Gallium oxide substrate nanoscale patterning method

Abstract

The invention provides a gallium oxide substrate nanoscale patterning method, and belongs to the technical field of semiconductor photoelectric material and device preparation. The method comprises the following steps: immersing a gallium oxide substrate into a precious metal nanoparticle colloidal solution to obtain a gallium oxide substrate dipped with precious metal nanoparticles; carrying out heat treatment on the gallium oxide substrate dipped with the noble metal nanoparticles to obtain a heat-treated substrate; carrying out metal auxiliary photochemical corrosion patterning on the heat treatment substrate to obtain a patterned substrate; and removing the noble metal nanoparticles on the patterned substrate. According to the method, the size-controllable regular patterned gallium oxide substrate is prepared by developing a mask-free continuous method, the acting force between the surface of the substrate and the precious metal nanoparticles is enhanced through heat treatment, the acting force can be larger than the metal-metal agglomeration acting force, and the substrate surface with regular metal particle arrangement is formed; and subsequent metal-assisted photochemical corrosion patterning is facilitated.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor optoelectronic materials and device preparation, and in particular to a method for nano-scale patterning of a gallium oxide substrate. Background Art

[0002] β-Ga2O3 is an emerging ultra-wide bandgap semiconductor with broad application prospects in the next generation of power electronics and optoelectronics. Due to its larger bandgap width, β-Ga2O3 has a Baliga's figure of merit (BFOM) of 3444, which is much larger than SiC (340) and GaN (870), the representative materials of the third generation semiconductors. When used as a power device, its maximum breakdown electric field strength can reach 8MV / cm. According to the functional relationship between on-resistance and maximum breakdown voltage, under the same electric field strength, the conduction loss of β-Ga2O3 is one order of magnitude lower than that of SiC and GaN. When the carrier (electron) concentration is 10 15 ~10 16 cm -3 When the electron mobility of β-Ga2O3 is measured experimentally, it is 300cm 2 ·V -1 ·s -1 In addition, compared with the high-temperature sublimation method used in SiC substrate equipment, and the high-pressure nitrogen method, ammonothermal technology method, and hydride vapor phase epitaxy method used in GaN substrate preparation, β-Ga2O3 single crystals can be grown on a large scale using a low-cost and stable melt method. In summary, β-Ga2O3 has the potential to prepare the next generation of high-voltage and high-power semiconductor devices, and also has great application prospects in photodetectors.

[0003] As gallium oxide devices are gradually gaining attention, the patterning process of substrates is receiving more and more attention. The study of the etching process is of great research significance for the preparation of gallium oxide patterned substrates with micron and nanostructures to meet the needs of high-performance devices. At present, the more commonly used methods include dry and wet etching. The defects of dry etching include the introduction of dangling bonds, defects, surface traps, carrier leakage paths and recombination centers in the material. In addition, compared with wet etching, dry etching will cause inevitable damage to the substrate surface caused by free ion bombardment, and its cost is relatively high.

[0004] The prior art (such as CN114496805A) discloses that a gallium oxide substrate is subjected to metal-assisted photochemical etching to form a nanometer-scale pattern, which requires the preparation of a metal aluminum template and has the problem of a complicated process. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a method for nano-scale patterning of a gallium oxide substrate. The present invention utilizes heat treatment to enhance the interaction between the substrate and the noble metal nanoparticles, which can be greater than the metal-metal agglomeration interaction force, forming a substrate surface with a regular arrangement of metal particles, which is beneficial to the subsequent metal-assisted photochemical etching patterning.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for nano-scale patterning of a gallium oxide substrate, comprising the following steps:

[0008] Immersing the gallium oxide substrate in a colloidal solution of noble metal nanoparticles to obtain a gallium oxide substrate impregnated with noble metal nanoparticles;

[0009] heat-treating the gallium oxide substrate impregnated with the noble metal nanoparticles to obtain a heat-treated substrate;

[0010] performing metal-assisted photochemical etching patterning on the heat-treated substrate to obtain a patterned substrate;

[0011] The noble metal nanoparticles on the patterned substrate are removed.

