Abstract:
A light emitting device comprises a flip-chip light emitting diode (LED) die mounted on a submount. The top surface of the submount has a reflective layer. Over the LED die is molded a hemispherical first transparent layer. A low index of refraction layer is then provided over the first transparent layer to provide TIR of phosphor light. A hemispherical phosphor layer is then provided over the low index layer. A lens is then molded over the phosphor layer. The reflection achieved by the reflective submount layer, combined with the TIR at the interface of the high index phosphor layer and the underlying low index layer, greatly improves the efficiency of the lamp. Other material may be used. The low index layer may be an air gap or a molded layer. Instead of a low index layer, a distributed Bragg reflector may be sputtered over the first transparent layer.

Description:
FIELD OF THE INVENTION 
     This invention relates to light emitting diodes (LEDs) with an overlying layer of phosphor to wavelength convert the LED emission and, in particular, to a technique of improving efficiency of an LED lamp with a remote phosphor. 
     BACKGROUND 
     To produce white light using a blue LED die, it is well known to deposit a YAG phosphor, or red and green phosphors, directly over the led die by, for example, spraying or spin-coating the phosphor in a binder, electrophoresis, applying the phosphor in a reflective cup, or other means. It is also known to affix a preformed tile of phosphor (e.g., a sintered phosphor powder) on the top of the LED die. Such phosphor layers are non-remote since they directly contact the surface of the semiconductor die. Blue light leaking through the phosphor, combined with the phosphor light, produces white light. Problems with such non-remote phosphors include: 1) there is significant backscattering of blue light from the phosphor layer, which is then partially absorbed by the LED, submount, and metal electrodes; 2) there is a significant amount of light generated by the phosphor that is partially absorbed by the LED, submount, and metal electrodes; 3) the photon density is very high for high power LEDs and saturates the phosphor; 4) the LED is very hot and phosphors may react to the heat to cause darkening of the polymer binder layer (e.g., silicone) in which the phosphor particles are imbedded; and 5) due to the various angles of blue light rays passing through different thicknesses of phosphors (a normal blue light ray passing through the least thickness), the color varies with viewing angle. 
     It is also known to infuse phosphor powder in a silicone binder and mold the silicone over the LED die to form a lens, such as described in U.S. Pat. No. 7,344,902, by Grigoriy Basin et al., assigned to the present assignee and incorporated herein by reference. The phosphor is distributed at a very low density in the lens. Such a remote phosphor creates a relatively large light source, whose brightness per unit area is greatly reduced compared to a LED die with a thin coating of phosphor. Also, since the phosphor overlies a large area of the submount, the phosphor light is partially absorbed by the submount, as well as the LED die and electrodes, so the efficiency of the white light LED is reduced. 
     The paper entitled, “A Nearly Ideal Phosphor-Converted White Light-Emitting Diode,” by Allen et al., Applied Physics Letters 92, 143309 (2008), describes a bare LED die surrounded by an air gap and a hemispherical phosphor layer encapsulated by a transparent layer. There is poor light extraction from the LED into the air gap, and light is absorbed by the submount. It is also very difficult to economically manufacture the device. 
     What is needed is a technique to create a phosphor-converted LED, using a remote phosphor, that is very efficient by having less light absorbed by the LED and submount. It is also desirable to provide a remote phosphor where the resulting light source is smaller than a light source having phosphor infused in a silicone lens. 
     SUMMARY 
     In one embodiment, a blue LED die is mounted on a submount. The submount is provided with a reflective surface surrounding the die. The LED die has molded over it a thin hemispherical encapsulant, such a silicone or another high index of refraction transparent material. A thin reflective layer is then created over the encapsulant layer that allows blue light to pass through but reflects phosphor light from above. The reflective layer may a low index of refraction (low n) layer that totally reflects light at greater than the critical angle (e.g., an air gap or porous layer), or the reflective layer may be a distributed Bragg reflector. A phosphor layer is then deposited or molded over the reflective layer. The phosphor layer may be dense and thin so as not to create a large light source. A clear outer layer, such as a molded silicone lens, is then formed over the phosphor layer to protect the phosphor layer and provide optical properties such as creating a desired emission pattern and increasing light extraction. 
