JP2004014796A - Method for manufacturing nitride semiconductor and method for manufacturing nitride semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a nitride semiconductor device capable of enhancing productivity and a production yield. <P>SOLUTION: Prior to a photolithography process, a mask 16 composed of a SiO<SB>2</SB>film is formed on a coating grown layer 14. The surface of the mask 16 is polished by mechanical abrasion (lapping) so that the height of a projection 15 is set to 1 μm or less, for example. The mask 16 is removed by using a hydrogen fluoride solution. The projection 15 formed on the surface of the coating grown layer 14 is removed, and the surface of the coating grown layer 14 is flattened. In a post-photolithography process, when a photomask is brought into contact with a substrate coated with a nitride semiconductor layer by using a contact-type exposure device, since pressure is equally applied to the whole surface of a substrate 11, cracks of the substrate are prevented. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for manufacturing a nitride semiconductor used for a semiconductor laser or the like, and a method for manufacturing a nitride semiconductor device in which a nitride semiconductor layer is formed on a substrate made of a nitride semiconductor.
[0002]
[Prior art]
A nitride semiconductor is a semiconductor material of direct transition and has a feature that a band gap extends from 1.9 to 6.2 eV. Therefore, this nitride semiconductor can emit light in a visible region to an ultraviolet region, and is attracting attention as a material for forming a semiconductor light emitting device such as a semiconductor laser (laser diode; LD) or a light emitting diode (light emitting diode; LED). Have been. In addition, nitride semiconductors have attracted attention as materials for electronic devices because of their high saturation electron velocity and high breakdown electric field.
[0003]
In such a nitride semiconductor device, particularly a nitride semiconductor light emitting device, the fact that the surface of the substrate made of a nitride semiconductor or the surface of the nitride semiconductor layer formed on the substrate is flat is an important factor in the process. Very important.
[0004]
[Problems to be solved by the invention]
By the way, in the nitride semiconductor crystal growth method, in order to remove lattice defects, the seed crystal is grown laterally with respect to a seed crystal portion on which growth is based, that is, in a direction horizontal to a layer surface to be formed. A crystal growth method utilizing a small number of dislocations originating from a part has been performed. When a nitride semiconductor crystal is grown on a substrate by such a crystal growth method, abnormal growth occurs at an edge portion of the substrate, and a projection is generated. Further, when crystal growth is performed on a region where dust adheres to the surface of the substrate, the crystal grows with dust as a nucleus, abnormal growth occurs, and projections occur. Furthermore, when dust adheres to the surface of the substrate, the dust itself becomes a projection.
[0005]
When a plurality of processes are performed on the substrate when the height of the protrusion generated on the surface of the substrate is larger than, for example, 1 μm, various problems occur. In particular, a problem occurs when the process is performed by bringing the surface of the substrate on which the protrusions are present into contact with a predetermined plane. For example, as shown in FIG. 11, when a process is performed by bringing a photomask 151 and a substrate 111 into contact with each other by a photolithography method using a contact-type exposure apparatus, when pressure is applied from above the substrate 111. In addition, if the protrusions 115 are present on the surface of the substrate 111, pressure may not be applied uniformly to the entire surface of the substrate 111, and there is a risk that the substrate may be cracked. Also, in the case where the back surface of the substrate is ground by attaching to the base in the substrate thinning step, when pressure is applied from above the substrate, if there is a protrusion on the back surface of the substrate, as in the case of the photolithography step, There has been a problem that the projections do not evenly apply pressure to the entire back surface of the substrate, and the substrate is cracked. As a result, there has been a problem that the productivity and the yield in the production of the nitride semiconductor or the nitride semiconductor device are reduced.
[0006]
The present invention has been made in view of such a problem, and an object of the present invention is to provide a method for manufacturing a nitride semiconductor and a method for manufacturing a nitride semiconductor element, which can improve productivity and yield.
[0007]
[Means for Solving the Problems]
The method for manufacturing a nitride semiconductor according to the present invention is a structural method including a plurality of steps of performing different treatments on a substrate made of a nitride semiconductor, wherein the surface of the substrate is preceded by a predetermined one of the plurality of steps. And the step of removing the projections generated in the step. Note that the substrate made of a nitride semiconductor also includes a substrate made of a material other than a nitride semiconductor, for example, a substrate made of Al ₂ O ₃ (sapphire) on which a nitride semiconductor is grown.
