A method for etching lithium tantalate wafer with high etching uniformity and controllable etching rate
By employing a three-stage progressive etching process and a multi-component synergistic regulation method, the problems of etching uniformity and rate control in the etching process of lithium tantalate wafers were solved, achieving efficient and uniform etching results and improving wafer surface quality and processing efficiency.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- TDG HLDG CO LTD
- Filing Date
- 2026-04-09
- Publication Date
- 2026-06-16
AI Technical Summary
The existing etching process for lithium tantalate wafers suffers from poor etching uniformity and difficulty in controlling the etching rate, which affects the surface quality and performance of the wafers and limits their application in high-end electronic and optical devices.
A three-stage progressive etching process is adopted, including rapid etching with strong acid, surface repair with composite etching solution, and complexation treatment with weak acid. The pH value and concentration of the etching solution are controlled by buffers and surfactants. The uniformity and stability of the etching solution are ensured by the surging function and the replenishment device. Finally, the surface smoothness is improved by the spin-drying step.
This achieves controllable corrosion uniformity and rate, improves the flatness and roughness of the wafer surface, reduces the difficulty of subsequent processing, and enhances processing efficiency and quality.
Abstract
Description
Technical Field
[0001] This invention relates to the field of semiconductor materials, and more specifically to a method for etching lithium tantalate wafers with high corrosion uniformity and controllable corrosion rate. Background Technology
[0002] Lithium tantalate (LiTaO3, abbreviated as LT) crystals possess excellent piezoelectric, photoelectric, and photorefractive effects, making them an important multifunctional crystal material. As a piezoelectric crystal, lithium tantalate has advantages such as fast response speed, low temperature coefficient, and good thermal stability. It can be used to manufacture filters, resonators, Q-switches, pyroelectric detectors, and other products, and is widely used in communication technology, electronics technology, remote sensing and telemetry, and other fields.
[0003] Etching is a crucial step in semiconductor manufacturing, typically involving a chemical reaction in a specific etching solution to process the surface or internal structure of the semiconductor material. After lithium tantalate slicing, the wafer surface may suffer mechanical and thermal damage, forming a damage layer that adversely affects crystal performance and surface quality. Etching can effectively remove this damage layer. Simultaneously, the wafer surface may exhibit some unevenness after slicing; etching can improve surface smoothness and roughness. Lithium tantalate slicing typically uses nitric acid, hydrofluoric acid, or hydrochloric acid for etching. These acidic solutions are highly corrosive to lithium tantalate, achieving high etching rates in a short time. Traditional etching methods struggle to precisely control the surface etching rate, resulting in poor surface roughness and uniformity, and easily leading to defects such as etching pits and back spots. This not only affects the performance and quality of lithium tantalate wafers but also restricts their application in high-end electronic and optical devices. Therefore, optimizing the etching process, controlling the etching rate, and improving the surface uniformity of lithium tantalate wafers are pressing issues that need to be addressed in lithium tantalate etching. The invention patent with patent number CN112133649A discloses a large-size wafer uniform high-temperature etching device and its etching method. The wafer is heated by a heating stage, and the rotation speed of the wafer fixing stage is coordinated with the translational and rotational speeds of the conveying arm to ensure that the etchant is sprayed evenly onto the wafer surface by the spray nozzle, thus achieving high-temperature etching. However, the method does not mention how to effectively control the etching rate, and the etching method through the spray system is affected by variables such as spray pressure, rotation speed, and evaporation of the etchant, which cannot completely guarantee the uniformity of etching. In addition, the etching efficiency of this method is low.
[0004] Therefore, there is a lack of existing corrosion technologies that can guarantee corrosion uniformity and controllable corrosion rate for lithium tantalate wafers. Summary of the Invention
[0005] This invention provides a method for etching lithium tantalate wafers with high corrosion uniformity and controllable corrosion rate to overcome the shortcomings of the prior art. This method has a stable processing flow, improves corrosion uniformity, and allows for controllable corrosion rate.
[0006] The technical solution adopted by this invention to solve the problem is: a method for etching lithium tantalate wafers with high corrosion uniformity and controllable rate, comprising the following steps:
[0007] a) Cleaning: After the lithium tantalate wafers have been sliced, they are placed in the cleaning solution and ultrasonically cleaned to remove surface residues, chips and slurry. After cleaning, pure water is used to rinse the wafer surface to remove residual cleaning agent.
