A sputtering cleaning system and cleaning method for the wall surface in an ionization chamber of a radio frequency ion thruster

Abstract

The present invention proposes a sputtering cleaning system and cleaning method for the inner wall of the ionization chamber of a radio frequency ion thruster. The system is composed of a gas distributor, a potential control line, a radio frequency coil, an ionization chamber, a screen grid, an acceleration grid, a positive DC voltage source, a radio frequency power supply, a negative DC voltage source and a conductive tape. The method includes: 1. The radio frequency ion thruster is started, and plasma is formed in the ionization chamber; 2. A positive voltage is applied to the screen grid, and ions are drawn out by the grid system, and the thruster enters a working state; 3. When the conductive film is thicker and the power input is weakened, the positive DC power supply is turned off, and no ions are drawn out of the thruster; 4. The negative DC power supply is turned on, and a negative voltage is slowly applied to the potential control line to increase the energy of ions hitting the inner wall of the ionization chamber; 5. The conductive film on the inner wall of the ionization chamber is bombarded by ions, and particles are sputtered out. The particles are neutral and are discharged from the ionization chamber under the action of the neutral particle flow, thereby achieving cleaning of the inner wall of the ionization chamber.

Description

Technical Field

[0001] The invention belongs to the field of space electric propulsion technology and plasma cleaning, and in particular relates to a sputtering cleaning system and a cleaning method for the wall surface of an ionization chamber of a radio frequency ion thruster. Background Art

[0002] The radio frequency ion thruster is an electric thruster used for space electric propulsion (or aerospace plasma propulsion). It has the advantages of high specific impulse, wide thrust variation range, simple structure, and mature technology. It has been widely used in the attitude and orbit control of spacecraft.

[0003] The ion thruster is highly similar to the ion source in structure and principle. It is an ion source dedicated to the field of space electric propulsion. It is characterized by using a grid system composed of a large number of small holes to accelerate the discharge of ions to obtain thrust. The grid system of the double-grid ion thruster consists of a screen grid and an acceleration grid. After the ions pass through the screen grid, most of them are discharged from the thruster through the acceleration grid. A small part of the ions are bombarded on the acceleration grid due to over-focusing, under-focusing and other factors. The metal particles on the surface of the acceleration grid are sputtered and flow back to the inside of the ionization chamber. The particles interact with the inner wall of the ionization chamber and are adsorbed by the wall. The number of adsorbed particles gradually increases, forming an island-net-layered film structure in sequence. This is the deposition process of the grid material on the inner wall of the ionization chamber.

[0004] The thin film deposited on the inner wall of the ionization chamber is the same material as the acceleration grid, which is a conductive material (commonly molybdenum, composite carbon material, stainless steel, etc.). As the thickness of the film increases, the shielding effect of the film on the RF coil gradually increases, which will greatly reduce the efficiency of the RF coil, resulting in the "three difficulties" of power feeding, thruster ignition and plasma maintenance, affecting the life of the thruster. Therefore, it is necessary to develop a sputtering cleaning method for the wall of the ionization chamber of the RF ion thruster.

[0005] At present, no public method for cleaning the inner wall of RF ion thrusters has been found, but there are similar processes in the fields of plasma sputtering and substrate surface sputtering cleaning:

[0006] 1. Sputtering cleaning of Hall ion source. Pre-vacuum to 5.0×10 -3 Pa, the temperature rises to 250℃, argon is introduced, the Hall ion source is turned on to generate argon ion beam, and the substrate is cleaned by ion beam sputtering. By controlling the vacuum degree to 0.5Pa, 0.1Pa and 0.05Pa respectively, 400V is generated,

[0007] 800V and 1200V discharge voltages, followed by ion beam bombardment at different discharge voltages. Figure 1As shown in the figure, the morphology of the substrate cleaned by sputtering changes under different discharge voltages. It is obvious that the cleaning of the argon ion beam can greatly reduce the contact angle of the substrate material, and the reduction of the contact angle increases with the increase of the discharge voltage. [1] .

