Patent Publication Number: US-2007096139-A1

Title: Light emitting diode encapsulation shape control

Description:
BACKGROUND  
      The present invention relates to light emitting diode (LED) devices. In particular, the present invention relates to an encapsulant for an LED having a controlled shape.  
      LEDs are a desirable choice of light source in part because of their relatively small size, low power/current requirements, rapid response time, long life, robust packaging, variety of available output wavelengths, and compatibility with modern circuit construction. These characteristics help explain the widespread use of LEDs over the past few decades in a multitude of different applications. Improvements to LEDs continue to be made in the areas of efficiency, brightness, and output wavelength, further enlarging the scope of potential applications.  
      LEDs are typically sold in a packaged form that includes an LED die or chip mounted on a metal header. The header can have a reflective cup in which the LED die is mounted, and electrical leads connected to the LED die. Some packages also include a molded transparent resin that encapsulates the LED die. The encapsulating resin can have a hemispherical or convex front surface (i.e., a convergent lens) to partially collimate light emitted from the die. The resin can also have a flat front surface or concave front surface (i.e., a divergent lens) to transmit a portion of the light through the sidewalls of the package.  
      An LED die may be encapsulated by filling the reflective cup with encapsulating resin such that the shape of the front opening of the reflective cup controls the shape of the front surface of the encapsulating resin. More specifically, the encapsulating resin forms a convex meniscus along the front opening of the reflective cup such that the larger the opening, the smaller the radius of curvature of the meniscus. However, completely filling the reflective cup with the encapsulating resin can distort the optics of the reflective cup and reduce the collimation of the light emitted from the open end of the reflective cup. This negatively affects the reflective cup&#39;s coupling efficiency into a waveguide or optical fiber with a limited numerical aperture.  
      An LED die may also be encapsulated by completely covering the die with encapsulating resin such that it only fills a portion of the reflective cup. However, because the reflective cup is typically coated with a highly reflective material such as silver or is formed from a polymer optical film, the encapsulating resin may wet the sides of the reflective cup to produce a concave meniscus. A concave meniscus tends to reflect light back toward the sidewalls of the reflective cup and the LED die due to total internal reflection, thereby reducing the potential light output of the LED. In the worst case, the concave meniscus may wick up the sides of the reflective cup enough to significantly impact the properties of the reflective cup, reducing the overall efficiency of the LED. This uncontrolled wicking leads to inconsistent and somewhat uncontrollable contours of the front surface of the encapsulating resin.  
     SUMMARY  
      Methods are disclosed herein for encapsulating a semiconductor optical device. The semiconductor optical device is disposed in a cavity defined by a cavity wall. The cavity wall is coated with a coating material having a first surface energy. An encapsulant having a second surface energy is introduced into the cavity and adjacent to the semiconductor optical device. The encapsulant is solidified to form an optical element having an outer surface with a shape that is a function of the first surface energy and the second surface energy.  
      Optical devices are disclosed in which a substrate includes a cavity defined by a cavity wall having a coating thereon. A light emitting semiconductor is positioned adjacent to the substrate. An encapsulant fills a portion of the cavity, encapsulates the light emitting semiconductor, and has an optically active surface with a shape that is a function of the relative surface energies of the coating and the encapsulant.  
      Associated components, systems, and methods are also disclosed.  
      These and other aspects of the present application will be apparent from the detailed description below. In no event, however, should the above summaries be construed as limitations on the claimed subject matter, which subject matter is defined solely by the attached claims, as may be amended during prosecution.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective cross-sectional view of a light emitting diode (LED) package.  
       FIG. 2  is a perspective cross-sectional view of an LED package that includes an LED die encapsulated by an encapsulating material having a convex surface.  
       FIG. 3  is a perspective cross-sectional view of an LED package that includes an LED die encapsulated by a phosphor material and an encapsulating material having a convex surface.  
       FIG. 4  is a perspective cross-sectional view of an LED package that includes an LED die encapsulated by an encapsulating material having a substantially planar surface.  
       FIG. 5  is a perspective cross-sectional view of an LED package that includes an LED die encapsulated by an encapsulating material having a substantially planar surface and a phosphor tab thereon. 
    
    
      In the figures, like reference numbers denote like parts.  
