LIGHT EMITTING DEVICE WITH INTEGRAL SHAPED REFLECTOR
FIELD OF THE INVENTION
This invention relates to the field of light emitting devices, and in particular to a wafer-level method for creating an integral coating that surrounds the light emitting device and provides a shaped radiation pattern.
BACKGROUND OF THE INVENTION
The increased use of solid-state light emitting devices (LEDs) for lighting
applications has created a highly competitive market in which cost and lighting efficiency are predominant factors. Techniques that reduce the cost of the device, as well as techniques that reduce the subsequent manufacturing and assembly costs are highly desirable, as are techniques that improve the light output efficiency of the devices and/or the lighting assembly.
U.S. patent application 2011/0062470, "REDUCED ANGULAR EMISSION CONE ILLUMINATION LEDS", published 17 March 2011 for Serge J. Bierhuizen and M. George Craford and incorporated by reference herein discloses a light emitting device package that includes an LED die 110 mounted on a support 120 with a reflector 150 that surrounds the LED die 110, as illustrated in FIG. 1. The reflector 150 includes a reflective surface 160 that provides a directed light output pattern. A lens 180 may also be situated above the reflector. The reflector 150 may be pre-formed on the support 120 and shaped to receive the LED die 110, or it may be molded around the LED die 110 after the die 110 is attached to the support 120. In an example embodiment, the reflective surface 160 may be sloped and may extend above the LED die 110 to redirect light that is emitted at a shallow angle from the die 110 in a desired direction, generally perpendicular to the light emitting surface. Although the above referenced application serves to increase the light output efficiency by directing the light in a desired direction, the creation of the reflector on the support increases the difficulty and cost of assembly. If the reflector is pre-formed, clearance must be allowed for receiving the LED die, and the reflector must be a material that is able to withstand the subsequent processing steps, including, for example, the soldering of the LED to the support. If the reflector is formed after the LED die is mounted to the support, the precision of situating the LED die with regard to the mold that is used to create the reflector must be carefully controlled, and the reflective material must not obscure the light emitting surface.
SUMMARY OF THE INVENTION
It would be advantageous to provide an LED die with an integral reflector. It would also be advantageous to provide an LED die with a coating that protects the LED die and allows for optimized shaping of the coated die.
To better address one or more of these concerns, in an embodiment of this invention, a reflective material is shaped around each LED die, or around each set of LED dies, before the dies are singulated from the supporting wafer. The reflective material may be a high- hardness and high-reflectivity dielectric material. A high-hardness material provides mechanical support and protection during subsequent handling of the singulated die. A high- reflectivity material reduces optical loss at the sides of the LED die, and can be shaped to provide a desired light output radiation pattern. By forming the integral reflector around each die while the dies are on the wafer, the manufacturing cost per die is reduced compared to forming individual reflectors after the dies are singulated. BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:
FIG. 1 illustrates an example prior art combination of an LED die on a support with a reflector that surrounds the LED die.
FIGs. 2A-2B illustrate example LED dies on a wafer surrounded by reflective material. FIGs. 3A-3B illustrate example connectivity schemes to singulated dies with reflective material.
FIGs. 4A-4C illustrate example profile shapes of reflective material surrounding LED dies. FIGs. 5A-5C illustrate example singulated LED structures. FIGs. 6A-6C illustrate an example method for molding reflective material to surround LED dies on a wafer.
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
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.
In a preferred embodiment of this invention, reflective walls 250 are created around light emitting device dies 110A - HOD while the dies 1 lOA-110D are situated on a supporting wafer 220, forming a plurality of light emitting structures 210A-210D, as illustrated in the example of FIGs. 2A-2B. FIG. 2A illustrates an example cross-section of the dies 1 lOA-110D and walls 250A-250D on the wafer 220, while FIG. 2B illustrates an example top view of the dies 110 and walls 250 on the wafer 220. Although only eight structures 210 are shown on the wafer 220, one of skill in the art will recognize that there may be hundreds or thousands of dies 110, and corresponding structures on a typical wafer 220.
The wafer 220 may be a wafer upon which the dies 110 are formed, typically termed a 'growth wafer', or a wafer to which the dies 110 are attached in a wafer-to-wafer attachment process, typically termed a 'carrier wafer'. Copending U.S. patent application 61/521,783, "Optimized wafer level processing method for the fabrication of LED Chip Size Packages", filed 10 August 2011 for Marc de Samber and Eric van Grunsven, Attorney docket
2011PF00975, and incorporated by reference herein, discloses a wafer- level bonding technique. In this copending application, the dies 110 are formed on a GaN-on-sapphire growth wafer with the light emitting GaN surface formed at the 'bottom' of the die 110, upon the sapphire growth wafer. A Si carrier wafer is subsequently attached to the 'tops' of the dies 110, the combination is 'flipped-over', and the sapphire growth wafer is removed, typically via a laser lift-off process, resulting in a Si carrier wafer 220 at the new 'bottom' of the dies 110, (i.e. the original "top") opposite the light emitting surfaces of the dies 110.
Regardless of whether the wafer 220 is a growth wafer or a carrier wafer, the dies 110 upon the wafer 220 are subsequently surrounded by reflective walls 250, using, for example, a reflective dielectric material that is molded upon the wafer 220, as detailed further below. Preferably, the material exhibits a high- hardness and a high-reflectivity; a silicone molding compound containing an inorganic filler with a high refraction index, such as Ti02, ZrO, and so on, is a commonly used dielectric material that is hard and reflective after curing.