[0008]
The method of manufacturing a nitride semiconductor device according to the present invention is a manufacturing method including a step of forming a nitride semiconductor layer on a substrate made of a nitride semiconductor, and a plurality of steps of performing different processes on the substrate. The method includes a step of removing a projection generated on the surface of the nitride semiconductor layer prior to the predetermined step of the above steps.
[0009]
In the method for manufacturing a nitride semiconductor or the method for manufacturing a nitride semiconductor device according to the present invention, prior to a predetermined step of the plurality of steps, the protrusion formed on the surface of the substrate or the protrusion formed on the nitride semiconductor layer is removed. Since the surface of the substrate or the nitride semiconductor layer is flattened, the photomask and the substrate are separated using a contact type exposure apparatus in a predetermined step, particularly, a photolithography step. Even when the contact is made, pressure is evenly applied to the entire surface of the substrate or the nitride semiconductor layer.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0011]
A method for manufacturing a nitride semiconductor laser as a method for manufacturing a nitride semiconductor device according to one embodiment of the present invention will be described with reference to FIGS. 3 is an enlarged schematic cross-sectional view of a part of FIG. 2, and FIG. 5 is an enlarged schematic cross-sectional view of a part of FIG. Further, the method for manufacturing a nitride semiconductor of the present invention is embodied by a method for manufacturing a nitride semiconductor device, and will be described together.
[0012]
Here, the nitride semiconductor is a gallium nitride-based compound containing gallium (Ga) and nitrogen (N), such as GaN, AlGaN (aluminum gallium nitride) mixed crystal, or AlGaInN (nitride). Aluminum / gallium / indium) mixed crystal. These may be, if necessary, n-type impurities made of Group IV and VI elements such as Si (silicon), Ge (germanium), O (oxygen), and Se (selenium), or Mg (magnesium), Zn (zinc) ), C (carbon), and the like.
[0013]
First, as shown in FIG. 1, a substrate 11 made of, for example, Al ₂ O ₃ is prepared. Other examples of the substrate 11 include Si (silicon), SiC (silicon carbide), GaAs (gallium arsenide), MgAl ₂ O ₄ (magnesium / aluminum composite oxide), LiGaO ₂ (lithium / gallium composite oxide), and GaN. Can be used. The substrate 11 includes a substrate made of any of these materials, on which the above-described nitride semiconductor is grown.
[0014]
Next, on the substrate 11 (for example, the (0001) plane), for example, GaN is grown to form a seed crystal layer 12 having a thickness of, for example, 2 μm. By the way, in the present embodiment, the growth of the nitride semiconductor crystal layer is performed using, for example, the MOCVD method. At this time, for example, (CH ₃ ) ₃ Ga (trimethylgallium, TMG) is used as a source gas of Ga (gallium), (CH ₃ ) ₃ Al (trimethylaluminum) is used as a source gas of aluminum, and a source gas of indium is used. (CH ₃ ) ₃ In (trimethylindium), and ammonia as a nitrogen source gas. Further, monosilane is used as a source gas for Si (silicon), and (C ₅ H ₅ ) ₂ Mg (bis = cyclopentadienyl magnesium) is used as a source gas for Mg (magnesium).
[0015]
Next, an SiO ₂ (silicon dioxide) film (not shown) having a thickness of, for example, 1 μm is formed on the seed crystal layer 12. The SiO ₂ film may be formed of Si _X N _Y (silicon nitride, x and y are arbitrary values), or may be formed as a stacked film of SiO ₂ and Si _X N _Y. Subsequently, a photoresist having a thickness of, for example, 1.3 μm is formed on the SiO ₂ film in order to form a seed crystal portion having a stripe shape in a positive shape with respect to the mask using a lithography method, for example, a photolithography method. A film (not shown) is formed.
[0016]
Subsequently, the SiO ₂ film is etched to partially remove the SiO ₂ film to form a mask (not shown). After forming the mask, the photoresist film is removed by oxygen ashing, treatment with acetone, or the like. Next, as shown in FIGS. 2 and 3, dry etching such as RIE (Reactive Ion Etching) is performed to remove a portion of the seed crystal layer 12 that is not covered with the mask. The stripe-shaped seed crystal portions 13 which are separated from each other are formed.