     Various techniques are described for forming the various layers over the LED die, with one method being a molding technique for forming all layers. 
     The inner hemispherical encapsulant, having an index between that of the LED die and the reflective layer, improves light extraction from the LED die. Since the phosphor “shell” is remote from the die, there is little backscattering of the blue light. Further, the phosphor light is reflected outward by the reflective layer and the reflective submount surface, improving efficiency. Further, any backscatter of blue light from the phosphor is reflected out by the reflective layer rather than going back into the LED since most of the backscattered light will not be normal to the reflective layer. Still further, since the remote phosphor is not infused in a lens, the phosphor shell may have a small diameter to create a bright light source. All the advantages of a remote phosphor are achieved while still creating a small light source. 
     The phosphor may be YAG, red, green, or any other color or combination of phosphors. 
     In one embodiment, the creation of the various layers is simultaneously performed on a wafer scale with hundreds of LED dies mounted on a submount wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a blue or UV flip-chip LED die, mounted on a submount, where the submount has a reflective top layer. 
         FIG. 2  illustrates a simplified submount wafer populated by an array of LED dies, such as 500-4000 LEDs, where all LED dies on the wafer are simultaneously processed. 
         FIG. 3  illustrates the submount wafer being brought against a mold for forming a first silicone layer for encapsulating the LED dies and spacing a phosphor layer from the LED dies. The same molding process, using different molds, may be used for forming all layers rather than the process of  FIGS. 5-8 . 
         FIG. 4  illustrates the LED dies after encapsulation. 
         FIG. 5  illustrates a solid hemispherical outer lens. 
         FIG. 6  illustrates the lens of  FIG. 5  that is machined to create a cavity. 
         FIG. 7  illustrates the cavity of  FIG. 6  having a phosphor layer deposited over it. 
         FIG. 8  illustrates the lens and phosphor layer being affixed over the encapsulated die. 
         FIG. 9  illustrates the lens and phosphor layer affixed over the encapsulated die, with an air gap between the phosphor layer and the encapsulant to provide TIR. 
         FIG. 10  illustrates various light rays in the light source of  FIG. 9  showing the reflection by the air gap and the submount surface. 
         FIG. 11  illustrates a light source with phosphor infused in the outer lens showing the reflection by the air gap and the submount surface. 
         FIG. 12  illustrates a light source where each layer is formed by a successive molding process over the LED die, and the low index of refraction (n) layer may be any moldable layer including a sacrificial sol-gel layer. 
         FIG. 13  illustrates a light source where the reflective layer is a distributed Bragg reflector (DBR). 
         FIG. 14  is a graph illustrating the improvement in lumen output vs. reflectivity of the reflective layer on the submount. 
     
    
    
     Elements that are the same or equivalent are labeled with the same numeral. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a conventional flip chip LED die  10  mounted on a portion of a submount wafer  12 . In a flip-chip, both the n and p contacts are formed on the same side of the LED die. 
     The LED die  10  is formed of semiconductor epitaxial layers, including an n-layer  14 , an active layer  15 , and a p-layer  16 , grown on a growth substrate, such as a sapphire substrate. The growth substrate has been removed in  FIG. 1  by laser lift-off, etching, grinding, or by other techniques. In one example, the epitaxial layers are GaN based, and the active layer  15  emits blue light. LED dies that emit UV light are also applicable to the present invention. 
     A metal electrode  18  electrically contacts the p-layer  16 , and a metal electrode  20  electrically contacts the n-layer  14 . In one example, the electrodes  18  and  20  are gold pads that are ultrasonically welded to anode and cathode metal pads  22  and  23  on a ceramic submount wafer  12 . The submount wafer  12  has conductive vias  24  leading to bottom metal pads  26  and  28  for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer  12  and 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. 
