the LED electrodes, that are typically bonded to a printed circuit board (after the submount wafer is diced) using conventional solder reflow or other means. 
         [0004]    It is known to provide reflective metal electrodes on the bottom surface of the LEDs so that light emitted downward by the LED active layer is reflected upward rather than being absorbed by the submount. Some of the LED&#39;s emitted light also impinges on the submount surface surrounding the LED die footprint. To reflect that light, it is known to deposit a reflective metal ring around the LED, such as silver or aluminum. 
         [0005]    Forming a metal reflector takes additional steps, and the metal must be insulated from the top metal pattern on the submount. 
         [0006]    What is needed is a better way to reflect light upwards from the surface of a submount or other LED mounting surface. 
       SUMMARY 
       [0007]    In one embodiment, a submount wafer is populated with LED dies. Instead of using a typical epoxy or silicone molding compound underfill for each LED die, a silicone molding compound containing TiO 2  powder as filler (or other white powder) is used as the underfill, and the underfill material also forms a substantially flat reflective layer surrounding each of the LEDs. In one embodiment, the underfill material is injection molded or compression molded over the wafer to form a layer of the underfill material over the entire wafer surface, including beneath each of the LEDs. 
         [0008]    If the percentage, by weight, of TiO 2  exceeds about 5% of the total filler content of underfill material, the layer is over 85% reflective. If the underfill material contains 10% of total filler content, by weight, TiO 2 , the layer is at least 90% reflective. 
         [0009]    After the reflective TiO 2 -infused layer is molded over the wafer (including below the LEDs), the exposed surface of the TiO 2 -infused layer is then microbead blasted to expose the top surface of each LED. The surface of the TiO 2 -infused layer surrounding the LEDs will be about at the same height as the LED semiconductor layers or less to form a reflective layer over the surface of the submount wafer. If the LED is a flip-chip, with a growth substrate on top after mounting on the submount, the growth substrate may then be removed. Since removal of the growth substrate puts a downward pressure on the LED semiconductor layers, the underfill supports the LED semiconductor layers during the substrate removal process. 
         [0010]    If the LED light is to be phosphor-converted, a phosphor layer is then molded over the exposed LED surface or deposited by any other means. 
         [0011]    A transparent lens is then molded over the LED, or formed using any suitable process. 
         [0012]    Since an underfill material must be injected beneath the LEDs anyway, there is no extra process step in forming the underfill material to also create a reflective layer surrounding each of the LEDs on the submount. Therefore, the reflective layer around the LEDs increases the light output, by preventing absorption of light by the submount, without any additional process step. Mixing particles of TiO 2  into the silicone underfill material prior to molding it onto the submount is a simple step. The TiO 2  particles are small to maximize surface area vs. volume. 
         [0013]    The submount wafer is then diced to separate out the individual LED/submounts, each having a reflective layer. The reflective layer is electrically insulating. 
         [0014]    A preferred underfill material that can be used with the present process is a silicone molding compound, which has a glass transition temperature around −10° C.-20° C. and the coefficient of thermal expansion is close to the substrate, so that there is very little thermal expansion of the silicone molding compound under worst case conditions, such as during AuSn or AgSn solder reflow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates a portion of a prior art submount wafer populated by an array of LEDs, such as 500-4000 LEDs, prior to an underfill step. 
           [0016]      FIG. 2A  illustrates a wafer-level injection molding process used to provide a reflective underfill material below the LEDs and over the surface of the submount wafer. 
           [0017]      FIG. 2B  illustrates an alternative type of wafer-level molding process, not using injection, to provide a reflective underfill material below the LEDs and over the surface of the submount wafer. 
           [0018]      FIG. 3  illustrates the LEDs on the wafer after being removed from the mold of  FIG. 2A  or  FIG. 2B . 
           [0019]      FIG. 4  illustrates the surface of the underfill material of  FIG. 3  being etched away by microbead blasting so as to expose the top surface of the LEDs yet leave a reflective layer over the surface of the submount wafer. 
           [0020]      FIG. 5  illustrates a laser lift-off technique for removing the growth substrates from the LEDs. 
           [0021]      FIG. 6  illustrates a phosphor layer (phosphor particles in a binder) formed over the exposed LED surfaces by a molding process similar to that of  FIG. 2B . 
           [0022]      FIG. 7  illustrates a lens formed over the LEDs by a molding process similar to that of  FIG. 2B . 
           [0023]      FIG. 8  is a cross-sectional view of a single LED mounted on a submount after the LEDs/submounts of  FIG. 7  have been singulated. The submount is shown soldered to a printed circuit board. 
       
    
    
       [0024]    Elements that are the same or equivalent are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0025]    As a preliminary matter, a conventional LED is formed on a growth substrate. In the example used, the LED is a GaN-based LED, such as an AlInGaN or InGaN LED, for producing blue light. Typically, a relatively thick n-type GaN layer is grown on a sapphire growth substrate using conventional techniques. The relatively thick GaN layer typically includes a low temperature nucleation layer and one or more additional layers so as to provide a low-defect lattice structure for the n-type cladding layer and active layer. One or more n-type cladding layers are then formed over the thick n-type layer, followed by an active layer, one or more p-type cladding layers, and a p-type contact layer (for metallization). 