[0012] Preferably, the heat treatment temperature is 60-400° C., and the heat preservation time is 1-10 hours.

[0013] Preferably, the illumination conditions for the metal-assisted photochemical etching patterning include: a wavelength of 254 to 260 nm, a power of 50 to 200 W, and a distance between the light source and the heat-treated substrate of 2 to 20 cm.

[0014] Preferably, the etching solution for metal-assisted photochemical etching patterning comprises an etchant, an oxidant and water, and the etchant comprises an acidic etchant or an alkaline etchant.

[0015] Preferably, the acidic corrosive agent includes one or more of hydrofluoric acid, sulfuric acid, hydrochloric acid and nitric acid, and the mass fraction of the acidic corrosive agent in the corrosive solution is 5-85%.

[0016] Preferably, the alkaline corrosive agent includes sodium hydroxide and / or potassium hydroxide, and the mass fraction of the alkaline corrosive agent in the corrosive solution is 20-75%.

[0017] Preferably, the oxidant includes one or more of potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, sodium peroxide, potassium peroxide, potassium permanganate, sodium hypochlorite, potassium hypochlorite and ammonium hypochlorite, and the mass fraction of the oxidant in the etching solution is 0.002-0.05%.

[0018] Preferably, the temperature of the metal-assisted photochemical etching patterning is 20 to 60° C., and the time is 1 to 48 hours.

[0019] Preferably, the particle size of the noble metal nanoparticles in the noble metal nanoparticle colloidal solution is 10 to 5000 nm.

[0020] Preferably, the process of removing the noble metal nanoparticles comprises: immersing the patterned substrate in aqua regia, heating to 40-50° C., and maintaining the temperature for 30-60 minutes.

[0021] The invention provides a method for nano-scale patterning of a gallium oxide substrate, comprising the following steps: immersing a gallium oxide substrate in a colloidal solution of noble metal nanoparticles to obtain a gallium oxide substrate impregnated with noble metal nanoparticles; heat-treating the gallium oxide substrate impregnated with noble metal nanoparticles to obtain a heat-treated substrate; performing metal-assisted photochemical etching patterning on the heat-treated substrate to obtain a patterned substrate; and removing the noble metal nanoparticles on the patterned substrate.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention develops a maskless, continuous method for preparing a size-controllable, regularly patterned gallium oxide substrate, which solves the problem that the current metal-assisted photochemical etching requires the preparation of a metal aluminum template, which is cumbersome and costly. The force between the substrate surface and the noble metal nanoparticles is enhanced by heat treatment, which can be greater than the metal-metal agglomeration force, thereby forming a substrate surface with a regular arrangement of metal particles, which is beneficial to the subsequent metal-assisted photochemical etching patterning. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a flow chart of a method for nanoscale patterning of a gallium oxide substrate according to an embodiment of the present invention;

[0025] Figure 2 This is a transmission electron image (TEM) of the platinum nanoparticles in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] The present invention provides a method for nano-scale patterning of a gallium oxide substrate, comprising the following steps:

[0027] Immersing the gallium oxide substrate in a colloidal solution of noble metal nanoparticles to obtain a gallium oxide substrate impregnated with noble metal nanoparticles;

[0028] heat-treating the gallium oxide substrate impregnated with the noble metal nanoparticles to obtain a heat-treated substrate;

[0029] performing metal-assisted photochemical etching patterning on the heat-treated substrate to obtain a patterned substrate;

[0030] The noble metal nanoparticles on the patterned substrate are removed.

[0031] The invention immerses the gallium oxide substrate into a colloidal solution of noble metal nanoparticles to obtain the gallium oxide substrate impregnated with the noble metal nanoparticles.

[0032] In the present invention, the gallium oxide substrate is preferably cleaned before use and dried for standby use. The present invention has no particular limitation on the specific cleaning method, and a method well known to those skilled in the art can be used.

[0033] In the present invention, the particle size of the noble metal nanoparticles in the noble metal nanoparticle colloidal solution is preferably 10 to 5000 nm.

