Abstract:
Lenses for LEDs are described that efficiently create a substantially uniform light emission across a surface of a backlight box. The backlight may illuminate an LCD. A wide-emitting lens refracts light emitted by an LED die to cause a peak intensity to occur within 35-65 degrees off the die&#39;s center axis, normal to the die&#39;s top surface, and an intensity along the center axis to be between 40% and 90% of the peak intensity. The lens is concave over the die and has smooth edges that transition into the lens sidewalls. The direct emissions of the lenses from a plurality of LEDs arranged on a base surface in a backlight box combine together to uniformly illuminate a light output surface of the backlight box.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to light emitting diodes (LEDs) and, in particular, to certain lens designs useful for backlighting. 
       BACKGROUND 
       [0002]    LED dies typically emit light in a lambertian pattern. It is common to use a lens over the LED die to narrow the beam or to make a side-emission pattern. A common type of lens for a surface mounted LED is preformed molded plastic, which is bonded to a package in which the LED die is mounted. One such lens is shown in U.S. Pat. No. 6,274,924, assigned to Philips Lumileds Lighting Company and incorporated herein by reference. 
         [0003]    When LEDs are the light source in a backlight, various techniques have been used to prevent the small LED die from appearing as a bright dot on the backlight&#39;s output surface. For example, for small backlights, LEDs may illuminate a solid, transparent light guide from the side edges. The LED light then mixes in the light guide and leaks out the top with a uniform emission profile. In larger backlights, designed by the present assignee, the LEDs are distributed on the base surface of a reflective backlight box, and each LED has a side emitting lens that greatly limits the light emission normal to the LED. The side emissions are mixed in the box and eventually leak out through the top opening of the box to create a uniform emission profile. In such a backlight, the reflected and mixed sidelight constitutes a vast majority of the light ultimately emitted by the backlight. It is inherent in such a design that light is attenuated by each reflection. 
         [0004]    The present assignee has developed an overmolding technique that molds a lens directly over the LED in virtually any shape. Various backlights using side-emitting lenses formed by overmolding are described in U.S. application publication number US 2006/0102914, assigned to Philips Lumileds Lighting Company and incorporated herein by reference. 
       SUMMARY 
       [0005]    A new lens surface shape is disclosed for an LED, where the lens is concave over the LED die, and the rim of the concave portion smoothly transitions into the sidewalls. The rim is at a particular radius from the center line to achieve the desired emission pattern. The shape of the lens results in a maximum intensity between 35-65 degrees with respect to the normal of the LED die surface. Instead of minimizing the emission at the normal of the LED, as is typically done with side-emitting lenses, the intensity along the normal is 40-90% of the maximum intensity. 
         [0006]    The lens is preferably silicone and formed by molding directly over the LED die. 
         [0007]    One or more of the LEDs incorporating the lens are used in a reflective backlight box, where the light emission from the lens directly illuminates a light emitting surface of the backlight (e.g., a diffuser sheet or a brightness enhancement film) and exits the backlight. Although there will be some reflected light inside the backlight box (reflected off the backlight box walls), such reflected light does not form the majority of the light that ultimately exits the backlight. In one embodiment, at least 50% of the light exiting the backlight box is from direct illumination by the LEDs. 
         [0008]    The LEDs in the backlight box may be blue, red, and green LEDs, or use phosphor conversion to create red, green, and blue light components. The optimum lens shape for each type of LED may be different to achieve the desired brightness profile. 
         [0009]    The thickness of the lens, the width of the lens, the shape of the lens, and the distance between the top of the lens and the top surface of the backlight are optimized to maximize the efficiency and brightness uniformity of the backlight. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-sectional view of a shaped LED die mounted on a submount, where the lens of the present invention is molded over the LED die. 
           [0011]      FIG. 2  is a cross-sectional view of an ultra-thin LED die mounted on a submount, where the lens of the present invention is molded over the LED die. 
           [0012]      FIGS. 3A and 3B  illustrate the emission pattern of the LED due to the lens shape. 
           [0013]      FIG. 4  is a cross-sectional view of a backlight incorporating an array of encapsulated LEDs similar to  FIG. 1  or  FIG. 2 . 
           [0014]      FIG. 5A  is a top down view of a simple backlight housing four LEDs having circular lenses, showing overlapping equi-brightness circular emission patterns resulting from the lenses. 
           [0015]      FIG. 5B  is a top down view of an LED with a circular lens. 
