# Patent

## Title
Manufacturing process for solid state lighting device on a conductive substrate

## Abstract

A method for fabricating a light emitting device includes forming a trench in a first surface on a first side of a substrate. The trench comprises a first sloped surface not parallel to the first surface, wherein the substrate has a second side opposite to the first side of the substrate. The method also includes forming light emission layers over the first trench surface and the first surface, wherein the light emission layer is configured to emit light and removing at least a portion of the substrate from the second side of the substrate to form a protrusion on the second side of the substrate to allow the light emission layer to emit light out of the protrusion on the second side of the substrate.

## Background
The present application is a Continuation-in-Part (CIP) application of and claims priority to commonly assigned pending U.S. patent application Ser. No. 12/691,269, entitled “Solid state lighting device on a conductive substrate”, filed Jan. 21, 2010, the content of which is incorporated herein by reference.


BACKGROUND
The present patent application is related to solid state light emission devices.
Solid-state light sources, such as light emitting diodes (LEDs) and laser diodes, can offer significant advantages over incandescent or fluorescent lighting. The solid-state light sources are generally more efficient and produce less heat than traditional incandescent or fluorescent lights. When LEDs or laser diodes are placed in arrays of red, green and blue elements, they can act as a source for white light or as a multi-colored display. Although solid-state lighting offers certain advantages, conventional semiconductor structures and devices used for solid-state lighting are relatively expensive. The high cost of solid-state light emission devices is partially related to the relatively complex and time-consuming manufacturing process for solid-state light emission devices.
Referring to FIG. 1, a conventional LED structure 100 includes a substrate 105, which may be formed of sapphire, silicon carbide, or spinel, for example. A buffer layer 110 is formed on the substrate 105. The buffer layer 110 serves primarily as a wetting layer, to promote smooth, uniform coverage of the sapphire substrate. The buffer layer 310 is typically deposited as a thin amorphous layer using Metal Organic Chemical Vapor Deposition (MOCVD). An n-doped Group III-V compound layer 120 is formed on the buffer layer 110. The n-doped Group III-V compound layer 120 is typically made of GaN. An InGaN quantum-well layer 130 is formed on the n-doped Group III-V compound layer 120. An active Group III-V compound layer 140 is then formed on the InGaN quantum-well layer 130. A p-doped Group III-V compound layer 150 is formed on the layer 140. A p-electrode 160 (anode) is formed on the p-doped Group III-V compound layer 150. An n-electrode 170 (cathode) is formed on the n-doped Group III-V compound layer 120.
A drawback in the conventional LED devices is that different thermal expansions between the group III-V nitride layers and the substrate can cause cracking in the group III-V nitride layers or delamination between the group III-V nitride layers from the substrate.
A factor contributing to complexity in some conventional manufacturing processes is that it requires a series of selective etch stages. For example, the cathode 170 in the conventional LED structure 100 shown in FIG. 1 is formed on the n-doped Group III-V compound layer 120 by selectively etching. These selective etch stages are complicated and time-consuming and, therefore, make the overall manufacturing process more expensive.
It is also desirable to increase active light emission intensities. The conventional LED device in FIG. 1, for example, includes non-light emission areas on the substrate 105 that are not covered by the InGaN quantum-well layer 130 to make room for the n-electrode 170. The p-electrode 160 can also block some of the emitted light from leaving the device. These design characteristics reduce the emission efficiency of the conventional LED devices.
Another requirement for LED devices is to properly direct inward-propagating light emission to the intended light illumination directions. A reflective layer is often constructed under the light emission layers to reflect light emission. One challenge associated with a metallic reflective layer is that the metals such as Aluminum have lower melting temperatures than the processing temperatures for depositing Group III-V compound layers on the metallic reflective layer. The metallic reflective layer often melts and loses reflectivity during the high temperature deposition of the Group III-V compound layers.
SUMMARY OF THE INVENTION
The disclosed light emitting device and associated manufacturing processes are intended to overcome above described drawbacks in conventional solid state lighting devices. Embodiments may include one or more of the following advantages. An advantage associated with the disclosed solid-state lighting structures and fabrication processes is that active light emitting areas and light emission efficiency can be significantly improved.
Another significant advantage associated with the disclosed solid-state lighting structures and fabrication processes is that a reflective layer can be properly formed under the light emission layers to effectively reflect the emitted light to intended light illumination directions.
Yet another significant advantage associated with the disclosed solid-state lighting structures and fabrication processes is that effective cooling can be provided by an entire conductive substrate during the lighting operation.
Moreover, the electrodes are arranged on the opposite sides of the disclosed light emission devices. Effective packaging techniques are provided without using wire bonding, which makes the packaged light emission modules more reliable and less likely to be damaged. Additionally, more than one light emission structure can be conveniently packaged in a single light emission module, which reduces packaging complexity and costs.
