Wafer cleaning method

Abstract

The invention provides a wafer cleaning method which comprises the following steps: a dry cleaning step: transferring a wafer after an etching process is completed into a reaction chamber, and introducing preset etching gas into the reaction chamber so as to etch and remove photoresist and / or residues of an anti-reflection layer on the surface of the wafer; wherein the preset etching gas comprises oxygen-containing gas; a wet cleaning step: transferring the wafer after the dry cleaning step into a cleaning device, and cleaning the surface of the wafer by adopting a preset cleaning solution to remove residues on the surface of the wafer; wherein the preset cleaning solution comprises a solution containing sulfuric acid and hydrogen peroxide, and / or an aqueous solution containing ammonium hydroxide and hydrogen peroxide. According to the method, the removal efficiency of the residues on the surface of the wafer is improved, the cleaning time of subsequent wet cleaning can be shortened, the amount of needed cleaning liquid can be reduced, the cleaning cost is saved, and the number of residual particles on the surface of the cleaned wafer can be effectively reduced.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a wafer cleaning method. Background Art

[0002] In the semiconductor manufacturing process, it is usually necessary to remove and clean the photoresist or anti-reflective coating to remove the photoresist residue or anti-reflective coating residue attached to the surface of the silicon wafer during the semiconductor manufacturing process. If the residue on the silicon wafer surface is not completely removed, it may affect the performance and manufacturing quality of the chip, leading to electrical performance problems or production defects.

[0003] Related photoresist cleaning technology usually uses specific chemical liquids to directly clean the photoresist during the residue cleaning process. However, using chemical liquids to directly clean silicon wafer residues requires a large amount of liquid, resulting in high cleaning costs, low efficiency in removing residues, and residues are still likely to remain after cleaning. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a wafer cleaning method, which improves the removal efficiency of residues on the wafer surface, reduces the cleaning time and the required amount of cleaning fluid in the wet cleaning step, saves cleaning costs, and can effectively reduce the number of residual particles on the wafer surface after cleaning.

[0005] In order to achieve the above objectives, the technical solutions adopted in the embodiments of the present invention are as follows:

[0006] In a first aspect, an embodiment of the present invention provides a wafer cleaning method, comprising:

[0007] A dry cleaning step, wherein the wafer after the etching process is introduced into a reaction chamber, and a predetermined etching gas is introduced into the reaction chamber to etch away the photoresist and / or anti-reflective layer residue on the surface of the wafer; wherein the predetermined etching gas includes an oxygen-containing gas;

[0008] A wet cleaning step is performed, wherein the wafer after the dry cleaning step is transferred into a cleaning device, and a preset cleaning liquid is used to clean the surface of the wafer to remove residues on the surface of the wafer; wherein the preset cleaning liquid includes a solution containing sulfuric acid and hydrogen peroxide and / or an aqueous solution containing ammonium hydroxide and hydrogen peroxide.

[0009] Furthermore, an embodiment of the present invention provides a first possible implementation manner of the first aspect, wherein the wet cleaning step includes a first wet cleaning step and a second wet cleaning step;

[0010] a first wet cleaning step of cleaning the surface of the wafer with the solution containing sulfuric acid and hydrogen peroxide to remove residues of the photoresist and / or anti-reflective layer;

[0011] In the second wet cleaning step, the surface of the wafer is cleaned using the solution containing ammonium hydroxide and hydrogen peroxide.

[0012] Furthermore, an embodiment of the present invention provides a second possible implementation of the first aspect, wherein the oxygen-containing gas includes oxygen, and the flow rate of the oxygen is in the range of 100 ml / min to 1000 ml / min.

[0013] Furthermore, an embodiment of the present invention provides a third possible implementation of the first aspect, wherein the preset etching gas further includes nitrogen and hydrogen;

[0014] The content of the oxygen-containing gas in the preset etching gas ranges from 80% to 90%.

