Patent Publication Number: US-9842973-B2

Title: Method of manufacturing ceramic LED packages with higher heat dissipation

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     The application is a divisional application of U.S. patent application Ser. No. 11/796,240, filed on Apr. 27, 2007 entitled “LED Packages with Mushroom Shaped Lenses and Methods of Manufacturing LED Light-Emitting Devices,” which is a continuation in part of U.S. patent application Ser. No. 11/260,101, filed on Oct. 26, 2005 entitled “Method of Manufacturing Ceramic LED Packages,” now U.S. Pat. No. 7,670,872, which in turn claims the benefit of U.S. Provisional Patent Application No. 60/623,266 entitled “1-5 Watt and Higher LED Packages,” U.S. Provisional Patent Application No. 60/623,171 entitled “3-10 Watt and Higher LED Packages,” and U.S. Provisional Patent Application No. 60/623,260 entitled “5-15 Watt and Higher LED Packages,” each filed on Oct. 29, 2004. The application is related to U.S. patent application Ser. No. 11/259,818 entitled “LED Package with Structure and Materials for High Heat Dissipation,” now U.S. Pat. No. 7,772,609, and U.S. patent application Ser. No. 11/259,842 entitled “High Power LED Package with Universal Bonding Pads and Interconnect Arrangement,” now U.S. Pat. No. 7,473,933, both filed on Oct. 26, 2005. The application is also related to U.S. patent application Ser. No. 11/036,559 filed on Jan. 13, 2005 and entitled “Light Emitting Device with a Thermal Insulating and Refractive Index Matching Material,” now U.S. Pat. No. 8,134,292. All applications noted above are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to light emitting diodes and more particularly to packages for high-power LEDs. 
     2. Description of the Prior Art 
     A light emitting diode (LED) is a semiconductor device that produces light when an electric current is passed therethrough. LEDs have many advantages over other lighting sources including compactness, very low weight, inexpensive and simple manufacturing, freedom from burn-out problems, high vibration resistance, and an ability to endure frequent repetitive operations. In addition to having widespread applications for electronic products as indicator lights and so forth, LEDs also have become an important alternative light source for various applications where incandescent and fluorescent lamps have traditionally predominated. 
     Using phosphors as light “converters,” LEDs can also serve to produce white light. In a typical LED-based white light producing device, a monochromatic LED is encapsulated by a transparent material containing appropriate phosphors. In some systems, an LED that produces a monochromatic visible light is encapsulated by a material containing a compensatory phosphor. The wavelength(s) of the light emitted from the compensatory phosphor is compensatory to the wavelength of the light emitted by the LED such that the wavelengths from the LED and the compensatory phosphor mix together to produce white light. For instance, a blue LED-based white light source produces white light by using a blue light LED and a phosphor that emits a yellowish light when excited by the blue light emitted from the LED. In these devices the amount of the phosphor in the transparent material is carefully controlled such that only a fraction of the blue light is absorbed by the phosphor while the remainder passes unabsorbed. The yellowish light and the unabsorbed blue light mix to produce white light. Another exemplary scheme uses an LED that produces light outside of the visible spectrum, such as ultraviolet (UV) light, together with a mixture of phosphors capable of producing either red, green, or blue light when excited. In this scheme, the light emitted by the LED only serves to excite the phosphors and does not contribute to the final color balance. 
     Recent advances in semiconductor technology have made it possible to manufacture high-power LEDs that produce light at selected wavelengths across the visible spectrum (400-700 nm). Such high-power LEDs can have reliability and cost advantages over existing technologies such as incandescent lamps, arc lamps, and fluorescent lamps in many lighting applications. High-power LEDs also offer advantages for design of next generation color display technologies such as active matrix thin film transistor liquid crystal displays (TFTLCDs) in applications such as consumer computer and television monitors, projection TVs, and large advertising displays. 
     Although high-power LED devices have been manufactured, their widespread use has been limited because of a lack of suitable packages for the LEDs. Current LED packages cannot handle the high-power density of LED chips. In particular, prior art packages provide inadequate heat dissipation away from the LED dies. Inadequate heat dissipation limits the minimum size of the package and therefore the density of LEDs per unit area in the device. One measure of how efficiently a package dissipates heat is the temperature rise across the package for a given input electrical power. This measure is generally in the range of 15 to 20 degrees centigrade per watt (° C./W) from the junction to the case in current LED packages, usually too high to provide adequate heat dissipation for an LED package having a power higher than 1 watt. 
     Without sufficient heat dissipation, devices incorporating high-powered LEDs can run very hot. Light output, LED efficiency, and LED life, are each dependent on the LED die junction temperature. Inadequate heat dissipation will cause the LED Die to operate at a higher temperature and therefore limits the performance of the LED die when the LED die is capable of operating at a power level exceeding the limits of the package. Insufficient heat dissipation by an LED package can cause the LED device to fail at an early stage or render it too hot to use safely. 
     Even under less severe conditions, inadequate heat conduction for an LED package may result in poor thermal stability of the phosphors, as well as encapsulation and lens materials, in those devices that employ phosphors. Specifically, exposure to high temperatures for extended periods tends to alter the chemical and physical properties of such phosphors, encapsulation, and lens materials, causing performance deterioration. For instance, the light conversion efficiency can decline and the wavelength of output light can shift, both altering the balance of the light mixture and potentially diminishing the intensity of the overall output. For example, currently available phosphors are often based on oxide or sulfide host lattices including certain rare earth ions. Under prolonged high temperature conditions, these lattices decompose and change their optical behavior. Other problems commonly found with LED-based white light sources are transient color changes and uneven color distributions, both caused by temperature gradients in the phosphor-containing material and degradation of the encapsulation and lens materials. Such behaviors often create an unsatisfactory illumination. The above-mentioned thermal problems worsen with increasing temperature and therefore are particularly severe for devices that incorporate high-power LEDs with phosphors. 
     Attempts have been made in current LED packages to alleviate the above problem. One example is to directly attach an LED die to a top surface of a metal heat slug such as a copper plate. The copper plate serves to spread the heat and to make electrical connections with the LED die. This design limits the selection of materials for the heat slug because the design relies at least partially on the conductive nature of the copper for making the conductive contacts between the LED die and the top surface of the copper heat slug. The use of copper heat slugs also has other limitations, such as a substantial mismatch between the coefficients of thermal expansion (CTE) of the LED die material and the copper onto which the LED die is attached. A large CTE mismatch can create high stresses upon heating a cooling at bonded interfaces. Cracks that form at these interfaces then render the LED package unreliable. In addition, the above design is relatively expensive and difficult to manufacture. 
     Other problems associated with LED packages relate to how lenses are attached. Typically, a layer of transparent adhesive is used to secure the lens to the package. Frequently, air bubbles form in the adhesive. The air bubbles increase internal reflections of the light in the adhesive layer, reducing the amount of light transmitted through the adhesive layer. Additionally, the lens can also become inadvertently detached from the package. This can happen when the lens experiences a shear force, for example a side impact that “pops” the lens off the package. 
     Given the importance of LEDs as light sources, particularly high-power LEDs, there is a need for improved LED packaging methods and materials to alleviate the above-identified problems by providing better thermal performance (e.g., improved thermal resistance from junction to case) and higher reliabilities (e.g., lower stresses in packaging materials). Such packaging methods and materials will allow LEDs to produce higher optical performance (Lumens/package) from a smaller package or footprint higher optical performance any light source applications. 
     BRIEF SUMMARY OF THE INVENTION 
     The present disclosure addresses the above problems by providing methods for forming LED packages and light emitting devices. According to an embodiment of the invention, a method for forming an LED package comprises forming a panel, defining a grid on a surface of the panel, and separating the LED package from the panel by breaking the panel along lines of the grid. Forming the panel includes forming a top layer, an intermediate body layer, and a thermally conducting layer, and bonding the intermediate body layer between the top and thermally conducting layers. Forming the panel can further include forming an alignment layer, and these embodiments also include bonding the alignment layer to the top layer opposite the intermediate layer. 
