Patent Publication Number: US-2011049545-A1

Title: Led package with phosphor plate and reflective substrate

Description:
FIELD OF THE INVENTION 
     This invention relates to light emitting diodes (LEDs) and, in particular, to providing a reflective layer on a mounting surface that confines side light and helps support the LED die and a phosphor plate over the LED die. 
     BACKGROUND 
     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. 
     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. 
     It is known to provide reflective metal electrodes on the bottom surface of each LED so that light emitted downward by the LED active layer is reflected upward rather than being absorbed by the submount. 
     It is also known to glue a thin phosphor plate to the top surface of the LED die to wavelength convert the light emitted from the LED. Such phosphor plates are thin and brittle, although much thicker than the semiconductor layers of the LED. The side light emitted by the LED semiconductor layers (e.g., blue) and the phosphor plate (e.g., white, red, green, etc.) creates non-uniformity of color in the emitted light and tends to distort the desired Lambertion emission of the LED. This increases non-uniformity in the light emission, even when shaped by an overlying lens. 
     What is needed is a way to reflect side light upwards so that virtually all light exits through the top surface of the phosphor plate. This will help create a more uniform light emission both in color and brightness. It is also desirable to help mechanically support the delicate LED die and phosphor plate. 
     SUMMARY 
     In one embodiment, a submount wafer is populated with flip chip LED dies. In one example, the active layer of the LEDs emits blue light. The growth substrate (e.g., sapphire for GaN LEDs) is then removed by laser lift-off or other technique. The exposed LED surface may be further process to roughen the surface for increased light extraction and to remove any damaged semiconductor material. 
     A thin phosphor plate is then glued to the exposed LED die surface. 
     A reflective mixture of silicone and TiO 2  is then spun over the entire submount surface to cover the tops of the phosphor plates and the sides of the LEDs. Alternatively, the reflective mixture may be molded over the submount surface or deposited using other methods. The reflective layer is electrically insulating. 
     A preferred reflective material is a silicone molding compound, which has a coefficient of thermal expansion close to that of the submount 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. If the percentage, by weight, of TiO 2  exceeds about 5% of the total filler content of the silicone, the layer is over 85% reflective. If the silicone contains 10% of total filler content, by weight, TiO 2 , the layer is at least 90% reflective. 
     After the reflective mixture is cured (hardened), the surface is etched by microbead blasting to expose the top surfaces of the phosphor plates. The resulting reflective material is substantially planar over the entire submount surface and about even with the top of the phosphor plates. Since the reflective material covers the sides of the semiconductor LEDs and phosphor plates, all side light will be reflected back into the LED and phosphor plate and ultimately upward through the top surface of the phosphor plate to create a substantially uniform Lambertion pattern. Any downward light emitted by the LED will be reflected upward by the LEDs&#39; reflective bottom metal electrodes (e.g., aluminum, silver, etc.). 
     The large area of the reflective material  32  ensures continued strong contact with the sides of the LED and phosphor plate. 
     In one embodiment, the underfill can also be a reflective mixture of silicone and TiO 2 , as described in U.S. application Ser. No. 12/503,951, filed on Jul. 16, 2009, entitled Reflective Substrate for LEDs, by Grigoriy Basin and Paul Martin (present co-inventors), assigned to the present assignee and incorporated herein by reference. 
     Lenses may then be molded over the reflective layer and phosphor plates. 
     The submount wafer is then diced to separate out the individual LEDs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a portion of a prior art submount wafer populated by an array of LEDs, such as 500-4000 LEDs, where the LEDs are undergoing a laser lift-off operation to remove the growth substrates. 
         FIG. 2  illustrates the submount wafer of  FIG. 1  after the growth substrates have been removed and after phosphor plates have been affixed to the semiconductor top surfaces of the LEDs. 
         FIG. 3  illustrates the submount wafer of  FIG. 2  after a reflective material (e.g., silicone containing TiO 2 ) has been deposited over the wafer and over the tops and sides of the LEDs. 
         FIG. 4  illustrates the submount wafer of  FIG. 3  undergoing microbead blasting to remove the reflective material over the phosphor plates and create a substantially planar surface. 
         FIG. 5  illustrates the submount wafer of  FIG. 4  after the microbead blasting process. 
