Patent Publication Number: US-2010109025-A1

Title: Over the mold phosphor lens for an led

Description:
FIELD OF THE INVENTION 
     This invention relates to light emitting diodes (LEDs) and, in particular, to a technique for forming a phosphor-converted LED (PC-LED). 
     BACKGROUND 
     It is known to form a silicone lens over an LED where the lens is infused with a phosphor powder. For example, the LED die may emit blue light, and the phosphor may emit yellow-green light (e.g., a YAG phosphor), or the phosphor may be a combination of red and green phosphors. The combination of the blue light leaking through the lens and the light emitted by the phosphor generates white light. Many other colors may be generated in this way by using the appropriate phosphors. However, such phosphor-converted LEDs (PC-LEDs) do not have a reproducible color from LED to LED over all viewing angles due to one or more of the following reasons: variations in the thickness of the phosphor coating, the phosphor being at different average distances from the LED die at different viewing angles, optical effects, misalignments and variations in LED die positioning with respect to the lens, and other factors. U.S. Pat. No. 7,322,902, assigned to the present assignee and incorporated herein by reference, describes a molding process for forming silicone lenses over LEDs. That patent describes a molding process for forming a hemispherical phosphor-infused lens over a hemispherical clear lens. However, that embodiment still does not produce a PC-LED having very consistent color vs. viewing angle. 
     Consistent color vs. viewing angle is extremely important where the light is not mixed and diffused, such as in a projector, a flashlight, automobile lights, or a camera flash where the light sources are directly magnified and projected onto a surface. Consistent color vs. viewing angle is also extremely important when multiple PC-LEDs are used together and need to be matched to create a uniform color across a screen. 
     Therefore, what is needed is a PC-LED that has very highly controlled color vs. viewing angle. 
     SUMMARY 
     A technique for forming multiple lenses, including a phosphor-infused lens, for a PC-LED is described where the characteristics and effects of the phosphor lens are more carefully controlled than in U.S. Pat. No. 7,322,902. 
     LED dice (e.g., GaN LEDs that emit visible blue light) are mounted on a submount wafer in an array. There may be hundreds of LED dice mounted on the wafer. The submount wafer may be a ceramic substrate, a silicon substrate, or other type of support structure with the LED dice electrically connected to metal pads on the support structure. 
     A first mold has first indentations in it corresponding to the ideal positions of the LED dice on the submount wafer. The indentations are filled with liquid or softened silicone. The submount wafer is precisely aligned with respect to the first mold so that the LEDs are immersed in the silicone. The silicone is then cured to form a hardened lens material. The indentations are substantially rectangular, with a planar surface, so a first clear lens is formed over each of the LEDs having a rectangular shape generally proportional to the LED shape. The depth and widths of the indentations are large enough so that the lens will cover the LEDs under worst case misalignments of the LEDs on the submount wafer in the x, y, and z directions. Misalignment in the z direction is caused by variations in the submount wafer surface and variations in the thicknesses of the metal bonds between the LEDs and the submount wafer. Since the submount wafer is precisely aligned to the mold, the “top” surface of the flat lenses will all be within a single reference plane. 
     A second mold has larger indentations that are precisely aligned to the first indentations in the first mold. The second indentations have a substantially rectangular shape proportional to the shapes of the LEDs and first indentations. The second indentations are filled with a liquid or softened mixture of silicone and phosphor. The submount wafer is then precisely aligned with respect to the second mold so that the LEDs and first lenses are immersed in the silicone/phosphor. The silicone is then cured to form a hardened second lens material. 
     Since the top surfaces of the first lenses were all in the same reference plane, and the first and second indentations are precisely aligned with each other, the inner and outer surfaces of the second lens (containing the phosphor) are completely determined by the molds rather than any x, y, z misalignments of the LEDs. Therefore, the thickness of the second lens (containing the phosphor) is predicable and precisely the same for all the LEDs on the submount wafer, and all lenses are formed concurrently. Further, the phosphor layer is substantially uniformly illuminated by the blue LED so that blue light uniformly leaks through the phosphor lens layer. Therefore, the resulting color (or chromaticity) of the PC-LED will be reproducible from LED to LED and uniform across a wide range of viewing angles. 
