Patent Publication Number: US-9899563-B2

Title: Method of fabricating light-emitting diode with a micro-structure lens

Description:
PRIORITY DATA 
     The present application is a divisional of U.S. patent application Ser. No. 13/087,564, filed on Apr. 15, 2011, now U.S. Pat. No. 8,969,894 issued Mar. 3, 2015, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a light emitting diode (LED) and more particularly to a LED with a micro-structure lens. 
     BACKGROUND 
     For a primary LED lens design, light-extraction efficiency and spatial color shift characteristics are important factors. The primary LED lens will affect secondary optical design and backend product applications. A conventional dome lens may improve light extraction performance, but it would have undesirable spatial color shift due to different optical path lengths in different directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram showing an exemplary LED with a micro-structure lens according to some embodiments; 
         FIG. 2A  is a schematic diagram showing a side view of an exemplary micro-structure lens for an LED according to some embodiments 
         FIG. 2B  is a schematic diagram showing a top view of the exemplary micro-structure lens for the LED in  FIG. 2A ; 
         FIG. 2C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens for the LED in  FIG. 2A ; 
         FIG. 3A  is a schematic diagram showing a side view of another exemplary micro-structure lens for an LED according to some embodiments; 
         FIG. 3B  is a schematic diagram showing a top view of the exemplary micro-structure lens for the LED in  FIG. 3A ; 
         FIG. 3C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens for the LED in  FIG. 3A ; 
         FIG. 4A  is a schematic diagram showing a side view of yet another exemplary micro-structure lens for an LED according to some embodiments; 
         FIG. 4B  is a schematic diagram showing a top view of the exemplary micro-structure lens for the LED in  FIG. 4A ; 
         FIG. 4C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens for the LED in  FIG. 4A ; 
         FIG. 5  is a plot showing Y/B ratio of various LEDs according to some embodiments; and 
         FIG. 6  is a flowchart of a method for fabricating the exemplary LED with a micro-structure lens in  FIG. 1  according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The making and using of various embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use, and do not limit the scope of the disclosure. 
       FIG. 1  is a schematic diagram showing an exemplary LED with a micro-structure lens according to some embodiments. An LED assembly  100  includes a substrate  102  and an LED die  108 . A reflective layer  104 , e.g., silver (Ag), is formed on the substrate  102 , e.g., silicon (Si). The LED die  108  is mounted on the substrate  102 , e.g., by bonding using a solder layer  106 . The size of the LED die  108  varies depending on applications, e.g., 300 μm-2000 μm. For low power applications, the LED die  108  may have a size of 300 μm-600 μm; while for high power applications, the LED die  108  may have a size of about 1000 μm or more. 
     A phosphor coating  110  is deposited over the LED die  108  to form a desired light color from the LED assembly  100 . For example, the color of light emitted by the LED die  108 , e.g., made of InGaN, may be blue, and a yellow phosphor material, e.g., cerium-doped yttrium aluminum garnet (Ce 3+ XAG), can be used to form a white light. Also, depending on the specific material and the thickness of the phosphor coating  110 , e.g., 30 μm-60 μm, the light color can be changed. The phosphor coating  110  can use various materials, which are known in the art. The phosphor particle size can range, e.g., 6 μm-30 μm, in some embodiments. The phosphor material can form a conformal coating on the LED die  108  by controlled dispensing of the phosphor, e.g., spraying. 
     A micro-structure lens  114 , including a micro-structure  116  on top and a lower portion  122 , is molded over the substrate  102  and the LED die  108 , which also forms a lens base layer  112  at the same time. The size of the micro-structure lens  114  varies depending on applications, and its diameter can be about 2.5 times of the size of the LED die  108 . The micro-structure lens  114  and the lens base layer  112  can comprise silicon (Si), for example. The micro-structure  116  includes at least one concentric ridge structures  120  in addition to a convex lens portion  118 . The micro-structure  116  can have a similar structure as a Fresnel lens structure, but not limited to it. Different structures are shown below in  FIGS. 2A-4A . The lower portion  122  is shaped to enhance light extraction efficiency, e.g., as a part of a dome lens, where the top part of the dome lens is replaced by the micro-structure  116 . 
