# Patent

## Title
Light-emitting devices on textured substrates

## Abstract

A device includes a textured substrate, which further includes a plurality of trenches. Each of the plurality of trenches includes a first sidewall and a second sidewall opposite the first sidewall. A plurality of reflectors configured to reflect light is formed, with each of the plurality of reflectors being on one of the first sidewalls of the plurality of trenches. The second sidewalls of the plurality of trenches are substantially free from any reflector.

## Background
PRIORITY DATA
The present application is a divisional of U.S. patent application Ser. No. 12/704,997, filed on Feb. 12, 2010, now U.S. Pat. No. 8,981,397, issued Mar. 17, 2015, the disclosure of which is hereby incorporated by reference in its entirety.


TECHNICAL FIELD
This disclosure relates generally to light-emitting devices (LEDs), and more particularly to the methods of manufacturing LEDs on textured substrates and the resulting structures.
BACKGROUND
Light-emitting devices (LEDs), such as light-emitting diodes or laser diodes, are widely used for many applications. As is well known to those skilled in the art, an LED may include a semiconductor light-emitting element having a plurality of semiconductor layers formed on a substrate. The substrate may be formed of, for example, gallium arsenide, gallium phosphide, alloys thereof, sapphire and/or silicon carbide. Continued development in LEDs has resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent and fluorescent lamps.
LEDs emit light from active layers. Accordingly, light may be extracted from the opposite sides of the active layers. However, in certain situations, for example, when used for illumination, it may be preferred that the light is directed to only one side of an active layer, with the light being scattered to achieve a more uniform light distribution. Conventionally, patterned package substrates are bonded onto the LED chips to direct light to desirable directions. This, however, will increase the cost and complexity in the formation of the package substrates and the bonding process for bonding LED chips onto the package substrates. In addition, the solutions in package substrates did not help improve the light-extraction efficiency.
SUMMARY
In accordance with one aspect, a device includes a textured substrate, which further includes a plurality of trenches. Each of the plurality of trenches includes a first sidewall and a second sidewall opposite the first sidewall. A plurality of reflectors configured to reflect light is formed, with each of the plurality of reflectors being on one of the first sidewalls of the plurality of trenches. The second sidewalls of the plurality of trenches are substantially free from any reflector.
Other embodiments are also disclosed.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1 through 5 are cross-sectional views and a top view of intermediate stages in the manufacturing of a light-emitting device (LED) in accordance with an embodiment, wherein a textured substrate is formed after the LED is formed; and
FIGS. 6 through 8 are cross-sectional views of intermediate stages in the manufacturing of a textured substrate in accordance with an alternative embodiment, wherein the textured substrate is pre-formed before the LED is bonded thereon.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments of the disclosure are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative and do not limit the scope of the disclosure.
A novel light-emitting device (LED) in accordance with an embodiment and the method of forming the same are presented. The intermediate stages of manufacturing an embodiment are illustrated. The variations and the operation of the embodiment are then discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements
FIG. 1 illustrates chip 100, which may be a part of an un-diced wafer comprising a plurality of identical chips. Alternatively, the structure shown in FIG. 1 may be a discrete chip that is already separated from a wafer. Chip 100 includes LED 110 formed over substrate 20. In various embodiments, substrate 20 is formed of sapphire (Al2O3). In alternative embodiments, substrate 20 is a silicon-containing substrate, such as a silicon carbide substrate, a silicon substrate, or a silicon germanium substrate. In yet other embodiments, substrate 20 comprises compound semiconductor materials comprising group-III and/or group-V elements, or also known as III-V compound semiconductor materials.
