JP3957565B2 - Plasma processing apparatus, processing apparatus, and processing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is a plasma processing apparatus, relates to processing apparatus and processing method, particularly for large-sized rectangular substrate, film deposition, surface modification, or a device for applying a treatment such as etching.
[0002]
[Prior art]
Conventionally, in order to perform plasma processing such as film deposition, surface modification, or etching in a manufacturing process of a semiconductor device or a liquid crystal display device, a parallel plate type high-frequency plasma processing device or an electron cyclotron resonance (ECR) plasma is used. A processing device or the like is used.
[0003]
However, in the parallel plate type plasma processing apparatus, the plasma density is low and the electron temperature is high (damage to the processing substrate is large), and in the ECR plasma processing apparatus, a DC magnetic field is required for plasma excitation, so a large area is required. Has a problem that it is difficult to process.
[0004]
On the other hand, in recent years, a plasma processing apparatus has been proposed that does not require a magnetic field for plasma excitation, and can generate a plasma having a high density and a low electron temperature.
[0005]
Hereinafter, such an apparatus will be described.
[0006]
<< Conventional First Plasma Processing Apparatus >>
4A is a top view of the first plasma processing apparatus, and FIG. 4B is a cross-sectional view.
[0007]
This conventional first plasma processing apparatus is described in Japanese Patent No. 2722070.
[0008]
Reference numeral 41 is a coaxial transmission line, 42 is a circular microwave radiation plate, 43 is a slit concentrically provided in the circular microwave radiation plate 42, 44 is an electromagnetic wave radiation window made of a dielectric, 45 is a vacuum vessel, and 46 is a gas introduction system. 47 is a gas exhaust system, 48 is a substrate for plasma processing, and 49 is a substrate mounting portion.
[0009]
In this plasma processing apparatus, microwave power is supplied from a coaxial transmission line 41 to a circular microwave radiation plate 42 having slits 43 arranged concentrically.
[0010]
In this plasma processing apparatus, the microwave introduced from the coaxial transmission path 41 toward the center of the circular microwave radiation plate 42 is propagated in the radial direction of the circular microwave radiation plate 42 and provided on the circular microwave radiation plate 42. The uniform plasma is generated in the vacuum container 45 by radiating from the slit 43.
[0011]
<< Conventional Second Plasma Processing Apparatus >>
FIG. 5A is a top view of the second plasma processing apparatus, and FIG. 5B is a cross-sectional view.
[0012]
This conventional second plasma processing apparatus is described in Japanese Patent No. 2857090.
[0013]
Reference numeral 51 denotes a rectangular waveguide, 52 denotes two slits provided in the rectangular waveguide 51, 53 denotes a waveguide antenna formed by arranging the slit 52 in the rectangular waveguide 51, 60 denotes a microwave source, and 54 denotes a dielectric. Electromagnetic radiation window made of a body, 55 is a vacuum vessel, 56 is a gas introduction system, 57 is a gas exhaust system, 58 is a substrate for plasma processing, 59 is a substrate mounting portion, and 82 is a reflection surface (short circuit) of the rectangular waveguide 51. Reference numeral 83 denotes an H plane (plane perpendicular to the electric field direction of the microwave) of the rectangular waveguide 51.
[0014]
This plasma processing apparatus supplies microwave power through an electromagnetic wave radiation window 54 from a waveguide antenna 53 including a rectangular waveguide 51 in which a slit 52 is arranged in a part of an H surface 83 of the rectangular waveguide 51. As a result, plasma is generated in the vacuum chamber 55.
[0015]
In this plasma processing apparatus, the width (opening area) of the two slits 52 provided on the H surface 83 of the rectangular waveguide 51 is changed in consideration of the reflection of microwaves on the reflecting surface 90 of the rectangular waveguide 51. Thus, the radiated power from the slit 52 of the microwave is to be made uniform. In FIG. 5A, the change in the width of the slit 52 is not shown, but as described in the publication, for example, the slit 52 is a reflection surface of the rectangular waveguide 51. It has a shape that changes into a stepped shape or a tapered shape so as to narrow toward 82.
[0016]
Thereby, if the generated plasma is sufficiently diffused, it becomes possible to generate a relatively uniform plasma by the microwave power radiated from the two slits 52.
