JP7604872B2 - Electrostatic Chuck Device - Google Patents

Description

The present invention relates to an electrostatic chuck device.

In semiconductor manufacturing processes, electrostatic chuck devices are used to hold plate-shaped samples in a vacuum environment and maintain the plate-shaped samples at a desired temperature. For example, Patent Document 1 discloses a structure in which a thin plate-shaped heater element is disposed between an electrostatic chuck section and a base section. Such electrostatic chuck devices can heat areas that are relatively cold within the surface of the plate-shaped sample, thereby reducing the temperature difference that can occur within the surface of the plate-shaped sample.

Patent No. 5163349

In conventional electrostatic chuck devices, if the thickness of the adhesive layer that secures the heater element is not uniform, there is a risk that the amount of heat transferred from the heater element to the plate-shaped sample will be uneven at each part.

The present invention was made in consideration of the above circumstances, and aims to provide an electrostatic chuck device that transfers heat from a heater element to a plate-shaped sample uniformly across the surface.

In order to solve the above problems, one aspect of the present invention is an electrostatic chuck device comprising an electrostatic chuck section that electrostatically attracts the plate-shaped sample, a base section that supports the electrostatic chuck section, and a heater section that is disposed between the electrostatic chuck section and the base section, the heater section comprising a heat generating layer, a first resin layer that is disposed on one side of the heat generating layer in the thickness direction, a second resin layer that is disposed on the other side of the heat generating layer in the thickness direction, a first adhesive layer that bonds the heat generating layer and the first resin layer, and a second adhesive layer that bonds the heat generating layer and the second resin layer, the heat generating layer having a band-shaped heater element that generates heat when a current flows therethrough, and a filling adhesive section that is filled between the heater elements, and the first resin layer, the second resin layer, the first adhesive layer, and the second adhesive layer are made of sheet materials.

In one aspect of the present invention, the filling adhesive portion may be made of a fluid adhesive.

In one aspect of the present invention, the first resin layer and the second resin layer may be configured to have amorphous fillers dispersed therein.

In one aspect of the present invention, the materials constituting the first resin layer and the second resin layer may be of the same composition, and the materials constituting the first adhesive layer, the second adhesive layer, and the filling adhesive portion may be of the same composition.

In one aspect of the present invention, the elastic modulus of the first resin layer and the second resin layer may be greater than the elastic modulus of the first adhesive layer, the second adhesive layer, and the filled adhesive portion.

In one aspect of the present invention, the side surface of the heater element may be a concave curved surface.

The present invention provides an electrostatic chuck device that ensures that the heat transferred from the heater element to the plate-shaped sample is uniform across the surface.

FIG. 1 is a cross-sectional view of an electrostatic chuck device according to one embodiment. FIG. 2 is a schematic cross-sectional view of a heater portion of an electrostatic chuck device according to an embodiment.

The electrostatic chuck device 1 according to the present invention will be described below with reference to the drawings. Note that the drawings used in the following description show enlarged views of characteristic parts for the sake of convenience, and the dimensional ratios of each component may differ from the actual ones. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited to them, and may be modified as appropriate without changing the gist of the invention.

FIG. 1 is a cross-sectional view of an electrostatic chuck device 1 according to the present embodiment.
The electrostatic chuck device 1 comprises an electrostatic chuck portion 2 that electrostatically adsorbs a plate-shaped sample W, a temperature adjustment base portion (base portion) 3 that supports the electrostatic chuck portion 2, and a heater portion 8 arranged between the electrostatic chuck portion 2 and the temperature adjustment base portion 3.

In the following explanation, each part of the electrostatic chuck device 1 will be described with the side on which the plate-shaped sample W is mounted relative to the electrostatic chuck portion 2 as the upper side, and the side on which the temperature adjustment base portion 3 is located as the lower side. However, the up-down direction here is used merely to simplify the explanation, and does not limit the position of the electrostatic chuck device 1 when in use.

The electrostatic chuck device 1 has a through hole 28 that penetrates from the temperature adjustment base part 3 to the electrostatic chuck part 2 in the thickness direction. For example, a lift pin for removing a plate-shaped sample is inserted into the through hole 28. In this case, a cylindrical insulator 29 is provided on the inner periphery of the through hole 28. In addition, a cooling gas for cooling the plate-shaped sample may be supplied to the through hole 28. The cooling gas supplied to the through hole 28 is ejected from the mounting surface 11a to cool the plate-shaped sample.

