Microwave plasma torch based on bias voltage regulation and control

Abstract

The invention discloses a microwave plasma torch based on bias voltage regulation and control. The microwave plasma torch structurally comprises a rectangular waveguide, a quartz tube, two electrode devices and a direct-current power supply. The quartz tube vertically penetrates through the center of the wide side of the rectangular waveguide, the two electrode devices are arranged at the upper end and the lower end of the quartz tube respectively and connected with a direct-current power source, and bias voltage is applied to a plasma reaction area in the quartz tube through the electrode devices. And the voltage generates a direct-current electric field along the axial direction of the quartz tube in the plasma reaction area. According to the microwave plasma torch, adjustable bias voltage is introduced to the two ends of the quartz tube of the microwave plasma torch, and a stable axial direct-current electric field is superposed on the basis of a microwave electric field, so that the movement direction, energy and spatial distribution of charged particles (especially electrons and positive ions) in plasma are effectively controlled; finally, the purposes of regulating and controlling the plasma chemical reaction path, optimizing the treatment uniformity and enhancing the specific reaction selectivity are achieved.

Description

Technical Field

[0001] This invention relates to the field of microwave plasma technology, and in particular to a microwave plasma torch based on bias voltage regulation. Background Technology

[0002] Microwave plasma torches, as efficient plasma generation devices without electrode contamination, have been widely used in fields such as material surface modification, thin film deposition, waste gas treatment, and chemical synthesis. Traditional rectangular waveguide microwave plasma torches typically consist of a waveguide, a quartz tube running through the waveguide, and a gas supply system. Microwave energy creates an electromagnetic field within the waveguide, ionizing the working gas flowing through the quartz tube, thereby generating plasma.

[0003] However, traditional rectangular waveguide microwave plasma torches have a significant limitation: it is difficult to actively and precisely control the reaction pathways within the plasma. Once excited, the spatial distribution of the plasma, the transport behavior of active particles (such as electrons, ions, and free radicals), and the chemical reaction pathways mainly depend on the inherent distribution of the microwave electric field, the gas flow field, and the physicochemical properties of the gas itself. This passive generation mode results in insufficient uniformity of plasma processing effects and weak selective control over specific reactions, leading to poor performance in applications involving complex gas compositions or requiring directional modification (such as asymmetric material processing, directional etching, and selective chemical reactions).

[0004] In existing technologies, methods such as adjusting microwave power, gas flow rate or mixing ratio, and reaction chamber pressure are commonly used to control plasma characteristics. However, these methods adjust the conditions from a macroscopic perspective and cannot directly and directionally intervene in the motion direction and energy distribution of charged particles within the plasma, resulting in low control efficiency and poor precision. Therefore, there is an urgent need for a microwave plasma torch that can actively and flexibly control the reaction path while the plasma is being generated. Summary of the Invention

[0005] To address the problem that existing microwave plasma torches cannot actively and flexibly control the plasma reaction path, this invention provides a microwave plasma torch based on bias voltage control. Specifically, it is a microwave plasma torch that actively controls the plasma reaction path by introducing an external bias voltage.

[0006] The present invention provides a microwave plasma torch based on bias voltage regulation, the structure of which includes a rectangular waveguide, a quartz tube, two electrode devices, and a DC power supply. The quartz tube vertically penetrates the center of the wide side of the rectangular waveguide. As a high-temperature resistant, microwave-transmitting insulating cavity, the quartz tube serves as both the flow channel for the working gas and the core reaction region for the conversion of microwave energy into plasma. The two electrode devices are respectively located at the upper and lower ends of the quartz tube, named the upper electrode device and the lower electrode device. The two electrode devices are connected to an external DC power supply. A bias voltage is applied to the plasma reaction region within the quartz tube through the electrode devices, generating a DC electric field along the axial direction of the quartz tube within the plasma reaction region.

[0007] The DC power supply is an adjustable high-voltage DC power supply, which is connected to the upper electrode device and the lower electrode device through a high-voltage wire.

