WO2013175338A1

WO2013175338A1 - Phosphor coating process for discrete light emitting devices

Info

Publication number: WO2013175338A1
Application number: PCT/IB2013/053796
Authority: WO
Inventors: Grigoriy Basin; Hideo Kageyama
Original assignee: Koninklijke Philips N.V.
Priority date: 2012-05-23
Filing date: 2013-05-10
Publication date: 2013-11-28

Abstract

Light emitting devices (LEDs) are fabricated on a wafer with one or more thick metal layers that provide structural support to each LED. The streets, or lanes, between individual LEDs do not include this metal, and the wafer can be easily sliced/diced into singulated self- supporting LEDs. These self-supporting LEDs are picked and placed upon an intermediate film, such as a conventional wafer sawing tape, with spacing between the LEDs. A phosphor film is laminated onto these LEDs on the intermediate film, then the self-supporting phosphor-laminated LEDs are singulated by slicing the phosphor film and removing the intermediate film.

Description

PHOSPHOR COATING PROCESS FOR DISCRETE LIGHT EMITTING DEVICES

FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to a method for producing phosphor coated light emitting devices. BACKGROUND OF THE INVENTION

U.S. Patent 7,344,952, "Laminating Encapsulant Film Containing Phosphor Over LEDs", issued 3 July 2008 to Haryanto Chandra, and incorporated by reference herein, discloses a technique for laminating a phosphor film to a set of light emitting devices on a submount. A variety of phosphor films are preformed, with varying wavelength conversion properties. Light emitting dies are tested and sorted ('binned') based on their light output characteristics, and dies having similar characteristics are attached to a submount. Thereafter, a particular phosphor film is selected to be applied to the dies on the submount with similar characteristics, so that the combination of the particular light emission of the light emitting dies and wavelength conversion of the selected phosphor film provide a desired composite light output. By pairing a group of similarly performing light emitting dies with a phosphor composition that is selected based on the particular characteristics of the group, the variance of the composite light output is substantially reduced.

The submount provides the structure required to support the individual dies, and electrical circuitry that allows an external power source to be coupled to the encapsulated light emitting die. After lamination, the submount with phosphor laminated light emitting dies is subsequently sliced/diced ("singulated") to produce individual light emitting devices that can be placed in lamps, attached to printed circuit boards, and so on.

The singulation of the light emitting devices, however, is hampered by the structural support provided by the the submount. The slicing apparatus must be able to cut through the submount, and a submount that is sufficiently thick and/or rigid to structurally support a group of light emitting devices through the lamination process is more difficult to slice than a non-structural substrate. SUMMARY OF THE INVENTION

It would be advantageous to provide phosphor laminated light emitting devices without requiring a structurally supporting submount that must be sliced.

To better address one or more of these concerns, in an embodiment of this invention, the LEDs are fabricated on a wafer substrate with one or more thick metal layers that provide structural support to each LED. The streets, or lanes, between individual LEDs do not include this metal, and the wafer can be easily sliced/diced into singulated self-supporting LEDs. These self-supporting LEDs are picked and placed upon an intermediate film, such as a conventional wafer sawing tape, with spacing between the LEDs. A phosphor film is laminated onto these LEDs on the intermediate film, then the self-supporting phosphor- laminated LEDs are singulated by slicing the phosphor film and removing the intermediate film.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIGs. lA-lD illustrate an example fabrication of a self-supporting light emitting device. FIGs. 2A-2E illustrate an example fabrication of self-supporting phosphor-laminated light emitting devices.

FIG. 3 illustrates an example flow diagram for the fabrication of self-supporting phosphor- laminated light emitting devices.

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.

FIGs. lA-lD illustrate an example fabrication of a self-supporting light emitting device, as disclosed in copending U.S. patent application 61/568,297, "FORMING THICK METAL LAYERS ON A SEMICONDUCTOR LIGHT EMITTING DEVICE", filed 8 December 2011 for Alexander Nickel, Jim Lei, Anneli Munkholm, Grigoriy Basin, Sal Akram, and Stefano Schiaffino, and incorporated by reference herein.

As illustrated in FIG. 1A, a light emitting structure 20 is formed on a substrate 40. Although only one light emitting structure 20 is illustrated, the substrate 40 may be a wafer upon which hundreds or thousands of light emitting structures are formed. The light emitting structure 20 may comprise, for example, an active layer that is sandwiched between an n-type region and a p-type region, and the substrate 40 may include, for example, a sapphire, SiC, Si, GaN, or composite substrate. Metal pads 84 and 86 provide electrical contact to the n-type and p-type regions, and are separated by a gap 89, which may be filled with an insulating material such as a dielectric, an oxide or nitride of silicon, air, or ambient gas. The light emitting structure 20 with accompanying pads 84 and 86 may be formed using any of a variety of techniques common in the art.

