JP2005291275A - Differential exhaust seal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a differential exhaust seal capable of reducing cost without reducing sealing function. <P>SOLUTION: An auxiliary pump P2 of a high vacuum pump P1 for reducing pressure inside a process chamber P is used as an air exhaust pump for operating the differential exhaust seal 44 to provide a low cost configuration. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a differential exhaust seal used in a processing apparatus that performs processing in a casing maintained in a vacuum atmosphere, for example.

  In a semiconductor manufacturing apparatus or the like, a workpiece is placed on a stage and moved to process in a process chamber maintained in a vacuum or a special gas atmosphere. Here, when a positioning device including a drive source is provided in the process chamber, the hermeticity with the outside of the process chamber is maintained, so that a vacuum or a special gas atmosphere can be maintained relatively easily.

  However, when a drive device including a drive source is provided in the process chamber, the process chamber itself becomes large, and it takes a long time to set the inside of the process chamber at a predetermined pressure, or a large amount of special gas that fills the inside of the process chamber is required. Or the maintenance of the positioning device is difficult.

On the other hand, when the volume of the process chamber is minimized, the above-described problem is solved, but a configuration for driving a table on which a workpiece provided in the process chamber is mounted from the outside of the process chamber is required. . As an example of such a configuration, a rotation shaft extending between the inside of the process chamber and the outside is provided via an opening formed in the wall of the housing communicating with the process chamber, and the rotation shaft is attached to the housing. In some cases, the table or the like inside the process chamber is rotated from the outside of the process chamber by rotating it relatively (Patent Document 1).
U.S. Pat. No. 4,726,689

  By the way, in the structure of patent document 1, the external foreign material is prevented from invading into a process chamber by attracting | sucking the gas between a housing | casing and a rotating shaft from a differential exhaust port. However, in Patent Document 1, the differential exhaust port is connected to a pump different from the exhaust pump that decompresses the process chamber. For this reason, it is performed to increase the number of pumps according to the number of differential pressure chambers. However, installing a plurality of pumps increases the cost of the entire semiconductor manufacturing apparatus.

  SUMMARY OF THE INVENTION In view of the above-described problems of the prior art, an object of the present invention is to provide a differential exhaust seal that can further reduce the cost without deteriorating the sealing function.

The differential exhaust seal of the present invention is
In the differential exhaust seal that seals between the casing whose inside is reduced to a predetermined pressure by the main pump and the movable member extending from the outside to the inside through the opening of the casing,
The back pressure of the main pump that decompresses the inside of the housing is maintained at a predetermined pressure lower than the pressure outside the housing by an auxiliary pump,
In the opening of the casing or the movable member, at least one differential pressure chamber formed to face the movable member or the casing, and the outside of the casing and the differential pressure chamber with respect to the differential pressure chamber On the other hand, a throttle part is provided on each of the inner sides of the casing, and the at least one differential pressure chamber communicates with the suction port of the auxiliary pump.

  The differential exhaust seal of the present invention seals between a casing whose inside is reduced to a predetermined pressure by a main pump and a movable member extending from the outside to the inside through the opening of the casing. In the differential exhaust seal, the back pressure of the main pump that decompresses the inside of the housing is maintained at a predetermined pressure lower than the pressure outside the housing by the auxiliary pump, and the opening of the housing or the In the movable member, at least one differential pressure chamber formed facing the movable member or the casing, the outside of the casing with respect to the differential pressure chamber, and the inside of the casing with respect to the differential pressure chamber Each side has a throttle portion and the at least one differential pressure chamber communicates with the suction port of the auxiliary pump, so that a separate exhaust pump is not required to operate the differential exhaust seal. ,Than It is possible to provide a cost configuration.

Here, the main pump is a pump whose inhaled gas is in a molecular flow region. Specifically, a turbo molecular pump, an oil diffusion pump, a sputter ion pump, a sour pump, a titanium supplement pump, a non-evaporable getter pump. A high vacuum pump such as a cryopump. The auxiliary pump is a pump suction gas is a gas of the viscous flow region, an oil rotary pump, the although dry pump such as the type does not matter, in which can be secured minimum suction amount Q _H of less preferred.

  Further, the “diaphragm” does not only indicate a case where the gap is small, but also includes a case where the gap is sealed so that there is no gap by, for example, an O-ring.