[0015] Furthermore, an embodiment of the present invention provides a fourth possible implementation of the first aspect, wherein the temperature of the solution containing sulfuric acid and hydrogen peroxide is 120°C to 130°C.

[0016] Furthermore, an embodiment of the present invention provides a fifth possible implementation of the first aspect, wherein the temperature of the aqueous solution containing ammonium hydroxide and hydrogen peroxide is 30°C to 70°C.

[0017] Furthermore, the embodiment of the present invention provides a sixth possible implementation of the first aspect, wherein the solution containing sulfuric acid and hydrogen peroxide is an SPM solution;

[0018] and / or,

[0019] The SPM solution is prepared by mixing concentrated sulfuric acid with a mass fraction of 98% and hydrogen peroxide with a mass fraction of 31% in a volume ratio of 3:1 to 5:1.

[0020] Furthermore, the embodiment of the present invention provides a seventh possible implementation of the first aspect, wherein the aqueous solution containing ammonium hydroxide and hydrogen peroxide is an SC1 cleaning agent;

[0021] and / or,

[0022] The SC1 cleaning agent is prepared by mixing 29% by mass of the ammonium hydroxide, 31% by mass of the hydrogen peroxide, and water in a volume ratio of 1:1:50 to 1:5:50.

[0023] Furthermore, an embodiment of the present invention provides an eighth possible implementation of the first aspect, wherein the cleaning time of the first wet cleaning step is 5 to 15 minutes.

[0024] Furthermore, an embodiment of the present invention provides a ninth possible implementation of the first aspect, wherein the cleaning time of the second wet cleaning step is 5 to 10 minutes.

[0025] An embodiment of the present invention provides a wafer cleaning method, which includes: a dry cleaning step, wherein a wafer that has completed an etching process is transferred into a reaction chamber, and a preset etching gas is introduced into the reaction chamber to etch away photoresist and / or anti-reflective layer residues on the wafer surface; wherein the preset etching gas includes an oxygen-containing gas; and a wet cleaning step, wherein the wafer that has completed the dry cleaning step is transferred into a cleaning device, and a preset cleaning liquid is used to clean the wafer surface to remove residues on the wafer surface; wherein the preset cleaning liquid includes a solution containing sulfuric acid and hydrogen peroxide, and / or an aqueous solution containing ammonium hydroxide and hydrogen peroxide. The present invention adds dry etching cleaning before wet cleaning. Since the residues of the photoresist and / or anti-reflective layer on the surface of the wafer are usually hydrocarbon organic matter, oxygen-containing gas is introduced into the chamber to effectively remove the residues. Since dry etching cleaning usually takes a shorter time than wet cleaning, the efficiency of removing residues on the wafer surface is improved, the cleaning time and the required amount of cleaning liquid for subsequent wet cleaning can be reduced, the cleaning cost can be saved, and the number of residual particles on the wafer surface after cleaning can be effectively reduced.

[0026] Other features and advantages of the embodiments of the present invention will be described in the following description, or some features and advantages can be inferred or determined without doubt from the description, or can be learned by implementing the above-mentioned technologies of the embodiments of the present invention.

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, preferred embodiments are given below and described in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 A flow chart of a wafer cleaning method provided by an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be described below with reference to the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0031] Currently, in the semiconductor manufacturing process, there are usually residues of photoresist and / or anti-reflective layer on the wafer after the etching process. The main function of the photoresist and anti-reflective layer is usually to transfer the pattern of the mask to the wafer, or to serve as a protective film for subsequent processes. The residues of photoresist and anti-reflective layer may come from various links in the photolithography process, such as coating, exposure and development. If the residues are not removed in time, it may affect the performance and manufacturing quality of the chip, leading to electrical performance problems or production defects.