[0017]
Recently, in plasma processing apparatuses used for manufacturing semiconductor devices and liquid crystal display devices, the size of the devices has been increased with the increase in the substrate size. An apparatus for processing the substrate is required. This is about 10 times the area of a 300 mm diameter substrate used for manufacturing a semiconductor device.
[0018]
Furthermore, a reactive gas such as monosilane gas, oxygen gas, hydrogen gas, and chlorine gas is used as the source gas for the plasma treatment. Many negative ions (O ⁻ , H ⁻ , Cl ^{− and the} like) exist in the plasma of these gases, and a manufacturing facility and a manufacturing method that take these behaviors into consideration are demanded.
[0019]
<< Conventional Third Plasma Processing Apparatus >>
6A is a top view of the third plasma processing apparatus, FIG. 6B is a cross-sectional view, and FIG. 7 is an enlarged view of the main part of FIG. 6A showing another example (FIG. 6A). And FIG. 7 differ in the number of slits).
[0020]
61 is a rectangular waveguide, 62a, 62b, 62c, and 62d (62a to 62e in FIG. 7) are slits provided in the rectangular waveguide 61, and 62 is a slit 62a to 62d (62a to 62d in the rectangular waveguide 61). 62e) is a waveguide antenna, 70 is a microwave source, 64 is an electromagnetic wave radiation window made of a dielectric, 65 is a vacuum vessel, 66 is a gas introduction system, 67 is a gas exhaust system, and 68 is a substrate for plasma processing. 69 is a substrate mounting portion, and 81 is a microwave propagation direction.
[0021]
[Problems to be solved by the invention]
However, the above conventional first to third plasma processing apparatuses have the following problems.
[0022]
<< Problem of Conventional First Plasma Processing Apparatus >>
When the microwave is propagated through conductors such as the coaxial transmission line 41 and the circular microwave radiation plate 42 as in the conventional first plasma processing apparatus shown in FIG. 4, the copper loss in these conductors, etc. Propagation loss occurs. This propagation loss becomes a serious problem as the frequency increases and the coaxial transmission distance and radiation plate area increase. Therefore, in the case of a large-sized device corresponding to a very large substrate such as a liquid crystal display device, the attenuation of microwaves is large and it is difficult to generate plasma efficiently.
[0023]
The plasma processing apparatus that radiates microwaves from the circular microwave radiation plate 42 is suitable for processing a circular substrate such as a semiconductor device, but is suitable for processing a rectangular substrate such as a liquid crystal display device. In this case, there is also a problem that the plasma becomes nonuniform at the corners of the substrate.
[0024]
Therefore, the conventional first plasma processing apparatus has a problem that it is difficult to process a large area substrate, particularly a square substrate.
[0025]
<< Problems of Conventional Second Plasma Processing Apparatus >>
In the case of a system in which the microwave propagated through the rectangular waveguide 51 is radiated from the two slits, that is, the waveguide antenna 53, as in the conventional second plasma processing apparatus shown in FIG. The propagation loss can be kept low. However, in the case of a reactive plasma in which many negative ions are present in the generated plasma, the ambipolar diffusion coefficient of the plasma becomes small, and therefore the problem is that the plasma is biased near the slit where microwaves are emitted. There is. This problem becomes even more serious when the plasma pressure is high. Therefore, there is a problem that it is difficult to perform a plasma treatment using a gas containing oxygen, hydrogen, chlorine, or the like that easily generates negative ions over a large area, particularly when the pressure is high.
[0026]
<< Problem of Conventional Third Plasma Processing Apparatus >>
In order to solve the above-mentioned problems in the conventional first and second plasma processing apparatuses, a plurality of rectangular waveguides 61 are provided for the rectangular waveguide 61 as in the conventional third plasma processing apparatus shown in FIGS. A plasma processing apparatus having a multi-slit waveguide antenna 63 in which slits 62a to 62d (62a to 62e in FIG. 7) are arranged has also been proposed. However, since these slits 62a to 62d cause a radiation loss with respect to the propagation of microwaves, the radiation due to the positions of the slits 62a to 62d increases as the length of the microwave propagation direction 81 of the rectangular waveguide 61 increases. The electromagnetic field intensity changes. Therefore, as shown in FIGS. 6A and 7, the length of the slits 62 a to 62 d (62 a to 62 e in FIG. 7) is changed along the propagation direction 81 of the microwave to thereby increase the electromagnetic field strength. In this case, since the opening areas of the slits 62a to 62d (62a to 62e) vary greatly depending on their positions, plasma generated in the vicinity of the slits 62a to 62d (62a to 62e). There is a problem that non-uniformity increases.