Furthermore, the disclosed LED structures and fabrication processes can overcome lattice mismatch between the group III-V layer and the substrate, and can prevent associated layer cracking and delamination that are found in some conventional LED structures.
In one general aspect, the present invention relates to a method for fabricating a light emitting device. The method includes forming a trench or a truncated trench in a first surface on a first side of a substrate, wherein the trench or a truncated trench comprises a first sloped surface not parallel to the first surface, wherein the substrate has a second side opposite to the first side of the substrate; forming light emission layers over the first trench surface and the first surface, wherein the light emission layer can emit light; and removing at least a portion of the substrate from the second side of the substrate to expose at least one of the light emission layers.
Implementations of the system may include one or more of the following. The method can further include forming a base electrode layer over the light emission layers before the step of removing at least a portion of the substrate from the second side of the substrate, wherein the base electrode layer at least partially fills the trench or a truncated trench on the first side of the substrate. The base electrode layer can be formed by electroplating or deposition over the light emission layers on the first side of the substrate. The base electrode layer can include a metallic material or a conducting polymer. The method can further include a reflective layer on the light emission layers, wherein the base electrode layer is formed on the reflective layer. The method can further include forming a transparent conductive layer over the light emission layers on the second side of the substrate after the step of removing at least a portion of the substrate from the second side of the substrate. The light emission layers can emit light in response to an electric current flowing across the base electrode layer and the transparent conductive layer. The substrate can include silicon, SiC, ZnO, Sapphire, or GaN. The first surface can be substantially parallel to a (100) crystal plane of the substrate, and wherein the first sloped surface is substantially parallel to a (111) crystal plane of the substrate. The substrate can have a (100) crystal plane and a (111) crystal plane, wherein the first surface is substantially parallel to the (111) crystal plane, and wherein the first sloped surface is substantially parallel to the (100) crystal plane. The light emission layers can include at least one quantum well formed by Group III-V compounds. The quantum well can include: a first III-V nitride layer; a quantum-well layer on the first III-V nitride layer; and a second III-V nitride layer on the quantum well layer. The method can further include forming a buffer layer on the first sloped surface before the step of forming light emission layers, wherein the light emission layers are formed on the buffer layer. The buffer layer can include a material selected from the group consisting of GaN, ZnO, AlN, HfN, AlAs, SiCN, TaN, and SiC. The step of removing can form a protrusion on the second side of the substrate. The protrusion can have the shape of a pyramid or a truncated pyramid, wherein the first sloped surface is a substantially flat face in part defining the pyramid or the truncated pyramid.
In another general aspect, the present invention relates to a method for fabricating a light emitting device. The method includes forming light emission layers having monolithic crystal structures on a silicon substrate, wherein the light emission layers can emit light when an electric current flows across the light emission layers, wherein the silicon substrate is on a first side of the light emission layers; forming abuse electrode layer over a second side of the light emission layers, the second side being opposite to the first side, wherein the base electrode layer comprises anon-crystalline conductive material; and removing at least a portion of silicon on the first side of the light emission layers to expose at least one of the light emission layers. The light emission layers can include a monolithic quantum well formed by Group III-V compounds. The non-crystalline conductive material can include a metallic material or a conducting polymer.
The method can further include a reflective layer on the first side of the light emission layers, wherein the base electrode layer is formed on the reflective layer; and forming a transparent conductive layer over the second side of the light emission layers after the step of removing at least a portion of the silicon substrate.
In another general aspect, the present invention relates to a method for making a light emission module. The method can include constructing one or more light emitting structures on a conductive substrate, wherein each of the one or more light emitting structures comprises light emission layers and a transparent conductive layer on the light emission layers; attaching the one or more light emitting structures to a mounting substrate by an electric interconnect, the mounting substrate having a first electrode and a second electrode; allowing the first electrode to be in electrical connection with the conductive substrate; and allowing the second electrode to be in electric connection with the transparent conductive layer, wherein the light emission layers in each of one or more light emitting structures can emit light when an electric current flows across the first electrode and the second electrode.
In another general aspect, the present invention relates to a light emitting device that includes a conductive substrate having a first substrate surface, wherein the conductive substrate includes a conductive material; a protrusion formed on the conductive substrate, wherein the protrusion can be defined in part by a first protrusion surface that is not parallel to the first substrate surface; and light emission layers disposed over the first protrusion surface, wherein the light emission layers can emit light when an electric field is applied across the light emission layers.
