Method for determining silicon surface and carbon surface of 3C-SiC single crystal wafer

Abstract

The invention provides a method for determining a silicon surface and a carbon surface of a 3C-SiC single crystal wafer, which comprises the following steps of: (1) immersing the 3C-SiC single crystal wafer into molten alkali for corrosion treatment; (2) taking out the corroded 3C-SiC single crystal wafer and observing the surface appearance of the 3C-SiC single crystal wafer; if the surface only shows the stacking fault characteristic, judging that the surface is a (C) surface; and if the surface has the characteristics of stacking fault and dislocation corrosion pits at the same time, determining that the surface is a (111) Si surface. According to the invention, the inherent difference of molten alkali on the corrosion morphology of the (111) Si surface and the (C) surface of the 3C-SiC single crystal is found and utilized for the first time: the (111) Si surface can present stacking fault and dislocation corrosion pits at the same time, and only the (C) surface has stacking fault. By observing the obvious morphology comparison, the two crystal faces can be quickly and accurately distinguished.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of semiconductor materials. Specifically, the present application relates to a method for determining the silicon face and the carbon face of a 3C-SiC (cubic silicon carbide) single crystal wafer. BACKGROUND

[0002] Silicon carbide (SiC) is widely used in new energy vehicles, photovoltaics, 5G communications and other fields due to its excellent properties such as wide band gap, high breakdown field strength, high saturated electron drift velocity and high thermal conductivity.

[0003] SiC crystal structures are numerous, and more than 250 types have been found so far, among which hexagonal system such as 4H-SiC and 6H-SiC, cubic system 3C-SiC and rhombohedral system 15R-SiC are the most common. Compared with 4H-SiC, 3C-SiC has higher carrier mobility (about 2-4 times) and lower interface state defect density (about one order of magnitude lower). Therefore, based on 3C-SiC, metal oxide semiconductor field effect transistor (MOSFET) is expected to solve the device reliability problem of 4H-SiC MOSFET caused by gate oxide interface defects.

[0004] Recent studies have shown that by high temperature solution growth method (HTSG) and adjusting the solid-liquid interface energy between the cosolvent and the crystal, 2 to 6 inch 3C-SiC single crystals can be successfully epitaxially grown on 4H-SiC seed crystals (see Energy Environ. Mater., 2024, 7(4): e12678; Crystal Growth and Design, 2025, 25, 1211-1216). Since this process is usually carried out on a 4H-SiC(000 ) face seed crystal, the obtained 3C-SiC single crystal grows along the <111> crystal direction, and the two surfaces of the wafer are (111) face and ( ) face. In 3C-SiC, the (111) face is terminated by silicon atoms [i.e. (111)Si face], while the ( ) face is terminated by carbon atoms [i.e. ( )C face].

[0005] The physical and chemical properties of the two crystal faces are quite different, so when they are used as seed crystals for crystal growth or subsequent device preparation process, homoepitaxial growth or heteroepitaxy must be accurately distinguished. However, the prior art does not provide how to effectively distinguish the two crystal faces.

[0006] In summary, there is an urgent need to develop a method that can accurately distinguish the (111)Si face and the ( )C face method, which will provide a key basis for the selection of 3C-SiC as seed crystal for growing single crystal and the selection of crystal face in subsequent epitaxial process, thereby accelerating the industrialization process of large-size 3C-SiC single crystal wafer. SUMMARY

[0007] The purpose of the present application is to provide a method for determining the silicon face and carbon face of a 3C-SiC single crystal wafer, which can clearly distinguish the (111) Si face and (0001) C face of a 3C-SiC single crystal by the difference in characteristic etching morphology (stacking fault and dislocation etching pit). The method of the present application is simple to operate and easy to implement, and can distinguish the (111) Si face and (0001) C face of 3C-SiC substrate wafer, polished wafer, ground wafer and cut wafer, and can also distinguish the (111) Si face and (0001) C face of 3C-SiC epitaxial wafer, and is suitable for industrial application.

[0008] The above purpose of the present application is achieved by the following technical solution.

