Abstract:
A method for fabricating a light emitting device is described where an array of flip-chip light emitting diode (LED) dies are mounted on a submount wafer. Over each of the LED dies is simultaneously molded a hemispherical first silicone layer. A preformed flexible phosphor layer, comprising phosphor powder infused in silicone, is laminated over the first silicone layer to conform to the outer surface of the hemispherical first silicone layer. A silicone lens is then molded over the phosphor layer. By preforming the phosphor layer, the phosphor layer may be made to very tight tolerances and tested. By separating the phosphor layer from the LED die by a molded hemispherical silicone layer, color vs. viewing angle is constant, and the phosphor is not degraded by heat. The flexible phosphor layer may comprise a plurality of different phosphor layers and may comprise a reflector or other layers.

Description:
FIELD OF THE INVENTION 
       [0001]    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 laminating a remote phosphor layer over the LED to achieve more precise color control and more uniform color vs. viewing angle. 
       BACKGROUND 
       [0002]    Prior art  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. 
         [0003]    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. 
         [0004]    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  24  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. 
         [0005]    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. 
         [0006]    To produce white light using the blue LED die  10 , it is well known to deposit a YAG phosphor, or red and green phosphors, directly over the die  10  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  10 . Such phosphor layers are non-remote since they directly contact the surface of the semiconductor die  10 . Blue light leaking through the phosphor, combined with the phosphor light, produces white light. Problems with such non-remote phosphors include: 1) the photon density is very high for high power LEDs and saturates the phosphor; 2) 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; 3) 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; and 4) it is difficult to create very uniform phosphor layer thicknesses and densities. 
         [0007]    It is also known to infuse phosphor powder in a silicone binder and mold the silicone over the LED die to form a lens. However, mold tolerances affect the thickness and alignment of the phosphor, which affect the overall color and color vs. viewing angle. Mold tolerances are generally 30-50 microns, and the desired phosphor thickness is only on the order of 100 microns, so it is difficult to achieve a ±50K target correlated color temperature (CCT) for a white LED over a certain viewing angle specified by a customer. 
         [0008]    Blue LED dies formed using the same process produce slightly different dominant wavelengths, and LEDs are sometimes binned according to their dominant wavelength. So if the same phosphor layer were applied to each blue LED die, the overall color temperature would be different for each bin of LED die. If white LEDs need to be matched, such as for backlights, such LEDs would have to come from the same bin. This effectively reduces yield for certain stringent applications. 
         [0009]    Additionally, reproducibility of the phosphor layer is difficult using the prior art processes. 
         [0010]    What is needed is a technique to create a phosphor-converted LED that does not suffer from the above-described drawbacks. 
       SUMMARY 
       [0011]    To achieve a more precise phosphor layer for use with a blue or UV LED die to create white light (or another color), a remote phosphor layer is used. The remote phosphor layer is spaced from the LED die so, compared to a phosphor that is formed directly on the LED die surface, there is a lower photon density and the phosphor experiences a lower temperature. The photon density is lower since the LED die light is spread out over a larger area before impinging on the remote phosphor layer. 
         [0012]    To achieve greater precision in the phosphor layer thickness, density, and wavelength conversion characteristics, the phosphor layer is a preformed, tested layer comprising phosphor powder infused in a silicone binder. A sheet of such a phosphor layer is formed to have a well-controlled thickness and phosphor density. The sheet is tested, such as by energizing it with blue light, to determine its dominant wavelength output. Phosphor sheets having different characteristics are then matched up with binned blue LED dies. In this way, a target white light CCT can be achieved using blue LEDs from different bins. 
         [0013]    To space the preformed phosphor layer from the LED die, a silicone layer is first molded over the LED die to encapsulate the die. In one embodiment, this first molded silicone layer has a substantially hemispherical shape. The matched phosphor sheet is laminated over the silicone layer using a vacuum, and the application of heat adheres the phosphor sheet to the silicone layer. Any typical imprecision in the mold or alignment (e.g., 30-50 microns) when forming the silicone layer does not significantly affect the white light CCT since the phosphor layer is remote and will also have a hemispherical shape. 
         [0014]    A second silicone layer is molded over the phosphor layer to protect the phosphor layer and serve as a lens. In one embodiment, the second silicon layer is substantially hemispherical so that the white LED outputs a Lambertian pattern. The shape of the second silicone lens may be formed to create any type of emission pattern 
         [0015]    The above process is performed simultaneously on an array of LED dies mounted on a submount wafer. The array of dies may be from a single bin. The phosphor layer may be a single sheet that spans the entire wafer. The wafer is then singulated to separate out the white light LEDs/submounts. 