[0008] b) First etching: Place the wafer cleaned in step a) into a corrosion-resistant container that can drain the solution, then place the container into a quartz tank containing a mixed strong acid etching solution and fix it. Etch at room temperature for 20~80 minutes. After etching, use pure water to clean the surface of the wafer to remove any residual etching solution.
[0009] c) Secondary etching: The wafer treated in step b) is placed in a quartz tank containing composite etching solution for secondary etching. The temperature is heated to 30~90℃ and turbulent etching is performed for 30~260 minutes. After etching, the wafer surface is cleaned with pure water to remove any residual etching solution.
[0010] d) Three-stage etching: The wafer treated in step c) is placed in a quartz bath containing a weak acid complex etching solution for three-stage etching. The temperature is heated to 40~60℃ and the etching is carried out by turbulence for 30~150 minutes. After the etching is completed, the wafer surface is cleaned with pure water to remove any residual etching solution.
[0011] e) Spin-drying: After the wafers in step d) have been cleaned with pure water, put them into a spin dryer to spin-dry them and obtain uniformly etched lithium tantalate wafers.
[0012] In step a) above, lithium tantalate wafers are usually cut using steel wire and slurry. Before cutting, the lithium tantalate crystal rod needs to be glued to the wafer loading table for cutting. After cutting, the wafer surface needs to be cleaned to remove residual glue, chips and slurry. The cleaning solution is one of potassium hydroxide, sodium hydroxide and tetramethylammonium hydroxide.
[0013] In step b) above, the mixed strong acid etching solution includes hydrochloric acid and nitric acid. Hydrochloric acid and nitric acid are highly corrosive and have a fast etching rate. This etching step is used to initially and quickly remove the line marks left on the surface of the wafer after cutting. The concentration of hydrochloric acid is 30~50wt%, the concentration of nitric acid is 50~65wt%, and the volume ratio of hydrochloric acid to nitric acid is (1~5):1. After etching, the wafer is placed in a pure water bath and rinsed for 3~5 minutes to remove the residual etching solution on the wafer surface.
[0014] In step c) above, the composite etching solution is composed of nitric acid, hydrofluoric acid, buffer and surfactant. The purpose of this etching step is to further remove residual cutting marks on the wafer surface and repair the flatness of the wafer surface. By adding surfactant, the etching solution can more easily cover the wafer surface, improve the uniformity of the etching process and the etching effect. The surfactant is one or more of polyvinyl alcohol, alkylphenol polyoxyethylene ether and sodium dodecyl sulfonate.
[0015] In step c) above, adding a buffer extends the activity time of the composite etching solution and reduces the rate of concentration change. The buffer stabilizes the H+ in the etching solution. + and F — The concentration is used to adjust the pH of the corrosive solution to maintain pH stability. This prevents the reaction from becoming too vigorous and out of control due to excessively low pH, and also prevents the reaction from becoming too high and triggering Ta. 5+ Hydrolysis and precipitation are employed to avoid abrupt changes in corrosion rate caused by rapid local HF consumption, ensuring the uniformity and stability of corrosion. This allows for precise control of material removal from the lithium tantalate wafer surface, achieving uniform removal, reducing material loss, and improving processing quality. The corrosion rate can be adjusted by regulating the amount of buffer used, which can be one or more of sodium fluoride, ammonium fluoride, potassium fluoride, sodium dihydrogen phosphate, and sodium acetate.
[0016] In step c) above, the concentration of hydrofluoric acid is 40~50wt%, and the concentration of nitric acid is 50~65wt%; the volume ratio of hydrofluoric acid, nitric acid, surfactant, and buffer is (2~5):(3~8):(0.05~0.2):(0.5~1).