[0009] The present application provides a method for determining the silicon face and carbon face of a 3C-SiC single crystal wafer, comprising the following steps:

[0010] (1) immersing the 3C-SiC single crystal wafer in molten alkali for etching treatment;

[0011] (2) taking out the etched 3C-SiC single crystal wafer and observing its surface morphology:

[0012] - if the surface only presents stacking fault characteristics, it is determined that the face is (0001) C face;

[0013] - if the surface presents both stacking fault and dislocation etching pit characteristics, it is determined that the face is (111) Si face.

[0014] The inventors of the present application unexpectedly found that immersing the 3C-SiC single crystal wafer in molten alkali for etching treatment can accurately determine the (111) Si face and (0001) C face of the 3C-SiC single crystal wafer by observing the morphology of the etched 3C-SiC single crystal wafer.

[0015] Preferably, in the method of the present application, the temperature of the molten alkali is 450-550°C.

[0016] ​​​​Preferably, in the method of the present application, the molten alkali is selected from one or more of KOH, NaOH and Na2O2. For example, the molten alkali can be a mixed alkali of KOH and Na2O2, wherein the mass fraction of Na2O2 in the mixed alkali is less than or equal to 16%.

[0017] Preferably, in the method of the present application, the etching treatment is performed for 1-60 min. More preferably, in the method of the present application, the etching treatment is performed for 1-10 min.

[0018] Preferably, in the method of the present application, the method further comprises, before step (1), the following step: performing a cleaning treatment on the surface of the 3C-SiC single crystal wafer.

[0019] Preferably, in the method of the present application, the cleaning treatment is performed by cleaning the wafer using an acidic solution and / or an alkaline solution.

[0020] Preferably, in the method of the present application, the method further comprises, after taking out the etched 3C-SiC single crystal wafer in step (2), the following step: placing the etched 3C-SiC single crystal wafer in an organic solvent for ultrasonic cleaning.

[0021] Preferably, in the method of the present application, the organic solvent is ethanol and / or acetone, and the ultrasonic cleaning is performed for 1-30 min.

[0022] Preferably, in the method of the present application, the method further comprises, after the ultrasonic cleaning, the following step: drying the ultrasonically cleaned wafer using a high-purity inert gas.

[0023] Preferably, in the method of the present application, the high-purity inert gas is high-purity argon and / or high-purity nitrogen.

[0024] Preferably, in the method of the present application, the purity of the high-purity nitrogen is greater than or equal to 99.999%, and the purity of the high-purity argon is greater than or equal to 99.999%.

[0025] Preferably, in the method of the present application, the observation in step (2) is performed by optical microscopy.

[0026] Preferably, in the method of the present application, the 3C-SiC single crystal wafer is n-type, p-type or semi-insulating type.

[0027] Preferably, in the method of the present application, the 3C-SiC single crystal wafer has a surface orientation of (111) crystal axis direction.

[0028] Preferably, in the method of the present application, the 3C-SiC single crystal wafer is a cutting wafer, a grinding wafer, a polishing wafer or a substrate wafer.

[0029] Preferably, in the method of the present application, the cutting wafer is a wafer obtained by cutting a 3C-SiC ingot by a single-wire saw or a multi-wire saw, and has a surface roughness of 0.2-2 μm.

[0030] Preferably, in the method of the present application, the grinding wafer is a wafer obtained by thinning a 3C-SiC cutting wafer by a single-side grinder, a double-side grinder or a thinner, and has a surface roughness of 20-80 nm.

[0031] Preferably, in the method of the present application, the polishing wafer is a wafer obtained by polishing a 3C-SiC grinding wafer by a mechanical polisher, and has a surface roughness of 1-5 nm.

[0032] Preferably, in the method of the present application, the substrate wafer is a wafer obtained by chemically mechanically polishing and cleaning a 3C-SiC polishing wafer, and has a surface roughness of less than 0.5 nm.

[0033] Preferably, in the method of the present application, the molten alkali in step (1) is molten in a crucible.

[0034] Preferably, in the method of the present application, the crucible is a nickel crucible, a silver crucible, a high-nickel alloy crucible or a graphite crucible.

[0035] Preferably, in the method of the present application, the molten alkali in step (1) is heated by a corrosion furnace.