[0017]
Subsequently, using the same mask, for example, dry etching such as RIE is performed, and the surface of the substrate 11 is slightly removed, for example, by about 200 nm to form the groove 11a. When the groove 11a is formed, the growth layer does not come into contact with the surface of the substrate 11 during the lateral growth from the seed crystal portion 13, so that there is no possibility that a defect due to stress distortion occurs in the layer. Next, the mask is removed using a hydrofluoric acid-based solution, for example, an aqueous solution of hydrogen fluoride.
[0018]
After removing the mask, as shown in FIGS. 4 and 5, GaN is grown on the seed crystal portion 13 to form a coating growth layer 14. At this time, since the seed crystal portion 13 is not formed at the edge of the substrate 11 on the surface of the coating growth layer 14 as described above, abnormal growth occurs, and for example, a projection 15 having a height larger than 1 μm is generated. . Further, when dust (not shown) adheres to the substrate 11, abnormal growth occurs with the dust as a nucleus, for example, a projection 15 having a height larger than 1 μm is generated. Further, when dust adheres to the surface of the coating growth layer 14, the dust itself becomes, for example, a projection 15 having a height larger than 1 μm.
[0019]
Next, as shown in FIG. 6A, a mask 16 made of a SiO ₂ film is formed on the overgrowth layer 14. Subsequently, as shown in FIG. 6B, the surface of the mask 16 is polished by, for example, mechanical polishing (lapping) so that the height of the projections 15 becomes 1 μm or less, for example. Subsequently, the mask 16 is removed using a hydrofluoric acid-based solution, for example, an aqueous solution of hydrogen fluoride. As a result, the protrusions 15 generated on the surface of the cover growth layer 14 are removed, and the surface of the cover growth layer 14 becomes flat. The mask 16 may be selectively formed only on the surface of the coating growth layer 14 other than the region where the protrusions 15 are formed.
[0020]
Here, the substrate 11 may be etched to remove the protrusions 15 of the coating growth layer 14. Specifically, the etching may be performed by using, for example, hydrochloric acid (HCl), sulfuric acid (H ₂ SO ₄ ), or a hydrogen fluoride (HF) aqueous solution as an acidic etching solution. The etching may be performed by using an aqueous solution of potassium hydroxide (KOH), an aqueous solution of sodium hydroxide (NaOH), or an aqueous solution of ammonia (NH ₃ ) as an alkaline etching solution. Furthermore, a mask may be formed on the surface of the coating growth layer 14, and the projection 15 may be selectively removed by performing etching using the mask by a reactive ion etching method. Among these etching methods, a method using an acidic etching solution or an alkaline etching solution is effective when the projection 15 is non-crystalline.
[0021]
Further, in order to remove the projections 15 of the coating growth layer 14, the projections 15 are removed by cutting out the area of the surface of the substrate 11 where the projections 15 are formed, for example, by dicing the edge of the substrate 11. You may. As described above, removing the protrusions 15 after the formation of the nitride semiconductor layer, particularly the overgrowth layer 14, is most effective in the method of manufacturing the nitride semiconductor laser. This will be described later in detail.
[0022]
Subsequently, as shown in FIG. 7A, a vertical structure of the nitride semiconductor laser is grown on the coating growth layer 14 having a flat surface by using, for example, the MOCVD method. That is, an n-side contact layer 21 made of GaN: Si having a thickness of 1.5 μm, an n-type cladding layer 22 made of n-type Al _0.08 Ga _0.92 N having a thickness of 1.0 μm, and then n-type GaN An n-type guide layer 23 having a thickness of 0.1 μm is formed. An active layer 24 having a multiple quantum well structure is formed thereon by a Ga _0.98 In _0.02 N / Ga _0.92 In _0.08 N multilayer film. Furthermore, a p-type guide layer 25 made of p-type GaN and having a thickness of 0.1 μm, a p-type cladding layer 26 made of p-type Al _0.14 Ga _0.86 N / GaN and having a thickness of 0.5 μm, A 0.1 μm thick p-side contact layer 27 of type GaN is grown. At this time, since the n-side contact layer 21 to the p-side contact layer 27 were sequentially grown as a vertical structure of the nitride semiconductor laser on the coating growth layer 14 having a flat surface, the surface of the p-side contact layer 27 was Becomes flat.
