GB2440542A

GB2440542A - Vacuum pump gearbox purge gas arrangement

Info

Publication number: GB2440542A
Application number: GB0615156A
Authority: GB
Inventors: Philip John Stephens
Original assignee: BOC Group Ltd; Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2006-07-31
Filing date: 2006-07-31
Publication date: 2008-02-06
Also published as: GB0615156D0

Abstract

A vacuum pump comprises a pumping chamber 16 having an inlet 16a and an outlet 16b leading to an exhaust line 18. The chamber houses a rotor 20 on a shaft 22 extending from the chamber into a gearbox 26. Purge gas is supplied to the gearbox and a passageway conveys purge gas from the gearbox to one of the chamber and the exhaust line. This passageway 42 extends at least partially along the shaft which comprises a first gas receiving opening 44 and a second opening 46 for discharging purge gas towards said one of the pumping chamber and the exhaust line. A pressure actuated flow control device 50 is located between the openings for varying the passageway conductance dependent on pressure differential between the openings. The device comprises an apertured (61 Fig. 2) elastomeric diaphragm (60) that closes passages (64) surrounding a sintered metal plug (62) at a large differential to reduce passage conductance.

Description

The present invention relates to a vacuum pump. Vacuum processing is commonly used in the manufacture of semiconductor devices and flat panel displays to deposit thin films on to substrates, and in metallurgical processes. Pumping systems used to evacuate relatively large process chambers to the desired pressure generally comprise at least one booster pump connected in series with at least one backing pump.

Vacuum pumps typically have oil-free pumping mechanisms, as any lubricants present in the pumping mechanism could cause contamination of the clean environment in which the vacuum processing is performed. Such “dry” vacuum pumps are commonly single or multi-stage positive displacement pumps employing inter-meshing pairs of rotors in the pumping mechanism. The rotors may have the same type of profile in each stage or the profile may change from stage to stage. One rotor of each pair is attached to a first shaft passing through the stage(s), with the second rotor of each pair being attached to a second shaft also passing through the stage(s). One of the shafts is driven by a motor and the other is usually driven synchronously in the opposite direction by means of timing gears attached to the shafts. A gearbox usually houses one end of each of the shafts, the timing gears, and bearings for supporting the shafts. The gearbox is normally positioned adjacent the exhaust pumping stage to operate substantially at atmospheric pressure.

In the semiconductor industries the increasing use of high flow rates of relatively corrosive gases such as chlorine, boron trichloride, hydrogen bromide, fluorine and chlorine trifluoride requires the gases passing through the pump to be isolated from the gearbox. However, for practical engineering reasons, the gearbox cannot be fully physically isolated from the exhaust pumping stage, in particular because of the slight leakage always associated with shaft seals which need to be present about the shafts and which are normally attached to a head plate located between the gearbox body and the exhaust stage. This is particularly true for seals of the non-contacting type, which are often used to minimise power consumption or because the speed of shaft rotation is too high for contact seals such as lip seals. In view of this, it is common for a purge gas, such as dry nitrogen, to be supplied to the gearbox to inhibit contamination of the gearbox. To prevent pressurisation of the gearbox, the purge gas must be allowed to escape from the gearbox. Due to the relatively harsh nature of the process gases that may be passing through the pumping stages of the vacuum pump, and which may become entrained within the purge gas, the purge gas is normally conveyed along a purge gas passageway to either the exhaust stage or the exhaust line leading from the exhaust stage to prevent uncontrolled and undesirable escape of any process gas to the atmosphere. The purge gas passageway may thus pass between at least one of the shaft seals and the shaft surrounded by these shaft seal(s). Consequently, the conductance of the purge gas passageway is preferably maximised in order to ensure that the pressure differential between the gearbox and the vacuum pump is as small as possible, as a high pressure differential may lead to excessive wear of these shaft seal(s).

The exhaust line normally contains a check valve, or non-return valve, to permit gas to be exhausted from the pump and inhibit any back-flow of the exhausted gas into the exhaust stage. In the event that the purge gas passageway passes from the gearbox directly to the exhaust line, the check valve will be located downstream from the purge gas passageway to prevent any back-flow of gas into the gearbox.

