Method for manufacturing high-conductivity conductive thin film

Abstract

The present invention relates to a method in which a charged molecular cluster aerosol of a size of several angstroms is prepared at room temperature / atmospheric pressure and deposited onto a substrate with acceleration by means of an electrical field, thereby enabling the deposition of a high-conductivity thin film without an additional process such as an annealing process.

Description

Method for manufacturing a high-conductivity conductive thin film

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2024-0045682, filed April 4, 2024, the entire disclosure of which is incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a high-conductivity conductive thin film, and more particularly, to a method for manufacturing a high-conductivity thin film by manufacturing a charged molecular cluster aerosol having a size of several Å at room temperature and pressure, accelerating the aerosol with an electric field at room temperature and pressure, and attaching the aerosol to a substrate.

[0003] As demand for electronic devices, including solar cells, batteries, displays, and memory, grows, the importance of conductive thin film formation technology is increasing. In particular, with the miniaturization and integration of devices, the importance of deposition technologies capable of uniformly forming high-conductivity, low-resistivity thin films is growing.

[0004] Existing thin film deposition technologies include chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD), and thermal deposition. All of the above technologies can create thin films with excellent conductivity from various materials, but they have limitations such as impurity contamination and high costs due to large vacuum equipment. For example, when depositing gold (Au) using vacuum thermal deposition, the resistivity is about three times that of the bulk material, but the deposition thickness is about 30 nm. When depositing a TiN thin film using the ALD method, the deposition thickness can be reduced to about 5 nm, but the resistivity is about 100 times that of the bulk material.

[0005] Meanwhile, spark discharge is a process technology that is attracting attention as it can solve the problems of purity reduction due to impurity contamination and high cost due to large vacuum equipment. According to the spark discharge method, a thin film can be deposited by producing nano-aerosols that are several to tens of nanometers in size at room temperature and pressure. However, since the nano-aerosols produced by the spark discharge method are several to tens of nanometers in size, the final thin film produced using it requires an additional annealing process to satisfy the required electrical properties due to high contact resistance. In the case of 3D gold nano-pillars produced by the spark discharge method, the resistivity value was found to be approximately 40 times higher than that of the bulk.

[0006] The present invention proposes a new method capable of depositing a high-conductivity thin film without an additional process such as an annealing process by producing a charged molecular cluster aerosol of several Å in size at room temperature and pressure and then accelerating it with an electric field to attach it to a substrate.

[0007] In order to solve the above problem, the present invention

[0008] A step of generating an aerosol of charged molecular clusters of a conductive material by causing a discharge in one of the electrodes by forming an asymmetric electric field under room temperature and pressure conditions in a discharge chamber having a discharge electrode section including a plurality of electrodes having different shapes, and

[0009] A method for depositing a conductive thin film is provided, comprising the step of introducing the molecular cluster aerosol into a deposition chamber equipped with a substrate and accelerating it with an electric field, and attaching the molecular cluster aerosol to the substrate, the deposition chamber being fluidly connected to the discharge chamber.

[0010] In the method of the present invention, the charged molecular cluster aerosol may be a charged fine particle having a size of 2 nm or less and being an aggregate of molecules having a size of less than 0.1 nm.

[0011] In the method of the present invention, the plurality of electrodes may include a first electrode connected to a power supply and a second electrode that is a ground electrode, the first electrode and the second electrode are spaced apart from each other by a predetermined interval, and the deposition chamber may have a third electrode in contact with the substrate.

[0012] In the method of the present invention, the electric field may be accelerated by applying a voltage of a polarity opposite to that applied to the first electrode to the third electrode.

[0013] In the present invention, a voltage of 1 to 10 kV is applied to the first electrode, a voltage of -1 to -10 kV is applied to the second electrode and the third electrode, the discharge voltage may be 1 to 1000 V, and the frequency may be 1 kHz to 500 MHz.

[0014] In the present invention, the first electrode may be in the shape of a wire having a predetermined diameter, and the second electrode may be in the shape of a plate having an opening corresponding to the shape of the wire.

[0015] In the present invention, the separation distance between the first electrode and the second electrode may correspond to the diameter of the first electrode.

[0016] The present invention can produce a high-conductivity thin film with a low resistivity and a thin thickness without an additional process by producing a charged molecular cluster aerosol of several Å in size from a low-temperature plasma by acceleration and collision of electrons at room temperature and pressure, and then accelerating the aerosol with an electric field to attach it to a substrate.