  It is preferable that the differential pressure chamber communicated with the suction port of the auxiliary pump is a differential pressure chamber closest to the outside of the housing.

The throttle portion has a gap, and is a flow rate from the outermost differential exhaust seal communicated with the auxiliary pump, and a flow rate Q _H represented by the following formula gives a necessary back pressure to the main pump. from the flow Q _pump of the auxiliary pump for obtaining said to be smaller than a value obtained by subtracting the exhaust flow rate Q ₀ from the main pump, preferably diaphragm portion shape are determined.
Q _H = (a ³ b / 24 μL) · P ₀ ² [Pa · m ³ / s] (1)
However, a: Clearance [m] between the throttle part and the movable member
b: Width [m] orthogonal to the gas moving direction in the throttle portion
L: Length in the moving direction of the gas [m] in the throttle part
μ: Viscosity of the passing gas [Pa · s]
P ₀ : External pressure of the housing [Pa]

  The movable member is a rotating shaft, and it is preferable that the shaft diameter d and the length L of each of the throttle portions satisfy L / d ≦ 6.

  Furthermore, the back pressure of the main pump is preferably 10 Pa or less.

  Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a front sectional view of a rotary introducer 10 including a differential exhaust seal according to the present embodiment, and the differential exhaust seal is shown in a simplified manner. As shown in FIG. 1, a rotary introducer 10 installed under atmospheric pressure has a process chamber P inside, and a housing 20 having an opening 20a that communicates the process chamber P with the outside thereof, and a housing A rotary shaft 30 which is a movable member extending through the opening 20a of the body 20 and extending from the process chamber P to the outside, a seal unit 40 attached to the housing 20 so as to shield the opening 20a, and a process chamber It has a high vacuum pump P1 that is a main pump for depressurizing P, and an auxiliary pump P2. Note that a valve V1 is disposed in a pipe that communicates the process chamber P and the suction port of the high vacuum pump P1, and a valve V2 is disposed in a pipe that communicates the discharge port of the high vacuum pump P1 and the suction port of the auxiliary pump P2. Has been placed. The discharge port of the auxiliary pump P2 is open to the atmosphere.

  The seal unit 40 includes a cylindrical main body 41 that is fixed to the housing 20 with bolts (not shown) or the like via a sealing element such as an O-ring, and the rotary shaft 30 is rotatable with respect to the main body 41. The bearings 42 and 43 are supported by each other, and a differential exhaust seal 44 is provided.

  The differential exhaust seal 44 has a circumferential groove-shaped differential pressure chamber 44a and land portions 44b and 44c arranged on both sides in the axial direction in the vicinity of the end portion of the main body 41 on the housing 20 side. The land portions 44b and 44c are close to the surface of the rotary shaft 30 with a small clearance (also referred to as a gap) a. The differential pressure chamber 44a communicates with the suction port of the auxiliary pump P2. That is, the suction port of the auxiliary pump P2 communicates with both the discharge port of the high vacuum pump P1 and the differential pressure chamber. Incidentally, the land portions 44b and 44c constitute the throttle portion of the present invention.

  The operation of this embodiment will be described. It is assumed that a workpiece (not shown) that is a rotating body is attached to the inner end of the process chamber P of the rotary shaft 30. From this state, the auxiliary pump P2 and the high vacuum pump P1 are operated to decompress the process chamber P. At this time, for example, the back pressure of the high vacuum pump P1, which is a turbo molecular pump, is maintained at a predetermined pressure lower than the atmospheric pressure by the auxiliary pump P2, whereby the performance of the high vacuum pump P1 can be exhibited. It is like that. Further, the differential exhaust seal 44 exhibits a sealing function by the operation of the exhaust pump P2.