[0008] 2. Material surface treatment based on ion thrusters. Ion thrusters have the advantage of easy adjustment of the acceleration voltage. Different acceleration voltages can be obtained by changing the potential of the screen grid. Figure 2 This is a typical schematic diagram of an ion beam cleaning device, from top to bottom, the ion source, grid system, tungsten cathode, ion beam, and substrate being cleaned. Typical operating conditions are as follows: grid system aperture 2mm, grid spacing 0.3–1.2mm, acceleration voltage 400–2300V, beam current 100–500mA [2],[3] .

[0009] 3. Capacitively coupled plasma cleaning system. The capacitively coupled plasma source has the advantages of simple structure, easy plasma generation, good economy and strong practicality. The typical plasma cleaning process is as follows: start the power supply, start the vacuum pump, open the isolation valve, open the inflation valve when the vacuum degree is less than 50Pa, adjust the flow rate, and turn on the RF power supply when the pressure stabilizes at 30Pa, and turn off the RF power supply after the cleaning time is reached. During the experiment, it was found that with the increase of RF power, the cleaning effect gradually optimized under the same cleaning time; but the upper limit of the cleaning effect is restricted by the power, that is, the same cleaning effect as that under high power cannot be achieved under low power. [4],[5] .

[0010] [1] Zhong Li, Dan Min, Shen Liru, Jin Fanya, Chen Meiyan, Liu Tong, Deng Zhi. Effect of Hall source sputtering cleaning process on bonding properties of ion-plated TiN coating[J]. Vacuum, 2020, 57(06): 5-10.

[0011] [2]Wilbur P,Buchholtz B.Surface engineering using ion-thrustertechnology[C] / / 30th Joint Propulsion Conference and Exhibit.1994:3235.

[0012] [3] Sun Hao, Bai Qingshun, Li Yuhai, et al. Plasma cleaning technology for organic pollutants on the surface of aluminum-coated diffraction gratings[J]. China Surface Engineering, 2022, 35(03): 235-244.

[0013] [4] Ye Rongkun. Research on the generation and cleaning effect of low-pressure radio frequency plasma[D]. Yanshan University, 2021.

[0014] [5] Liu Guoguang, Miao Junfang, Hu Wenping et al. Design of radio frequency plasma cleaning system[J]. Special Equipment for Electronic Industry, 2012, 41(07): 29-31.

[0015] At present, no publicly available sputtering cleaning method for the inner wall of a radio frequency ion thruster has been found. In the existing technologies, the plasma in the Hall ion source and ion thruster cleaning device does not directly contact the surface to be cleaned, and the plasma bombards the surface to be cleaned after acceleration to achieve the cleaning of the material surface; the capacitive coupling plasma cleaning system uses the method of plasma wrapping the surface to be cleaned, relying on the sheath acceleration between the plasma and the surface to be cleaned, so that the ions impact the surface to be cleaned to achieve the cleaning effect.

[0016] The shortcomings and problems of the prior art are as follows:

[0017] 1. The plasma is separated from the surface to be cleaned. This method cannot clean the wall of the ionization chamber.

[0018] 2. If the plasma is in direct contact with the surface to be cleaned, the ions are accelerated only by the plasma and the sheath of the surface to be cleaned, and the acceleration voltage cannot be adjusted, resulting in low ion energy, reduced cleaning efficiency, and affected cleaning results.

[0019] 3. When the surface to be cleaned is a metal film or an alloy film, a higher acceleration voltage is required to achieve a good cleaning effect. The acceleration voltage cannot be adjusted in the existing technology.

[0020] 4. The ion source often only has the function of generating plasma and has a single function.