In an exemplary embodiment, before LED 110 is formed, buffer layer 24 is formed on substrate 20. Buffer layer 24 may comprise un-doped gallium nitride (u-GaN). A plurality of layers for forming LED 110 is then formed layer-by-layer on buffer layer 24. For example, LED 110 includes cladding layer 26 of a first conductivity type (p-type or n-type). Cladding layer 26 may be an n-type group-III/V compound (III/V compound) layer, such as an n-GaN layer (GaN doped with an n-type impurity). At least one multiple quantum well (MQW) 28 that acts as an active layer for emitting light is formed on cladding layer 26. MQW 28 may be formed of, for example, InGaN. Cladding layer 30, which is of a second conductivity type opposite the first conductivity type, is formed on MQW 28. Cladding layer 30 may be a p-type III/V compound layer, such as a p-GaN layer (GaN doped with a p-type impurity).
An ohmic contact layer (not shown, for example, formed of GaAs) may optionally be formed over cladding layer 30. Reflector 32 is formed over (and may be formed over the ohmic contact layer) cladding layer 30. Reflector 32 may be formed of a metallic material, such as Al, Cu, Ti, TiN, TiW, or alloys thereof. Top electrodes (which are also bond pads) 34 and 38 are formed to electrically connect to cladding layers 30 and 26, respectively. Accordingly, by applying a voltage between electrodes 34 and 38, LED 110 may be activated to emit light.
The formation of layers 26, 28, 30, and 32 are known in the art, and hence are not described herein. In an exemplary embodiment, the formation methods of layers 26, 28, and 30 include epitaxial growth. It is realized that LED 110 may have many designs, which are also in the scope of the present disclosure. For example, the materials of each of layers 26, 28, and 30 may be different from the above-discussed materials.
Referring to FIG. 2, after LED 110 is formed, chip 100 is flipped upside down. Photo resist 40 is then formed on substrate 20, and is patterned to form openings 42. As shown in FIG. 3A, substrate 20 is then etched through openings 42. Trenches 44 are thus formed in substrate 20 and extend from top surface 20a of substrate 20 into substrate 20. Trenches 44 include inner sidewalls 44_1, outer sidewalls 44_2, and bottoms 44_3. Inner sidewalls 44_1 face away from center 48 of chip 100, while outer sidewalls 44_2 face toward center 48. Inner sidewalls 44_1, outer sidewalls 44_2, and bottoms 44_3 may be substantially straight in the cross-sectional view or curved slightly.
FIG. 3B schematically illustrates a top view of the structure shown in FIG. 3A, wherein the cross-sectional view is obtained from the plane crossing line 3A-3A in FIG. 3B. In an embodiment, trenches 44 (and the portion of un-etched substrate 20 between trenches 44) form closed loops, which may have four straight sides, each being parallel to one of edges 46 of chip 100. In alternative embodiments, trenches 44 may be in the form of circles. In yet other embodiments, trenches 44 may have near-square shapes with straight edges and curved corners. Trenches 44 may be symmetric relative to center 48 of chip 100.
Referring back to FIG. 3A, in an embodiment, outer sidewalls 44_2 form slant angles α (illustrated as α1, α2, α3, and an) with major surface 20a (the horizontal surface in FIG. 3A) of substrate 20. Slant angles α may be greater than about 45 degrees, and may be between about 45 degrees and 90 degrees. With such slant angles, the vertical light, as shown by arrows in FIG. 5, will be reflected upwardly, rather than downwardly.
Chip 100 has edges 46. In an embodiment, the slant angles α closer to edges 46 are smaller than the slant angles α closer to center 48. In a direction from edges 46 to center 48, the increase in slant angles α may also be gradual. The differences in slant angles α of outer sidewalls 44_2 may be achieved by controlling the pattern size of trenches 44 in different regions of chip 100. For example, under some etching conditions, a wider width (W1) would produce a trench with a smaller slant angle. Generally, the widths W1, W2, W3 . . . of trenches 44 (refer to FIG. 3A), which are arranged from edges 46 toward center 48, may be increasingly greater, smaller, stay the same, or change in a random fashion in the direction from edges 46 to center 48. Further, in the direction from edges 46 toward center line 48, spacings S1, S2, S3 . . . between neighboring trenches 44 (refer to FIG. 3A), may be increasingly greater, smaller, stay the same, or change in a random fashion. The slant angles α closest to edges 46 may be close to about 45 degrees, while the slant angles α closest to center 48 may be close to 90 degrees. Such arrangements result in the micro-loading effect in the etching of substrate 20 during the formation of trenches 44, and hence result in the difference in slant angles α. The optimum distribution of widths (W1/W2/W3 and so on) and spacings (S1/S2/S3 and so on) partially depend on the LED light direction requirements and may be found through experiments. After the formation of trenches 44, photo resist 40 is removed.