  The sealing function of the differential exhaust seal 44 will be described. As in the present embodiment, a circumferential groove (differential pressure chamber) 44a is provided in the middle of the through hole of the rotating shaft 30 that penetrates the partition walls of the two chambers (here, the process chamber P and the atmosphere) having a pressure difference, If the clearance a between the land portions 44b and 44c and the rotary shaft 30 is reduced as much as possible when the exhaust pump P2 is connected to the exhaust pump P2, the pressure resistance increases, and the gas flowing from the high-pressure chamber (atmosphere) side (generally Is less air). When most of the inflowing gas is exhausted from the differential pressure chamber 44a, the amount of gas flowing into the low pressure chamber (process chamber P) becomes extremely small. Therefore, it is possible to obtain a state in which the two chambers are hermetically isolated even though the rotating shaft 30 penetrates freely (or linearly). Such a configuration is called a differential exhaust seal. Since the differential exhaust seal 44 in the present embodiment is non-contact, there is no dust generation / outgas from the seal, it has a long life, and has a feature that the life does not depend on the temperature, but the land portion 44b. 44c may be provided with a contact type seal such as an O-ring, for example, so as to be in close contact with the opposing surface. In this way, what is exhausted from the differential pressure chamber 44a is also called a differential exhaust seal.

  After the inside of the process chamber P is depressurized to a predetermined pressure, the rotating shaft 30 is rotated from the outside by a drive source (not shown), so that a predetermined process is performed while the workpiece rotates in a vacuum environment in the process chamber P. Is to be given.

  According to the present embodiment, since the auxiliary pump P2 of the high vacuum pump P1 that decompresses the process chamber P is used as the exhaust pump for operating the differential exhaust seal 44, a lower cost configuration is provided. The

Here, the dimensions of the land portion 44c of the differential exhaust seal 44 will be considered. FIG. 2A is a schematic diagram of the differential exhaust seal, and FIG. 2B is a model diagram of a space between the throttle portion and the rotation shaft. Here, the minimum suction amount Q _H required to operate the differential pumping seal the auxiliary pump P2 is expressed by the following equation.
Q _H = (a ³ b / 24 μL) · P ₀ ² [Pa · m ³ / s] (1)
However, a: Clearance [m] between the throttle portion 44c and the rotary shaft 30
b: Width (namely, inner circumferential length) [m] orthogonal to the gas moving direction in the throttle 44c
L: length [m] in the gas moving direction in the throttle 44a
μ: Air viscosity [Pa · s]
P ₀ : Case 20 external pressure (= atmospheric pressure) [Pa]

A specific numerical calculation example is shown below.
(Calculation Example 1)
First, if there is little exhaust gas from the process chamber P in the configuration as FIG. 1 (if the exhaust flow rate Q ₀ from the main pump can be regarded as zero), the auxiliary pump of the effective pumping speed 1300 [L / min] Examples of calculations in some cases using P2 (dry pump) and changing the turbo molecular pump P1 and the outer diameter of the rotating shaft are shown.

That is, in the case where the outer diameter of the rotating shaft 30 is 10 mm and the axial length L of the land portion 44c is 10 mm, the auxiliary pump when four types of turbo molecular pumps P1 having different required back pressure (allowable exhaust port pressure) are used. Table 1 shows the calculation result of the allowable flow rate Q _{pump of} P2 and the upper limit value of the gap a. When the turbo molecular pump P1 having the required back pressure of 10 Pa and the axial length L = 10 mm of the land portion 44c is used, the rotation axis Table 2 shows calculation results of the allowable flow rate Q _pump of the auxiliary pump P2 and the upper limit value of the gap a when the outer diameter D of 30 is changed. That is, from the allowable flow rate Q _pump of the auxiliary pump P2 (that is, the allowable exhaust flow rate of the auxiliary pump P2 capable of obtaining the necessary back pressure of the main pump P1), the exhaust flow rate Q _o of the main pump P1 (Q _o ≈ in this example) 0) than the value obtained by subtracting the determined by (1) Q _H is the upper limit of the size of the gap a is defined to be smaller.

In Tables 1 and 2, the allowable flow rate Q _pump of the dry pump P2 is determined from the effective exhaust speed of the dry pump P2 when the required back pressure of the turbo molecular pump P1 is given. Further, μ = 1.82 × 10 ⁻⁵ [Pa · S] and P 2 _O = 10 ⁵ [Pa] (atmospheric pressure) were set.

(Calculation Example 2)
In the configuration shown in FIG. 1, Table 3 and Table 4 are the same as those in Calculation Example 1 except that the outgas and exhaust gas from the process entrance P are Q ₀ = 3000 [Pa · L / min]. Shown in

(Calculation Example 3)
Table 5 shows the results when the conditions other than the necessary back pressure of the turbo molecular pump P1 are the same as those in the calculation example 1, except that the auxiliary pump P2 having an effective exhaust speed of 4200 [L / min] is used. Table 6 shows the results when the outer diameter D of the rotating shaft 30 is changed when the axial length L of 44c is 10 mm and the turbo molecular pump P1 having a required back pressure of 5 Pa is used.