[0032] In related wafer cleaning technologies, specific chemical solvents are usually used for cleaning, which can dissolve photoresist residues, keep the wafer surface clean, and ensure the quality and performance of subsequent processes. For example, SPM (Sulfuric Peroxide Mixture, sulfuric acid hydrogen peroxide mixture) solution is used to clean the wafer for 20 minutes to oxidize the residues, and then another new SPM solution is used to continue to oxidize the residues for 10 minutes. Finally, the wafer is cleaned with SC1 (RCA Standard Cleaning-1, a mixture of ammonium hydroxide and hydrogen peroxide) cleaning agent for 10 minutes to remove the photoresist and anti-reflective layer residues on the wafer surface. However, the removal efficiency of directly using chemical solvents to clean the residues is low. The residue value after cleaning the wafer is usually 200-300 granularity. In addition, SPM solution is used twice in the cleaning process, which requires a large amount of cleaning solution and acid, and the cost is relatively high.

[0033] In order to improve the above problems, an embodiment of the present invention provides a wafer cleaning method, which is described in detail below.

[0034] This embodiment provides a wafer cleaning method. Figure 1 The wafer cleaning method flow chart shown in FIG. 1 mainly includes the following steps:

[0035] Step S102, a dry cleaning step, wherein the wafer after the etching process is transferred into a reaction chamber, and a predetermined etching gas is introduced into the reaction chamber to etch away the photoresist and / or anti-reflective layer residues on the surface of the wafer;

[0036] The preset etching gas includes an oxygen-containing gas; the wafer may include a semiconductor device formed with an anti-reflective layer and a photoresist layer, and the main components of the residues of the anti-reflective layer and the photoresist layer are usually hydrocarbon organic matter. By introducing the oxygen-containing gas into the reaction chamber where the wafer is located, the oxygen-containing gas can be ionized into oxygen atoms that react with the hydrocarbon organic matter to generate by-products such as carbon monoxide, carbon dioxide, and water. The reaction by-products are then pumped away by a vacuum pump, thereby completing the cleaning and removal of the residues of the photoresist and / or anti-reflective layer. The resulting chemical reaction formula is:

[0037] C x H y +(x+y / 2)O2→xCO2+y / 2H2O

[0038] The process time of the dry cleaning step is related to the thickness of the photoresist and / or anti-reflective layer. A thicker film requires a longer process time. The process time of the dry cleaning step can range from 30 seconds to 5 minutes. When ionizing the oxygen-containing gas, the power of the upper and lower electrodes can range from 500 to 1000 W.

[0039] The wafer after the etching process that needs to be cleaned usually includes a photoresist and / or an anti-reflection layer. The photoresist and anti-reflection layer can include SOC (System on Chip), SIARC (Spin-on Anti-Reflection Coating), BARC (Bottom Anti-Reflection Coating, a coating located between the Si substrate and the photoresist) and PR (Photoresist).

[0040] Step S104, a wet cleaning step, wherein the wafer after the dry cleaning step is transferred to a cleaning device and the wafer surface is cleaned with a preset cleaning liquid to remove residues on the wafer surface;

[0041] The preset cleaning solution includes a solution containing sulfuric acid and hydrogen peroxide and / or an aqueous solution containing ammonium hydroxide and hydrogen peroxide.

[0042] After using an oxygen-containing gas to remove the residue of the photoresist and / or anti-reflective layer, the wafer after dry cleaning (also known as dry etching cleaning) is transferred to a cleaning device (such as a spray cleaning device or a tank cleaning device) and spray-cleaned or tank-cleaned with a preset cleaning liquid to remove residual organic matter. The preset cleaning liquid includes either one or both of a solution containing sulfuric acid and hydrogen peroxide and an aqueous solution containing ammonium hydroxide and hydrogen peroxide. Generally, dry etching cleaning takes less time and is more efficient than wet cleaning. Using wet cleaning to remove organic residues after the ashing process (i.e., the reaction of carbon with oxygen to form carbon monoxide and carbon dioxide) can greatly reduce the number of residual particles on the wafer surface after cleaning.

