WO2014184698A1

WO2014184698A1 - Chip scale light emitting device package in molded leadframe

Info

Publication number: WO2014184698A1
Application number: PCT/IB2014/061155
Authority: WO
Inventors: Salman Akram; Jyoti Kiron Bhardwaj; Johannes Willem Herman SILLEVIS SMITT
Original assignee: Koninklijke Philips N.V.
Priority date: 2013-05-14
Filing date: 2014-05-02
Publication date: 2014-11-20

Abstract

Light Emitting Diodes (LEDs) (20) are fabricated on a wafer substrate (40) with one or more thick metal layers (28,30) that provide structural support to each LED (20). The streets (33), or lanes, between individual LEDs (20) do not include this metal, and the wafer can be easily sliced/diced into singulated self-supporting LEDs (20). Before singulation, further processes may be applied at the wafer- level; after singulation, these self-supporting LEDs (20) can be picked and placed directly upon a leadframe (210). The leadframe (210) may be molded to provide a particular shape that enhances the light output efficiency and/or facilitates subsequent fabrication processes.

Description

CHIP SCALE LIGHT EMITTING DEVICE PACKAGE IN MOLDED LEADFRAME

FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to a method for producing chip scale light emitting devices using a molded leadframe. BACKGROUND OF THE INVENTION

Thin-Film light emitting devices, including thin-film flip-chip devices, are conventionally tested, singulated (diced), then attached to a submount, typically via a pick- and-place process that attaches hundreds of light emitting dies on the submount. The submount provides the structure required to support the individual light emitting dies, and the electrical circuitry that allows an external power source to be coupled to the light emitting dies. The submount also allows for subsequent processes, such as lamination and

encapsulation, to be applied to all of the devices on the submount concurrently, significantly reducing fabrication costs. After such processing, the submount with finished light emitting devices 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 on the submount, 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 finished 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. Because these devices are self-supporting, a separate support submount is not required.

Before singulation, further processes may be applied at the wafer-level; after singulation, these self-supporting LEDs may be picked and placed directly upon a leadframe. The leadframe may be molded to provide a particular shape that enhances the light output efficiency and/or facilitates subsequent fabrication processes.

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-lE illustrate an example fabrication of a self-supporting light emitting device. FIGs. 2A-2B illustrate an example placement of a self-supporting light emitting device within a molded leadframe.

FIGs. 3 A-3B illustrate example encapsulations of self-supporting light emitting devices in the molded leadframe.

FIGs. 4A-4B illustrate the addition of optical elements upon the molded leadframe with encapsulated light emitting device.

FIGs. 5A-5D illustrate alternative molded leadframes.

FIG. 6 illustrates an example flow diagram for the fabrication of self-supporting light emitting devices in molded leadframes.

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-lE 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 (Attorney Docket 2011P00972), and incorporated by reference herein.

As illustrated in FIG. 1 A, a light emitting structure 20 is formed on a substrate 40. 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. 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. Gaps 88 between the light emitting structures 20 may be filled with non-conductive material to provide a uniform surface across the substrate 40.

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 may be formed above the pads 84, 86. The base layer 22 may be a conductive adhesion layer, and may include, for example, Ti, W, and alloys such as TiW. The base layer 24 may be 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, or they may be patterned to isolate regions of the light emitting structure 20, as detailed further below.

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

In FIG. IB, the thick metal layers 28, 30 may be 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.

Other techniques may be used to form thick metal elements to support the light emitting structure 20 and provide conductivity to each of the pads 84, 86 of these structures. In copending U.S. patent application 61/656,691, "CHIP SCALE LIGHT EMITTING

DEVICE WITH METAL PILLARS IN A MOLDING COMPOUND FORMED AT WAFER LEVEL", filed 7 June 2012, for Jipu Lei, Stefano Schiaffino, Alexander Nickel, Mooi Guan Ng, Grigoriy Basin, and Sal Akram, (Attorney docket 2012P00450) and incorporated by reference herein, discloses the creation of multiple pillars upon the pads 84, 86, and embedding these pillars in a molding compound. The pillars provide the mechanical support and conductivity, while the molding compound prevents distortions in the pillar structure.

The conductive base layers 22, 24 electrically couple the thick metal layers 28, 30 to the pads 84 and 86, respectively. 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.