In embodiments of this invention, thick metal layers are formed above the pads 84, 86. To facilitate the formation of these thick metal layers, two base layers 22, 24 are formed above the pads 84, 86. The base layer 22 is a conductive adhesion layer, and may include, for example, Ti, W, and alloys such as TiW. The base layer 24 is a seed layer on which the thick metal layers may be formed. For example, if the thick metal layers are formed by copper plating, the seed layer 24 may be copper. The base layers 22, 24 may be formed to cover the entire surface of the wafer of semiconductor devices, and subsequently etched to electrically isolate the pads, as detailed further below.

A removable material 26, such as a photoresist, is applied in a select pattern to provide distinct regions upon which the thick metal layers are formed. As illustrated, the removable material may be coincident with the gaps 89 between the pads 84, 86. This removable material may also be placed in the regions ("streets") between the individual light emitting structures 20.

In FIG. IB, the thick metal layers 28, 30 are formed in the regions defined by the removable material 26; as illustrated the metal layer 28 is above the pad 86 and the metal layer 30 is above the pad 84. The thickness of these layers 28, 30 is greater than 50 microns in some embodiments, and greater than 100 microns in some embodiments.

The conductive base layers 22, 24 electrically couple these thick metal layers 28, 30 to the pads 84 and 86. Optionally, either or both of the base layers 22, 24 may be coupled directly to the light emitting element 20, serving a dual role as connector pad and base/seed layer; similarly, the pads 84 and 86 may comprise the seed material, obviating the need for the base layers 22, 24.

However, in this example embodiment, the conductive base layers 22, 24 extend across the entire wafer 40, and thus create a conductive path among all of the pads 84, 86 of all of the light emitting elements. In FIG. 1C, the removable material 26 is removed, exposing the base layers 22, 24 in the regions between the pads 84, 86 and in the streets between devices on the wafer. The exposed regions of the base layers 22, 24 are removed by conventional etching, creating electrically isolated thick metal regions 28, 30 atop the pads 86, 84.

One of skill in the art will recognize that if the conductive base layers 22, 24 had been patterned to be situated only atop the pads 84, 86, the use of a removable material 26 and subsequent removal and etching process need not be performed.

In FIG. ID, an electrically insulating material 33 is formed over the wafer, providing support between the thick metal regions 28, 30, and between the individual light emitting devices on the wafer. This material 33 may be applied over the entire surface of the wafer, then planed down to expose the metal regions 28, 30 using conventional techniques, such as microbead blasting, fly cutting, cutting with a blade, or chemical mechanical polishing. Metal contact pads 36, 38 may be formed on the thick metal layers 28, 30, to facilitate connection to a structure such as a PC board, for example by reflow-soldering. Contact pads 36 and 38 may be, for example, gold microbumps or solder, and may be formed by any suitable technique, including, for example, plating or screen printing.

The substrate 40 may be removed, exposing the surface of the light emitting structure

20 from which light will be emitted when the device 100 is externally powered via the pads 36, 38. The light emitting surface of the light emitting structure 20 may be roughened to enhance the extraction of light, thereby improving the light output efficiency. In the alternative, substrate 40 may be transparent to the light emitted by the light emitting structure 20 and may remain in place.

The individual devices 100 may be singulated by laser scribing and dicing. Of particular note, because the thick metal regions 28, 30 do not extend into the streets between the devices 100, conventional semiconductor dicing techniques may be used.

When the substrate 40 is removed and the individual devices 100 are singulated, each individual device 100 will have sufficient structural integrity to be self-supporting for subsequent processes, and in particular, to be self-supporting during the application of a phosphor layer.

In an embodiment of this invention, a phosphor film is laminated over a plurality of self-supporting light emitting devices 100, as illustrated in FIGs. 2A-2E.

FIG. 2A illustrates the placement of a plurality of light emitting devices 100 on a removable adhesive substrate 210, such as a conventional semiconductor sawing or dicing tape held in a frame (not shown). The tape 210 is selected to be able to withstand the lamination process detailed below, typically exposure to 150°C for up to four hours, including, for example, a kapton tape with a silicone glue layer. A conventional pick and place process may be used to place the devices 100 on the tape 210 with a space between each device 210 that allows the phosphor film to extend down the sidewalls of each device 100. In the alternative, tape 210 is a stretchable tape that is connected to the wafer prior to singulation. After singulation the tape 210 may be stretched to separate the die. Picking and placing may be performed after the individual devices 100 have been tested, to avoid lamination of faulty devices 100. Optionally, this testing may include characterizing each device 100 with respect to its light output characteristics, and

subsequently placing devices with similar characteristics on the tape 210, to provide an array of devices with uniform characteristics. In this manner, the particular characteristics of the phosphor film may be selected based on this uniform characteristic of the light emitting devices, as disclosed in the above cited U.S. patent 7,344,952.