Implementations of the system may include one or more of the following. The conductive material can include a metallic material or a conducting polymer. The protrusion can have the shape of a pyramid or a truncated pyramid, wherein the first substrate surface is a substantially flat face in part defining the pyramid or the truncated pyramid. The first protrusion surface can have an angle between 20 degrees and 80 degrees relative to the first substrate surface. The light emission layers can include at least one quantum well formed by Group III-V compounds. The quantum well can be formed by a first III-V nitride layer; a quantum-well layer on the first III-V nitride layer; and a second III-V nitride layer on the quantum well layer. The light emitting device can further include a reflective layer formed between the conductive substrate and the light emission layers. The reflective layer can include aluminum, silver, gold, mercury, or chromium. The light emitting device can further include a transparent conductive layer formed over the light emission layers, wherein the electric field across the light emission layers is produced by a voltage applied between the transparent conductive layer and the conductive substrate. The light emitting device can further include an electrode layer around the protrusion, wherein the electrode layer is in electric connection with the transparent conductive layer.
In another general aspect, the present invention relates to a light emitting device that includes a non-crystalline conductive substrate; and light emission layers having monolithic crystal structures disposed over the conductive substrate, wherein the light emission layers can emit light when an electric field is applied across the light emission layers.
Implementations of the system may include one or more of the following. The light emission layers can include at least one monolithic quantum well formed by Group III-V compounds. The monolithic quantum well can be formed by a first nitride layer; a quantum-well layer on the first III-V nitride layer; and a second III-V nitride layer on the quantum well layer. The light emitting device can further include a reflective layer formed between the conductive substrate and the light emission layers. The light emitting device can further include a transparent conductive layer formed over the light emission layers, wherein the electric field across the light emission layers is produced by a voltage applied between the transparent conductive layer and the conductive substrate. The light emitting device can further include a protrusion formed on the conductive substrate, wherein the conductive substrate comprises a first substrate surface outside of the protrusion, wherein the protrusion is defined in part by a first protrusion surface that is not parallel to the first substrate surface. The protrusion can have the shape of a pyramid or a truncated pyramid. The non-crystalline conductive substrate can include a metallic material or a conducting polymer
In another general aspect, the present invention relates to a light emission module that includes a mounting substrate having a first electrode and a second electrode; one or more light emitting structures constructed on a conductive substrate, wherein each of the one or more light emitting structures comprises light emission layers and a transparent conductive layer on the light emission layers, wherein the light emission layers in each of one or more light emitting structures can emit light when a voltage is applied between the transparent conductive layer and the conductive substrate; and an electric interconnect that can attach or clamp the one or more light emitting structures to the mounting substrate to allow the first electrode to be in electrical connection with the conductive substrate and the second electrode to be in electric connection with the transparent conductive layer.
Implementations of the system may include one or more of the following. The one or more light emitting structures can include one or more protrusions on the conductive substrate, wherein the light emission layers are formed on the one or more protrusions. The one or more light emitting structures can further include an electrode layer around the protrusion, the electrode layer in electric connection with the transparent conductive layer, wherein the electric interconnect can electrically connect the electrode layer to the second electrode. The electric interconnect can include a window over the light emission layers in the one or more light emitting structures to allow light emitted from the light emission layers to pass through when the electric interconnect clamps the one or more light emitting structures to the mounting substrate.



BRIEF DESCRIPTION OF THE DRAWINGS
The following drawings, which are incorporated in and from a part of the specification, illustrate embodiments of the present invention and, together with the description, serve to explain the principles of the invention.
FIG. 1 is a cross-sectional view of a conventional LED structure.
FIG. 2 is a flowchart for fabricating the light emission devices in accordance with an aspect of the present invention.
FIGS. 3A-3S are cross-sectional and perspective views illustrating the light emitting structures at different steps in the flowchart in FIG. 2 for fabricating the light emission device.
FIG. 4A is a detailed cross-sectional view illustrating a light emission structure with small lateral dimensions.
FIG. 4B is a detailed cross-sectional view illustrating a light emission structure with a flat conductive surface.
FIG. 5A shows the packaging of the light emission devices into light emitting modules.
FIG. 5B shows the packaged light emission devices from FIG. 5A,
FIG. 6 is a flowchart for fabricating the light emission devices in accordance with another aspect of the present invention.
FIGS. 7A-7J are cross-sectional views illustrating the light emitting structures at different steps in the flowchart in FIG. 6 for fabricating the light emission device.
FIG. 8A shows the packaging of the light emission devices into light emitting modules.
FIG. 8B shows the packaged light emission devices from FIG. 8A,
FIG. 9 is a schematic diagram illustrating angular distribution of light emission from the light emitting device in accordance with the present invention.