[0036] Preferably, in the method of the present application, the corrosion furnace is a muffle furnace or a tube furnace.

[0037] In some embodiments of the present application, the method of the present application comprises the following steps:

[0038] (1) cleaning the surface of the 3C-SiC wafer;

[0039] (2) placing the alkali in a crucible;

[0040] (3) setting the temperature of the corrosion furnace to 450-550°C;

[0041] (4) after the temperature of the corrosion furnace reaches the set temperature, placing the crucible containing the alkali into the corrosion furnace;

[0042] (5) after the alkali is molten, immersing the 3C-SiC single crystal wafer into the molten alkali for corrosion;

[0043] (6) After keeping the etching at the set temperature of 450-550 °C for 1-60 min, the 3C-SiC single crystal wafer is taken out from the molten alkali;

[0044] (7) The etched 3C-SiC single crystal wafer is put into ethanol and ultrasonically treated for 1-30 min;

[0045] (8) The 3C-SiC single crystal wafer is taken out and dried by blowing with high-purity inert gas;

[0046] (9) The wafer surface morphology is observed by using an optical microscope: if only stacking fault features appear on the surface, the surface is a (0001) C surface; if stacking fault and dislocation etching pits exist on the surface at the same time, the surface is a (111) Si surface.

[0047] In some specific embodiments of the present application, the cleaning of the 3C-SiC wafer surface in step (1) is achieved by a method comprising the following steps:

[0048] The 3C-SiC wafer is boiled in a mixture of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 for 10-20 min at a temperature of 75-85 °C; then washed with deionized water for 5 min; then boiled in a mixed solution of ammonia, hydrogen peroxide and deionized water with a volume ratio of 0.5:1:5 for 5 min at a temperature of 85 °C; then washed with deionized water for 5 min; finally, the 3C-SiC wafer is dried by blowing with high-purity inert gas.

[0049] The present application has the following beneficial effects:

[0050] (1) A clear and reliable crystal surface distinguishing method is provided for the first time. The present application first discovers and utilizes the inherent difference in etching morphology of (111) Si surface and (0001) C surface of 3C-SiC single crystal in molten alkali: the (111) Si surface will exhibit stacking fault and dislocation etching pits at the same time, while the (0001) C surface will only exhibit stacking fault. By observing this significant morphology contrast, rapid and accurate discrimination of the two crystal surfaces can be achieved.

[0051] (2) It has excellent universality and wide applicability. This method is not only suitable for the final substrate wafer, but also can effectively distinguish wafers at different processing stages (such as cutting, grinding and polishing), and can identify epitaxial wafers. This wide applicability enables it to be seamlessly embedded into the existing wafer preparation and device production process.

[0052] ​​​(3) Simple operation and easy to industrialize. The method has a simple process, requires conventional equipment, does not require high operator skills, and has good process repeatability and low cost. These characteristics make it very suitable for large-scale industrial production, providing key and practical technical support for the industrialization of 3C-SiC single crystal materials. Attached Figure Description

[0053] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0054] Figure 1 The image shows an optical photograph of the (111) Si surface obtained by etching a 3C-SiC single crystal wafer using Example 1. The long strips in the image represent stacking fault corrosion morphology, and the irregular and regular triangles represent dislocation corrosion pits.

[0055] Figure 2 The method used in Example 1 to etch a 3C-SiC single crystal wafer ( Optical photograph of surface C, where the long strips represent stacking fault corrosion morphology;

[0056] Figure 3 The image shows an optical photograph of the (111) Si surface obtained by etching a 3C-SiC single crystal wafer using Example 2. The long strips in the image represent stacking fault corrosion morphology, and the irregular and regular triangles represent dislocation corrosion pits.

[0057] Figure 4 The 3C-SiC single crystal obtained by etching in Example 2 ( Optical photograph of surface C, where the long strips represent stacking fault corrosion morphology;

[0058] Figure 5 The image shows an optical photograph of the (111) Si surface obtained by etching a 3C-SiC single crystal wafer using Example 3. The long strips in the image represent stacking fault corrosion morphology, and the irregular and regular triangles represent dislocation corrosion pits.