     In some embodiments of the method of forming the LED package forming the thermally conducting layer includes preparing a sheet of AlN. Forming the thermally conducting layer can also include forming a sheet with a square array of vias disposed therethrough. Forming the thermally conducting layer can further include forming a metallization pattern on a top surface of the thermally conducting layer, and in these embodiments bonding the intermediate body layer includes bonding the intermediate body layer to the top surface of the thermally conducting layer. In some of these embodiments forming the thermally conducting layer further includes forming a metallization pattern on a bottom surface of the thermally conducting layer. 
     In some embodiments of the method of forming the LED package forming the intermediate body layer includes preparing a sheet of AlN. Forming the intermediate body layer can also include forming a sheet with a square array of vias disposed therethrough and an aperture disposed within each square defined by the array. Likewise, forming the top body layer can include preparing a sheet of AlN, and can also include forming a sheet with a square array of vias disposed therethrough and an aperture disposed within each square defined by the array. In some of these latter embodiments the aperture within each square has an inclined sidewall, and forming the top body layer can further include metallizing a sidewall of the aperture within each square. Forming the top body layer can further include forming a metallization pattern on a top surface of the top body layer, and in these embodiments bonding the intermediate body layer between the top and thermally conducting layers includes bonding the intermediate body layer to a bottom surface of the top body layer. 
     In other embodiments of the method of forming the LED package bonding the intermediate body layer between the top and thermally conducting layers includes co-firing. Bonding the intermediate body layer between the top and thermally conducting layers can also include aligning a square array of vias defined in each of the layers. The step of bonding the intermediate body layer between the top and thermally conducting layers can alternatively include applying an adhesive between two of the layers. 
     In still other embodiments of the method of forming the LED package forming the intermediate body layer includes forming a metal sheet with a square array of vias disposed therethrough an aperture disposed within each square defined by the array. In these embodiments bonding the intermediate body layer between the top and thermally conducting layers includes applying an electrically insulating adhesive between the intermediate body layer and the thermally conducting layer. 
     In yet other embodiments of the method of forming the LED package defining the grid on the surface of the panel includes scribing snap lines on the surface of the panel. Where one of the top, intermediate body, or thermally conducting layers is a non-ceramic layer, the step of forming the non-ceramic layer includes defining a grid thereon. In some of these embodiments defining the grid on the surface of the panel includes aligning the grid on the surface of the panel with the grid defined on the non-ceramic layer. 
     In still other embodiments of the method of forming the LED package forming the panel includes forming a square array of vias disposed therethrough. In some of these embodiments defining the grid on the surface of the panel includes scribing snap lines on the surface of the panel that intersect the vias. In these embodiments the method can further include plating metal into the vias. 
     According to another embodiment of the invention, a method for forming a light emitting device comprises forming a panel having a square array of vias disposed therethrough and a cavity disposed within each square defined by the array, defining a grid on a surface of the panel, bonding an LED die to a floor of each cavity, and separating the light emitting device from the panel by breaking the panel along lines of the grid. In some embodiments the method further comprises encapsulating each LED die. In some of these embodiments encapsulating each LED die includes forming a thermally insulating layer over each LED die, and forming a luminescent layer over each thermally insulating layer. Some embodiments of the method further comprise forming a lens over the LED die. In some of these embodiments forming the lens includes injection molding or printing with masks. 
     According to another embodiment of the invention, a light-emitting device comprises a package and a lens. The package includes a light-emitting side, an encapsulated LED configured to emit light toward the light-emitting side of the package, and a socket including a sidewall and a bottom surface, the socket disposed on the light-emitting side of the package. The lens includes a cap and a plug, where the plug is disposed within the socket. In some embodiments, the shape of the lens comprises a mushroom shape. An angle defined between the sidewall and the bottom surface of the socket can be between about 45 degrees to about 140 degrees. Likewise, an angle defined between a sidewall of the plug and a lower surface of the lens can also be between about 45 degrees to about 140 degrees. The light-emitting device can also include an adhesive layer, such as silicone, disposed between the lens and the package. 
     According to another embodiment of the invention, a method of fabricating a light-emitting device is provided. The exemplary method comprises providing a lens including a plug and a cap, and providing a package including an encapsulated LED configured to emit light toward a light-emitting side of the package, and a socket disposed on the light-emitting side of the package. The method further comprises depositing an adhesive within the socket, and attaching the lens to the light-emitting side of the package, such that the plug is disposed within the socket. In some embodiments, providing the lens comprises machining a lens blank, such as by turning the lens blank, to form the plug of the lens. 
     Another exemplary method of fabricating a light-emitting device also comprises providing a package including an encapsulated LED configured to emit light toward a light-emitting side of the package, and a socket disposed on the light-emitting side of the package. In this exemplary method, a mold defining a shape of a cap is placed over the socket, material is flowed into a space formed by the mold and the socket, and the material is cured to form the lens in place. 
     Still another exemplary method is directed to attaching an LED lens to an LED package to form a light-emitting device. This exemplary method comprises introducing a bead of adhesive, such as silicone, having a convex surface and a viscosity of about 2000 to 4000 centipoise onto a surface of the LED package, contacting a point on the convex surface of the bead of adhesive with a surface of the LED lens, and spreading the adhesive between the surface of the LED package and the surface of the LED lens. In some embodiments, the contacted point on the convex surface of the bead of adhesive is at about an apex of the convex surface of the bead of adhesive. In various embodiments a width of the bead of adhesive is between about 30 percent to about 55 percent of a length of the surface of the LED package, and a height of the bead of adhesive is between about 20 percent to about 35 percent of a width of the bead of adhesive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an LED die bonded to an exemplary LED package according to an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of an exemplary embodiment of the LED package of the present disclosure. 
         FIG. 3  is a top view of an exemplary LED package of the present disclosure. 
         FIGS. 4A and 4B  are exemplary metallization patterns for a top surface of a thermally conducting layer of an LED package according to embodiments of the present disclosure. 
         FIG. 5  is an exemplary metallization pattern for the bottom surface of a thermally conducting layer of an LED package according to an embodiment of the present disclosure. 
         FIGS. 6A-6C  are cross-sectional views of several exemplary embodiments of an LED package of the present disclosure. 
         FIG. 7  is a top view of an LED package in accordance with another embodiment of the present disclosure. 
         FIG. 8  is a top view of a plurality of LED packages manufactured in parallel during an exemplary embodiment of a fabrication process. 
         FIG. 9  is a flowchart for a method according to an exemplary embodiment of the invention. 
         FIGS. 10 and 11  show cross-sectional views of an LED package and a lens in accordance with an embodiment of the present disclosure. 
         FIG. 12  is a cross-sectional view of an LED package and a lens prior to assembly in accordance with another embodiment of the present disclosure. 
         FIG. 13  is a cross-sectional view of the lens and LED package of  FIG. 12  assembled, in accordance with another embodiment of the present disclosure. 
         FIG. 14  is a cross-sectional view of an LED package and a lens prior to assembly in accordance with another embodiment of the present disclosure. 
         FIG. 15  is a cross-sectional view of the lens and LED package of  FIG. 14  assembled, in accordance with another embodiment of the present disclosure. 
         FIGS. 16-19  illustrate successive stages of bonding a lens to an LED package using an adhesive, in accordance with an embodiment of the present disclosure. 
         FIGS. 20-21  illustrate successive steps in fabricating a lens using an injection mold in accordance with an embodiment of the present disclosure. 
         FIG. 22  is a cross-sectional view of an exemplary embodiment of a lens blank of the present disclosure. 
         FIG. 23  is a bottom plan view of the lens blank of  FIG. 22 , in accordance with an embodiment of the present disclosure. 
         FIG. 24  is a side elevation of a lens machined from the lens blank of  FIG. 22 , in accordance with an embodiment of the present disclosure. 
         FIG. 25  is a bottom plan view of the lens of  FIG. 24 , in accordance with another embodiment of the present disclosure. 
         FIG. 26  is a bottom plan view of the lens of  FIG. 24 , in accordance with another embodiment of the present disclosure. 
         FIG. 27  is a flowchart for a method according to an exemplary embodiment of the invention. 
         FIG. 28  is a flowchart for a method according to an exemplary embodiment of the invention. 
         FIG. 29  is a flowchart for a method according to an exemplary embodiment of the invention. 