The electrostatic chuck section 2 has a mounting plate 11, the upper surface of which serves as a mounting surface 11a on which a plate-shaped sample W such as a semiconductor wafer is placed, a support plate 12 which is integrated with the mounting plate 11 and supports the bottom side of the mounting plate 11, an electrostatic adsorption electrode (electrostatic adsorption internal electrode) 13 provided between the mounting plate 11 and the support plate 12, and an insulating layer 14 which insulates the periphery of the electrostatic adsorption electrode 13.

The mounting plate 11 and the support plate 12 are disk-shaped with the same shape of the overlapping surfaces and are made of an insulating ceramic sintered body that has mechanical strength and durability against corrosive gases and their plasma, such as an aluminum oxide-silicon carbide (Al ₂ O ₃ --SiC) composite sintered body, an aluminum oxide (Al ₂ O ₃ ) sintered body, an aluminum nitride (AlN) sintered body, or an yttrium oxide (Y ₂ O ₃ ) sintered body.

The mounting surface 11a of the mounting plate 11 has multiple protrusions 11b formed at predetermined intervals, each protrusion having a diameter smaller than the thickness of the plate-shaped sample, and these protrusions 11b support the plate-shaped sample W.

The overall thickness including the mounting plate 11, support plate 12, electrostatic adsorption electrode 13, and insulating material layer 14, i.e., the thickness of the electrostatic chuck portion 2, is, for example, 0.7 mm or more and 5.0 mm or less.

For example, if the thickness of the electrostatic chuck portion 2 is less than 0.7 mm, it becomes difficult to ensure the mechanical strength of the electrostatic chuck portion 2. If the thickness of the electrostatic chuck portion 2 exceeds 5.0 mm, the heat capacity of the electrostatic chuck portion 2 becomes large, the thermal responsiveness of the plate-shaped sample W placed on it deteriorates, and due to the increase in lateral heat transfer of the electrostatic chuck portion, it becomes difficult to maintain the in-plane temperature of the plate-shaped sample W at the desired temperature pattern. Note that the thicknesses of each portion described here are merely examples and are not limited to the above ranges.

The electrostatic adsorption electrode 13 is used as an electrode for an electrostatic chuck to generate an electric charge and fix the plate-shaped sample W by electrostatic adsorption force, and its shape and size are appropriately adjusted depending on the application.

The electrostatic attraction electrode 13 is preferably formed from conductive ceramics such as an aluminum oxide-tantalum carbide (Al ₂ O ₃ -Ta ₄ C ₅ ) conductive composite sintered body, an aluminum oxide-tungsten (Al ₂ O ₃ -W) conductive composite sintered body, an aluminum oxide-silicon carbide (Al ₂ O ₃ -SiC) conductive composite sintered body, an aluminum nitride-tungsten (AlN-W) conductive composite sintered body, an aluminum nitride-tantalum (AlN-Ta) conductive composite sintered body, or an yttrium oxide-molybdenum (Y ₂ O ₃ -Mo) conductive composite sintered body, or a high-melting point metal such as tungsten (W), tantalum (Ta), or molybdenum (Mo).

The thickness of the electrostatic adsorption electrode 13 is not particularly limited, but may be, for example, 0.1 μm or more and 100 μm or less, and a thickness of 5 μm or more and 20 μm or less is more preferable.

If the thickness of the electrostatic adsorption electrode 13 is less than 0.1 μm, it becomes difficult to ensure sufficient conductivity. If the thickness of the electrostatic adsorption electrode 13 exceeds 100 μm, cracks are likely to occur at the bonding interfaces between the electrostatic adsorption electrode 13 and the mounting plate 11 and support plate 12 due to the difference in thermal expansion coefficient between the electrostatic adsorption electrode 13 and the mounting plate 11 and support plate 12.

An electrostatic attraction electrode 13 of this thickness can be easily formed by a film formation method such as sputtering or vapor deposition, or a coating method such as screen printing.