In alternative embodiments, instead of using etching to form trenches 44, other applicable methods, such as laser micro-machining may also be used. For example, a mask (not shown, which can be a polymer mask) may be formed over the flat substrate 20. Patterns may be formed in the mask, with the patterns in the mask being similar to the desirable pattern of trenches 44. The mask and the underlying substrate 20 are then etched using an etchant attacking both substrate 20 and the mask. As a result, the portions of substrate 20 over which the mask is thinner are etched more than the portions of substrate 20 over which the mask is thicker. Accordingly, the pattern in the mask is transferred to substrate 20.
In FIG. 4, reflective layer 50 is formed on substrate 20. Reflective layer 50 may be a conformal layer formed using, for example, electro or electroless plating or a physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), or other known semiconductor deposition method. The materials of reflective layer 50 may include metals, such as Al, Cu, Ti, TiN, and/or alloys thereof. The thickness of reflective layer 50 may be between about 50 nm and about 2,000 nm, for example. Reflective layer 50 extends into trenches 44, and is formed on sidewalls 44_1 and 44_2 and bottoms 44_3. Next, photo mask 52 is formed and patterned. After patterning, photo mask 52 may leave the portions of reflective layer 50 on bottoms 44_3 and inner sidewalls 44_1 of trenches 44 uncovered. Also, the flat portions of top surface 20a of substrate 20 are not covered. The portions of reflective layer 50 on outer sidewalls 44_2 are covered by photo mask 52. Next, the exposed portions of reflective layer 50 are removed, while the covered portions of reflective layer 50 remain to form a plurality of reflectors 50′. The resulting structure is shown in FIG. 5.
FIG. 5 schematically illustrates the light reflected from reflectors 50′. It is observed that reflectors 50′, with different slant angles α, result in the light to be redirected to different directions. With the reflection of reflectors 50′, the light (symbolized by arrows 60) emitted by LED 110 is scattered and is redistributed more uniformly. In locations having no reflectors 50′, light (symbolized by arrows 62) is not reflected. With the redirection of the light by reflectors 50′, the light extraction efficiency is improved so that less light is absorbed in the LED structure. Since from edges 46 toward center 48, the slant angles α of outer sidewalls 44_2 become increasingly greater, reflectors 50′ closer to edges 46 of chip 100 will not block the light reflected from reflectors 50′ closer to center 48. Further, the gradual increase in slant angles α improves the uniformity in the distribution of the light.
FIGS. 6 through 8 illustrate the intermediate stages in the formation of an LED structure in accordance with an alternative embodiment. Unless specified otherwise, reference numerals in this embodiment represent like elements as in the embodiment shown in FIGS. 1 through 5. In this embodiment, the textured substrate is pre-formed and then bonded onto the LED. Referring to FIG. 6, textured substrate 20′ is provided, on which reflectors 50′ are formed. The formation processes of reflectors 50′ may be essentially the same as shown in FIGS. 2 through 5, and are not repeated herein. In this embodiment, since substrate 20′ is not used to grow an LED thereon, substrate 20′ be formed of materials that are more transparent, which materials do not have to be suitable for growing LEDs thereon. For example, substrate 20′ may be formed of silicon, glass, or the like, although it may also be formed of the same materials as substrate 20 as in FIG. 1.
FIG. 7 illustrates the laser lift off of an LED from a substrate. For example, LED 110 is formed on substrate 120, wherein buffer layer 24 may be formed of un-doped GaN. LED 110 may be essentially the same as shown in FIG. 5. In an exemplary embodiment, substrate 120 is exposed to a light energy, such as a laser beam (symbolized by arrows) projecting from the bottom of substrate 120. The laser beam penetrates through substrate 120 to reach layer 24. As a result, buffer layer 24 is decomposing by the heat resulting from the laser beam, and hence LED 110 is separated from substrate 120. In an embodiment, the laser is a KrF laser with a wavelength of about 248 nm. Next, as shown in FIG. 8, LED 110 is bonded onto textured substrate 20′, for example, using transparent adhesive 66.
It is observed that the embodiments may be implemented directly onto the substrates on which the LEDs are grown. Accordingly, the embodiments provide a manufacturing and packaging process with a low cost and a low complexity.
Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the embodiments as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the disclosure.