     In general, the reflection at the surface of an LED lens is reduced by using a dome-shaped (half-sphere or hemisphere) package with the LED at the center so that the outgoing light rays strike the surface perpendicularly, at which angle the reflection is minimized. The reflective layer  104  on the substrate  102  increases the LED efficiency. The refractive index of the package material can also match the refractive index of the LED (semiconductor), to minimize back-reflection. An anti-reflection coating may be added as well. 
     The micro-structure lens  114  is designed to make optical path length (OPL) between the LED die  108  and the lens surface uniform as much as possible, thus improving the spatial color uniformity and still have high light extraction efficiency from the LED assembly  100 . For example, a first length OPL 1  from an edge of the LED die  108  to the top center of the micro-structure lens  114  (or the convex lens portion  118 ) and a second length OPL 2  from the edge of the LED die  108  to the side of the micro-structure lens  114  are substantially the same. 
     In comparison, a conventional dome lens would have a hemispherical structure and the same radius from the center of the hemisphere to the top and a side of the dome lens. Because of the size of the LED die  108 , the OPL from an edge of the LED die  108  to the top and a side of the dome lens would be substantially different. For example, the diameter of the conventional dome lens can be about 2.5 times of the length of the LED die  108 . Assuming the length of the LED die  108  (positioned at the center of a hemispherical dome lens) is about 300 μm, the diameter of the dome lens can he about 750 μm, and the difference of OPL can be more than 100 μm, Due to the large difference in OPL, a significant special color shift will result for the conventional dome lens, e.g., a yellow ring at the edge of a white light LED. 
     In one exemplary design for the micro-structure lens  114 , a dome type lens is designed first, using an optical design software. The lens shape is hemispherical to enhance the light extraction efficiency of the LED assembly  100 . Then the height of the dome lens can be reduced. by using a micro-structure  116 . The optical design software can be used to optimize the lens structure performance (e.g., high light extraction efficiency and enhanced color uniformity). 
     For the fabrication of the LED assembly  100 , a substrate (e.g., Si)  102  is provided, and a reflective layer  104  is formed on the substrate  102 . A bare LED die  108  is bonded on a substrate  102 , e.g., using a solder layer  106  that is disposed over the reflective layer  104 . A conformal phosphor coating  110  is formed. The phosphor coating  110  has a relatively small particle size, e.g., 6 μm-30 μm, forming a thin phosphor layer, e.g., 30 μm-60 μm, in some embodiments. 
     For the fabrication of micro-structure lens  114 , the lens shape is molded using a lens molding instrument. A lens molding process is known in the art. If the details of the micro-structure lens  114 , e.g., the ridge structures  120 , are not well defined with precision, a dry/wet etching process can be used to obtain better defined structures after molding. The ridge structures  120  have a height/depth of about 3 μm-5 μm in some embodiments. 
       FIG. 2A  is a schematic diagram showing a side view of an exemplary micro-structure lens  114  for an LED according to some embodiments. The micro-structure lens  114  has three concentric ridge structures  120  and a convex lens portion  118  on top in  FIG. 2A . The three concentric ridge structures  120  have pointed peaks, similar to a Fresnel lens structure. As mentioned above, if the details of the micro-structure lens  114 , e.g., the ridge structures  120 , are not well defined with precision after the lens molding, a dry/wet etching process can be used to obtain better defined structures.  FIG. 2B  is a schematic diagram showing a top view of the exemplary micro-structure lens  114  for the LED in  FIG. 2A .  FIG. 2C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens  114  for the LED in  FIG. 2A . 