[0059] Figure 6 The method used in Example 3 to etch a 3C-SiC single crystal wafer ( The image shows an optical photograph of surface C, where the long strips represent stacking fault corrosion morphology.

[0060] Figure 7 To obtain by etching a 3C-SiC single crystal using Comparative Example 1 ( The optical photograph of side C shows no corrosion defects.

[0061] Figure 8 The image shows an optical photograph of the (111) Si surface obtained by etching a 3C-SiC single crystal using Comparative Example 1. No etching defects were found. Detailed Implementation

[0062] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0063] Example 1

[0064] (1) The known (111)Si surface with a surface roughness of 0.2 nm is compared with ( The 3C-SiC single crystal substrate on the C-side was boiled in a solution of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 for 10 minutes at a temperature of 80°C; then rinsed with deionized water for 5 minutes; then boiled in a mixed solution of ammonia, hydrogen peroxide and deionized water with a volume ratio of 0.5:1:5 for 5 minutes at a temperature of 85°C; then rinsed with deionized water for 5 minutes; finally, the 3C-SiC wafer was dried with high-purity N2.

[0065] (2) Put potassium hydroxide (KOH) and sodium peroxide (Na2O2) into a nickel (Ni) crucible, wherein the weight ratio of KOH to Na2O2 is 4:1; then, set the temperature of the muffle furnace to 550°C, and after the temperature of the corrosion furnace reaches the set temperature, put the Ni crucible containing KOH and Na2O2 into the muffle furnace.

[0066] (3) After KOH and Na2O2 are melted, the 3C-SiC single crystal wafer is immersed in the molten KOH and Na2O2 and etched at 550℃ for 30 min.

[0067] (4) Take out the 3C-SiC single crystal wafer, then clean it with ethanol for 10 minutes by ultrasonic cleaning, and then dry the wafer with high-purity N2.

[0068] (5) Observe the surface morphology of the wafer using an optical microscope, such as Figure 1 and Figure 2 As shown, ( The C-surface shows only stacking fault corrosion morphology, while the Si-surface shows both stacking fault and dislocation corrosion pit morphology.

[0069] It can be seen that the (111)Si plane of the 3C-SiC single crystal wafer, as determined by the testing method of the present invention, is different from the (111)Si plane. The C-plane is accurate and conforms to the (111) Si plane of the 3C-SiC single crystal wafer that was determined before conducting the test method of this invention. The fact of side C.

[0070] Example 2

[0071] (1) The known (111)Si surface with a surface roughness of 2 μm is compared with ( The 3C-SiC wafers on the C-side were boiled in a solution of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 for 10 minutes at 80°C; then rinsed with deionized water for 5 minutes; subsequently boiled in a mixed solution of ammonia, hydrogen peroxide, and deionized water with a volume ratio of 0.5:1:5 for 5 minutes at 85°C; then rinsed with deionized water for 5 minutes; and finally dried with high-purity N2.

[0072] (2) Put potassium hydroxide (KOH) into a nickel (Ni) crucible, and then set the temperature of the muffle furnace to 550°C. After the temperature of the corrosion furnace reaches the set temperature, put the Ni crucible containing KOH into the muffle furnace.

[0073] (3) After KOH is melted, the 3C-SiC single crystal wafer is immersed in the molten KOH and etched at 550°C for 30 min.

[0074] (4) Take out the 3C-SiC single crystal wafer, then clean it with acetone for 30 minutes by ultrasonic cleaning, and then dry the wafer with high-purity N2.

[0075] (5) Observe the surface morphology of the wafer using an optical microscope, such as Figure 3 and Figure 4 As shown, ( The C-surface shows only stacking fault corrosion morphology, while the Si-surface shows both stacking fault and dislocation corrosion pit morphology.

[0076] It can be seen that the (111)Si plane of the 3C-SiC single crystal wafer, as determined by the testing method of the present invention, is different from the (111)Si plane. The C-plane is accurate and conforms to the (111) Si plane of the 3C-SiC single crystal wafer that was determined before conducting the test method of this invention. The fact of side C.