[0023]
Next, for example, after performing treatment with acetone and UV (Ultraviolet) ozone treatment, annealing is performed to activate impurity atoms introduced into the nitride semiconductor layer formed on the substrate 11. After the annealing, a treatment with an aqueous solution of potassium hydroxide or an aqueous solution of hydrogen fluoride is performed.
[0024]
Next, using a contact-type exposure apparatus, a lithographic method, for example, a photolithographic method, is used to form a stripe-shaped region on the low-defect region of the overgrowth layer 14, that is, a region corresponding to a region with few defects between the seed crystal portions 13. A mask is formed to form a current constriction. Specifically, as shown in FIG. 7B, for example, an SiO ₂ film 41 and a photoresist film 42 are sequentially formed on the p-side contact layer 27 having a flat surface, and then the photoresist film 42 is formed. A photomask 51 having an opening 51A is brought into contact therewith, and exposure is performed from above the photomask 51 in order to form a mask in a positive shape with respect to the opening 51A. Here, since the SiO ₂ film 41 and the photoresist film 42 were sequentially formed on the p-side contact layer 27 having a flat surface in the previous step, the surface of the photoresist film 42 was flat, and therefore, When the photomask 51 is brought into contact with the photoresist film 42, pressure is evenly applied to the entire surface of the substrate 11.
[0025]
After selective removal of the photoresist film 42, as shown in FIG. 8, by etching the SiO ₂ film 41, by selectively removing the SiO ₂ film 41 to form a mask 41A. Subsequently, the photoresist film 42 is removed by oxygen ashing, treatment with acetone, or the like.
[0026]
Next, a part of the p-side contact layer 27 and a part of the p-type cladding layer 26 are patterned into a narrow band by performing dry etching such as RIE, for example, to form a ridge-shaped stripe-shaped current confining portion. Here, the current constriction portion is formed above the low confinement region so as to correspond to the low defect region, by adjusting the position of the light emitting region determined according to the position of the current confinement portion to the low defect portion of the active layer 24. This is because deterioration of characteristics can be prevented.
[0027]
Subsequently, predetermined portions of the p-type cladding layer 26 to the n-side contact layer 21 are removed by photolithography or the like to expose the n-side contact layer 21, and a region for forming the n-side electrode 33 is provided. Subsequently, the entire exposed portion from the n-side contact layer 21 to the p-side contact layer 27 is covered with the insulating film 32, and an n-side electrode 33 is formed on the n-side contact layer 21. The p-side electrode 31 is formed. Here, the n-side electrode 33 is formed, for example, by sequentially depositing Ti, Pt, and Au (gold) by a vacuum deposition method. The p-side electrode 31 is formed by, for example, sequentially depositing Pd (palladium), Pt, and Au by a vacuum deposition method.
[0028]
Next, a projection (not shown) generated on the back surface of the substrate 11 is removed by performing the same method as that used for removing the projection 15 of the coating growth layer 14. For example, after forming a mask (not shown) made of a SiO ₂ film, the surface of the mask is polished by mechanical polishing so that the height of the projections is, for example, 1 μm or less. As a result, the protrusions formed on the back surface of the substrate 11 are removed, and the back surface of the substrate 11 becomes flat.
[0029]
When the back surface of the substrate 11 is etched using an acidic etching solution or an alkaline etching solution to remove projections on the back surface of the substrate 11, the p-side formed on the surface of the substrate 11 is removed. It is necessary to form a mask on the side electrode 31 and the n-side electrode 33.
[0030]
Next, the substrate 11 is attached to a base (not shown), and the back surface of the substrate 11 is ground so that the thickness of the substrate 11 is, for example, about 80 μm. At this time, since the back surface of the substrate 11 is flat, pressure is evenly applied to the entire back surface of the substrate 11 and the substrate does not crack.
