JP2013152948A - Method of producing cathode body for magnetron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that a cathode body used for a magnetron has many disadvantages in increasing power.SOLUTION: A cathode body for a magnetron is obtained, which uses as a base material a high melting point metal having an electron emitting material contained therein and has a surface coated with a rare-earth boride. Preferably, the electron emitting material and the high melting point metal are LaOand W respectively, and the rare-earth boride is LaB.

Description

  The present invention relates to a magnetron that is a microwave oscillation device used in a microwave oven or an industrial plasma generator, and more particularly to a magnetron that is reliable and has a long life.

  Referring to FIG. 1, the structure of a conventional magnetron is partially shown. 101 is a cathode body, 102 is an anode vane forming an anode, 103 is a cooling water coil for cooling the anode, and 104 is a current introduction terminal for heating the cathode body in order to emit thermoelectrons from the cathode body. is there. Usually, the anode is grounded. On the other hand, the cathode body has a filament structure, and a high negative DC voltage of about several kV to 10 kV is applied to the anode, and a large alternating current of about several tens of A to 100 A is applied to the filament through an insulating transformer. The filament is heated by flowing to emit thermoelectrons.

  Since a predetermined DC vertical magnetic field is applied to the interaction space 106 between the filament and the anode by the electromagnet 105, the emitted electrons are accelerated in the anode direction by the electric field between the filament and the anode, and are generated by the DC vertical magnetic field. When the Lorentz force works, the interaction space turns. The anode vanes are arranged symmetrically about a plurality of axes, and the resonance frequency of the space formed by the adjacent anode vanes is set to the frequency of the extracted microwave. When the swirling electron current passes around the end of the anode vane, it vibrates strongly at the resonance frequency of this resonance cavity, that is, the frequency of the extracted microwave. The oscillated high-frequency power is output to a waveguide or the like provided outside the tube by an antenna such as a loop or a slit.

Japanese Patent Laid-Open No. 11-283516 JP 2008-53129 A JP 2003-100224 A

In the above prior art, the filament is usually formed of W containing an electron emitting material such as ThO ₂ inside. Although the work function of the electron emitting material ThO ₂ is relatively low, the work function of W is as high as 4.6 eV, so that the filament must be heated to emit sufficient electrons. For this reason, there is a problem that the filament life is shorter than problems such as filament evaporation and embrittlement.

  Further, in order to sufficiently heat the filament, the alternating current to be energized must be very large. In particular, as an industrial magnetron capable of obtaining a large power of several tens to 100 kW, a magnetron having an oscillation frequency of 915 MHz is often used rather than a magnetron having an oscillation frequency of 2.45 GHz. Thus, as an industrial magnetron, The 100 kW output high power magnetron used requires a current of as much as 100 A. When a large current of 100 A flows through the filament, various problems peculiar to a high-power magnetron arise.

  For example, the voltage drop in the filament due to a large current is about 10 V, which is sufficiently small compared to the high voltage applied to the cathode for generating an electric field, so the influence on the electric field applied between the anode and the filament can be ignored. When such a large current flows through the filament, the AC magnetic field generated in the interaction space is not negligible compared to the externally applied vertical magnetic field, and ripples occur in the output power and oscillation frequency. there were. Furthermore, the cooling water tube for performing anode cooling has an inner diameter of about 10 mm or less, but in order to take heat generated in the magnetron, typically a large flow rate of water of about 10 L / min is required, and the load of the cooling water pump However, there is a problem that when a large flow rate of water flows through a tube having an inner diameter of about 10 mm or less, the flow rate of water is large, heat cannot be efficiently removed, and anode control with good controllability cannot be performed.

On the other hand, in the conventional magnetron, as electron emission member for improving the electron emission of the cathode, as shown in Patent Document 1 or the like, ThO ₂ other materials, for example, hafnium oxide, zirconium oxide, lanthanum oxide, cerium oxide, etc. Although it is known to be used alone or in combination with tungsten, the electron emission characteristics are still insufficient. In particular, in an industrial magnetron that requires a large power of 100 kW, whether or not the above-described electron-emitting member can maintain a sufficient life has not been sufficiently studied.

