Abstract:
The substrate that is used to support the growth of the LED structure is used to support the creation of a superstructure above the LED structure. The superstructure is preferably created as a series of layers, including conductive elements that forma conductive path from the LED structure to the top of the superstructure, as well as providing structural support to the light emitting device. The structure is subsequently inverted, such that the superstructure becomes the carrier substrate for the LED structure, and the original substrate is thinned or removed. The structure is created using materials that facilitate electrical conduction and insulation, as well as thermal conduction and dissipation.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to the field of solid state light emitting devices, and in particular to a light emitting device in a chip scale package, and a method of manufacturing such a device 
       BACKGROUND OF THE INVENTION 
       [0002]    Light emitting devices (LEDs), and particularly those operating at greater than approximately a quarter watt, generally include a semiconductor element that provides the light, and one or more non-semiconductor elements that provide mechanical support, electrical connections, thermal dissipation, wavelength conversion, and so on. 
         [0003]    As the popularity and the field of use of solid-state LEDs continue to expand, the potential for profit from large quantity sales increases, as does the competition for such sales among manufacturers. In such an environment, the even minor savings in per-unit costs can have a major impact on profitability. Accordingly, manufacturers of LEDs strive to reduce material costs and manufacturing costs. 
         [0004]      FIG. 1  illustrates a conventional medium-to-high power LED comprising semiconductor elements  110  and at least two non-semiconductor elements: a ceramic substrate  120 , and a pair of electrodes  122 . As can be seen, in this embodiment, the ceramic substrate  120  is well over twice the area of the light emitting semiconductor structure  110 ; the extra area primarily being used to facilitate external connections to the semiconductor structure  110  via the electrodes  122 . Accordingly, the substrate  120  accounts for a relatively significant portion of the material cost of the device. Additionally, placing the semiconductor structure  110  on the substrate  120  generally requires a precise pick-and-place process, which adds to the manufacturing cost of the device. 
         [0005]    U.S. Pat. No. 7,329,905, “CHIP-SCALE METHODS FOR PACKAGING LIGHT EMITTING DEVICES AND CHIP-SCALE PACKAGED LIGHT EMITTING DEVICES”, issued 12 Feb. 2008 to Ibbetson et al. discloses a technique that uses wafer bonding to eliminate the pick-and-place process, and to reduce the size of the supporting substrate. As illustrated in  FIG. 2A , a first wafer includes a substrate  212  upon which multiple LED structures  216  are formed, with contacts  218  at the top of the structures. A second wafer includes a carrier substrate  220  that includes through-hole vias  222 , with contacts  228 ,  238  at the top and bottom of the carrier substrate, respectively. As illustrated in  FIG. 2B , the first wafer is inverted and bonded to the second wafer, the contacts  218  of the LED structures being coupled to corresponding contacts  228  at the top of the carrier substrate. Optionally, to reduce interference with the light output from the top of the LED structures, the growth substrate  212  of the first wafer can be thinned or removed. The resultant wafer bonded structure is subsequently diced/singulated (dashed lines) into individual light emitting devices, with contacts  238  at the bottom of the carrier substrate for external connections to the LED structure. These devices can then be placed upon a printed circuit board and coupled to corresponding electrodes on the board, generally using solder reflow techniques. 
         [0006]    Although the techniques of U.S. Pat. No. 7,329,905 eliminate the need to pick-and-place individual LED structures, and reduce the substrate area beyond the LED structure, compared to the conventional structure of  FIG. 1 , further cost reductions, or simplifications, in material and/or manufacturing, would be advantageous. 
       SUMMARY OF THE INVENTION 
       [0007]    It would be advantageous to eliminate the need to provide through-hole vias in a chip-scale packaged light emitting device. It would also be advantageous to provide more options with regard to materials used for the substrate, and with regard to coupling through the substrate to the light emitting structure. 
         [0008]    In an embodiment of this invention, the substrate that is used to support the growth of the LED structure is used to support the creation of a superstructure above the LED structure. The superstructure is preferably created as a series of layers, including conductive elements that form a conductive path from the LED structure to the top of the superstructure. The structure is subsequently inverted, such that the superstructure becomes the carrier substrate for the LED structure, and the original substrate is thinned or removed. The structure is created using materials that facilitate electrical conduction and insulation, as well as thermal conduction and dissipation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
           [0010]      FIG. 1  illustrates an example prior art light emitting device. 
           [0011]      FIGS. 2A-2B  illustrate another example prior art light emitting device. 