[0031]
Lastly, the substrate 11 is cleaved with a predetermined width in the direction perpendicular to the length direction of the p-side electrode 31, and a reflection film (for example, Al ₂ O _{3 having} a reflectance of 10%) made of, for example, Al ₂ O _{3 is formed} on an end surface for outputting laser light. On the other end face, reflection films having a reflectivity of 96%, each having, for example, four laminated films of an Al ₂ O ₃ film and a TiO ₂ (titanium dioxide) film are formed. Thus, nitride semiconductor laser 10 shown in FIG. 9 is obtained.
[0032]
In the nitride semiconductor laser 10, when a predetermined voltage is applied between the p-side electrode 31 and the n-side electrode 33, a current is injected into the active layer 24, and light emission occurs due to electron-hole recombination. This light is reflected by a reflecting mirror film (not shown), oscillates as a laser, and is emitted outside as a beam.
[0033]
As described above, the present embodiment includes a predetermined step of the plurality of steps, in particular, the step of removing the protrusion 15 generated on the cover growth layer 14 prior to the photolithography step. Layer 14 becomes flat. Therefore, in the subsequent photolithography process, when the photomask 51 is brought into contact with the substrate 11 on which the nitride semiconductor layer is formed by using a contact type exposure apparatus, pressure is uniformly applied to the entire surface of the substrate 11. In addition, it is possible to prevent the occurrence of a substrate crack. As a result, the productivity and yield of manufacturing the nitride semiconductor laser 10 can be improved.
[0034]
Although the present invention has been described with reference to the embodiment, the present invention is not limited to the above embodiment and can be variously modified. For example, in the above embodiment, the protrusions are removed before each of the step of forming the n-side contact layer 21 on the coating growth layer 14 and the step of polishing the back surface of the substrate 11. The protrusions may be removed before any of a plurality of steps performed for manufacturing the nitride semiconductor laser 10.
[0035]
For example, in the above embodiment, a mask is formed on the seed crystal layer 12 provided on the substrate 11, and the seed crystal layer 12 is selectively removed by performing etching using the mask. Thus, the seed crystal part 13 is formed, but another method may be used. For example, as shown in FIG. 10, a seed crystal portion 62 may be formed by forming a growth suppressing layer 61 made of, for example, SiO ₂ on a seed crystal layer 12 made of a nitride semiconductor. The growth suppressing layer 61 is formed in a pattern having an opening, and a portion where the seed crystal layer 12 is exposed from the opening of the pattern becomes a seed crystal portion 62. After the seed crystal portion 62 is formed in this manner, the nitride semiconductor laser 60 shown in FIG. 10 is manufactured by sequentially forming the coating growth layer 14 and the vertical structure of the laser in the same manner as in the above embodiment. It may be.
[0036]
Furthermore, in the above embodiment, the nitride semiconductor laser is described as a specific example of the semiconductor element. However, the present invention can be applied to other nitride semiconductor elements such as a light emitting diode and a high frequency device.
[0037]
【The invention's effect】
As described above, the method for manufacturing a nitride semiconductor according to any one of claims 1 to 10 or the method for manufacturing a nitride semiconductor device according to any one of claims 11 to 20 is provided. According to the method, prior to the predetermined step of the plurality of steps, the method includes the step of removing the protrusion generated on the substrate or the nitride semiconductor layer, so that the substrate or the nitride semiconductor layer becomes flat and In the step, when the surface of the substrate is brought into contact with a predetermined plane, pressure is evenly applied to the entire surface of the substrate, thereby preventing the occurrence of substrate cracking. As a result, it is possible to improve the productivity and the yield in manufacturing the nitride semiconductor or the nitride semiconductor device.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view for explaining a method for manufacturing a nitride semiconductor device according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of a step that follows the step of FIG.
FIG. 3 is an enlarged sectional view of a part of FIG. 2;
FIG. 4 is a sectional view of a step that follows the step of FIG. 2;
FIG. 5 is an enlarged sectional view of a part of FIG. 4;
FIG. 6 is a sectional view of a step that follows the step of FIG. 4;
FIG. 7 is an enlarged sectional view of a step that follows the step of FIG. 6;
FIG. 8 is an enlarged sectional view of a step that follows the step of FIG. 7;
FIG. 9 is a sectional view of a nitride semiconductor laser.
FIG. 10 is a sectional view of a modification of the nitride semiconductor laser.
