Abstract:
One or more LED dice are mounted on a support structure. The support structure may be a submount with the LED dice already electrically connected to leads on the submount. A mold has indentations in it corresponding to the positions of the LED dice on the support structure. The indentations are filled with a liquid optically transparent material, such as silicone, which when cured forms a lens material. The shape of the indentations will be the shape of the lens. The mold and the LED dice/support structure are brought together so that each LED die resides within the liquid silicone in an associated indentation. The mold is then heated to cure (harden) the silicone. The mold and the support structure are then separated, leaving a complete silicone lens over each LED die. This over molding process may be repeated with different molds to create concentric shells of lenses. Each concentric lens may have a different property, such as containing a phosphor.

Description:
FIELD OF THE INVENTION 
     This invention relates to light emitting diodes (LEDs) and, in particular, to a technique for forming a lens over an LED die. 
     BACKGROUND 
     LED dies typically emit light in a Lambertian pattern. It is common to use a lens over the LED die to narrow the beam or to make a side-emission pattern. A common type of lens for a surface mounted LED is preformed molded plastic, which is bonded to a package in which the LED die is mounted. One such lens is shown in U.S. Pat. No. 6,274,924, assigned to Lumileds Lighting and incorporated herein by reference. 
     SUMMARY 
     A technique for forming a lens for surface mounted LEDs is described herein. 
     One or more LED dice are mounted on a support structure. The support structure may be a ceramic substrate (a submount) or other type of support structure with the LED dice electrically connected to metal pads on the support structure. 
     A mold has indentations in it corresponding to the positions of the LED dice on the support structure. The indentations are filled with a liquid, optically transparent material, such as silicone, which when cured forms a hardened lens material. The shape of the indentations will be the shape of the lens. The mold and the LED dice/support structure are brought together so that each LED die resides within the liquid lens material in an associated indentation. 
     The mold is then heated to cure (harden) the lens material. The mold and the support structure are then separated, leaving a complete lens over each LED die. This general process will be referred to as over molding. 
     The over molding process may be repeated with different molds to create concentric shells of lenses. Each lens may have a different property, such as containing a phosphor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of four LED dice mounted on a support structure, such as a submount, and a mold for forming a lens around each LED die. 
         FIG. 2  is a side view of the LED dice being inserted into indentations in the mold filled with a liquid lens material. 
         FIG. 3  is a side view of the LED dice removed from the mold after the liquid has been cured, resulting in a lens encapsulating each LED die. 
         FIG. 4  is a perspective view of an array of LED dice on a submount or circuit board with a molded lens formed over each LED die. 
         FIG. 5  is a close-up side view of a flip-chip LED die mounted on a submount, which is, in turn, mounted on a circuit board, and where a molded lens is formed over the LED die. 
         FIG. 6  is a close-up side view of a non-flip-chip LED die mounted on a submount, which is, in turn, mounted on a circuit board, where wires electrically connect n and p metal on the LED die to leads on the circuit board, and where a molded lens is formed over the LED die. 
         FIGS. 7 ,  8 ,  9 ,  10 , and  11  are cross-sectional views of an LED die with different lenses formed over it. 
         FIG. 12  is a cross-sectional view of a side-emitting lens molded onto the LED die using the inventive techniques. 
         FIG. 13  is a cross-sectional view of a collimating lens molded onto the LED die using the inventive techniques. 
         FIG. 14  is a cross-sectional view of a preformed side-emitting lens affixed over a lambertian lens that has been molded onto the LED die using the inventive techniques. 
         FIG. 15  is a cross-sectional view of a backlight for a liquid crystal display or other type of display using the LED and side-emitting lens of  FIG. 14 . 
         FIG. 16  is a perspective view of a cell phone with a camera that uses as a flash an LED with a molded lens. 
     
    
    
     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 package (or 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. 1-3  for simplicity. 
     Examples of forming LEDs are described in U.S. Pat. Nos. 6,649,440 and 6,274,399, both assigned to Lumileds Lighting and incorporated by reference. 
     Optionally, a conductive substrate is bonded to the LED layers (typically to the p-layers) and the sapphire substrate is removed. One or more LED dice may be bonded to a submount, with the conductive substrate directly bonded to the submount, to be described in greater detail with respect to  FIGS. 5 and 6 . One or more submounts may be bonded to a printed circuit board, which contains metal leads for connection to other LEDs or to a power supply. The circuit board may interconnect various LEDs in series and/or parallel. 
     The particular LEDs formed and whether or not they are mounted on a submount is not important for purposes of understanding the invention. 