[0043] The above-mentioned wafer cleaning method provided in this embodiment adds dry etching cleaning before wet cleaning. Since the residues of the photoresist and / or anti-reflective layer on the wafer surface are usually hydrocarbon organic matter, oxygen-containing gas is introduced into the chamber to effectively remove the residues. Since dry etching cleaning usually takes a shorter time than wet cleaning, the efficiency of removing residues on the wafer surface is improved, the cleaning time and the required amount of cleaning liquid for subsequent wet cleaning can be reduced, the cleaning cost can be saved, and the number of residual particles on the wafer surface after cleaning can be effectively reduced.

[0044] In one embodiment, the wet cleaning step provided in this embodiment includes a first wet cleaning step and a second wet cleaning step;

[0045] A first wet cleaning step is to clean the surface of the wafer using a solution containing sulfuric acid and hydrogen peroxide to remove residues of the photoresist and / or anti-reflective layer;

[0046] In the second wet cleaning step, a solution containing ammonium hydroxide and hydrogen peroxide is used to clean the surface of the wafer.

[0047] A solution containing sulfuric acid and hydrogen peroxide (i.e., hydrogen peroxide) is used to clean organic residues on the wafer surface after dry cleaning. Sulfuric acid and hydrogen peroxide react to form peroxide, which has strong oxidizing properties and can destroy the carbon-hydrogen bonds of organic matter. It can effectively remove organic matter remaining in the photoresist and / or anti-reflective layer.

[0048] A solution containing ammonium hydroxide and hydrogen peroxide is used to clean the etching residues on the wafer surface. Ammonium hydroxide (i.e., ammonia water) and hydrogen peroxide (i.e., hydrogen peroxide) have a slight etching effect on the wafer surface, and the particles are peeled off from the wafer surface through the electrostatic repulsion between the particles and the surface.

[0049] The first wet cleaning step and the second wet cleaning step may both be performed by spray cleaning or tank cleaning.

[0050] In one embodiment, the oxygen-containing gas provided in this embodiment includes oxygen, and the flow rate of oxygen is in the range of 100 ml / min to 1000 ml / min.

[0051] By controlling the oxygen flow rate into the chamber at 100ml / min to 1000ml / min, the pressure of the chamber is stabilized at 1000 to 5000 Pa, thereby effectively improving the cleaning rate of dry cleaning.

[0052] In one embodiment, the preset etching gas provided in this embodiment further includes nitrogen and hydrogen;

[0053] The content of the oxygen-containing gas in the preset etching gas is in the range of 80% to 90%.

[0054] During the dry cleaning process, while introducing an oxygen-containing gas into the chamber, other gases that do not react with the residue can also be introduced simultaneously to remove the byproducts of the reaction from the chamber. The nitrogen and hydrogen content of the preset etching gas can be in the range of 10% to 20%.

[0055] In one embodiment, the temperature of the solution containing sulfuric acid and hydrogen peroxide provided in this embodiment is 120°C to 130°C.

[0056] During the wet cleaning process, in order to improve the cleaning effect, when the wafer is cleaned with a solution containing sulfuric acid and hydrogen peroxide, the temperature of the solution containing sulfuric acid and hydrogen peroxide is 120°C to 130°C to obtain a better cleaning effect.

[0057] In one embodiment, the temperature of the aqueous solution containing ammonium hydroxide and hydrogen peroxide provided in this embodiment is 30°C to 70°C.

[0058] During the wet cleaning process, in order to improve the cleaning effect, when the wafer is finally cleaned with an aqueous solution containing ammonium hydroxide and hydrogen peroxide, the temperature of the aqueous solution containing ammonium hydroxide and hydrogen peroxide is 30°C to 70°C to avoid damage to other film layers.

[0059] The solution containing sulfuric acid and hydrogen peroxide can be obtained by mixing sulfuric acid and hydrogen peroxide in any proportion. In one embodiment, in order to improve the cleaning effect, the solution containing sulfuric acid and hydrogen peroxide provided in this embodiment is an SPM solution.

