Patent Document

FIELD OF THE INVENTION 
       [0001]    This invention relates to forming a phosphor layer or other wavelength conversion layer over light emitting diodes (LEDs) and, in particular, to a method for forming such a layer over LED dies mounted on a submount tile. 
       BACKGROUND 
       [0002]    It is common for the light from an LED die to be converted to a different color, such as white light, by depositing a phosphor over the LED die. 
         [0003]    It is known to mount individual LED dies, singulated from an LED wafer, on a submount tile and then further process the dies on the tile to speed up processing. Such submount tiles may be populated with, for example,  2000  LED dies. 
         [0004]    While the LED dies are mounted on the submount tile, phosphor is typically sprayed over the dies, molded over the dies, printed over the dies, deposited by electrophoresis, or deposited using other methods. In some cases, phosphor particles are infused in a heat-curable silicone binder, and the mixture is deposited over the LED dies. The mixture is then cured by heat. 
         [0005]    In cases where the phosphor layer is deposited over the entire surface of the tile, as well as over the LED dies, the phosphor undesirably covers reflective metallization on the tile that is intended to reflect any downward LED light away from the submount after the tile is singulated. Additionally, the phosphor may cover a small wire bond electrode on a top surface of the LED dies. Additionally, it may be desirable to only provide a constant thickness of the phosphor around the sides of the LED dies so as to provide uniform color around the LED dies. In all such cases, a special masking and etching process must be performed to remove the undesired phosphor. Etching processes are time-consuming and relatively expensive, so add cost to the resulting LEDs. 
         [0006]    After all tile-level processes have been completed, the tile is singulated to create the individual phosphor-converted LEDs. 
         [0007]    What is needed is a process that does not require etching of the phosphor over the tile or LED dies. 
       SUMMARY 
       [0008]    This summary describes only a few embodiments of the invention. Other embodiments are envisioned. 
         [0009]    LED dies are mounted a single submount tile (or wafer). The LED dies have a light emitting top surface. In one embodiment, the growth substrate is removed from the semiconductor surface by laser lift-off after the LED dies are mounted on the submount tile. A pre-formed sheet of UV sensitive silicone infused with phosphor is then laminated over the tile, including over the tops and sides of the LED dies. Only the silicone/phosphor layer over the top and sides of the LED dies is desired, so the silicone/phosphor layer directly on the tile needs to be removed. 
         [0010]    In another embodiment, a liquid silicone/phosphor layer is spun on, sprayed on, screen printed on, molded over, or deposited on the tile and LED dies in other ways. 
         [0011]    The silicone/phosphor layer is then masked to expose the areas that are to remain. UV light is then applied to the silicone/phosphor layer exposed through the mask, which creates a cross-linked material that will remain after the silicone/phosphor layer is developed using a solvent. The silicone acts as a negative photoresist. The unexposed silicone/phosphor layer is then dissolved with a solvent, and the remaining silicone/phosphor layer over the top and sides of the LED dies is then rinsed and dried. 
         [0012]    In one embodiment, the LEDs are flip dies, and no area on the top surfaces of the LED dies needs to be electrically contacted. In another embodiment, if the LED dies require one or more top wire bonds, the mask blocks the UV light in those areas, so the silicone/phosphor layer is removed from the wire bond areas. 
         [0013]    The selective removal of the silicone/phosphor layer: 1) allows reflective metallization surrounding the LED dies to reflect the downward LED light for increased efficiency; 2) causes the light emitted by the LED and phosphor to be more uniform around the LED die; and 3) enables wire bonding to top areas of the LED dies. Since no etching is needed, the process is performed quickly, cleanly, and inexpensively. 
         [0014]    After any additional wafer-level processes, the tile is then singulated to produce individual phosphor-converted LEDs. 
         [0015]    Instead of phosphor particles being infused in the silicone, light scattering particles or other materials may be infused. In an embodiment where the silicone is just used as an encapsulant and/or a lens, there may be no materials infused in the silicone. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a cross-sectional view of a small portion of a submount tile, showing two LED dies having their electrodes bonded to pads on the submount tile. 
           [0017]      FIG. 2  illustrates a supporting film, a layer of UV sensitive silicone infused with phosphor deposited over it, and a protective film over the silicone/phosphor layer. 
           [0018]      FIG. 3  illustrates the silicone/phosphor layer laminated over the submount tile. 
           [0019]      FIG. 4A  illustrates a mask selectively exposing the silicone/phosphor layer over the entire top surface of the LED die to UV light. 