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 may be 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 removal and etching process need not be performed. Likewise, the process may allow groups of light emitting devices to remain connected.

In FIG. ID, if the material 26 has been removed, an electrically insulating material 33 may be 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 or etched to expose the metal regions 28, 30 using conventional techniques, such as microbead blasting, fly cutting, cutting with a blade, or chemical mechanical polishing. If the layers 22, 24 have been patterned and the material 26 remains, the material 26 forms the illustrated material 33.

As illustrated in FIG. ID, 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.

As illustrated in FIG. IE, the individual devices 100 may be singulated, using, for example, 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, 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 picking and placing process used for coupling the device 100 to a leadframe, as illustrated in FIGs. 2A-2B. FIGs. 2A-2B illustrate the placement of a self supporting light emitting device 100 within a molded leadframe 210. FIG. 2A illustrates a cross section of the leadframe 210, while FIG. 2B illustrates a view from above the leadframe 210, before mounting the light emitting device 100. Although only one molded leadframe 210 is illustrated, an array of leadframes 210 will generally be formed in a single mold, allowing for subsequent processes to be performed concurrently for each of the leadframes 210 in the array.

The example molded leadframe 210 includes openings 215 through which leads 250A, 250B (collectively, leads 250) extend. The leads 250 may be formed on the leadframe 210 using conventional printed circuit techniques, wherein a pattern of removable insulating material is printed on the leadframe 210, and a layer of metal is formed within each region formed by the insulating material. Techniques for creating molded frames with leads (i.e. leadframes) are common in the art.

The molded leadframe 210 also includes walls 220 that create a cavity 230 within the molded leadframe 210. In this example leadframe 210, the walls 220 are sloped to reflect light emitted from the sides of the light emitting device 100 toward the open end of the cavity 230.

The metal contact pads 36, 38 of the light emitting device 100 are coupled to the leads 250 of the leadframe 210, typically by soldering, thereby allowing the coupling of an external power source (not illustrated) to the light emitting device 100 via the segments of the leads 250 that are on the lower surface of the leadframe 210. Of particular note, because the leads 250 extend beyond the contact pads 36, 38, the coupling of the leadframe 210 to a printed circuit board, or other substrate allows for easing the tolerance requirements for the spacing of the conductors on the printed circuit board that couple to the leads 250.

After coupling the light emitting device 100 to the leads 250 of the leadframe 210, the light emitting device 100 may be encapsulated as illustrated in the following figures.

FIGs. 3 A-3B illustrate an example encapsulation of the light emitting device in the leadframe 210. An encapsulating material 310, such as silicone, is deposited within the cavity formed by the walls 220 of the leadframe 210. The encapsulating material 310 may include one or more wavelength conversion materials that absorb light emitted by the light emitting device at a first wavelength, and emit light at a second wavelength. To increase the likelihood of the light being emitted from the upper surface 315 of the encapsulating material 310, the walls 220 and/or the lower surface 225 of the leadframe 210 may be reflective, using, for example, a reflective material to form the leadframe 210, a reflective coating applied to the walls 220 and/or surface 225 of the leadframe 210, a material having a refractive index that increases the likelihood of total internal reflection (TIR) to form the leadframe 210, or other techniques common in the art.

Optionally, a second material 320 may also be used as a base layer for the material 310. This second material 320 may be reflective, and may be less expensive and/or easier to apply than the material 310. The material 310, for example, may contain phosphors as wavelength conversion material. However, the likelihood of receiving light from the light emitting device at the regions adjacent the thick metal 28, 30 may be minimal. Accordingly, filling this region with non-phosphor material 320, such as a white dielectric, could reduce the cost without substantially impacting the light output efficiency. To enhance the light output efficiency, or to achieve a desired light output pattern, optical elements may be situated on the light emitting surface 315 of the encapsulating material 310.

FIG. 4A illustrates an optical element 410 that comprises a plurality of hemispherical lens elements that serve to scatter the light output from the surface 315, providing for a wide beam-width light output.

FIG. 4B illustrates an optical element 420 that comprises a plurality of curved surfaces situated such that the light output from the surface 315 is collimated to provide a narrow beam-width light output.

Other optical element shapes may be used to achieve a desired light output pattern, using techniques common in the art.