FIG. 11 is a view for explaining a problem in a conventional nitride semiconductor manufacturing method.
[Explanation of symbols]
10, 60: nitride semiconductor laser, 11: substrate, 11a: groove, 12: seed crystal layer, 13, 62: seed crystal part, 14: coating growth layer, 15 ... · Projection, 16, 41A ··· Mask, 21 ··· n-side contact layer, 22 ··· n-type cladding layer, 23 ··· n-type guide layer, 24 ··· active layer, 25 ··· p Mold guide layer, 26 ... p-type clad layer, 27 ... p-side contact layer, 31 ... p-side electrode, 32 ... insulating film, 33 ... n-side electrode, 41 ... SiO ₂ film, 42 ... photoresist film, 51 ... photomask, 51A ... opening, 61 ... growth inhibiting layer

Claims (20)

A method for manufacturing a nitride semiconductor including a plurality of steps of performing different processes on a substrate made of a nitride semiconductor,
A method for manufacturing a nitride semiconductor, comprising a step of removing a protrusion generated on a surface of the substrate before a predetermined step of the plurality of steps. 2. The nitriding method according to claim 1, wherein the step of removing the protrusion includes a step of forming a mask on the surface of the substrate and a step of polishing the surface of the substrate on which the mask is formed. Of manufacturing semiconductor products. 2. The method according to claim 1, wherein the step of removing the projection is a step of etching the surface of the substrate. 4. The method according to claim 3, wherein an acidic etching solution is used. The method according to claim 4, wherein hydrochloric acid, sulfuric acid, or an aqueous solution of hydrogen fluoride is used as the acidic etching solution. 4. The method for producing a nitride semiconductor according to claim 3, wherein an alkaline etching solution is used. 7. The method for manufacturing a nitride semiconductor according to claim 6, wherein an aqueous solution of potassium hydroxide, an aqueous solution of sodium hydroxide, or an aqueous solution of ammonia is used as the alkaline etching solution. 4. The method according to claim 3, wherein a mask is formed on the surface of the substrate, and the surface of the substrate is selectively etched by a reactive ion etching method using the mask. . 2. The method for manufacturing a nitride semiconductor according to claim 1, wherein the step of removing the protrusion is a step of cutting out a region of the surface of the substrate where the protrusion has occurred. 2. The method according to claim 1, wherein the predetermined step is a step of forming a pattern on a surface of the substrate by a lithography method. A method of forming a nitride semiconductor layer on a substrate made of a nitride semiconductor, and a method of manufacturing a nitride semiconductor device including a plurality of steps of performing different processing on the substrate,
A method of manufacturing a nitride semiconductor device, comprising a step of removing a projection formed on a surface of the nitride semiconductor layer before a predetermined step of the plurality of steps. The step of removing the protrusion is a step including a step of forming a mask on the surface of the nitride semiconductor layer and a step of polishing the surface of the nitride semiconductor layer on which the mask is formed. A method for manufacturing a nitride semiconductor device according to claim 11. The method for manufacturing a nitride semiconductor device according to claim 11, wherein the step of removing the protrusion is a step of etching the nitride semiconductor layer. 14. The method for manufacturing a nitride semiconductor device according to claim 13, wherein an acidic etching solution is used. The method according to claim 14, wherein hydrochloric acid, sulfuric acid, or an aqueous solution of hydrogen fluoride is used as the acidic etching solution. 14. The method for manufacturing a nitride semiconductor device according to claim 13, wherein an alkaline etching solution is used. 17. The method for manufacturing a nitride semiconductor device according to claim 16, wherein an aqueous solution of potassium hydroxide, an aqueous solution of sodium hydroxide, or an aqueous solution of ammonia is used as the alkaline etching solution. 14. The nitride semiconductor device according to claim 13, wherein a mask is formed on the nitride semiconductor layer, and the nitride semiconductor layer is selectively etched using the mask by a reactive ion etching method. Production method. The method of manufacturing a nitride semiconductor device according to claim 11, wherein the step of removing the protrusion is a step of cutting out a region where the protrusion has occurred from the surface of the nitride semiconductor layer. 12. The method according to claim 11, wherein the predetermined step is a step of forming a pattern on a surface of the nitride semiconductor layer by a lithography method.

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