         FIG. 30  is a flowchart for a method according to an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure provides LED packages with structures and materials that provide higher heat dissipation than presently available. A further benefit of the present invention is improved matching of the coefficients of thermal expansion (CTES) of the LED dies and the materials to which they are bonded for higher reliability. Due to the improved heat conduction, the packages of the present invention allow high-power LEDs to operate at full capacity. Improved heat conduction also allows for both smaller packages and devices within which packages are placed more closely together. 
     One measure of how efficiently a package dissipates heat is the temperature rise across the package. Using this measure, in current high-power LED packages the thermal resistance from the junction to the case is generally in the range of 15 to 20° C./W. By comparison, an exemplary embodiment of the present disclosure has a lower thermal resistance of only about 6° C./W or 3° C./W for a four LED dice package. Therefore, the present disclosure enables LED devices for new applications in both high temperature environments (such as in an automobile engine compartment) and also in environments that cannot accommodate high temperature components (such as a dental curing light for use in a patient&#39;s mouth). 
     Accordingly, exemplary packages for high-power LEDs according to the present disclosure have the following features: 1) They offer higher performance by enabling 50% or greater luminosity per LED die as compared to prior art packages; 2) they provide a high thermal conductivity path to conduct heat away from LED dies; 3) they redirect light emitted at low solid angles (tangential light) into directions more nearly perpendicular to the surface of the LED die; and 4) they provide a material layer, for bonding to the LED die, having a CTE that is closely matched to the CTE of the LED die to minimize interfacial stresses and improve reliability. 
     The present disclosure provides embodiments for a package for a single high-power LED die in the 1 to 7 watt output power range that provides the desirable features discussed above. The present disclosure also provides embodiments to stabilize the wavelength (i.e., color) of LED dies. In the case of white LED applications, the present disclosure provides embodiments for improving white light LED efficiency. 
     The present disclosure also provides embodiments for a package for multiple high-power LED dies with a combined output in the 1 to 15 watt output power range. These packages have very small form factors and can be fabricated at low cost. The small form factors enable the design of light source optics with more compact sizes. Therefore, the present invention enables a new class of high-power LED-based light source and display applications to emerge. 
     The packages of the present invention can be used with LED devices that operate over the range of wavelengths from ultraviolet (UV) to Infrared (IR) which covers the range from 200 to 2000 nanometers. Further, packages of the present invention can include bonding pads configured to accommodate any of a number of different LED die designs that are presently available in the market. The present disclosure, in some embodiments, also provides a versatile package design whereby the thermal and electrical paths are separated. In this way, the package can be attached to a heat sink of a circuit board using either a thermally and electrically conductive epoxy or solder, or a thermally conductive and electrically non-conductive epoxy. 
       FIG. 1  is a perspective view of an exemplary LED package  100  according to an embodiment of the invention. To form a light emitting device, an LED die  110  is bonded to the LED package  100  as shown. The LED package  100  comprises a body  120  having a cavity  130  extending downward from a top surface  140  thereof. The cavity  130  includes a floor  150  for bonding to the LED die  110 . In some embodiments, the LED package  100  has a square footprint enabling multiple light emitting devices to be densely arranged in a square array. The LED package  100  is intended primarily for LED dies that produce 1-5 watts of power, but is not limited thereto. 
     In the embodiment shown in  FIG. 1 , a sidewall  160  of the cavity  130  is inclined at an angle so that the cavity  130  takes the shape of an inverted and truncated cone. The sidewall can also be vertical, or nearly so. In some embodiments the sidewall  160  of the cavity  130  is inclined at a 45° angle. Preferably, the sidewall  160  is highly reflective at a wavelength emitted by the LED die  110 . This can be achieved, for example, with a coating of a highly reflective material such as silver, though other materials can be used, depending on the wavelength of the light produced by the LED die  110 . Thus, the sidewall  160  can serve to redirect light emitted from the edges of the LED die  110 . The light from the edges of the LED die  110  is redirected in a direction perpendicular to a top surface of the LED die  110  so that the light emitted from the side surfaces of the LED die  110  adds to the light emitted from the top surface of the LED die  110 . In other embodiments the sidewall  160  takes a parabolic shape to better focus the redirected light. 
       FIG. 2  is a cross-sectional view of one exemplary embodiment of an LED package  200  of the present disclosure. It can be seen from  FIG. 2  that the LED package  200  comprises three layers (embodiments with four layers are described elsewhere herein) designated from top to bottom as a top body layer  210 , an intermediate body layer  220  and a thermal conduction layer  230 . The thermal conduction layer  230  has a bottom surface  235 . A LED die  240  can be bonded to a top surface of thermal conduction layer  230  within a cavity  250  formed through layers  210  and  220 . A thickness of intermediate body layer  220  is designed to be approximately the same as a thickness of a die attach layer  245  that bonds the LED die  240  to the thermal conduction layer  230 . Also, in some embodiments a metallization layer on a sidewall  255  of the top body layer  210  extends from a top rim  260  at a top surface  280  of the top layer  210  to a bottom rim  270  near a bottom surface of the top body layer  210 . 
       FIG. 3  is a top view of the exemplary LED package  200  of  FIG. 2 . The top rim  260  and the bottom rim  270  correspond to the outer diameter and the inner diameter of the cavity  250  and are represented by two circles  260  and  270 , respectively. It can be seen that the LED die  240  is positioned within the inner diameter  270 . This embodiment also includes partial vias  290 ,  292 ,  294 , and  296 , one at each of the four corners of the LED package  200 . The partial vias  290 ,  292 ,  294 , and  296  are metallized, in some embodiments, to serve as electrical paths. 
     The thermal conduction layer  230  includes a thermally conductive material, which preferably has a thermal conductivity greater than about 14 W/m° K, and more preferably has a thermal conductivity greater than 150 W/m° K. Depending on applications, power density, desired package size and thickness of the several layers, a variety of thermally conductive materials can be used to form the thermal conduction layer  230 . Such materials include, but are not limited to, aluminum nitride (AlN), alumina (Al 2 O 3 ), Alloy 42, copper (Cu), copper-tungsten (Cu/W) alloy, aluminum silicon carbide, diamond, graphite, and beryllium oxide. In addition to thermal conductivity, the coefficient of thermal expansion (CTE), the fracture toughness, Young&#39;s modulus, and cost are other parameters to be considered in selecting the material for the thermal conduction layer  230 . 
     Matching the CTE of the thermally conductive material with that of the LED die reduces interfacial stresses and therefore improves reliability. Preferably, the CTE of the thermally conductive material should be less than 15 parts per million per degree centigrade (ppm/° C.) in order to more closely match the CTE of typical LED die materials such as silicon. The mismatch in the CTEs between the LED package and the LED die according to embodiments of the present disclosure is about 4.7:3, whereas for prior art packages the best ratios are about 17:3. Improved heat dissipation allows packages of the present disclosure to have a smaller footprint and to be thinner than prior art packages. An exemplary embodiment of the present disclosure has dimensions of 4.4 mm×4.4 mm×0.9 mm vs. prior art packages that measure 14 mm×7 mm×2.5 mm. 
     The thermal conduction layer  230 , with the help of layers  210  and  220  in some embodiments, dissipates much of the heat generated by the LED  240 . For applications that demand the highest thermal dissipation capabilities, each of the three layers  210 ,  220 , and  230  comprise ceramic AlN. AlN is desirable because it combines high thermal conductivity with a CTE that is very similar to that of LED substrate materials, such as SiC, sapphire, or silicon, the material from which solid-state LEDs are most frequently fabricated. However, Al 2 O 3  can also be used for these layers for other applications. For some applications, thermal conduction layer  230  is made from either AlN or Al 2 O 3  while layers  210  and  220  are made of other suitable materials including plastics and metals such as copper, aluminum, and Alloy 42. For some applications it is desirable to use the thermal conduction layer  230  as the primary thermal conduction path away from the LED die  240  in order to prevent heat from being directed towards the top of the package  200 . For example, it may be desirable to keep the top of the light emitting device cool to the touch. 
     It will be appreciated that the package  200  does not need to be formed from three layers as illustrated by  FIG. 2 ; more or fewer layers also can be used. For example, an embodiment with four layers is also described herein. Ceramic processing techniques can also be used to form the body as an integral unit. However, a layered configuration is desirable for the ease of fabrication. For some applications with secondary lenses, layers  210  and  220  are optional. 