[0027]
An object of the present invention is to solve the above-mentioned problems and to provide a plasma processing apparatus , a processing apparatus and a processing method capable of generating highly uniform plasma even when processing a large area substrate.
[0028]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention adopts a configuration as described in the claims.
[0029]
In other words, the plasma processing apparatus according to claim 1 comprises a waveguide antenna composed of a plurality of slits arranged along the propagation direction of the electromagnetic wave, and an electromagnetic wave radiation window made of a dielectric, and the waveguide In the plasma processing apparatus for generating plasma by the electromagnetic wave radiated from the antenna through the electromagnetic wave radiation window, the angle of the slit with respect to the propagation direction is changed stepwise.
[0030]
The plasma processing apparatus according to claim 2 is characterized in that, in the plasma processing apparatus according to claim 1, the angle of the slit at the position where the electromagnetic field intensity is the smallest is 90 °.
[0031]
According to a third aspect of the present invention, there is provided a processing apparatus comprising: a transmitter that outputs a microwave; a waveguide that propagates the microwave from the transmitter; and a plurality of the waveguides that are provided in the microwave propagation direction of the waveguide. And a vacuum vessel provided so that microwaves are emitted from each of the slits through an electromagnetic wave emission window, and the plurality of slits are provided in the propagation direction of the microwaves. The angle with respect to the direction is provided in a stepwise manner.
According to a fourth aspect of the present invention, there is provided a processing apparatus comprising: a transmitter that outputs microwaves; a waveguide that propagates microwaves from the transmitter; and a plurality of waveguides provided in the microwave propagation direction of the waveguides. And a vacuum vessel provided so that microwaves are radiated from each of these slits through an electromagnetic wave radiation window, and the plurality of slits have tips where the microwaves of the waveguide are the weakest An angle with respect to the propagation direction of the microwave is provided so that the radiation efficiency of the microwave from the slit becomes the best, and the angle of the slit with respect to the propagation direction changes stepwise .
According to a fifth aspect of the present invention, there is provided a processing apparatus comprising: a transmitter that outputs a microwave; a waveguide that propagates the microwave from the transmitter; and a plurality of waveguides that are provided in the microwave propagation direction of the waveguide. And a vacuum vessel provided so that microwaves are radiated from each of these slits through an electromagnetic wave radiation window, and the power and power density radiated from each of the slits are substantially constant, The plurality of slits are provided so that the electromagnetic field distribution in the vicinity of each slit is substantially equal , and the angle of the slit with respect to the propagation direction changes stepwise .
A processing apparatus according to claim 6 is the processing apparatus according to any one of claims 1 to 3, wherein the opening area of each slit is substantially constant.
The processing apparatus according to claim 7 is the processing apparatus according to any one of claims 1 to 4, wherein the plurality of slits gradually step from 45 degrees to 90 degrees with respect to a propagation direction of the microwave. It is provided so that it may increase in number.
The processing method according to claim 8 is provided with an electromagnetic wave propagation step of propagating an electromagnetic wave from an electromagnetic wave source in the waveguide, and an angle with respect to the propagation direction of the electromagnetic wave is changed stepwise in the waveguide. The method includes a step of radiating an electromagnetic wave having a uniform electromagnetic field intensity from a plurality of slits, and a step of radiating the uniform electromagnetic wave into a vacuum vessel to generate plasma in the vacuum vessel.
In the plasma processing apparatus according to claims 1 and 2, it is possible to generate highly uniform plasma with the above-described configuration, respectively, and the processing apparatus according to claims 3 to 7 and the processing method according to claim 8. Then, it becomes possible to perform a highly uniform process by the above structures respectively.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.
[0033]
FIG. 1A is a top view of a plasma processing apparatus according to an embodiment of the present invention, and FIG.