     In accordance with one embodiment of the invention, a reflective layer  29  (e.g., R&gt;90%) is formed over the surface of the submount wafer  12  to reflect light generated by a remote phosphor layer. Submounts are typically ceramic, silicon, or other light absorbing material. The reflective layer  29  may be a sputtered metal mirror (e.g., Al or Ag), a dielectric mirror, a metal/dielectric combination, or a non-absorbing diffuser. The reflective layer  29 , in one embodiment, extends to all regions of the wafer  12  except over the LED die  10 . In another embodiment, the reflective layer  29  is a specular ring around each LED die that extends at least under where the phosphor layer contacts the submount. When depositing the reflective layer  26 , a mask (not shown) may be temporarily formed over the LED die areas, prior to attachment of the LED dies, to prevent the reflective layer  29  from covering the metal pads  22  and  23 , or a printing process may be used to form the reflective layer  29 . The reflective layer  29  will increase the efficiency of the lamp. 
       FIG. 2  is a simplified illustration of a submount wafer  12  on which is mounted an array of LED dies  10 . There may be 500-4000 LEDs on a single submount wafer  12 . All LEDs on the wafer  12  will be processed simultaneously using the method described below. 
     A first silicone layer is molded over the LED dies  10  to encapsulate the dies  10  as follows. 
       FIG. 3  illustrates a portion of the submount wafer  12  and LED dies  10  being positioned over a mold  30  having cavities  32  filled with liquid silicone  34 , or softened silicone  34 , or powered silicone  34 , or silicone in tablets. If the silicone  34  is not dispensed in liquid or softened form, the mold  30  is heated to soften the silicone  34 . The submount wafer  12  is brought against the mold  30  so that the LED dies  10  are immersed in the silicone  34  in each cavity  32 . The wafer  12  and mold  30  are pressed together to force the silicone  34  to fill all voids. A perimeter seal allows the pressure to be high while allowing all air to escape as the silicone  34  fills the voids. A vacuum may also be pulled between the wafer  12  and the mold  30  using a vacuum source around the seal. 
     The mold  30  is then heated to cure the silicone  34 , depending on the type of silicone  34  used. If the original silicone  34  was a solid (e.g., a powder or tablets) at room temperature, the mold  30  is cooled to harden the silicone  34 . Alternatively, a transparent mold may be used and the silicone  34  may be cured with UV light. 
     The mold  30  is then removed from the wafer  12 , resulting in the structure of  FIG. 4 , where the resulting silicone layer  36  encapsulates each LED die  10 . In the embodiment shown, the silicone layer  36  is formed to have a substantially hemispherical shape. The thickness of the silicone layer  36  is not critical since the LED light expands in a Lambertian pattern through the transparent silicone layer  36 . 
     The wafer  12  may then be subjected to a post-cure temperature of about 250° C. to additionally harden the silicone layer  36 , depending on the type of silicone  34  used. Materials other than silicone may be used such as an epoxy molding compound in powder form or another suitable polymer. 
     The silicone layer  36  may also be formed using injection molding, where the wafer  12  and mold are brought together, a liquid silicone is pressure-injected into the mold through inlets, and a vacuum is created. Small channels between the mold cavities allow the silicone to fill all the cavities. The silicone is then cured by heating, and the mold is separated from the wafer  12 . 
     The silicone layer  36  (a polymer) may instead be formed of a high index glass, epoxy, or other material. 
     One technique for forming additional layers over the LED die  10  and silicone layer  36  is described with respect to  FIGS. 5-8 . An all-molding process is described later. 
     In  FIG. 5 , a solid hemispherical dome  38  is formed by molding or another technique. The dome  38  may have a diameter on the order of 5 mm. The dome may be silicone, epoxy, sapphire, or other suitable transparent material. 