[0017] FIG. 1A is a schematic diagram of a pressure plasma discharge chamber for producing a charged molecular cluster aerosol and a deposition chamber for accelerating the produced charged molecular cluster aerosol with an electric field and then attaching it to a substrate to produce a high-conductivity thin film, according to one embodiment.

[0018] Figure 1b is a schematic diagram showing the electric field scheme within the deposition chamber of Figure 1a.

[0019] Figure 2a is an image (A) of a normal pressure plasma used in the present method and a graph (B) showing the discharge voltage between two electrodes over time measured with an oscilloscope.

[0020] Figure 2b is a graph showing a conventional spark discharge image (C) and the spark discharge voltage between two electrodes over time measured with an oscilloscope (D).

[0021] Figure 3 is a TEM image of the Au molecular cluster aerosol manufactured in Example 1.

[0022] Figures 4a to 4c are scanning probe microscopy (AFM) images comparing the surface roughness of the Au thin film according to Example 1 and the Au thin films according to Comparative Examples 1 and 2, respectively.

[0023] Figure 5 shows the resistivity measurement results for the thin films of Figure 4.

[0024] Figure 6 is a TEM image of a TiN molecular cluster aerosol according to Example 2.

[0025] Figure 7 shows the resistivity measurement results for the thin film of Figure 6.

[0026] Hereinafter, the present invention will be described with reference to specific examples and drawings.

[0027] The present invention relates to a method for producing a high-conductivity thin film having a thin thickness without an additional process by producing a charged molecular cluster aerosol of several Å in size from a low-temperature plasma by acceleration and collision of electrons at room temperature and pressure, and then accelerating it with an electric field to attach it to a substrate, thereby reducing the contact resistance of the thin film.

[0028] The method according to the present invention comprises the steps of generating a charged molecular cluster aerosol of a conductive material by causing a discharge in one of the electrodes by forming an asymmetric electric field under room temperature and pressure conditions in a discharge chamber having a discharge electrode section including a plurality of electrodes having different shapes, and the steps of introducing the molecular cluster aerosol into a deposition chamber fluidly connected to the discharge chamber and equipped with a substrate, accelerating it with an electric field, and depositing it on the substrate.

[0029] In the method according to the present invention, the term "charged molecular cluster aerosol" refers to fine particles having a size of less than 2 nm and formed by agglomeration of several charged molecules. Here, "size" refers to the diameter of the fine particles, and in the case of irregular shapes, refers to the maximum diameter. Unlike nanoaerosols having a size of several nm (at least larger than 2 nm) generated by conventional spark discharges, the charged molecular cluster aerosols are fine particles having a size of less than 2 nm, or about 1 nm or less, formed by agglomeration of several molecules, and are charged.

[0030] In the present invention, room temperature means a temperature that is not heated or cooled, and can generally mean 15 to 30°C or 20 to 25°C. In addition, normal pressure means atmospheric pressure (100 to 110 kPa) rather than a reduced pressure state.

[0031] FIG. 1 schematically illustrates a device configuration and electric field scheme in one embodiment of the present invention, wherein the device comprises a discharge chamber (100) for producing a charged molecular cluster aerosol (A) and a deposition chamber (200) for producing a thin film by introducing and accelerating the electric field of the produced molecular cluster aerosol (A).

[0032] The discharge chamber (100) has a discharge electrode section (10), and the discharge electrode section (10) includes a first electrode (11) connected to a power supply and a second electrode (12) which is a ground electrode. The first electrode (11) may be connected to an RLC circuit or may be directly connected to the power supply without an RLC circuit. The first electrode (11) and the second electrode (12) have different shapes, and since voltage is applied only to the first electrode (11) and the second electrode (12) is grounded, as a result, electric fields near the first electrode (11) and near the second electrode (12) may be created asymmetrically, and discharge may occur only at the electrode where a high electric field is generated. In addition, the first electrode (11) and the second electrode (12) are spaced apart from each other by a predetermined distance. The first electrode (11) and the second electrode (12) do not need to be made of the same material.