  For anode heat dissipation, it is known that air is cooled by providing a large number of fins on the anode, and it is known that the liquid cooling structure is improved as disclosed in Patent Documents 2 and 3, but the industrial magnetron Then, there was a possibility that sufficient cooling could not be performed. Accordingly, an object of the present invention is to provide a magnetron having excellent electron emission characteristics over a long period of time.

  Another object of the present invention is to provide a magnetron having an efficient anode cooling structure.

  Still another object of the present invention is to provide a magnetron that prevents ripples in output power and oscillation frequency.

  The inventors of the present invention previously described in Japanese Patent Application No. 2007-99778 and the like, by moving the ring-shaped plasma region on the target with time, the local wear of the target is prevented and the plasma density is increased. Then, a sputtering apparatus capable of improving the film formation rate was proposed. The sputtering apparatus has a configuration in which a target is disposed opposite to a substrate to be processed and a magnet member is provided on the opposite side of the target from the substrate to be processed.

  Specifically, the magnet member of the sputtering apparatus described above includes a rotating magnet group in which a plurality of plate magnets are spirally attached to the surface of the rotating shaft, a periphery of the rotating magnet group in parallel with the target surface, and And a fixed outer peripheral plate magnet magnetized perpendicularly to the target. According to this configuration, by rotating the rotating magnet group, the magnetic field pattern formed on the target by the rotating magnet group and the fixed outer peripheral plate magnet is continuously moved in the direction of the rotation axis. The plasma region can be continuously moved in the direction of the rotation axis with time.

  By using the rotating magnet type sputtering apparatus, the target can be used uniformly over a long period of time, and the film formation rate can be improved.

  According to the first aspect of the present invention, there is obtained a magnetron characterized by having a cathode body whose base material is a refractory metal containing an electron emitting material and whose surface is coated with a rare earth boride. .

According to a second aspect of the present invention, there is provided a magnetron wherein the electron emitting material is La ₂ O ₃ and the rare earth boride is LaB ₆ .

According to the third aspect of the present invention, an electrode having an electrode member mainly containing tungsten or molybdenum and containing La ₂ O ₃ and a rare earth element boride film formed on the surface of the electrode member by sputtering. Can be obtained as a cathode body.

According to a fourth aspect of the present invention, in the third aspect, the rare earth element boride is at least one boron selected from the group consisting of LaB ₄ , LaB ₆ , YbB ₆ , GaB ₆ , and CeB _6. A magnetron characterized in that it contains a chemical is obtained.

According to a fifth aspect of the present invention, there is obtained a magnetron characterized in that the cathode body contains 4 to 6% La ₂ O ₃ by volume ratio.

According to the sixth aspect of the present invention, there is obtained a method for manufacturing a magnetron cathode body, characterized in that a LaB ₆ film is formed by sputtering on the surface of a cathode body mainly composed of tungsten or molybdenum by a plasma sputtering apparatus.

According to a seventh aspect of the present invention, there is provided the method for manufacturing a magnetron cathode body according to the sixth aspect, comprising the step of annealing the LaB ₆ film formed by sputtering in an inert gas atmosphere. can get.

  According to the eighth aspect of the present invention, there is obtained the method for manufacturing a magnetron cathode body according to the seventh aspect, wherein the annealing temperature is set to 400 ° C. to 1000 ° C. in the annealing step.

According to a ninth aspect of the present invention, in accordance with the sixth or seventh aspect, the LaB ₆ film is formed by sputtering with a standardized ion dose of 5 to 17 by RF-DC coupled discharge. A method of manufacturing a magnetron cathode body is obtained.

  According to a tenth aspect of the present invention, there is provided the magnetron according to any one of the first to fifth aspects, wherein the anode is cooled by contacting a plurality of tubes through which cooling water flows in parallel with the anode. Is obtained.

  According to an eleventh aspect of the present invention, there is provided the magnetron according to any one of the first to fifth aspects, wherein a cylindrical jacket is provided outside the anode, and the anode is cooled by flowing cooling water through the jacket. Thus, a magnetron can be obtained.

  According to a twelfth aspect of the present invention, there is provided the magnetron according to the tenth or eleventh aspect, wherein the Reynolds number of the passing cooling water is 1000 to 5000 in a portion of the flow path through which the cooling water contacts the anode. A magnetron characterized by being set within a range is obtained.