           [0012]      FIG. 3  illustrates an example flow diagram for creating a light emitting device with a superstructure that is suitable for supporting the light emitting device and for providing external contacts for coupling the light emitting device to a power source. 
           [0013]      FIGS. 4A-4H  illustrate example views of the light emitting device during manufacture. 
           [0014]      FIGS. 5-8  illustrate example alternative structures for forming a light emitting device. 
       
    
    
       [0015]    Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
       DETAILED DESCRIPTION 
       [0016]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
         [0017]    The process and device of this invention may be best understood with reference to the example flow diagram of  FIG. 3 , and the corresponding light emitting device structures of  FIG. 4 . Although this invention is particularly well suited for creating multiple light emitting devices on a wafer or other carrier,  FIG. 4  and the accompanying description will address creation of a single example light emitting device. One of skill in the art will recognize that the number of devices being created on the carrier is irrelevant to the principles of this invention. 
         [0018]    At  310 , the light emitting element  416  and associated electrode contacts  418 A,  418 B are created on a substrate  412 , typically a growth substrate that facilitates the creation of the semiconductor device and interconnection layers. The light emitting element  416  is illustrated as a stack of layers, corresponding to the typical sandwich of a light emitting substance between an anode and cathode. Any number of known techniques may be used to create the structure of  FIGS. 4A and 4B ,  FIG. 4A  being a side view and  FIG. 4B  being a top view. 
         [0019]    In this example embodiment, the device is configured to provide a set of four contact areas  418 A for coupling to the one of the electrodes (e.g. anode) of the light emitting element  416 , and a larger surrounding area  418 B for coupling to the other electrode (e.g. cathode). A gap  415  isolates these electrodes  418 A,  418 B. The use of four contact areas  418 A and large area  418 B facilitates a more uniform current density distribution within the device; in some embodiments, the contact areas  418 A may be coupled to individual light emitting devices that provide different light output wavelengths (colors). For ease of reference, it is assumed herein that these electrodes  418 A are intended to be coupled to a common source of power. 
         [0020]    At  320 , the created light emitting elements  416  may be tested, although testing may alternatively be performed after completing the creation of the superstructure, detailed below. At  330  ( FIG. 4C ), an insulating material  420 , such as a dielectric, is applied to the structure, to isolate the electrodes from subsequent conductive layers except at select locations  428 A,  428 B. Conventional lithographic techniques may be used to provide this patterned layer of insulating material  420 . As detailed further below, the light output is intended to exit the device in a direction away from the electrodes  418 A-B and insulation  420 ; accordingly, the electrodes  418 A-B and insulating  420  layers are preferably reflective to reduce the amount of light lost or absorbed within the device. Alternatively, the electrodes  418 A-B or the insulating layer  420  may be transparent, relying on subsequent layers to provide such reflections. As also detailed further below, the insulation  420  preferably conducts heat and does not conduct electricity. 
         [0021]    At  340  ( FIG. 4D ), relative tall insulating/isolating dividers  430  are created at select locations on the structure. In a typical embodiment, the light emitting element  416  may be in the order of about five microns thick, whereas the height of the dividers  430  may be in the order of a hundred microns or more. Lithographic techniques may be used to create these dividers  430 , using a slurry, such as an epoxy resin, that is cured at the select locations. Although the dividers  430  are illustrated as having a rectangular cross-section, one of skill in the art will recognize that these dividers  430  may have a trapezoidal shape with a larger base than top area. 
         [0022]    At  350  ( FIG. 4E ), the spaces between the dividers  430  are filled with metal  438 A,  438 B. Conventional application of a seed layer within these spaces, followed by an overplating of metal, such as copper, may be used. This overplating may purposely extend above the dividers  430 , and then planed, mechanically or chemically, or both, to expose the dividers  430 , isolating the regions  438 A,  438 B. The metal  438 A extends into the gap(s)  428 A in the insulating layer  420 , thereby contacting the electrode contact(s)  418 A of the light emitting element  416 . In like manner, the metal  438 B extends into the gap(s)  428 B, contacting the electrode contact(s)  418 B. 
         [0023]    At  360  ( FIG. 4F ), another insulating layer  442  is applied above the metal  438 A,  438 B, with gaps  448 A,  448 B at select locations. As with the insulation  420 , the insulation  442  preferably conducts heat and does not conduct electricity. For example, the insulation  442  layer may include a resin or an inorganic material, such as SiO 2  or Si 3 N 4 . 