[0033] According to one embodiment, the first electrode (11) may be a wire shape having a predetermined diameter, and the second electrode (12) may be a plate shape having an opening corresponding to the diameter. Since the diameter of the first electrode (11) is smaller than that of the second electrode (12), when a potential difference is applied between the two electrodes, an asymmetric electric field can be formed in which an electric field having a greater intensity is formed around the first electrode (11) than around the second electrode (12). Since a greater electric field is generated near the first electrode (11), an atmospheric pressure plasma is generated only near the first electrode (11), thereby vaporizing the material of the first electrode (11) and generating a molecular cluster aerosol having a size of several Å near the first electrode (11). In the opposite case, the size of the second electrode (12) can be made smaller than that of the first electrode (11), thereby generating an atmospheric pressure plasma only near the second electrode (12) and vaporizing the material of the second electrode (12) and generating a molecular cluster aerosol.

[0034] The diameter of the first electrode (11) and the diameter of the opening of the second electrode (12) may or may not be the same, and may be independently 0.1 mm or more, 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, 0.5 mm or more, 0.6 mm or more, 0.7 mm or more, 0.8 mm or more, 0.9 mm or more, or 1 mm or more, and 5 mm or less, 4 mm or less, 3 mm or less, 2 mm or less, or 1.5 mm or less. According to a preferred embodiment, it may be 0.8 mm or more and 2 mm or less. The diameter of the first electrode (11) and the diameter of the opening of the second electrode (12) are not limited thereto, and may be any size that can cause discharge.

[0035] The gap between the first electrode (11) and the second electrode (12) may be 0.1 mm or more, 0.2 mm or more, 0.3 mm or more, 0.4 mm or more, or 0.5 mm or more, 5 mm or less, 4 mm or less, 3 mm or less, 2 mm or less, or 1.5 mm or less, based on the gap between the center of the end of the first electrode (11) close to the second electrode (12) and the center of the opening of the second electrode, and may preferably be 0.5 to 1 mm, but is not necessarily limited thereto.

[0036] When the first electrode (11) is wire-shaped and the second electrode (12) is plate-shaped, the width of the second electrode (12) is 9 mm 2 ~ 400 mm 2 The range may be, but is not necessarily limited to, the plate shape, including square, rectangular, and circular.

[0037] The second electrode (12) is larger than the first electrode (11) and has an asymmetrical structure in which, when voltage is applied to the first electrode (11) from the power supply when it is grounded, a strong electric field is generated only near the first electrode (11).

[0038] The discharge chamber (100) generates a molecular cluster aerosol (A) by applying a predetermined voltage to the first electrode (11) under room temperature and pressure conditions.

[0039] The voltage applied to the first electrode (11) may be a positive voltage or a negative voltage. The magnitude of the voltage is 1 to 10 kV in absolute value, and specifically, 1.1 kV or more, 1.2 kV or more, 1.3 kV or more, 1.4 kV or more, or 1.5 kV or more, and 6 kV or less, 5.5 kV or less, 5 kV or less, 4.5 kV or less, or 4 kV or less, for example, 3 to 4 kV.

[0040] The magnitude of the voltage applied to the first electrode (11) can be determined within a range that prevents a spark from forming between the first electrode (11) and the second electrode (12), and this can be affected by the gap between the first electrode (11) and the second electrode (12). That is, as the gap between the first electrode (11) and the second electrode (12) increases, the magnitude of the voltage at which a spark occurs increases, and therefore the magnitude of the voltage applied to the first electrode (11) can also increase.

[0041] The materials of the first electrode (11) and the second electrode (12) in the discharge chamber (100) may be the same or different, and are composed of a conductive material including a metal such as gold (Au), titanium (Ti), silver (Ag), palladium (Pd), copper (Cu), a metal alloy, or a metal oxide such as indium tin oxide (ITO).

[0042] The discharge electrode unit (10) including the first electrode (11) and the second electrode (12) may be one, and if necessary, two or more may be provided.

[0043] The discharge voltage occurring in the discharge chamber (100) can form a low-temperature plasma by accelerating and colliding electrons near the first electrode (11) due to the potential difference between the first electrode (11) and the second electrode (12). The discharge voltage is very low, such as 1 to 1 kV, for example, 1 to 500 V, and has a very fast frequency of 1 kHz or more, for example, 1 to 500 MHz.

[0044] The carrier gas injected into the discharge chamber may be an inert gas including nitrogen, argon, helium, hydrogen, oxygen, etc., and may be injected at 0.1 to 100 l / min per electrode, for example, 1 to 10 l / min. The 'amount of carrier gas per electrode' means the value obtained by dividing the total amount of carrier gas injected into the discharge electrode section (10) by the number of electrodes. When there are multiple discharge electrode sections (10), each discharge electrode section may independently inject the carrier gas.