(Calculation Example 4)
Tables 7 and 8 show the results when the outgas and the exhaust gas from the process chamber P are 1000 [Pa · L / min] corresponding to the calculation example 3.

  The above is an example in the case where the movable member is a rotating shaft, but the present invention can also be applied to the case where the facing surfaces of the housing and the movable member are planar (moves in a two-dimensional direction along the plane). . Next, a calculation example in the case where the facing surface of the housing and the movable member is planar is shown. FIG. 3 is a schematic cross-sectional view of a positioning device including a differential exhaust seal when the movable member is planar, and FIG. 4 is a schematic view of a facing surface of the differential exhaust seal. In the figure, 120a is an opening of the housing 120, 144a is a differential pressure chamber, 144b and 144c are land portions (throttle portions), and a slider 130 as a movable member is in the left-right direction in FIG. 3 and the land portions 144b and 144c. In a state where a gap a is maintained between them, a guide device (not shown) can slide.

(Calculation Example 5)
A combination of a turbo molecular pump P1 having a required back pressure of 10 [Pa] and a dry pump P2 having an effective exhaust speed of 1300 [L / min] with little exhaust gas from the inside of the process chamber P (accordingly, allowable flow rate QH = 7000 [Pa] Table 9 shows the case where the size of the opening was changed in (L / min]). However, the length L of the land portion 144c in FIG. 4 is 10 [mm]. In this case, b in the formula (1) is set to b = (W2 + H2) × 2.

  When the opposing surface is flat, the present invention can also be applied to a rotation introducing machine that supports the slider rotatably instead of or in addition to sliding the slider 130 left and right as shown in FIG. It is. In this case, when only rotation is possible, the shape of the differential exhaust seal is preferably a concentric circle instead of the shape as shown in FIG. Needless to say, it changes accordingly.

  As described above, based on the equation (1), the shape of the differential exhaust seal and how to determine the size of the throttle gap from the selected turbo molecular pump P1 and dry pump P2 are determined. be able to. Alternatively, when the clearance a is given based on the formula (1), the turbo molecular pump P1 and the dry pump P2 and the differential exhaust seal shape (D, L, H1, W1, H2, W2, etc.) are selected. You may make it do.

  Here, the required back pressure of the high vacuum pump (in the above example, the turbo molecular pump P1) as the main pump is determined based on the compression ratio of the high vacuum pump and the required vacuum level in the housing.

For example, if the inside of the casings 20 and 120 (vacuum chamber) requires a high vacuum of 10 ⁻⁵ [Pa] or less, the back pressure is 100 [Pa] if the compression ratio of the main pump is 10 ^7. It must be less than In reality, it is more preferable to obtain a back pressure of about 10 [Pa] for further safety.

  Here, when the inlet pressure of the auxiliary pump P2 (that is, the internal pressure of the differential pressure chamber 44a) is set to 10 Pa or less, most of the main pump in the case where a turbo molecular pump or the like is used as the high vacuum pump P1. It is preferable because it fits. The reason will be described in detail.

  In general, a high vacuum pump cannot release its exhaust directly to the atmosphere, so it is necessary to connect another pump (referred to as an auxiliary pump) to the exhaust port to lower the back pressure. A pump that has a maximum allowable value of the pressure at the exhaust port of the high vacuum pump and has an ultimate pressure higher than the pressure value cannot be used as an auxiliary pump. This pressure value is called the allowable exhaust port pressure (required back pressure). The allowable exhaust port pressure is one of the specifications of the high vacuum pump.

  The turbo molecular pump used as a high vacuum pump in the present embodiment has two types of cooling methods, water cooling and air cooling. Generally, the water cooling type pump has a higher allowable exhaust port pressure, for example, When comparing the allowable exhaust port pressure with a turbo molecular pump having the same exhaust speed execution exhaust speed of 550 [L / s], there is an example of 200 Pa in the water cooling system and 10 Pa in the air cooling system. When the allowable exhaust pressure is high, a connectable auxiliary pump is allowed even at a higher ultimate pressure, and the choice of the auxiliary pump is widened. Therefore, it can be said that the water cooling method is more advantageous, but the water cooling pump is often more expensive than the air cooling method.