A problem may arise if the pump is switched off when the process chamber is being evacuated by the pump, and thus when the pumping stage(s) and the foreline leading from the process chamber to the pump, are under vacuum. In this event, the exhaust line check valve will close, and the pressure will rapidly ·· ·

equalise throughout the pump at a value close to the ultimate vacuum of the pump. Due to the high conductance of the purge gas passageway, this can result in the purge gas being rapidly drawn along the purge gas passageway from the gearbox into the exhaust stage. In view of the relatively high flow rate of the purge gas, oil and/or grease may be picked up from the gearbox by the purge gas and transferred into the stage(s) of the pump, thereby undesirably contaminating the previously “dry” pump.

It is an aim of at least the preferred embodiment of the present invention to seek to solve this problem.

The present invention provides a vacuum pump comprising: a pumping chamber having a chamber inlet and a chamber outlet leading to an exhaust line, the pumping chamber housing at least one rotor located on a shaft extending from the pumping chamber into a gearbox; means for conveying a purge gas to the gearbox; and a purge gas passageway for conveying purge gas from the gearbox to one of the pumping chamber and the exhaust line; wherein the purge gas passageway extends at least partially along the shaft, the shaft comprising a first opening for receiving purge gas from the gearbox, a second opening for discharging purge gas from the shaft towards said one of the pumping chamber and the exhaust line, and a pressure actuated flow control device located between the openings for varying the conductance of the passageway dependent on a pressure differential between the openings.

The flow control device is preferably designed such that the passageway has a relatively low conductance when the pressure differential between the openings is relatively high, and a relatively high conductance when the pressure differential between the openings is relatively low. Consequently, when there is a low pressure differential between the openings, for example during normal operation of the pump, the relatively high conductance of the passageway can enable purge gas to pass relatively freely from the gearbox to the selected one of the pumping ·

chamber and the exhaust line. However, when there is a relatively high pressure differential between the openings, for example if the pump has been switched off during operation, the relatively low conductance of the passageway inhibits the purge gas from “rushing” out from the gearbox, and thus reduces the likelihood that oil or other contaminants will be picked up from the gearbox as the purge gas enters the passageway.

The flow control device preferably defines, at least in part, a first, relatively low conductance path and a second, relatively high conductance path for gas passing through the passageway, and preferably comprises pressure actuated means for selectively closing one of the paths depending on the pressure differential between the openings. The flow control device may comprise a porous body, for example a sintered metal plug, with at least part of the first path passing through the porous body and at least part of the second path passing around the porous body. The flow control device may comprise at least one channel, preferably a plurality of parallel channels, for conveying gas around the porous body.

The pressure actuated means preferably comprises a valve body, for example an annular elastomeric diaphragm, moveable against a valve seat dependent on the pressure differential between the openings to close the second path.

Shaft seals may be located along the shaft for isolating the gearbox from the pumping chamber, with first and second annular plenum chambers spaced along the shaft and located between adjacent shaft seals. Purge gas may be conveyed to the first plenum chamber, from which the purge gas is supplied to the gearbox, and the purge gas may be discharged from the second opening into the second plenum chamber, from which purge gas is conveyed to the selected one of the pumping chamber and the exhaust line. The second plenum chamber is preferably located closer to the pumping chamber than the first plenum chamber.

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view of a vacuum pump;

Figure 2 is a schematic cross-sectional view of a flow control device located within a bore of the shaft of the pump of Figure 1 , with a valve member of the device in a first position;

Figure 3 illustrates the gas flow channels around the sintered plug of the flow control device of Figure 2; and

Figure 4 is a schematic cross-sectional view of the flow control device of Figure 2, with the valve member in a second position.

Figure 1 illustrates an embodiment of a vacuum pump 10. The vacuum pump 10 may comprise a single pumping stage within a pumping chamber. Alternatively, the pump may comprise a plurality of pumping stages, the pumping stages comprising an inlet stage adjacent to the pump inlet and an exhaust stage adjacent the pump outlet. Ports and gas passageways may link the pumping stages so that gas can be pumped from the pump inlet to the pump outlet sequentially through the stages. The, or each, pumping stage may comprise a single rotor or a pair of intermeshing rotors, which may have a Roots or a Northey (or “claw”) profile.