DESCRIPTION OF THE INVENTION
Referring to FIGS. 2 and 3A-3D (FIGS. 3A and 3B are cross-sectional views along the A-A direction in FIG. 3C), a silicon substrate 300 has a first side 310 and a second side 320 opposing to the first side 310. The silicon substrate 300 can for example be about 750 μm in thickness. The silicon substrate 300 includes a surface 301 on the first side 310. The substrate 300 can be a (100) silicon wafer, that is the surface 301 is along a (100) crystalline plane. A mask layer 302 is formed and patterned on the surface 301. The mask layer 302 can be formed by a silicon nitride layer, or a silicon oxide layer. The mask layer 302 has an opening 305 that exposes the silicon substrate 300 on the first side 310. The silicon substrate 300 is then etched through the opening 305 to form a trench 308 (step 210). The trench 308 has a plurality of substantially flat surfaces 331 that are not parallel or be sloped relative to the surface 301. The surfaces 331 can form a reverse pyramid, truncated trench, or a truncated reverse pyramid having a surface 332 that is substantially parallel to the surface 301. If the substrate 300 is a (100) silicon wafer, the surfaces 331 are (111) silicon surfaces and the surface 332 is a (100) silicon surface. The surfaces 331 are at a 54.7° angle relative to the surface 301 and the surface 332. The mask layer 302 is then removed, leaving a trench having sloped surfaces 331 in the substrate 300 on the first side 310 of the substrate 300 (step 215, FIG. 3D).
Referring to FIGS. 2, 3E and 3F, one or more buffer layers 335 are next formed on the surface 301 and surfaces 331 (step 220). The buffer layer(s) 335 can for example comprise AlN in a thickness range between about 1 nm and about 1000 nm, such as 10 to 100 Angstroms. The buffer layer(s) can include a thinner AlN layer (e.g. about 30 nm) formed on the substrate 300 at a lower substrate temperature (e.g. 700° C.) followed by a deposition of a thicker AlN layer (e.g. about 70 nm) formed on the first thinner AlN layer at a higher substrate temperature (e.g. 1,200° C.). The buffer layer(s) 335 can also be formed of GaN, ZnO, HfN, AlAs, TaN, or SiC.
A plurality of light emitting layers 340 are next formed on the buffer layer 335 (step 225). The light emitting layers 340 include semiconductor quantum well layers that can produce and confine electrons and holes under an electric field. The recombination of the electrons and the holes can produce light emission. The emission wavelengths are determined mostly by the bandgap of the material in the quantum-well layers. Exemplified light emitting layers 340 can include, from the buffer layer 335 and up, an AlGaN layer (about 4,000 A in thickness), a GaN:Si layer (about 1.5 μm in thickness), an InGaN layer (about 50 A in thickness), a GaN:Si layer (about 100 A in thickness), an AlGaN:Mg layer (about 100 A in thickness), and GaN:Mg (about 3,000 A in thickness). The GaN:Si layer (about 100 A in thickness) and the InGaN layer can be repeated several times (e.g. 3 to 7 times) on top of each other to form a periodic quantum well structure.
The buffer layer 335 and the light emitting layers 340 can be formed using atomic layer deposition (ALD), Metal Organic Chemical Vapor Deposition (MOCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Chemical Vapor Deposition (CVD), or Physical vapor deposition (PVD). The formation of the buffer layer(s) 335 between the substrate 300 and the light emitting layers 340 can reduce mechanical strain between the (111) silicon surfaces of the substrate 300 and the light emitting layers 340, and prevent cracking and delamination in the light emitting layers 340. As a result, the quantum-well layers can have monolithic crystal structures with matched crystal lattices. Light emitting efficiency of the LED device can be improved. Details of forming trenches, the buffer layer, and the light emitting layers are disclosed in patent application Ser. No. 12/177,114, titled “Light Emitting Device” filed on Jul. 21, 2008 and patent application Ser. No. 11/761,446, titled “Silicon Based Solid State Lighting” filed on Jun. 12, 2007 both by the present inventor, the disclosures of which are incorporated herein by reference.
A reflective layer 345 is next, referring to FIGS. 2 and 3G, formed on the light emitting layers 340 (step 230). The reflective layer 345 can be formed by a layer of Aluminum approximately 500 nm in thickness. The reflective layer 345 can also include materials such as Ag, Au, Hg, or Cr.
A base electrode layer 350 is next formed on the reflective layer (FIG. 3H, step 235). The base electrode layer 350 can include a metallic material such as copper, aluminum, nickel, and iron, and can be formed by electroplating. The base electrode layer 350 can also include a conductive polymer material, which can be formed by coating. The base electrode layer 350 can have a layer thickness about 200 μm. As described below, the base electrode layer 350 can be used as one of the electrodes for applying electric field across the light emitting layers 340 and for cooling the light emitting device during operation. The base electrode layer 350 can fill at least a portion of the trench 308, which can leave a dimple 355, as shown in FIGS. 3H-3J. A plurality of trenches 308 and related light emission layers 340 can be simultaneously formed on a single substrate (300) such as a silicon wafer, as shown in FIG. 3J.