Although illustrated as a discrete lens 410, 420, the optical element 410, 420 may be formed using the material 310 when the material 310 is placed within the leadframe 210, using a conventional molding process. FIGs. 5A-5D illustrate alternative leadframes.

In FIG. 5A, the leadframe 510 is a simple substantially flat leadframe with leads 250 that provide external contact to the light emitting device 100. In this embodiment, a lens element 515 is provided upon the leadframe 510 after the light emitting device 100 is mounted on the leadframe 510. The lens element 515 may be molded over the light emitting device 100, or it may be preformed, with a recess for receiving the light emitting device 100. The upper surface of the leadframe 510, or the lower surface of the lens element 515 may be reflective. A wavelength conversion sheet 530 may be incorporated between the lens element 515 and the light emitting device 100.

In FIG. 5B, the leadframe 520 is a leadframe with recesses 525 for receiving light emitting elements 100. In this example embodiment, a substantially flat surface 521 is provided, and a preformed wavelength conversion sheet 530 is laminated upon this flat surface 531 to encapsulate the light emitting elements 100 within the leadframe 520.

Although not illustrated, a plurality of lens elements may also be formed upon the sheet 530, or the preformed sheet 530 may have a shaped upper surface to achieve a particular optical effect.

In some embodiments, all of the light emitting devices that are situated on a given leadframe 520 are selected based on a common light emitting characteristic, and the preformed sheet 530 is selected based on this common light emitting characteristic. 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 preforming a variety of phosphor films, with varying wavelength conversion properties, and selecting a particular film to provide a particular light output based on the combination of the lights produced by the light emitting elements and the wavelength conversion material.

In this example embodiment, light emitting elements 100 may be tested and sorted

('binned') based on their light output characteristics. Thereafter, multiple light emitting elements 100 having similar characteristics are placed on a single leadframe 520, and a particular phosphor sheet 530 is selected to be applied to the leadframe 520, 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 among devices formed from the leadframe 520 is substantially reduced. In the example embodiment of FIG. 5B, the leadframe 520 may include features 522 that facilitate the singulation of the individual light emitting devices on the leadframe. A slicing of the leadframe 520 through the wavelength conversion layer 530 and the thinner thickness provided by the features 522, thereby reducing the wear and/or load on the tool used to perform the slicing.

FIGs. 5C and 5D illustrate a profile and top view of a leadframe 540 that includes a recess 545 for receiving multiple light emitting elements 100 (100A-100D), and features 542 that facilitate singulation. When the individual light emitting devices are singulated from the leadframe 540, each device will include the multiple light emitting elements. Although the leadframe 540 of FIG. 5D is illustrated with multiple recesses 545 arranged horizontally, one of skill in the art will recognize that the leadframe 540 may include an array of horizontally and vertically arranged recesses 545.

Although contacts may be provided to allow each of the multiple light emitting elements within each recess 545 to be independently connected to an external power source, in the example embodiment of FIG. 5D, the leadframe 540 includes a single pair of external contacts 250N, 250P, and a conductive pattern 550A-550C on the upper surface of the recess 545 that interconnects the multiple light emitting elements 100A-100D (illustrated in dashed schematic form in FIG. 5D, to illustrate current flow). In this example embodiment, the conductive pattern 550A-550C couples each of the four light emitting elements 100A-100D in series, by connecting the P-contact of light emitting element 100 A to the N-contact of light emitting element 100B via conductive pattern 550A, the P-contact of light emitting element 100B to the N-contact of light emitting element lOOC via conductive pattern 550B, and the P- contact of light emitting element lOOC to the N-contact of light emitting element 100D via conductive pattern 550C. Other conductive patterns may be formed to provide other electrical configurations of the multiple light emitting elements 100.

The embodiments presented herein are merely examples of forms and features that may be provided by leadframes that are designed to receive self-supporting light emitting elements 100 with thick metal pillars. One of skill in the art will recognize that other forms and features, as well as other combinations of forms and features may be used for creating such leadframes. FIG. 6 illustrates an example flow diagram for the fabrication of encapsulated self- supporting light emitting devices.

At 610, a conventional light emitting structure is formed with N and P contacts on the same surface of the structure.

Upon these N and P contacts, a thick metal layer is formed, at 630, with insulating material between the formed layers. If necessary a seed layer is applied to the N and P contacts to facilitate the formation of the thick metal layer, at 620.