The insulating layer 14 surrounds the electrostatic attraction electrode 13 to protect it from corrosive gases and their plasma, and also bonds and integrates the boundary between the mounting plate 11 and the support plate 12, i.e., the peripheral area other than the electrostatic attraction electrode 13, and is made of an insulating material that has the same composition or the same main component as the material that constitutes the mounting plate 11 and the support plate 12.

A power supply terminal 15 for applying a DC voltage to the electrostatic attraction electrode 13 is connected to the electrostatic attraction electrode 13. The power supply terminal 15 is inserted into a through hole 16 that penetrates the temperature adjustment base part 3, the heater part 8, and the support plate 12 in the thickness direction. An insulating insulator 15a is provided on the outer periphery of the power supply terminal 15, and the power supply terminal 15 is insulated from the metal temperature adjustment base part 3 by this insulator 15a.

The material of the portion (extraction electrode) of the power supply terminal 15 that is connected to the electrostatic attraction electrode 13 and inserted into the support plate 12 is not particularly limited as long as it is a conductive material with excellent heat resistance, but it is preferable that the thermal expansion coefficient is close to that of the electrostatic attraction electrode 13 and the support plate 12. For example, it is made of a conductive ceramic material such as Al ₂ O ₃ —TaC.

The portion of the power supply terminal 15 that is inserted into the temperature adjustment base 3 is made of a metal material such as tungsten (W), tantalum (Ta), molybdenum (Mo), niobium (Nb), or Kovar alloy.

The temperature adjustment base 3 is a thick, disk-shaped part for adjusting the electrostatic chuck 2 to a desired temperature. A suitable example of the temperature adjustment base 3 is a water-cooled base with a flow path 3A formed therein for circulating water.

The material constituting the temperature adjustment base part 3 is not particularly limited as long as it is a metal with excellent thermal conductivity, electrical conductivity, and workability, or a composite material containing such a metal, and for example, aluminum (Al), an aluminum alloy, copper (Cu), a copper alloy, stainless steel (SUS), etc. are preferably used. At least the surface of the temperature adjustment base part 3 that is exposed to the plasma is preferably anodized or has an insulating film such as alumina formed thereon.

FIG. 2 is a schematic cross-sectional view of the heater portion 8. As shown in FIG.
The heater section 8 includes a heat generating layer 85 , a first resin layer 81 , a second resin layer 82 , a first adhesive layer 83 , and a second adhesive layer 84 .

The first resin layer 81 is disposed on one side (upper side) of the heat generating layer 85 in the thickness direction. The second resin layer 82 is disposed on the other side (lower side) of the heat generating layer 85 in the thickness direction. The first adhesive layer 83 is disposed between the heat generating layer 85 and the first resin layer 81, and bonds the heat generating layer 85 and the first resin layer 81 together. The second adhesive layer 84 is disposed between the heat generating layer 85 and the second resin layer 82, and bonds the heat generating layer 85 and the second resin layer 82 together.

Although not shown, it is preferable that an adhesive layer be provided between the first resin layer 81 and the electrostatic chuck portion 2 to bond them to each other. Similarly, it is preferable that an adhesive layer be provided between the second resin layer 82 and the temperature adjustment base portion 3 to bond them to each other.
Each layer of the heater section 8 will now be described in detail.

The heat generating layer 85 has a band-shaped heater element 5 and a filling adhesive portion 86. The thickness of the heat generating layer 85 is, for example, about 30 μm to 60 μm. The thickness of the heater element 5 and the filling adhesive portion 86 is the same as the thickness of the heat generating layer 85.

The heater element 5 is composed of a strip-shaped resistance heating member. The heater element 5 is manufactured by processing a non-magnetic metal sheet, such as an Inconel (registered trademark) sheet, a titanium sheet, a tungsten (W) sheet, or a molybdenum (Mo) sheet, into a desired heater shape, for example, by processing the overall outline of a meandering strip-shaped conductive sheet into a ring shape using photolithography or laser processing. The heater element 5 is arranged in a meandering manner within the heat generating layer 85. The heater element 5 generates heat when a current flows through it.

Power supply terminals 17 are connected to both ends of the heater element 5. The power supply terminals 17 pass current through the heater element 5. A cylindrical insulating insulator 18 is attached to the outer periphery of the power supply terminal 17, insulating the temperature adjustment base 3 from the power supply terminal 17.