[0021] In summary, the current sputtering cleaning method for the inner wall of the ionization chamber of the RF ion thruster is vacant and has many disadvantages and problems. A sputtering cleaning method with adjustable particle acceleration voltage and good cleaning effect should be developed. Summary of the invention

[0022] The purposes of the present invention are: 1. To solve the problem of poor cleaning effect when the plasma is in direct contact with the surface to be cleaned. 2. To solve the problem that the particle acceleration voltage cannot be directly adjusted. 3. To solve the problem that the ion source has only a single purpose and realize the switchability of functions. 4. To solve the problem of difficulty in power feeding, thruster ignition and plasma maintenance after the ion thruster has been working for a long time.

[0023] To achieve the above-mentioned purpose, the present invention proposes a sputtering cleaning system and a cleaning method for the inner wall of an ionization chamber of a radio frequency ion thruster. Different from the existing sputtering cleaning method, the present invention is characterized in that after a conductive film is deposited on the inner wall of the ionization chamber, the inner wall potential is adjustable, so the particle acceleration voltage is adjustable, which optimizes the cleaning effect and improves the cleaning efficiency.

[0024] The schematic diagram of the ion thruster ionization chamber wall cleaning system of the present invention is as follows: Figure 5 As shown, it is composed of a gas distributor 1, a potential control line 2, a radio frequency coil 3, an ionization chamber 4, a screen grid 5, an acceleration grid 6, a positive DC voltage source 7, a radio frequency power source 8, a negative DC voltage source 9, and a conductive tape 10.

[0025] The gas distributor 1 is in a "T" shape with threads, and is connected to the ionization chamber 4 by a bolt structure (nuts are not drawn). The potential control line 2 is a flexible wire, which passes through the threaded surface of the gas distributor 1, passes through the inside of the gas distributor 1 and enters the ionization chamber 4, and is connected to the inner wall of the ionization chamber 4 through the conductive tape 10. The screen grid 5 and the acceleration grid 6 are coaxial with the ionization chamber 4, and the two are positioned by positioning pins; wherein the screen grid 5 is in direct contact with the flange of the ionization chamber 4, and the screen grid 5 and the acceleration grid 6 are 1mm apart. The RF coil 3 is a spiral structure with 8 turns, coaxial with the ionization chamber 4, and 3mm away from the outer wall of the ionization chamber 4. The positive DC voltage source 7, the radio frequency power source 8, and the negative DC voltage source 9 are external circuit components, wherein the positive DC voltage source 7 is connected to the screen grid 5 and the acceleration grid 6, the screen grid 5 applies a 600-2000V positive DC voltage, and the acceleration grid 6 is grounded, which is used to draw out ions in the ionization chamber 4 to generate thrust; the radio frequency power source 8 and its matching network are connected to the radio frequency coil 3, and the two ends of the radio frequency coil 3 are connected to the 2.0MHz radio frequency power source 8, which is used to maintain the inductively coupled discharge of the working fluid gas in the ionization chamber 4; the negative DC voltage source 9 is connected to the potential control line 2, and the potential control line 2 is connected to a 0-60V negative DC voltage, which is used to change the potential of the conductive film on the inner wall of the ionization chamber 4.

[0026] Figure 6 The cross-sectional view of the main structure of the present invention includes a gas distributor 1, a potential control line 2, an ionization chamber 4 and a conductive tape 10. The gas distributor 1 fits with the bottom of the ionization chamber 2, and there are threads on the gas distributor 1, so that the gas distributor 1 and the ionization chamber 4 are connected with a bolt structure (the nut is not shown in the figure); the potential control line 2 passes through the threaded surface of the gas distributor 1, extends from the inside of the gas distributor 1 into the ionization chamber 4, and is connected to the inner wall of the ionization chamber 4 with a conductive tape 10.

[0027] Figure 7 The main structural isometric diagram of the present invention includes a gas distributor 1, a potential control line 2, an ionization chamber 4 and a conductive tape 10, and the connection method is similar to Figure 6 same.

