Abstract:
A light emitting packaged diode ids disclosed that includes a light emitting diode mounted in a reflective package in which the surfaces adjacent the diode are near-Lambertian reflectors. An encapsulant in the package is bordered by the Lambertian reflectors and a phosphor in the encapsulant converts frequencies emitted by the LED chip and, together with the frequencies emitted by the LED chip, produces white light. A substantially flat meniscus formed by the encapsulant defines the emitting surface of the packaged diode.

Description:
BACKGROUND 
     This application claims priority from Ser. No. 60/824,390 filed Sep. 1, 2006 and from Ser. No. 11/739,307 filed Apr. 24, 2007. 
    
    
     The present invention relates to the geometry of encapsulant materials in light emitting diodes (LEDs) and particularly to the light pattern emitted from surface mount side view LEDs that produce white light. 
     Light emitting diodes represent a class of semiconductor materials in which the application of current across a p-n junction drives recombinations between electrons and holes with at least some of the recombinations generating photons. In accordance with well-understood principles of electronics and physics, the wavelength (and thus the frequency) of the photons is based upon the energy change of the recombination. In turn, the energy is defined or constrained by the bandgap of the semiconductor materials; i.e., the energy difference between the material&#39;s valence band and its conduction band. 
     As a result, the color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy red and yellow portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum. 
     In some applications, an LED is more useful when its output is moderated or converted to a different color. In particular, as the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as backlighting for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color elements (often formed by liquid crystals, “LCDs”). 
     In the present application, the term “white light” is used in a general sense. Those familiar with the generation of colors and of color perception by the human eye will recognize that particular blends of frequencies can be defined as “white” for precise purposes. Although some of the diodes described herein can produce such precise output, the term “white” is used somewhat more broadly herein and includes light that different individuals or detectors would perceive as having a slight tint toward, for example, yellow or blue. 
     In many conventional applications, the light emitting diode (which in its basic semiconductor structure is typically referred to as the chip), is packaged for its intended use. As used herein, the term package typically refers to the placement of the semiconductor chip on an appropriate electrical structure (sometimes as simple as a small piece of metal) along with a plastic lens (resin, epoxy, encapsulant) that provides some physical protection to the diode and can optically direct the light output. 
     In many conventional applications, the lens is at least partially formed of a hemisphere. An example is the classic T1 ¾ package which is widely recognized and is incorporated in a large number of LED applications. 
     More recently, light emitting diodes are being used for illumination purposes. In particular, LEDs that can produce white light are used for back lighting flat panel displays (computer screens, personal digital assistants, cellular telephones) via some other type of device (typically a liquid crystal) to generate or display color. In many of these applications, the LEDs are mounted perpendicular to the face of the relevant screen. In this orientation, and instead of being directed at the rear of the screen itself, the LEDs are directed towards the edges of a light guide—often a planar piece of plastic—so that when the light from the LEDs enters the edge of the light guide, the light guide in turn redirects it perpendicularly towards the plane of the display screen. 
     Light emitting diodes packaged for this purpose are referred to as side view surface mount LEDs or side lookers. Many surface mount side view diodes incorporate a concave meniscus within their housing. A concave meniscus can tend to focus light and yield a higher intensity, but at a cost of overall flux. Nevertheless, a concave meniscus also protects the encapsulant from mechanical damage (the encapsulant is generally more fragile than other portions of the package). 
     Side mount surface view LEDs that emit white light typically do so by incorporating a blue-emitting LED chip with a yellow-emitting phosphor. The blue light from the chip excites the phosphor to emit yellow light. This produces a combination of blue and yellow frequencies that together generate an appropriate hue of white light. 
     It has been discovered, however, that when a concave conventional meniscus is used in a side view surface mount LED in combination with certain higher brightness diodes, the concave shape of the encapsulant becomes a disadvantage because it may reduce both flux and color uniformity. 
     These disadvantages are particularly noticeable for chips with far field profiles that are more Lambertian than typical LEDs. Lambertian refers to the degree to which a surface adheres to the Lambert cosine law which states that the reflected or transmitted luminous intensity (flux) in any direction from an element of a perfectly diffusing surface varies as the cosine of the angle between that direction and the normal vector of the surface. The Lambert cosine law is often expressed as the formula N=N0cosA, where N is the radiant intensity, N0 is the radiance normal to the emitting surface and A is the angle between the viewing direction and the normal to the emitting surface. 
