Abstract:
A high brightness LED phosphor coupling device. A semiconductor light source is encapsulated by a medium of first index of refraction, a layer of phosphor surrounded by a second medium of second index of refraction of optical index less than the first index and a light coupler for redirecting most of the light from the light source to an area of the phosphor about equal to the area of the light source multiplied by the square of the ratio of the first to the second index of refraction.

Description:
This application claims priority to U.S. Provisional Patent Application No. 60/469,649, filed on May 12, 2003, and incorporated herein by reference. 

   This invention is directed to a high brightness LED-phosphor coupling device. More particularly the invention is directed to a semiconductor light source encapsulated by a medium of first index of refraction, a layer of phosphor surrounded by a second medium of second optical index less than the first index and a light coupler for redirecting most of the light from the light source to an area of the phosphor about equal to the area of the light source multiplied by the square of the ratio of the first to the second index of refraction. 
   BACKGROUND AND PRIOR ART 
   Light-emitting diodes (LEDs) are a commonly used light source in applications including lighting, signaling, signage, and displays. LEDs have several advantages over incandescent and fluorescent lamps, including high reliability, long lifetime, and high efficiency. A typical prior art LED package is shown in  FIG. 1 . The die is a piece of semiconductor material that actually produces the emitted light. The contact and bond wire carry electrical current to the die. The substrate provides a physical mounting surface for the die and helps conduct away the heat generated by the die. The substrate shown in  FIG. 1  is cup-shaped, which helps collect light from the die and redirect it upwards. In some preferred cases, including the example in  FIG. 1 , the substrate also conducts electrical current. The die is surrounded by an encapsulant, typically a polymer but can be other materials such as glass, which is transparent to the light wavelengths of interest. The encapsulant protects the die from mechanical damage, moisture, and atmospheric exposure. It also increases light extraction efficiency from the die relative to a die in air. The semiconductor die is typically a high-optical-index material such as sapphire, SiC, or GaP. Light inside the high-index material can only escape when it is incident on the die surface at an angle of incidence θ&lt;θ C =arcsin[n surround /n die ], where n surround  and n die  are the indices of refraction of the surround and die. Light extraction efficiency is improved when this critical angle is as large as possible. Encapsulants have a higher index than air, and therefore they increase light extraction. The encapsulant is generally chosen to have as high an index as possible, but selection is typically constrained by other requirements such as transparency, resistance to thermal and photochemical degradation, hardness, and ease of application and curing in a manufacturing process. 
   The semiconductor die in an LED typically produces only a narrow spectrum of light, which is perceived by the eye as a single color such as red, blue, green, amber, etc. However, for many applications, especially lighting, broadband white light is preferred. A common approach to providing white light with LEDs is to overlay the die with a phosphor which absorbs some or all of the LED light and emits light at lower wavelengths, thus providing a mixture of colors that the eye perceives as white. This combination of LED phosphor is commonly referred to as a “white LED.” White LEDs can be made from a number of LED-phosphor combinations, including blue LED+yellow phospor, blue LED+a combination of red and green phosphors, and UV LED+a combination of red, green, and blue phosphors. The various approaches and difficulties of different materials combinations are well known in the art, such as described by Regina Mueller-Mach et al. in “High-Power Phosphor-Converted Light-Emitting Diodes Based on III-Nitrides,”  IEEE Journal On Selected Topics In Quantum Electronics,  Vol. 8, No. 2, March/April 2002. 
   Performance and reliability of white LEDs also depend on the method for applying the phosphor as well as the materials. The phosphor typically comes in the form of fine powder. The most commonly used method is to disperse this powder in a polymer binder and dispense the mixture directly onto the LED. The polymer binder can be the same material used for the encapsulant, and in fact the phosphor-binder mixture becomes functionally part of the encapsulant.  FIGS. 2A and 2B  show two examples. In previously developed white LEDs, as shown in  FIG. 2A , the phosphor-containing volume was usually much larger than the die, which in turn caused the effective light source area to be much larger than the die. It is well-known in the art of optical design that in a high-collection optical systems the output beam has an etendue E out =n out   2 A out Ω out , where n out  is the index of the medium in which the beam emerges (typically air, with n≅1), A out  is the area of the beam, and Ω out  is the solid angle of the beam. For high collection efficiency, it is accepted that E out  must be greater than or equal to the etendue of the light source itself. The phosphor layer is substantially Lambertian ±90°, which means that the etendue of the phosphor layer is approximately n binder   2 ×A phosphor ×π where n binder  is the optical index of the medium in which the phosphor is immersed. Since n binder  is typically equal to or at least similar to n encapsulant , and A phosphor  is larger than the area of the die, the etendue of the phosphor layer is clearly much larger than the etendue of the light emerging from the surface of the die into the encapsulant. This larger light source is unfavorable for many applications, especially those requiring that the light be collected and redirected into a highly collimated beam of light within a limited output aperture, for example flashlights, spotlights, and automotive headlight high beams. 
   Recent improvements have produced white LEDs in which the phosphor-binder mixture is limited to a small region directly on the die surface, largely co-extensive with the die. An example is shown in  FIG. 2B . Note that this method of applying the phosphor is much easier when the die is a “flip chip” with all its contacts on the bottom surface, and therefore no bond wire on top. However, applying the phospor directly to the surface of the die has several disadvantages. The phosphor/binder mixture is subjected to the high temperatures of the die, which produces large amounts of heat and is typically the highest-temperature point in the package. The resulting high temperature causes color shifts due to the temperature sensitivity of the phosphor light emission. High temperatures also subject both binder and phosphor to temperature related degradation, which decreases light output over time. White LEDs accoding to  FIG. 2  are also subject to color non-uniformity, as light emitted directly by the LED has a different spatial or angular pattern from light emitted by the phosphor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a prior art typical single-color LED lamp package without a phosphor layer. 
       FIG. 2A  shows one example of a prior art white LED package including a phosphor layer and  FIG. 2B  shows another form of prior art with phosphor layer. 
       FIG. 3  shows an array of LED light sources constructed with the non-imaging coupler constructed as a single layer. 
       FIG. 4  shows a single LED light source with the non-imaging light coupler constructed in two layers, leaving a ledge between layers to allow room for attaching a bond wire. 
   

