Patent Publication Number: US-2010127288-A1

Title: Light-emitting diode devices and methods for fabricating the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a light-emitting diode (LED) device and more particularly to LED devices with improved adhesion of a lens thereon. 
     2. Description of the Related Art 
     Light-emitting diode (LED) devices are solid-state light sources with multiple-advantages. They are capable of reliably providing light with high brightness and thus are applied in displays, traffic lights and indicators. LED devices typically include an LED die electrically bonded on a support substrate and the LED die may have an n-contact formed on one side and a p-contact formed on the opposite side therein or have both contacts formed on the same side therein. 
     The LED die in an LED device typically emit light in a lambertian pattern. It is common to provide a lens over the LED die of the LED device to narrow the beam or to make a side-emission pattern. A common type of lens for a surface mounted LED is a molded plastic lens, which is bonded to a package in which the LED die is mounted. 
       FIG. 1  is a cross section of a conventional LED device disclosed in U.S. Pat. No. 7,344,902, issued to Basin et al. As shown in  FIG. 1 , the LED device has four LED die  10  mounted on a support structure  12  such as ceramic or silicon substrate with metal leads, a metal heat sink, a printed circuit board, or any other structure. A molded lens  22  is provided over each LED die  10  to cover each LED die  10 . 
       FIG. 2  is a perspective view of a conventional LED device where the support structure  12  supports an array of LED die, each having a molded lens  22 . The mold used normally has a corresponding array of indentations. If the support structure  12  were a ceramic or silicon submount, each LED die  10  (with its underlying submount portion) can be separated by sawing or breaking of the submount  12  to form individual LED devices. 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  10  and refracts the light to create a desired emission pattern, but also encapsulates the LED die  10  to protect the die from contaminants, adds mechanical strength, and protects wire bonds. 
     However, since the support structure  12  shown in  FIGS. 1 and 2  is provided with a substantially planar top surface, the adhesion between the lens  22  covering the LED die  10  and the support structure  12  is mainly maintained by physical forces such as van der Waals forces. Thus, because the physical bonding force for adhering the lens  22  to the underlying support structure  12  is not strong enough, the lens  22  easily peels off from the support structure  12  when an external force F is applied thereto. In addition, an interface  50  between the lens  22  and the underlying support structure  12  provides a leakage path for environmental moisture to seep in. This causes reliability problems of the LED product. Thus, poor reliably of the LED device is affected by adhesion of the lens  22  to the underlying support structure 
     Therefore, there is a need for a novel LED device addressing the above problems. 
     BRIEF SUMMARY OF THE INVENTION 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. A light-emitting diode (LED) device and a method for fabricating the same are provided. An embodiment of an LED device comprises a support structure with at least one LED die mounted thereon, a recess formed in a part of the support structure from a side of the LED die, and a lens formed over the support structure to encapsulate the LED die and the recess, thereby forming a protrusion in the support structure. 
     An embodiment of a method for fabricating an LED device comprises providing a support structure with at least one LED die mounted thereon. A recess is formed in a part of the support structure from a side of the LED die. A lens is formed over the support structure to encapsulate the LED die and the recess, thereby forming a protrusion in the support structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a cross section of a conventional LED device; 
         FIG. 2  is a perspective view of a conventional LED device; 
         FIGS. 3-5  are cross sections showing a method for fabricating LED devices according to an embodiment the invention; 
         FIG. 6  is a cross section of a method for fabricating LED devices according to another embodiment of the invention; 
         FIGS. 7 and 8  are enlarged cross sections showing an LED device according to various embodiments of the invention, respectively; and 
         FIGS. 9-11  are enlarged top views showing an LED device according to various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIGS. 3-5  are cross sections showing an exemplary method for fabricating LED devices. As shown in  FIG. 3 , a cross section showing four LED die  102  mounted on a support structure  100  is illustrated. The support structure  100  can be, for example, a submount (e.g., ceramic or silicon substrate with metal pads), a metal heat sink, a printed circuit board, or any other support structure. In this embodiment, the support structure  100  is a silicon submount with metal pads (not shown). As shown in  FIG. 3 , a plurality of recesses  104  are formed in the support structure  100 , respectively adjacent to an opposite side of each LED die  102 . The recesses  104  can be formed by methods such as an etching, laser cutting, or mechanical cutting depending on the material of the support structure  100  and are spaced from the LED die  102  by the support structure  100 . 
     In  FIG. 4 , a mold  106  having a plurality of indentations  107  corresponding to the desired shape of a lens over each LED die  102  is provided. The mold  106  can be made of materials such as metal, plastic or polymer and can be opaque or light-transmissive. A very thin non-stick film (not shown) having the general shape of the mold  106 , can be optionally placed over the mold  106  to prevent molding lens material to stick to the mold  106 . The mold indentions  107  are then filled with a heat-curable or light-curable liquid lens material  108 . The liquid lens material  108  can be any suitable optically transparent material such as silicone, an epoxy, or a hybrid silicone/epoxy. A hybrid may be used to achieve a matching coefficient of thermal expansion (CTE). Silicone and epoxy have a sufficiently high index of refraction (greater than 1.4) to greatly improve the light extraction from an AlInGaN or AlInGaP LED as well as act as a lens. 
     In this embodiment, the support structure  100  and the mold  106  are pressed against each other so that each LED die  102  is inserted into the liquid lens material  108  and the lens material  108  is under compression. At this time, the liquid lens material  108  fills the recesses  104  formed in the support structure  100 . The mold  106  is then heated to about 150° C. or other suitable temperature, or subjected to a suitable light source such as a ultraviolet (UV) light for a period of time to harden the lens material  108 . 