           [0016]      FIG. 6A  is a top down view of a simple backlight housing four LEDs having rectangular lenses, showing equi-brightness rectangular emission patterns resulting from the lenses. 
           [0017]      FIG. 6B  is a top down view of an LED with a rectangular lens. 
           [0018]      FIGS. 7 and 8  are detailed cross-sectional views of one type of flip-chip LED die encapsulated by the new lens, where phosphor is either dispersed in the lens material ( FIG. 7 ) or located as a layer over the die ( FIG. 8 ). 
       
    
    
       [0019]    Elements labeled with the same numeral in the various figures may be the same or equivalent. 
       DETAILED DESCRIPTION 
       [0020]    As a preliminary matter, a conventional LED is formed on a growth substrate. In the example used, the LED is a GaN-based LED, such as an AlInGaN LED, for producing blue or UV light. Typically, a relatively thick n-type GaN layer is grown on a sapphire growth substrate using conventional techniques. The relatively thick GaN layer typically includes a low temperature nucleation layer and one or more additional layers so as to provide a low-defect lattice structure for the n-type cladding layer and active layer. One or more n-type cladding layers are then formed over the thick n-type layer, followed by an active layer, one or more p-type cladding layers, and a p-type contact layer (for metallization). 
         [0021]    Various techniques are used to gain electrical access to the n-layers. In a flip-chip example, portions of the p-layers and active layer are etched away to expose an n-layer for metallization. In this way the p contact and n contact are on the same side of the chip and can be directly electrically attached to the package (or submount) contact pads. Current from the n-metal contact initially spreads laterally through the n-layer. In contrast, in a vertical injection (non-flip-chip) LED, an n-contact is formed on one side of the chip, and a p-contact is formed on the other side of the chip. Electrical contact to one of the p or n-contacts is typically made with a wire or a metal bridge, and the other contact is directly bonded to a package (or submount) contact pad. A flip-chip LED is used in the various examples for simplicity, although a non-flip-chip LED may be used instead. 
         [0022]    Examples of forming LEDs are described in U.S. Pat. Nos. 6,649,440 and 6,274,399, both assigned to Philips Lumileds Lighting Company and incorporated herein by reference. 
         [0023]    Optionally, the metal pads on the LED dice are bonded to pads on a submount wafer, and the sapphire substrate is removed. The submount wafer is then singulated by sawing to separate out the LEDs. Electrodes of one or more submounts may then be bonded to a printed circuit board, which contains metal leads for connection to other LEDs and to a power supply. The circuit board may interconnect various LEDs in series and/or parallel. 
         [0024]    The particular LEDs formed and whether or not they are mounted on a submount is not important for purposes of understanding the invention. 
         [0025]    In the preferred embodiment for forming a lens over each LED die, an array of LEDs is mounted on a submount wafer. The submount may be a ceramic substrate, a silicon substrate, or other type of support structure with the LED dice electrically connected to metal pads on the submount. A lens is then overmolded onto each LED die simultaneously using the overmolding process described in U.S. application publication number US 2006/0102914, assigned to Philips Lumileds Lighting Company. 
         [0026]    In this overmolding process, a mold has indentations in it corresponding to the positions of the LED dice on the submount wafer. The indentations are filled with a liquid, optically transparent material, such as silicone, which when cured forms a hardened lens material. The shape of the indentations will be the shape of the lens. The mold and the LED dice/support structure are brought together so that each LED die resides within the liquid lens material in an associated indentation. 
         [0027]    The mold is then heated to cure (harden) the lens material. The mold and the substrate wafer are then separated, leaving a complete lens over each LED die, completely encapsulating the die. This general process is referred to as overmolding. The submount wafer is then singulated to separate out the LEDs. 
         [0028]    In one embodiment, the inventive lens is the only overmolded lens encapsulating the LED. In another embodiment, a hemispherical lens is first overmolded on the LED to encapsulate the LED, followed by molding the inventive lens over the hemispherical lens. 
         [0029]      FIG. 1  is a cross-sectional view of an LED  20  comprising a semiconductor LED die  22  mounted to a submount  24  and encapsulated by an overmolded lens  26 , in accordance with one embodiment of the invention. The die  22  is shaped to increase light extraction. Such chip shaping is described in U.S. Pat. No. 6,570,190, assigned to the present assignee and incorporated by reference. 