[0017] In step d) above, the weak acid complexing etching solution is composed of a weak acid, a complexing agent, and an etching aid. The complexing agent includes ethylenediaminetetraacetic acid (EDTA), the weak acid includes citric acid, and the etching aid includes polyethylene glycol (PEG). The purpose of this etching step is to optimize the flatness and roughness of the wafer surface, reduce the amount of material removed during polishing, and improve the polishing efficiency. This etching step uses citric acid, which has a relatively weak acidity, in combination with EDTA to complex the wafer surface. PEG is added to improve the wettability of the wafer surface, ensuring uniform etching and improving the flatness and roughness of the wafer surface. After etching, a smoother and more uniform etched surface can be obtained. By adjusting the volume ratio of each component, the etching rate can be precisely controlled, reducing the amount of material removed during polishing and improving the efficiency of subsequent polishing processes. The volume ratio of EDTA, citric acid, and PEG is (5~10):(1.5~3):1.
[0018] As a preferred embodiment, steps c) and d) involve activating the surging function during the corrosion process to ensure that the corrosion solution is in a convection state, thus avoiding uneven temperature distribution or uneven concentration distribution of the corrosion solution, which could lead to poor corrosion uniformity.
[0019] As a preferred embodiment, in steps c) and d), the quartz tank is equipped with a concentration detection device and a liquid replenishment device during the etching process. Since the etching liquid used in this invention is volatile, some of the etching liquid will evaporate during the churning process, causing the concentration of the etching liquid to decrease. The concentration detection device will detect the concentration of the etching liquid in real time. When the concentration is lower than the set value, the liquid replenishment device will automatically replenish the etching liquid to the set concentration.
[0020] As a preferred embodiment, in step e), the spin-drying speed is accelerated from 50 to 300 rpm / s to 1000 to 3000 rpm, and then high-purity nitrogen gas at a temperature of 20 to 40°C is introduced at a flow rate of 0.5 to 5 L / min. The blowing direction is at an angle of 15° to 40° with the rotation direction for 30 to 60 seconds, and the spin-drying time is 3 to 8 minutes.
[0021] Compared with the prior art, the present invention has the following beneficial technical effects:
[0022] Firstly, the present invention proposes a method for etching lithium tantalate wafers with high corrosion uniformity and controllable rate. This method employs a three-stage progressive etching process: "rapid etching with strong acid - surface repair with composite etching solution - surface optimization with weak acid complexation treatment." This process ensures high etching efficiency while progressively improving surface quality. Rapid etching with strong acid removes line marks, providing a smooth substrate for subsequent etching. The composite etching solution completely removes line marks and initially repairs flatness, solving the problems of uncontrollable strong etching and low efficiency of weak etching. The weak acid complexation treatment removes surface damage and optimizes roughness, enabling the regulation of the etching rate and reducing the amount removed by subsequent polishing. This solves the problems of corrosion uniformity and corrosion quality control, and reduces the difficulty of subsequent processing.
[0023] Secondly, in the lithium tantalate wafer etching method with high corrosion uniformity and controllable rate proposed in this invention, the composite etching solution for secondary etching is matched with a buffer and a surfactant to achieve multi-component synergistic regulation. The buffer stabilizes pH and F. — Concentration is used to control the corrosion rate, while surfactants are used to enhance wetting and microstructure penetration capabilities, ensuring corrosion uniformity, achieving precise and controllable corrosion process, and improving wafer processing quality.
[0024] Thirdly, the lithium tantalate wafer etching method with high corrosion uniformity and controllable rate proposed in this invention adopts a weak acid complex etching system for the three-stage etching process, using citric acid, ethylenediaminetetraacetic acid and polyethylene glycol. Through complexation, the surface damage removal and the dual optimization of flatness and roughness are achieved. By controlling the etching rate by the component ratio, the amount of subsequent polishing removal is effectively reduced, and the overall wafer processing efficiency is greatly improved. Detailed Implementation
[0025] The present invention will be further described below with reference to embodiments, but these should not be construed as limiting the scope of protection of the present invention.
[0026] Example 1:
[0027] A method for etching lithium tantalate wafers with high corrosion uniformity and controllable corrosion rate includes the following steps:
[0028] a) Cleaning: The sliced wafers are placed in sodium hydroxide cleaning solution for ultrasonic cleaning to remove surface residues, chips and slurry. After cleaning, pure water is used to remove residual cleaning agent from the wafer surface.
