Abstract:
A light-emitting package ( 510 ) includes a substantially transparent substrate ( 526 ) having a first surface ( 550 ) and a second surface ( 552 ), and a light-emitting diode (LED) ( 518 ) adapted to emit light having a predetermined wavelength, the LED ( 518 ) being secured over the first surface ( 550 ) of the substantially transparent substrate ( 526 ). The second surface ( 552 ) of the substrate defines a principal light emitting surface of the package, and includes a grating pattern ( 554 ) that matches the predetermined wavelength of the light emitted from the LED ( 518 ) for controlling the emission geometry of the light emitted by the package.

Description:
[0001]    The present invention relates to making semiconductor packages and particularly relates to methods of making light emitting diodes packages having optimized light extraction characteristics.  
         BACKGROUND OF THE INVENTION  
         [0002]    Referring to FIG. 1, conventional light emitting diodes or “LEDs” include thin layers of semiconductor material of two opposite conductivity types, typically referred to as p-type layers  20  and n-type layers  22 . The layers  20 ,  22  are typically disposed in a stack, one above the other, with one or more layers of n-type material in one part of the stack and one or more layers of p-type material at an opposite end of the stack. Each LED includes a junction  24  provided at the interface of the p-type and n-type layers. The various layers of the stack may be deposited in sequence on a substantially transparent substrate  26 , such as a sapphire substrate, to form a wafer. The wafer is then cut apart to form individual dies which constitute separate LEDs.  
           [0003]    In operation, electric current passing through the LED package is carried principally by electrons in the n-type layer  22  and by electron vacancies or “holes” in the p-type layer  24 . The electrons and holes move in opposite directions toward the junction  24 , and recombine with one another at the junction. Energy released by electron-hole recombination is emitted from the LED as light  28 . As used herein, the term “light” includes visible light rays, as well as light rays in the infrared and ultraviolet wavelength ranges. The wavelength of the emitted light  28  depends on many factors, including the composition of the semiconductor materials and the structure of the junction  24 .  
           [0004]    [0004]FIG. 2 shows a typical LED package  10  including p-type and n-type semiconductor layers  20 ,  22  mounted atop a substantially transparent substrate  26 . The LED is surrounded by a substantially transparent encapsulant  30 . Each layer of the package has its own unique index of refraction. As used herein, the term “refraction” means the optical phenomenon whereby light entering a transparent medium has its direction of travel altered. The LED  18  has an index of refraction designated n 1 , the transparent substrate  26  has an index of refraction designated n 2  and the encapsulant layer  30  has an index of refraction designated n 3 . Because the index of refraction n 2  of the substantially transparent substrate  26  is greater than the index of refraction n 3  of the transparent encapsulant  30 , many of the light rays generated by LED  18  will not be emitted from the LED package  10 , but will be subject to total internal reflection. The optical phenomenon, known as total internal reflection, causes light incident upon a medium having a lesser index of refraction (e.g. encapsulant layer) to bend away from the normal so that the exit angle is greater than the incident angle. The exit angle will then approach 90° for some critical incident angle θ c , and for incident angles θ i  greater than critical angle θ c  there will be total internal reflection of the light ray. The critical angle can be calculated using Snell&#39;s Law. Referring to FIG. 2, a light ray subject total internal reflection is designated as light ray  32 .  
           [0005]    Thus, in many LED packages the light rays generated by the LED are never emitted from the package because such light rays are totally internally reflected within the package. Thus, there is a need for LED packages having designs that optimize the amount of light that may be extracted from the packages. There is also a need for LED packages having means for tailoring the emission geometry for higher efficiency in advanced packages.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with certain preferred embodiments of the present invention, a light-emitting diode (LED) package includes a substantially transparent substrate, such as a sapphire substrate, having a first surface and a second surface remote therefrom. The second surface of the substrate includes a lens. The lens at the second surface of the substantially transparent substrate preferably includes at least one radial surface. In other words, the lens defines a convex surface having an apex at a remote point from the first surface of the substantially transparent substrate. In other preferred embodiments, the lens may include an array of micro-lenses provided at the second surface of the substantially transparent substrate, each of the micro-lenses including a radial or convex surface.  