[0034] In the present invention, the noble metal nanoparticles preferably include one or more of Ir, Pt, Au, Ni and Rh.

[0035] In the present invention, the loading amount of the noble metal nanoparticles in the gallium oxide substrate impregnated with the noble metal nanoparticles is preferably 0.001 wt % to 0.2 wt %.

[0036] The present invention has no special limitation on the preparation method of the noble metal nanoparticle colloidal solution. The hydrothermal method well known to those skilled in the art can be used. The size and morphology of the noble metal nanoparticles synthesized by the hydrothermal method are controllable. By regulating the synthesis conditions of the hydrothermal method, uniform nanoscale patterning of the surface with various morphologies can be obtained. Taking the noble metal nanoparticles as Pt as an example, the preparation method of the noble metal nanoparticle colloidal solution preferably includes the following steps:

[0037] (a) Na2PtCl6 or HAuCl4 or Pt(acac)2 is prepared as a precursor, deionized water, ethylene glycol (EG), N,N-dimethylformamide (DMF) or tetraethylene glycol (TEG) is used as a solvent, polyvinyl pyrrolidone (PVP) is used as a protective agent, and halogen atoms, sodium dodecyl sulfate (SDS), sodium acetate, amino acid, oleylamine (OAm), oleic acid (OA), hexadecyltrimethylammonium bromide (CTAB), sodium citrate, acrylic acid (AA) or ascorbic acid (AC) are used as additives;

[0038] (b) The precursor, protective agent, additive and solvent in step (a) are mixed in a stirrer and placed in a reaction kettle to obtain a solution, and the solution is heated to 100-200° C. and maintained for 1-10 hours for hydrothermal reaction to obtain a dark colloidal solution, i.e., the noble metal nanoparticles are successfully synthesized.

[0039] In the present invention, the concentration of the precursor in the solution is preferably 0.01 to 0.5 mol·L -1 .

[0040] In the present invention, the concentration of the protective agent in the solution is preferably 0.2 to 0.8 mol·L -1 .

[0041] In the present invention, the concentration of the additive in the solution is preferably 0.2 to 0.8 mol·L -1 .

[0042] In the present invention, the mixing time is preferably 20 to 40 minutes.

[0043] In the present invention, the gallium oxide substrate is preferably first immersed in the noble metal nanoparticle colloidal solution, allowed to stand, and then lifted up at a uniform speed.

[0044] In the present invention, the immersion time is preferably 1 to 180 minutes.

[0045] The present invention adopts a dipping method to form noble metal nanoparticles with uniform surface coverage, which is simple to operate and can reduce costs.

[0046] After obtaining the gallium oxide substrate impregnated with the noble metal nanoparticles, the present invention performs heat treatment on the gallium oxide substrate impregnated with the noble metal nanoparticles to obtain a heat-treated substrate.

[0047] In the present invention, the temperature of the heat treatment is preferably 60 to 400° C., more preferably 200 to 300° C., and the holding time is preferably 1 to 10 h, more preferably 5 to 6 h.

[0048] In the present invention, the heating rate from room temperature to the temperature of the heat treatment is preferably 0.1 to 10° C. / min.

[0049] In the present invention, the atmosphere of the heat treatment is preferably air, hydrogen, nitrogen, oxygen or argon. The role of the heat treatment is to strengthen the interaction between the noble metal nanoparticles and the substrate surface, so that the interaction force between the noble metal nanoparticles and the substrate surface is greater than the interaction force between the noble metal nanoparticles and the noble metal nanoparticles, thereby avoiding the shedding and agglomeration of the noble metal nanoparticles and removing the protective agent and additives in the colloidal solution of the noble metal nanoparticles.

[0050] After obtaining the heat-treated substrate, the present invention performs metal-assisted photochemical etching patterning on the heat-treated substrate to obtain a patterned substrate.

[0051] In the present invention, the illumination conditions for the metal-assisted photochemical etching patterning preferably include: a wavelength of 254 to 260 nm, a power of 50 to 200 W, and a distance of 2 to 20 cm between the light source and the heat-treated substrate; providing external light energy can cause the energy band of the gallium oxide substrate to bend, and holes to move to the surface of the gallium oxide substrate, thereby promoting selective patterned etching.