         [0024]    At  370  ( FIG. 4F ), a final metal layer is applied above the insulating layer  442 . In this example, three conductive contacts  444 ,  458 A,  458 B are formed. The metal at contact  458 A extends into the gap  448 A, providing contact through the metal  438 A to the electrode  418 A, and the metal at contact  458 B extends into the gap  448 B, providing contact through the metal  438 B to the electrode  418 B. These contacts  458 A,  458 B serve as the external contacts for coupling a power source to the light emitting element  416 . One of skill in the art will recognize that although two contacts  458 A,  458 B are discussed above and illustrated in these figures, additional contacts may also be provided. For example, the light emitting elements  416  may include multiple segments, for providing different levels of illumination, different colors and combinations of colors, and so on. 
         [0025]    The metal pad at  444  is not coupled to the underlying metal structures  438 A,  438 B, and serves to provide an external contact for heat dissipation. That is, assuming minimal heat insulation via the insulating layers  420 ,  442 , the metal structures  438 A,  438 B will serve to conduct heat generated by the light emitting element  416  to the metal pad  444 , and from there to the underlying substrate, such as a printed circuit board. 
         [0026]    At  380  ( FIGS. 4G-4H ), the structure is inverted, such that the core metal structures  438 A,  438 B provide the structural support for the light emitting device, allowing the original growth substrate  412  to be removed, or reduced in thickness, thereby reducing optical losses as the light exits the ‘top’ of the light emitting element  416 , in a direction opposite the core metal structures  438 A,  438 B. As illustrated in the bottom view of  FIG. 4H , the contacts  444 ,  458 A,  458 B may extend across the width of the device, to facilitate external connections to the device. 
         [0027]    Of particular note, a light emitting device created using the principles of this invention does not require wafer-bonding, and the location and orientation of the external contacts  458 A,  458 B are substantially independent of the location and orientation of the internal electrodes  418 A,  418 B, thereby providing substantial design flexibility, compared to the use of through-hole vias ( 222  in  FIGS. 2A-2B ). 
         [0028]    The structure of  FIG. 4G  may be further processed as required. For example, a layer of wavelength conversion material (e.g. phosphor) may be applied, to generate different color(s) from the color produced by the light emitting element  416 , so as to produce, for example a combination of colors that produce a white light emitting device. In like manner, a lens may be created atop the structure, to provide particular optical qualities, and/or to protect the upper layers of the device. 
         [0029]    One of skill in the art will recognize that the particular structure illustrated in  FIGS. 4A-4H  is merely an example structure.  FIGS. 5-8  illustrate a few alternative structures that may be created using the techniques discussed above. For ease of reference, in these figures, the anode elements are shaded with a light shading, the cathode elements are shaded with a medium shading, and the thermal elements are shaded with a dark shading. Insulating sections are illustrated without shading. 
         [0030]      FIG. 5  illustrates an example structure that does not have a separate thermal element, per se. In this example, a wall  520  extends around the perimeter of the device, and is coupled to the cathode structure  528 B. This wall  520  is configured to dissipate heat through the outer perimeter of the device. An external heat sink or fin structure (not illustrated) may be affixed to the perimeter to further facilitate heat dissipation. One of skill in the art will recognize that the wall  520  may alternatively be insulated from the structures  528 A,  528 B, thereby forming a separate heat dissipation element that is not electrically coupled to the light emitting element  516 . 
         [0031]      FIG. 6  illustrates an example structure that provides for external connections to the anode  528 A and cathode  528 B structures via the edges of the device. In this example, a thermal element  644  extends across the bottom of the device. 
         [0032]      FIG. 7  illustrates another example of an edge-connected device, attached to a printed circuit board  710 . In this example, only the cathode structure  728 B extends to the edges, the anode structure  728 A extending to a contact  758 A at the bottom of the device. The cathode  728 B may be coupled to conductors  712 B on the printed circuit board  710  via solder joints  730 , and the anode contact  758 A may be coupled to the conductor  712 A on the printed circuit board  710  via solder balls  740 . A variety of methods of coupling the coupling the structure to the printed circuit board may be used, including the use of solder balls or a continuous solder film. 
         [0033]      FIG. 8  illustrates a bottom view of a multiple-anode device. As noted above, the light emitting device may include a plurality of light emitting elements. By providing separate contacts  858 A 1 - 4 , the intensity or color can be varied by selectively activating one or more combinations of anodes  858 A 1 - 4 . In this example, a common cathode contact  858 B is illustrated, although one of skill in the art will recognize that multiple cathode contacts may be provided to facilitate a variety of different configurations. In this example thermal element  844  is placed between anode and cathode contacts. 
         [0034]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0035]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.