           [0007]    The light emitting diode package also includes a light emitting diode secured over the first surface of the substantially transparent substrate so that the second surface of the substantially transparent substrate is remote from the light emitting diode. The light emitting diode preferably emits light that is extracted from the package. In one preferred embodiment the light emitting diode emits light having a predetermined wave length and the lens includes a grating pattern that is matched to the wave length of light emitted by the light emitting diode so as to control the emission geometry of the light emitted by the package. The grating pattern may have a radial configuration or a linear configuration wherein the gratings are substantially parallel to one another. In other embodiments, the grating pattern may include a series of ridges formed on the second surface of the substantially transparent substrate. In still other embodiments, the lens may be a fresnel lens formed at the second surface of the substantially transparent substrate. All of the above-mentioned lenses may be formed using a subtractive etching process that removes a portion of the substantially transparent substrate. In other embodiments, the lens may be formed by using an additive process that deposits material over the second surface of the substantially transparent substrate.  
           [0008]    The LEDs may include materials selected from the group consisting of semiconductors such as III-V semiconductors, as for example, materials according to the stoichiometric formula Al a In b Ga c N x As y P z  where (a+b+c) is about 1 and (x+y+z) is also about 1. Most typically, the semiconductor materials are nitride semiconductors, i.e., III-V semiconductors in which x is 0.5 or more, most typically about 0.8 or more. Most commonly, the semiconductor materials are pure nitride semiconductors, i.e., nitride semiconductors in which x is about 1.0. The term “gallium nitride based semiconductor” as used herein refers to a nitride based semiconductor including gallium. The p-type and n-type conductivity may be imparted by conventional dopants and may also result from the inherent conductivity type of the particular semiconductor material. For example, gallium nitride based semiconductors typically are inherently n-type even when undoped. N-type nitride semiconductors may include conventional electron donor dopants such as Si, Ge, S, and O, whereas p-type nitride semiconductors may include conventional electron acceptor dopants such as Mg and Zn. The substrate is preferably substantially transparent and may be selected from a group of materials including sapphire, GaN, AIN, ZnO and LiGaO. In certain preferred embodiments, the LEDs are GaN LEDs and the substrate is made of sapphire.  
           [0009]    The light emitting diode package is preferably adapted to be flip-chip mounted to a microelectronic element, such as a printed circuit board, so that the second surface of the substantially transparent substrate overlies the light emitting diode portion of the package and so that the second surface of the substantially transparent substrate faces away from the microelectronic element. The package may also include a substantially transparent encapsulant encapsulating at least a portion of the substantially transparent substrate. In certain preferred embodiments, the transparent encapsulant covers both the substantially transparent substrate and the light emitting diode. The substantially transparent encapsulant may also encapsulate any conductive elements used to electrically interconnect the light emitting diode package with the microelectronic element. The substantially transparent encapsulant may include epoxies, elastomers and polymers.  
           [0010]    In other preferred embodiments of the present invention, a method for making a light emitting diode package includes providing a substantially transparent substrate having a first surface and a second surface, mounting a light emitting diode over the first surface of the substantially transparent substrate, and forming a lens at the second surface of the substantially transparent substrate for optimizing light extraction from the package. The method may also include flip-chip mounting the light emitting diode package to a microelectronic element so that the lens at the second surface of the substantially transparent substrate is remote from the microelectronic element. The method also preferably includes encapsulating the light emitting diode package with a substantially transparent encapsulant such as an epoxy, elastomer and polymer. The encapsulant may be provided in a liquid form and then cured. These and other preferred embodiments of the present invention will be described in more detail below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 shows a front elevation view of a conventional light emitting diode (LED) having a p-type layer, an n-type layer and a substantially transparent substrate.  
         [0012]    [0012]FIG. 2 shows a front elevation view of a conventional LED package including the LED of FIG. 1 mounted atop a microelectronic element and sealed in an encapsulant.  
         [0013]    [0013]FIG. 3 shows a front elevation view of a conventional LED package.  