  The allowable exhaust port pressure of the air-cooled pump is usually several tens of Pa, which is about 10 Pa, which is the lowest pressure value. Therefore, if the exhaust system is configured so that the exhaust port pressure of the high vacuum pump (turbo molecular pump) is 10 Pa or less, it can be applied to process chambers using most turbo molecular pumps. For example, due to the limitation of the allowable exhaust port pressure, it cannot be applied only to a process chamber in which a water-cooled turbo molecular pump is used.

Here, the exhaust system is described in one word, but the following requirements are necessary to realize the exhaust system (= exhaust system in which the exhaust port pressure of the turbo molecular pump is 10 Pa or less).
(1) First, it is necessary as a premise that the ultimate pressure of the auxiliary pump is 10 Pa or less.
(2) The inlet pressure of the auxiliary pump determined by the flow rate of gas flowing from the turbo molecular pump (= exhaust of the turbo molecular pump) needs to be 10 Pa or less.
(3) It is necessary that the inlet pressure of the auxiliary pump determined by the gas flow rate from the differential exhaust seal (Q _{H in the} above formula (1)) is 10 Pa or less. For this purpose, the ultimate pressure of the auxiliary pump is as low as possible, and it is easier to realize when the exhaust speed is as high as possible. However, from the original purpose of the present invention, the auxiliary pump originally exists as an auxiliary pump of the turbo molecular pump. Since the remaining capacity of the pump that has been used is used to exhibit the function of the differential exhaust seal, in that case, it is preferable to reduce the inflow gas flow rate from the differential exhaust seal as much as possible. For this purpose, the following is effective.
(4) The inner peripheral length b of the throttle portion 44c is reduced. This is because the inflow increases in proportion to the inner circumferential length b.
(5) Increase the length L of the aperture 44c. This is because the inflow amount increases in inverse proportion to the length L.
(6) The clearance a of the aperture 44c is reduced. This is because the inflow increases in proportion to the cube of the clearance a.

  The necessary and sufficient value of the above numerical value cannot be determined unconditionally because it varies depending on the ultimate pressure of the auxiliary pump (= differential exhaust pump) and the exhaust speed performance, but at least the size of the clearance (referred to as seal clearance) a is 5 It is necessary to be about ˜50 μm.

  The present invention has been described above with reference to the embodiments. However, the present invention should not be construed as being limited to the above-described embodiments, and can be modified or improved as appropriate.

  For example, when the movable member as shown in FIG. 1 is a rotating shaft, or a flat movable member as shown in FIG. 3 is arranged to close the opening, it can slide in the direction along the outer wall of the housing. In addition to the above, the present invention can also be applied to a movable member that can reciprocate in the penetrating direction on a shaft or a prismatic movable member that penetrates the opening.

  In the case of a flat movable member as shown in FIG. 3, the differential pressure chamber may be provided on the movable member side. However, within the moving range of the movable member, it is necessary to prevent the throttle portion adjacent to the differential chamber from being always exposed inside or outside the housing. The area when viewed from the direction as shown in FIG. In that respect, it is preferable to provide a differential pressure chamber in the case on the fixed side, because the area can be minimized in order to obtain the same performance.

  Further, in the example of FIGS. 1 and 3, one differential pressure chamber is provided, but the present invention can also be applied to the case where two or three or more differential pressure chambers are provided. In this case, any differential pressure chamber may be configured to communicate with the suction port of the auxiliary pump. For example, when there is a large amount of exhaust from the chamber, the differential pressure chamber closest to the outside (atmosphere side) is used. Is preferably communicated with the suction port of the auxiliary pump. On the other hand, when there is no exhaust from the inside, the cleanliness of the process chamber P is such that the differential pressure chamber on the inner side (near the process chamber P side) communicates with the suction port of the auxiliary pump. Is preferable. FIG. 5 shows an example of a rotation introducing machine in which the movable member is a rotating shaft and two circumferential groove-shaped differential pressure chambers are provided.

  Most of the exhaust is performed in the differential pressure chamber 44a, and the pressure in the differential pressure chamber 44d is sufficiently lower than the atmospheric pressure. Therefore, a turbo molecular pump P3, which is a high vacuum pump, is used for exhausting the differential pressure chamber 44d. However, since the required ultimate pressure is not as low as that of the process chamber, a large turbo molecular pump P3 is not required.