     In  FIG. 6 , the dome  38  is machined or processed to form a cavity  40 , having a diameter on the order of 3 mm. In one embodiment, the molding process for dome  38  may create a thin connector between domes in an array of domes that match the locations of the LED dies  10  on the submount wafer  12  to simplify handling. 
     In  FIG. 7 , a thin phosphor layer  42 , on the order of a few hundred microns, is formed in the cavity to a substantially uniform thickness. This may be done using a lamination of a preformed flexible sheet of phosphor infused in a silicone binder. The phosphor may also be deposited by spraying phosphor in a silicone binder, electrophoresis, deposition followed by machining, or by other techniques. As in all embodiments, the phosphor layer  42  may comprise a plurality of different phosphor layers or a mixture of phosphors, such as YAG, red, and/or green phosphors to produce white light. If a UV LED were used, a blue phosphor would also be used to create white light. 
     The completed cap  44  is then aligned with each LED die  10 , as shown in  FIG. 8 , and the cap  44  is affixed to the surface of the submount wafer  12  surrounding each LED die  10 . Silicone may be used as an adhesive. 
     As shown in  FIG. 9 , there is an air gap  46  between the phosphor layer  42  and the silicone layer  36 . The LED die  10  has sides about 1 mm, the silicone layer  36  has a diameter of about 2 mm, the phosphor layer  42  is a few hundreds of microns, and the cavity  40  has a diameter of about 3 mm, leaving an air gap  46  of about 0.2-0.5 mm around the silicone layer  36 . Since the index of refraction (n) of the air gap  46  is about 1, and the n of the phosphor layer  42  is on the order of 1.7-2, any phosphor light generated towards the air gap  46  at greater than the critical angle will be totally reflected back and not be absorbed by the LED die  10 , electrodes, or other elements. 
     By making the silicone layer  36  substantially hemispherical around the LED die  10 , there will be very little TIR of the LED light at the interface of the air gap  46  and the silicone layer  46 . The silicone layer  36  improves the extraction of light from the LED die  10  since its index of refraction (e.g., &gt;1.5) is closer to the index of refraction of the LED die  10  (e.g., &gt;2.2). 
       FIG. 10  shows how various light rays generated will be reflected in the lamp  48  of  FIG. 9 . Ray  50  is a blue ray from the LED die  10 , and leaks through the phosphor layer  42 . Ray  52  is an emission (e.g., yellow, red, green, etc.) from a phosphor particle that is in a direction away from the air gap  46 . Ray  54  is an emission from the phosphor particle that reflects off the air gap  46  interface at greater than the critical angle and exits the lamp  48  without impinging on the LED  10  or submount wafer  12 . Ray  56  is an emission from a phosphor particle that reflects off the reflective layer  29  ( FIG. 1 ) on the submount wafer  12 . 
     Also, ray  54  may be a backscattered blue ray from the LED die. Although the blue ray generally enters the phosphor layer near normal incidence, the backscatter from the phosphor is generally isotropic, so the backscattered light is at a wide range of angles. Any backscattered blue light greater than the critical angle is reflected out by the air gap  46  interface (or other reflective layer described herein) rather than going back into the LED. 
     The combination of the low index layer (air gap  46 ), silicone layer  36 , and reflective layer  29  greatly increase the light extraction from the lamp  48 . 
       FIG. 11  illustrates that, instead of forming the cap of  FIG. 7 , a thicker cap of a low density of phosphor particles in a silicone lens  58  may be used. The air gap  46  serves as a reflector as in  FIG. 10 . Ray  50  is a blue ray from the LED die  10  leaking through the lens  58 . Rays  60  and  62  are rays from a phosphor particle (or backscattered blue light) that have reflected off the air gap  46  interface. Ray  64  is a ray from a phosphor particle that reflected off the reflective layer  29  ( FIG. 1 ) on the submount wafer  12 . Since the silicone lens  58  is much wider than the phosphor layer  42  in  FIG. 10 , the brightness of the lamp  66  per unit area will be less than that of  FIG. 10 , which may be advantageous or disadvantageous depending on the application. 