[0060] In one embodiment, in order to improve the convenience of preparing the cleaning solution, considering that concentrated sulfuric acid with a mass fraction of 98% and hydrogen peroxide with a mass fraction of 31% are commonly used solutions, the SPM solution provided in this embodiment is prepared by mixing commonly used concentrated sulfuric acid with a mass fraction of 98% and hydrogen peroxide with a mass fraction of 31% in a volume ratio of 3:1 to 5:1.

[0061] Taking into account both cleaning efficiency and cleaning effect, the cleaning time of the first wet cleaning step provided in this embodiment is 5 to 15 minutes, preferably 10 minutes. Because the wafer is first dry-cleaned to remove most of the residue on the wafer surface, the cleaning time of the first wet cleaning step can be greatly shortened, thereby improving cleaning efficiency while ensuring cleaning effect.

[0062] The above-mentioned solution containing ammonium hydroxide and hydrogen peroxide can be obtained by mixing ammonium hydroxide and hydrogen peroxide in any proportion. In one embodiment, the aqueous solution containing ammonium hydroxide and hydrogen peroxide provided in this embodiment is SC1 cleaning agent;

[0063] In one embodiment, to improve the convenience of preparing the cleaning solution, the SC1 cleaning agent provided in this embodiment is prepared by mixing 29% by mass of ammonium hydroxide, 31% by mass of hydrogen peroxide, and water in a volume ratio of 1:1:50 to 1:5:50.

[0064] In one embodiment, the second wet cleaning step provided in this embodiment has a cleaning time of 5 to 10 minutes, preferably 5 minutes. Because the wafer is first dry cleaned and the first wet cleaning step removes most of the residue on the wafer surface, the cleaning time of the second wet cleaning step can be shortened, effectively improving cleaning efficiency.

[0065] The wafer cleaning method provided in this embodiment effectively improves the residue removal rate on the wafer surface by adding a dry cleaning step before wet cleaning, reduces the cleaning steps and cleaning time of wet cleaning, thereby improving the overall cleaning efficiency, saving cleaning costs, and optimizing the photoresist cleaning process in the process.

[0066] Based on the above embodiment, this embodiment provides an example of applying the above wafer cleaning method to improve the efficiency of photoresist removal, which can be specifically performed with reference to the following steps:

[0067] Step 1: Dry etching and cleaning: oxygen, nitrogen and hydrogen are introduced into the chamber, the upper electrode power and the lower electrode power are controlled to 500-1000W, and the process time is 30s-5min;

[0068] Photoresist is primarily composed of hydrocarbons. Oxygen ionizes into oxygen atoms, which react with the photoresist to produce carbon monoxide, carbon dioxide, and water. These are then removed via a vacuum pump, completing the photoresist removal process. Nitrogen and hydrogen content is 10%-20%, and the oxygen flow rate is 100-1000ml / min.

[0069] Step 2, wet cleaning;

[0070] Use SPM to clean organic residues after dry etching of photoresist. Sulfuric acid and hydrogen peroxide react to form carboxylic acid, which has strong oxidizing properties and can destroy the carbon-hydrogen bonds of organic matter, effectively removing organic matter. The SPM cleaning time is shortened to 10 minutes, and only one cleaning is required.

[0071] The SPM solution is prepared by mixing concentrated sulfuric acid with a mass fraction of 98% and hydrogen peroxide with a mass fraction of 31% in a volume ratio of 3:1 to 5:1. The temperature of the SPM solution is 120° C. to 130° C.

[0072] Use SC1 to clean etching residue or particles from the wafer surface. Ammonium hydroxide and hydrogen peroxide have a mild etching effect on the wafer surface, removing particles from the wafer surface through electrostatic repulsion between the particles and the surface. SC1 cleaning time is shortened to 5 minutes.

