LED wafer with laminated phosphor layer

An LED wafer with a growth substrate is attached to a carrier substrate by, for example, a heat-releasable adhesive so that the LED layers are sandwiched between the two substrates. The growth substrate is then removed, such as by laser lift-off. The exposed surface of the LED layers is then etched to improve light extraction. A preformed phosphor sheet, matched to the LEDs, is then affixed to the exposed LED layer. The phosphor sheet, LED layers, and, optionally, the carrier substrate are then diced to separate the LEDs. The LED dice are released from the carrier substrate by heat or other means, and the individual LED dice are mounted on a submount wafer using a pick-and-place machine. The submount wafer is then diced to produce individual LEDs. The active layer may generate blue light, and the blue light and phosphor light may generate white light having a predefined white point.

FIELD OF THE INVENTION

This invention relates to light emitting diodes (LEDs) with an overlying layer of phosphor to wavelength-convert the LED emission and, in particular, to a technique of laminating a phosphor layer over the LEDS.

BACKGROUND

Prior artFIG. 1illustrates a conventional flip chip LED die10mounted on a portion of a submount wafer12. In a flip-chip, both the n and p contacts are formed on the same side of the LED die.

The LED die10is formed of semiconductor epitaxial layers, including an n-layer14, an active layer15, and a p-layer16, grown on a growth substrate, such as a sapphire substrate. The growth substrate has been removed inFIG. 1by laser lift-off, etching, grinding, or by other techniques. In one example, the epitaxial layers are GaN based, and the active layer15emits blue light. LED dies that emit UV light are also applicable to the present invention.

A metal electrode18electrically contacts the p-layer16, and a metal electrode20electrically contacts the n-layer14. In one example, the electrodes18and20are gold pads that are ultrasonically welded to anode and cathode metal pads22and24on a ceramic submount wafer12. The submount wafer12has conductive vias24leading to bottom metal pads26and28for bonding to a printed circuit board. Many LEDs are mounted on the submount wafer12and will be later singulated to form individual LEDs/submounts.

Further details of LEDs can be found in the assignee's U.S. Pat. Nos. 6,649,440 and 6,274,399, and U.S. Patent Publications US 2006/0281203 A1 and 2005/0269582 A1, all incorporated herein by reference.

While an array of LED dies10are mounted on the submount wafer12or after the wafer12is diced, it is well known to deposit a phosphor over each LED die to generate any desired light color. To produce white light using the blue LED die10, it is well known to deposit a YAG phosphor, or red and green phosphors, directly over the die10by, for example, spraying or spin-coating the phosphor in a binder, electrophoresis, applying the phosphor in a reflective cup, or other means. It is also known to affix a preformed tile of phosphor (e.g., a sintered phosphor powder or phosphor powder in a binder) on the top of the LED die10. Blue light leaking through the phosphor, combined with the phosphor light, produces white light. Problems with creating the phosphor layer over the LED die10include the difficulty in creating very uniform phosphor layer thicknesses and densities. Any variation in the thickness or density will result in color non-uniformity over the surface of the LED die. A preformed tile of phosphor may be made more uniform and allows color testing of the tile prior to affixing it to the LED die; however, it is difficult and time-consuming to precisely affix each tile (e.g., 1 mm2) to the top surface of an LED die10.

Additionally, if a phosphor layer is deposited over all the LED dies10while the LED dies10are mounted on the submount wafer12, prior to dicing the wafer12, much of the phosphor will be wasted since it would be deposited on portions of the wafer12in-between the LED dies10.

What is needed is a technique to create a phosphor layer over an LED die that does not suffer from the drawbacks of the prior art.

SUMMARY

In one embodiment of the invention, LED layers are grown over a growth substrate, such as sapphire, SiC, GaN, spinel, or other known substrate, to form an LED wafer. The type of substrate used depends on the type of LEDs to be formed. The n and p-layers are contacted by cathode and anode metal electrodes so as to create perhaps thousands of LEDs on a single substrate wafer.

The surface of the LED wafer, prior to dicing, is adhesively fixed to a flat carrier substrate, such as by a releasable adhesive. Suitable releasable adhesives include those releasable by UV, heat, or a solvent. The LED layers are now sandwiched between the growth substrate and the carrier substrate. The carrier substrate may be a silicon wafer with an adhesive layer. Other carrier substrates include those composed of metal, glass, plastic, or any other suitable material.

The growth substrate is then removed, while the carrier substrate provides mechanical support. In one example, the growth substrate is sapphire, the LED layers are GaN layers optionally containing Al and In, and the sapphire substrate can be removed by laser lift-off.

The exposed surface of the LED layers can be thinned and roughened, such as by etching or a combination of processes, to increase light extraction and remove damage caused by the laser lift-off.