The side surface 5a of the heater element 5 is a concave curved surface. Here, the side surface 5a of the heater element 5 refers to the surface facing both sides in the width direction of the heater element 5.

The filling adhesive portion 86 is filled between the heater elements 5 that extend in a serpentine manner. The filling adhesive portion 86 has fluidity in an uncured state. In other words, the filling adhesive portion 86 is made of a fluid adhesive. The filling adhesive portion 86 holds the heater elements 5 when it hardens. In addition, the filling adhesive portion 86 contributes to uniform heating of the heating layer 85 by filling the gaps between the heater elements 5.

The filled adhesive portion 86 contacts the side surface 5a of the heater element 5. As described above, since the side surface 5a is a concave curved surface, the filled adhesive portion 86 restricts the movement of the heater element 5 in the thickness direction. The heater element 5 is made of a metal material, and the filled adhesive portion 86 is made of a resin material. Therefore, there is a large difference between the thermal expansion coefficient of the heater element 5 and the thermal expansion coefficient of the filled resin portion 86. According to this embodiment, since the side surface 5a of the heater element 5 is a concave curved surface, it is possible to prevent the heater element 5 and the filled adhesive portion 86 from peeling off from each other even if stress occurs between them due to the difference in thermal expansion coefficient.

The first resin layer 81 and the second resin layer 82 sandwich the heat generating layer 85 from both sides in the thickness direction. As a result, the first resin layer 81 and the second resin layer 82 increase the withstand voltage of the heater section 8.

The first resin layer 81 and the second resin layer 82 each have a thickness of about 40 μm. The first resin layer 81 and the second resin layer 82 are made of an insulating resin material. The first resin layer 81 and the second resin layer 82 are made of an adhesive resin material that has heat resistance and insulating properties, such as polyimide resin, silicone resin, and epoxy resin.

The first resin layer 81 and the second resin layer 82 have irregular filler 7 dispersed therein. As the filler 7, irregular particles are preferably used, and calcium phosphate, aluminum nitride, silicon oxide, aluminum oxide, YAG, SmAlO ₃ , etc. are suitable, and one or more of these can be selected. Here, irregular means that the shape and size of the dispersed filler 7 are not uniform. Each filler 7 has a volume equivalent to a diameter of 1 μm to 5 μm or less when converted into a sphere. In addition, each filler 7 has an aspect ratio of about 1.5 to 2. The filler 7 is preferably contained in the first resin layer 81 and the second resin layer 82 in an amount of 0.01 vol % or more and 5.0 vol % or less, and more preferably in an amount of 0.01 vol % or more and 3.0 vol % or less.

The first resin layer 81 and the second resin layer 82 contain the filler 7, and thus have a higher thermal conductivity than when the filler 7 is not contained. According to this embodiment, the increased thermal conductivity of the first resin layer 81 allows the heat of the heat generating layer 85 to be efficiently transferred to the plate-shaped sample W via the electrostatic chuck portion 2. In addition, the increased thermal conductivity of the first resin layer 81 allows the responsiveness of the heating of the plate-shaped sample W due to the heat generated by the heater element 5 to be improved. Furthermore, the increased thermal conductivity of the first resin layer 81 and the second resin layer 82 allows the cooling efficiency of the plate-shaped sample W by the temperature adjustment base portion 3 to be improved.

In the first resin layer 81 and the second resin layer 82, it is preferable to align the long axis direction of the filler 7 along a direction perpendicular to the thickness direction of each layer (hereinafter referred to as the surface direction). This improves the heat transfer along the surface direction of the first resin layer 81 and the second resin layer 82, and can improve the thermal uniformity in the surface direction of the first resin layer 81 and the second resin layer 82. As a result, this can contribute to improving the thermal uniformity of the plate-shaped sample W mounted on the electrostatic chuck portion 2.

Some of the amorphous filler 7 contained in the first resin layer 81 is exposed at the interface with the first adhesive layer 83. This forms fine irregularities on the surface of the first resin layer 81, and the anchor effect acts to increase the bonding strength between the first resin layer 81 and the first adhesive layer 83. Similarly, since the second resin layer 82 contains the filler 7, fine irregularities are formed between the second resin layer 82 and the second adhesive layer 84, and the bonding strength with the second adhesive layer 84 is increased.