     DETAILED DESCRIPTION  
       FIG. 1  is a perspective cross-sectional view of a light emitting diode (LED) package  10 . LED package  10  includes LED die  12  and substrate  14 . Substrate  14  includes carrier or header  16  that includes cavity  18  defined by cavity wall  20 . Cavity wall  20  typically comprises a highly reflective material. Cavity wall  20  has sloped sides such that the cross-sectional area of cavity  18  proximate to LED die  12  is less than the cross-sectional area of cavity  18  at the opening of cavity  18 . LED die  12  generates light when an electrical current is applied through electrical contacts (not shown). Electrical connections to LED die  12  can be made through substrate  14 , by wire bonds or, in the case of a flip chip configuration, by conductive strips connected to contact pads at the bottom of LED die  12 .  
      LED die  12  is encapsulated by encapsulating material  25 . Encapsulating material  25  is a low viscosity material that is introduced into cavity  18  in liquid form. Encapsulating material  25  completely covers LED die  12  and fills a portion of cavity  18 . The reflective material of cavity wall  20  may comprise such materials as silver or a polymer optical film. These materials have high surface energies relative to encapsulating material  25 , which causes the molecules of the liquid encapsulating material  25  to have a stronger attraction to the molecules of cavity wall  20  than to each other. Thus, encapsulating material  25  has a tendency to wet or stick to the highly reflective material of cavity wall  20 . Consequently, when encapsulating material  25  is cured, a concave meniscus is formed at front surface  28 . The concave meniscus tends to reflect light back toward cavity wall  20  and LED die  12  due to total internal reflection, which reduces the potential light output of LED package  10 . In addition, this uncontrolled wicking up cavity wall  20  leads to inconsistent and somewhat uncontrollable contours of front surface  28  of encapsulating material  25 .  
       FIG. 2  is a perspective cross-sectional view of LED package  40  including an encapsulated LED die  12 . LED package  40  includes substrate  44  comprising carrier or header  46 . Carrier  46  includes cavity  48  defined by cavity wall  50 . Cavity wall  50  is sloped or otherwise shaped such that the cross-sectional area of cavity  48  proximate to LED die  12  is less than the cross-sectional area of cavity  48  distal from LED die  12 . Cavity wall  50  thus forms a minor opening in which LED die  12  is mounted and a major opening opposite from the minor opening. In some embodiments, cavity  48  can have a square cross-section proximate to LED die  12  and a round cross-section at the opening of cavity  48 . In other embodiments, the cross-sectional area of cavity  48  proximate to LED die  12  can be substantially similar to the cross-sectional area of cavity  48  distal from LED die  12 .  
      LED package  40  is fabricated by etching, boring, or otherwise forming cavity  48  in carrier  46 , and then coating or treating the resulting cavity wall  50  with coating material  52 . Coating material  52  is in some cases a low surface energy material that limits wetting of materials on cavity wall  50 . Coating material  52  can also be selected to have a moderate or high surface energy depending on the desired shape of the encapsulant meniscus. In some embodiments, coating material  52  can be a monolayer coating of material (i.e., one molecule thick). Next, carrier  46  is positioned adjacent to LED die  12  such that LED die  12  is positioned in the minor opening of cavity  48  and opposite from the major opening of cavity  48 . A curable encapsulating material  55  is then dispensed into cavity  48  in liquid form (such as by a dispensing nozzle) to form a liquid mass that completely covers LED die  12  and fills a portion of cavity  48 . In one embodiment, encapsulating material is a light transmissive material having a refractive index of at least approximately 1.5. The liquid encapsulating material  55  has a shape determined to some extent by the density, viscosity, volume, and surface tension of the curable material, as well as environmental conditions such as local gravity and temperature.  
      In addition, the shape of liquid encapsulating material  55  is determined by the size of cavity  48  and the relative surface energies of coating material  52  and liquid encapsulating material  55 . Thus, coating material  52  can be selected to have a surface energy relative to encapsulating material  55  to produce a meniscus at the front surface of the encapsulating material  55  having the desired shape. In the embodiment shown in  FIG. 2 , coating material  52  has a low surface energy relative to liquid encapsulating material  55 . Consequently, the molecules of liquid encapsulating material  55  are more strongly attracted to each other than to cavity wall  50  (i.e., the cohesive forces are stronger than the adhesive forces). The size of cavity  48  and the relative surface energies of coating material  52  and encapsulating material  55  cause the liquid encapsulating material  55  to form a convex meniscus at front surface  58 .  
      LED package  40  is then exposed to radiation of sufficient energy and suitable wavelength to cause full curing of the liquid encapsulating material  55 . Useful wavelengths and intensities of radiation used for curing encapsulating material  55  include any that induce curing chemistry and are absorbed by some portion of encapsulating material  55 . In some embodiments, useful radiation includes ultraviolet (UV) light with a wavelength of less than 400 nm.  