Next, the silicon material in the substrate 300 is removed from the second side 320 below the buffer layer 335 and the light emitting layers 340 to expose the buffer layer 335 (FIG. 3K, step 240). As shown in a bottom perspective view of FIG. 3L, the buffer layer 335 and the light emitting layers 340 are thus disposed on the pyramids 360 on the second side 320 of the substrate 300 (not shown in FIG. 3L because the silicon material in the substrate 300 has been removed). As a result, a light emitting structure 370 is partially formed. The light emitting structure 370 is also shown in a top perspective view in FIG. 3M and in a cross-sectional view in FIG. 3N with the first side 310 and the second side 320 are reversed in position (the reflective layer 345 is not shown in FIGS. 3L and 3M due to drawing scale).
Next, the buffer layer(s) 335 are removed from the second side 320 of the light emitting structure 370 (FIG. 3O, step 245). The light emitting layers 340 are exposed to the second side 320. A transparent conductive layer 375 comprising for example indium tin oxide (ITO) is next formed on the light emitting layers on the second side of the substrate (FIG. 3P, step 250). The removal of the buffer layer(s) 335 allows the transparent conductive layer 375 to be in contact with the light emitting layers 340 to allow a voltage to be applied across the light emitting layers 340.
A conductive ring layer (ring electrode) 380 is next formed on the transparent conductive layer 375 around the pyramids as shown in FIGS. 3Q-3S (step 255). (The light emitting layer 340 and the reflective layer 345 in FIG. 3R are not shown due to drawing scale). The ring electrode 380 can be formed by the same material (e.g. copper) as the base electrode layer 350 or Al. A notable feature of the light emitting structure 370 is that the quantum-well layers having monolithic crystal structures are formed over a non-crystalline conductive substrate that are made of metals or conductive polymers. The monolithic crystal structure of the quantum well layers allows the light emission layers 340 that comprise the monolithic quantum layers to have high light emission efficiency. The non-crystalline conductive substrate functions as one of the two electrodes for applying the electric field, and can provide cooling to the light emission device during operation.
It should be understood that the shape and the size the dimple 355 may vary with the dimension of the light emission structure 370. A light emission structure having a lateral dimension of 2 mm or larger may have a large and deep dimple 355 in the base electrode layer 350 as shown in FIG. 3H. A light emission structure 370 having a lateral dimension smaller than 2 mm may have a large and deep dimple 355 in the base electrode layer 350 as shown in FIG. 4A. Furthermore, as shown in FIG. 4B, the base electrode layer 350 can be flattened on the first side 310 by for example chemical mechanical polishing a flat conductive surface. The base electrode layer 350 can thus have a substantially flat surface 390 on the first side 310 opposing to the light emission side (the second side 320) of the base electrode layer 350.
FIG. 5A shows an exploded view light emission modules 500 and 550, and the packaging of light emission structures 370 into light emitting modules 500 and 550 (step 260 in FIG. 2). An insulating substrate 400 includes on its upper surface electrode layers 410, 420, 430. The insulating substrate 400 can be made of an insulating ceramic material, which can act as a heat sink. The light emission structure 370 includes a pyramid 360 on the base electrode layer 350. A plurality of light emission layers (not shown) are formed on the pyramid 360. The transparent conductive layer 375 is formed on the light emission layers. The ring electrode 380 is formed around the pyramid 360 and in contact with the transparent conductive layer 375. The light emission structure 370 can be mounted directly on the electrode layer 420 on the insulating substrate 400 such that the electrode layer 420 is in electric contact with the base electrode layer 350. An electric interconnect 450 includes a window frame 451 having an opening therein and connected with two arms 460 and 470. The electric interconnect 450 is made of an electrically conductive material such as copper. The electric interconnect 450 can be clamped down such that the window frame 451 is in electric contact with the ring electrode 380. The two arms 460 and 470 become respectively in contact with the electrode layers 410 and 430.
When the electric interconnect 450 and the light emission structure 370 are tightly clamped to the substrate 400, the electrode layers 410 and 430 are connected with the transparent conductive layer 375. The electrode layer 420 is connected with the base electrode layer 350. An electric voltage applied across the electrode layer 420 and the electrode layers 410 and 430 can thus produce an electric field across the light emission layers 340 (in FIGS. 3Q and 4, not shown in FIG. 5A), which can cause light emission in the light emission module 500 (shown in FIG. 5B).
In some embodiments, referring to FIGS. 6 and 7A-7J, a silicon substrate 700 has a first side 710 having a surface 701 and a second side 720 opposing to the first side 710. The substrate 700 can be a (100) silicon wafer with the surface 701 is along a (100) crystalline plane. The substrate 700 can also be formed by SiC, Sapphire, or GaN. SiN layers are deposited on both the first side and the second side of the silicon substrate 700 (step 605, FIG. 7A). A mask layer 702M is formed by patterning and selecting etching the SiN layer 702 on the surface 701 (step 610, FIG. 7B). The mask layer 702M has an opening 705 that exposes the silicon substrate 700 on the first side 710.