These reflective walls 250 serve to reflect light emitted by each die 110 in a desired direction, as well as to provide a structure that supports and protects each die 110. The walls 250 include a reflective surface 260 that may be shaped in any of a variety of desired forms, as illustrated further below. In the example of FIG. 2B, the walls 250 include a curved outline 255 that serves to form a conic light output pattern, although one of skill in the art will recognize that different outline shapes may also be used to achieve different light output patterns. Likewise reflective surface 260 may be composed of multiple surfaces or faceted.
After the walls 250 are formed, the wafer 220 may be processed to provide individual ('singulated') LED structures 210 for subsequent use in light emitting applications, each singulated structure 210 comprising an LED die 110 surrounded by a wall 250 having a reflective surface 260. Of particular note, because the walls 250 surround the dies 110, the walls 250 protect the dies 110 during subsequent handling, such as pick-and-place processes.
The walls 250 may also be sized to accommodate subsequent processes and/or assemblies. For example, a lighting fixture may be designed with cavities that are designed to receive light emitting devices of a particular size and shape. In such an application, the walls 250 may be sized and shaped so that the singulated structure 210 conforms to this particular size and shape.
For ease of illustration, FIGs. 3A-3B, 4A-4C, and 5A-5C illustrate example device structures 210 after singulation, although one of skill will recognize that these structures 210 are formed as a plurality of structures upon a wafer 220, as illustrated in the examples of FIGs. 2A-2B. The wafer 220 preferably includes conductors that are arranged to facilitate contact with the LED dies 110, as illustrated in FIGs. 3A-3B. In FIG. 3 A, the wafer 220 includes through-holes, or vias, that include conductors 310 that serve to connect external contacts 320 to corresponding contacts (not illustrated) on the LED die 110. In FIG. 3B, the wafer 220 includes surface conductors 330 that are coupled to contacts (not illustrated) on the LED die 110, and extend beyond the walls 250. Other techniques for coupling to an LED die 110 are common in the art, including, for example, wire bonding to contacts (not illustrated) on the upper surface of the LED die 110.
For ease of illustration, connections on the wafer 220 are not illustrated in the subsequent figures.
FIGs. 4A-4C illustrate example structures for the walls surrounding the LED dies 110. In FIG. 4A, a wall with an asymmetric profile is illustrated, wherein one side 451 of the wall extends to a different height above the LED die 110 than the other side 452 of the wall. The corresponding reflective surface 460 will provide a light output pattern that directs more light to one side than the other.
In FIG. 4B, the reflective wall 453 does not extend above the upper surface of the LED die 110. This structure will produce a light output pattern similar to a conventional flat- surfaced LED, but with less light loss due to emissions from the sides of the LED die 110. As noted above, as compared to a conventional LED die, the reflective wall 453 will also serve to protect the LED die during subsequent handling.
In FIG. 4C, the wall 456 surrounds a plurality of LED dies 110, such as an array of dies 110 that are used to provide more light output than a single LED die. The area 458 between the dies 110 may also be filled with reflective material to reduce optical loss and to reflect light in the preferred upward direction.
FIGs. 5A-5C illustrate example lighting structures after subsequent processing. This processing is preferably applied while the structures 210 are still on or connected to the wafer 220, although some or all of the processing may be performed after the structures 210 are singulated.
In FIG. 5 A, a layer of material 570 is applied over the LED die 110, using the walls 250 to contain the material 570. This material 570 may be applied for a number of reasons, including protecting the light-emitting surface, improving the light extraction efficiency, converting some or all of the emitted light to a different wavelength, and so on. In FIGs. 5B and 5C, a layer of transparent material 580, such as a silicone epoxy, is applied, with or without the illustrated material 570, to achieve a desired optical effect, to further bind the wall 250 to the LED die 110, and/or to further protect the LED die 110. In FIG. 5B, the material 580 extends to the wall 250, whereas in FIG. 5C, the material 580 covers the entire structure 210.
The reflective walls that surround the LED dies may be formed using a variety of techniques, including, for example, conventional photo-lithography techniques. In an example embodiment, the walls may be molded upon the wafer, as illustrated in FIGs. 6A- 6C.
In FIG. 6A, the wafer 220 containing the LED dies 11 OA- 1 IOC are coated with a moldable material 650, such as the aforementioned silicone molding compound with a high refraction index filler, and a mold 610 is applied to create the desired shape of the walls surrounding the LED dies 110.
FIG. 6B illustrates the wafer 220 after the mold 610 of FIG. 6A is removed. As illustrated, the example mold 610 is shaped to provide only a small amount of material 650 atop the LED dies 11 OA- 1 IOC and in the areas between the LED dies 11 OA- 1 IOC.
While the material 650 is not yet fully hardened, the surface of the wafer is brushed to remove the material 650 from atop the LED die 110 and in the areas between the LED dies 110, producing the structures 210A-210C, each structure 21 OA- IOC comprising an LED die that is surrounded by a wall 250 with a reflective surface 260, as illustrated in FIG. 6C. The brushing process will also reduce the amount of material 650 that forms the walls, and the mold 610 of FIG. 6A is shaped to compensate for this reduction.
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. For example, one of ordinary skill in the art will recognize that, in the example of a growth wafer being replaced by a support wafer, the reflective walls may be formed on the support wafer before the LED dies are bonded to the support wafer, such that the bonding of the wafers effect the formation of the walls surrounding the LED dies. In like manner, particularly in the example of FIG. 4B, the reflective walls may be formed on the growth layer before the bonding to the support layer, thereby avoiding the need to remove the reflective material from the surface of the LED dies. Such an application of a dielectric material over the growth layer may also facilitate the bonding of these wafers.
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. A single processor or other unit may fulfill the functions of several items recited in the claims. 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. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless
telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.