A preformed phosphor sheet approximately the size of the entire LED wafer is then affixed over the exposed surface of the LED layers. The LEDs making up the LED layers form a continuous surface since they have not been diced, so there is little waste of phosphor. The phosphor sheet may be pretested and selected to match the particular color characteristics of the LEDs on the wafer. In one embodiment, the phosphor sheet is somewhat flexible and comprises phosphor powder infused in a silicone binder. The phosphor sheet may be affixed to the LED layer surface using a thin layer of silicone.

In one embodiment, the phosphor sheet contains a YAG phosphor (yellow-green). In another embodiment, the phosphor sheet contains mixed red and green phosphors. In another embodiment, the phosphor sheet comprises multiple layers, such as a layer of red and a separate layer of YAG to produce a warm white color. The process can be used to make any color light using any type of phosphor.

The bottom surface of the carrier substrate may then be affixed to a tacky, stretchable sheet. Support surfaces other than a stretchable sheet may be used instead.

The phosphor sheet, LED layers, and carrier substrate are then diced (e.g., by sawing) to separate out the LEDs. The stretchable sheet may then be pulled in the x and y directions to physically separate the LEDs by a predetermined distance. Alternately, the phosphor sheet and LED layers may be singulated on the carrier substrate (e.g., by sawing) without the carrier substrate being singulated. In such a case, the stretchable sheet is not necessary.

The carrier substrate is then subjected to UV, heat, or a solvent to release the LED dice from the carrier substrate (whether or not the carrier substrate is diced).

An automatic pick and place machine then removes each LED die and mounts the die to a submount wafer. The LED die metal electrodes may be bonded to the submount wafer metal electrodes by ultrasonic bonding. Further processing may be performed on the LED dies while mounted on the submount wafer, such as forming a lens over each die. The submount wafer is then diced.

Accordingly, any phosphor waste is minimized, and it is straight forward to affix the phosphor sheet to the LED wafer. After dicing, the phosphor layer over each LED die is inherently aligned with the edges of the LED die. The phosphor layer may be uniformly thick and may have a substantially uniform density of phosphor. The resulting phosphor layer can be matched for each LED wafer so that the resulting color (e.g., white point) meets a target color. This can be important for applications where many identical LEDs are needed, such as for backlighting a large LCD television.

The LEDs may be flip-chips, or have top and bottom electrodes, or have top electrodes only.

Elements that are the same or equivalent are labeled with the same numeral.

DETAILED DESCRIPTION

FIG. 2illustrates LED layers30grown over a growth substrate32. In one embodiment, the LEDs emit blue or UV light and are formed by epitaxial GaN layers, such as shown inFIG. 1. The substrate32may be sapphire, GaN, SiC, or other suitable growth substrate. The substrate32is typically a circular wafer. Metal electrodes34are formed in electrical contact with the n and p LED layers for each LED die area. The metal electrodes34may be similar to the electrodes18and20inFIG. 1.

In another embodiment, the LEDs are not flip-chips but may have top and bottom electrodes or top electrodes only.

The boundaries between LEDs are shown by dashed lines35, where the LED wafer will be later sawed or scribed and broken.

InFIG. 3, the metal electrodes34are affixed to an adhesive layer36on a carrier substrate38. The carrier substrate38may be silicon since a silicon wafer can be made very flat and is relatively inexpensive. The adhesive layer36is preferably a non-tacky material that softens when heated to adhere to the metal electrodes34. Upon reheating the adhesive layer36, the metal electrodes34will be released. Such adhesives are well known. The carrier substrate38should be at least as large as the growth substrate32.

After the LED wafer is affixed to the carrier substrate38, so that there is good mechanical support for the thin LED wafer, the top surface of the GaN LED layers is exposed to pulses of excimer laser light42through the transparent growth substrate32. The laser light causes the surface GaN molecules to break down, and the gas released forces the growth substrate32off the LED layers30. The growth substrate32is then easily taken off the LED layers30. Such a laser lift-off process is well known.

InFIG. 4, the exposed surface of the LED layers30is thinned and roughened such as by reactive ion etching44or other suitable process. The LED layers30may instead be first thinned by mechanical polishing followed by an etching process to roughen the surface to achieve a controlled degree of roughness. The thinning removes portions that have been damaged by the laser lift-off and improves light extraction. Roughening the surface reduces internal reflections and further improves light extraction. A simplified magnified portion46of the LED surface is shown inFIG. 4.

A phosphor sheet is separately formed.FIG. 5illustrates the preformed phosphor sheet48. If white light is to be produced by the resulting LED, and the LED active layer emits blue light, the phosphor sheet48may be formed of one or more phosphors that emit red and green light when excited by blue light, and the phosphor layer must be thin enough or of sufficient low density to allow some blue light to pass through and combine with the red and green components. Suitable phosphors include a YAG phosphor (produces yellow-green light), combinations of red and green phosphors, or a combination of a YAG phosphor with a red phosphor to produce a warmer white light. If the LED generates UV light, a blue phosphor may also be included in the phosphor sheet48.