     After the support structure  100  is separated from the mold  106  (not shown), an LED device with a lens  110  over each LED die  102  is formed as illustrated in  FIG. 5 . The lens  110  can be any size or shape, and is not limited to that illustrated in  FIG. 5 . As shown in  FIG. 5 , the lenses  110  are all formed with protrusions formed into the support structure  100 , thereby improving adhesion between the lens  110  and the support structure  100 . 
     The LED device shown in  FIG. 5  can be formed with a perspective view as that shown in  FIG. 2 , wherein the support structure  100  supports an array of LED dies, each having a lens  110  covered thereon. In this case, the mold (not shown) used would have a corresponding array of indentations. If the support structure  100  is a ceramic or silicon submount, each LED device (with its underlying submount portion) can be separated by sawing or breaking of the submount  100  to form individual LED device. Alternatively, the support structure  100  may be separated/diced to support subgroups of the LED device or may be used without being separated/diced. The lenses  110  are not only improves the light extraction from the LED die and refracts the light to create a desired emission pattern, but also encapsulates the LED die to protect the die from contaminants, adds mechanical strength, and protects conductive bonding therein. 
       FIG. 6  shows another exemplary method for fabricating the LED device illustrated in  FIG. 5 . In this embodiment, a gel-dispensing method is used to form the lenses  110  by providing a modified mold  112  having a plurality of indentations  107  corresponding to the desired shape of a lens over each LED die  102 . In each of the indentations  107 , a plurality of through holes  114  and  116  are provided. The mold  112  can be provided over the structure shown in  FIG. 3 . At this time, a pair of holes  114  and  116  is provided from a side for each of the indentations  107 , respectively, having a substantially L shape or reversed-L shape. One of the through hole pairs  114  and  116  functions as an inlet channel to fill the gel-like lens materials into the indentations  107  and the other through hole functions as a suction channel connected to the vacuum surroundings (not shown). Thus, the lens materials can be filled into the indentations  107  formed in the mold  112  and then hardened by the methods as that discussed in  FIG. 4 , thereby forming the lens  110  covering each LED die  102  in the embodiment. 
       FIG. 7  is an enlarged cross section showing an exemplary LED device having a single flip-chip LED die  102  formed on a submount  100 ′ of any suitable material, such as a ceramic or silicon. In one embodiment, the submount  100 ′ may be function as the support structure  100  illustrated in  FIGS. 3-6  and may be separated from the LED device, as illustrated in  FIG. 5 , by sawing. The LED die  102  in  FIG. 7  has a bottom p-contact layer  120 , a p-metal contact  122 , p-type layers  124 , a light emitting active layer  126 , n-type layers  128 , and an n-metal contact  130  contacting the n-type layers  128 . Metal pads (not shown) on the submount  100 ′ are directly metal-bonded to contacts  122  and  130 , respectively. The lens  110 , formed using the technique of  FIGS. 3-6 , encapsulates the LED die  102  and has a plurality of protrusions filling the recesses  104  in the submount  100 ′. 
     The LED die  102  in  FIG. 7  may also be a non-flip-chip die, with a wire  152  connecting the n-layer and the p-layer with a metal pad (not shown) on the submount&#39;  100 . The lens  110  may encapsulate the wire.  FIG. 8  is an enlarged cross section showing another exemplary LED device having a non flip-chip LED die  102  having a top n-metal contact  132  connected to a metal lead by the wire  150  and an exposed p-metal contact  134  connected to another metal pad by a wire  152 . The LED die  102  is mounted on the submount  100 ′. As that illustrated in  FIG. 8 , the lens  110  encapsulates the LED die  102  and has a plurality of protrusions filling the recesses  104  formed in the support structure  100 . 
     As shown in  FIGS. 7-8 , since the lens  110  is now formed with protrusions penetrating into the support structure  100  thereunder, adhesion between the lens  110  and the underlying support structure  100  is thus improved. Additionally, an undesired moisture leakage path formed at an interface  150  between the lens  110  and the underlying support structure  110  is blocked by the protrusions, thus improve the reliability of the LED device. 
     In one embodiment, the recesses  104  are typically formed with a depth O of about 100˜300 μm from the top surface the submount  100 ′ or the support structure  100  and with a distance T of about 500˜1000 μm from an outer edge of the submount  100 ′ or the support structure  100 . 
       FIGS. 9-11  are schematic top views showing various embodiments of an LED device. As shown in  FIG. 9 , a plurality of regions is defined over the submount  100 ′ or the support structure  100 . In the embodiment illustrated in  FIG. 9 , the recess  104  is formed as a continuous trench in the support structure  100 . The continuous trench illustrated in  FIG. 9  surrounds all sides of the LED die  102 , thereby improving adhesion of the lens  110  and the support substrate  100 /submount  100 ′. The recess  104  can be formed as several non-continuous trenches  104  formed in the submount  100 ′ or the support structure  100 . The non-continuous trenches  104  respectively surround each side of the LED die  102 , thereby substantially surrounding the LED die  102  and improving adhesion of the lens  110  and the support substrate  100 /submount  100 ′, as shown in  FIG. 10 . In  FIG. 11 , another embodiment showing an LED device having an array of LED die surrounded by the recess  104  is illustrated. In this embodiment, the recess  104  is formed as a continuous trench surrounding a plurality LED die  102  and is not limited thereto, the recess  104  can be also formed as several non-continuous trenches  104  as that illustrated in  FIG. 10 . The LED die  102  illustrated in  FIG. 11  is electrically mounted to the support substrate  100 /submount  100 ′ by the methods illustrated in  FIGS. 7 and 8 . 
     While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.