[0034]
1 is a rectangular waveguide, 2a, 2b, 2c and 2d are slits provided in the rectangular waveguide 1, 3 is a waveguide antenna formed by arranging slits 2a to 2d in the rectangular waveguide 1, and 10 is a micro Wave source 4 is an electromagnetic wave radiation window made of a dielectric, 5 is a vacuum vessel, 6 is a gas introduction system, 7 is a gas exhaust system, 8 is a substrate for plasma processing, 9 is a substrate mounting portion, and 11 is a microwave propagation direction. (Traveling direction), L is the length of the rectangular waveguide 1, and θ is the angle of the slits 2 a to 2 d with respect to the microwave propagation direction 11.
[0035]
A gas introduction system 6 for introducing the raw material gas and a gas exhaust system 7 for exhausting the introduced gas are connected to the vacuum vessel 5 in which plasma is generated. Microwaves oscillated by an oscillator, which is a microwave source 10, are transmitted through the rectangular waveguide 1 in the microwave propagation direction 11, and pass through the slits 2 a to 2 d constituting the waveguide antenna 3. It is radiated into the vacuum vessel 5 through the electromagnetic wave radiation window 4 made of a body material.
[0036]
In the present embodiment, the wavelength of the microwave propagating in the rectangular waveguide 1 as the waveguide antenna 3 on the H surface (see 83 in FIG. 5) of the rectangular waveguide 1 having a length L of 32 cm. Are provided with slits 2a to 2d for microwave radiation, and the angle (rotation angle) θ formed by these slits 2a to 2d with respect to the propagation direction 11 of the microwave is shown in FIG. As shown, there is a gradual change (gradual increase). The angle θ of each of the slits 2a to 2d is set so that the electromagnetic field intensity is substantially equal between the slits 2a to 2d. In the present embodiment, the microwave tends to have a weakened electromagnetic field intensity as it propagates through the rectangular waveguide 1 due to radiation loss in the slits 2a to 2d. Therefore, the angle θ of the slit 2d located at the tip of the rectangular waveguide 1 where the electromagnetic field strength of the microwave is the weakest (the right end of FIG. 1A) is as shown in FIG. In addition, by making the radiation efficiency of the electromagnetic wave the highest at 90 ° and decreasing the angle θ of the slit as the electromagnetic field intensity becomes stronger (as it goes in the direction opposite to the propagation direction 11 of the microwave), each slit 2a˜ The electromagnetic field intensity radiated from 2d was adjusted to be constant.
[0037]
Thereby, in this Embodiment, since the opening area of each slit 2a-2d can be kept substantially constant (all the opening areas of the slits 2a-2d are the same), since the emitted electromagnetic field intensity can be controlled to be constant, Plasma unevenness in the vicinity of the slits 2a to 2d was reduced, and highly uniform plasma was obtained.
[0038]
FIG. 2 is a view showing another example of the waveguide antenna 3 of the present embodiment different from FIG. In FIG. 1A, four slits 2a to 2d are arranged, but in FIG. 2, five slits 2a to 2e are arranged.
[0039]
More specifically, in the rectangular waveguide 1 having a length L of 40 cm, five slits 2 a to 2 e for microwave radiation are ½ of the wavelength of the microwave propagating in the rectangular waveguide 1. Each is provided at an interval of about 8 cm. In addition, the angle θ between the microwave propagation direction 11 and the slits 2a to 2e is adjusted so that the electromagnetic field intensity of the microwaves radiated from the slits 2a to 2e is uniform. The opening area of 2e is almost constant. Thereby, not only the electric power radiated | emitted from each slit 2a-2e but the electric power density of each slit 2a-2e can be made substantially constant. For this reason, the electromagnetic field distribution in the vicinity of each of the slits 2a to 2e becomes substantially equal for each of the slits 2a to 2e, and energy supply to the plasma can be performed uniformly over a large area. As a result, plasma with high uniformity can be generated even in plasma that is likely to generate high pressure and negative ions.
[0040]
FIG. 3 is a diagram showing the relationship between the slit angle θ and the electric field strength. From the figure, it can be seen that the electric field strength increases as the slit angle θ increases.