[0052] In the present invention, the etching solution for metal-assisted photochemical etching patterning preferably includes an etchant, an oxidant and water, and the etchant preferably includes an acidic etchant or an alkaline etchant.

[0053] In the present invention, the acidic corrosive agent preferably includes one or more of hydrofluoric acid, sulfuric acid, hydrochloric acid and nitric acid, and the mass fraction of the acidic corrosive agent in the etching solution is preferably 5-85%.

[0054] In the present invention, the alkaline corrosive agent preferably includes sodium hydroxide and / or potassium hydroxide, and the mass fraction of the alkaline corrosive agent in the corrosive solution is preferably 20-75%.

[0055] In the present invention, the oxidant preferably includes one or more of potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, sodium peroxide, potassium peroxide, potassium permanganate, sodium hypochlorite, potassium hypochlorite and ammonium hypochlorite, and the mass fraction of the oxidant in the etching solution is preferably 0.002-1.5%.

[0056] In the present invention, the temperature of the metal-assisted photochemical etching patterning is preferably 20 to 60° C., more preferably room temperature, and the time is preferably 1 to 48 hours.

[0057] In the present invention, by metal-assisted photochemical etching patterning, noble metal nanoparticles can be used as a catalyst, and a mixture of an etchant and an oxidant can be used as an etching solution at room temperature to achieve uniform and controllable nanoscale patterning on the surface of a gallium oxide substrate. The corrosion of gallium oxide in the present invention is as follows: the key factor affecting the corrosion behavior of gallium oxide crystals is the concentration of holes. However, since gallium oxide crystals belong to an intrinsic n-type structure, holes as minority carriers are very rare in the valence band, which makes it difficult to corrode gallium oxide. In order to generate more electron-hole pairs, a light source with an energy higher than the bandgap width of gallium oxide needs to be used. According to the work function, the concentration of holes can be selected. The required metal, when the metal with a work function higher than that of gallium oxide contacts gallium oxide, the electric field generated in the space charge region will cause the energy band to bend, causing a potential difference between the surface and the interior of the semiconductor, and the electrons are transferred from the semiconductor conduction band to the metal, reducing the recombination of photogenerated electrons and holes, and the holes move toward the semiconductor surface along the direction of the energy band bending to ensure the occurrence of corrosion, and the hole aggregation area on the surface of gallium oxide is oxidized to produce oxygen, and the oxidant in the etching solution is reduced, and the metal-assisted photochemical etching is completed. The patterning degree of the gallium oxide substrate depends on the shape and size of the precious metal nanoparticles deposited on the surface of gallium oxide. In the present invention, during the metal-assisted photochemical etching patterning process, the precious metal nanoparticles act as a catalyst and no loss occurs. At the same time, since the metal-assisted photochemical etching patterning occurs at room temperature and lower temperatures, the precious metal nanoparticles are not easy to diffuse to the surface of gallium oxide; and the metal-assisted photochemical etching patterning process has good controllability, and the fine control of the surface patterning can be achieved by regulating the composition and concentration of the etching solution, controlling the distance of the light source, and regulating the etching time.

[0058] In the present invention, the metal-assisted photochemical corrosion patterning preferably uses an ultraviolet lamp as a light source. The use of ultraviolet irradiation can ensure that the corrosion is carried out at room temperature, avoiding the experimental risk caused by the excessively high temperature of the corrosion liquid, reducing the cost, reducing the diffusion behavior of the precious metal, and improving the quality of the surface nano-scale patterning. The entire corrosion process needs to be carried out in a light-proof light box. At the same time, the corrosion liquid contains hydrofluoric acid and an oxidant, so it is preferred to use polytetrafluoroethylene material to make the corrosion container. In the present invention, the corrosion container is preferably a corrosion pool.

[0059] After obtaining the patterned substrate, the present invention removes the noble metal nanoparticles on the patterned substrate.