[0029] b) First etching: Prepare a mixed strong acid etching solution by mixing 40wt% hydrochloric acid and 60wt% nitric acid at a volume ratio of 1.5:1. Place the wafer treated in step a) into a corrosion-resistant basket, then place the basket into the etching solution and etch at room temperature for 30 minutes. After etching, use pure water to clean the wafer surface to remove any residual etching solution.
[0030] c) Secondary etching: Prepare a composite etching solution by mixing 40wt% hydrofluoric acid, 60wt% nitric acid, alkylphenol polyoxyethylene ether, and sodium fluoride in a volume ratio of 2:6:0.05:0.7. Place the wafer and basket treated in step b) into the composite etching solution, turn on the surging function, start the concentration detection device and the liquid replenishment device, heat the etching solution to 40°C, and etch at 40°C for 60 minutes. After etching, use pure water to clean the surface of the wafer to remove the residual etching solution.
[0031] d) Three-stage etching: Prepare a weak acid complex etching solution by mixing ethylenediaminetetraacetic acid, citric acid, and polyethylene glycol in a volume ratio of 8:2:1. Place the wafer and basket treated in step c) into the weak acid complex etching solution, turn on the surging function, start the concentration detection device and the liquid replenishment device, heat the etching solution to 60°C, and etch at 60°C for 50 minutes. After etching, use pure water to clean the surface of the wafer to remove the residual etching solution.
[0032] e) Spin-drying: After the wafers in step d) are cleaned with pure water, they are placed in a spin dryer and the spin-drying speed is accelerated from 100 rpm / s to 1000 rpm. High-purity nitrogen gas at 30°C is introduced at a flow rate of 1 L / min. The blowing direction is at a 20° angle to the rotation direction and is continued for 30 seconds. Spin-drying is then carried out for 5 minutes to obtain a uniformly etched lithium tantalate wafer.
[0033] The lithium tantalate wafers after etching in this embodiment were tested and found to have no corrosion marks or pits on the surface, with a TTV of 3.37 μm and a surface roughness of 0.34 μm.
[0034] Example 2:
[0035] A method for etching lithium tantalate wafers with high corrosion uniformity and controllable corrosion rate includes the following steps:
[0036] a) Cleaning: The sliced wafers are placed in sodium hydroxide cleaning solution for ultrasonic cleaning to remove surface residues, chips and slurry. After cleaning, pure water is used to remove residual cleaning agent from the wafer surface.
[0037] b) First etching: Prepare a mixed strong acid etching solution by mixing 35wt% hydrochloric acid and 55wt% nitric acid at a volume ratio of 2:1. Place the wafer treated in step a) into a corrosion-resistant basket, then place the basket into the etching solution and etch at room temperature for 50 minutes. After etching, use pure water to clean the wafer surface to remove any residual etching solution.
[0038] c) Secondary etching: Prepare a composite etching solution by mixing 45wt% hydrofluoric acid, 55wt% nitric acid, alkylphenol polyoxyethylene ether, and sodium fluoride in a volume ratio of 3:6:0.1:0.8. Place the wafer and basket treated in step b) into the composite etching solution, turn on the surging function, start the concentration detection device and the liquid replenishment device, heat the etching solution to 40°C, and etch at 40°C for 50 minutes. After etching, use pure water to clean the surface of the wafer to remove the residual etching solution.
[0039] d) Three-stage etching: Prepare a weak acid complex etching solution with a volume ratio of ethylenediaminetetraacetic acid, citric acid, and polyethylene glycol of 6:3:1. Place the wafer and basket treated in step c) into the weak acid complex etching solution, turn on the surging function, start the concentration detection device and the liquid replenishment device, heat the etching solution to 50°C, and etch at 50°C for 60 minutes. After etching, use pure water to clean the surface of the wafer to remove the residual etching solution.
[0040] e) Spin-drying: After the wafers in step d) are cleaned with pure water, they are placed in a spin dryer and the spin-drying speed is accelerated from 100 rpm / s to 1500 rpm. High-purity nitrogen gas at 30°C is introduced at a flow rate of 1 L / min. The blowing direction is at a 20° angle to the rotation direction and is continued for 30 seconds. Spin-drying is then carried out for 3 minutes to obtain a uniformly etched lithium tantalate wafer.