         FIG. 6  illustrates additional detail of a singulated LED module after lenses have been molded or otherwise formed over each LED. Side emission light rays are shown being reflected by the reflective material formed over the sides of the LED and phosphor plate. 
     
    
    
     Elements that are the same or equivalent are labeled with the same numeral. 
     DETAILED DESCRIPTION 
     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). 
     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. 
     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. 
     The LEDs are then singulated and mounted on a submount wafer. 
     Prior art  FIG. 1  illustrates conventional flip chip LEDs  10  mounted on a portion of a submount wafer  12 . The wafer  12  is typically a ceramic or silicon. The LED  10  is formed of semiconductor epitaxial layers grown on a growth substrate  14 , 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. 
     A metal electrode  16  is formed on the LED  10  that electrically contacts the p-layer, and a metal electrode  18  is formed on the LED  10  that electrically contacts the n-layer. In one example, the electrodes are reflective metal with gold bumps that are ultrasonically welded to anode and cathode metal pads  20  and  22  on the submount wafer  12 . The submount wafer  12 , in one embodiment, has conductive vias leading to bottom metal pads (not shown) for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer  12 , and the wafer  12  will be later singulated to form individual LEDs/submounts. 
     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. 
     An underfill  24  is then injected under each LED  10  or molded so as to be dispersed under each LED  10 . The underfill  24  may be silicone, epoxy, or other suitable material that provides mechanical support of the thin LED layers during a laser lift-off process. In one embodiment, the underfill is a reflective material, such as a silicone molding compound with particles of TiO 2  (appearing white under white light), or other reflective particles such as ZrO 2 , as described in U.S. application Ser. No. 12/503,951, filed on Jul. 16, 2009, entitled Reflective Substrate for LEDs, by Grigoriy Basin and Paul Martin (present co-inventors), assigned to the present assignee. 
     The LEDs  10  then undergo a laser lift-off process to remove the growth substrate  14 . The laser pulses are shown by arrows  28 . 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  14  away from the LEDs  10 . After the growth substrates  14  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. Laser lift-off is well known. 
     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 imperfections due to the growth process and the damage caused by the laser lift-off process. The resulting top surface is desirably roughened to increase the light extraction efficiency. Roughening may be achieved by suitable etching, including photo-electrochemical etching (PEC). 
     As shown in  FIG. 2 , to create phosphor-converted light, a preformed phosphor plate  30  is affixed to the exposed top surface of each LED  10 . The plate  30  typically has a thickness of a few hundred microns, while the LED semiconductor layers have a thickness of only a few tens of microns. The phosphor plate  30  may be formed of sintered phosphor powder, phosphor powder infused in a clear binder (e.g., silicone), a dried phosphor slurry, or formed in other ways. In one embodiment, the phosphor provides red and green components or yellow (YAG) so that the combination of the phosphor light and the blue LED light leaking through the phosphor creates white light. 
     The phosphor plate  30  may be affixed to the LED  10  by silicone, epoxy, a high index glass, sintering, or by other means. 
     The light emitted from the top surface of the phosphor plate  30  of  FIG. 2  is substantially uniform in color and has a Lambertian distribution. However, the blue LED light from the sides of the LED and the light from the sides of the phosphor plate  30  create non-uniformity in the overall color output. And, the side light distorts the desired Lambertian distribution. Additionally, when a lens is used, the side light is not adequately shaped by the lens. Also, the phosphor plate  30  is delicate and subject to delamination, especially under conditions such as those experienced when used in automobiles. 
     To overcome these drawbacks of the structure of  FIG. 2 , a reflective material  32 , shown in  FIG. 3  is deposited over the surface of the submount wafer  12  to overlie the top and sides of the phosphor plates  30  and the sides of the LEDs  10 . In one embodiment, the reflective material  32  is deposited in a liquid form and then spun on the wafer  12  to create a substantially planar layer. In other embodiments, the reflective material  32  is sprayed on, molded over (using wafer scale compression molding or injection molding), or dispensed in other ways. 
     In one embodiment, the reflective material  32  is a silicone molding compound containing TiO 2  so as to appear white under white light. 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 material  32 , 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 the 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. Other formulations of an electrically insulating, reflective material  30  may be used. White inorganic powders other than TiO 2  may also be used. 