[0045] The carrier gas enables the charged molecular cluster aerosol (A) generated in the discharge chamber (100) to move smoothly into the deposition chamber (200).

[0046] The deposition chamber (200) includes an inlet (21) through which a charged molecular cluster aerosol generated in the discharge chamber (100) flows, a substrate (22), and a third electrode (20) connected to a power supply capable of applying voltage to the substrate (22).

[0047] The substrate (22) can be a conductive substrate such as a metal, semiconductor, or metal oxide, or a non-conductive substrate such as a polymer or glass, and can be used in various ways depending on the purpose of the final product. For example, a silicon substrate on which a silicon dioxide thin film is deposited can be used.

[0048] A voltage opposite to the voltage applied to the first electrode (11) is applied to the third electrode (20) located under the substrate (22). For example, when a positive voltage is applied to the first electrode (11), a negative voltage is applied to the third electrode (20). The applied voltage may be -1 to -10 kV. For example, when a positive voltage of 3 to 4 kV is applied to the first electrode (11), the voltage applied to the third electrode (20) may be -4.0 to -6.0 kV. By doing so, the charged molecular clusters are accelerated and attached to the substrate (22) due to the electric field formed between the third electrode (20) and the second electrode (12).

[0049] The substrate (22) may be made of a conductive material including a metal such as gold (Au), titanium (Ti), silver (Ag), palladium (Pd), copper (Cu), a metal alloy, or a metal oxide such as indium tin oxide (ITO), and does not need to be the same material as the first electrode (11) and the second electrode (12) in the discharge chamber.

[0050] The thin film (24) formed by depositing a molecular cluster aerosol (A) on a substrate (22) has a very low surface roughness because the molecular cluster aerosol is attached to fine particles of molecular size formed by vaporization and adiabatic expansion of the first electrode (11). For example, the root mean square (RMS) surface roughness (unit: nm) measured from an atomic force microscope (AFM) image may be 2.5 or less, and preferably 2.4 or less, 2.3 or less, or 2.25 or less.

[0051] The thickness of the thin film can be formed from several nm to several hundred nm, and for example, it can be 4 to 30 nm. The thin film manufactured by the method according to the present invention can have a resistivity of 50 nΩ·m or less even at a thin thickness of, for example, 5 nm. This level of resistivity is similar to the resistivity that can be achieved in a 30 nm thick thin film obtained by vacuum thermal evaporation, which is known to be able to obtain the lowest resistivity among existing methods.

[0052] Below, a specific example of manufacturing a thin film using a device such as that illustrated in Fig. 1 is described. The following example is merely intended to aid understanding of the present invention and does not limit the scope of the present invention.

[0053] <Example 1> Manufacturing of Au thin film according to the present invention

[0054] An Au wire electrode (first electrode, 11) with a diameter of about 1 mm was prepared in a discharge chamber (100) at room temperature and pressure. As the second electrode (12), an Au plate electrode (thickness 1 mm, area 25 mm) with a hole (opening) with a diameter of about 1 mm in the center was prepared. 2 ) was prepared. High-purity nitrogen (purity 99.99%) was used as the carrier gas and injected at a rate of 5 l / min per electrode. The first electrode (11) and the second electrode (12) are separated by about 1 mm.

[0055] A positive voltage of approximately 3.5 kV was applied to the Au wire electrode (first electrode, 11) via a power supply, and the second electrode (12) was grounded. Under these conditions, it was confirmed that no spark discharge occurred and a low-temperature plasma was formed only near the Au wire electrode (11) (see Fig. 2a). The discharge voltage between the two electrodes, measured with an oscilloscope, was low, at several tens of V (e.g., 20 to 25 V), and exhibited characteristics of a bar-like oscillation frequency of several hundred MHz (e.g., 50 to 100 MHz).

[0056] As can be seen from (A) and (B) of Fig. 2a, discharge occurred only near the first electrode (11), and Au of the first electrode (11) was vaporized and adiabatically expanded, producing a positively charged Au molecular cluster aerosol of several Å in size. Fig. 3 is a TEM image of the generated Au molecular cluster aerosol.

