# Patent

## Title
Method for manufacture of bright GaN LEDs using a selective removal process

## Abstract

A method of fabricating LED devices includes using a laser to form trenches between the LEDs and then using a chemical solution to remove slag creating by the laser.

## Background
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor processes and, more particularly, to a method for singulating die from a wafer in furtherance of fabricating light emitting diodes.
Although saw dicing has traditionally been effectively used for singulating dice it has given way to singulation processes employing different methods. For example the laser has replaced the mechanical saw to avoid the deleterious effects of vibration and particulate contaminates generated by saw dicing. Edge defects that often occurred as a result of saw dicing are avoided, as are micro-cracks.
Laser cutting may be employed with a cleaving process such that the laser is used to form a trench in the wafer, but does not cut completely through the wafer. A cleaving process is implemented to complete the singulation process after formation of the trench. Often the laser cutting process leaves a residue on the die formed from the singulation process. The residue may prove problematic to the desired operation of optical semiconductor dice, such as light emitting diodes. These and other limitations may be exist with the conventional cleaving process.
Accordingly, it is desirable to provide an improved method for singulating dice.
BRIEF SUMMARY OF THE INVENTION
This invention provides techniques for manufacturing high brightness optical devices. More particularly, the method provides a processing method for removing slag material to enhance light output and efficiency of light emitting diodes, commonly termed LEDs.
The invention provides a manufacturing method for light emitting diode devices. The manufacturing method includes a technique for singulating dice from a substrate, e.g., a sapphire substrate or bulk gallium nitride. The method includes providing the substrate having a thickness of gallium and nitrogen containing material. Preferably, the substrate includes a plurality of die, with each die providing an LED device. The method also includes forming trenches by scanning a coherent beam of energy in a repetitive manner to respective portions of the surface. The scanned coherent beam, preferably derived from a laser, causes ablation of selected materials of the surface. The trenches extend from the surface of the substrate toward an opposing surface, without separating the die. The die may be held together by a thickness of adhesive material or a remaining thickness of the substrate. The trenches can be of desired shape, e.g., v-shaped.
The method causes formation of slag material along the trenches during their formation. The method also subjects the trenches and the slag to a chemical solution to selectively remove the slag without significantly changing the cross-section of the trenches. The chemical is preferably a potassium hydroxide solution, a potassium ferricyanide solution, or a hydrochloric acid solution, or combinations of them. Preferably, the chemical solution is a buffered potassium ferricyanide solution.
The method may further include increasing the temperature of the chemical solution from approximately 50° C. to a temperature below the boiling point of the solution. The die may then be broken apart. The method also includes mounting the substrates to a flexible adhesive thickness of material from which they are later removed.
In a specific embodiment, the invention provides a method which includes forming a plurality of trenches by scanning a coherent beam of energy in a repetitive manner across respective portions of the surface regions. Each of the trenches extends from the surface of the semiconductor substrate toward an opposing surface. Slag is formed along the trenches, but is later removed by subjecting the trenches and the slag to a chemical solution. The chemical solution comprises a potassium hydroxide solution, a potassium ferricyanide solution, or a hydrochloric acid solution. Upon completion each of the die is substantially free from slag.
The chemical solution has certain preferably characteristics. As an example, the potassium hydroxide solution consists of 45% potassium hydroxide by weight and 55% water by weight. The potassium ferricyanide solution consists of potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight and water in a range of 55%-69% by weight. The method also includes heating the hydrochloric acid solution to approximately 65° C. The hydrochloric acid solution consists of about 35% hydrochloric acid by weight and 65% water by weight.
The present method provides for a high efficiency bright light emitting diode device, e.g. with 100 lumens per Watt output. Preferably, the method maintains the shape of each of the trenches to within about 98% of their original shape even after removing the slag. That is, there is less than 2% and preferably less than 1% removal of the gallium and nitrogen containing material. Depending upon the embodiment, the present method achieves over a 100% increase in light output compared to no removal of the slag.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a single light emitting diode;
FIG. 2 is a flow diagram showing the steps used to manufacture the light emitting diode shown in FIG. 1;
FIG. 3 is a cross-sectional view showing a substrate upon which the light emitting diode of FIG. 1 is produced;
FIG. 4 is a cross-sectional view showing the substrate in FIG. 3 with a p-type gallium nitride layer thereon;
FIG. 5 is a cross-sectional view of the substrate shown in FIG. 4 having a patterned photoresist layer thereon;
FIG. 6 is a cross-sectional view of the substrate shown in FIG. 4 having a plurality of metal layers thereon;
FIG. 7 is a cross-section view of the substrate shown in FIG. 6 after a lift-off process;
FIG. 8 is a cross-sectional view of the wafer shown in FIG. 6 with slag present;
FIG. 9 is a top view of the wafer shown in FIG. 8;
FIG. 10 is a cross-sectional view of the wafer shown in FIG. 8 after removal of slag; and
FIG. 11 is a diagram illustrating device performance.



DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a light emitting diode 10 on a substrate 12 of n-type gallium nitride GaN manufactured. An active layer 14 is formed upon substrate. Active layer 14 may comprise a single quantum well or multiple quantum wells, e.g. 2-10 quantum wells. A layer of p-type gallium nitride 16 is formed upon quantum wells 14, and a metal stack 18 is positioned upon layer 16. The stack is formed of three separate metal layers 20, 22 and 24. Layers 20 and 24 are platinum and layer 22 is silver.
Substrate 12 may have a large-surface orientation between ten degrees and 0.2 degree (or less) of (0 00 1), (0 00−1), {1−1 0 0}, {1 1−2 0}, {1−1 0.+−.1}, {1−1 0.+−.2}, {1−1 0.+−.3}, {2 0−2.+−.1}, or {1 1 −2.+−.2}. In one embodiment, the substrate has a semipolar large-surface orientation, designated by (hkil) Bravais-Miller indices, where i=−(h+k), 1 is nonzero and at least one of h and k are nonzero. The substrate preferably has a dislocation density below 104 cm2 and an optical absorption coefficient below 100 cm10−1 at wavelengths between about 465 nm and about 700 nm. The nitride base crystal has an optical absorption coefficient below 100 cm−1 at wavelengths between about 700 nm and about 6667 nm. The surface of substrate 12 has a dislocation density below 105 cm−2 and is substantially free of low-angle grain boundaries, or tilt boundaries, over a length scale of at least 3 millimeters. The substrate 12 may be doped with any suitable n-type dopants from group VI and group IV atoms, e.g., sulfur, selenium, tellurium, silicon, germanium. In the present embodiment, substrate 12 is doped with Si and O providing a dopant concentration of approximately of 3 E18 cm-3.
Active layer 14 preferably comprises InGaN wells and GaN barrier layers. In other embodiments, the well layers and barrier layers comprise AlwInxGa1-w-xN and AlyInzGa1-y-zN, respectively, where 0≦w,x,y,z,w+x,y+z≦1, where w<u,y and/or x>v,z so that the bandgap of the well layer(s) is less than that of the barrier layer(s) and the n-type substrate. The well layers and barrier layers each have a thickness between about 1 nm and about 20 nm. In another embodiment, active layer 14 comprises a double heterostructure, with an InGaN or AlwInxGa1-w-xN and AlyInzGa1-y-zN layer about 20 nm to about 500 nm thick surrounded by GaN or AlyInzGa1-y-zN layers, where w<u, y and/or x>v, z. The composition and structure of active layer 14 are chosen to provide light emission at a preselected wavelength. Active layer 14 may be left undoped (or unintentionally doped) or may be doped n-type or p-type. Active layer 14 is formed upon substrate 12 using standard processing techniques.
Layer 16 may be doped with any suitable p-type dopant, such as group II or IV atoms, e.g., magnesium, zinc, cadmium, silicon, and germanium. In the present example, layer is doped with magnesium to provide a dopant concentration of approximately 1e20 cm-3.
Referring to FIGS. 1, 2 and 3, during fabrication, light emitting diode 10 is fabricated concurrently with a plurality of other light emitting diodes on a common semiconductor wafer. Wafer 15 is doped with n-type dopants using known techniques, at step 100. At step 102, active layer 14 is formed upon wafer 15 using known techniques. Following formation of active layer 14, p-type gallium nitride layer 16 is formed thereupon, shown in FIG. 4, at step 104 of FIG. 2. At step 106 surface of layer is exposed to an acid-containing cleaning solution. The cleaning solution consists essentially of 15% of nitric acid by weight, 27% of hydrochloric acid by weight and 58% of water by weight. This provides cleaned surface 26.
Referring to both FIG. 2 and FIG. 5, at step 108 a patterned photo resist layer 28 is formed upon cleaned surface 26. Layer 28 has a shape of a battlement leaving portions 30 of cleaned surface 26, with segments 32 of photo resist material being present between adjacent portions 30. Following formation of patterned photo resist layer 28, wafer 15 regions 30 and segments 32 are exposed to a buffered oxide etch process, at step 110. To that end, wafer 15 dipped into a solution consisting essentially of 2% hydrofluoric acid by weight and 8.75% ammonium fluoride by weight, with the rest being water. At step 112, three metal layers are sequentially deposited upon portions and segments 32. Specifically, a platinum layer 34 is deposited, followed by deposition of a silver layer 36. Another platinum layer 38 is deposited upon silver layer 36.
