Robust LED structure for substrate lift-off

[0001] This invention relates to flip-chip light emitting diodes (LEDs) and, in particular, to a process for preparing the LED for removal of the growth substrate and adding an optical element in its place.

[0002] Philips Lumileds Lighting Company, LLC has developed a technique for forming high efficiency LEDs, where the LEDs are formed as flip-chips and the growth substrate is removed after the flip-chip is mounted on a submount. In a flip-chip, both the n and p contacts are formed on the same side of the LED die opposite to the growth substrate side.

[0003] Prior art FIGS. 1-3 illustrate the general substrate lift-off process and the problems associated with adding an optical element in place of the removed substrate. Further details can be found in the assignee's U.S. patent Publications US 2006/0281203 A1 and 2005/0269582 A1, both incorporated herein by reference.

[0004] In FIG. 1, LED semiconductor epitaxial layers 10, including an n-layer, an active layer, and a p-layer, are grown on a growth substrate 12, such as a sapphire substrate. In the example, the layers 10 are GaN based, and the active layer emits blue light.

[0005] Metal electrodes 14 are formed that electrically contact the p-layer, and metal electrodes 16 are formed that electrically contact the n-layer. In the example, the electrodes are gold bumps that are ultrasonically welded to anode and cathode metal pads 18 and 20 on a ceramic submount 22. The submount 22 has conductive vias 24 leading to bottom metal pads 26 and 28 for bonding to a printed circuit board.

[0006] An underfill material 30 is then injected under and around the LED for structural support, to fill in air gaps, and to protect the chip from contaminants. The underfill 30 may be liquid silicone that is then cured to harden.

[0007] The substrate 12 is then removed using a laser lift-off process. The photon energy of the laser (e.g., an excimer laser) is selected to be above the band gap of the LED material and below the absorption edge of the sapphire substrate (e.g., between 3.44 eV and 6 eV). Pulses from the laser through the sapphire are converted to thermal energy within the first 100 nm of the LED material. The generated temperature is in excess of 1000° C. and dissociates the gallium and nitrogen. The resulting high gas pressure pushes the substrate away from the epitaxial layers to release the substrate from the layers, and the loose substrate is then simply removed from the LED structure. The underfill helps prevent the thin LED layers from breaking under the high pressure.

[0008] The growth substrate may instead be removed by etching, such as reactive ion etching (RIE). Other techniques may be used depending on the type of LED and substrate. In one example, the substrate is Si-based and an insulating material between the substrate and the LED layers is etched away by a wet etch technique to remove the substrate.

[0009] The exposed LED material may be further etched to remove damaged material and to thin the LED to enhance the light output. FIG. 2 shows the resulting structure.

[0010] Since the underfill 30 originally covered the sides of the substrate 12, the edges of the underfill remain after the substrate is removed to effectively form walls around the LED layers 10. Providing a precise amount of underfill to only fill under and around the LED layers without contacting the substrate is extremely difficult and so the structure of FIG. 2 is typical. The underfill 30 typically extends further laterally than shown in the figures in actual devices.

[0011] As shown in FIG. 3, a transparent adhesive material 32 (e.g., silicone) is deposited on the exposed LED surface layer. A preformed phosphor plate 34 is intended to be precisely positioned over the LED and glued to the top surface of the LED. Any misalignment of the plate 34 causes the plate 34 to not seat properly on the LED due to the raised underfill walls around the LED. The phosphor plate 34 may be formed of a YAG phosphor either sintered or in a transparent binder. The YAG phosphor emits yellow-green light when energized by the blue LED. The yellow-green light combined with the blue light leaking through the plate 34 produces white light. As a result of the plate 34 not seating properly, the light emission properties of the LED will not be optimal and the plate 34 may easily delaminate from the LED.

[0012] What is needed is an improved technique that avoids the above-mentioned alignment problems when affixing a phosphor plate or any other optical element to the surface of the LED in place of the removed substrate.

[0013] An LED structure is disclosed that offers a greater tolerance for affixing a phosphor plate or other optical element to the top surface of an LED chip after the substrate is removed.

[0014] An extra etching step is performed on the LED/substrate wafer prior to the LEDs/substrates being singulated and mounted onto a submount. The etching step etches through the LED epitaxial layers entirely around each LED on the substrate wafer to form a gap between each LED on the wafer. The substrate is not etched. The LEDs are then singulated by cutting (or breaking) the substrate wafer approximately halfway between the etched gaps between the LEDs so that each LED will have edges of the substrate extending out beyond edges of the LED by a small amount (e.g., 0.1-0.25 mm). The substrate for each LED appears as an enlarged substrate compared to the prior art. Since the LED already undergoes etching to expose the n-layer to form the flip-chip n-electrode, the extra etching step does not require extra handling of the LED, just an extra masking and etching step.

[0015] Each singulated LED/substrate is then mounted on a submount. After an underfill is deposited and cured, the underfill fills in under the LED and covers at least a portion of the sides of the “enlarged” substrate. When the substrate is removed, there is then a gap between the walls of the underfill surrounding the LED and the LED itself. This gap relaxes the tolerance when placing the phosphor plate over the top of the LED so that the plate is affixed flush onto the surface of the LED. Additionally, the phosphor plate may be formed larger than the LED surface to prevent the blue side-emission from the LED passing around the edges of the plate. Hence, the resulting light color will be more uniform.

[0016] Variations of this technique are also described, such as the singulation step occurring after the LEDs are mounted on a submount wafer.

[0017] Instead of a phosphor plate, a lens, a reflector, or other optical element may benefit from the increased tolerance provided by the invention.