[0057] Copper (Cu) was used as the substrate electrode (20) in the deposition chamber (200), and a silicon substrate on which a silicon dioxide (SiO2) thin film was deposited to a thickness of 100 nm was used as the substrate (22). A negative voltage of 5 kV was applied to the third electrode (20), thereby generating an electric field between the second electrode (12) and the third electrode (20), and a positively charged molecular cluster aerosol generated in the discharge chamber (100) was accelerated and attached in the electric field, thereby depositing a 5 nm thick Au thin film on the silicon substrate.

[0058] <Comparative Example 1> Fabrication of Au thin film by spark discharge

[0059] A 25 nm thick Au thin film was deposited in the same manner as in Example 1, except that the voltage applied to the first electrode was 6.5 kV, and a spark discharge was generated by the acceleration and collision of electrons and cations at the two electrodes to form a charged nano-aerosol. As can be seen in (C) of Fig. 2b, discharge was observed near both the first and second electrodes, and as can be seen in (D) of Fig. 2b, the discharge voltage oscillated in a sawtooth shape, and the spark discharge voltage was very high, reaching 3.5 kV, and the frequency was very low, 30 Hz.

[0060] From FIG. 2a and FIG. 2b, it can be seen that the discharge phenomenon of Comparative Example 1 is completely different from the discharge phenomenon observed in the present invention.

[0061] <Comparative Example 2> Fabrication of Au thin film by vacuum thermal evaporation

[0062] For the same substrate as Example 1, temperature 800 to 900°C, pressure 10 -8 Under conditions of less than 0.0000013332 Torr (less than 0.0000013332 Pa), a 30 nm thick Au thin film was deposited through vacuum thermal evaporation.

[0063] Characterization of Au thin films: surface roughness, resistivity, and bulk resistivity

[0064] The surface roughness of the Au thin films manufactured in Example 1 and Comparative Examples 1 and 2 was measured using an atomic force microscope (AFM).

[0065] Figures 4a to 4c are scanning probe microscopy (AFM) images comparing the surface roughness of the Au thin film according to Example 1 and the Au thin films according to Comparative Examples 1 and 2, respectively. The measured RMS surface roughness (unit: nm) was 2.25 for Example 1 (the present invention), 2.667 for Comparative Example 1 (spark discharge), and 2.832 for Comparative Example 2 (vacuum thermal deposition). It can be confirmed that the thin film manufactured by the method according to Example 1 has a low surface roughness value despite being very thin.

[0066] In addition, the resistivity and bulk resistivity were measured using a four-point probe. According to the measurement results of Fig. 5, the 5 nm thick Au thin film according to Example 1 was measured to be 45 nΩ·m, which is similar to the resistivity of the 30 nm thick Au thin film (Comparative Example 2) manufactured by vacuum thermal deposition. Since the resistivity increases as the thickness of the thin film decreases, the fact that the resistivity of the 5 nm thin film of Example 1 and the 30 nm thick thin film of Comparative Example 2 are similar shows that the thin film manufactured in the present invention has very excellent conductivity. Meanwhile, it was confirmed that the Au thin film of Comparative Example 1 manufactured by the existing spark discharge method had a resistivity that was about 30 times higher than that of Example 1 or Comparative Example 2.

[0067] From the above results, it can be seen that according to the present invention, a thinner and more conductive thin film can be manufactured than vacuum thermal deposition without an additional process at room temperature / atmospheric pressure.

[0068] <Example 2> Fabrication of TiN thin film

[0069] Except for using a Ti electrode to produce TiN molecular cluster aerosols, the remaining conditions were the same as in Example 1, and positively charged TiN molecular cluster aerosols of several Å in size could be produced (see Fig. 6). TiN molecular clusters produced at atmospheric pressure were accelerated and attached by an electric field to produce TiN thin films of 5 nm and 10 nm thickness, respectively.

[0070] Fig. 6 is a TEM image of a TiN molecular cluster aerosol according to Example 2. Copper (Cu) was used as the third electrode (20) in the deposition chamber (200), and a silicon substrate on which a silicon dioxide (SiO2) thin film was deposited to a thickness of 100 nm was used as the substrate (21). By applying a negative voltage of 5 kV to the substrate electrode, the positively charged molecular cluster aerosol generated in the discharge chamber (100) was accelerated and attached by an electric field, thereby manufacturing TiN thin films with thicknesses of 5 nm and 10 nm, respectively.