         [0014]    [0014]FIG. 4 shows a front elevation view of a LED package, in accordance with certain preferred embodiments of the present invention.  
         [0015]    [0015]FIG. 5 shows front elevation view of a LED package, in accordance with further preferred embodiments of the present invention.  
         [0016]    [0016]FIG. 6 shows a front elevation view of a LED package, in accordance with still further preferred embodiments of the present invention.  
         [0017]    [0017]FIG. 7A shows a plan view of a substantially transparent substrate of a LED package, in accordance with certain preferred embodiments of the present invention.  
         [0018]    [0018]FIG. 7B shows a plan view of a substantially transparent substrate of a LED package, in accordance other preferred embodiments of the present invention.  
         [0019]    [0019]FIG. 8 shows a front elevation view of a LED package, in accordance with other preferred embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    [0020]FIG. 3 shows a front elevation view of a conventional flip-clip LED package including a LED having a semiconductor material of a first conductivity type in a lower region  120  and a second conductivity type in an upper region  122 . The lower region  120  may be formed from a p-type semiconductor material whereas the upper region  122  may be formed from an n-type semiconductor material. The LED includes a junction  124  between the lower region  120  and the upper region  122 . The lower and upper regions  120 ,  122  may abut one another so that they define the junction  124  at their mutual border. In alternative embodiments, however, the junction  124  may include multi-layered structures in the mutually adjacent portions of regions  120 ,  122  or between these regions. Thus, the junction  124  may be a simple homojunction, a single heterojunction, a double heterojunction, a multiple quantum well or any other type of junction structure. The upper region  122  may incorporate a “buffer layer” at the interface with substantially transparent substrate  126 .  
         [0021]    The LED is preferably flip-chip mounted atop a substrate  128  having contacts  129  and encapsulated in a substantially transparent encapsulant material  130  to form a LED package. The characteristics of encapsulant layer  130  are selected so that light generated by the LED may be emitted from the package. Encapsulant layer  130  has an index of refraction n 2  that is less than the index of refraction n 1  of transparent substrate  126 . As a result, when the incident angle θ i  of a light ray  146  at interface  164  is greater than the critical angle θ c , the light ray  146  is totally internally reflected back into the substrate  126  and does not pass into the encapsulant layer  130  where the ray can be emitted from the LED package  128 . As a result, many of the light rays generated by the LED are never emitted from encapsulant layer  130  of the LED package. Thus, there is a need for a LED package design which optimizes the amount of light that may be extracted from the package.  
         [0022]    [0022]FIG. 4 shows a flip-chip mounted LED package including a lens formed on a second surface of a substantially transparent substrate for optimizing the amount of light extracted from the package. The LED package includes LED  218  having first layer  220 , second layer  222 , and junction  224  between the first and second layers. In certain preferred embodiments, LED  218  is preferably a GaN LED. The LED package includes substantially transparent substrate  226  such as a sapphire substrate having a first surface  250  and a second surface  252  remote from first surface  250 . During assembly, the second layer  222  of LED  218  is abutted against the first surface  250  of substantially transparent substrate  226 . The second surface  225  of substantially transparent substrate  226  preferably includes a lens having a convex or radial surface.  
         [0023]    The LED package is preferably mounted atop substrate  228  having contacts  229 . LED package may be electrically interconnected with substrate  228  using fusible conductive masses  254  such as solder balls. The electrically interconnected LED package may be encapsulated using substantially transparent encapsulant layer  230 . The encapsulant  230  may be initially in a liquid state and may be cured to form a solid encapsulant layer. Preferred encapsulants include epoxies, elastomers and polymers.  
         [0024]    Providing a flip-chip mounted LED package having a radial surface  252  optimizes the amount of light extracted from the package. For example, light ray  246  is emitted from junction  224  at substantially the same angle as light ray  246  shown in FIG. 3. However, because the second surface  252  of substrate  226  is convex, light ray  246  engages interface  264  at less than the critical angle and is able to pass through the interface  264  between substrate  226  and encapsulant layer  230 .  