[0077] Example 3

[0078] (1) The known (111)Si surface with a surface roughness of 2 μm is compared with ( The 3C-SiC wafers on the C-side were boiled in a solution of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 for 10 minutes at 80°C; then rinsed with deionized water for 5 minutes; subsequently boiled in a mixed solution of ammonia, hydrogen peroxide, and deionized water with a volume ratio of 0.5:1:5 for 5 minutes at 85°C; then rinsed with deionized water for 5 minutes; finally, the 3C-SiC wafers were dried using high-purity N2.

[0079] (2) Put potassium hydroxide (KOH) into a nickel (Ni) crucible, and then set the temperature of the muffle furnace to 450°C. After the temperature of the corrosion furnace reaches the set temperature, put the Ni crucible containing KOH into the muffle furnace.

[0080] (3) After KOH is melted, the 3C-SiC single crystal wafer is immersed in KOH alkali and etched at 450℃ for 10 min.

[0081] (4) Take out the 3C-SiC single crystal wafer, then clean it with acetone for 5 minutes by ultrasonic cleaning, and then dry the wafer with high-purity N2.

[0082] (5) Observe the surface morphology of the wafer using an optical microscope, such as Figure 5 and Figure 6 As shown, ( The C-surface shows only stacking fault corrosion morphology, while the Si-surface shows both stacking fault and dislocation corrosion pit morphology.

[0083] It can be seen that the (111)Si plane of the 3C-SiC single crystal wafer, as determined by the testing method of the present invention, is different from the (111)Si plane. The C-plane is accurate and conforms to the (111) Si plane of the 3C-SiC single crystal wafer that was determined before conducting the test method of this invention. The fact of side C.

[0084] Example 4

[0085] (1) The known (111)Si surface with a surface roughness of 2 μm is compared with ( The 3C-SiC wafers on the C-side were boiled in a solution of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 for 10 minutes at 80°C; then rinsed with deionized water for 5 minutes; subsequently boiled in a mixed solution of ammonia, hydrogen peroxide, and deionized water with a volume ratio of 0.5:1:5 for 5 minutes at 85°C; then rinsed with deionized water for 5 minutes; and finally dried with high-purity N2.

[0086] (2) Put potassium hydroxide (KOH) into a nickel (Ni) crucible, and then set the temperature of the muffle furnace to 500°C. After the temperature of the corrosion furnace reaches the set temperature, put the Ni crucible containing KOH into the muffle furnace.

[0087] (3) After KOH is melted, the 3C-SiC single crystal wafer is immersed in the molten KOH and etched at 500℃ for 5 min.

[0088] (4) Take out the 3C-SiC single crystal wafer, then clean it with ethanol for 10 minutes by ultrasonic cleaning, and then dry the wafer with high-purity N2.

[0089] (5) Observe the surface morphology of the wafer using an optical microscope. The C-surface shows only stacking fault corrosion morphology, while the Si-surface shows both stacking fault and dislocation corrosion pit morphology.

[0090] It can be seen that the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer determined by the test method of the present application are accurate, and consistent with the fact that the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer have been determined before the test method of the present application is performed. ) C surface of the 3C-SiC single crystal wafer determined by the test method of the present application are accurate, and consistent with the fact that the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer have been determined before the test method of the present application is performed. ) C surface of the 3C-SiC single crystal wafer determined by the test method of the present application are accurate, and consistent with the fact that the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer have been determined before the test method of the present application is performed.

[0091] Comparative Example 1

[0092] (1) A 3C-SiC single crystal substrate wafer with a known (111) Si surface and a (100) C surface and a surface roughness of 0.2 nm was boiled in a mixed solution of sulfuric acid and hydrogen peroxide with a volume ratio of 4:1 at a temperature of 80°C for 10 min, and then washed with deionized water for 5 min; thereafter, the wafer was boiled in a mixed solution of ammonia, hydrogen peroxide and deionized water with a volume ratio of 0.5:1:5 at a temperature of 85°C for 5 min, and then washed with deionized water for 5 min; finally, the wafer was blown dry with high-purity N2. (2) Potassium hydroxide (KOH) and sodium peroxide (Na2O2) were placed in a nickel (Ni) crucible, wherein the weight ratio of KOH to Na2O2 was 4:1; then, the temperature of the muffle furnace was set to 440°C, and after the temperature of the etching furnace reached the set temperature, the Ni crucible containing KOH and Na2O2 was placed in the muffle furnace.