           [0020]      FIG. 4B  illustrates a mask selectively exposing the silicone/phosphor layer to UV light over only a portion of the top surface of the LED die to allow wire bonding to a top electrode of the LED dies. In this example, the LED dies are vertical dies with one electrode on the bottom and a wire bond electrode on top. 
           [0021]      FIG. 5A  illustrates the silicone/phosphor layer of  FIG. 4A  after the unexposed silicone/phosphor layer is removed by a solvent. 
           [0022]      FIG. 5B  illustrates the silicone/phosphor layer of  FIG. 4B  after the unexposed silicone/phosphor layer is removed by a solvent and a wire is bonded to the top electrode. 
           [0023]      FIG. 6  is a flowchart identifying steps in two alternative processes in accordance with some embodiments of the invention. 
       
    
    
       [0024]    Elements that are the same or equivalent are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0025]      FIGS. 1-6  represent only a few embodiments of the invention. Many other embodiments are envisioned. The process shown in  FIGS. 1-5  will be described while referring to the flowchart of  FIG. 6 .  FIG. 6  illustrates two alternative process flows, a lamination flow  20  and a molding flow  31 . Several process steps are common to both flows. 
         [0026]      FIG. 1  illustrates a submount tile  10  populated with an array of conventional LED dies  12 , as identified in step  14  of  FIG. 6 . There may be thousands of dies  12  bonded to the tile  10 . Typical shapes of the submount tile  10  are rectangular and circular. The submount base material may be ceramic, silicon, insulated aluminum, or other material. The LED dies  12  comprise semiconductor layers that have been epitaxially grown over a growth substrate, such as sapphire in the case of GaN based LED dies. An active layer sandwiched between a p-type layer and an n-type layer generates light. The LED dies  12  are mounted to the submount tile  10 . After mounting the LED dies  12  to the tile  10 , the growth substrate is removed from over the top surface of the semiconductor layers by laser lift-off. The dies  12  in  FIG. 1  are flip chips, with both electrodes  16  formed on the bottom surface. The LED die electrodes  16  may be ultrasonically bonded to metal pads formed on the submount tile  10  surface. The submount tile  10  pads lead to more robust electrodes on the bottom surface of the tile  10  (using conductive vias) for surface mounting to a printed circuit board. 
         [0027]    An underfill material  18  is injected or molded between the LED dies  12  and the surface of tile  10 . 
         [0028]    In one example, the submount tile  10  has a reflective metal surface  19  outside the footprint of the LED dies  12  to reflect any downward light to increase efficiency. The reflective metal may be an enlarged bond pad of the submount tile  10 . It is therefore desirable for such reflective metal to not be covered with any phosphor. There may be other reasons for not wanting a layer of phosphor over the surface of tile  10 . 
         [0029]    In one embodiment, identified in lamination flow  20  of  FIG. 6 , a silicone/phosphor layer is laminated over the tile  10  and dies  12 .  FIG. 2  illustrates the construction of one embodiment of the silicone/phosphor layer. A non-stick supporting film  24 , such as a plastic film with a layer of Teflon or other suitable material, provides a temporary support for the silicone/phosphor layer. (Step  25  in  FIG. 6 .) A liquid slurry of a UV sensitive silicone infused with phosphor particles is prepared and sprayed one, spun on, printed on, or deposited over the supporting film  24  by any other technique to form a silicone/phosphor layer  26 . (Step  27  in  FIG. 6 .) The type of phosphor used depends on the desired color to generate. For example, for an LED die  12  that generates blue light, a YAG phosphor, or a combination of red and green phosphors, may be used to create white light. The phosphor particle density and thickness of the layer  26  determines the color emitted by the complete device. Typically, the silicone/phosphor layer  26  will be between 20-200 microns thick. The layer  26  will be of uniform thickness. The layer  26  may include other materials, such as light scattering TiO2, ZrO, or silica particles. UV sensitive silicones and suitable phosphors are commercially available. 
         [0030]    The silicone/phosphor layer  26  is then dried by heat to form a solid layer. (Step  30  in  FIG. 6 .) A protective film  32  is then attached over the silicone/phosphor layer  26 . (Step  32  in  FIG. 6 .) The resulting layered structure may be cut to the dimensions of the submount tile  10  and stacked for later use. 