As illustrated, the devices 100 are mounted on the tape 210 with the light emitting layer 20 exposed, and the contacts 36, 38 adhered to the tape 210.

FIG. 2B illustrates the placement of a phosphor film 220 over the light emitting surfaces of the devices 100. The phosphor film 220 may be a very thin silicone film containing phosphor or other inert inorganic material or combination of multiple phosphors and inorganic materials. The phosphor/silicone lamination film may be manufactured by applying the mixture of silicone, phosphor or multiple phosphor or other inorganic additives such as silica or other scattering materials such as Ti0₂ onto a support film. The mixture of all the above components is deposited to create a thin layer on top of the support film. The support film facilitates the handling of the phosphor mixture which is typically about 50um thick, and prevents film deformation during the initial lamination steps.

After the thin layer of mixed materials is dried, but not fully cured, the mixture is covered by a cover film (protective layer), forming the phosphor film 220. The wavelength conversion characteristics of the phosphor film 220 is determined by the type and quantity of the phosphor within the mixture. This technique of creating a preformed film 220 allows for characterizing the light conversion characteristics of the film 220 prior to applying it to the light emitting surfaces of the devices 100, thereby allowing for the selection from among different films 220 to best match the optical characteristics of the particular light emitting devices 100 on the tape 210 to provide a desired color and color temperature. Phosphor film 220 maybe prepared in a seperate location or "in place" on top of the device 100. Before applying the phosphor film 220 to the devices 100, the aforementioned cover film may be removed, and the phosphor film 220 is softened by a temperature treatment that make it suitable for the conformal coating (in 3 dimensions) over the individual devices 100. The film 220 may be cured at 100-150°C for 1-10 minutes in order to achieve a film hardness that prevents the silicone from melting and maintains the desired film thickness over the devices 100. The film 220 is positioned on top of the devices 100 on the tape 210 such that the aforementioned support film is facing up and the phosphor/silicone mixture is facing the light emitting surface of the devices 100.

After the phospor film 220 is positioned and aligned to the devices 100, the structure 210-100-220, hereinafter "tape 250", may be placed into a chamber of the vacuum laminator that is heated to 40-120°C. The chamber is then outgased to allow the film to achieve sufficient adhesion to the surface of the devices 100. The phosphor film 220 may be forced toward the surface of the devices 100 by mechanical pressure via a diaphragm or by non- contact pressure provided by compressed air as shown in FIG. 2C.

The tape 250 with the placed LEDs and phosphor film is removed from the laminator and allowed to cooled to room temperature. After removing the aforementioned support film from the phosphor film 220, the tile 250 is placed into a second lamination chamber for a final conformal lamination process. The chamber is at set up at 70-130°C and is outgassed for sufficient time to eliminate the air between the phosphor layer 220 and the tape 210. After the vacuum cycle is completed, air is allowed into the chamber, which pushes the heated phosphor layer into the gaps between the devices 100 on the tape 210. The tape 250 is again placed into an oven for a final cure of 1-4 hours at 150°C. As noted above, the tape 210 is selected to be able to withstand this high temperature cure without substantial distortion.

After curing, the tape 250 is transferred to the singulation process to separate the devices 100 on the tape 210 by sawing through the layer of phosphor layer 220, as illustrated in FIG. 2D. Of particular note, as contrast to the process disclosed in USP 7,344,952, because the devices 100 are self-supporting, and placed on the tape 210 with a space between the devices 100, the sawing need only penetrate the phosphor layer 220, and need not cut through a supporting substrate.

After removing the tape 210, the singulated light emitting devices 100 are ready for further assembly processes as self-supporting phosphor-coated light emitting devices 100, as illustrated in FIG. 3E. FIG. 3 illustrates an example flow diagram for the fabrication of self-supporting phosphor-laminated light emitting devices.

At 300, light emitting structures are formed on a growth substrate; these light emitting structures will have electrically isolated contact pads on an upper surface of the structure.

At 310, a conductive base/seed layer is applied, and may cover the upper surface of all of the structures.

At 320, partitions are created atop the conductive base/seed layer. These partitions are configured to define regions within which a thick metal layer will be formed; preferably, the thick metal layers are in regions coincident with the contact pads of each of the light emitting structures, and these partitions facilitate the electrical isolation of these thick metal regions.

At 330, the thick metal layers are formed within the regions defined by the partitions.

At 340, if the conductive base/seed layer was formed such that pads that should be isolated are electrically coupled, the partition material is removed to expose the conductive base/seed layer, and portions of the base/seed layer are removed, typically by etching. To fill the gaps left by the removal of the partition material, an insulating material is applied, thereby restoring the structural integrity of each device.