         [0025]    Referring to FIG. 5, a LED package  310  in accordance with another preferred embodiment of the present invention includes LED  318  having first surface  320  and second surface  322 . LED  318  has a junction  324  extending between first and second layers  320 ,  322 . LED package includes a substantially transparent substrate  326  having a first surface  350  and a second surface  352  remote therefrom. The second layer  322  of LED  318  is attached to first surface  350  of substantially transparent substrate  326 . Second surface  352  of substrate  326  includes an array of microlenses  370  formed thereon. The microlenses may be formed at the second surface  352  using a subtractive etching process or by depositing material atop second surface  352 .  
         [0026]    In a conventional package, a light ray  346  generated at junction  324  of LED  318  would be totally and internally reflected back into substrate  326  if second surface  352  were substantially flat. However, due to the array of microlenses  370 , the light ray  346  is able to pass through the interface  364  between substantially transparent substrate  326  and encapsulant layer  330 . Thus, a flip-chip LED package having an array of microlenses formed on a second surface of a substantially transparent substrate results in a greater number of light rays escaping from the package  310 .  
         [0027]    [0027]FIG. 6 shows a LED package including LED  418  having first layer  420  and second layer  422 , with junction layer  424  extending therebetween. LED  410  includes a substantially transparent substrate  426  having a first surface  450  abutted against second layer  422  and second surface  452  remote therefrom. The second surface  452  of substantially transparent substrate  426  is subjected to a subtractive etching process or an additive process for forming a grating at second surface  452 . The grating at second surface  452  increases the number of light rays emitted from the sides of the LED package  410 . As shown in FIG. 6, light ray  446  strikes the interface  454  between substrate  426  and encapsulant layer  430 . The granting directs light ray  446  through a side  470  of substrate  426  and passes through encapsulant layer  430 . Although the present invention is not limited by any particular theory of operation, it is believed that providing a grating structure at second surface  452  optimizes the amount of light rays extracted from one or more sides of LED package  410 .  
         [0028]    Referring to FIGS. 7A and 7B, in accordance with certain preferred embodiments of the present invention, the grating pattern formed at the second surface of substantially transparent substrate  426  may have various configurations depending upon the desired light extraction characteristics for the package. In the embodiment shown in FIG. 7A, gratings  454  extend in directions substantially parallel to one another. Using this particular grating pattern, light generated at the junction of an LED will preferably be directed through sidewalls of an LED package in the directions indicated by arrows R 1  and R 2 .  
         [0029]    [0029]FIG. 7B shows another substantially transparent substrate  426 ′ whereby the grating pattern  454 ′ formed at the second surface of substrate  426 ′ has a radial configuration including a series of concentric circles. In this particular embodiment, light generated at the junction of an LED will preferably be directed through sidewalls of an LED package in the directions indicated by arrows R 1 -R 4 . In other words, the light rays are preferably emitted from all sides of substantially transparent substrate  426 ′.  
         [0030]    [0030]FIG. 8 shows a LED package  510 , in accordance with further preferred embodiments of the present invention. The LED package includes LED  518  having first layer  520 , second layer  522  and junction  524  extending between first and second layer  520 ,  522 . A substantially transparent substrate  526  is formed atop second layer  522  of LED  518 . Substantially transparent substrate  526  includes first surface  550  and second surface  552  remote therefrom. A grating pattern  554  is formed atop second surface  552  of substrate  526 . The grating pattern is preferably formed by an additive process that adds material to the second surface of the substrate  526 . The grating pattern results in light rays generated by LED  518  being reflected through sidewall  570  of substantially transparent substrate  526 . The pattern and/or spacing of the gratings  554  is matched with the wavelength of the light emitted from junction  524 . In other word, the pattern of the grating  554  is matched with the wavelength of the LED  518  so that an optimum amount of light is transmitted through the sidewall  570  of substrate  526  and through the sides of LED package  510 .  
         [0031]    These and other variations and combinations of the features discussed above can be utilized without departing from the present invention. Thus, the foregoing description of the preferred embodiments should be taken by way of illustration rather than by way of limitation of the invention as defined by the claims.