         [0030]    The lens  26  has a concave shape above the die  22  and has a rounded rim at a certain radius, where the lens  26  is the thickest, then falls off. The sidewalls of the lens  26  are substantially vertical, such as at an angle of 10-15% with respect to vertical. 
         [0031]    In one example, the surface of the lens  26  is described by the following equation, where Z is the vertical distance of the lens surface to the top of the LED die and R (radius) is the distance from the centerline. The dimensions are given relative to a center height of 1.0. 
         [0000]        Z ( R )=1.0+0.4* R   4 −0.0497* R   14   eq. 1 
         [0032]      FIG. 2  is a cross-sectional view of an LED  28  comprising an ultra-thin semiconductor LED die  30  mounted to a submount  24  and encapsulated by an overmolded lens  31 , in accordance with one embodiment of the invention. The die  30  may be made very thin by removing the growth substrate. The concavity of the lens  31  is less than that of lens  26  since the LED die is wider and further from the lens surface. In one example, the surface of the lens  31  is described by the following equation: 
         [0000]        Z ( R )=1.0+0.2* R   6 −0.0921* R   12   eq. 2 
         [0033]    Depending on the specific requirements for a radiation pattern, the values of the polynomial coefficients can be optimized. Therefore, the general polynomial function is: 
         [0000]        Z ( R )= C 0+ C 2* R   2   +C 4* R   4   +C 6* R   6   +C 8* R   8   +C 10* R   10   +C 12* R   12   +C 14* R   14   eq. 3 
         [0034]    The lens curvature is not restricted to polynomial functions. 
         [0035]      FIG. 3A  is a graph of the relative intensity (lm/sr) versus the angle away from the normal of the LED die top surface.  FIG. 3B  illustrates the same emission pattern where the pattern represents equal intensity of the emission at various angles. The LED top surface, simplified to be a point source, is at the 0,0 point. 
         [0036]    Since the light is broadly concentrated within a certain angle, the light pattern is about 1.5-2 times wider than a Lambertian pattern for the same brightness level contour. 
         [0037]    In the preferred embodiment, the peak intensity is between 35-65 degrees off the normal. The intensity along the normal is 10-60 percent less than the peak intensity (i.e., 90-40% of the peak intensity). With typical wide emitting lenses, the brightness along the normal is made as small as possible. The present lens is designed to produce a substantially uniform intensity upon a flat output surface of a backlight box ( FIG. 4 ) where a majority of light emitted from the backlight is due to direct illumination by the LED rather than through reflected light in the backlight box. In one embodiment, the intensity along the normal is 50-70% of the peak intensity. 
         [0038]    The dimensions of the concave lens are selected to optimally illuminate a flat surface at a particular distance from the LED die. Any change to the lens thickness, width, or curvature will typically change the angle of peak intensity. 
         [0039]      FIG. 4  is a cross-sectional view of a backlight  40  containing an array of encapsulated LEDs similar to LED  20  or  28  in  FIGS. 1 and 2 . Selected light rays  41  are also shown. The light rays  41  may or may not have the same brightness levels, and the direct light from a plurality of LEDs overlap on the top output surface of the backlight  40  to create a substantially uniform brightness profile across the output surface. Each LED shown in the backlight  40  may output white light using phosphor conversion, or the LEDs may output different colors. If the LEDs form an array of red, green, and blue LEDs, the light rays overlap to create substantially uniform white light across the output surface of the backlight  40 . Since LED dies of different types may have different emission profiles and different brightness levels, the lenses for one type of LED may be different from the lenses for a different type of LED to achieve the optimal overlap and color contribution necessary for a substantially uniform white point across the backlight output surface. 
         [0040]    Backlight  40  is formed of reflective inner surfaces  42 , a top diffuser sheet  44  (e.g., a roughened plastic sheet), and one or more brightness enhancement films (BEFs)  46 . The diffuser sheet  44  and each BEF  46  may be very thin (less than 1 mm). The diffuser sheet  44  improves the brightness uniformity across the surface of the backlight. The BEFs  46  may be formed by a micro-prism pattern in a plastic sheet that redirects light within a narrow angle toward the viewer. A liquid crystal display  48  overlies the backlight  40  and, essentially, has a controllable shutter at each pixel location for the RGB pixels for displaying a color image. If the backlight  40  emits white light (containing RGB components), a red, green, or blue filter at the corresponding RGB pixel locations only passes the intensity-modulated red, green, or blue component. 