[0073] The SC1 cleaning agent is a mixture of 29% ammonium hydroxide by mass, 31% hydrogen peroxide by mass, and water in a volume ratio of 1:1:50 to 1:5:50. The temperature of the SC1 cleaning agent is 30° C. to 70° C.

[0074] Wet cleaning methods may include spray cleaning and / or tank cleaning.

[0075] The wafer cleaning method provided in this embodiment adds a shorter dry etching cleaning process. After cleaning the residues on the wafer surface, the number of residues on the wafer surface is 20 to 30 particles. Compared with the 200 to 300 particles of residues on the wafer surface in related wafer cleaning technologies, the cleaning effect is improved by 90%. The cleaning time using SPM solution can be shortened to 10 minutes, and the cleaning time using SC1 cleaning agent can be shortened to 5 minutes, saving the use of SPM solution. The cleaning process time is shortened from 40 minutes to 15 minutes, greatly shortening the wafer cleaning time and improving the cleaning efficiency.

[0076] In addition, in the description of the embodiments of the present invention, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; they may refer to mechanical connections or electrical connections; they may refer to direct connections or indirect connections through an intermediate medium; and they may refer to internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0077] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0078] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A wafer cleaning method, characterized in that: include: A dry cleaning step, wherein the wafer after the etching process is introduced into a reaction chamber, and a predetermined etching gas is introduced into the reaction chamber to etch away the photoresist and / or anti-reflective layer residue on the surface of the wafer; wherein the predetermined etching gas includes an oxygen-containing gas; A wet cleaning step is performed, wherein the wafer after the dry cleaning step is transferred into a cleaning device, and a preset cleaning liquid is used to clean the surface of the wafer to remove residues on the surface of the wafer; wherein the preset cleaning liquid includes a solution containing sulfuric acid and hydrogen peroxide and / or an aqueous solution containing ammonium hydroxide and hydrogen peroxide.

2. The method according to claim 1, characterized in that The wet cleaning step includes a first wet cleaning step and a second wet cleaning step; a first wet cleaning step of cleaning the surface of the wafer with the solution containing sulfuric acid and hydrogen peroxide to remove residues of the photoresist and / or anti-reflective layer; In the second wet cleaning step, the surface of the wafer is cleaned using the solution containing ammonium hydroxide and hydrogen peroxide.

3. The method according to claim 1, characterized in that The oxygen-containing gas includes oxygen, and the flow rate of the oxygen is in the range of 100 ml / min to 1000 ml / min.

4. The method according to claim 1, wherein The preset etching gas also includes nitrogen and hydrogen; The content of the oxygen-containing gas in the preset etching gas ranges from 80% to 90%.

5. The method according to claim 1, wherein The temperature of the solution containing sulfuric acid and hydrogen peroxide is 120°C to 130°C.

6. The method according to claim 1, characterized in that The temperature of the aqueous solution containing ammonium hydroxide and hydrogen peroxide is 30°C to 70°C.

7. The method according to claim 1, characterized in that The solution containing sulfuric acid and hydrogen peroxide is an SPM solution; and / or, The SPM solution is prepared by mixing concentrated sulfuric acid with a mass fraction of 98% and hydrogen peroxide with a mass fraction of 31% in a volume ratio of 3:1 to 5:

1.

8. The method according to claim 1, characterized in that The aqueous solution containing ammonium hydroxide and hydrogen peroxide is SC1 cleaning agent; and / or, The SC1 cleaning agent is prepared by mixing 29% by mass of the ammonium hydroxide, 31% by mass of the hydrogen peroxide, and water in a volume ratio of 1:1:50 to 1:5:

50.

9. The method according to claim 2, characterized in that The cleaning time of the first wet cleaning step is 5 to 15 minutes.

10. The method according to claim 2, characterized in that The cleaning time of the second wet cleaning step is 5 to 10 minutes.

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