[0027] Elements that are the same or equivalent are labeled with the same numeral.

[0028] As a preliminary matter, a conventional LED is formed on a growth substrate. In the example used, the LED is a GaN-based LED, such as an AlInGaN or InGaN LED, for producing blue light. Typically, a relatively thick n-type GaN layer is grown on a sapphire growth substrate using conventional techniques. The relatively thick GaN layer typically includes a low temperature nucleation layer and one or more additional layers so as to provide a low-defect lattice structure for the n-type cladding layer and active layer. One or more n-type cladding layers are then formed over the thick n-type layer, followed by an active layer, one or more p-type cladding layers, and a p-type contact layer (for metallization).

[0029] For a flip-chip, portions of the p-layers and active layer are etched away to expose an n-layer for metallization. In this way, the p contact and n contact are on the same side of the chip and can be directly electrically attached to the submount contact pads. Current from the n-metal contact initially flows laterally through the n-layer.

[0030] Other types of LEDs that can be used in the present invention include AlInGaP LED, which can produce light in the red to yellow range.

[0031] Examples of forming LEDs are described in 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 assigned to Philips Lumileds and incorporated by reference.

[0032] FIG. 4 is a top down view of a wafer 36 comprising a substrate, such as a sapphire growth substrate, with epitaxial LED layers formed thereon. The LED layers include n-layers, an active layer, and p-layers. Light is generated in the active layer. The wafer 36 is cross-hatched to show the boundaries of individual LEDs 40. In an actual example, there may be hundreds or thousands of LEDs formed on a single wafer.

[0033] FIG. 5 is a cross-sectional view of a small portion of the wafer 36. The LED layers are masked and subjected to a dry etch 42, such as RIE, to etch through the epitaxial layers to create a narrow gap 44 completely around each LED 40. The masking and etching are performed using conventional techniques applicable to the particular LED material and substrate 46. The substrate 46 is not etched. The LEDs 40 may have a top surface area on the order of 1 mm² or less. The gap 44 may have any suitable width, such as 0.1 mm-0.5 mm. The etching step, described previously, to gain access to the n-layers for the n-electrode can be performed before, after, or as part of the etching step shown in FIG. 5.

[0034] The various metallization steps are then performed to create the metal connections and electrodes on the surface of the LED, described with respect to FIG. 1.

[0035] The places where the substrate 46 will ultimately be broken or cut for singulation are shown with dashed lines 48. The lines 48 are approximately through the middle of the gaps 44. Thus, the substrate 46 will extend beyond each edge of the singulated LED die by about half the gap between adjacent LEDs on the wafer (e.g., 0.05 mm-0.25 mm).

[0036] In one embodiment, the LEDs on the wafer 36 are all bonded to corresponding pads on a submount wafer at the same time before singulation. Bonding may be by ultrasonic welding, as previously described. Then, the structure is singulated by sawing or scribing/breaking to create individual LEDs mounted on submounts. Alternatively, the substrate 46 of FIG. 5 is broken along the lines 48 and the singulated LEDs/substrates are then individually mounted on respective submounts.

[0037] The LED layers will generally have a thickness between 5-20 microns, and the substrate 46 has a thickness greater than 100 microns.

[0038] FIG. 6 illustrates the resulting LED 40 and substrate 46 after being mounted on a conventional submount 22, such as that described with respect to FIG. 1. The submount is singulated to gain access to the LED 40 for the underfill process. An insulating underfill 52 is injected under and around the LED 40 for structural support, to fill in air gaps, and to protect the chip from contaminants. The underfill 52 may be liquid silicone that is then cured to harden. The underfill 52 contacts the sides of the substrate 46 to ensure adequate coverage of the underfill 52.

[0039] FIG. 6A is a top down view of FIG. 6 showing how the substrate 46 is longer and wider than the LED 40 and how the underfill 52 surrounds the substrate 46.

[0040] An excimer laser beam 54 is applied to the surface of the sapphire substrate 46, as described with respect to FIG. 1, to remove the substrate 46. The substrate 46 may be removed by any of the aforementioned methods. After the substrate is removed, the LED epitaxial layers may be etched the remove damaged material and to thin the LED for improved light extraction. The LED 40 may be similar to the LED 10 described with respect to FIGS. 1 and 2 except for its dimensions relative to the removed substrate 46.

[0041] As seen in FIG. 7, the allowable area for a phosphor plate or any other optical element is wider than the area shown in FIG. 3, since the walls of the underfill 52 are separated from the edges of the LED 40. This allows the positioning of the plate or optical element to have a less stringent tolerance compared to the positioning required in FIG. 3.

[0042] As shown in FIG. 8, a suitable transparent adhesive 56 is injected, sprayed, or otherwise deposited over the LED. Then, an optical element 58, such as a phosphor plate, a Fresnel lens or other type of lens, or even a reflector, is positioned over the LED 40, using conventional automated pick and place equipment. The element 58 is then supplied downward pressure, and the adhesive 56 is cured.

[0043] The larger allowable area for positioning the element 58 enables the element 58 to be wider than the LED 40 itself. Therefore, any upward light emitted from the sides of the LED is still converted by the optical element. The allowable placement area may be any suitable size to obtain the desired yield from the pick and place step. There is a tradeoff between the size of the placement area and lost LED material due to the etching.

[0044] In one embodiment, the LED 40 die emits blue light, and the phosphor emission from the phosphor plate in conjunction with the blue light results in a white light being generated. For example, the phosphor plate may contribute a yellow component or red and green components to the blue light to create white light. One such suitable phosphor is a YAG phosphor.

[0045] While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.