   DESCRIPTION OF THE INVENTION 
     FIG. 3  shows one embodiment of the current invention, shown generally at  10 . The phosphor is deposited in a layer  12  separated from the die  14  by a significant thickness of clear encapsulant  16 . The phosphor  12  is immersed in a medium  18  having an optical index n 2  lower than that of the encapsulant  16 . In one embodiment of  FIG. 3  the medium  18  is air. The phosphor  12  spatial area is larger than the die  14 , but by virtue of this lower index the phosphor layer  12  has an optical etendue comparable to that of the die  14  itself. A non-imaging optical coupler  20  collects the majority of the light from the die  14  and redirects it to the phosphor  12  while maintaining the etendue comparable to the die  14 . 
   The non-imaging optical coupler  20  can have a variety of forms as described in many known publications and U.S. patents. The shape of the coupler  20  should redirect most of the light from the die  14  from very large angles inside the encapsulant  16  to angles smaller than the critical angle arcsin[n 2 /n encapsulant ], so that the light is not trapped in the encapsulant  16  by total internal reflection (TIR). In the embodiment of  FIG. 3  the non-imaging optical coupler  20  has the cross-section of a compound parabolic concentrator (CPC). In three dimensions the coupler  20  can have cylindrical or rectangular symmetry (CPC or “crossed CPC”). The input aperture  22  of the coupler  20  has diameter and angle D in  and ±θ in , where D in  is equal or somewhat larger than the diameter of the die  14  and θ in  is 60–90°, corresponding to the die  14  output angular distribution measured inside the encapsulant  16 . The output aperture  24  of the coupler  20  has diameter D out  comparable to, 
               n   encapsulant       n   2       ⁢       D     i   ⁢           ⁢   n         sin   ⁢           ⁢     θ   2               
where θ 2  is typically 70–90°. Thus the output etendue n 2   2 π 2 (D out   2 /4) sin 2 θ 2  is comparable to the etendue of the die  14  itself.
 