[0008] The upper electrode device includes a metal electrode rod, an insulating ceramic, and an adapter. The adapter is a cylindrical shape with openings at both ends, and a gas outlet is provided in the middle of the adapter for evacuating a vacuum before the reaction and maintaining the required working pressure or discharging reaction products during the reaction. The lower end of the adapter is fitted onto the upper end of the quartz tube and fixedly connected. The metal electrode rod is vertically inserted from the upper port of the adapter, extending into the plasma reaction region inside the quartz tube, with the top of the metal electrode rod located outside the adapter. The insulating ceramic is annular, fitted and fixed to the top of the metal electrode rod, and covers and seals the upper port of the adapter; the top of the metal electrode rod is connected to a DC power supply.

[0009] The metal electrode rod serves as a high-voltage electricity introduction terminal, with its front end extending into the plasma reaction region and making direct electrical contact with the internal space of the quartz tube, but without short-circuiting with the tube wall. It is used to directly apply a controllable axial bias voltage to the plasma to regulate the behavior of charged particles. The metal electrode rod can be made of a metal resistant to plasma corrosion (such as stainless steel, tungsten, or nickel).

[0010] The adapter has an insulating limiter in its center to restrict the position of the metal electrode rod. The limiter is circular, and the metal electrode rod passes vertically through the central through-hole of the limiter. The inner diameter of the central through-hole is equal to the outer diameter of the metal electrode rod. The edge of the limiter is embedded in the wall of the adapter for fixed installation. Multiple air holes are also distributed on the limiter for gas transmission. The limiter is preferably made of polytetrafluoroethylene (PTFE) material, used to precisely fix and high-voltage insulate the metal electrode rod while simultaneously connecting the gas path.

[0011] Specifically, the adapter has connectors for flange connection at both the upper and lower ends, which are integrally formed with the adapter. The lower end of the adapter is fitted onto the upper end of the quartz tube and fixedly connected by the first flange. An annular step is formed on the surface of the connector at the upper end of the adapter along the edge of the adapter opening. The inner diameter of the annular step is equal to the outer diameter of the insulating ceramic. The insulating ceramic is installed inside the annular step, completely covering the opening of the adapter. A second flange is installed on the upper surface of the insulating ceramic and bolted to the connector of the adapter.

[0012] The structure of the lower electrode device is exactly the same as that of the upper electrode device. A gas inlet is provided in the middle of the adapter in the lower electrode device for introducing working gas (such as argon, oxygen, nitrogen or a mixture) into the quartz tube.

[0013] Compared with the prior art, the advantages of the present invention are: (1) Active control of plasma reaction path is realized: This invention introduces an adjustable axial bias voltage independent of the microwave system into the conventional rectangular waveguide microwave plasma torch. By spatial superposition of microwave electric field and DC bias electric field, and by changing the polarity, amplitude or waveform (such as DC or pulse) of bias voltage, the movement and energy of charged particles can be directly and actively controlled, thereby realizing precise control of chemical reaction path, spatial distribution of active particles and surface treatment directionality, breaking through the limitation of traditional microwave plasma torches that can only passively rely on microwave field distribution.

[0014] (2) Significantly improves the selectivity of the treatment: By adjusting the bias voltage, certain reaction channels can be selectively enhanced or inhibited, thereby improving the selectivity of the target product, which is particularly valuable in material synthesis and waste gas purification.

[0015] (3) The present invention designs a special flange sealing connector. This flange not only realizes the sealing and fixing of the quartz tube and the connection with the gas supply / vacuum system, but more importantly, it integrates a metal electrode that contacts the plasma and ensures electrical insulation from the quartz tube wall.

[0016] (4) Expanded equipment functions and application scope: This device not only retains the advantages of microwave plasma in terms of high efficiency and no electrode pollution, but also has the electric field control capability similar to DC or radio frequency plasma. It can adapt to more complex process requirements, such as workpieces that require asymmetric surface treatment, applications that require ion implantation or directional bombardment, and complex chemical processes that require precise control of reaction intermediates.

[0017] (5) Simple structure and easy integration: The main technical point of this invention is the introduction of electrode flange and power supply, which is easy to modify and upgrade on the basis of the main structure of traditional microwave plasma torch, with low implementation cost and high reliability.