         [0031]    Prior to laminating the silicone/phosphor layer  26  onto the tile  10 , the protective film  32  is removed, and the silicone/phosphor layer  26  is pre-cured at 50-150° C. for 1-10 minutes to achieve a film hardness that prevents the silicone from deforming during the lamination step and maintains the target thickness of layer  26  over the LED dies  12 . (Step  34  in  FIG. 6 .) 
         [0032]    The silicone/phosphor layer  26  is then positioned over the tile  10  with the supporting film  24  facing away from the tile  10 . The layer  26  is then laminated under heat and pressure to adhere to the tile  10  and LED dies  12 , as shown in  FIG. 3 . Air is removed by performing the lamination process under vacuum conditions. The layer  26  will be conformal over the surface, including over the LED dies  12 . (Step  36  of  FIG. 6 .) The supporting film  24  is then removed. 
         [0033]    As an alternative to laminating the pre-formed layer  26  onto the tile  10  as in lamination flow  20 , the liquid UV sensitive silicone infused with phosphor may be prepared as a slurry (step  38 , molding flow  31  of  FIG. 6 ), then deposited over the tile  10  and dies  12  in any number of ways. Such ways include molding, spraying, spinning on, screen printing, and other suitable deposition techniques (step  40  of  FIG. 6 ). The deposited liquid silicone/phosphor layer  26  of  FIG. 3  is then dried. (Step  42  of  FIG. 6 .) 
         [0034]      FIG. 4A  illustrates a mask  50  positioned over the tile  10 . (Step  51  of  FIG. 6 .) The mask  50  may be chrome-plated glass. The mask  50  has transparent portions  52  that define areas of the silicone/phosphor layer  26  that will remain after the development step. Opaque portions  54  define areas of the silicone/phosphor layer  26  which will be removed. A UV source emitting light  56  having a peak wavelength of about  365  nm selectively exposes the silicone/phosphor layer  26  over the entire top surface of the LED dies  12  to create cross-linkages in the silicone/phosphor layer  26 . (Step  57  of  FIG. 6 .) 
         [0035]    As shown in  FIG. 4B , if the LED dies  58  were vertical LED dies, with a top wire bond electrode and a large bottom reflector electrode  59 , the silicone/phosphor layer  26  must be removed from the wire bond electrode.  FIG. 4B  illustrates a mask  60  selectively exposing the silicone/phosphor layer  26  to the UV light  56  over only a portion of the top surface of the LED dies  58  to ultimately allow wire bonding to the top electrode. 
         [0036]    After the exposure step, the tile  10  is dipped in a solvent to dissolve the unexposed silicone/phosphor layer  26 . This step may also be referred to as developing the silicone/phosphor layer  26 . The solvent may be heptaine, which does not dissolve the cross-linked silicone/phosphor layer  26 . (Step  60  of  FIG. 6 .) The remaining silicone/phosphor layer  26  is then rinsed with di-ionized water and dried. The drying may be at 120-150° C. for 1-4 hours. 
         [0037]      FIG. 5A  illustrates the silicone/phosphor layer  26  of  FIG. 4A  after the unexposed silicone/phosphor layer  26  is removed by the solvent. 
         [0038]      FIG. 5B  illustrates the silicone/phosphor layer  26  of  FIG. 4B  after the unexposed silicone/phosphor layer  26  is removed by a solvent and a wire  64  is bonded to the top electrode by conventional techniques. 
         [0039]    In one embodiment, silicone lenses are then molded over the LED dies  12 / 58  while on the tile  10 , and the tile  10  is then sawed to singulate the LED/submount units. (Step  66  of  FIG. 6 .) Additional tile level processes may be performed. 
         [0040]    The resulting silicone/phosphor layer  26  is conformal and produces substantially uniform color around the LED die. 
         [0041]    Although UV sensitive silicone is used in the example, other UV sensitive materials may also be used, but such materials should be non-yellowing when exposed to prolonged light and heat from the LED dies. 
         [0042]    Instead of a wavelength converting phosphor infused in the silicone, other particles may be use for wavelength conversion or for other purposes, such as light scattering. Further, the silicone may not contain any particles and may only be used as a lens or an encapsulant. 
         [0043]    The UV sensitive layer need not cover the entire tile, but it is desirable that it cover at least all the LED dies and the tile surface between the LED dies. 
         [0044]    Although a negative photo-material has been described that becomes non-dissolvable by exposure to the radiation, a positive photo-material may instead be used that becomes dissolvable when exposed to the radiation. If such a material is used, the transparent and opaque portions of the masks of  FIGS. 4A and 4B  would be reversed. 
         [0045]    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.

Technology Category: 5