Referring to FIG. 2, at step 114 a lift-off process is used to remove segments 32 and the portions of layers 34, 36 and 38 in superimposition therewith, leaving a plurality of spaced-apart metal stacks 40. As a result, regions 42, shown in FIG. 7, of exposed wafer 15 remain between the spaced-apart metal stacks 40.
Referring to both FIG. 2 and FIG. 8, at step 116, a trench 44 is formed in each regions 42, using desired techniques, such as laser etching using a coherent beam of energy in the ultra violet spectrum. It should be understood that a trench 44 is adjacent to each metal stack 40 and is positioned between each adjacent pair of metals stacks 40. As result, a plurality of trenches 44 are formed in wafer 15 to form a grid pattern, shown in FIG. 9. Referring again to FIG. 8, each trench 44 extends from surface 46 of wafer 15 toward an opposing surface 48. Each trench 44 includes a nadir 50 that is spaced-apart from surface 48, defining a bulwark 52. Bulwark 52 extends between nadir 50 and surface 48. After formation of trenches 44, bulwark 52 is cleaved using a three point bending technique to segment light emitting diode 10 from wafer 15, referred to as a die. Typically, all bulwarks 52 are subjected to the cleaving process concurrently so as to produce a plurality of dies. To assist in affixing the spatial location of each die after segmentation it is common to include a layer of flexible adhesive tape 58 upon surface 48.
A problem encountered with using laser etching is the creation of contaminants that remains on the light emitting diode 10, referred to as slag 45. Slag 45 includes gallium metal, GaNx and GaOx and causes reduced light output. Although slag 45 is shown primarily in trenches 44, in practice slag 45 may be present in virtually any region of wafer 15. To remove the slag, wafer 15 is subjected to a chemical solution at step 115. The chemic solution is selected from a set of solutions consisting essentially of a potassium hydroxide solution, a potassium ferricyanide solution and a hydrochloric acid solution. The potassium hydroxide solution consists of 45% of potassium hydroxide by weight and 55% water by weight. The potassium ferricyanide solution consists of potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight and water in a range of 55%-69% by weight; and the hydrochloric acid solution consists of 35% of hydrochloric acid by weight and 65% water by weight. It is desired that the hydrochloric acid solution is heated to approximately 65° C. in increase the efficacy of removing the slag. Although exposing wafer 15 to the chemical solution is done following formation of trenches 44, this may occur before or after cleaving of bulwark 52. As a result of exposure to chemical solution, slag 45 is removed from trenches 44 and the remaining portions of wafer. In another embodiment, it is possible to grow the quantum wells, scribe the wafer, dip the wafer in the chemicals to remove the slag, put down photo resist, deposit the metals, and then cleave the wafer.
Example
To prove the principles and operation of the invention, certain experiments were performed. These experiments are merely examples of the process. In this example, gallium and nitrogen containing substrates with LED devices were fabricated. The substrates were subjected to scribing using a laser process. Scribing occurred at an energy density of ablation, i.e. 0.075 W/um2. The laser process used a 6 um spot size 2.1 W laser with a 25 um pulse width set at 80 Khz. The ranges of laser output that the slag removal would work is from 400 nm to 200 nm with average power from 25 mw to 10 W. Preferably, the laser beam ablates a portion of the gallium and nitrogen containing material. After formation of the scribes and slag, removal occurred using potassium ferricyanide species in a solution including a potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight, and water in a range of 55%-69% by weight, which may be mixed and/or agitated, although is not required. The temperature of the solution ranges from about 60 to 75 degrees Celsius and is preferably about 65 degrees Celsius. Referring to FIG. 11, we achieved over a 100% improvement in relative light output once the slag was removed using the method of the invention. Also, the efficiency of conversion, commonly external quantum efficiency, exceeded 30% and is preferably about 40 to 50% and greater.
It should be understood that the description recited above is an example of the invention and that modifications and changes may be undertaken which are within the scope of the claimed invention. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements, including a full scope of equivalents.