[0041] The lithium tantalate wafers after etching in this embodiment were tested and found to have no corrosion marks or pits on the surface, with a TTV of 3.52 μm and a surface roughness of 0.29 μm.
[0042] Comparative Example 1:
[0043] A method for etching lithium tantalate wafers includes the following steps:
[0044] a) Cleaning: Same as in Example 1;
[0045] b) First etching: Prepare a mixed strong acid etching solution by mixing 40% hydrochloric acid and 60% nitric acid at a volume ratio of 1.5:1. Place the wafer treated in step a) into a corrosion-resistant basket, then place the basket into the etching solution and etch for 60 minutes at room temperature. After etching, use pure water to clean the wafer surface to remove any residual etching solution.
[0046] c) Spin-drying: After the wafers in step b) have been cleaned with pure water, put them into a spin dryer and spin-dry them at a speed of 1000 rpm for 5 minutes to obtain the etched lithium tantalate wafers.
[0047] The lithium tantalate wafers after this comparative etching process were tested and found to have etching marks and pits on the wafer surface. The TTV was 13.22 μm and the front surface roughness was 1.76 μm. The analysis showed that the etching rate was too fast and the etching solution was too corrosive, resulting in uneven etching. Some areas were over-etched, forming etching marks or pits, which caused excessive TTV and front surface roughness of the wafer.
[0048] Comparative Example 2:
[0049] A method for etching lithium tantalate wafers includes the following steps:
[0050] a) Cleaning: Same as in Example 1;
[0051] b) Primary corrosion: Same as in Example 1;
[0052] c) Secondary etching: A composite etching solution is prepared by mixing 40wt% hydrofluoric acid, 60wt% nitric acid, alkylphenol polyoxyethylene ether, and sodium fluoride in a volume ratio of 2:6:0.05:0.7. The wafer and basket treated in step b) are placed in the composite etching solution, the etching solution is heated to 40°C, and etched at 40°C for 60 minutes. After etching, the wafer surface is cleaned with pure water to remove any residual etching solution.
[0053] d) Three-stage etching: Prepare a weak acid complex etching solution by mixing ethylenediaminetetraacetic acid, citric acid and polyethylene glycol in a volume ratio of 8:2:1. Place the wafer and basket treated in step c) into the weak acid complex etching solution, heat the etching solution to 60°C, and etch at 60°C for 50 minutes. After etching, use pure water to clean the surface of the wafer to remove any residual etching solution.
[0054] e) Same as Example 1.
[0055] The lithium tantalate wafers after etching in this comparative example were tested and found to have significantly different etching effects on their surfaces. The area near the bottom of the quartz tank was more heavily etched and had some etching pits. The TTV was 9.23 μm and the surface roughness was 0.65 μm. The reason for this was that the surging function and the liquid replenishment device were not activated in this example. The temperature and concentration of the etching solution at different depths in the quartz tank varied greatly, resulting in significant differences in the etching effect in different areas of the wafer during the etching process.
[0056] Comparative Example 3:
[0057] A method for etching lithium tantalate wafers includes the following steps:
[0058] a) Cleaning: Same as in Example 1;
[0059] b) Primary corrosion: Same as in Example 1;
[0060] c) Secondary etching: Prepare a composite etching solution by mixing 40wt% hydrofluoric acid and 60wt% nitric acid at a volume ratio of 1:3. Place the wafer and basket treated in step b) into the composite etching solution, turn on the surging function, start the concentration detection device and liquid replenishment device, heat the etching solution to 60°C, and etch for 60 minutes. After etching, use pure water to clean the surface of the wafer of residual etching solution.
[0061] d) Tertiary corrosion: Same as Example 1;
[0062] e) Spin-dry: Same as in Example 1.
[0063] The lithium tantalate wafers after the comparative etching process were tested and found that some wafer surfaces had etching marks, with a TTV of 8.38 μm and a front surface roughness of 0.67 μm. The reason for this was that no surfactant and buffer were used in step c), which resulted in a poorer repair effect on the wafer surface flatness and a higher TTV value.