         [0016]    In one embodiment, the phosphor layer contains a YAG phosphor (yellow-green). In another embodiment, the phosphor layer contains mixed red and green phosphors. In another embodiment, the phosphor layer comprises multiple layers, such as a layer of red and a separate layer of YAG to produce a warm white color. The process can be used to make any color light using any type of phosphor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a cross-sectional view of a prior art blue or UV flip-chip LED die, mounted on a submount. 
           [0018]      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. 
           [0019]      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. 
           [0020]      FIG. 4  illustrates the LED dies immersed in the silicone filling the mold indentions. 
           [0021]      FIG. 5  illustrates a preformed, thin, and flexible phosphor layer being laminated over the molded silicone layer using a vacuum and heat, such that the phosphor layer conforms to the outer surface of the silicone layer. 
           [0022]      FIG. 6  illustrates a phosphor sheet with a layer of red phosphor and a layer of a YAG phosphor (or a green phosphor). 
           [0023]      FIG. 7  illustrates a multi-layer phosphor sheet where the top layer is formed having microlenses. 
           [0024]      FIG. 8  illustrates a multi-layer phosphor sheet where there is a reflective layer on the bottom that passes blue light but reflects red, green, and yellow light. 
           [0025]      FIG. 9  illustrates a multi-layer phosphor sheet where the top surface is formed to have varying thicknesses to match characteristics of the individual LED dies. 
           [0026]      FIG. 10  illustrates a phosphor layer with an overlying pigmented layer. 
           [0027]      FIG. 11  illustrates a white light LED after undergoing the processes described herein. 
       
    
    
       [0028]    Elements that are the same or equivalent are labeled with the same numeral. 
       DETAILED DESCRIPTION 
       [0029]      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. 
         [0030]    A first silicone layer is molded over the LED dies  10  to encapsulate the dies  10  as follows. 
         [0031]      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 , as shown in  FIG. 4 , 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. 
         [0032]    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. 
         [0033]    The mold  30  is then removed from the wafer  12 , resulting in the structure of  FIG. 5 , 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 . 
         [0034]    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. 
         [0035]    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 . 
         [0036]    The silicone layer  36  serves to separate a uniform phosphor layer from the LED die, as described below. 
         [0037]      FIG. 5  illustrates a preformed phosphor layer  38  being laminated to the surface of the wafer  12  and to the silicone layer  36 . The phosphor layer  38  may be the same size as the wafer  12 . The phosphor layer  38  is formed of a suitable phosphor powder, such as YAG, red, or green phosphor, or any combination of phosphors, to achieve the target color emission. To create the phosphor layer  38 , the phosphor powder is mixed with silicone to achieve a target density, and the phosphor layer  38  is formed to have a target thickness. The desired thickness may be obtaining by spinning the mixture on a flat surface or molding the phosphor layer. 
         [0038]    After the phosphor layer  38  is cured, the phosphor layer  38  may be tested by energizing the phosphor layer  38  using a blue light source and measuring the light emission. Since blue LEDs generally emit slightly different dominant wavelengths, the blue LEDs may be tested prior to being mounted on the submount wafer  12 , and the LEDs are binned according to their dominant wavelengths. Preformed phosphor layers of varying thicknesses or phosphor densities are then matched up with LEDs from particular bins so that the resulting color emissions may all be the same target white point (or CCT). If all LED dies on the submount wafer  12  are from the same bin and the phosphor layer  38  was previously matched to that bin, the color emission will be a target CCT. 
         [0039]    In one embodiment, the phosphor layer  38  is on the order of a few hundred microns thick and highly flexible. 
         [0040]    As shown in  FIG. 5 , the matched phosphor layer  38  is placed over the wafer  12 , and a vacuum is drawn between the phosphor layer  38  and the wafer  12  to remove all air. This will conformally coat the silicone layer  36  and wafer  12 . The structure is then heated to adhere the silicone in the phosphor layer  38  to the silicone layer  36 . 