## Claims

1. A method of manufacturing high brightness LED devices, the method of manufacturing comprising:
providing a gallium and nitrogen substrate having a surface with a plurality of light emitting diodes;
treating the substrate to form a plurality of trenches among the light emitting diodes, the plurality of trenches extending from the surface toward an opposing surface, by scanning a coherent beam of energy onto the surface wherein the step of treating causes formation of slag along the plurality of trenches; and
subjecting the trenches and the slag to a chemical solution including potassium ferricyanide to substantially remove the slag without substantially changing the trenches,
wherein the temperature of the chemical solution ranges from about 60 to 75 degrees Celsius, and the chemical solution comprises potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight, and water in a range of 55%-69% by weight.

2. The method of claim 1 wherein the substrate comprises GaN, the slag is formed overlying each of the trenches, and at least some trenches have a protruding portion of slag, the slag having a lower density than a density of the substrate.

3. The method of claim 1 further comprising separating the plurality of light emitting diodes from each other.

4. The method of claim 3 wherein the step of separating comprises breaking the substrate apart using the trenches.

5. The method of claim 4 further comprising;
mounting the substrate to a flexible adhesive material prior to breaking the substrate apart, and
removing the light emitting diodes from the flexible adhesive material after breaking the substrate apart.

6. The method of claim 1 further comprising heating the chemical solution to approximately 65° C.

7. A method of separating individual semiconductor die having light emitting diodes formed on a surface of a substrate, wherein the substrate comprises gallium nitride, the method comprising:
forming a plurality of trenches in the substrate by scanning the substrate with a coherent beam of energy, the plurality of trenches extending from the surface of the substrate toward an opposing surface, the step of forming the plurality of trenches also forming slag along the plurality of trenches;
subjecting the substrate, including the trenches and the slag, to a chemical solution to selectively remove substantially all of the slag without substantially changing the trenches; and
separating the individual semiconductor die from each other,
wherein the temperature of the chemical solution ranges from about 60 to 75 degrees Celsius, and the chemical solution consists of potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight and water in a range of 55%-69% by weight.

8. The method as recited in claim 7 further comprising heating the chemical solution to approximately 65° C.

9. The method as recited in claim 7 further comprising, after the step of forming the plurality of trenches, a step of mounting the substrate on a flexible adhesive layer.

10. A method of manufacturing light emitting diode (LED) devices, the method comprising:
providing a sapphire substrate including a layer of material, the layer containing gallium and nitrogen and having a plurality of LEDs thereon;
forming a plurality of trenches in at least the layer of material by scanning the substrate with a coherent beam of energy, thereby causing ablation of surface regions of the substrate and formation of slag along the plurality of trenches;
subjecting the substrate, including the trenches and the slag, to a chemical solution to selectively remove the slag without substantially changing the trenches; and
separating the plurality of LEDs from each other using the trenches,
wherein the temperature of the chemical solution ranges from about 60 to 75 degrees Celsius, and the chemical solution comprises potassium hexacyanoferrate in a range of 30% to 40% by weight, potassium hydroxide in a range of 1% to 5% by weight and water in a range of 55%-69% by weight.

11. The method as recited in claim 10 wherein:
an adhesive tape is affixed to a bottom surface of the substrate; and
the LEDs are separated from each other by the trenches and then each LED is removed from the tape.

12. The method as recited in claim 10 further comprising heating the chemical solution to above about 65° C.

13. The method as recited in claim 10 wherein the plurality of trenches are formed using a pulsed laser.