     It will also be appreciated that heat produced by the LED die  240  is dissipated from the package  200  primarily through the thermal conduction layer  230 . Consequently, layer  230  preferably has a thickness that is optimized for thermal conductivity therethrough. It has been found that for a given material, the thermal conductivity decreases if layer  230  is either too thin or too thick and, accordingly, there is an optimal thickness for optimal thermal conductivity. In the embodiment where AlN ceramic is used for a thermal conduction layer  230 , the optimal thickness of layer  230  is in a range of 0.2 mm to 0.4 mm, and ideally about 0.3 mm. 
     It will be appreciated that the LED package  200  may be further attached to a heat sink (not shown) along the bottom surface  235 . In addition, to optimize heat dissipation from the package  200  to the heat sink, the die attach layer  245  is preferably also thermally conductive. In the present disclosure, for a thin layer to be characterized as being thermally conductive, the material of the layer should have a thermal conductivity of at least 0.5 W/m° K, and ideally about 50 W/m° K. 
     In some embodiments, the thermal conductivity of the die attach layer  245  is desirably at least 1 W/m° K. The die attach layer  245  can comprise, for example, an electrically conductive epoxy, a solder, a thermally conductive and electrically non-conductive epoxy, or a nano-carbon-fiber filled adhesive. In some embodiments as discussed below, where the LED die  240  needs to make an electrical connection with the thermal conduction layer  230  through a central pad, the die attach layer  245  is also electrically conductive. In this disclosure, a thin layer material is considered to be electrically conductive if it has a volume resistivity less than 1×10 −2  ohm-meter. A material for an electrically conductive die attach layer  245  desirably has a volume resistivity less than 1×10 −4  ohm-meter. 
     The thermal conduction layer  230 , in accordance with the present disclosure, may be either electrically conductive or electrically nonconductive. As described below, where the thermal conduction layer  230  is electrically nonconductive, the present disclosure uses a metallization pattern for the top surface of the thermal conduction layer  230  to provide necessary electrical contacts. This unique design makes it possible to fabricate the thermal conduction layer  230  from thermally conductive materials that are not electrically conductive, such as ceramics. Electrically nonconductive materials have conventionally been considered unsuitable for making heat slugs. 
       FIG. 4A  illustrates an exemplary metallization pattern for the top surface of thermal conduction layer  230  of the LED package  200  of  FIGS. 2 and 3 . It can be seen that a generally square central pad  410  is connected by a trace  420  to one of the four partial vias ( 290 ,  292 ,  294 , and  296 ), and partial via  294  particularly in  FIG. 4A . Nickel and tungsten are exemplary metals for the metallization. The bottom surface of the LED die  240  is bonded, for example by solder, a thermally and electrically conductive adhesive, or a thermally conductive and electrically non-conductive adhesive, to the central pad  410 . It will be appreciated that in those embodiments in which the central pad  410  for bonding the LED die  240  is not electrically conductive, the central pad  410  can be merely a region on the floor of the cavity rather than a patterned layer of some material on the floor of the cavity. In other words, the die attach layer  245  bonds the LED die  240  directly to the floor of the cavity in the central pad region. 
     The central pad  410  is surrounded on three sides by three bonding pads  430 ,  440 , and  460 , each connected to one of the remaining three partial vias  290 ,  292 , and  296 . An electrical contact (not shown) on the top surface of the LED die  240  is wire bonded to one of these three bonding pads  430 ,  440 , and  460  where exposed on the floor of the cavity  250  (i.e., within the circle  270 ). The four partial vias  290 ,  292 ,  294 , and  296  connect the bonding pads  430 ,  440 , and  460  to external electrical contacts (not shown) on either the top of layer  210  or the bottom of layer  230 , or both. These external electrical contacts provide leads to a power source on a circuit board. It can be seen from  FIGS. 2-4  that after the package  200  is fully assembled most of the metallization pattern shown in  FIG. 4A  is sandwiched between layers  230  and  220  and hidden from view. 
     In the embodiment shown above in  FIG. 4A , the central pad  410  serves both as an electrical connector and a thermal bonding pad between the LED die  240  and the top surface of the thermal conduction layer  230 . To facilitate electrical connection, the LED die  240  may be either directly bonded to the central pad  410  or attached thereto using an electrically conductive adhesive. In this disclosure, an adhesive is considered to be electrically conductive if it has a volume resistivity less than 1×10 −2  ohm-meter. For better performance, an electrically conductive adhesive desirably should have a volume resistivity less than 1×10 −4  ohm-meter. It should be understood, however, that in some embodiments the central pad  410  serves as a thermal bonding pad but not as an electrical connector, as described elsewhere herein. In such embodiments, the central pad  410  is not connected to one of the partial vias  290 ,  292 ,  294 , and  296 . Instead, all partial vias  290 ,  292 ,  294 , and  296  are connected to a respective side pad (such as the side pads  430 ,  440 , and  460 ). 
       FIG. 4B  illustrates another exemplary metallization pattern for the top surface of thermal conduction layer  230  of the LED package  200  of  FIGS. 2 and 3 . In this embodiment a first pad  470  is connected to two partial vias  292 ,  294 , and a second pad  480  is connected to the other two partial vias  290 ,  296 . An exemplary spacing between the first and second pads  470  and  480  is 0.10 mm. Nickel, tungsten, and silver are exemplary metals for the metallization. In some embodiments, silver is coated over another metal, such as nickel. Line  490  indicates where the bottom surface of the LED die  240  is bonded to the first pad  470 . One benefit of the exemplary metallization pattern of  FIG. 4B , compared to the metallization pattern shown in  FIG. 4A , is that a greater area of the floor of the cavity within the inner diameter  270  is metallized, which serves to reflect a greater amount of light upward and out of the package. 
       FIG. 5  illustrates an exemplary metallization pattern for the bottom surface  235  of the thermal conduction layer  230 . In this embodiment, a centrally located pad  510  provides a thermal path from the bottom  520  of layer  230  to a substrate (not shown) to which the package  200  is attached. The substrate can include a heat sink. The pad  510  is circular or square in some embodiments, but is not limited to any particular shape. 
     Each of the four partial vias  290 ,  292 ,  294 , and  296  at the corners of the package  200  connect to one of the separate semi-circular electrical contacts  530 ,  540 ,  550 , and  560 , respectively. One of the four semi-circular electrical contacts,  550  in this particular embodiment, is connected through one of the four partial vias ( 294  in this case) and trace  420 , as shown in  FIG. 4A , to the central pad  410 , while the other three semi-circular electrical contacts ( 530 ,  540 , and  550  in this embodiment) connect to the three bonding pads  430 ,  440 , and  460 , respectively. Thus, when attached to the substrate, the centrally located pad  510  is soldered (or otherwise bonded, such as with a thermally conductive epoxy) to the substrate for heat dissipation and two of the four semi-circular electrical contacts  530 ,  540 ,  550 , and  560  are connected to electrical contacts on the substrate to provide an electrical path through the LED package  200  and to the LED die  240 . One of the two semi-circular electrical contacts ( 550  in this embodiment) connects through the central pad  410  to the bottom of the LED die  240 , while the other (any one of  530 ,  540 , and  560 ) is connected through its respective side bonding pad ( 430 ,  440 , and  460 ) to the top of the LED die  240  by a wire bond (not shown). The particular semi-circular electrical contact  530 ,  540 , or  560  that is used to connect to the LED die  240  is determined according to the characteristics and the requirements of the particular LED die  240 . 
     It will be understood that by having an arrangement of several bonding pads in a number of different locations enables the same package to be used with different LED designs. Thus, an LED from one manufacturer may be bonded to one set of bonding pads while an LED from another manufacturer may be bonded to another set of bonding pads. In this respect the package is universal to different LEDs from different sources. Further still, the design of the package of the present invention allows for flexible and simple processes for attaching LEDs to the packages. 