     In practical terms, the emission from a Lambertian emitter is more uniform than that of a less-Lambertian or non-Lambertian emitter. 
     Accordingly, a concave meniscus tends to reduce or eliminate the far field advantages of those LEDs that produce nearly Lambertian far field patterns. 
     Accordingly, a need exists to complement the brightness and far field characteristics of high quality diodes with an appropriate lens or encapsulant shape that enhances, rather than hinders, the light output for the intended purpose. 
     SUMMARY 
     The invention is a light emitting packaged diode that includes an LED chip with a near-Lambertian far field pattern mounted in a reflective package in which the surfaces adjacent the diode are also near-Lambertian reflectors; an encapsulant in the package and bordered by the Lambertian reflectors; a phosphor in the encapsulant that converts frequencies emitted by the LED chip and that together with the frequencies emitted by the LED chip produce white light; and a substantially flat meniscus formed by the encapsulant that defines the emitting surface of the package diode. 
     The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the followed detailed description taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional diagram of a conventional side view surface mount light emitting diode. 
         FIGS. 2 ,  3  and  4  are cross-sectional diagrams of side view surface mount light emitting diodes according to the present invention. 
         FIG. 5  is a perspective view of diodes according to the present invention in conjunction with a light guide. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross sectional view of a conventional light emitting diode of the type referred to as a surface mount side view diode. The diode is broadly designated at  10 . The packaged diode  10  includes a light emitting diode chip  11  in a white resin package  12 . Respective electrical contacts  13  and  14  are included for connecting the packaged diode  10  to a circuit or device. For the sake of clarity,  FIG. 1  refrains from illustrating all of the wiring, but does include a wire  15  typically used to connect the chip  11  to one electrode while the chip itself rests on a second electrode (not shown). As in many such conventional packages, the chip  11  is covered by an encapsulant  16  that fills a recess defined by the walls  17  and floor  20  of the resin package  12 . A phosphor for light conversion is indicated as the black dots  21 . 
     In such a conventional package  10 , the encapsulant forms a concave meniscus  22 . As noted earlier, in such conventional diodes, the concave meniscus  22  can help focus the light in a particular direction, but may reduce the color uniformity. Nevertheless, this also reduces flux in exchange for higher directional intensity. The concave meniscus  22  also defines the encapsulant within the borders of the white resin package  12 . This can help protect the encapsulant from physical contact with other objects and thus provides, at least to some extent, some physical protection to the diode  10 . 
       FIG. 2  illustrates a packaged diode  25  according to the present invention. Where identical, elements carry the same reference numerals as in  FIG. 1 . Accordingly, in the side view surface mount embodiment of  FIG. 2 , the diode  25  includes the white resin package  12 , the LED chip  11 , the respective contacts  13  and  14 , and the recess in the package  12  defined by the side walls  17  and the floor  20 . The walls  17  and floor  20  of the package  12  are preferably near-Lambertian in their reflection characteristics. The white resins used for such packages are widely available and well understood in this art and will not be otherwise discussed in detail. 
     In  FIG. 2  the encapsulant is designated at  26  because it carries a different shape from the conventional meniscus illustrated in  FIG. 1 . In particular, in the invention the encapsulant is formed into a substantially flat lens  27 . The latter design provides advantages during manufacturing, better far field uniformity, and a greater luminant flux. 
     The encapsulant can be any material that is suitable for the purposes of the invention and that does not otherwise interfere with the operation of the chip. As set forth in co-pending and commonly assigned application Ser. No. 60/824,385 filed Sep. 1, 2006 for, “Phosphor Position In Light Emitting Diodes,” when the LED chip emits in the higher energy portions of the spectrum (e.g., blue, violet, and ultraviolet), the encapsulant should be less reactive or inert to the photons emitted at such frequencies. Thus, the polysiloxane (“silicone”) resins tend to be particularly well suited for the encapsulant. 
     Although the inventors do not wish to be bound by any theory, it has been discovered that the use of the flat or lens  27  complements and enhances chips with Lambertian or near-Lambertian far field characteristics such as the EZBRIGHT™ and EZR™ LED chips available from Cree Inc., Durham N.C., the assignee of the present invention. 