[0018] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0019] Figure 1 This is an overall external view of the microwave plasma torch based on bias voltage regulation according to the present invention.

[0020] Figure 2 This is a diagram of the internal structure of the microwave plasma torch based on bias voltage regulation according to the present invention.

[0021] Figure 3 This is a structural diagram showing the fixing method of a quartz tube and a rectangular waveguide.

[0022] Figure 4 This is a comparison diagram of the potential distribution in plasma simulation experiments.

[0023] Figure 5 This is a comparison chart of electron density distribution in plasma simulation experiments.

[0024] Numbering on the map: 1-Rectangular waveguide, 2-Quartz tube, 3-Upper electrode device, 4-Lower electrode device, 5-Metal electrode rod, 6-Insulating ceramic, 7-Adapter, 8-Gas outlet, 9-Connector, 10-First flange, 11-Annular step, 12-Second flange, 13-Power connector, 14-Limiter, 15-Air hole, 16-Air inlet, 17-Metal cylinder, 18-O-ring, 19-Fixed component. Detailed Implementation

[0025] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0026] like Figure 1-3As shown, the microwave plasma torch based on bias voltage regulation of the present invention includes a rectangular waveguide 1 and a quartz tube 2, with the quartz tube 2 perpendicularly penetrating the center of the wide side of the rectangular waveguide 1. In this embodiment, the rectangular waveguide 1 is a standard BJ26 rectangular waveguide with an operating frequency of 2.45 GHz, used for transmitting and confining microwave energy. The inner diameter of the quartz tube 2 is 16 mm and the outer diameter is 20 mm, and its section inside the rectangular waveguide 1 constitutes the plasma reaction region; the quartz material has good microwave transmittance, electrical insulation, and high temperature resistance. In a specific embodiment, the quartz tube is fixed to the rectangular waveguide as follows: a through circular hole is opened on the top and bottom plates of the rectangular waveguide 1, and a metal cylinder 17 is installed on the circular hole. The quartz tube 2 is inserted into the metal cylinder 17, penetrating the rectangular waveguide 1. The outer wall surface of the metal cylinder 17 is threaded. After the quartz tube 2 passes through the rectangular waveguide 1, an O-ring 18 made of elastic material is fitted at the top end of the metal cylinder 17. A bottle cap-shaped fastener 19 with a central through hole is used to fit into the quartz tube 2 through its central through hole. The fastener 19 is rotated so that its threads are threaded to the metal cylinder 17 to compress the O-ring 18, thereby achieving the purpose of fixing the quartz tube 2.

[0027] Two identical electrode devices are respectively set at the upper and lower ends of the quartz tube 2 (the parts extending out of the rectangular waveguide 1), and are named the upper electrode device 3 and the lower electrode device 4 respectively.

[0028] The upper electrode device 3 includes a metal electrode rod 5, an insulating ceramic 6, and an adapter 7. The adapter 7 is a cylindrical shape with openings at the top and bottom, and an opening in the middle serves as a gas outlet 8. The gas outlet 8 is used to evacuate the system before the reaction and to maintain the required working pressure or discharge reaction products during the reaction. The adapter 7 has connectors 9 at its upper and lower ends for flange connection, which are integrally formed with the adapter 7. The lower end of the adapter 7 is fitted onto the upper end of the quartz tube 2 and fixedly connected by a first flange 10. Specifically, a gasket is installed between the connector 9 and the first flange 10, and the connector 9 is fixedly connected to the first flange 10 by bolts. The metal electrode rod 5 is vertically inserted from the upper port of the adapter 7, extending into the plasma reaction region inside the quartz tube 2, with the top of the metal electrode rod 5 located outside the adapter 7. The insulating ceramic 6 is annular and fitted and fixed to the top of the metal electrode rod 5. An annular step 11 is formed along the edge of the adapter opening on the upper end of the connector surface of the adapter 7. The inner diameter of the annular step is equal to the outer diameter of the insulating ceramic 6. The insulating ceramic 6 is installed inside the annular step 11, completely covering the opening of the adapter 7. A second flange 12 is installed on the upper surface of the insulating ceramic 6. Similarly, an annular step 11 is also formed on the second flange 12 to accommodate the upper half of the insulating ceramic 6. The second flange 12 is bolted to the connector 9 of the adapter to achieve a sealed connection at the upper port of the adapter 7. The top of the metal electrode rod 5 is a power connector 13 for connecting to a DC power supply. In this embodiment, the metal electrode rod 5 is made of tungsten.