  According to a thirteenth aspect of the present invention, the magnetron according to the twelfth aspect is characterized in that, in the cooling water tube that supplies cooling water to the magnetron, the Reynolds number of the passing cooling water is set to 1000 or less. A magnetron is obtained.

  According to a fourteenth aspect of the present invention, in order to heat the cathode body and emit thermoelectrons, a direct current is passed through the cathode body to heat it. The magnetron according to any one of 10 to 13 is obtained.

  In the present invention, a cathode that can maintain a long life even when a large current of 100 A is applied by coating a rare earth boride on the surface of a refractory metal containing an electron emitting material. The body is obtained.

It is the schematic which shows a part of conventional magnetron. It is the schematic which shows the filament part which concerns on Example 1 of this invention. It is a graph explaining the thermoelectron emission characteristic of W containing ThO ₂ and the thermoelectron emission characteristic of W containing La ₂ O ₃ . It is the schematic for demonstrating the magnetron which concerns on Example 2 of this invention. It is the schematic for demonstrating the magnetron based on Example 3 of this invention. It is the schematic for demonstrating the magnetron which concerns on Example 4 of this invention. It is the schematic which shows the rotating magnet sputtering device used when manufacturing the cathode body for magnetrons which concerns on this invention. In the case of performing sputtering film formation by DC discharge is a diagram showing a (100) plane pressure dependence of the peak intensity and a sheet resistance of the LaB ₆ film. LaB ₆ is a diagram showing a normalized ion dose dependency of the peak intensity and a sheet resistance of the (100) face film.

  Embodiments of the present invention will be described below with reference to the drawings.

A first embodiment of the present invention will be described with reference to FIG. 201 is a filament, 203 is a lower end seal, 204 is a center lead, 205 is a side lead, 206 is an upper end seal, 207 is a getter material, 208 is a solder for connecting the end seal and lead, and the end seal and filament. It is a material. The inventor of the present invention has formed a LaB ₆ thin film of about 100 nm on the surface of the base material of LaF ₃ by magnetron sputtering by introducing La ₂ O ₃ in a volume ratio of 4 to 6%, preferably 2 to 3%. It has been found that a sufficient radiation current for microwave oscillation can be obtained by forming the film stably at a low temperature of about 1400 ° C. as compared with the conventional W containing ThO ₂ .

Fig. 3 shows the thermoelectron emission characteristics of W with ThO ₂ and W with La ₂ O ₃ into a discharge electrode with a diameter of 1.6 mm and a hyperbola at the tip to cause thermionic emission a number of times. It is the result of investigating the change of voltage. An increase in the discharge voltage indicates that the tip is worn and the discharge performance is deteriorated. As can be seen from FIG. 3, in the case of W with ThO ₂ , discharge performance deteriorates by discharging 100 times, whereas in the case of W with La ₂ O ₃ , the discharge voltage is stable up to 600 times, It can be seen that stable thermal electron emission has been realized. When the electrode tip temperature during arc discharge was measured with a radiation thermometer, it was confirmed that it was 3700 ° C. in the case of W containing ThO ₂ , whereas that of La ₂ O ₃ containing W was 3000 ° C. ing. It is considered that this temperature difference occurred due to the shape of the generated discharge and the difference in thermal conductivity. In the figure, W introduced with 3% La ₂ O ₃ is used as a filament.

Next, the effect of depositing LaB ₆ on the surface will be described. LaB ₆ crystal is a chemically stable low work function material (work function of about 2.7 eV), and it is known that high thermionic emission current density can be obtained. However, until now, the technology for forming a high-quality LaB ₆ thin film has not been established.

On the other hand, the inventor irradiates the surface of the film when the normalized ion dose (LaB ₆ ) is formed in a newly developed rotating magnet sputtering apparatus (described below) that does not cause plasma damage. It was found that a thin film having excellent crystallinity and a work function of 2.8 eV can be formed by controlling the number of incident Ar ions with respect to LaB ₆ (expressed as Ar ⁺ / LaB ₆ ) and ion irradiation energy.

In this embodiment, a LaB ₆ thin film having a film thickness of 100 nm is formed on the surface of a W ₂ filament containing La ₂ O ₃ by using a rotary magnet sputtering apparatus. By using this filament, a stable electron emission current was obtained at a low temperature of about 1400 ° C., and a long-life magnetron was realized.