       FIG. 1  is a side view of four LED dice  10  mounted on a support structure  12 . The support structure may be a submount (e.g., ceramic or silicon with metal leads), a metal heat sink, a printed circuit board, or any other structure. In the present example, the support structure  12  is a ceramic submount with metal pads/leads. 
     A mold  14  has indentations  16  corresponding to the desired shape of a lens over each LED die  10 . Mold  14  is preferably formed of a metal. A very thin non-stick film  18 , having the general shape of mold  14 , is placed over mold  14 . Film  18  is of a well known conventional material that prevents the sticking of silicone to metal. 
     In  FIG. 2 , the mold indentions  16  have been filled with a heat-curable liquid lens material  20 . The lens material  20  may be any suitable optically transparent material such as silicone or an epoxy. Silicone has a sufficiently high index of refraction (e.g., 1.76) to greatly improve the light extraction from an AlInGaN or AlInGaP LED as well as act as a lens. 
     A vacuum seal is created between the periphery of the support structure  12  and mold  14 , 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 is then heated to about 150 degrees centigrade (or other suitable temperature) for a time to harden the lens material  20 . 
     The support structure  12  is then separated from mold  14 . Film  18  causes the resulting hardened lens to be easily released from mold  14 . Film  18  is then removed. 
       FIG. 3  illustrates the resulting structure with a molded lens  22  over each LED die  10 . In one embodiment, the molded lens is between 1 mm and 5 mm in diameter. The lens  22  may be any size. 
       FIG. 4  is a perspective view of a resulting structure where the support structure  12  supports an array of LED dice, each having a molded lens  22 . The mold used would have a corresponding array of indentations. If the support structure  12  were a ceramic or silicon submount, each LED (with its underlying submount portion) can be separated by sawing or breaking the submount  12  to form individual LED dice. Alternatively, the support structure  12  may be separated/diced to support subgroups of LEDs or may be used without being separated/diced. 
     The lens  22  not only improves the light extraction from the LED die and refracts the light to create a desired emission pattern, but the lens also encapsulates the LED die to protect the die from contaminants, add mechanical strength, and protect any wire bonds. 
       FIG. 5  is a simplified close-up view of one embodiment of a single flip-chip LED die  10  on a submount  24  formed of any suitable material, such as a ceramic or silicon. In one embodiment, submount  24  acted as the support structure  12  in  FIGS. 1-4 , and the die/submount of  FIG. 5  was separated from the structure of  FIG. 4  by sawing. The LED die  10  of  FIG. 5  has a bottom p-contact layer  26 , a p-metal contact  27 , p-type layers  28 , a light emitting active layer  30 , n-type layers  32 , and an n-metal contact  31  contacting the n-type layers  32 . Metal pads on submount  24  are directly metal-bonded to contacts  27  and  31 . Vias through submount  24  terminate in metal pads on the bottom surface of submount  24 , which are bonded to the metal leads  40  and  44  on a circuit board  45 . The metal leads  40  and  44  are connected to other LEDs or to a power supply. Circuit board  45  may be a metal plate (e.g., aluminum) with the metal leads  40  and  44  overlying an insulating layer. The molded lens  22 , formed using the technique of  FIGS. 1-3 , encapsulates the LED die  10 . 
     The LED die  10  in  FIG. 5  may also be a non-flip-chip die, with a wire connecting the top n-layers  32  to a metal pad on the submount  24 . The lens  22  may encapsulate the wire. 
     In one embodiment, the circuit board  45  itself may be the support structure  12  of  FIGS. 1-3 . Such an embodiment is shown in  FIG. 6 .  FIG. 6  is a simplified close-up view of a non-flip-chip LED die  10  having a top n-metal contact  34  connected to a metal lead  40  on circuit board  45  by a wire  38 . The LED die  10  is mounted on a submount  36 , which in the example of  FIG. 6  is a metal slab. A wire  42  electrically connects the p-layers  26 / 28  to a metal lead  44  on circuit board  45 . The lens  22  is shown completely encapsulating the wires and submount  36 ; however, in other embodiments the entire submount or the entire wire need not be encapsulated. 
     A common prior art encapsulation method is to spin on a protective coating. However, that encapsulation process is inappropriate for adding a phosphor coating to the LED die since the thickness of the encapsulant over the LED die is uneven. Also, such encapsulation methods do not form a lens. A common technique for providing a phosphor over the LED die is to fill a reflective cup surrounding the LED die with a silicone/phosphor composition. However, that technique forms a phosphor layer with varying thicknesses and does not form a suitable lens. If a lens is desired, additional processes still have to create a plastic molded lens and affix it over the LED die. 