[0028] The present invention provides a sputtering cleaning method for the inner wall of an ionization chamber of a radio frequency ion thruster, comprising the following steps:

[0029] Step 1: The RF ion thruster is started, and plasma is formed in the ionization chamber. At this time, there is no conductive film on the wall of the ionization chamber.

[0030] Step 2: Apply positive voltage to the screen grid, the ions are drawn out by the grid system, and the thruster enters working state.

[0031] Step 3: When the conductive film is thicker and the power input is weakened, the positive DC power supply can be turned off, and no ions are extracted from the thruster, but the plasma in the ionization chamber is still maintained.

[0032] Step 4: Turn on the negative DC power supply and slowly apply a negative voltage to the potential control line. The energy of the ions hitting the inner wall of the ionization chamber increases.

[0033] Step 5: Under the bombardment of ions, the conductive film on the inner wall of the ionization chamber sputters out particles. The particles are neutral and are discharged out of the ionization chamber under the action of the neutral particle flow, thereby cleaning the inner wall of the ionization chamber.

[0034] In engineering practice, the acceleration grid is generally made of sputtering-resistant materials such as molybdenum or carbon composite materials, so that fewer sputtered particles enter the ionization chamber and the deposition rate of the conductive film is slow. In the early stage of the laboratory research and development process, the thruster iteration speed is fast and the number of iterations is large. The economic cost of using molybdenum or carbon composite materials is too high, so stainless steel, aluminum and other low-cost but sputtering-resistant materials are often selected. This leads to the formation of a thicker conductive film on the inner wall of the ionization chamber, which restricts the time of a single test. Due to the limitations of test conditions, the physical strength of the test personnel and other factors, when a thicker conductive film is generated, the thruster is often taken out, cleaned with chemical reagents, and the test is suspended. Therefore, the present invention can increase the time of a single test by cleaning the conductive film on the inner surface of the ionization chamber under the condition of using non-sputtering-resistant materials, thereby reducing economic and time costs.

[0035] At present, there are few studies on the deposition mechanism of the inner wall of the ionization chamber in the world. To fill this gap, it is necessary to select materials with high sputtering yield and fast deposition speed as acceleration grid materials, so that a conductive film can be quickly generated on the inner wall of the ionization chamber, so as to quickly obtain the film formation process and clarify the film formation mechanism. After a certain experiment, a conductive film was generated on the inner wall of the ionization chamber. Without cleaning with reagents, the ionization chamber was used to conduct a plasma density measurement experiment. During the experiment, the grid system did not work, and only plasma existed in the ionization chamber. After the experiment was completed, it was found that the conductive film on the inner wall of the ionization chamber became thinner and the resistance of the conductive film increased.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] 1. The present invention can clean the inner wall surface of the ionization chamber to a certain extent without taking out the ionization chamber.

[0038] 2. The present invention can improve the electrical efficiency of the thruster by reducing the thickness of the conductive film and increasing the feed power of the radio frequency coil.

[0039] 3. The present invention improves the ionization rate of the working gas and increases the gas mass utilization rate while improving the coil feed power.

[0040] 4. The present invention is easy to control and simple to operate. The cleaning rate can be adjusted by adjusting the negative DC power supply, adjusting the wall potential in the ionization chamber, changing the particle acceleration voltage, and so on.

[0041] 5. The present invention has a wide range of applications. As long as the deposited thin film is conductive, the present invention can be used to clean the thin film.

[0042] 6. The present invention has a simple structure and is easy to assemble.

[0043] 7. The present invention helps to improve the overall performance of radio frequency ion thrusters. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is the reference of Zhong Li, which shows the comparison of contact angles of substrate surfaces after cleaning.

[0045] Figure 2 This is a document by Wilbur P, a schematic diagram of ion source plasma sputtering cleaning technology.

[0046] Figure 3 This is Ye Rongkun’s document, schematic diagram of capacitively coupled plasma source.