     A third substantially rectangular lens is then molded over the phosphor-infused second lens, which may be harder than the other lenses and have a lower index of refraction. 
     The submount wafer is then diced to separated out the individual PC-LEDs. The submount/PC-LED may then be mounted on a circuit board or packaged. 
     The inventive technique applies equally to PC-LEDs where most or virtually all LED light (e.g., blue or UV) is absorbed by the phosphor layer, and the resulting light is primarily the light emitted by the phosphor layer. Such PC-LEDs would use a high density of phosphor particles in the phosphor lens layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of four LED dice mounted on a submount wafer, where the LED dice are shown inadvertently mounted at different heights and/or slightly misaligned. 
         FIG. 2  is a side view of the LED dice being inserted into indentations in a first mold filled (or partially filled) with a liquid (or softened) inner lens material for forming a planarized first clear lens. 
         FIG. 3  is a side view of the LED dice submerged in the liquid lens material and the lens material being cured. 
         FIG. 4  is a side view of the LED dice, after removal from the first mold, being inserted into indentations in a second mold filled (or partially filled) with a liquid (or softened) lens material containing phosphor powder, where the first clear lens causes the resulting phosphor filled lens to have precise inner and outer dimensions. 
         FIG. 5  is a side view of the LED dice, after removal from the second mold, being inserted into indentations in a third mold filled (or partially filled) with a liquid (or softened) outer lens material. 
         FIG. 6  is a side view of the LED dice submerged in the outer lens material while curing the outer lens material. 
         FIG. 7  is a side view of the LED dice with the three molded lenses. 
         FIG. 8  is a front view of the submount wafer populated with an array of the LED dice with the three molded lenses. 
         FIG. 9  is a cross-sectional view of a single flip chip LED/submount separated from the submount wafer and mounted on a circuit board. 
     
    
    
     Elements labeled with the same numerals are the same or equivalent. 
     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 LED, for producing blue or UV 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). 
     Various techniques are used to gain electrical access to the n-layers. In a flip-chip example, 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. In contrast, in a vertical injection (non-flip-chip) LED, an n-contact is formed on one side of the chip, and a p-contact is formed on the other side of the chip. Electrical contact to one of the p or n-contacts is typically made with a wire or a metal bridge, and the other contact is directly bonded to a package (or submount) contact pad. A flip-chip LED is used in the examples of  FIGS. 1-9  for simplicity. 
     Examples of forming LEDs are described in U.S. Pat. Nos. 6,649,440 and 6,274,399, both assigned to Philips Lumileds Lighting, LLC and incorporated by reference. 
       FIG. 1  is a side view of four LED dice  10  mounted on a submount wafer  12 . The submount wafer  12  is typically ceramic or silicon, with metal leads for connection to a printed circuit board, a package leadframe, or any other structure. The substrate wafer  12  may be circular or rectangular. Prior to mounting on the submount wafer  12 , the LED dice  10  are separated from other LEDs grown on the growth substrate (e.g., sapphire) by a standard sawing or scribing-breaking operation and positioned on the submount wafer  12  by an automatic placement machine. The metal pads on the LED dice  10  are bonded to corresponding gold bumps on the submount wafer  12  by ultrasonic bonding. The combined metal pads and gold bumps are shown as metal bonds  14 . The gold bumps are connected, by conductive vias through the submount wafer  12 , to bonding pads on the bottom surface of the submount wafer  12  for surface mounting to a circuit board. Any configuration of metal may be used on the submount wafer  12  for providing terminals to connection to a power supply. In the preferred embodiment, the growth substrate is removed from the flip-chip LEDs after mounting on the wafer  12 . 
     There is some misalignment of the LED dice  10  on the submount wafer  12  due to tolerances, and the heights of the LED dice  10  above the wafer  12  surface vary somewhat due to the tolerances of the metal pads, gold bumps, and ultrasonic bonding. Such non-uniformity is shown in  FIG. 1 . 