       FIGS. 7-11  illustrate various lenses that may be formed using the above-described techniques. 
       FIG. 7  illustrates an LED die  10  that has been coated with a phosphor  60  using any suitable method. One such method is by electrophoresis, described in U.S. Pat. No. 6,576,488, assigned to Lumileds Lighting and incorporated herein by reference. Suitable phosphors are well known. A lens  22  is formed using the techniques described above. The phosphor  60  is energized by the LED emission (e.g., blue or UV light) and emits light of a different wavelength, such as green, yellow, or red. The phosphor emission alone or in conjunction with the LED emission may produce white light. 
     Processes for coating an LED with a phosphor are time-consuming. To eliminate the process for coating the LED die with a phosphor, the phosphor powder may be mixed with the liquid silicone so as to become embedded in the lens  62 , shown in  FIG. 8 . 
     As shown in  FIG. 9 , to provide a carefully controlled thickness of phosphor material over the LED die, an inner lens  64  is formed using the above-described techniques, and a separate molding step (using a mold with deeper and wider indentations) is used to form an outer phosphor/silicone shell  66  of any thickness directly over the inner lens  64 . 
       FIG. 10  illustrates an outer lens  68  that may be formed over the phosphor/silicone shell  66  using another mold to further shape the beam. 
       FIG. 11  illustrates shells  70 ,  72 , and  74  of red, green, and blue-emission phosphors, respectively, overlying clear silicone shells  76 ,  78 , and  80 . In this case, LED die  10  emits UV light, and the combination of the red, green, and blue emissions produces a white light. All shells are produced with the above-described methods. 
     Many other shapes of lenses can be formed using the molding technique described above.  FIG. 12  is a cross-sectional view of LED  10 , submount  24 , and a molded side-emitting lens  84 . In one embodiment, lens  84  is formed of a very flexible material, such as silicone, which flexes as it is removed from the mold. When the lens is not a simple shape, the release film  18  ( FIG. 1 ) will typically not be used. 
       FIG. 13  is a cross-sectional view of LED  10 , submount  24 , and a molded collimating lens  86 . 
       FIG. 14  illustrates how a preformed lens  88  can be affixed over a molded lambertian lens  22 . In the example of  FIG. 14 , lens  22  is formed in the previously described manner. Lens  22  serves to encapsulate and protect LED  10  from contaminants. A preformed side-emitting lens  88  is then affixed over lens  22  using a UV curable adhesive or a mechanical clamp. This lens-forming technique has advantages over conventional techniques. In a conventional technique, a preformed lens (e.g., a side emitting lens) is adhesively affixed over the LED die, and any gaps are filled in by injecting silicone. The conventional process is difficult to perform due to, among other reasons, carefully positioning the separated die/submount for the lens placement and gap-filling steps. Using the inventive technique of  FIG. 14 , a large array of LEDs ( FIG. 4 ) can be encapsulated simultaneously by forming a molded lens over each. Then, a preformed lens  88  can be affixed over each molded lens  22  while the LEDs are still in the array ( FIG. 4 ) or after being separated. 
     Additionally, the molded lens can be made very small (e.g., 1-2 mm diameter), unlike a conventional lens. Thus, a very small, fully encapsulated LED can be formed. Such LEDs can be made to have a very low profile, which is beneficial for certain applications. 
       FIG. 14  also shows a circuit board  45  on which submount  24  is mounted. This circuit board  45  may have mounted on it an array of LEDs/submounts  24 . 
       FIG. 15  is a cross-sectional view of a backlight for a liquid crystal display (LCD) or other display that uses a backlight. Common uses are for televisions, monitors, cellular phones, etc. The LEDs may be red, green, and blue to create white light. The LEDs form a two-dimensional array. In the example shown, each LED structure is that shown in  FIG. 14 , but any suitable lens may be used. The bottom and sidewalls  90  of the backlight box are preferably coated with a white reflectively-diffusing material. Directly above each LED is a white diffuser dot  92  to prevent spots of light from being emitted by the backlight directly above each LED. The dots  92  are supported by a transparent or diffusing PMMA sheet  94 . The light emitted by the side-emitting lenses  88  is mixed in the lower portion of the backlight, then further mixed in the upper portion of the backlight before exiting the upper diffuser  96 . Linear arrays of LEDs may be mounted on narrow circuits boards  45 . 
       FIG. 16  illustrates an LED  10  with a molded lens  22  being used as a flash in a camera. The camera in  FIG. 16  is part of a cellular telephone  98 . The cellular telephone  98  includes a color screen  100  (which may have a backlight using the LEDs described herein) and a keypad  102 . 
     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.