         [0026]    For a flip-chip, portions of the p-layers and active layer are etched away to expose an n-layer for metallization. In this way, the p contact and n contact are on the same side of the chip and can be directly electrically attached to the submount contact pads. Current from the n-metal contact initially flows laterally through the n-layer. The LED bottom electrodes are typically formed of a reflective metal. 
         [0027]    Other types of LEDs that can be used in the present invention include AlInGaP LEDs, which can produce light in the red to yellow range. Non-flip-chip LEDs can also be used. 
         [0028]    The LEDs are then singulated and mounted on a submount wafer. 
         [0029]    Prior art  FIG. 1  illustrates a conventional LED  10  flip chip mounted on a portion of a submount wafer  22 . The LED  10  is formed of semiconductor epitaxial layers grown on a growth substrate  12 , such as a sapphire substrate. In one example, the epitaxial layers are GaN based, and the active layer emits blue light. Any other type of LED is applicable to the present invention. 
         [0030]    Metal electrodes  14  are formed on the LED  10  that electrically contact the p-layer, and metal electrodes  16  are formed on the LED  10  that electrically contact the n-layer. In one example, the electrodes are gold bumps that are ultrasonically welded to anode and cathode metal pads on the submount wafer  22 . The submount wafer  22 , in one embodiment, has conductive vias leading to bottom metal pads for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer  22 , and the wafer  22  will be later singulated to form individual LEDs/submounts. 
         [0031]    Further details of LEDs can be found in the assignee&#39;s U.S. Pat. Nos. 6,649,440 and 6,274,399, and U.S. Patent Publications US 2006/0281203 A1 and 2005/0269582 A1, all incorporated herein by reference. 
         [0032]    A reflective underfill material is prepared. In one embodiment, particles of TiO 2  (appearing white under white light), or other reflective particles such as ZrO 2 , are added to a silicone molding compound that is suitable for underfilling. A typical silicone molding compound contains about 82%-84% SiO 2  by weight, which creates a very stable material in the high-photon energy, high-heat environment of a power LED. To create the reflective properties of the underfill, TiO 2  is included in the silicone molding compound to replace some of the SiO 2  to cause the TiO 2  to be about 5-10% or higher by weight of the total amount of filler in silicone molding compound. The TiO 2  plus the SiO 2  should equal about 80%-84% by weight of the silicone compound. A 5% addition of TiO 2  results in about an 85% reflectivity of the silicone compound, and a 10% addition of TiO 2  results in over 90% reflectivity of the silicone compound. Significantly more TiO 2  begins to reduce the desirable characteristics of the silicone compound for used as an underfill. Other formulations of an electrically insulating, reflective underfill material may be used. 
         [0033]      FIG. 2A  illustrates one type of suitable injection molding process for creating the underfill and reflective layer for each LED. A mold  36  has cavities  38  that define the shape of the hardened underfill material after the molding process. The mold  36  may be formed of aluminum. The mold  36  has a perimeter seal  37  that seals against the submount wafer  22  when the mold  36  is aligned with the wafer  22  and pressed against the wafer  22 . 
         [0034]    The mold  36  has at least one inlet  40 , for injecting the reflective liquid underfill material  41 , and at least one outlet  42  connected to a vacuum source. Once the mold  36  is sealed against the wafer  22 , a vacuum is created within the mold  36 , and the underfill material  41  is injected through inlet  40 . The underfill material  41  flows into all the cavities  38  via channels  44  between the cavities, assisted by the vacuum and the injection pressure of the material  41 . The vacuum removes almost all the air in the mold  36 . Ultimately, the entire mold  36  will be filled with the underfill material  41 , including all voids under the LEDs. 
         [0035]    The mold  36  is then heated to cure the liquid underfill material. The temperature of the mold  36  during curing is about 150° C. Alternatively, a transparent mold may be used and the underfill material may be cured with UV light. 
         [0036]      FIG. 2B  illustrates an alternative wafer-level molding process that does not use pressure injection of the underfill material. In  FIG. 2B , the mold  48  has cavities  50  that are first filled with particles (e.g., powder or small tablets) of solid underfill material  41  at atmospheric pressure. The solid material is then heated in the mold to soften it. The submount wafer  22  is brought against the mold  48  so that the LEDs are immersed in the underfill material in each cavity  50 . The wafer  22  and mold  48  are pressed together to force the underfill material to fill all voids. A perimeter seal  53  allows the pressure to be high while allowing all air to escape as the underfill material fills the voids. A vacuum may also be pulled between the wafer  22  and the mold  48  using a vacuum source around the seal  53 . 
         [0037]    The mold  48  is then cooled to solidify the underfill material. Certain materials harden automatically after the heating and compression process. Handling the underfill material as a solid has various benefits. Further, some suitable materials that can be used for an underfill are not a liquid at room temperatures prior to curing, so heating up a solid material in the mold followed by compression greatly increases the number of possible materials that can be used as an underfill. 