         [0041]    The lens shape, the spacing between LEDs, and the distance to the top of the backlight are selected so that the emissions from adjacent LEDs merge to form a substantially uniform illumination over the backlight top surface. Since the light emitted through the center of the lens  26  normal to the LED surface travels the least distance to the backlight top surface, that light has the least spreading, while the peak intensity light emitted at a 35-65 degree angle travels further to impinge upon the top surface of the backlight and thus spreads out more before impinging on the backlight top surface. The combination of the intensity profile ( FIG. 3B ) and the different distances the light rays travel to impinge on the top surface of the backlight results in the intensity pattern across the top surface of the backlight to be substantially uniform within a defined area. 
         [0042]      FIG. 5A  is a top down view of a backlight  50  containing only four LEDs with circular lenses  26 . It is assumed in this simplified embodiment that each LED outputs the same color light (e.g., white light) so mixing of different colors of LED light is not a concern. The circles  52  represent an equi-brightness pattern of each LED impinging on the diffuser sheet  44  of the backlight.  FIG. 5B  is a top down view of LED  20  of  FIG. 1  comprising a single LED die  22  with a circular lens  26 . As seen in  FIG. 5A , adjacent circular emission patterns may overlap, abut, or be slightly separated; however, the resulting illumination pattern at the output of the backlight appears fairly uniform due to the smoothly varying emission pattern and the diffuser sheet  44 . 
         [0043]    To further improve the brightness uniformity, each lens may have a generally rectangular shape, as shown in the top down view of a single LED  56  with lens  58  in  FIG. 6B . The equi-brightness emission from each LED impinging on a diffuser sheet is shown by the rectangles  60  in  FIG. 6A . The backlight  62  in  FIG. 6B  has improved brightness uniformity since the overlap of the rectangular emissions from the individual LEDs is more consistent over the surface of the backlight. 
         [0044]    In one embodiment, the thickness of a lens  26  is 0.5 to 1 mm, and the width of the lens is about 2-3 mm, given that an LED die is about 1 mm×1 mm. The lens dimensions may be different depending on the LED type, the backlight configuration, the pitch of the LEDs, and other factors. In one embodiment, the distance between the top of the lens  26  and the backlight diffuser sheet  44  is 1-3 cm. In one embodiment, the total thickness of the backlight box is 3.5 cm. The optimum pitch of the LEDs on the backlight base, the number of LEDs, and the size of the lens may be determined empirically depending on the required size of the backlight and the required brightness of the backlight emission. 
         [0045]    To create white light from each LED, the LED die may emit blue light, and phosphor particles energized by the blue light generate red, green, and/or yellow components that combine with the blue light to create white light, as illustrated in  FIG. 7 . 
         [0046]      FIG. 7  is a simplified close-up view of one embodiment of a flip-chip LED die  59  on a submount  24 , where the submount  24  is formed of any suitable material, such as a ceramic or silicon. The LED die  59  has a bottom p-contact layer  64 , a p-metal contact  66 , p-type layers  68 , a light emitting active layer  70 , n-type layers  72 , and an n-metal contact  74  contacting the n-type layers  72 . Metal pads on submount  24  are directly metal-bonded to contacts  66  and  74 . Vias through submount  24  terminate in metal pads on the bottom surface of submount  24 , which are bonded to the metal leads  76  and  78  on a circuit board  80 . The metal leads  76  and  78  are connected to other LEDs or to a power supply. Circuit board  80  may be a metal plate (e.g., aluminum) with the metal leads  76  and  78  overlying an insulating layer. The molded lens  26  encapsulates the LED die  59 . The board  80  may be a strip, and identical strips may be arranged in a desired pattern on the base surface of a backlight box of any size. 
         [0047]    In  FIG. 7 , phosphor particles  82  are dispersed in the liquid lens material before the lens material is dispensed in the mold. A YAG phosphor generates a yellow-green light when energized by blue light and may be suitable for producing white light. Red phosphor may be added to create a warmer white light. 
         [0048]    In  FIG. 8 , the phosphor is formed either as a preformed sheet  86  that is affixed over the LED die  59 , or the entire surface of the LED die  59  is coated with phosphor  86 / 88  by, for example, electrophoresis or any other coating technique. 
         [0049]    In another embodiment, the LED die may be a non-flip-chip die, with a wire connecting the top n-layers to a metal pad on the submount. The lens then also encapsulates the wire. 
         [0050]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.