The first resin layer 81 and the second resin layer 82 are made of sheet material. Therefore, the first resin layer 81 and the second resin layer 82 have a uniform thickness throughout. The first resin layer 81 is formed in a sheet shape in advance, and is laminated and assembled on the first adhesive layer 83, which is the lower layer. Similarly, the second resin layer 82 is formed in a sheet shape in advance, and is laminated and assembled on the temperature adjustment base part 3, which is the lower layer.

The first adhesive layer 83 is provided to adhere the heat generating layer 85 including the heater element 5 to the first resin layer 81. Similarly, the second adhesive layer 84 is provided to adhere the heat generating layer 85 to the second resin layer 82. The thickness of the first adhesive layer 83 and the second adhesive layer 84 is about 10 μm. The first adhesive layer 83 and the second adhesive layer 84 are made of an adhesive resin material having heat resistance and insulating properties, such as fluororesin, polyimide resin, silicone resin, epoxy resin, etc.

The first adhesive layer 83 and the second adhesive layer 84 do not contain filler. That is, the heater section 8 has adhesive layers (the first adhesive layer 83 and the second adhesive layer 84) that do not contain filler between the heat generating layer 85 and the first resin layer 81, and between the heat generating layer 85 and the second resin layer 82. Since the first adhesive layer 83 and the second adhesive layer 84 do not contain filler, they are more flexible than when they contain filler. Therefore, the first adhesive layer 83 can follow the unevenness and undulations of the surfaces of the heater element 5 and the first resin layer 81 and uniformly bond them. Similarly, the second adhesive layer 84 can follow the unevenness and undulations of the surfaces of the heater element 5 and the second resin layer 82 and uniformly bond them. This can suppress the decrease in thermal conductivity and the unevenness of thermal conductivity caused by uneven bonding.

In this embodiment, not only the first adhesive layer 83 and the second adhesive layer 84, but also the filled adhesive portion 86 do not contain a filler and are more flexible than the first resin layer 81 and the second resin layer 82. That is, the elastic modulus of the first resin layer 81 and the second resin layer 82 is greater than the elastic modulus of the first adhesive layer 83, the second adhesive layer 84, and the filled adhesive portion 86. The filled adhesive portion 86 is made of an adhesive resin material having heat resistance and insulating properties, such as, for example, fluororesin, polyimide resin, silicone resin, or epoxy resin.

According to this embodiment, the first adhesive layer 83, the second adhesive layer 84, and the filled adhesive portion 86 have a low elastic modulus and are flexible, so that the thermal stress between the first resin layer 81 and the second resin layer 82 and the heater element 5 can be sufficiently alleviated. This makes it possible to suppress deformation (warping) of the heater portion 8 caused by thermal stress.

The first adhesive layer 83 and the second adhesive layer 84 are made of a sheet material. That is, the first adhesive layer 83 and the second adhesive layer 84 are sheet-like adhesives. Therefore, the first adhesive layer 83 and the second adhesive layer 84 have a uniform thickness throughout. The first adhesive layer 83 is formed in a sheet shape in advance, and is laminated and attached onto the heating layer 85, which is the lower layer. Similarly, the second adhesive layer 84 is formed in a sheet shape in advance, and is laminated and attached onto the second resin layer 82, which is the lower layer.

The first adhesive layer 83 and the second adhesive layer 84 are bonded by heat pressing while being laminated together with the first resin layer 81, the heat generating layer 85, and the second resin layer 82. This allows each layer to be bonded to one another while maintaining the uniformity of the thicknesses of the first resin layer 81, the second resin layer 82, the first adhesive layer 83, and the second adhesive layer 84.

According to this embodiment, the first resin layer 81, the second resin layer 82, the first adhesive layer 83, and the second adhesive layer 84 are made of sheet material, so that the variation in thickness is suppressed. As a result, the heat transferred from the heater element 5 to the plate-shaped sample W can be made uniform within the plane. In addition, when the plate-shaped sample W is cooled in the temperature adjustment base part 3, the heat transferred from the plate-shaped sample W to the temperature adjustment base part 3 can be made uniform within the plane, and the temperature distribution of the plate-shaped sample W can be made uniform.