FIG. 6 is a view showing another example of a rotation introducing machine in which two circumferential groove-shaped differential pressure chambers are provided. In this example, the differential pressure chamber 44a close to the atmosphere side is sucked by the auxiliary pump P3 with respect to the configuration of FIG. The effective pumping speed of the turbo molecular pump P1 is 550 L / s, the effective pumping speed of the auxiliary pump P2 is 265 L / min, the ultimate pressure is 0.2 Pa, the effective pumping speed of the differential pump P3 is 5 L / min, the ultimate pressure When the 33 L process chamber is evacuated with a configuration of 4100 Pa, the process chamber is 1.3 × 10 ⁻⁵ Pa (= 1.0 × 10 ⁻⁷ Torr, even higher vacuum in the high vacuum region. The relationship between the seal length (L) and the shaft diameter (d) for maintaining the pressure below the pressure region covering the normal semiconductor manufacturing process was investigated for various combinations. The list is shown in Table 10. As the seal clearance, 10 μm and 20 μm, which are usually used, are adopted. In any case, it can be seen that (L / d) is 6 or less. In other words, if (L / d) satisfies 6 or less, it can be said that the differential exhaust in the present invention using at least one of the auxiliary pump and the differential exhaust pump is established.

1 is a front sectional view of a rotary introducer 10 including a differential exhaust seal according to an embodiment of the present invention. FIG. 2A is a schematic diagram of the differential exhaust seal, and FIG. 2B is a model diagram of a space between the throttle portion and the counterpart member. It is a schematic sectional drawing of the positioning device containing the differential exhaust seal at the time of making a movable member planar. It is the schematic of the opposing surface of the differential exhaust seal of FIG. It is a schematic block diagram which shows the example of the rotation introducer which provided two circumferential groove-shaped differential pressure chambers when a movable member is a rotating shaft. It is a schematic block diagram which shows another example of the rotation introducer which provided the two circumferential groove-shaped differential pressure chambers in the case where a movable member is a rotating shaft.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Rotation introducing machine 20 Case 30 Rotating shaft 40 Seal unit 44 Differential exhaust seal

Claims (5)

In the differential exhaust seal that seals between the casing whose inside is reduced to a predetermined pressure by the main pump and the movable member extending from the outside to the inside through the opening of the casing,
The back pressure of the main pump that decompresses the inside of the housing is maintained at a predetermined pressure lower than the pressure outside the housing by an auxiliary pump,
In the opening of the casing or the movable member, at least one differential pressure chamber formed to face the movable member or the casing, and the outside of the casing and the differential pressure chamber with respect to the differential pressure chamber On the other hand, the differential exhaust seal is characterized in that each of the inner sides of the housing has a throttle portion, and the at least one differential pressure chamber communicates with the suction port of the auxiliary pump.   2. The differential exhaust seal according to claim 1, wherein the differential pressure chamber communicated with the suction port of the auxiliary pump is a differential pressure chamber closest to the outside of the housing. The throttle portion has a gap, and is a flow rate from the outermost differential exhaust seal communicated with the auxiliary pump, and a flow rate Q _H represented by the following formula gives a necessary back pressure to the main pump. 3. The differential according to claim 2, wherein the shape of the throttle portion is determined to be smaller than a value obtained by subtracting the exhaust flow rate Q ₀ from the main pump from the flow rate Q _{pump of the} auxiliary pump to obtain. Exhaust seal.
Q _H = (a ³ b / 24 μL) · P ₀ ² [Pa · m ³ / s] (1)
However, a: Clearance [m] between the throttle part and the movable member
b: Width [m] orthogonal to the gas moving direction in the throttle portion
L: Length in the moving direction of the gas [m] in the throttle part
μ: Viscosity of the passing gas [Pa · s]
P ₀ : External pressure of the housing [Pa]   2. The differential exhaust seal according to claim 1, wherein the movable member is a rotating shaft, and a shaft diameter d thereof and a length L of each of the throttle portions satisfy L / d ≦ 6. The differential exhaust seal according to any one of claims 1 to 4, wherein a back pressure of the main pump is set to 10 Pa or less.

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