[0047] Figure 4 This is a photograph of the conductive film observed by field emission scanning electron microscopy.

[0048] Figure 5 It is a schematic diagram of the inner wall cleaning system of the ion thruster ionization chamber of the present invention.

[0049] Figure 6 It is a cross-sectional view of the main structure of the present invention.

[0050] Figure 7 It is an axonometric diagram of the main structure of the present invention.

[0051] The numbers in the figure are explained as follows:

[0052] Gas distributor 1 potential control line 2 radio frequency coil 3

[0053] Ionization chamber 4 screen grid 5 acceleration grid 6

[0054] Positive DC voltage source 7 RF power supply 8 Negative DC voltage source 9

[0055] Conductive tape 10 DETAILED DESCRIPTION

[0056] The parameters of the positive DC voltage source, the radio frequency power source, and the negative DC voltage source in the present invention have wide adaptability. The typical voltage range of the positive DC voltage source applied to the screen grid is 300-3000V, usually 1000V; the typical power range of the radio frequency power source applied to the radio frequency coil is 70-100W, usually 80W, and the frequency is 2.0MHz; the typical voltage range of the negative DC voltage source applied to the potential control line is 0--60V, usually -20V. The following are the specific operating steps, and xenon is selected as the thruster working fluid.

[0057] The operating steps of the present invention are:

[0058] 1. Thruster ignition: Xenon gas with a flow rate of 1.5 sccm is introduced into the thruster, and the RF coil is applied with a power of 80 W to form a plasma in the ionization chamber.

[0059] 2. Ion extraction. A positive voltage of 1000V is applied to the screen grid, and the ions are extracted from the ionization chamber. Among them, some ions are accelerated by the screen grid and the acceleration grid, bombarding the surface of the acceleration grid, and the atoms on the surface of the acceleration grid are sputtered and flow back into the ionization chamber.

[0060] 3. Generate a conductive film. The sputtered atoms are deposited on the inner wall of the ionization chamber. After 210 minutes, a conductive film with a thickness ranging from 600nm to 2000nm is generated, such as Figure 4 As shown; the power input was reduced from 57.54W to 56.54W, the ion beam current was reduced from 38.8mA to 30.5mA, and the thruster performance was significantly reduced.

[0061] 4. Turn off the grid system. Adjust the voltage applied to the screen grid to 0, and ions will no longer be extracted.

[0062] 5. Sputtering cleaning. Turn on the negative DC voltage source, slowly adjust the potential control line to -20V, and enter the cleaning state. Since the potential control line is connected to the inner wall of the ionization chamber through conductive tape, the metal conductive film on the inner wall of the ionization chamber is at the same potential as the potential control line. Maintain the potential of the potential control line at -20V for 1 hour, and then turn off the negative DC voltage source.

[0063] 6. Sputtering cleaning is completed.

[0064] The present invention provides a sputtering cleaning system and cleaning method for the inner wall of an ionization chamber of a radio frequency ion thruster, the main innovations of which are:

[0065] 1. Design a method for changing the potential of the conductive film on the inner wall of the ionization chamber. The existing technology cannot actively control the potential of the conductive film on the inner wall of the ionization chamber. The design is to use a potential control line connected to the inner wall of the ionization chamber, and then change the potential of the conductive film through a negative DC voltage source.

[0066] 2. Improve the ion acceleration method. Currently, there is no method to use the sheath to accelerate ions and then bombard the conductive film.

[0067] 3. Design a way for the potential control line to enter the ionization chamber. The prior art does not have a way for the potential control line to enter the ionization chamber. It is designed to enter the ionization chamber through a gas distributor, which may allow the potential control line to enter the ionization chamber from the side wall of the ionization chamber; or from the exit of the ionization chamber. The potential control line is connected to the inner wall of the ionization chamber through a conductive tape, and there may be some mechanical structure (such as threads, buckles, etc.) to tightly connect the two. In the current design, there is only one potential control line, and there may be multiple potential control lines distributed along the circumference of the ionization chamber, which can make the potential distribution of the inner wall of the ionization chamber more uniform.