[0060] In the present invention, the removing preferably comprises the following steps: immersing the patterned substrate in aqua regia, heating to 40-50° C., and maintaining for 30-60 minutes.

[0061] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0062] Figure 1 Flow chart of the method for nanoscale patterning of a gallium oxide substrate in an embodiment of the present invention.

[0063] Example 1

[0064] Configuration 0.01mol·L -1 Sodium chloroplatinate aqueous solution, 0.2 mol·L -1 CTAB aqueous solution, 0.2 mol·L -1 AA aqueous solution, 1.6mL sodium chloroplatinate aqueous solution, 1.0mL CTAB aqueous solution, 1.0mL AA aqueous solution, placed in a 50mL round-bottom flask, add deionized water to make the total reaction volume reach 10mL. Mix evenly under the stirring of the stirrer for 30min to form a uniform milky yellow solution, transfer the reaction mixture in the beaker to a 25mL reactor, heat to 100℃ in an oven, and then keep it for 1h to obtain a dark brown colloidal solution, that is, platinum nanoparticles are successfully synthesized. The colloidal solution in the reactor is ultrasonically homogenized, and then the gallium oxide substrate is immersed in the colloid and taken out, and the gallium oxide impregnated with platinum nanoparticles (platinum nanoparticle loading is 0.01wt%) is placed in a muffle furnace for heat treatment (400℃, 1h). The substrate with deposited Pt particles was placed in an etching solution composed of 20% hydrofluoric acid by mass and 0.012% potassium persulfate by mass as an oxidant. A 120W mercury lamp (254nm) was used as the light source. The substrate was irradiated at a distance of 5cm and the etching time was 5h. After the etching was completed, the gallium oxide substrate was placed in the prepared aqua regia and heated to 40°C for 60min. Finally, the wafer with the metal nanoparticles removed was taken out, rinsed with deionized water, and dried with nitrogen to obtain a square nanoscale patterned gallium oxide substrate with a side length of 10nm and an etching depth of 1μm.

[0065] Figure 2 This is a transmission electron image (TEM) of the platinum nanoparticles in Example 1, and it can be seen that platinum nanoparticles are prepared.

[0066] Example 2

[0067] Prepare 0.5 mol·L -1 Platinum acetylacetonate aqueous solution, 0.4 mol·L -1 Ethylene glycol aqueous solution, 0.4 mol·L -1AC aqueous solution, 1.6mL sodium chloroplatinate aqueous solution, 1.0mL ethylene glycol aqueous solution, 1.0mL AC aqueous solution, placed in a 50mL round-bottom flask, add deionized water to make the total reaction volume reach 10mL. Stirred by a stirrer for 30min to mix evenly to form a uniform milky yellow solution, the reaction mixture in the beaker was transferred to a 25mL reactor, heated to 150°C in an oven, and then kept for 2h to obtain a dark brown colloidal solution, that is, platinum nanoparticles were successfully synthesized. The colloidal solution in the reactor was ultrasonically homogenized, and then the gallium oxide substrate was immersed in the colloid and taken out, and the gallium oxide impregnated with platinum nanoparticles (platinum nanoparticle loading was 0.2wt%) was placed in a muffle furnace for heat treatment (300°C, 3h). The substrate with deposited Pt particles was placed in a corrosion solution composed of 40% hydrofluoric acid by mass and 1.5% potassium permanganate by mass as an oxidant. A 120W mercury lamp (254nm) was used as a light source. The substrate was irradiated at a distance of 10cm and the corrosion reaction lasted for 10 hours. After the corrosion was completed, the gallium oxide substrate was placed in the prepared aqua regia and heated to 50°C for 30 minutes. Finally, the wafer with the metal nanoparticles removed was taken out, rinsed with deionized water, and dried with nitrogen to obtain a square nanoscale patterned gallium oxide substrate with a side length of 20nm and a corrosion depth of 1.5μm.