      As a result of the curing process, encapsulating material  55  is solidified and produces a front surface  58  having a smooth, convex shape (i.e., a convergent lens shape). In alternative embodiments, coating material  52  can be selected to have a surface energy such that encapsulating material  55  forms a substantially planar shape or a concave shape (i.e., a divergent lens shape). By coating cavity wall  50  with an appropriate coating material  52 , the shape of front surface  58  is highly controllable without the need to etch, mold, machine, or otherwise modify the shape of the solidified encapsulating material  55 . The finish of front surface  58  is also of high quality since it is formed without touching any other surfaces. Thus, an LED package  40  having preferred optical properties may be produced by treating the cavity wall  50  with coating material  52  having a surface energy based on the surface energy of encapsulating material  55 , resulting in a higher proportion of the light exiting from the LED package  40  at desirable angles. In some embodiments, carrier  46  is made of a dissolvable material, and is removed after the curing process by a dissolving step such that the encapsulated LED die  12  may be isolated.  
       FIG. 3  is a perspective cross-sectional view of another LED package  60 . Carrier  46  of LED package  60  is prepared similarly to carrier  46  of LED package  40  ( FIG. 2 ). In particular, cavity wall  50  is first coated with coating material  52 , which is a low surface energy material that limits wetting of materials on cavity wall  50 . In one embodiment, coating material  52  is a monolayer coating of material (i.e., one molecule thick). Next, carrier  46  is positioned adjacent to LED die  12  such that LED die  12  is positioned in cavity  48  opposite from the opening of cavity  48 .  
      Phosphor material  62  is then introduced into cavity  48  and adjacent to LED die  12 . Phosphor material  62  may be incorporated into LED package  60  to generate white light from a blue LED die, for example. One example of a suitable phosphor material  62  is a Y 3 Al 5 O 12 :Ce 3+  (YAG:Ce) phosphor. Another example of a suitable phosphor material  62  is SrGa 2 S 4 :Eu (HPL63/F-F1) from Phosphor Technology Ltd., Herts, England. A curable encapsulating material  65  is then dispensed into cavity  48  in liquid form (such as by a dispensing nozzle) to form a liquid mass that completely covers phosphor material  62  and fills a portion of cavity  48 . In one embodiment, encapsulating material is a light transmissive material having a refractive index of at least approximately 1.5. As described above, the liquid encapsulating material  65  has a shape determined to some extent by chemical and environmental conditions, as well as the size of cavity  48  and the relative surface energies of coating material  52  and liquid encapsulating material  65 . Consequently, in the embodiment shown in  FIG. 3 , the relative surface energies of coating material  52  and encapsulating material  65  cause the liquid encapsulating material  65  to form a convex meniscus at front surface  68 . LED package  60  is then exposed to radiation of sufficient energy and suitable wavelength to cause full curing of the liquid encapsulating material  65 . As a result of the curing process, encapsulating material  65  is solidified such that front surface  68  maintains a smooth, convex shape (i.e., a convergent lens shape)  
      A heavily doped phosphor, such as phosphor material  62 , is a highly thixotropic material that becomes less viscous when agitated and thus does not provide the desired convex front surface for emitting light from LED package  60 . However, by depositing encapsulating material  65  on top of the phosphor material  62 , the desired convex front surface is formed due to the relative surface energies of coating material  52  and encapsulating material  65 . The addition of the phosphor is advantageous in that some or all of the light output from LED die  12  is converted to longer light wavelengths based on the phosphor used, typically to produce an overall white light output.  
       FIG. 4  is a perspective cross-sectional view of another LED package  70 , which includes substrate  74  comprising carrier or header  76 . Carrier  76  includes cavity  78  defined by cavity wall  80 . Cavity wall  80  has substantially planar sides proximate to the minor opening of cavity  78  and curved sides that extend to the major opening of cavity  78 . Cavity wall  80  forms a minor opening in which LED die  12  is mounted and a major opening opposite from the minor opening. In one embodiment, cavity  78  has a square cross-section proximate to the minor opening.  