The thick metal layer may comprise any electrically conductive material, and preferably a material that exhibits high thermal conductivity to dissipate heat generated by the light emitting structure. Suitable materials include, for example, copper, nickel, gold, palladium, nickel-copper alloy, or other metals and alloys.

At 640, pads are formed on these thick metal pillars to facilitate external connection to the light emitting device 100. Depending upon the material used for forming the thick metal layers, the pads may merely be an extension of the pads beyond the aforementioned insulating material between the metal layers, or they may be a material selected to satisfy particular requirements, such as the requirement to provide non-oxidizing material, such as gold, as the external contacts. Each pad may occupy a surface area greater or smaller than the surface area of the corresponding thick metal layer, depending upon the dimensional requirements of the particular application for the device.

The devices 100 with the thick metal pillars and pads are singulated into individual devices, at 650. Of particular note, because of the thick metal pillars, the individual devices are self supporting, and thus can be directly picked and placed onto a leadframe, at 660, without the need to provide a supporting substrate structure, as required in the prior art.

At 670, the devices are encapsulated while on the leadframe, and after encapsulation, the encapsulated devices with leadframes are singulated, at 680.

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, although the example embodiments illustrate a single thick metal layer above each of the N and P contacts, one of skill in the art will recognize that this thick metal layer may comprise a plurality of individual thick metal pillars, and that other thick metal layers may be formed that are not coupled to the N and P contacts, as disclosed in the above referenced copending application of Jipu Lei et al. In some embodiments, multiple metal materials may be used to form a composite single thick metal layer. Other embodiments include metal and dielectric stacks for form a multilayer stack.

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 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;

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 each of the first and second pads; first and second contact pads that are electrically coupled to the first and second pads via the thick metal layer, the first and second contact pads being on a surface of the structure opposite the light emitting surface; and

a leadframe that includes first and second leads that are electrically coupled to the first and second contact pads and enable external coupling to the first and second contact pads,

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

2. The device of claim 1, wherein the first and second leads extend through the leadframe to enable external coupling to the first and second contact pads on a surface of the leadframe that is opposite to a surface upon which the light emitting structure is situated.

3. The device of claim 1, wherein the leadframe includes reflective walls that form a cavity in which the light emitting structure is situated, and the device includes an encapsulating material within the cavity.

4. The device of claim 3, wherein the encapsulating material includes a wavelength conversion material.

5. The device of claim 3, including one or more optical elements situated on a light emitting surface of the encapsulating material .

6. The device of claim 5, wherein the one or more optical elements are formed by the encapsulating material.

7. The device of claim 6, wherein the one or more optical elements include a collimator.

8. The device of claim 1, including a wavelength conversion film situated upon the light emitting surface.

9. The device of claim 1, wherein the light emitting structure is one of a plurality of light emitting structures, and the leadframe includes a conductive pattern that provides coupling among the plurality of light emitting structures.

10. The device of claim 9, wherein the leadframe includes reflective walls that form a cavity in which the plurality of light emitting structures are situated, and the device includes an encapsulating material within the cavity.

11. A leadframe comprising:

a plurality of leadframe elements, and

a plurality of self-supported light emitting structures,

wherein:

each self-supported light emitting structure includes contact pads that are coupled to a light emitting element via thick metal elements of at least 50 microns that provide structural support to the self- supported light emitting structure;

each leadframe element includes leads that provide external coupling to the contact pads of one or more of the plurality of self-supported light emitting structures situated at the leadframe element; and

each leadframe element with associated one or more self- supported light emitting structures forms an independently operable light emitting device.

12. The leadframe of claim 11, including a plurality of reflective walls separating the leadframe elements, the reflective walls forming a cavity within which the one or more light emitting structures are situated.

13. The leadframe of claim 12, wherein each cavity is filled with an encapsulating material.

14. The leadframe of claim 11, wherein a pair of reflective walls are situated between each pair of leadframe elements, a space between each pair of reflective walls serving to facilitate slicing of the leadframe to singulate the leadframe elements with associated one or more self- supported light emitting structures.

15. The leadframe of claim 11, wherein each leadframe element includes two or more of the light emitting structures, and the leadframe element includes a conductive pattern that provides coupling among the two or more light emitting structures.