     In alternative embodiments, the top surface  280  of the top body layer  210  has a metallization pattern to provide electrical contacts rather than the bottom surface of the thermal conduction layer  230 . Each of the partial vias  290 ,  292 ,  294 , and  296  at the corners and sides of the LED package  200  connect to a separate electrical contact on the top surface  140  of the top body layer  210 . In these embodiments wire bonds to the electrical contacts on the top surface  140  of the top body layer  210  connect the LED package  200  to a power source or a circuit board. Locating the electrical contacts on the top of the package  200  rather than the bottom provides a greater area of contact between the bottom surface  235  and the substrate for even greater heat dissipation. The LED package  200  in these embodiments can be bonded to a substrate, for example, by solder or thermally conductive epoxy. The bond does not have to be electrically conductive. 
     It will be appreciated that the packages of the present disclosure provide improved heat dissipation in several ways, some of which are listed as follows. In some embodiments, the use of a material having superior thermal conductivity for the thermal conduction layer  230  improves heat dissipation. In other embodiments, the accommodation for an electrically nonconductive material for thermal conducting makes it possible to use unconventional thermally conductive materials, for example AlN ceramic, to form the thermally conducting layer. In other embodiments, optimizing the thickness of the thermal the conducting layer  230  further improves heat dissipation. In still other embodiments, providing a large area of contact between the bottom surface  235  of thermal conduction layer  230  and the substrate to which it attaches can further improve heat dissipation. In some embodiments, the packages of the present disclosure also direct a greater percentage of light out of the package, both reducing the heating of the package from absorbed light and increasing the light production efficiency. 
     Because of the improved heat dissipation, exemplary packages according to the present disclosure exhibit thermal resistances of about 6° C./W at an output greater than 1 watt per package. Exemplary packages according to the present disclosure with four LED dice exhibit a thermal resistance of 3° C./W, with outline dimensions of 7 mm×7 mm×1 mm. The present disclosure also makes highly compact LED packaging possible. In some exemplary packages, the square LED package has a width and length of about 4.4 mm and a thickness of about 1 mm (with thicknesses of about 0.5 mm, 0.1 mm and 0.3 mm for the top body layer, the intermediate body layer and the thermally conducting layer, respectively). The present disclosure therefore enables high-power LEDs to be used in higher-temperature environments, such as in automotive engine compartments, as well as in applications where high-temperature components cannot be tolerated, such as in dental applications, for example, in an illumination device used to cure dental cements. 
     The features disclosed in the present disclosure can be combined with other techniques of LED packaging. For example, the package of the present disclosure can further use encapsulating techniques as described in the U.S. patent application Ser. No. 11/036,559, entitled “Light Emitting Device with a Thermal Insulating and Refractive Index Matching Material,” filed on Jan. 13, 2005, which is incorporated by reference herein. 
       FIG. 6A  is a cross-sectional view of another exemplary embodiment of the LED package of the present disclosure. From top to bottom, the LED package  600  comprises layers  610 ,  620 , and  630 . Similar to the LED package  200  of  FIG. 2 , layer  610  is a top body layer, layer  620  is an intermediate body layer, and layer  630  is a thermal conducting layer. An LED die  640  mounted to a top surface of thermal conducting layer  630  through an LED die attach layer  645 . A thermal insulation layer  650  and a luminescent layer  655  are placed in a tapered cavity having the shape of an inverted cone. The cavity has a side wall extending from a top rim  660  to a bottom rim  670 . The LED package  600  also has an auxiliary member  680  enclosing the package from the top. The auxiliary member  680  is optional and can be, for example, an optical lens for focusing the light emitted from the LED package  600 . The auxiliary member  680  can also serve as a protective capping layer. 
     It can be seen that the thermal insulation layer  650  is disposed between the luminescent layer  655  and a top surface of the LED die  640 . The thermal insulation layer  650  at least partially protects the luminescent material in the luminescent layer  655  from the heat produced by the LED die  640 , thus, better maintaining thermal properties, such as light conversion efficiency and output wavelength, at or near optimal values far longer than under the prior art. The thermal insulating material of thermal insulation layer  650  can also be a material with an index of refraction chosen to closely match that of the material of the LED die  640 . 
     The use of a thermal insulating material to protect the luminescent material within the encapsulant member from the heat produced by the LED is made particularly effective when applied in the LED packages of the present disclosure. It will be appreciated that prior art light emitting devices do not include thermal insulation to protect phosphors from the heat generated by the LEDs because heat dissipation has been an overriding concern in such devices. Put another way, designers of prior art light emitting devices have sought to dissipate as much heat as possible through the phosphor-containing layers (e.g., luminescent layer  655 ) because to do otherwise would require too much heat dissipation through the remainder of the light emitting device. However, where the thermally conducting layer  630  provides sufficient heat conduction, it is no longer necessary to conduct heat through the phosphor-containing luminescent layer  655 , and thermal insulation can be introduced to shield the luminescent materials. 
     The thermal insulation layer  650  is preferably transparent, or nearly so, to the light emitted from the LED die  640 . The thermal insulating material is therefore preferably transparent to at least one wavelength emitted by the LED die  640 . The wavelengths emitted by various available LEDs extend over a wide spectrum, including both visible and invisible light, depending on the type of the LED. The wavelengths of common LEDs is generally in a range of about 200 nm-2000 nm, namely from the infrared to the ultraviolet. 
     In order to effectively thermally insulate the luminescent layer  655 , the thermal insulating material of the thermal insulation layer  650  should have a low thermal conductivity, desirably with a thermal conductivity of no more than 0.5 watt per meter per degree Kelvin (W/m° K), and more desirably with a thermal conductivity of no more than 0.15 W/m° K. The thermal insulating material for the thermal insulation layer  650  desirably also has high heat resistance, preferably with a glass transition temperature, T g , above 170° C., and more preferably a glass transition temperature above 250° C. Furthermore, in order to have good thermal compatibility and mechanical compatibility between the thermal insulation layer  650  and other components, especially the LED die  640 , which are typically semiconductor materials, the thermal insulating material desirably has a coefficient of thermal expansion no greater than 100 ppm/° C., and more desirably a coefficient of thermal expansion no greater than 30 ppm/° C. 
     Luminescent materials suitable for the present invention include both fluorescent materials (phosphors) and phosphorescent materials. Phosphors are particularly useful for LED-based white light sources. Common phosphors for these purposes include Yttrium Aluminum Garnet (YAG) materials, Terbium Aluminum Garnet (TAG) materials, ZnSeS+ materials, and Silicon Aluminum Oxynitride (SiAlON) materials (such as .alpha.-SiAlON). 
     The present invention also provides a light emitting device comprising a package of the invention configured with an LED die and a luminescent material. In one embodiment, light emitting device produces white light based on a monochromatic LED. This can be done, for example, by using a visible light LED and a compensatory phosphor, or by using an invisible light LED together with RGB phosphors. For instance, a blue LED-based white light source produces white light by using a blue light LED and a phosphor that produces a yellowish light. 
       FIGS. 6B and 6C  show cross-sections of additional embodiments of the LED package  600 . In  FIG. 6B  the top body layer  610  includes a circular notch  685  to receive a lens  690 . The lens  690  can be glass or plastic, for example. The notch  685  beneficially provides a guide that centers the lens  690  over the LED die  640  during assembly. In some of these embodiments, the top body layer  610  comprises a metal such as a copper-tungsten (Cu/W) alloy. The tapered cavity and the notch  685 , in some of these embodiments, are formed by a stamping operation. In further embodiments, the intermediate body layer  620  and the thermal conducting layer  630  are also made of alumina. 
     In  FIG. 6C  the LED package  600  comprises an alignment layer  695  placed above the top body layer  610 . A circular aperture in the alignment layer  695  creates essentially the same guide for the lens  690  as described above with respect to  FIG. 6B . The alignment layer  695  can include, for example, metal or ceramic. In those embodiments in which layers  610 ,  620 , and  630  include AlN, the alignment layer  695  can also include AlN. 