At 350, the growth substrate is removed, and the individual devices are singulated. The streets between the devices may be laser scribed before or after the growth substrate is removed to facilitate this singulation. Optionally, each device may be tested, before or after singulation, and sorted/binned according to their particular optical characteristics.

At 360, the individual devices are picked and placed on a dicing/sawing tape, or other substrate that is suitable for heating to 150°C for four hours or more without substantial deformation. The devices are spaced apart to allow the phosphor film to cover the sidewalls of each device, the particular separating distance being dependent upon the composition and thickness of the selected phosphor film. Optionally, if the devices have been tested and binned according to their individual characteristics, the devices that are placed on a particular tape will preferably be picked from the same bin, thereby exhibiting common light output characteristics. At 370, a phosphor laminate film is selected, based on the characteristics of the devices on the tape and one or more desired composite light output characteristics, the composite light being formed from the wavelength converted light produced by the phosphors in the film, and the remainder light emitted by the device and not converted by the phosphors. As noted above, the phosphor laminate film may be pre-heated before it is placed atop the light emitting devices, to prevent unwanted flow of the phosphor material during the lamination process.

At 380, the phosphor laminate film atop the devices is thermally cured and vacuum processed to conformally join the phosphor material to the surface and sidewalls of the light emitting devices, as detailed above with respect to FIGs. 2B-2C.

At 390, the phosphor material that is situated in the streets between the devices is sliced/sawn to singulate each of the light emitting devices, and the dicing/sawing tape is removed, thereby forming self-supporting phosphor-coated light emitting devices that can be subsequently assembled into lamps and other lighting systems and devices.

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, it is possible to operate the invention in an embodiment wherein additional films are laminated over the phosphor-coated devices before singulation. These additional films may be other wavelength-conversion films, or films with other optical functions. In some embodiments, the additional films include a light scattering film for improving color uniformity over a wider viewing angle.

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.

Claims

CLAIMS:

1. A method comprising:

forming a plurality of self-supporting light emitting devices on a first substrate, singulating the plurality of self-supporting light emitting devices,

removing the first substrate,

placing a group of devices of the plurality of self-supporting light emitting devices on an intermediate substrate, with a space between the devices on the intermediate substrate, laminating a wavelength-conversion film over the group of devices on the intermediate substrate,

singulating the laminated group of devices into individual light emitting devices, and removing the intermediate substrate.

2. The method of claim 1, wherein the intermediate substrate includes a dicing/sawing tape.

3. The method of claim 1, wherein the intermediate substrate includes a material that can be subjected to a temperature of 150C for up to four hours without substantial deformation.

4. The method of claim 1, wherein the intermediate substrate include a kapton film with a silicone glue layer.

5. The method of claim 1, wherein the wavelength-conversion film includes a silicone and phosphor mixture on a support film.

6. The method of claim 1, wherein the laminating includes:

heating the wavelength-conversion film to limit a flow of material in the film, placing the wavelength-conversion film atop the group of devices, forming a tile comprising the intermediate substrate, the group of devices, and the wavelength-conversion film, and

heating the tile in a vacuum chamber.

7. The method of claim 1, including selecting the group of devices based on each device of the group having similar light output characteristics.

8. The method of claim 1, including selecting the wavelength-conversion film based on one or more light output characteristics of the group of devices.

9. The method of claim 1, wherein singulating the laminated group of devices includes sawing material of the wavelength-conversion film that is located in the spaces between the devices.

10. The method of claim 1, wherein the self-supporting light emitting devices include a thick metal layer that has a thickness of at least 50 microns.

11. The method of claim 10, wherein forming the self-supporting light emitting devices includes creating partitions above a light emitting structure and depositing the thick metal layer in regions formed by the partitions.

12. The method of claim 11, wherein the partitions are formed of a photoresist material, and the forming of the self-supporting light emitting devices includes a photoresist removal and etching process.

13. The method of claim 1, including laminating an other film over the wavelength conversion film.

14. The method of claim 1, wherein the other film includes a light scattering material.

15. A light emitting device comprising:

a light emitting structure that includes an active layer between an n-type layer and a p-type layer and includes a light emitting surface and sidewalls;

a phosphor coating that covers the light emitting surface and sidewalls;

at least a first pad that is electrically coupled to the n-type layer and a second pad that is electrically connected to the p-type layer;

a thick metal layer built upon the first and second pads;

wherein the thick metal layer has a thickness of at least 50 microns.

16. The light emitting device of claim 15, wherein the phosphor coating has a thickness that is substantially uniform on the surface and on the sidewalls.

17. The light emitting device of claim 15, including a second coating above the phosphor coating.

18. The light emitting device of claim 17, wherein the second coating includes a light scattering material.