     In  FIG. 2 , a first mold  16  has indentations  18  corresponding to the desired shape of a first lens over each LED die  10 . The mold  16  is preferably formed of a metal. A very thin non-stick film (not shown), having the general shape of mold  16 , may be placed over the mold  16  to prevent the sticking of silicone to metal, if needed. The film is not needed if a non-stick mold coating is used or if a mold process is used that results in a non-stick interface. In the preferred embodiment, the shape of each indentation is substantially rectangular to achieve a planarized top surface of the first lenses. For purposes of easier release and to avoid any bright points, the edges of the substantially rectangular indentations are slightly rounded. 
     In  FIG. 2 , the mold indentions  18  have been filled (or partially filled to reduce waste) with a heat-curable liquid (or softened) lens material  20 . The lens material  20  may be any suitable optically transparent material such as silicone, an epoxy, or a hybrid silicone/epoxy. A hybrid may be used to achieve a matching coefficient of thermal expansion (CTE). Silicone and epoxy have a sufficiently high index of refraction (greater than 1.4) to greatly improve the light extraction from an AlInGaN or AlInGaP LED. One type of suitable silicone has an index of refraction of 1.76. In the preferred embodiment, the lens material  20  is soft when cured to absorb differences in CTE between the LED dice  10  and the cured lens material  20 . 
     In  FIG. 3 , the edges of the substrate wafer  12  are precisely aligned with the edges (or other reference points) on the mold  16 . Note that the LED dice  10  are not precisely aligned with the indentations  18  in the x, y, and z directions due to the tolerances of the LED dice  10  mounting. 
     A vacuum seal is created between the periphery of the submount wafer  12  and the mold  16 , and the two pieces are pressed against each other so that each LED die  10  is inserted into the liquid lens material  20 , and the lens material  20  is under compression. 
     The mold  16  is then heated to about 150 degrees centigrade (or other suitable temperature) for a time to harden the lens material  20 . 
     The submount wafer  12  is then separated from the mold  16 , and the lens material  20  may be further cured by UV or heat to form a first clear lens  22  ( FIG. 4 ) over each LED die  10 . The lens  22  encapsulates the LED die  10  for protection and for heat removal and has outer dimensions precisely aligned with respect to the edges of the submount wafer  12  (or other reference points on the wafer  12 ). The first clear lens  22  has approximately the same shape as the LED die but slightly larger to cover the entire LED under worst case positioning of the LED die. Importantly, the outer “top” surfaces of all the first clear lenses  22  over the LED dice  10  are within the same planarized reference plane, since all the indentations  18  were identical. 
     In  FIG. 4 , in a second molding process identical to the first molding process, mold indentions  24  in a second mold  26  are filled (or partially filled to reduce waste) with a heat-curable liquid (or softened) lens material  28  containing phosphor powder. The lens material  28 , other than the phosphor, may be similar to that used for the inner lens material  20  or may cure to form a harder lens. The phosphor may be a conventional YAG phosphor that emits a yellow-green light, or may be a red phosphor, a green phosphor, a combination of red and green phosphors, or any other phosphor, depending on the desired color of light to be produced. The blue light from the LED die  10  leaks through the phosphor to add a blue component to the overall light. The density of the phosphor and the thickness of the phosphor layer determine the overall color of the PC-LED. It is imperative for reproducible color from LED to LED that the phosphor layer thickness be always the same from one LED to the next at least across the top surface of the LED. Further, for uniformity of color over a wide range of viewing angles, the phosphor thickness should be uniform across the entire surface of each LED die, and substantially the same amount of LED light should illuminate all portions of the phosphor layer. Therefore, the shape of the phosphor layer should have approximately the same relative dimensions as the LED die  10 , which is substantially rectangular. 
     As with the first molding process, the edges of the submount wafer  12  are precisely aligned with the edges (or other reference points) on the mold  26 . Note that the first clear lenses  22  are now precisely aligned with the indentations  24  due to the indentations  18  and  24  being precisely aligned with respect to the molds&#39; edges (or other reference points for alignment with the submount wafer  12 ). 