     The LED package of the present invention, in some embodiments, can support multiple LED dies within a single package to further increase the output level and density.  FIG. 7  is a top view of an LED package  700  in accordance with another embodiment of the present disclosure. The LED package  700  is similar to the LED package  200  in  FIGS. 2-5 , except that the LED package  700  contains multiple LEDs ( 710 A,  710 B,  710 C, and  710 D) instead of a single LED. The top view of the LED package  700  shows the cavity  730 , the top surface  740 , the outer diameter  760  and the inner diameter  770  of the cavity  730 , and the four partial vias  790 ,  792 ,  794 , and  796 . In the particular embodiment shown in  FIG. 7 , the LED package  700  includes four LEDs  710 A,  710 B,  710 C, and  710 D, although in principle any other number of LEDs may be arranged in a package of the present invention. The four LEDs  710 A,  710 B,  710 C, and  710 D can be the same or different, and in some embodiments are independently operable. For example, the multiple LEDs ( 710 A,  710 B,  710 C and  710 D) may be selectively operable and may be operable in any combination. The LED package  700  is intended to provide an LED package capable of producing an output of 1-15 watts with a thermal resistance of 3° C./W, but is not limited thereto. 
     Methods are disclosed for fabricating a layered LED package as described with reference to  FIGS. 2-7 . The methods vary depending upon the materials selected for each layer, specific designs, such as the pattern of metallization and the location and routing of the electrical connections, and applications of the LED package. In those embodiments shown in  FIGS. 2-5  and in which all three layers  210 ,  220 , and  230  are made of a ceramic, for example, the layers  210 ,  220 , and  230  can be manufactured separately, stacked together, and co-fired (sintered) to bond the layers  210 ,  220 , and  230  together. When non-ceramic materials are used for layers  210  and  220 , however, the layers  210  and  220  can be bonded together with suitable adhesives or solders. 
     In one embodiment of the method of the invention, multiple LED packages are formed together in a batch process in which the individual LED packages are fabricated in parallel as a panel  800  from which individual LED packages can later be separated.  FIG. 8  shows a top view of a plurality of LED packages  810  manufactured in parallel during an exemplary embodiment of a fabrication process. In this embodiment, the LED packages  810 , which can be fabricated to include LED dies  820 , are assembled in a square grid pattern separated by snap lines  830 . Rows or columns of the packages  810  can be snapped apart along the snap lines  830 , and then further sub-divided into individual LED packages  810 . According to this embodiment, each of the top body layer (e.g.,  210 ), the intermediate body layer (e.g.,  220 ) and the thermally conducting layer (e.g.  230 ) for the plurality of LED packages  810  is produced as a whole piece, and each layer is independently fabricated as a sheet and then bonded together. LED dies  820  can be added to the grid of LED packages  810  before the grid is separated into individual LED packages  810 . 
     Easily fractured materials, such as ceramics, are particularly suited for the above described embodiment. Separating the grid into the individual LED packages  810  would be difficult if a metal, such as copper, is used to form a bottom plate for heat dissipation. If a material that is not easily fractured is used for any of the three layers (e.g., the top body layer  210 , the intermediate body layer  220  and the thermal conduction layer  230 ), it may be necessary to prepare such layers along the snap lines  830  with deep grooves or perforations to facilitate separation. 
     The grid in  FIG. 8  also includes an array of vias (holes)  840  along the snap lines  830 . Each via  840  is shared by four neighboring LED packages  810 , except for those located at an edge or corner which would be shared by either one or two neighboring LED packages  810 . After the individual LED packages  810  are separated along the snap lines  830 , the vias  840  are separated apart to become partial vias (e.g.,  290 ,  292 ,  294  and  296 ). 
     To produce a thermally conducting layer (e.g.,  230  or  630 ) using a ceramic material according to a particular embodiment, for example, a ceramic layer of a material such as AlN is prepared with a square array of vias  840  disposed therethrough. The vias  840  sit at the intersections of the snap lines  830  in  FIG. 8 . Ultimately, when the LED packages  810  are separated from one another, each via  840  becomes a partial via (e.g.,  290 ,  292 ,  294 , and  296 ) of four different neighboring packages  810 . The top and bottom surfaces of the ceramic layer are patterned, in exemplary embodiments, with metallization as shown in  FIGS. 4 and 5 . Patterning can be achieved, for example, by plating. Suitable metals for the metallization include tungsten and nickel. These patterns are repeated for each package  810  that will be produced. 
     Various patterns of metallization may be used to achieve different effects and to suit the different requirements of the LED dies  820 . In some embodiments, for example, the central pad (e.g., the central pad  410  in  FIG. 5 ) serves both as a thermal contact and an electrical contact. In these embodiments, the central pad on the top surface of the thermally conducting layer ( 230 ) is connected by a trace ( 420 ) to one of the partial vias ( 294 ) so that an electrical connection extends from the central pad to the opposite surface of the thermally conducting layer. If desirable, the electrical connection may be further extended to the central pad ( 510 ). In these embodiments, a small patch of AlN, or another material, can be placed over the trace ( 420 ) between the central pad and the partial via to prevent solder from flowing along the trace during soldering. 
     To produce an intermediate body layer (e.g., layer  220 ), according to this embodiment, a layer of a material such as AlN is prepared with a square array of vias disposed therethrough. The square array of vias matches the square array of vias in the thermally conducting layer. Additionally, a square array of apertures is defined in the layer such that each aperture is centered in a square defined by four adjacent vias. These apertures correspond to the inner diameter of the cavity (e.g., the inner diameter  270  in  FIGS. 2-5 ) of the respective LED package. 
     To produce a top body layer (e.g., layer  210 ), according to this embodiment, a layer of a material such as AlN is prepared with a square array of vias disposed therethrough. The square array of vias matches the square arrays in the thermally conducting layer and the intermediate body layer. Additionally, a square array of apertures is defined in the layer such that each aperture is centered in a square defined by four adjacent vias. The array of apertures on the top body layer match the array of apertures on the intermediate body layer but have a different diameter. These apertures are preferably inclined or otherwise shaped to provide a sidewall as discussed above with respect to  FIGS. 1-3 . Specifically, in a preferred embodiment, each inclined aperture has a top rim that corresponds to the outer diameter (e.g., the outer diameter  260  in  FIGS. 2-5 ) of the cavity in the respective LED package, and a bottom rim that corresponds to the inner diameter (e.g., the inner diameter  270  in  FIGS. 2-5 ) of the cavity in the respective LED package  810 . The top body layer is then metallized to provide sidewall metallization and any electrical contacts for the top surface. For the embodiments that do not require electrical contacts for the top surface of the top body layer, no electrical contacts are formed on the top surface. 
     Once the thermally conducting layer, the intermediate layer and the top body layer are individually prepared, the three layers are brought together in an assembly, the vias in each layer are aligned, and the three layers are bonded together. As noted above, where all three layers are ceramic the assembly can be co-fired, else the layers can be bonded together with a suitable adhesive or solder. In the latter embodiments, the adhesive can serve to electrically insulate the metallization on the top surface of the thermally conducting layer (e.g., metallization pattern shown in  FIG. 4A ) from an intermediate layer comprising a metal such as copper. Once the layers have been bonded to one another, the vias  840  can be plated to provide electrical connections between metallizations on the various surfaces of the layers. 
     Although the LED packages  810  can be separated at this point for subsequent fabrication into light emitting devices, it is often desirable to first attach LED dies  820  to form an entire panel  800  of light emitting devices in parallel. To create a panel  800  of light emitting devices, solder flux or a thermally conductive die-attach is dispensed and the LED dies  820  are bonded to the LED packages  810 . Then, each LED die  820  is wire bonded to the appropriate bonding pads. Preferably, the cavities of the LED packages  810  are next filled to encapsulate the LED dies  820 . In some embodiments this process includes forming a thermally insulating layer over the LED die  820 , forming a luminescent layer over the thermally insulating layer, and then forming a lens over the luminescent layer. Finally, the assembly is diced along the snap lines  830 . It will be appreciated that the light emitting devices of the present invention can be manufactured with fewer processing steps than prior art devices, in some instances fewer than half as many steps. 
     To produce an embodiment such as that shown in  FIG. 6C , in which a ceramic alignment layer  695  is included, the method described above can be modified so that the alignment layer  695  is co-fired together with the thermally conducting, intermediate, and top body layers. Alternately, a metal alignment layer  695  can be bonded to the top body layer with a suitable adhesive or solder. 
     In those embodiments that include an alignment mechanism for aligning a lens such as lens  690  in  FIGS. 6B and 6C , the lens can be added to the package  810  in a number of different ways. In some embodiments, a vacuum tool is used to pick up a lens and move the lens into position. In other embodiments a number of lenses are held on a strip of tape; a lens on the tape is aligned with the package  810  and a tool presses the lens into the guide to transfer the lens from the tape and-to the package  810 . It will be appreciated that lens transfer by vacuum tool or from tape can be achieved either before or after the LED packages  810  are separated from one another. 