The silicon substrate 700 is then etched through the opening 705 to form a trench 708 (step 615, FIG. 7C). The trench 708 has a plurality of substantially flat surfaces 731 that are not parallel or be sloped relative to the surface 701. The surfaces 731 can form a reverse pyramid. If the substrate 700 is a (100) silicon wafer, the surfaces 731 are (111) silicon surfaces. The surfaces 731 are at a 54.7° angle relative to the surface 701. Optionally, the mask layer 702M can then be removed, leaving a trench having sloped surfaces 731 in the substrate 700 on the first side 710 of the substrate 700.
One or more buffer layers (not shown for clarity) are next formed on the surface 701, and surfaces 731 or the remaining hard mask layer 702M (step 620). The buffer layer(s) can comprise AlN in a thickness range between about 1 nm and about 1000 nm, such as 10 to 100 Angstroms. The buffer layer and the light emitting layers 740 can be formed using atomic layer deposition (ALD), Metal Organic Chemical Vapor Deposition (MOCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), Chemical Vapor Deposition (CVD), or Physical Vapor Deposition (PVD).
A plurality of light emitting layers 740 are next formed on the buffer layer (step 625, FIG. 7D). The light emitting layers 740 include semiconductor quantum well layers that can produce and confine electrons and holes under an electric field. The recombination of the electrons and the holes can produce light emission. The emission wavelengths are determined mostly by the bandgap of the material in the quantum-well layers. Exemplified light emitting layers 740 can include, from the buffer layer, an AlGaN layer (about 4,000 A in thickness), a GaN:Si layer (about 1.5 μm in thickness), an InGaN layer (about 50 A in thickness), a GaN:Si layer (about 100 A in thickness), an AlGaN:Mg layer (about 100 A in thickness), and GaN:Mg (about 3,000 A in thickness). The GaN:Si layer (about 100 A in thickness) and the InGaN layer can be repeated several times (e.g. 3 to 7 times) on top of each other to form a periodic quantum well structure. The light emitting layers 740 can for example be formed by MOCVD.
The formation of the buffer layer(s) between the substrate 700 and the light emitting layers 740 can reduce mechanical strain between the (111) silicon surfaces of the substrate 700 and the light emitting layers 740, and prevent cracking and delamination in the light emitting layers 740. As a result, the quantum-well layers can have monolithic crystal structures with matched crystal lattices. Light emitting efficiency of the LED device can be improved. Details of forming trenches, the buffer layer, and the light emitting layers are disclosed in patent application Ser. No. 12/177,114, titled “Light Emitting Device” filed on Jul. 21, 2008 and patent application Ser. No. 11/761,446, titled “Silicon Based Solid State Lighting” filed on Jun. 12, 2007 both by the present inventor, the disclosures of which are incorporated herein by reference.
After MOCVD layers, a surface treatment on top of GaN:Mg (about 3,000 A in thickness) can be applied to further enhance light emitting efficiency. This treatment can be dry or wet etch with or without patterning.
A reflective layer 745 is next formed on the light emitting layers 740 (step 630, FIG. 7E). The reflective layer 745 can be formed by a layer of Aluminum approximately 500 nm in thickness. The reflective layer 345 can also include materials such as Ag, Au, or Cr. The reflective layer 345 can be formed by MOCVD, Electroplating, or PVD.
A base electrode layer 750 is next formed on the reflective layer (FIG. 7F, step 635). The base electrode layer 750 can include a metallic material such as copper, nickel, aluminum, chromium, and steel, and can be formed by electroplating. The base electrode layer 750 can also be formed by a deposition method such as PVD, MBE, CVD, and PECVD. The base electrode layer 750 can also include a conductive polymer material, which can be formed by coating. The base electrode layer 750 can have a layer thickness from 50 to 500 μm. The base electrode layer 750 can be used as one of the electrodes for applying electric field across the light emitting layers 740 and for cooling the light emitting device during operation. The base electrode layer 750 can fill at least a portion of the trench 708, which can leave a dimple 755.
Next, the silicon material in the substrate 700 and the SiN layer 702 are removed from the second side 720 below the buffer layer and the light emitting layers 740 (FIG. 7G, step 640) (the device structure is flipped upside down from FIG. 7F to FIG. 7G). The light emitting layers 740 are thus disposed on the pyramids 760 on the second side 720 of the substrate 700. As a result, a light emitting structure 770 is partially formed. The buffer layer (not shown for clarity) is removed from the second side 720 of the light emitting structure 770 (step 645) to expose the light emitting layers 740 the second side 720 (FIG. 7G).