     The incorporation of phosphors is well understood in the art and will not be discussed in detail herein. For blue light emitting diodes cerium-doped yttrium-aluminum-garnet (YAG:Ce) is one example of a phosphor material that is appropriate because it is responsive to the higher energy frequencies and emits yellow frequencies in response. As noted earlier herein, the combination of these blue and yellow frequencies can produce appropriate shades of white light. 
       FIG. 3  illustrates another embodiment of the invention. Again and where appropriate, like reference numerals will identify identical elements. In  FIG. 3  the packaged diode is broadly designated at  30  and includes the LED chip  11  in the white resin package  12 . In  FIG. 3 , however, the encapsulant is designated at  34  and is formed into a moderate dome lens  31 . It has been discovered that a very slight meniscus (convex or concave) that is maintained within about 50 microns of the top surface  32  of the white resin package  12  still offers the uniformity benefits of the flat surface illustrated in  FIG. 2 . The 50 micron dimension is defined between the highest (or lowest) portion of the meniscus  33  and the top surface  32  of the white resin package  12 . As a proportional comparison, the package  12  will have dimensions on the order of about 2-4 millimeters in length, width and height. 
     It has further been discovered according to the invention that radiation mixing with the dome lens  31  can be enhanced with a light diffuser indicated by the dots in the encapsulant  34 . Generally speaking, a diffuser is any set of physical objects, potentially including bubbles in the encapsulant, that physically scatter light within the encapsulant  34 . 
       FIGS. 2 and 3  illustrates that the phosphor  21  is preferably positioned immediately upon or very close to the chip  11  and the floor  20  of the white resin package  12 . The method for positioning of the phosphor in this manner is set forth in previously incorporated application No. 60/824,385 for Phosphor Position in Light Emitting Diodes. 
       FIG. 4  is a cross-sectional view of a diode  40  according to the present invention that is similar to  FIG. 3  with respect to the dome lens  31 . In  FIG. 4 , however, the encapsulant is designated at  41  because it does not include the diffuser. The phosphor is designated at  42  because it covers only the emissive area of the chip  11 . Limiting the position of the phosphor  42  in this manner permits the dome lens  31  to be somewhat larger while still producing good color uniformity. 
     Light extraction and thus efficiency can be optimized with such a modest dome  31  ( FIGS. 3 and 4 ) and modest domes can offer the benefit of focusing the light in a particular direction while defocusing it in others. This in turn maximizes coupling into planar components such as the light guides that are used as part of the backlighting unit for flat displays, particularly including those that include liquid crystals. In such applications, the invention offers the capability of focusing the light tightly within the plane of the light guide while spreading it across the plane for maximum efficiency and uniformity. 
       FIG. 5  illustrates this concept is in which two diodes, according to the present invention (each designated at  45 ) are positioned along a rear edge  46  of a light guide  47 . As indicated by the vector arrows  50 , the slightly domed meniscus ( 31  in  FIGS. 3 and 4 ) can produce more light in the predominant plane of the light guide  47  while avoiding wasting light above and below that plane, thus increasing the efficiency of the combination of the diodes  45  and the light guide  47 . 
       FIG. 5  also illustrates the fact some embodiments, a side view surface mount diode is characterized by a rectangular profile rather than a square or circular profile. In such cases, the light emitting surface of the moderately domed encapsulant forms an arc-shaped profile that extends in the direction of primary emission and, as further illustrated by  FIG. 5 , toward the edge of the light guide that the diode is intended to illuminate. 
     In such cases, if the encapsulant were to be viewed individually, the extent to which it extended from the plane of the white resin package (e.g.  12  in  FIGS. 1-4 ) would define a solid polygon with one rectangular side, two arc shaped planes perpendicular to the rectangular side and a vertical curved wall extending from end to end of the rectangular side and following the arc of the two arc-shaped planes. This distinguishes the domes of the present invention from hemispheres or other partial spheres that are more typical in surface mount LEDs and in surface mount side view LEDs. 
     It will be understood that the dome lens  31  or flat lens  27  according to the invention need not be formed in a single step. Because variations in the manufacture of the meniscus  27 ,  31  are almost inevitable, initial variability in the color point, color uniformity, intensity, and far field profile (or any combination of these) can result. For example, variations as small as 10 microns in the height or position of the encapsulant  10  can define the difference between useful and failed diodes. Accordingly, in the invention the encapsulant can be reworked by successive applications of thin layers of additional encapsulant until the desired characteristics are produced. 
     In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms have been employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.