     In an exemplary batch process that can be performed before the LED packages  810  are separated from the panel  800 , the lenses are formed by injection molding. In this process a mold having an array of lens-shaped wells is sealed to the panel  800  so that one well is aligned with each of the packages  810 . A suitable plastic is injected into the mold to fill the wells. The plastic is then cured to form the lenses. In another exemplary batch process, the lenses are formed by mask printing. 
       FIG. 9  depicts a method  900  according to an exemplary embodiment of the invention. Method  900  comprises a step  910  of forming a panel, a step  920  of defining a grid on a surface of the panel, an optional step  930  of bonding an LED die within a cavity of the panel, and a step  940  of separating a unit from the panel by breaking the panel along lines of the grid. In those embodiments in which method  900  is directed to forming an LED package, step  930  is omitted and the resulting unit, the LED package, does not have an LED die. The LED die can be subsequently added to the package to form a light emitting device. In those embodiments in which method  900  is directed to forming a light emitting device, the LED die is added to a cavity within the panel in step  930  before the unit, in this case the light emitting device, is separated from the panel in step  940 . 
     The present disclosure further provides LED package structures and lens structures that provide stronger attachments between the lenses and the LED packages. The improved attachment provides increased resistance to separation of the lenses from the LED packages caused by mismatched CTEs, or thermal effects such as adhesive degradation. The improved attachment further provides resistance to separation due to mechanical stresses to the devices, or due to shear forces between the lenses and the LED packages. The present disclosure further provides methods for lens fabrication. The present disclosure also provides a method for applying an adhesive layer between a lens and an LED package that reduces internal reflection of light within the adhesive layer, which increases total luminosity and overall efficiency of the LED device. 
       FIGS. 10 and 11  show cross-sectional views of an LED package  1010  and a lens  1050  of the present disclosure.  FIG. 10  shows the lens  1050  and the LED package  1010  prior to assembly and  FIG. 11  shows the lens  1050  mounted on the LED package  1010 . The LED package  1010  includes an encapsulated LED  1040 , a socket  1020 , and a top surface  1035 . The encapsulated LED  1040  is configured to emit light toward a light-emitting side of the LED package  1010 . The socket  1020  is disposed on the light-emitting side of the LED package  1010  and is defined by a sidewall  1025  and a socket floor  1030 . The sidewall  1025  can be a continuous surface around the periphery of the socket floor  1030  of the socket  1020 . The top surface  1035  is exterior to the socket  1020  and may form a generally annular surface about the periphery of the socket  1020 . 
     In some embodiments, the LED package  1010  includes various layers (e.g., a thermal conducting layer, a thermal insulation layer, a luminescent layer, protective capping layer, an intermediate body layer, a top body layer, etc.) as described elsewhere herein. As discussed with respect to  FIG. 6C , an alignment layer with an aperture, such as the alignment layer  695 , can be used to form the socket  1020 . 
     The lens  1050  includes a cap  1060 , a plug  1070 , and a lower surface  1065 . In plan view, the lens  1050  can be circular, oval, rectangular, or various other shapes. In various embodiments, the cap  1060  is convex, concave, or various other shapes configured to focus, disperse, mask, or otherwise modify light emitted from the LED package  1010 . Examples of other shapes for the cap  1060  include an asymmetric shape, a Fresnel surface, a collimating lens, etc. In some embodiments, the lens  1050  has a mushroom shape. In some embodiments, the surface of the cap  1060  is configured to diffuse light exiting the lens  1050 , for example using a textured surface. 
     The plug  1070  is configured to fit into the socket  1020  and includes a sidewall  1075 . The sidewall  1075  forms a continuous surface around the periphery of the plug  1070 . The plug  1070  mechanically stabilizes the attachment of the lens  1050  to the LED package  1010  and serves to resist shear forces between the lens  1050  and the LED package  1010  that can result in the lens  1050  becoming loose and/or separating from the LED package  1010 . The lens  1050  can be secured to the LED package  1010  using a press fit, a friction fit, or an interference fit between the sidewall  1075  of the plug  1070  and the sidewall  1025  of the socket  1020 . 
     The plug  1070  further provides additional bonding surfaces such as the lower surface  1065  and the sidewall  1075  for securing the lens  1050  to the LED package  1010 . For example, the lens  1050  can be secured to the LED package  1010  using an adhesive  1110  between the lens  1050  and the LED package  1010 . In some embodiments, the adhesive  1110  is applied to the top surface  1035  and bonds the cap  1060  to the LED package  1010 . Alternatively, the adhesive  1110  can form a layer between the lens  1050  and the LED package  1010 . In some embodiments, the adhesive  1110  forms a layer that extrudes between the top surface  1035  of the LED package  1010  and the cap  1060 . While the cap  1060  is illustrated in  FIG. 11  as having a width  1080  that is less than a width  1085  of the LED package  1010 , the width  1080  of the cap  1060  can be greater than the width  1085  of the LED package  1010 . 
       FIGS. 12 and 13  show cross-sectional views of an LED package  1210  and a lens  1250  of the present disclosure.  FIG. 12  is a cross-sectional view of the LED package  1210  and the lens  1250  prior to assembly and  FIG. 13  is a cross-sectional view of the lens  1250  mounted to the LED package  1210 . The sidewall  1225  defines an angle “Φ” with respect to a socket floor  1230 , and a sidewall of the lens  1250  defines at an angle “13” with respect to a lower surface  1265 . The angles Φ and β as illustrated in  FIGS. 12 and 13  are about 135 degrees. However, the angles Φ and β can include a range of angles. For example, the range of angles can include about 90 degrees to about 140 degrees. In some embodiments, the angle Φ is the same as the angle β. When the angles Φ and β are less than 90 degrees, the lens sidewall  1275  and the socket sidewall  1225  can form an interference or “snap” fit. While the lens  1250  is illustrated in  FIG. 13  as having a width  1280  that is less than a width  1285  of the LED package  1210 , width  1280  of the lens  1250  can be greater than the width  1285  of the LED package  1210 . 
       FIGS. 14 and 15  show cross-sectional views of the LED package  1010  of  FIG. 10  and lens  1250  of  FIG. 12 .  FIG. 14  is a cross-sectional view of the LED package  1010  and the lens  1250  prior to assembly and  FIG. 15  is a cross-sectional view of the lens  1250  mounted to the LED package  1010 . As discussed elsewhere herein, the angle β includes a range of angles, for example, greater than 90 degrees to about 140 degrees. The inclined sidewall  1275  can accommodate horizontal misalignment between the lens  1250  and the LED package  1010  during assembly of the lens  1250  with the LED package  1010 . For example, the inclined sidewall  1275  can guide the lens  1250  into place in the socket  1020  when using vacuum handling equipment to place the lens  1250 , as discussed elsewhere herein. While the lens  1250  is illustrated in  FIG. 15  as having a width  1280  that is less than the width  1085  of the LED package  1010 , width  1280  of the lens  1250  can be greater than the width  1085  of the LED package  1010 . 
     Air bubbles in a transparent, adhesive layer between the LED package  1010  and the lens  1050  can reduce the transparency of the adhesive layer. The number of air bubbles can be reduced by depositing an adhesive bead having a preferred shape and size on the LED package  1010 .  FIGS. 16-19  illustrate successive stages of bonding a lens  1050  to an LED package  1010 .  FIG. 16  is a cross-sectional view of an adhesive bead  1610  applied to an LED package  1010  before attaching the lens  1050 . The adhesive bead  1610  is characterized by a convex bead surface  1620 , a width  1630 , and a height  1640 . In some embodiments, the width  1630  of the adhesive bead  1610  is in a range of about 30 percent to about 50 percent of a width  1650  of the socket  1020 . In some embodiments, the height  1640  of the adhesive bead  1610  is in a range of about 20 percent to about 35 percent of the width  1630  of the adhesive bead  1610 . In some embodiments, the viscosity of the adhesive bead  1610  is in a range of about 2000 to 4000 centipoise. 