  FIG. 7 is a view showing an example of a rotating magnet sputtering apparatus used in the present invention. The rotating magnet sputtering apparatus shown in FIG. 7 includes a target 1, a polygonal (for example, regular hexagonal) columnar rotating shaft 2, and a plurality of helical plates that are spirally attached to the surface of the columnar rotating shaft 2. The outer peripheral magnet group 3 including the magnet group and the fixed outer peripheral plate magnet 4 and the fixed outer peripheral plate magnet 4 arranged on the outer periphery of the rotating magnet group 3 so as to surround the rotating magnet group 3 are on the side opposite to the target 1. An outer peripheral paramagnetic material 5 is provided. Further, a backing plate 6 is bonded to the target 1, and portions other than the columnar rotating shaft 2 and the spiral plate magnet group 3 other than the target 1 side are covered with a paramagnetic material 15. Covered by.

  When viewed from the target 1, the fixed outer peripheral plate magnet 4 has a structure surrounding the rotating magnet group 3 constituted by a spiral plate magnet group, and is magnetized so that the side of the target 2 becomes an S pole. . The fixed outer peripheral plate magnet 4 and each plate magnet of the spiral plate magnet group are formed of Nd—Fe—B based sintered magnets.

  Furthermore, a plasma shielding member 16 is provided in the illustrated space 11 in the processing chamber, a cathode body manufacturing jig 19 is installed, and the pressure is reduced to introduce plasma gas.

  The illustrated plasma shielding member 16 extends in the axial direction of the columnar rotating shaft 2, and defines a slit 18 that opens the target 1 to the cathode manufacturing jig 19. In a region that is not shielded by the plasma shielding member 16 (that is, a region that is opened with respect to the target 1 by the slit 18), a plasma having a high magnetic field strength and a high density and a low electron temperature is generated. This is a region where charge-up damage and ion irradiation damage do not occur in the cathode body member provided at, and at the same time, a region where the film formation rate is fast. By shielding the region other than this region with the plasma shielding member 16, it is possible to perform film formation without damage without substantially reducing the film formation rate.

  The backing plate 6 is formed with a refrigerant passage 8 through which a refrigerant passes, and an insulating material 9 is provided between the housing 7 and the outer wall 14 forming the processing chamber. The feeder line 12 connected to the housing 7 is drawn to the outside through the cover 13. A DC power source, an RF power source, and a matching unit (not shown) are connected to the feeder line 12.

  In this configuration, plasma excitation power is supplied from the DC power source and the RF power source to the backing plate 6 and the target 1 through the matching unit, the feeder line 12 and the housing, and the plasma is excited on the target surface. Plasma excitation is possible only with DC power or RF power alone, but it is desirable to apply both from the viewpoint of film quality controllability and film formation rate controllability. The frequency of the RF power is usually selected from several hundred kHz to several hundred MHz, but a high frequency is desirable from the viewpoint of high density and low electron temperature of plasma. In this embodiment, a frequency of 13.56 MHz is used. doing.

  As shown in FIG. 7, a plurality of filaments 201 forming the cathode body are attached to the cathode body manufacturing jig 19 installed in the space 11 in the processing chamber.

As a film forming condition by sputtering of the LaB ₆ film, it is preferable to first clean the electrode material surface with plasma before film forming. For example, 90 mTorr (12 Pa) and RF 300 W are suitable for Ar plasma. The pressure in the chamber during sputtering is about 20 mTorr (2.7 Pa) (Ar plasma has an electron temperature of about 1.9 eV and ion irradiation energy of about 10 eV), and the specific resistance is minimized (about 200 μΩcm before annealing). At this time, the film forming rate is 90 nm / min. However, if the pressure is set to 10 mTorr (1.3 Pa), the film forming rate is further increased to 100 nm / min or more, and the specific resistance increases only slightly. Therefore, the pressure is preferably 5 to 35 mTorr (0.67 Pa to 4.7 Pa). When the substrate temperature (stage temperature) is increased, the specific resistance further decreases, and the substrate temperature is about 175 μΩcm at 300 ° C. with Ar 20 mTorr (2.7 Pa). Furthermore, the specific resistance is further lowered by annealing after film formation, and it is about 100 μΩcm by annealing at a temperature of 800 ° C. in high purity Ar. The annealing temperature is preferably 400 ° C to 1000 ° C. The annealing time may be 30 minutes or longer. For example, 3 hours or less is sufficient. The atmosphere of annealing is preferably an inert gas.