[0029] An insulating limiter 14, made of polytetrafluoroethylene, is provided in the center of the adapter 7 to restrict the position of the metal electrode rod. The metal electrode rod 5 passes vertically through the central through hole of the limiter 14. The inner diameter of the central through hole of the limiter is equal to the outer diameter of the metal electrode rod. The edge of the limiter 14 is embedded in the wall of the adapter 7 for fixed installation. Multiple air holes 15 are also distributed on the limiter.

[0030] The structure of the lower electrode device is exactly the same as that of the upper electrode device. The opening in the middle of the adapter in the lower electrode device serves as the air inlet 16, and the bottom end of the metal electrode rod is the power connector. Both metal electrode rods are located on the central axis of the quartz tube, and the front ends of the two electrode rods are located at the upper and lower parts of the plasma reaction region, respectively, without direct contact, and there is a gap between them.

[0031] In this embodiment, an adjustable high-voltage DC power supply is used, which is connected to the metal electrode rods in the upper and lower electrode devices via high-voltage wires. This power supply can provide a continuously adjustable DC bias voltage within the range of 0 to ±5000 V (typical value). By changing the polarity (positive or negative) and amplitude of the power supply, a controllable and stable DC electric field can be established in the axial direction (i.e., the direction of gas flow / the direction perpendicular to microwave propagation) of the plasma reaction cavity inside the quartz tube.

[0032] The working principle of the microwave plasma torch based on bias voltage regulation of the present invention is as follows: Microwaves are input through a rectangular waveguide port, forming a specific electromagnetic field mode (such as the TE10 mode) within the waveguide. The electric field component of this mode acts perpendicularly to the quartz tube, ionizing the gas inside and generating microwave plasma. Simultaneously, an adjustable DC high-voltage power supply applies a bias voltage to the metal electrode rods at both ends of the quartz tube. This voltage generates an additional DC electric field along the axial direction of the quartz tube within the plasma reaction chamber. This DC electric field superimposes with the microwave electric field (perpendicular to the axial direction), acting together on the charged particles in the plasma: generating a directional acceleration force along the axial direction for electrons and ions, altering their drift velocity and spatial distribution; influencing the energy distribution function of charged particles, thereby changing the rates of excitation, ionization, and dissociation reactions; and guiding active particles (such as ions) towards specific electrode directions, achieving directional control of the reaction path (e.g., promoting surface modification of materials in a set direction, or guiding specific chemical reactions towards the desired product direction). By adjusting the polarity and magnitude of the bias voltage in real time, or even using a pulsed mode, dynamic and flexible control of plasma characteristics can be achieved, which is impossible with traditional pure microwave devices.

[0033] Plasma simulation experiments were conducted using COMSOL Multiphysics simulation software. The simulation conditions are shown in Table 1.

[0034] Table 1. Simulation Experiment Conditions

[0035] See the comparison chart of simulation test results. Figure 4 and Figure 5 Under no bias voltage, the electron density exhibits an axisymmetric double-ring structure, with the maximum values ​​located on both sides of the central axis of the quartz tube, forming a typical "donut" shaped distribution. The plasma core region is concentrated near the waveguide coupling port (axial ±15 mm range), with a full width at half maximum (FWHM) of approximately 25 mm. The symmetry along the waveguide central plane (z=0) is greater than 95%, conforming to TE... 10 Model electric field distribution characteristics. The maximum electron density is 2.5 × 10⁻⁶. 17 m -3 The electron density appears at a radial radius of approximately 4 mm and an axial radius of approximately ±8 mm. In terms of spatial uniformity, the electron density fluctuation in the core region is less than ±15%, making it suitable for large-area uniform processing. With the introduction of a bias voltage, the overall electron density significantly migrates towards the upper electrode (+10 V side), completely disrupting symmetry and forming a comet-tail-like asymmetric distribution. The plasma offset shows the density center point shifting upwards by approximately 18 mm, and the core region extending to a region 10 mm below the upper electrode. Electrode sheath strengthening occurs, with a high-density sheath appearing near the upper electrode, and the local electron density increasing by 92% compared to the region without bias voltage. The maximum electron density is 4.8 × 10⁻⁶. 17 m -3 Located 5 mm below the upper electrode, it provides a 1.92-fold increase compared to the unbiased voltage group. Attenuation occurs in the lower region, with the electron density near the lower electrode decreasing to 0.8 × 10⁻⁶. 17 m -3 The decline reached 68%, showing a clear "suction effect".