[0041]
In the case of the present embodiment shown in FIGS. 1A and 2, the angle θ formed by the slits 2a to 2e with respect to the microwave propagation direction 11 is 4 so that the angle gradually increases from 45 ° to 90 °. The angle formed by the slits 2d or 2e is changed stepwise or stepwise so that the microwave radiation efficiency in the slit 2e at the tip where the microwave in the rectangular waveguide 1 becomes weakest is the highest. θ is 90 °.
[0042]
Note that the angle θ formed by the slit is not limited to the angle and arrangement of the present embodiment, and the angle θ can be changed between 0 ° and 90 ° according to the microwave intensity in the waveguide. That's fine.
[0043]
The angle θ formed with respect to the microwave propagation direction 11 of the slit 2d or 2e corresponding to the position where the electromagnetic field intensity is the smallest in the rectangular waveguide 1 is 90 °. However, the present invention is not limited to this.
[0044]
In this embodiment, a rectangular shape is used as the shape of the slits 2a to 2e. However, the shape is not limited to this, and a long shape having a rounded corner, an ellipse, a rhombus, a trapezoid, or the like is used. Also good.
[0045]
As described above, in the plasma processing apparatus of the present embodiment, a waveguide composed of a plurality of slits 2a to 2d (2a to 2e in FIG. 2) arranged along the propagation direction 11 of microwaves (electromagnetic waves). In a plasma processing apparatus that includes an antenna 3 and an electromagnetic wave radiation window 4 made of a dielectric, and generates plasma by electromagnetic waves radiated from the waveguide antenna 3 through the electromagnetic wave radiation window 4, slits 2a to 2d (2a to 2e) ) With respect to the microwave propagation direction 11 changes stepwise. Further, the angle θ of the slit 2d (2e in FIG. 2) at the position where the electromagnetic field intensity is the smallest is 90 °.
[0046]
In the plasma processing apparatus of the present embodiment, a waveguide is used for microwave transmission, and microwave power is radiated into the plasma from slits 2a to 2d (2a to 2e) provided in the waveguide. It becomes possible to radiate high-power microwaves efficiently. The waveguide antenna 3 includes a plurality of slits 2a to 2d (2a to 2e) along the microwave propagation direction 11, and these slits 2a to 2d (2a to 2e) are directions of microwave propagation. Since the angle θ made with respect to 11 changes stepwise, the aperture ratio (opening area) of the slits 2a to 2d (2a to 2e) can be changed even when the rectangular waveguide 1 becomes larger (longer). In addition, uniform electromagnetic waves can be emitted from the slits 2a to 2d (2a to 2e). Thereby, even in a plasma processing apparatus that processes a large-area substrate, it is possible to generate plasma with extremely high uniformity. This is a very useful device for manufacturing a liquid crystal display device whose substrate area increases year by year.
[0047]
Although the present invention has been specifically described above based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various modifications can be made without departing from the scope of the invention. For example, the structure of the plasma processing apparatus shown in FIG. 1 to which the present invention is applied is merely an example structure, and is not limited to this.
[0048]
【The invention's effect】
As described above, according to the present invention, a plasma processing apparatus capable of generating a highly uniform plasma even on a large area substrate, and a processing apparatus and a process capable of performing a highly uniform process. A method can be provided.
[Brief description of the drawings]
FIG. 1A is a top view of a plasma processing apparatus according to an embodiment of the present invention, and FIG.
FIG. 2 is a diagram showing another example of the waveguide antenna according to the present embodiment.
FIG. 3 is a diagram illustrating a relationship between a slit angle θ and an electric field intensity.
4A is a top view of a plasma processing apparatus of a first conventional example, and FIG. 4B is a cross-sectional view.
5A is a top view of a second conventional plasma processing apparatus, and FIG. 5B is a cross-sectional view.
6A is a top view of a plasma processing apparatus of a third conventional example, and FIG. 6B is a cross-sectional view.