[0068] Example 3

[0069] Configuration 0.01mol·L -1 Sodium chloroplatinate aqueous solution, 0.8 mol·L -1 CTAB aqueous solution, 0.8 mol·L -1AC aqueous solution, 1.6mL sodium chloroplatinate aqueous solution, 1.0mL CTAB aqueous solution, 1.0mL AC aqueous solution, placed in a 50mL round-bottom flask, add deionized water to make the total reaction volume reach 10mL. Stirring with a stirrer for 30min, mix evenly to form a uniform milky yellow solution, transfer the reaction mixture in the beaker into a 25mL reactor, heat to 160°C in an oven, and then keep it for 2h to obtain a dark brown colloidal solution, that is, platinum nanoparticles are successfully synthesized. The colloidal solution in the reactor is ultrasonically homogenized, and then the gallium oxide substrate is immersed in the colloid and taken out, and the gallium oxide impregnated with platinum nanoparticles (platinum nanoparticle loading is 0.001wt%) is placed in a muffle furnace for heat treatment (200°C, 5h). The substrate with deposited Pt particles was placed in a corrosion solution composed of 50% sodium hydroxide and 0.001% sodium hypochlorite as an oxidant, and a 120W mercury lamp (254nm) was used as a light source. The substrate was irradiated at a distance of 2cm, and the corrosion reaction lasted for 1h. After the corrosion was completed, the gallium oxide substrate was placed in the prepared aqua regia and heated to 45°C for 40min. Finally, the wafer with the metal nanoparticles removed was taken out, rinsed with deionized water, and dried with nitrogen to obtain a square nanoscale patterned gallium oxide substrate with a side length of 40nm and a corrosion depth of 0.5μm.

[0070] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for nanoscale patterning of a gallium oxide substrate, characterized in that: The following steps are involved: immersing the gallium oxide substrate in a colloidal solution of noble metal nanoparticles to obtain a gallium oxide substrate impregnated with noble metal nanoparticles; heat-treating the gallium oxide substrate impregnated with the noble metal nanoparticles to obtain a heat-treated substrate; performing metal-assisted photochemical etching patterning on the heat-treated substrate to obtain a patterned substrate; The noble metal nanoparticles on the patterned substrate are removed.

2. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The heat treatment temperature is 60-400° C., and the heat preservation time is 1-10 hours.

3. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The illumination conditions for the metal-assisted photochemical etching patterning include: a wavelength of 254 to 260 nm, a power of 50 to 200 W, and a distance between the light source and the heat-treated substrate of 2 to 20 cm.

4. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The etching solution for metal-assisted photochemical etching patterning includes an etchant, an oxidant and water, and the etchant includes an acidic etchant or an alkaline etchant.

5. The method for nano-scale patterning of a gallium oxide substrate according to claim 4, characterized in that: The acidic corrosive agent includes one or more of hydrofluoric acid, sulfuric acid, hydrochloric acid and nitric acid, and the mass fraction of the acidic corrosive agent in the corrosive solution is 5-85%.

6. The method for nano-scale patterning of a gallium oxide substrate according to claim 4, characterized in that: The alkaline corrosive agent includes sodium hydroxide and / or potassium hydroxide, and the mass fraction of the alkaline corrosive agent in the corrosive solution is 20-75%.

7. The method for nano-scale patterning of a gallium oxide substrate according to claim 4, wherein: The oxidant includes one or more of potassium persulfate, sodium persulfate, ammonium persulfate, hydrogen peroxide, sodium peroxide, potassium peroxide, potassium permanganate, sodium hypochlorite, potassium hypochlorite and ammonium hypochlorite. The mass fraction of the oxidant in the etching solution is 0.002-0.05%.

8. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The temperature of the metal-assisted photochemical etching patterning is 20 to 60° C., and the time is 1 to 48 hours.

9. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The particle size of the noble metal nanoparticles in the noble metal nanoparticle colloidal solution is 10 to 5000 nm.

10. The method for nano-scale patterning of a gallium oxide substrate according to claim 1, wherein: The removal comprises the following steps: immersing the patterned substrate in aqua regia, heating to 40-50° C., and maintaining the temperature for 30-60 minutes.

Images