     The technique of  FIGS. 5-11  may be difficult due to the handling and various alignments. 
     Successive molding processes, represented by  FIG. 3 , may be used to form the structure shown in  FIG. 12 , where, instead of an air gap, a low index material is molded directly over the silicone layer  36 . In one embodiment, the silicone layer  36  is molded as previously described. Next, a mold having larger cavities  32  ( FIG. 3 ) is filled with sol-gel. Sol-gel is well known and comprises nano-particles in a solvent to form a gel. Such a substance can be molded. The solvent is then dried by heat, resulting in some shrinkage and crystals formed by the nano-particles. The resulting layer will be extremely porous and effectively acts like an air gap. The index of refraction is very low since the structure is mostly space. The sol-gel layer is shown as layer  68  in  FIG. 12 . Instead of sol-gel, another low index material can be used, as long as the index is lower than the phosphor layer. 
     Next, another mold with slightly larger dome shaped cavities is filled with phosphor particles infused in silicone. The submount wafer  12  with molded sol-gel domes is then brought against the mold as discussed with respect to  FIG. 3 . The phosphor layer  70  is then cured by heat. A final silicone lens  72  is then molded over the phosphor layer, or the phosphor layer may be the final layer. The operation of the resulting lamp  74  is similar to that shown in  FIG. 10  or  FIG. 11 , depending if the phosphor layer were the final layer. 
       FIG. 13  illustrates that the low index layer ( 46  or  68 ) can instead be a deposited Bragg reflector (DBR)  76 . A DBR  76  can be made very thin using conformal sputtering and may consists of 10 pairs of SiO 2 /Ta 2 O 5  (indicies n=1.5 and 2, respectively) with thicknesses 98 nm and 64 nm (provides reflectively at 450 nm (blue) around 40 degrees), followed by 6 pairs of SiO 2 /Ta 2 O 5  with thicknesses 129 nm and 81 nm (provides reflectivity at 550 nm (green) around 45 degrees). The DBR  76  is substantially transparent at 450 nm around normal incidence of the LED light (R&lt;10%) until an angle of 15 degrees. The DBR  76  is reflective at greater than 15 degrees. The phosphor layer  78  and outer silicone lens  80  are then molded as previously described.  FIG. 13  illustrates various light rays passing through the DBR  76 , and being reflected off the DBR  76 , and being reflected off the submount surface. 
     For all these designs to be efficient, the extraction efficiency from the inner silicone dome must be high. This requires that LED light impinge at this interface with an angle less than the critical angle and, therefore, the radius of the inner dome must be large enough. Therefore, in general, a trade-off exists between the requirement of small incidence angles and small source size. If the die is 1×1 mm and the radius of the inner dome is 2 mm, a large fraction of the light impinges on the dome at small angles (less than 15 degrees), and only a few rays impinge at angles as high as 20 degrees. This is smaller than the angle of total internal refraction for an epoxy (or silicone)/air interface (about 41 degrees) and smaller than the maximum angle of high transmission for the DBR. Therefore, such dimensions are suitable for the implementations described in this application. 
       FIG. 14  is a graph illustrating the approximate improvement in lumen output vs. reflectivity of the reflective layer  29  ( FIG. 1 ) on the submount when used with the remote phosphor embodiments of the present invention. 
     Various combinations of all the embodiments may be used to create a remote phosphor lamp with high efficiency. 
     In addition to the improved efficiency, the remote hemispherical phosphor layer, having a substantially uniform thickness, enables uniform color vs. viewing angle, and the phosphor is not degraded by heat. 
     The submount wafer  12  is then singulated to form individual LEDs/submounts, where the various figures can represent the individual LEDs/submounts. 
     In this disclosure, the term “submount” is intended to mean a support for at least one LED die, where electrical contacts on the submount are bonded to electrodes on the LED dies, and where the submount has electrodes that are to be connected to a power supply. 
     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.