A transparent conductive layer 775 comprising for example ITO is next formed on the light emitting layers on the second side of the substrate (FIG. 7H, step 650). The transparent conductive layer 775 is in contact with the light emitting layers 740 to allow a voltage to be applied across the light emitting layers 740.
A conductive ring layer (ring electrode) 780 is next formed on the transparent conductive layer 775 around the pyramids (FIG. 7I, step 655). The ring electrode 780 can be formed by the same material (e.g. copper) as the base electrode layer 750 or Al. The light emitting structure 770 is then diced to its final form. (FIG. 7J, step 660)
A notable feature of the light emitting structure 770 is that multiple pyramids 760 can be formed on a single device in a series of common processing steps. The light emitting layers formed on the multiple pyramids can significantly increase lighting intensity. The number of pyramids in a single light emitting structure can be varied to customize the dimensions of the lighting device.
Another notable feature of the light emitting structure 770 is that the quantum-well layers having monolithic crystal structures are formed over a non-crystalline conductive substrate that are made of metals or conductive polymers. The monolithic crystal structure of the quantum well layers allows the light emission layers that comprise the monolithic quantum layers to have high light emission efficiency. The non-crystalline conductive substrate functions as one of the two electrodes for applying the electric field, and can provide cooling to the light emission device during operation.
Referring to FIGS. 8A and 8B, a light emission structure 470 includes a plurality of pyramids 360 formed on a common base electrode layer 350, as described above. A plurality of light emission layers (not shown due to drawing scale) are formed on the pyramids 360. The transparent conductive layer 375 is formed on the light emission layers. A common ring electrode 380 is formed around the pyramids 360 and in contact with the transparent conductive layer 375.
When the electric interconnect 450 and the light emission structure 470 are tightly clamped to the substrate 400, the electrode layers 410 and 430 are connected with the transparent conductive layer 375. The electrode layer 420 is connected with the base electrode layer 350. An electric voltage applied across the electrode layer 420 and the electrode layers 410 and 430 can thus produce an electric field across the light emission layers 340 (in FIGS. 3Q and 4, not shown in FIG. 8A), which can cause light emission in the light emission module 550 (FIG. 8B).
An advantage of the light emission modules 500 and 550 is that there is no need for wire bonding to electrically connect the light emission structures 370 to external electrodes (410-430). As it is known that wire bonding is easily damaged in the handling, the disclosed light emission modules are thus more reliable than some conventional solid-state light emitting devices.
The packaging of light emitting modules (step 260 in FIG. 2) can also include dicing of light emitting structures on a substrate into dies each containing smaller number of light emitting structures. For example, the light emitting structures 370 on the conductive substrate 350 in FIG. 4B can be diced into dies each containing one or a few light emitting structures which can subsequently form a light emitting module as shown in FIGS. 5A and 5B.
FIG. 9 is a schematic diagram illustrating angular distribution of light emission from the light emitting device in accordance with the present invention. A light emitting device 370 includes a pyramid 360 formed on a base electrode layer 350. Light emitting layers having light emission surfaces 910, 920 are formed on the sloped surfaces of the pyramid 360. If the light emission structure 370 is formed with a (100) silicon wafer, the upper surface 930 is along the (100) crystalline plane and the light emission surfaces 910, 920 parallel to the (111) crystalline planes. The light emission surfaces 910, 920 are at a 54.7° angle relative to the upper surface 930. For the same foot print on the upper surface, the sum of the areas of the emission surfaces on the light emission surfaces 910, 920 is approximately 1.73 times the area of the upper surface 930 under the pyramid 360. The light emission from the light emission surfaces 910, 920 can assume a broad distribution 1280 as shown in FIG. 9.
An advantage associated with the disclosed light emission device and fabrication processes is that light emitting layers are constructed on surfaces sloped relative to the substrate, which can significantly increase light emission areas and efficiency. Another advantage of the disclosed light emission device and fabrication processes is that silicon wafers can be used to produce solid state LEDs. Manufacturing throughput can be much improved since silicon wafer can be provided in much larger dimensions (e.g. 8 inch, 12 inch, or larger) compared to the substrates used in the conventional LED structures. Furthermore, the silicon-based substrate can also allow driving and control circuit to be fabricated in the substrate. The light emission device can thus be made more integrated and compact than conventional LED devices. Another advantage associated with the disclosed devices and fabrication processes is that the disclosed light emitting structures can be fabricated using existing commercial semiconductor processing equipment such as ALD and MOCVD systems. The disclosed fabrication processes can thus be more efficient in cost and time that some conventional LED structures that need customized fabrication equipments. The disclosed fabrication processes are also more suitable for high-volume semiconductor lighting device manufacture. Yet another advantage of the disclosed light emitting structures and fabrication processes is that multiple buffer layers can be formed to smoothly match the crystal lattices of the silicon substrate and the lower group III-V nitride layer. Yet another advantage of the disclosed light emitting structures and fabrication processes is that a surface treatment is applied to p-doped GaN to enhance light emitting efficiency. Yet another advantage of the disclosed LED structures and fabrication processes is that a transparent conductive layer formed on the light emitting layers and a reflective layer formed under the light emitting layers can maximize light emission intensity from the upper surfaces of the LED structures. Yet another advantage of the disclosed light emitting structures and fabrication processes is that light emitting layers are directly in contact with conductive metal substrate, which insures the best thermal conductivity during LED operation. This can increase both LED life time and efficiency, especially for high brightness and high power LEDs. Yet another advantage of the disclosed light emitting structures and fabrication processes is that there is a wafer level common electrode to use wireless wafer lever packaging.