[0036] In summary, the microwave plasma torch of the present invention introduces an adjustable bias voltage at both ends of the quartz tube of a conventional microwave plasma torch, and superimposes a stable axial DC electric field on the microwave electric field, thereby achieving effective control over the motion direction, energy and spatial distribution of charged particles (especially electrons and positive ions) in the plasma, ultimately achieving the purpose of regulating the plasma chemical reaction path, optimizing the processing uniformity and enhancing the selectivity of specific reactions. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A microwave plasma torch based on bias voltage regulation, characterized in that, It includes a rectangular waveguide, a quartz tube, two electrode devices, and a DC power supply. The quartz tube vertically penetrates the center of the wide side of the rectangular waveguide. The two electrode devices are respectively located at the upper and lower ends of the quartz tube. The two electrode devices are connected to the DC power supply. A bias voltage is applied to the plasma reaction region inside the quartz tube through the electrode devices. This voltage generates a DC electric field along the axial direction of the quartz tube in the plasma reaction region.

2. The microwave plasma torch based on bias voltage regulation as described in claim 1, characterized in that, The electrode devices located at the top and bottom of the quartz tube are named the upper electrode device and the lower electrode device, respectively, and the two electrode devices have the same structure.

3. The microwave plasma torch based on bias voltage regulation as described in claim 2, characterized in that, The upper electrode device includes a metal electrode rod, an insulating ceramic, and an adapter. The adapter is a cylindrical shape with openings at the top and bottom, and a gas outlet is provided in the middle of the adapter. The lower end of the adapter is fitted onto the upper end of the quartz tube and fixedly connected. The metal electrode rod is vertically inserted from the upper port of the adapter and extends into the plasma reaction region inside the quartz tube, with the top of the metal electrode rod located outside the adapter. The insulating ceramic is annular and is fitted onto the top of the metal electrode rod, covering and sealing the upper port of the adapter. The top of the metal electrode rod is connected to a DC power supply.

4. The microwave plasma torch based on bias voltage regulation as described in claim 3, characterized in that, An insulating limiter is provided in the middle of the adapter to restrict the position of the metal electrode rod. The limiter is circular, and the metal electrode rod passes vertically through the central through hole of the limiter. The inner diameter of the central through hole of the limiter is equal to the outer diameter of the metal electrode rod. The edge of the limiter is embedded in the wall of the adapter to achieve fixed installation. Multiple air holes are also distributed on the limiter for gas transmission.

5. The microwave plasma torch based on bias voltage regulation as described in claim 4, characterized in that, The limiter is a limiter made of polytetrafluoroethylene material.

6. The microwave plasma torch based on bias voltage regulation as described in claim 3, characterized in that, The adapter is provided with a joint for flange connection at its upper and lower ends, and the joint is integrally formed with the adapter; the lower end of the adapter is fitted onto the upper end of the quartz tube and fixedly connected through the first flange.

7. The microwave plasma torch based on bias voltage regulation as described in claim 6, characterized in that, An annular step is formed on the upper end of the adapter surface along the edge of the adapter opening. The inner diameter of the annular step is equal to the outer diameter of the insulating ceramic. The insulating ceramic is installed inside the annular step to completely cover the opening of the adapter. A second flange is installed on the upper surface of the insulating ceramic and is bolted to the adapter's connector.

8. The microwave plasma torch based on bias voltage regulation as described in claim 1, characterized in that, The rectangular waveguide is a standard BJ26 rectangular waveguide.

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