      Similar to other embodiments, LED package  70  can be fabricated by etching, boring, or otherwise forming cavity  78  in carrier  76 , and then coating or treating the resulting cavity wall  80  with coating material  82 . Coating material  82  can be a low surface energy material that limits wetting of materials on cavity wall  80 . Coating material  82  can alternatively be selected to have a moderate or high surface energy depending on the desired shape of the encapsulant meniscus. In some embodiments, coating material  72  is a monolayer coating of material (i.e., one molecule thick). Next, carrier  76  is positioned adjacent to LED die  12  such that LED die  12  is positioned proximate to the minor opening of cavity  78  opposite the major opening of cavity  78 . A curable encapsulating material  85  is then dispensed into cavity  78  in liquid form (such as by a dispensing nozzle) to form a liquid mass that completely covers LED die  12  and fills a portion of cavity  78 . In one embodiment, encapsulating material is a light transmissive material having a refractive index of at least approximately 1.5. The liquid encapsulating material  85  has a shape determined to some extent by chemical and environmental conditions, as well as by the size of cavity  78  and the relative surface energies of coating material  82  and liquid encapsulating material  85 . In the embodiment shown in  FIG. 4 , the relative surface energies of coating material  82  and encapsulating material  85  cause the liquid encapsulating material  85  to form a substantially planar meniscus at front surface  88 .  
      LED package  70  is then exposed to radiation of sufficient energy and suitable wavelength to cause full curing of the liquid encapsulating material  85 . As a result of the curing process, encapsulating material  85  is solidified such that front surface  88  maintains a smooth, substantially planar shape. A substantially planar shape can minimize the increase in area of the effective emitting surface (i.e., front surface  88 ) of encapsulating material  85 , thereby minimizing the increase in étendue of the source. In addition, the distance between the minor and major openings of cavity  78  may be minimized so that the effective LED emission area is the major opening of cavity  78 . The planar front surface  88  provides a reduction in light emitted through front surface  88  and provides an associated increase in light emitted through the sides of encapsulating material  85 .  
       FIG. 5  is a perspective cross-sectional view of another LED package  90 . LED package  90  is substantially similar to LED package  70  of  FIG. 4 , but includes the addition of phosphor tab  92  affixed to front surface  88  of encapsulating material  85 . An advantage of this embodiment is that front surface  95  of phosphor tab  92  may be shaped as desired while having little or no losses due to Fresnel reflections or total internal reflection at the interface between encapsulating material  85  and phosphor tab  92  (provided the refractive indices of encapsulating material  85  and phosphor tab  92  are substantially matched). In addition, a wavelength-selective multilayer film can optionally be introduced between encapsulating material  85  and phosphor tab  92  to improve the overall efficiency of LED package  90 .  
     EXAMPLE AND COMPARATIVE EXAMPLE  
      A Revision 8 Cree reflector was used as a carrier. This carrier had a cavity extending through it with a minor opening shaped as a 350 μm square and a major opening shaped as a circle with a diameter of 650 μm. The length of the cavity, measured from the minor opening to the major opening, was about 0.9 mm. The cavity wall was coated with a low surface energy coating of a mixture of mono- and di[2-(perfluorooctyl)ethyl]phosphate dissolved in methyl tertbutyl ether at a concentration of 0.25% weight to weight (w/w). The entire package was dried overnight at room temperature. The coated cavity was then partially filled with Dow Corning Sylgard® 182 silicone adhesive, which was used as an encapsulant. A 0.002 inch thick wet hand spread of this adhesive was formed, and the carrier was then pressed into this adhesive layer allowing the adhesive to fill the cavity through its minor opening until the cavity was about ¾ full. The reflector with adhesive was then cured at 150° C. for 30 minutes.  
      In order to examine the shape of the meniscus formed by the adhesive layer, the reflector was cut in half, and the cured encapsulant was removed. Examination revealed that the above preparation produced a convex meniscus shape at the outer surface of the encapsulant.  
      For purposes of comparison, the process was repeated, except that the cavity wall was left uncoated. This comparison preparation produced a deeply concave meniscus shape at the outer surface of the encapsulant. The concave shape was formed by the wicking action of the liquid encapsulant up the sidewall of the uncoated cavity.  
      In summary, semiconductor optical devices can be encapsulated by disposing the semiconductor optical device in a cavity defined by a cavity wall. The cavity wall can be coated with a coating material having a first surface energy. An encapsulant having a second surface energy can be introduced into the cavity adjacent to the semiconductor optical device. The encapsulant is solidified to form an outer surface with a shape that is a function of the first surface energy and the second surface energy. The coating material, the encapsulant, or both are selected based on their surface energies to produce a desired outer surface shape, thus producing an optical element having desired optical properties.  
      Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.