     A vacuum seal is created between the periphery of the submount wafer  12  and the mold  26 , and the two pieces are pressed against each other so that each LED die  10  and first clear lens  22  are inserted into the liquid lens material  28 , and the lens material  28  is under compression. 
     The mold  26  is then heated to about 150 degrees centigrade (or other suitable temperature) for a time to harden the lens material  28 . 
     The submount wafer  12  is then separated from the mold  26 , and the lens material  28  may be further cured by UV or heat to form a phosphor-infused second lens  32  ( FIG. 5 ), having precise inner and outer dimensions, over each first clear lens  22 . The inner dimensions are dictated by the first clear lens  22 . The outer dimensions are dictated by the indentions  24 , so the second lenses  32  all have identical thicknesses. 
     In  FIGS. 5 and 6 , a third molding step is performed identical to the previous molding steps, but the outer lens material  34  (e.g., a silicone) should have a lower index of refraction than the inner two lens materials to better couple light into the air (n=1). The third mold  36  indentations  38  are slightly larger than the indentations  24  of the second mold  26 . The indentations  38  are filled with a clear liquid (or softened) lens material  34 , and the submount wafer  12  and mold  36  are brought together under a vacuum.  FIG. 6  shows the submount wafer  12  aligned with the third mold  36  so that the indentations  38  are aligned with both the inner clear lens  22  and the phosphor-infused second lens  32 . The resulting outer lens  40  ( FIG. 7 ) should be formed of a silicone that cures hard to provide protection and stay clean. 
     In one embodiment, the range of hardness of the first clear lens  22  is Shore 00 5-90, and the hardness of the clear outer lens  40  is greater than Shore A 30. The second lens  32  may be hard or have an intermediate hardness to absorb differences in CTE. 
       FIG. 7  shows the submount wafer  12  after separation from the mold  36  and after complete curing to create the hard outer lenses  40  for protection and improved light extraction from the PC-LEDs  50  The outer lens  40  may also contain molded features, such as roughening, prisms, or other features from indentations  38  that increase the extraction of light or diffuse the light for improved color and brightness uniformity across a wide viewing angle. The outer lens  40  may be any shape, such as rectangular, hemispherical, collimating, side-emitting, or other shape desired for a particular application. 
     The thickness of each of the first and second lens layers will typically be between 100-200 microns; however, in some instances the range may be 50-250 microns or thicker, depending on the amount of phosphor needed and other factors. The outer clear lens may have any thickness, such as from 50 microns to more than several millimeters, depending on its desired optical properties. 
       FIG. 8  is a front view of the submount wafer  12  with the completed, wafer-processed PC-LEDs  50  of  FIG. 7 . The submount wafer  12  is then diced to separate out the individual LEDs/submounts for mounting on a circuit board or for packaging. 
       FIG. 9  is a simplified close-up view of one embodiment of a single flip-chip PC-LED  50  on a submount  52 , separated from the submount wafer  12  by sawing. The PC-LED  50  has a bottom p-metal contact  54 , a p-contact layer  55 , p-type layers  56 , a light emitting active layer  57 , n-type layers  58 , and an n-metal contact  59  contacting the n-type layers  58 . Metal pads on submount  52  are directly metal-bonded to contacts  54  and  59 . Vias  62  through the submount  52  terminate in metal pads on the bottom surface of the submount  52 , which are bonded to the metal leads  64  and  65  on a printed circuit board  66 . The metal leads  64  and  65  are connected to other LEDs or to a power supply. Circuit board  66  may be a metal plate (e.g., aluminum) with the metal leads  64  and  65  overlying an insulating layer. 
     The inventive technique applies equally to PC-LEDs where most or virtually all LED light (e.g., blue or UV) is absorbed by the phosphor layer, and the resulting light is primarily the light emitted by the phosphor layer. Such a PC-LED would use a high density of phosphor in the phosphor layer. Such PC-LEDS may emit amber, red, green, or another color light other than white light. 
     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.