The foregoing descriptions and drawings should be considered as illustrative only of the principles of the invention. The invention may be configured in a variety of shapes and sizes and is not limited by the dimensions of the preferred embodiment. Numerous applications of the present invention will readily occur to those skilled in the art. Therefore, it is not desired to limit the invention to the specific examples disclosed or the exact construction and operation shown and described. Rather, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention. For example, the n-doped and the p-doped group III-V nitride layers can be switched in position, that is, the p-doped group III-V nitride layer can be positioned underneath the quantum-well layer and n-doped group III-V nitride layer can be positioned on the quantum-well layer. The disclosed LED structure may be suitable for emitting green, blue, and emissions of other colored lights. In another example, a (111) silicon wafer can be used as a substrate to allow trenches having (100) sloped surfaces to form in the substrate.
Moreover, the sloped protrusion surface can be at an angle between 20 degrees and 80 degrees, or as a more specific example, between 50 degrees and 60 degrees, relative to the upper surface of the substrate. The emission surfaces on a protrusion in the disclosed light emitting device can be more than 1.2, or 1.4, or 1.6 times of the base area of the protrusion. The large emission surface areas in the described light emitting devices allow the disclosed light emitting device can thus generate much higher light emission intensity than conventional LED devices.
The disclosed systems and methods are compatible with a wide range of applications such as laser diodes, blue/UV LEDs, Hall-effect sensors, switches, UV detectors, micro electrical mechanical systems (MEMS), and RF power transistors. The disclosed devices may include additional components for various applications. A laser diode based on the disclosed device can include reflective surfaces or mirror surfaces for producing lasing light. For lighting applications, the disclosed system may include additional reflectors and diffusers.

## Claims

1. A method of fabricating a light emitting device, comprising:
forming a trench in a first surface on a first side of a substrate, wherein the trench comprises a first sloped surface not parallel to the first surface, wherein the substrate has a second side opposite to the first side of the substrate;
forming light emission layers over the first trench surface, wherein the light emission layer is configured to emit light;
removing at least a portion of the substrate from the second side of the substrate to expose at least one of the light emission layers forming a transparent conductive layer and over the exposed at least one of the light emission layers.

2. The method of claim 1, wherein the light emission layers are configured to emit light in response to an electric current flowing across the base electrode layer and the transparent conductive layer.

3. A method for fabricating a light emitting device, comprising:
forming light emission layers having monolithic crystal structures on a silicon substrate, wherein the light emission layers are configured to emit light when an electric current flows across the light emission layers, wherein the silicon substrate is on a first side of the light emission layers;
forming a reflective layer on the second side of the light emission layers, the second side being opposite to the first side; forming a base electrode layer on the reflective layer, wherein the base electrode layer comprises a conductive material;
removing at least a portion of silicon substrate on the first side of the light emission layers to expose at least one of the light emission layers; and
forming a transparent conductive layer over the exposed at least one of the light emission layers.

4. A method for making a light emission module, comprising:
constructing one or more light emitting structures on a conductive substrate, wherein each of the one or more light emitting structures comprises light emission layers and a transparent conductive layer on the light emission layers, wherein the light emission layers are formed on one or more protrusions on the conductive substrate, wherein the one or more light emitting structures further comprise an electrode layer around the protrusion, the electrode layer in electric connection with the transparent conductive layer, wherein the electric interconnect is configured to electrically connect the electrode layer to the second electrode;
attaching the one or more light emitting structures to amounting substrate by an electric interconnect, the mounting substrate having a first electrode and a second electrode;
allowing the first electrode to be in electrical connection with the conductive substrate; and
allowing the second electrode to be in electric connection with the transparent conductive layer, wherein the light emission layers in each of one or more light emitting structures are configured to emit light when an electric current flows across the first electrode and the second electrode.

5. The method of claim 4, wherein the electric interconnect includes a window to allow light emitted from the light emission layers to pass through when the one or more light emitting structures is clamped to the mounting substrate by the electric interconnect.