Claims

1. A sputtering cleaning system for the inner wall of the ionization chamber of a radio frequency ion thruster, Features: It consists of a gas distributor, a potential control line, a radio frequency coil, an ionization chamber, a screen grid, an acceleration grid, a positive DC voltage source, a radio frequency power source, a negative DC voltage source and a conductive tape; wherein, The gas distributor is in a "T" shape with threads on it, and is connected to the ionization chamber through a bolt structure; the potential control line is a flexible wire, which passes through the threaded surface of the gas distributor, enters the ionization chamber through the inside of the gas distributor, and is connected to the inner wall of the ionization chamber through a conductive tape; the screen grid, the acceleration grid and the ionization chamber are coaxial, and the two are positioned by positioning pins; the positive DC voltage source, the radio frequency power source, and the negative DC voltage source are external circuit components; A positive DC voltage source is connected to the screen grid and the accelerating grid; a negative DC voltage source is connected to a potential control line, and the potential control line is connected to a 0-60 V negative DC voltage for changing the potential of the conductive film on the inner wall of the ionization chamber; The cleaning steps are: Step 1: The RF ion thruster is started, and plasma is formed in the ionization chamber. At this time, there is no conductive film on the wall of the ionization chamber; Step 2: Apply positive voltage to the screen grid, the ions are drawn out by the grid system, and the thruster enters the working state; Step 3: When the conductive film is thicker and the power input is weakened, the positive DC power supply is turned off, and no ions are extracted from the thruster, but the plasma in the ionization chamber is still maintained; Step 4: Turn on the negative DC power supply and slowly apply a negative voltage to the potential control line, so that the energy of ions hitting the inner wall of the ionization chamber increases; Step 5: Under the bombardment of ions, the conductive film on the inner wall of the ionization chamber sputters out particles. The particles are neutral and are discharged out of the ionization chamber under the action of the neutral particle flow, thereby cleaning the inner wall of the ionization chamber.

2. A sputtering cleaning system for the inner wall of an ionization chamber of a radio frequency ion thruster according to claim 1, Features: The screen grid is in direct contact with the flange of the ionization chamber, and the distance between the screen grid and the acceleration grid is 1 mm.

3. A sputtering cleaning system for the inner wall of an ionization chamber of a radio frequency ion thruster according to claim 1, Features: The RF coil is a spiral structure with 8 turns, coaxial with the ionization chamber and 3 mm away from the outer wall of the ionization chamber.

4. A sputtering cleaning system for the inner wall of an ionization chamber of a radio frequency ion thruster according to claim 1, Features: A positive DC voltage of 600-2000 V is applied to the screen grid, and the accelerating grid is grounded to extract ions in the ionization chamber to generate thrust.

5. The sputtering cleaning system for the inner wall of the ionization chamber of a radio frequency ion thruster according to claim 1, Features: The RF power supply and its matching network are connected to the RF coil, and both ends of the RF coil are connected to a 2.0 MHz RF power supply for maintaining the inductively coupled discharge of the working fluid gas in the ionization chamber.

6. A sputtering cleaning system for the inner wall of an ionization chamber of a radio frequency ion thruster according to claim 1, Features: The acceleration grid is made of molybdenum or carbon composite material.

7. The sputtering cleaning system for the inner wall of the ionization chamber of a radio frequency ion thruster according to claim 1, Features: Xenon is selected as the thruster working fluid.

8. The sputtering cleaning system for the inner wall of the ionization chamber of a radio frequency ion thruster according to claim 1, Features: The typical voltage range of the positive DC voltage source applied to the screen grid is 300-3000 V; the typical power range of the RF power source applied to the RF coil is 70-100 W; the typical voltage range of the negative DC voltage source applied to the potential control line is 0-60 V.

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