[0093] (3) After the KOH and Na2O2 were melted, the 3C-SiC single crystal wafer was immersed in the molten KOH and Na2O2 for etching at 440°C for 30 min.

[0094] (4) The 3C-SiC single crystal wafer was taken out, and then ultrasonically cleaned with ethanol for 10 min, and then blown dry with high-purity N2.

[0095] (5) The wafer surface morphology was observed using an optical microscope, as shown in Figs. 1 and 2, and since the etching temperature was not high enough, the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer did not appear etching dislocations or stacking faults, and the (111) Si surface and the (100) C surface of the 3C-SiC single crystal wafer could not be distinguished.

[0096] Figure 7 and Figure 8 ​​​​

Claims

1. A method for determining the silicon and carbon faces of a 3C-SiC single crystal wafer, comprising the following steps: (1) Immerse the 3C-SiC single crystal wafer in molten alkali for etching treatment; (2) Remove the etched 3C-SiC single crystal wafer and observe its surface morphology: - If the surface only exhibits stacking fault characteristics, then the surface is determined to be ( C-side; - If the surface has both stacking faults and dislocation corrosion pits, then the surface is determined to be the (111)Si surface.

2. The method according to claim 1, wherein, The temperature of the molten alkali is 450-550℃; Preferably, the molten alkali is selected from one or more of KOH, NaOH, and Na2O2; Preferably, the corrosion treatment is performed for 1-60 minutes; More preferably, the corrosion treatment is performed for 1-10 minutes.

3. The method according to claim 1, wherein, The method also includes the following steps prior to step (1): cleaning the surface of the 3C-SiC single crystal wafer.

4. The method according to claim 3, wherein, The cleaning process is performed by cleaning the wafer using an acidic solution and / or an alkaline solution.

5. The method according to claim 1, wherein, The method further includes the following steps after removing the etched 3C-SiC single crystal wafer in step (2): placing the etched 3C-SiC single crystal wafer in an organic solvent for ultrasonic cleaning.

6. The method according to claim 5, wherein, The organic solvent is ethanol and / or acetone, and the ultrasonic cleaning is performed for 1-30 minutes.

7. The method according to claim 5, wherein, The method further includes the following step after ultrasonic cleaning: drying the ultrasonically cleaned wafer with high-purity inert gas. Preferably, the high-purity inert gas is high-purity argon and / or high-purity nitrogen; More preferably, the purity of the high-purity nitrogen gas is greater than or equal to 99.999%, and the purity of the high-purity argon gas is greater than or equal to 99.999%.

8. The method according to claim 1, wherein, The observation in step (2) is performed using an optical microscope.

9. The method according to claim 1, wherein, The 3C-SiC single crystal wafer is n-type, p-type, or semi-insulating; Preferably, the surface orientation of the 3C-SiC single crystal wafer is along the (111) crystal axis direction; Preferably, the 3C-SiC single crystal wafer is a diced wafer, a grinding wafer, a polished wafer, or a substrate wafer; More preferably, the dicing wafer is a wafer obtained by dicing a 3C-SiC ingot using a single-wire dicing machine or a multi-wire dicing machine, with a surface roughness of 0.2-2 μm; More preferably, the grinding wafer is a wafer obtained by thinning a 3C-SiC diced wafer using a single-sided grinding machine, a double-sided grinding machine, or a thinning machine, with a surface roughness of 20-80 nm; More preferably, the polishing wafer is a wafer obtained by polishing a 3C-SiC grinding wafer with a mechanical polishing machine, and the surface roughness is 1-5 nm; More preferably, the substrate is a 3C-SiC polished wafer that has undergone chemical mechanical polishing and cleaning, with a surface roughness of less than 0.5 nm.

10. The method according to claim 1, wherein, The molten alkali in step (1) is molten in a crucible; Preferably, the crucible is a nickel crucible, a silver crucible, a high-nickel alloy crucible, or a graphite crucible; More preferably, the molten alkali in step (1) is heated by a corrosion furnace; More preferably, the corrosion furnace is a muffle furnace or a tube furnace.

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