## Claims

1. A method, comprising:
forming a plurality of recesses in a first side of a substrate, wherein the recesses each include a first sidewall surface and a second sidewall surface;
forming a reflective layer over the first side of the substrate, wherein the reflective layer are formed on the first and second sidewall surfaces of the recesses;
removing portions of the reflective layer formed on the second sidewall surfaces of the recesses; and
forming a light-emitting diode (LED) on a second side of the substrate, wherein the second side being opposite to the first side.

2. The method of claim 1, wherein the first sidewall surfaces are outer sidewall surfaces, and wherein the second sidewall surfaces are inner sidewall surfaces.

3. The method of claim 1, wherein the forming of the recesses is performed in a manner such that at least some of the recesses have different shapes than other recesses.

4. The method of claim 3, wherein the recesses are formed with different slant angles.

5. The method of claim 1, wherein: the forming of the recesses is performed in a manner such that each recess further includes a bottom surface that adjoins the first sidewall surface and the second sidewall surface; and the removing of the portions of the reflective layer is performed such that at least a portion of each of the bottom surface is free of the reflective layer.

6. The method of claim 1, wherein the forming of the recesses is performed such that at least some of the recesses are circumferentially surrounded by other recesses in a top view.

7. The method of claim 6, wherein each of the recesses is a closed loop in the top view.

8. The method of claim 1, wherein the step of forming the light-emitting diode (LED) on a second side of the substrate comprises bonding the second side of the substrate to the light-emitting diode (LED).

9. The method of claim 1, wherein the substrate includes a sapphire substrate, a silicon substrate, a silicon carbide substrate, a silicon germanium substrate, or a substrate containing III-V compound materials.

10. A method, comprising:
forming a plurality of trenches in a substrate, wherein the trenches each extend from a first side of the substrate toward a second side of the substrate opposite the first side, wherein the trenches each include a first sidewall surface, a second sidewall surface, and a bottom surface adjoining the first and second sidewall surfaces;
forming a reflective material in each of the trenches, the reflective material covering the first sidewall surface, the second sidewall surface, and the bottom surface in each of the trenches;
removing portions of the reflective material from each of the trenches in a manner such that the second sidewall surface and at least a portion of the bottom surface in each trench is free of being covered with the reflective material; and
forming a light-emitting diode (LED) on a second side of the substrate, wherein the second side being opposite to the first side.

11. The method of claim 10, wherein the first sidewall surfaces are outer sidewall surfaces, and wherein the second sidewall surfaces are inner sidewall surfaces.

12. The method of claim 10, wherein the forming of the trenches is performed in a manner such that the trenches have different slant angles.

13. The method of claim 10, wherein each of the trenches is a closed loop in a top view, and wherein at least some of the trenches are encircled by at least some other trenches in the top view.

14. The method of claim 10, wherein the step of forming the light-emitting diode (LED) on the second side of the substrate comprises bonding the second side of the substrate to the light-emitting diode (LED).

15. A method of forming a semiconductor device, the method comprising:
providing a substrate;
forming a plurality of trenches extending from a surface of the substrate into the substrate, wherein the plurality of trenches comprises inner sidewalls and outer sidewalls;

forming a reflective layer on the surface of the substrate and on the inner sidewalk and the outer sidewalls;
removing the reflective layer from one of the inner sidewalls of the plurality of trenches; and
forming a light-emitting diode (LED) on a second side of the substrate, wherein the second side being opposite to the first side.

16. The method of claim 15, wherein the step of forming the light-emitting diode on the second side of the substrate comprises bonding the light-emitting diode (LED) onto the substrate, with the plurality of trenches and the light-emitting diode (LED) being on opposite sides of the substrate.

17. The method of claim 15, wherein the step of forming the plurality of trenches comprises laser micro-machining.