     The reflective material  32  is then heated to cure (harden) it. Alternatively, the reflective material  32  may be cured with UV light. 
     As shown in  FIG. 4 , the entire surface of the wafer  12  is then etched using high-velocity microbeads  36  in a process called microbead blasting. In one embodiment, the microbeads  36  have diameters between 1-20 microns and are formed of NaHCO 3 . The microbeads  36  are accelerated through a nozzle by air at a pressure of about 50-100 psi or less. The nozzle may be large to etch the reflective material  32  from over the phosphor plates  30  without the nozzle moving, or a smaller nozzle may be used to etch the reflective material  32  off only a few plates  30  at a time followed by the nozzle moving to a next position over the wafer  12 . Removing excess material of any kind using microbeads is a known process. 
     As shown in  FIG. 5 , the reflective material  32  has been etched to create a substantially planar reflective layer over the submount wafer  12  surface between the LEDs  10  and about even with the top surface of the phosphor plates  30 . The large area of the reflective material  32  ensures continued strong contact with the sides of the LED  10  and phosphor plate  30 . 
     Transparent lenses (e.g., silicone) may then be molded over each LED  10  to increase the light extraction from the LED, protect the phosphor plate  30  and LED  10 , and create a desired light emission pattern. The lens may be any shape, such as the hemispherical shape shown in  FIG. 6 . The lenses are molded on a wafer scale prior to dicing the wafer  12 . 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. 
     In one embodiment, the lens material also contains phosphor particles to further wavelength convert the light emitted by the LEDs  10 . In one embodiment, the phosphor plate  30  is a YAG phosphor (emits yellow-green light) and the phosphor in the molded lens emits a red light when energized by the blue LED light to create a warmer white light. 
     Other wafer-level processes may also be performed on the LED array while mounted on the submount wafer  12 . 
     The submount wafer  12  is then singulated along the dashed lines in  FIG. 5 , such as by sawing, to form individual LEDs/submounts, as shown in  FIG. 6 .  FIG. 6  shows additional detail of the resulting LED and submount  38 . Metal vias  40  extend through the ceramic submount  38  and terminate in robust bonding pads  42  on the bottom surface of the submount  38  for bonding to pads on a circuit board. A hemispherical lens  43  has been molded over the LED. 
     A light ray  44  is shown being emitted from the side of the semiconductor layers of the LED  10  and reflecting off the reflective material  32  toward the phosphor plate  30 . Another light ray  46  is shown being emitted from the side of the phosphor plate  30  and reflected upward by the reflective material  32 . Downward light rays will ultimately be reflected up by the reflective material  32  or the reflective LED electrodes  16  and  18 .  FIG. 6  also illustrates the transparent glue layer  50  that affixes the phosphor plate  30  to the LED  10 , and side light from the glue layer  50  is also reflected by the reflective material  32 . The reflective material  32  confines the light from the LED so that virtually all light is emitted from the top surface of the phosphor plate  30 . 
     Since the reflective material  32  is formed of a transparent material containing reflective powder, light enters the material  32  at various depths before being reflected. The depth depends on the percentage of TiO 2 . 
     The reflective material  32  not only confines the light to improve color uniformity and overall light output uniformity, but the entire LED  10  and phosphor plate  30  has additional mechanical support to prevent delamination, edge chipping, etc. The reflective material  32  also increases the light output and efficiency of the LED due to the side light from the LED and plate being ultimately reflected upward so as to be useful, as well as due to the reflective layer surface surrounding the plate reflecting light and because there is less light absorption by the submount wafer. 
     In another embodiment, the growth substrate is not removed, and the reflective material  32  is formed over the wafer  12  surface to cover the wafer up to the top surface of the LEDs, such as up to the top of the growth substrate, the top of a phosphor plate over the substrate, or the top of another optical element. The reflective material  32  still channels any side light through the top surface of the LED to achieve improved light emission. 
     In another embodiment, the phosphor plate is a phosphor layer that was not preformed but was deposited over the LED surface such as by electrophoresis, spraying, or other process. 
     In another embodiment, the reflective material  32  also forms the underfill in a single compression or injection molding step. 
     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.