         [0038]    The mold of  FIG. 2A  or  2 B is then removed from the wafer  22 , resulting in the structure of  FIG. 3 , having the hardened underfill material  54  encapsulating each LED. There is also a layer of hardened underfill material  54  on the wafer  22  surface between each LED. 
         [0039]    To perform a laser lift-off process to remove the growth substrates  12 , the underfill material  54  over the growth substrate  12  must first be removed. If the growth substrate  12  will be removed by grinding or another mechanical etch process, such grinding may be used to simultaneously remove the excess underfill material  54 . 
         [0040]      FIG. 4  illustrates the removal of the excess underfill material  54  by blasting the entire surface of the wafer  22  with high-velocity microbeads  58 . In one embodiment, the microbeads  58  have diameters between 1-20 microns and are formed of NaHCO 3 . The microbeads  58  are accelerated through a nozzle by air at a pressure of about 100 psi or less. The nozzle may be large to etch the underfill material  54  from over the LEDs  10  without the nozzle moving, or a smaller nozzle may be used to etch the underfill material  54  off only a few LEDs at a time followed by the nozzle moving to a next position over the wafer  22 . Removing excess material of any kind using microbeads is a known process. The underfill material  54  is etched to leave a reflective layer over the submount surface between the LEDs  10 . The thickness of the remaining layer should be sufficient to reflect at least 80% of the impinging light. In one embodiment, the thickness of the reflective layer over the submount surface is about 30-50 microns (which may be approximately the thickness of the underfill beneath the LED die), and the overall top surface of the reflective layer is substantially flat, as determined by the mold shape and effects of the microbead blasting. 
         [0041]      FIG. 5  illustrates a laser lift-off process. The laser pulses are shown by arrows  60 . During the laser lift-off, the surface of the GaN absorbs heat, causing the surface layer to decompose into the Ga and N 2 . The N 2  pressure pushes the sapphire growth substrates  12  away from the LEDs. After the growth substrates  12  become detached from the semiconductor LED layers during the lift-off process, they are removed by, for example, an adhesive sheet or some other suitable process. The underfill mechanically supports the thin LED semiconductor layers during the lift-off process. 
         [0042]    The exposed LED layers are then thinned by, for example, RIE or a mechanical etch, since the exposed top layer is a relatively thick n-layer, and the surface has been damaged by the laser lift-off process. The resulting top surface may then be roughened to increase the light extraction efficiency. 
         [0043]    If it is desired to create phosphor-converted light, a mold similar to mold  48  in  FIG. 2B  is then provided to mold a phosphor layer over the LEDs  10 . For example, the LEDs  10  may emit blue light and it may be desired to create white light by depositing a layer of YAG phosphor (generates yellow light) or a layer of red and green phosphors. The blue light leaks through the phosphor to combine with the phosphor-generated light. 
         [0044]    The phosphor mixture may be phosphor particles infused in a binder such as silicone. The mold cavities then define the shape of the phosphor over the LEDs  10 , and the phosphor mixture is cured to harden the phosphor layer.  FIG. 6  illustrates a molded phosphor layer  62  over the LEDs. If the tolerances of the mold process cause there to be a thin phosphor layer over the underfill material  54 , such thin phosphor may be removed using a microbead blasting step. 
         [0045]    A transparent lens  66  ( FIG. 7 ) may then be molded over each LED  10  to increase the light extraction from the LED, protect the phosphor layer  62  and the LED  10 , and create a desired light emission pattern. The lens  66  may be any shape, such as the hemispherical shape shown in  FIG. 7 . In one embodiment, the lens  66  is molded from silicone using the same general process shown in  FIG. 2B . 
         [0046]    In one embodiment, the lens material also contains phosphor particles to wavelength convert the light emitted by the LEDs  10 . 
         [0047]    Other wafer-level processes may also be performed on the LED array while mounted on the submount wafer  22 . 
         [0048]    The submount wafer  22  is then singulated to form individual LEDs/submounts, such as shown in  FIG. 8 . As shown in  FIG. 8 , the entire surface of the submount wafer  22  portion is covered by the reflective underfill material  54 . A light ray  70  is shown being emitted by the phosphor layer  62  in a downward direction and being reflected upward by the underfill material  54 . Light rays from the sides of the LED  10  will also be reflected upward and exit through the top surface of the underfill material  54 . 
         [0049]      FIG. 8  also shows the electrodes  72  and  73  on the surface of the submount wafer  22  portion, the vias  76  leading to the bottom electrodes  78  and  79 , and a printed circuit board  82  having pads soldered to the electrodes  78  and  79 . The board  82  may have an aluminum core for sinking heat. 
         [0050]    The invention also applies to forming a reflective material around LEDs mounted on a substrate whether or not the reflective material is also used as an underfill. For example, the bottom electrodes on the LED may take up virtually the entire backside of the LED and an underfill may not be needed. 
         [0051]    Details of a wafer-level lens molding process are described in Patent Publication US 2006/0105485, entitled Overmolded Lens Over LED Die, by Grigoriy Basin et al., assigned to the present assignee and incorporated herein by reference. 
         [0052]    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.