In this embodiment, the materials constituting the first resin layer 81 and the second resin layer 82 have the same composition, and the materials constituting the first adhesive layer 83, the second adhesive layer 84, and the filling adhesive portion 86 have the same composition. Therefore, in the heater portion 8, the compositions of the layers disposed on one side and the other side in the thickness direction of the heat generating layer 85 match each other. For this reason, when the heater portion 8 is bonded by heat pressing, residual thermal stress inside can be suppressed, and the durability of the heater portion 8 can be increased.

According to this embodiment, the material constituting the filling adhesive portion 86 has the same composition as the material constituting the first adhesive layer 83 and the second adhesive layer 84. Therefore, it is possible to suppress the occurrence of thermal stress at the boundary between the filling adhesive portion 86 and the first adhesive layer 83, and at the boundary between the filling adhesive portion 86 and the second adhesive layer 84, and to suppress peeling between the layers.

The above describes an embodiment of the present invention, but each configuration and their combinations in the embodiment are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible without departing from the spirit of the present invention. Furthermore, the present invention is not limited to the embodiment.

1...electrostatic chuck device, 2...electrostatic chuck part, 3...temperature adjustment base part (base part), 5...heater element, 5a...side surface, 7...filler, 8...heater part, 81...first resin layer, 82...second resin layer, 83...first adhesive layer, 84...second adhesive layer, 85...heat generating layer, 86...filling adhesive part, W...plate-shaped sample

Claims (5)

an electrostatic chuck portion that electrostatically attracts a plate-shaped sample;
A base portion supporting the electrostatic chuck portion;
a heater portion disposed between the electrostatic chuck portion and the base portion,
The heater unit includes:
A heating layer;
a first resin layer disposed on one side of the heat generating layer in a thickness direction;
a second resin layer disposed on the other side of the heat generating layer in the thickness direction;
a first adhesive layer that bonds the heat generating layer and the first resin layer;
a second adhesive layer that adheres the heat generating layer and the second resin layer,
The heat generating layer is
A strip-shaped heater element that generates heat when an electric current flows through it;
and a filling adhesive portion filled between the heater elements,
the first resin layer, the second resin layer, the first adhesive layer, and the second adhesive layer are made of sheet materials;
Amorphous fillers are dispersed in the first resin layer and the second resin layer.
Electrostatic chuck device. an electrostatic chuck portion that electrostatically attracts a plate-shaped sample;
A base portion supporting the electrostatic chuck portion;
a heater portion disposed between the electrostatic chuck portion and the base portion,
The heater unit includes:
A heating layer;
a first resin layer disposed on one side of the heat generating layer in a thickness direction;
a second resin layer disposed on the other side of the heat generating layer in the thickness direction;
a first adhesive layer that bonds the heat generating layer and the first resin layer;
a second adhesive layer that adheres the heat generating layer and the second resin layer,
The heat generating layer is
A strip-shaped heater element that generates heat when an electric current flows through it;
and a filling adhesive portion filled between the heater elements,
the first resin layer, the second resin layer, the first adhesive layer, and the second adhesive layer are made of sheet materials;
The elastic modulus of the first resin layer and the second resin layer is greater than the elastic modulus of the first adhesive layer, the second adhesive layer, and the filling adhesive portion.
Electrostatic chuck device. an electrostatic chuck portion that electrostatically attracts a plate-shaped sample;
A base portion supporting the electrostatic chuck portion;
a heater portion disposed between the electrostatic chuck portion and the base portion,
The heater unit includes:
A heating layer;
a first resin layer disposed on one side of the heat generating layer in a thickness direction;
a second resin layer disposed on the other side of the heat generating layer in the thickness direction;
a first adhesive layer that bonds the heat generating layer and the first resin layer;
a second adhesive layer that adheres the heat generating layer and the second resin layer,
The heat generating layer is
A strip-shaped heater element that generates heat when an electric current flows through it;
and a filling adhesive portion filled between the heater elements,
the first resin layer, the second resin layer, the first adhesive layer, and the second adhesive layer are made of sheet materials;
The side surface of the heater element is a concave curved surface.
Electrostatic chuck device. The filling adhesive portion is made of a fluid adhesive.
The electrostatic chuck device according to any one of claims 1 to 3 . the first resin layer and the second resin layer are made of the same material;
The materials constituting the first adhesive layer, the second adhesive layer and the filling adhesive portion have the same composition.
The electrostatic chuck device according to any one of claims 1 to 4 .

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