FIG. 7 is a diagram showing another example of a third conventional waveguide antenna.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Rectangular waveguide, 2a, 2b, 2c, 2d, 2e ... Slit, 3 ... Waveguide antenna, 4 ... Electromagnetic radiation window, 5 ... Vacuum container, 6 ... Gas introduction system, 7 ... Gas exhaust system, 8 DESCRIPTION OF SYMBOLS ... Board | substrate, 9 ... Board | substrate mounting part, 10 ... Microwave source, 11 ... Microwave propagation direction, L ... Length of rectangular waveguide, (theta) ... Angle with respect to microwave propagation direction,
DESCRIPTION OF SYMBOLS 41 ... Coaxial transmission path, 42 ... Circular microwave radiation plate, 43 ... Slit, 44 ... Electromagnetic radiation window, 45 ... Vacuum container, 46 ... Gas introduction system, 47 ... Gas exhaust system, 48 ... Substrate, 49 ... Substrate mounting Part,
DESCRIPTION OF SYMBOLS 51 ... Rectangular waveguide, 52 ... Slit, 53 ... Waveguide antenna, 54 ... Electromagnetic radiation window, 55 ... Vacuum container, 56 ... Gas introduction system, 57 ... Gas exhaust system, 58 ... Substrate, 59 ... Substrate mounting 60, a microwave source,
61 ... Rectangular waveguide, 62a, 62b, 62c, 62d, 62e ... Slit, 63 ... Waveguide antenna, 64 ... Electromagnetic radiation window, 65 ... Vacuum container, 66 ... Gas introduction system, 67 ... Gas exhaust system, 68 DESCRIPTION OF SYMBOLS ... Board | substrate, 69 ... Board | substrate mounting part, 70 ... Microwave source, 81 ... Microwave propagation direction, 82 ... Reflecting surface (short-circuit surface, R surface), 83 ... H surface.

Claims (8)

A waveguide antenna having a plurality of slits arranged along the propagation direction of the electromagnetic wave;
An electromagnetic radiation window made of a dielectric,
In the plasma processing apparatus for generating plasma by the electromagnetic wave radiated from the waveguide antenna through the electromagnetic wave radiation window,
The plasma processing apparatus, wherein an angle of the slit with respect to the propagation direction changes stepwise.   The plasma processing apparatus according to claim 1, wherein the angle of the slit at a position where the electromagnetic field intensity is the smallest is 90 °. A transmitter that outputs microwaves;
A waveguide that propagates microwaves from the transmitter;
A plurality of slits provided in the microwave propagation direction of the waveguide;
A vacuum vessel provided so that microwaves are radiated from each of these slits through an electromagnetic wave radiation window;
Comprising
The processing apparatus, wherein the plurality of slits are provided in a propagation direction of the microwave, and an angle with respect to the propagation direction is changed stepwise. A transmitter that outputs microwaves;
A waveguide that propagates microwaves from the transmitter;
A plurality of slits provided in the microwave propagation direction of the waveguide;
A vacuum vessel provided so that microwaves are radiated from each of these slits through an electromagnetic wave radiation window,
The plurality of slits are provided with an angle with respect to the propagation direction of the microwave so that the radiation efficiency of the microwave from the slit at the tip where the microwave of the waveguide becomes the weakest is the best , A processing apparatus characterized in that the angle changes stepwise . A transmitter that outputs microwaves;
A waveguide that propagates microwaves from the transmitter;
A plurality of slits provided in the microwave propagation direction of the waveguide;
A vacuum vessel provided so that microwaves are emitted from each of these slits through an electromagnetic wave radiation window,
The plurality of slits are provided such that the power and power density radiated from each slit are substantially constant, and the electromagnetic field distribution in the vicinity of each slit is substantially equal , and the angle of the slit with respect to the propagation direction is stepwise. The processing apparatus characterized by having changed into . The processing apparatus according to claim 1, wherein an opening area of each slit is substantially constant. 5. The process according to claim 1, wherein the plurality of slits are provided so as to gradually increase from 45 degrees to 90 degrees with respect to a propagation direction of the microwave. apparatus. An electromagnetic wave propagation process for propagating electromagnetic waves from an electromagnetic wave source in the waveguide;
A plurality of the waveguides provided with the angle with respect to the propagation direction of the electromagnetic wave changed stepwise. Radiating electromagnetic waves with uniform electromagnetic field intensity from the slit;
Radiating the uniform electromagnetic wave into the vacuum vessel to generate plasma in the vacuum vessel;
The processing method characterized by comprising.

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