[0064] Comparative Example 4:
[0065] A method for etching lithium tantalate wafers includes the following steps:
[0066] a) Cleaning: Same as in Example 1;
[0067] b) Primary corrosion: Same as in Example 1;
[0068] c) Secondary corrosion: Same as in Example 1;
[0069] d) Spin-drying: After the wafers in step c) have been cleaned with pure water, put them into a spin dryer and spin-dry them at a speed of 1000 rpm for 5 minutes to obtain the etched lithium tantalate wafers.
[0070] The lithium tantalate wafers after this comparative etching process were tested and found to have etching pits on the wafer surface. The surface TTV of the wafer was 5.20 μm, and the roughness of the front side was 1.38 μm. The reason for this was that a third etching was not performed. The composite etching solution in step c) can adjust the etching rate, but it is difficult to guarantee the etching uniformity. In some areas, etching pits were formed, and the roughness of the front side of the wafer was relatively large.
[0071] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A method for etching lithium tantalate wafers with high corrosion uniformity and controllable rate, characterized in that, Includes the following steps: a) Cleaning: Provide the sliced lithium tantalate wafers and clean them with a cleaning solution; b) Primary etching: The cleaned wafer obtained in step a) is etched using a mixed strong acid etching solution; c) Secondary etching: The wafer obtained after the primary etching in step b) is etched using a composite etching solution, wherein the composite etching solution includes acid, surfactant and buffer. d) Third etching: The wafer obtained after the second etching in step c) is etched using a weak acid complexing etching solution, wherein the weak acid complexing etching solution includes a weak acid, a complexing agent and an etching aid. e) Spin dry: Spin dry the wafer obtained after three etching steps d) to obtain a uniformly etched lithium tantalate wafer; In steps c) and d), the surging function is activated to put the corrosive liquid into a convection state, and a liquid replenishment device is set up.
2. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, In step a), ultrasonic cleaning is used, and the cleaning solution includes one of potassium hydroxide, sodium hydroxide, and tetramethylammonium hydroxide solution.
3. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, In step b), the mixed strong acid corrosion solution includes hydrochloric acid and nitric acid, the corrosion temperature is room temperature, and the corrosion time is 20~80 min; wherein the concentration of hydrochloric acid is 30~50wt%, the concentration of nitric acid is 50~65wt%, and the volume ratio of hydrochloric acid to nitric acid is (1~5):
1.
4. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, In step c), the corrosion temperature is 30~90℃ and the corrosion time is 30~260min; the acid includes nitric acid and hydrofluoric acid; the buffer includes one or more of sodium fluoride, ammonium fluoride, potassium fluoride, sodium dihydrogen phosphate, and sodium acetate; and the surfactant includes one or more of polyvinyl alcohol, alkylphenol polyoxyethylene ether, and sodium dodecyl sulfonate.
5. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 4, characterized in that, In step c), the concentration of hydrofluoric acid is 40-50 wt%, and the concentration of nitric acid is 50-65 wt%; the volume ratio of hydrofluoric acid, nitric acid, surfactant, and buffer is (2-5):(3-8):(0.05-0.2):(0.5-1).
6. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, In step d), the weak acid includes citric acid, the complexing agent includes ethylenediaminetetraacetic acid, and the corrosion aid includes polyethylene glycol; the corrosion temperature is 40~60℃, and the corrosion time is 30~150min.
7. The lithium tantalate wafer etching method with high corrosion uniformity and controllable rate as described in claim 6, characterized in that, In step d), the volume ratio of ethylenediaminetetraacetic acid, citric acid, and polyethylene glycol is (5~10):(1.5~3):
1.
8. The method for etching lithium tantalate wafers with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, Steps c) and d) are carried out in a quartz tank, which is also equipped with a concentration detection device. When the concentration detection device detects that the concentration of the etching solution is lower than the set value, the etching solution is automatically replenished to the set concentration through the replenishment device.
9. The method for etching lithium tantalate wafers with high corrosion uniformity and controllable rate as described in claim 1, characterized in that, In step e), the spin-drying speed is accelerated from 50 to 300 rpm / s to 1000 to 3000 rpm, and then high-purity nitrogen gas at a temperature of 20 to 40°C is introduced at a flow rate of 0.5 to 5 L / min. The blowing direction is at an angle of 15° to 40° with the rotation direction for 30 to 60 seconds, and the spin-drying time is 3 to 8 minutes.