Next, in order to verify the optimum conditions for LaB ₆ film formation by sputtering, the following experiment was performed. A SiO ₂ film having a thickness of 90 nm was formed on the Si substrate by thermal oxidation, and a LaB ₆ film having a thickness of 80 nm was formed thereon using the rotating magnet sputtering apparatus shown in FIG. At that time, the following parameters were changed to measure the orientation (XRD measurement) and resistivity.

Film deposition pressure (5 mTorr to 90 mTorr, SI unit 0.67 Pa to 12 Pa)
・ Ion irradiation energy (9eV-80eV)
・ Standardized ion irradiation amount (Ar ⁺ / LaB ₆ = 1 to 20)

As a result of the XRD measurement, the LaB ₆ film formed by sputtering with the rotary magnet sputtering apparatus has the crystal planes of (210), (200), and (110) having extremely low strength, while the (100) crystal plane is strong. Was very large and the film quality was found to be excellent. This is one of the features of the present invention as compared with the (100) weakness in the conventional sputtering film formation.

FIG. 8 shows the pressure dependence of such (100) peak intensity and sheet resistance in a LaB ₆ film according to the present invention. This is data when plasma is formed by applying DC 900 W using Ar gas. As shown in FIG. 8, in DC discharge of about Ar20 mTorr (2.7 Pa) or less, the sheet resistance is very low (specific resistance value is about 200 μΩcm), but (100) peak intensity is small and crystallinity is poor. I understand. On the other hand, in a DC discharge in the vicinity of Ar50 mTorr (6.7 Pa), an approximately (100) -oriented LaB ₆ film is obtained, but the resistance increases (specific resistance value is about 1000 μΩcm).

  On the other hand, FIG. 9 showing changes in (100) peak intensity and sheet resistance when the normalized ion irradiation dose is changed from about 1 to about 20.

In FIG. 9, when the ion irradiation energy is suppressed to about 10 eV or less by RF-DC coupled discharge and the normalized ion irradiation amount is increased to about 5 to 17, the resistance decreases (specific resistance value is 300 to 400 μΩcm). It was found that the crystallinity was also improved. The results of FIG. 9 are that the pressure is Ar50 mTorr (6.7 Pa), the ion irradiation energy is all about 9.0 eV, and the target power density is all about 2 W / cm ² . In FIG. 9, DC discharge is performed at 900 W, and the normalized ion irradiation dose (Ar ⁺ / LaB ₆ ) at that time is 1.3. On the other hand, in the RF-DC discharge, the RF frequency is 13.56 MHz and the RF power is 600 W.

When the normalized ion irradiation amount (Ar ⁺ / LaB ₆ ) is 8.3, DC is −270 V, 10.2 is −240 V, and 16.5 is −180 V.

  A second embodiment of the present invention will be described with reference to FIG. Reference numeral 406 denotes a magnetron with 30 kW output, 401 denotes an inlet tube for introducing cooling water for anode cooling of the magnetron to the magnetron, and 404 denotes an outlet tube for cooling water. Reference numerals 402, 403, and 403 denote first, second, and third cooling tubes, respectively, which are connected in parallel between the inlet tube 401 and the outlet tube 404 so as to be wound around the anode. The cooling water is introduced by a pump (not shown).

  In the prior art, as shown in FIG. 1, for example, a single tube having an inner diameter of 6 mm is simply wound around the anode a plurality of times.

  Here, in the case of a 30 kW output magnetron, it is necessary to remove about 6 kW of heat flow with water. If the water inlet and outlet temperatures are 25 ° C. and 60 ° C., respectively, a cooling water amount of 2.5 L / min is required. When this is introduced with a single tube having an inner diameter of 6 mm, the water flow rate in the tube is 1.5 m / sec and the Reynolds number is about 8000. The Reynolds number is a dimensionless number that represents the degree of turbulent flow. When the Reynolds number is about 1000 or more, the Reynolds number is turbulent. In a laminar flow with a Reynolds number of 1000 or less, the pressure loss of water is small and the pump load is small, but the cooling efficiency is poor.