       FIG. 3A  is a schematic diagram showing a side view of another exemplary micro-structure lens  114  for an LED according to some embodiments. Compared to the exemplary micro-structure lens in  FIG. 2A , the top of the concentric ridge structures  120  are not pointed, but rather curved in  FIG. 3A . The height of the concentric ridge structures  120  are the same as each other, but he top of the convex lens portion  118  is higher than the top of the ridge structures  120 . The different lens designs are selected for different implementations and applications, based on performances (simulated or experimental), and ease of fabrication, etc.  FIG. 3B  is a schematic diagram showing a top view of the exemplary micro-structure lens  114  for the LED in  FIG. 3A .  FIG. 3C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens  114  for the LED in  FIG. 3A . 
       FIG. 4A  is a schematic diagram showing a side view of yet another exemplary micro-structure lens  114  for an LED according to some embodiments. The top of the concentric ridge structures  120  are circular instead of triangular, and the height of the concentric ridge structures  120  are different from each other in  FIG. 4A . The diameter and height of the concentric ridge structures  120  increase as they get closer to the center of the micro-structure lens  114 . The top of the convex lens portion  118  is higher than the top of the ridge structures  120 .  FIG. 4B  is a schematic diagram showing a top view of the exemplary micro-structure lens  114  for the LED in  FIG. 4A .  FIG. 4C  is a schematic diagram showing a three-dimensional view of the exemplary micro-structure lens  114  for the LED in  FIG. 4A . 
       FIG. 5  is a plot showing a yellow/blue (Y/B) ratio of various LEDs according to some embodiments. A Y/B ratio plot  502  of an LED without any lens shows a very low value (predominantly blue) at the center (low view angle), and increases (predominantly yellow) to either side edges (high view angles on either side). The Y/B ratio varies from about 0.13 to about 0.75. A Y/B ratio plot  504  of an LED with a conventional lens shows similar trends, and the Y/B ratio varies from about 0.23 to about 0.58. Compared to those two plots, a Y/B ratio plot  506  of an LED with an exemplary micro-structure lens  114  shows improved color distribution uniformity. The WB ratio varies from about 0.38 to about 0.53 in this example. 
       FIG. 6  is a flowchart of a method for fabricating the exemplary LED with a micro-structure lens in  FIG. 1  according to some embodiments. At step  602 , a light emitting diode (LED) die on a substrate is provided. At step  604 , the micro-structure lens that has at least one concentric ridge is molded over the LED die. A first optical path length from an edge of the LED die to a top center of the micro-structure lens is substantially the same as a second optical path length from the edge of the LED die to a side of the micro-structure lens. 
     In various embodiments, a reflective layer is formed on the substrate. A solder layer is formed on the reflective layer. The LED die is bonded to the substrate. The LED die is wire bonded. A phosphor coating is formed on the LED die. The micro-structure lens is etched after molding. 
     According to some embodiments, a light emitting diode (LED) with a micro-structure lens includes a LED die and a micro-structure lens. The micro-structure lens includes a convex lens portion, at least one concentric ridge structure surrounding the convex lens portion, and a lower portion below the convex lens portion and the at least one concentric ridge structure. The lower portion is arranged to be disposed over the LED die. A first optical path length from an edge of the LED die to a top center of the micro-structure lens is substantially the same as a second optical path length from the edge of the LED die to a side of the micro-structure lens. 
     According to some embodiments, a method of fabricating a light emitting diode (LED) with a micro-structure lens includes providing a light emitting diode (LED) die on a substrate. The micro-structure lens is molded that has at least one concentric, ridge over the LED die. A first optical path length from an edge of the LED die to a top center of the micro-structure lens is substantially the same as a second optical path length from the edge of the LED die to a side of the micro-structure lens. 
     A skilled person in the art will appreciate that there can be many embodiment variations of this disclosure. Although the embodiments and their features have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the embodiments. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosed. embodiments, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. 
     The above method embodiment shows exemplary steps, but they are not necessarily required to be performed in the order shown. Steps may be added, replaced, changed order, and/or eliminated as appropriate, in accordance with the spirit and scope of embodiment of the disclosure. Embodiments that combine different claims and/or different embodiments are within the scope of the disclosure and will be apparent to those skilled in the art after reviewing this disclosure.