   The shape of the non-imaging coupler  20  need not be a CPC. Because the die  14  emits light from the sides  26  and  28  as well as the top  30 , it may be preferred to adjust the shape  20  to optimally collect this side emission along with the top emission. In some embodiments the bottom portion  31  of the coupler  20  will approach the sides  26  and  28  of the die  14  very closely. In this case it may be preferred to cut out a small region of one sidewall  32  to leave room for the bond wire  34  shown in the  FIG. 3 , with the coupler  20  preferably placed on the substrate  36  after the die  14  has been attached and wire-bonded. 
   Phosphor powder  12  can be applied to a solid surface  38  or  40  but be substantially immersed in air using various application methods. For example, the phosphor can be dispersed in a binder, spread on the surface; and then the binder can be burned off, as is common in manufacturing flouorescent lamps. The phosphor can also be dispersed in a solvent, spread on the surface, and the solvent can be evaporated off. By these methods the phosphor can be applied to a solid surface, such as the surface  38  of the encapsulant  16 . In the embodiment of  FIG. 3  the phosphor  12  is supported by a glass layer  40 , which facilitates application and provides additional sealing to the die region. 
   In the embodiment of  FIG. 3  the phosphor  12  region is further coupled to a non-imaging collimator secondary optic  42 . This secondary optic  42  was described in detail in our copending patent application “Compact Non-Imaging Light System” filed May 5, 2004. 
   The embodiment of  FIG. 3  is simple, compact, manufacturable, efficient, and well-adapted for applications needing collimated beams. With the die  14  coupled to a low thermal resistance substrate  36 , the die  14  can be driven very hard to produce high lumens/mm 2 , and the optics  20  and  42  will preserve the beam density. The simplicity of the design also makes it easy to tailor to different output distributions, and to adjust the design as LED efficiency continues to improve. 
   Some important advantages of this device and approach include:
         1) The phosphor  12  is thermally isolated from the die  14 , so temperature stability of the phosphor  14  is less of an issue than in conventional packages, allowing a greater range of phosphors to be used.   2) The glass  42  seals the system, protecting the phosphor  12  from moisture and other contaminants.   3) Unlike the approach shown in  FIG. 2B , this approach can be used equally well when there is a bond wire  34  on top of the die  14  (not just for flip chips as in  2 B).   4) The phosphor  12  makes a nice, uniform, Lambertian source for this second non-imaging array  42 , hiding the bond wire  34  or the contact electrode patterns typically found on the surfaces  30  of the die  14 .   5) For headlights and similar systems with complex output distributions, individual elements  42 ,  44 , and  46  of the molded non-imaging array  48  can be tilted at different axes.       

     FIG. 4  shows another embodiment in which the non-imaging coupler  20  is made in two layers  50  and  52 . The bottom layer  50  can be part of the circuit board  54 . The die  14  is placed on the circuit board  54  as shown in  FIG. 4  and wire-bonded to the top  56  of the first layer  50 , within approximately 0.2 mm of the edge  60  of the ledge  58 . Then a molded second layer  52  is added. The advantage is the wire bond  34  can be placed at a point in the assembly sequence where there is plenty of room for access by most common types wire-bonding equipment. The step  58  can be integrated into the non-imaging design  20 , so that optical performance shows very little degradation. 
   While preferred embodiments of the invention have been shown and described, it will be clear to those skilled in the art that various changes and modifications can be made without departing from the invention in its broader aspects.