         [0041]    By laminating a preformed phosphor layer rather than forming the phosphor over the LED die, uniform phosphor thickness and density are guaranteed. It is very easy to create a uniform phosphor sheet. By spacing the phosphor layer  38  from the LED die  10  using the silicone layer  36 , the photon density at the phosphor layer  38  is reduced, there are no thermal degradation problems with the phosphor, the refractive index of the silicone layer  36  can be tailored to increase the extraction efficiency, and there are no mold tolerances that affect the phosphor layer  38  performance. Since no mold misalignment affects the phosphor layer, there is improved color uniformity. The color vs. viewing angle is consistent since the blue LED light passes through equal thicknesses of the phosphor layer  38  at all angles. 
         [0042]    Another advantage of the preformed laminated phosphor layer  38  is that the phosphor layer may be formed of multiple layers, each layer being customized and precisely formed.  FIGS. 6-10  illustrate some multi-layered phosphor layers that can be laminated onto the wafer  12 . In the preferred embodiment, the multi-layer sheet is preformed, due to the ease of laminating the layers together, and the sheet is tested and then laminated as a single sheet to the wafer  12 . Alternatively, the multiple layers may be individually laminated onto the wafer  12 . 
         [0043]      FIG. 6  illustrates a red phosphor layer  40  with an overlying YAG phosphor layer  42 . The red phosphor layer  40  is customized to create a warmer white, since the yellow-green YAG phosphor tends to create a harsh white. A green phosphor may be used instead of YAG. Any number of phosphor layers may be formed to create the desired color characteristics. In one embodiment, a UV LED die is used and one of the layers is a blue phosphor layer. The multiple phosphor layers may be separately formed and laminated together using heat and pressure and/or a vacuum. 
         [0044]      FIG. 7  illustrates that the top phosphor layer  44  may be molded to have tiny lenses (or other optical elements) over its surface to reduce TIR or to achieve increase light scattering or other optical effects. 
         [0045]      FIG. 8  illustrates that one of the laminated layers may be a chromatic reflector  46  that allows blue light to pass but reflects longer wavelength light. In this way, the light produced by the phosphors is not absorbed by the LED die  10  but is always reflected upward. 
         [0046]      FIG. 9  illustrates that the top phosphor layer  48  may be molded to have different thicknesses to be matched with individual blue LED dies  10  on the wafer  12  to achieve the same target CCT for each LED. 
         [0047]      FIG. 10  illustrates that a phosphor layer  42  may be laminated with a non-phosphor optical layer  50  that may be a pigmented color filter, a light scattering layer (e.g., silicone containing particles of TiO 2 ), or other type of layer. 
         [0048]      FIG. 11  illustrates the wafer  12  with the laminated phosphor layer  38  being brought against a mold  60  in order to form a silicone lens over the LEDs. This will protect the laminated phosphor layer  38 , create any desired emission pattern, and increase light extraction by tailoring the refractive index of the silicone and the shape of the lens. 
         [0049]    In  FIG. 11 , the mold  60  contains cavities  62  filled with silicone  64  for forming a hemispherical lens  66  ( FIG. 12 ). The molding process may be the same as describe with respect to  FIG. 3 . The lens  66  may instead be a side-emitting lens or any other type of lens. The lens  66  may even have phosphor powder (e.g., red phosphor) in it to shift the output color temperature. 
         [0050]      FIG. 12  shows the wafer  12  removed from the mold  60  after curing. 
         [0051]    In one embodiment, the first silicone layer  38  has a refractive index of 1.4, and the lens  66  has an index of 1.5 to reduce the percentage of blue photons that are internally reflected. The mold for the outer lens  66  may create a roughened outer surface to increase light extraction efficiency. 
         [0052]    By using lamination of the preformed phosphor layer  38 , mold tolerances do not affect the color emission or color vs. viewing angle. Since many LEDs from the same bin are processed simultaneously on a wafer scale, and the phosphor layer  38  is laminated as a large sheet, the LEDs generate a target CCT to very tight tolerances (less than 50K), and processing is relatively easy. 
         [0053]    The submount wafer  12  is then singulated to form individual LEDs/submounts, where one such LED is shown in  FIG. 13 . Note that the phosphor layer  38  continues to the edges of the singulated submount. 
         [0054]    In this disclosure, the term “submount wafer” is intended to mean a support for an array of LED dies, where electrical contacts on the wafer are bonded to electrodes on the LED dies, and the wafer is later singulated to form one or more LEDs on a single submount, where the submount has electrodes that are to be connected to a power supply. 
         [0055]    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.