     The adhesive  1110  can be transparent to light in wavelengths emitted by the encapsulated LED  1040  and/or emitted by a luminescent layer within the LED package  1010 . In some embodiments, the adhesive includes luminescent material and/or forms a luminescent layer. Other examples of adhesive properties include, an electrically conductive epoxy, a solder, a solder mixed with glass beads, a thermally conductive and electrically non-conductive epoxy, or a nano-carbon-fiber filled adhesive. Suitable adhesives include silicone, epoxy, etc. In some embodiments, a preformed ring including solder mixed with glass beads can be disposed on the top surface  1035  and form a hermetic seal between the LED package  1010  and the lens  1050 , or between the LED package  1210  and the lens  1250 , or between the LED package  1010  and the lens  1250 . 
       FIG. 17  is a cross-sectional view of the lens  1050  in contact with the adhesive bead  1610 . In  FIG. 17 , the lower surface  1065  of the lens  1050  is illustrated making initial contact with the adhesive bead  1610  at a point at or near an apex of the convex bead surface  1620 .  FIG. 18  is a cross-sectional view of the lens  1050  applying a force to the adhesive bead  1610 . The force “F” spreads the adhesive bead  1610  across the lower surface  1065  of the lens  1050  and the socket floor  1030  of the LED package  1010 . Surface tension of the adhesive bead  1610  can combine with wetting of the lower surface  1065  of the lens  1050  (and the socket floor  1030  of the LED package  1010 ) to maintain the convex shape in the convex bead surface  1620  thereby reducing the likelihood of trapping air bubbles in the adhesive material. 
       FIG. 19  is a cross-sectional view of an assembled light-emitting device  1900  including an adhesive layer  1910  between the lens  1050  and the LED package  1010 . Excess adhesive material can form an adhesive fillet  1920  around the periphery of the lens  1050  as illustrated. While  FIGS. 16-19  illustrate bonding a lens  1050  including a plug  1070  to an LED package  1010  including a socket  1020 , in some embodiments, other mating surfaces may be used for the lens  1050  and the LED package  1010 . For example, the lens  1050  can include a generally flat surface that omits the plug  1070  and the LED package  1010  can include a generally flat surface that omits the socket  1020 . 
     As discussed elsewhere herein, a lens can be formed by injection molding. In this process a mold having a lens-shaped well is sealed to an LED package having a socket. A suitable material, e.g., a plastic in a fluid state, is injected into the mold to fill the well and the socket. The injected material is then cured to form the lens. The process can be performed as a batch process on an array of LED packages, using a mold having an array of lens shaped wells. 
       FIGS. 20-21  illustrate successive steps in fabricating a lens using an injection mold.  FIG. 20  is a cross-sectional view of a mold  2010  sealed to an LED package  1210  prior to injection of a fluid lens material. The mold  2010  includes an injection port  2020  configured to admit the lens material such as a plastic, silicone, or epoxy, configured to cure to a solid shape. The LED package  1210  and the mold  2010  combine to form an injection mold. 
       FIG. 21  is a cross-sectional view of a completed LED device including a molded lens  1250  after the mold  2010  has been separated from the LED package  1210 . The sidewall of the LED package  1210  is inclined at an angle Φ. The angle Φ between the sidewall  1225  and the socket floor  1230  is illustrated as less than 90 degrees, which provides an inference between the lens  1250  and the LED package  1210 . However, the angle Φ can include other angles, for example a range from about less than 90 degrees to about 140 degrees. The angle Φ is illustrated in  FIGS. 20 and 21  as being fabricated using molding techniques. However other methods of fabrication can be employed to achieve an angle Φ less than 90 degrees. 
     Another method of manufacturing a lens includes forming a lens blank and then machining the blank to remove excess material. For example, a glass blank can be turned to remove material to form the plug.  FIG. 22  is a cross-sectional view of an exemplary embodiment of a lens blank  2210  prior to machining, and  FIG. 23  is a bottom plan view of the lens blank  2210 .  FIG. 24  is a front elevation illustrating the resulting lens  1050 . In some embodiments, the lens blank  2210  is formed by molding, e.g., injection molding. Alternatively, the lens blank  2210  can be cast. In various embodiments, the lens blank  2210  is fabricated from materials including glass, plastic, silicone, epoxy, etc. 
     The lens blank  2210 , as illustrated, is a hemisphere having radial symmetry. Other suitable shapes having radial symmetry include a circular cylinder, a spherical section, a concave spherical section, etc. A plug  1070  is formed by removing the material in an annular volume defined by a region  2220  as illustrated in  FIGS. 22 and 23 . The lens blank  2210  in  FIG. 22  includes a region  2220  of material, which can be removed to fabricate the lens  1050  from the lens blank  2210 . When the lens blank  2210  has radial symmetry, the material can be removed by turning the lens blank  2210 . Alternatively, the material in the region  2220  can be removed by grinding. In other examples, when the lens blank  2210  does not have radial symmetry, the material in the region  2220  can be removed, for example, by using an endmill. 
       FIGS. 25-26  show bottom plan views illustrating alternative shapes for the lens  1050 .  FIG. 25  illustrates a generally oval shape for the lens  1050 . The oval shape for lens  1050  has a diameter D 1  in a minor axis, and a diameter D 2  in a major axis. The diameter D 2 , as illustrated, is greater than the diameter D 1 . In some embodiments, the ratio of the diameter D 1  to the diameter D 2  is about 3 to 4, reflecting an aspect ratio of a standard television screen. In another embodiment, the ratio of the diameter D 1  to the diameter D 2  is about 9 to 16, reflecting an aspect ratio of a wide screen television. Other ratios of the diameter D 1  to the diameter D 2  can be employed. Moreover, the lens  1050  can be fabricated in other shapes of varying complexity, for example a hexagon. The lens  1250  can also be described by shapes similar to those illustrated in  FIGS. 22 and 23  including an allowance for the inclined sidewall  1275 . 
       FIG. 26  illustrates a generally rectangular shape for the lens  1050 , having a width “W” and a length “L.” In some embodiments, the ratio of the width “W” to the length “L” is about 3 to 4, reflecting an aspect ratio of a standard television screen. In another embodiment, the ratio of the width “W” to the length “L” is about 9 to 16, reflecting an aspect ratio of a wide screen television. Other ratios of the width “W” to the length “L” can be achieved. In various embodiments, the aspect ratio of the lens  1050  and/or shape of the cap  1060  can be configured to shape a beam light, for example, for use in a projector or a headlight of an automobile. 
       FIG. 27  depicts an exemplary method  2700  for fabricating a light-emitting device. The method  2700  comprises a step  2710  of providing an LED package with a socket, a step  2720  of providing a lens including a plug and a cap, an optional step  2730  of depositing an adhesive in the socket, and a step  2740  of attaching the lens to the LED package. In those embodiments in which the method  2700  is directed to mechanically securing the lens in an LED package, the step  2730  is omitted and the resulting light-emitting device does not include an adhesive layer. 
       FIG. 28  depicts a method  2800  for fabricating a lens. The method  2800  comprises a step  2810  of forming a lens blank, and a step  2820  of machining the lens blank to form the lens. In some embodiments, the lens blank is formed from glass, and the lens blank is turned to remove material from an annular region to form the plug. 
       FIG. 29  depicts a method  2900  for molding a lens. The method  2900  comprises a step  2910  of providing an LED package with a socket, a step  2920  of placing a mold over the socket, a step  2930  of introducing a fluid material into the mold, and a step  2940  of curing the fluid material into a shape of the lens. 
       FIG. 30  depicts a method  3000  for forming an adhesive layer between a lens and an LED package. The method  3000  comprises a step  3010  of introducing a bead of adhesive onto the LED package, a step  3020  of contacting a point on the bead of adhesive with the lens, and a step  3030  of spreading the adhesive using the lens. Although  FIGS. 10-26  illustrate lenses having a plug and  FIGS. 10-21  illustrate LED packages having a socket it will be understood that an adhesive layer may be formed between other mating surfaces using the method  3000 . For example, a lens and an LED package may each include a substantially flat mating surface. 
     In the foregoing specification, the present invention is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the present disclosure is not limited thereto. Various features and aspects of the above-described invention may be used individually or jointly. Further, the present invention can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art. It will be further recognized that “LED” and “LED die” are used interchangeably herein.