  On the other hand, if the turbulent flow is used, the cooling efficiency is increased. However, the pump load increases as the Reynolds number increases, and the pump power increases. In the end, the Reynolds number is in the range of about 1000 to 5000, preferably about 2000 to 3000, and the cooling efficiency is good and the pump load is small.

  In this embodiment, since three tubes having an inner diameter of 6 mm are connected in parallel to the inlet tube 401 and the outlet tube 405, the water flow rate is 1/3 in each tube, so the flow rate is also 1/3. The Reynolds number was about 3000. In addition, the inlet tube 401 and the outlet tube 405 had an inner diameter of 55 mm, and the Reynolds number was about 900, realizing a laminar flow. As a result, anode cooling with low pump load and excellent cooling efficiency was realized.

  A third embodiment of the present invention will be described with reference to FIG. Reference numerals 501 and 502 denote an inlet tube for introducing cooling water for anode cooling of the magnetron to the magnetron and an outlet tube for cooling water. Reference numeral 503 denotes a cylindrical jacket outside the anode, and the anode is cooled by flowing cooling water through the jacket. Reference numeral 504 denotes a baffle plate provided at the entrance of the jacket, which is installed to make the flow distribution in the jacket uniform. The tubes 501 and 502 have an inner diameter of 55 mm and a Reynolds number of about 900. On the other hand, the flow path interval is set so that the Reynolds number is 2500 in the jacket. As a result, anode cooling with low pump load and excellent cooling efficiency was realized.

  A fourth embodiment of the present invention will be described with reference to FIG. Reference numeral 601 denotes a DC magnetic field applied from the outside, 602 a DC power source for supplying a filament current for heating the filament, 604 a filament, and 603 a DC magnetic field generated by the filament current. 605 is an anode vane and 606 is an interaction space. In the prior art, the filament was heated by passing an alternating current through the filament. However, because the filament current is large, the magnetic field in the interaction space fluctuates due to the generation of the alternating magnetic field, and the trajectory of the swirling electrons Fluctuation has been induced. This caused a ripple in the magnetron output power and frequency. For example, when the filament diameter is 5 mm, the number of turns is 10, and the filament current is 100 A, the magnetic field strength generated by the filament current in the interaction space 1 mm away from the filament is 91 gauss, which is a value that cannot be ignored with respect to the external magnetic field. .

  In the present embodiment, since a DC power source is used, no magnetic field fluctuation occurs, and the magnetic field generated by the filament and the external magnetic field are made the same in direction to prevent the magnetic field intensity from decreasing. This realized stable magnetron oscillation with no ripple in output power and oscillation frequency.

  The present invention can be applied not only to a large power industrial magnetron and a large magnetron having an oscillation frequency of 915 MHz, but also to a household magnetron having an oscillation frequency of 2.45 GHz.

201 Filament 203 Lower end seal 204 Center lead 205 Side lead 206 Upper end seal 207 Getter material 208 Brazing material 401 Inlet tube 402 First cooling tube 403 Second cooling tube 404 Third cooling tube 405 Outlet tube 406 Magnetron 501 tube 502 Tube 503 Jacket 504 Baffle plate 601 DC magnetic field 602 DC power source 603 DC magnetic field 604 Filament 605 Anode vane 606 Interaction space 1 Target 2 Columnar rotating shaft 3 Rotating magnet group 4 Fixed outer magnet 5 Outer paramagnetic body 6 Backing plate 7 Housing 8 Refrigerant passage 9 Insulating material 11 Space in processing chamber 12 Feeder wire 13 Cover 14 Outer wall 15 Paramagnetic material 16 Plasma shielding member 18 Slit

Claims (4)

A method for producing a cathode body for magnetron, comprising forming a LaB ₆ film by sputtering on a surface of a cathode body mainly composed of tungsten or molybdenum by a plasma sputtering apparatus. Method for manufacturing a magnetron cathode for body according to claim 1, characterized in that it comprises a step of annealing the LaB ₆ film is sputtered in an inert gas atmosphere.   The method for manufacturing a magnetron cathode body according to claim 2, wherein the annealing temperature is set to 400 ° C to 1000 ° C in the annealing step. The method for producing a magnetron cathode body according to claim 1 or 2, wherein the LaB ₆ film is formed by sputtering with an RF-DC coupled discharge with a normalized ion irradiation dose of 5 to 17.

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