[0071] The resistivity of these TiN thin films was measured to be 228 μΩ·cm and 45 μΩ·cm, respectively. Figure 7 shows a comparison of the resistivity of the TiN thin films fabricated in this manner with that of thin films reported in previous studies. When compared at the same thickness, the resistivity was 2 to 3 times lower than that of metalorganic chemical vapor deposition (MOCVD) or Fast Atomic Sequential Technology (FAST), as well as atomic layer deposition (ALD), which is known to have the lowest resistivity in previous studies.

[0072] As described above, the present invention enables the production of high-quality, high-conductivity thin films of conductive materials, such as metals, metal alloys, or metal oxides, using simple equipment and under mild conditions. Therefore, it can be utilized in a variety of fields requiring nanoparticle fabrication and high-conductivity thin film deposition, including environmental, energy, food, cosmetics, semiconductor, and medical applications.

[0073] While specific aspects of the present invention have been described in detail above, it will be apparent to those skilled in the art that these specific descriptions merely represent preferred embodiments and are not intended to limit the scope of the present invention. Therefore, the actual scope of the present invention is defined by the claims and their equivalents.

[0074] The present invention is the result of research conducted with the support of the following tasks.

[0075] (1) Project Number: RS-2023-00282896

[0076] Assignment ID: 1711200108

[0077] Ministry of Science and ICT

[0078] Project Management (Professional) Institution Name: National Research Foundation of Korea

[0079] Research Project Name: 2023 Nano and Introducing Technology Development Project NanoConnect

[0080] Research Project Title: Development of a High-Conductivity, High-Transmittance Flexible Transparent Electrode Incorporating 3D Nanoelectrodes and Development of an Ultra-Lightweight, High-Efficiency, High-Stability Flexible Perovskite Solar Cell Using the Electrode

[0081] Project Implementation Organization: Multiscale Energy Systems Research Group

[0082] Research period: August 1, 2023 - June 30, 2026

[0083] (2) Assignment Number: RS-2023-00279529

[0084] Announcement No. 2023-0524

[0085] Project Name: Excellent Research - Mid-career Research (Type 1-2)

[0086] Project Name: Pulse Engineering for Ensuring Long-Term Stability of Large-Area, Wide-Bandgap Perovskite Solar Cells

[0087] Total project period: September 1, 2023 - February 28, 2027

[0088] Agreement Period: September 1, 2023 - February 28, 2027

[0089] Lead Research and Development Institution: Multiscale Energy Systems Research Group

[0090] Business Registration Number: 1198208369

[0091] Representative: Choi Man-soo

[0092] (Chief) Research Director: Man-soo Choi

Claims

1. A step of generating an aerosol of charged molecular clusters of a conductive material by causing a discharge in one of the electrodes by forming an asymmetric electric field under room temperature and pressure conditions in a discharge chamber having a discharge electrode section including a plurality of electrodes having different shapes, and A method for depositing a conductive thin film, comprising the step of introducing the molecular cluster aerosol into a deposition chamber equipped with a substrate and fluidly communicating with the discharge chamber, accelerating it with an electric field, and attaching it to the substrate.

2. In paragraph 1, The method of claim 1, wherein the charged molecular cluster aerosol is a charged particle having a size of 2 nm or less and is an aggregate of molecules having a size of less than 0.1 nm.

3. In paragraph 1, A method wherein the plurality of electrodes include a first electrode connected to a power supply and a second electrode that is a ground electrode, the first electrode and the second electrode being spaced apart from each other by a predetermined interval, and the deposition chamber has a third electrode in contact with the substrate.

4. In paragraph 3, A method of accelerating an electric field by applying a voltage of polarity opposite to that applied to the first electrode to the third electrode.

5. In paragraph 3, A method in which a voltage of 1 to 10 kV is applied to the first electrode, a voltage of -1 to -10 kV is applied to the second electrode and a voltage of -1 to -10 kV is applied to the third electrode, the discharge voltage is 1 to 1000 V, and the frequency is 1 kHz to 500 MHz.

6. In paragraph 3, A method wherein the first electrode is in the shape of a wire having a predetermined diameter, and the second electrode is in the shape of a plate having an opening corresponding to the shape of the wire.

7. In paragraph 6, A method wherein the distance between the first electrode and the second electrode corresponds to the diameter of the first electrode.

Images