In one embodiment, to create the phosphor sheet48, the phosphor powder is mixed with silicone to achieve a target phosphor density, and the phosphor sheet48is formed to have a target thickness. The desired thickness may be obtaining by spinning the mixture on a flat surface or molding the phosphor sheet. Alternatively, the phosphor sheet48may be sawed from an elongated boule of phosphor to the desired thickness. In another embodiment, the phosphor sheet48is formed of sintered phosphor powder and may be sawed from a boule of sintered phosphor.

After the phosphor sheet48is formed, the phosphor sheet48may be tested by energizing the phosphor sheet48using a blue light source and measuring the light emission. Since blue LEDs in different wafers generally emit slightly different dominant wavelengths, the blue LEDs may be tested while part of the LED wafer. Preformed phosphor sheets of varying thicknesses or phosphor densities are then matched up with particular LED wafers so that the resulting color emissions may all have the same target white point (or CCT). Producing LEDs that output substantially identical white points is particularly valuable for applications that require matched LEDs such as for backlighting a large LCD television.

In one embodiment, the phosphor sheet48is on the order of a few hundred microns thick and somewhat flexible. The phosphor sheet48is preferably the same size as the LED wafer or larger.

As shown inFIG. 5, the matched phosphor sheet48is placed over the LED layers30, and a vacuum can be drawn between the phosphor sheet48and the LED layers30to remove all air. The phosphor sheet48can then be laminated to the LED layers30using heat and pressure (assuming there is sufficient silicone in the phosphor sheet48). This will conform the phosphor sheet48to the top surface of the LED layers30. For a phosphor sheet48not containing an appropriate type of silicone or for a sintered phosphor sheet48, a thin layer of silicone is applied over the LED layers30or phosphor sheet48to act as an adhesive for laminating the phosphor sheet48to the LED layers30using pressure. The silicone may be cured by heat or UV.

By laminating a preformed phosphor sheet onto the LED layer30prior to the LEDs being diced, at least the following advantages result: 1) there is little wasted phosphor since almost all the phosphor coats an LED; 2) the phosphor over each LED may have a uniform thickness and density; 3) it is fairly easy to properly position the phosphor sheet over the LED wafer and affix it to the LED wafer; 4) the phosphor sheet may be color-matched to the particular LED color emission; 5) when the LEDs are diced, the phosphor layer will precisely align with the edges of the LED to produce uniform color; and 6) the phosphor sheet may be formed of multiple layers, each layer being customized and precisely formed. In one embodiment, a multi-layer phosphor sheet is preformed by lamination, and the sheet is tested and then laminated as a single sheet to the LED layers30. Alternatively, the multiple layers may be individually laminated over the LED layers30. The multiple layers may be a YAG layer and a red phosphor layer.

FIG. 6illustrates the phosphor sheet48(assuming it is somewhat flexible) conforming over the surface of the LED layers30and carrier substrate38. InFIG. 6, the bottom surface of the carrier substrate38is affixed to a tacky stretchable sheet52. This may be done before or after the phosphor sheet48is affixed to the LED layers30. A suitable stretchable sheet52is commercially available for supporting dice during the dicing process. Support structures other than a stretchable sheet may also be used.

The phosphor sheet48, LED layers30, adhesive layer36, and carrier substrate38are then diced along the dashed lines54by any suitable technique. If the metal electrodes34extend to the edges of each LED, the metal electrodes34are also separated by the dicing process. The stretchable sheet52may be flexed over a curved surface to break the carrier substrate38after the carrier substrate38is partially sawed.

As shown inFIG. 7, the stretchable sheet52is stretched in the x and y directions to separate the LEDs by a predetermined amount. Note that the phosphor layer56(separated from the phosphor sheet48) is inherently aligned with the edges of the LED die58. Therefore, the resulting light emission will be substantially uniform.

The structure is then heated by, for example, an infrared lamp, to release the metal electrodes34from the adhesive layer36, and the structure is accessed by a pick-and-place machine programmed to automatically remove each LED die58, shown inFIG. 8, and mount the LED die58on a submount wafer60.

In another embodiment, the carrier substrate38is not singulated, and the sawing is only through the phosphor sheet48and LED layers30. The adhesive layer36is then released from the diced LED layers30using UV, heat, etc. The pick-and-place machine can then remove each LED die individually from the carrier substrate38. In such an embodiment, there is no need to mount the carrier substrate on the stretchable sheet52.

FIG. 8shows the submount wafer60having top metal electrodes62matched to the LED's electrodes34. The bonding may be by ultrasonic welding or other technique. The submount wafer60may be ceramic and have metal vias64that lead to bottom electrodes66for attachment to a printed circuit board.

FIG. 9is a top down view of the submount wafer60after being populated with an array of LED dice58. A lens may be formed over each LED die58while on the submount wafer60.

FIG. 10is a cross-sectional view of a single LED die58and submount12after dicing the submount wafer60and after the LED die58and phosphor layer56are encapsulated by a silicone lens72. The LED can be other than a flip-chip LED and may be formed of any suitable material.