In this embodiment, the vacuum pump 10 comprises a stator 12 housing a multistage rotor assembly. The stator 12 comprises a plurality of transverse walls 14 which divide the stator 12 into a plurality of pumping chambers 16. In this embodiment, the stator 12 is divided into four pumping stages, although the stator 12 may be divided into any number of pumping stages required to provide the pump 10 with the desired pumping capacity. In Figure 1 , only one of the pumping stages 17 is illustrated for clarity purposes, this stage providing an exhaust pumping stage 17 of the pump 10. Each pumping chamber 16 has a chamber inlet 16A for receiving gas to be pumped within that chamber 16, and a chamber outlet 16B from which pumped gas is exhausted from the chamber 16. Gas exhaust from chamber outlet 16B from the exhaust pumping stage 17 enters an exhaust line 18 from which gas is exhausted from the pump 10. The rotor assembly comprises two intermeshing sets of rotors, each pumping stage comprising a pair of intermeshing rotors. One rotor 20 of each pair is attached to a first shaft 22 and the other rotor of each pair is attached to a second shaft; the second shaft and its attached rotors are not visible in Figure 1 as they are hidden by the first shaft 22 and its attached rotor 20. Each shaft is supported by bearings 24 for rotation relative to the stator 12.

One of the shafts is driven by a motor (not shown) connected to one end of that shaft. The other shaft is connected to that shaft by means of meshed timing gears (not shown) so that the shafts are rotated synchronously but in opposite directions within the stator 12. A gearbox 26 is located adjacent the exhaust pumping stage 17, the gearbox 26 being separated from the exhaust pumping stage 17 by a head plate 28 through which the shafts pass.

Shaft seals 30, 32, 34 are located about each of the shafts to contain oil vapours in the gearbox 26 and generally to minimise any escape of oil towards the exhaust pumping stage 17 to preserve the cleanliness of the pumping chamber 16. Each shaft seal is preferably of a close tolerance, non-contact design, and preferably comprises a metal ring having an internal diameter which is only slightly larger than the external diameter of the shaft and which is centred about the shaft in a non-contacting manner. Each ring is held within a groove in the head plate 28 which allows for limited movement of the ring within the grooves.

A first annular plenum volume 36 and a second annular plenum volume 38 extend about the shaft 22, each plenum volume being located within the head plate 28 and between two adjacent shaft seals. The first plenum volume 36 is located closer to the gearbox 26 than the second plenum volume 38. In order to prevent the pumped gases from entering the gearbox 26, a purge gas, for example dry and/or filtered nitrogen, is conveyed into the first plenum volume 36 from a source thereof along a first purge gas conduit 40 passing through the stator 12 and head plate 28. The purge gas flows across the shaft seal 34 and enters the gearbox 26, and flows across the shaft seal 32 into the second plenum volume 38.

To inhibit pressurisation of the gearbox 26, the purge gas must be discharged from the gearbox 26. For this purpose, a purge gas passageway is established between the gearbox 26 and one of the exhaust pumping stage 17 and the exhaust line 18 to enable the purge gas to be discharged from the gearbox 26 in a controlled manner. In the illustrated embodiment, the purge gas passageway passes from the gearbox 26 directly to the exhaust line 18. The shaft 22 includes a bore 42 which passes part way along the shaft 22, preferably co-axially along the shaft 22. A first set of radial bores 44 are located towards one end of the shaft 22 to provide first openings through which purge gas may enter the bore 42 from the gearbox 26. An annular filter element 45 may be attached to the shaft 22 about the radial bores 44 to prevent the flow of oil into the radial bores 44 whilst allowing gas to passes through the filter element 45 whilst the shaft 22 is rotating. For example, the filter 45 may be a coalescing filter.

A second set of radial bores 46 are located towards the exhaust pumping stage 17 to provide second openings through which purge gas is discharged from the bore 42 towards the selected one of the pumping stage 17 and the exhaust line 18. In this embodiment, the radial bores 46 are positioned so that purge gas is discharged from the bore 42 into the second plenum volume 38. A second purge gas conduit 48 passing through the head plate 28 and the stator 12 conveys purge gas from the second plenum volume 38 to the exhaust line 18, and so the gas passageway, in this embodiment, is provided by a combination of the bore 42, first and second radial openings 44, 46, and the second gas conduit 48. In the event that the gas passageway was to extend between the gearbox 26 and the pumping chamber 16, the second gas conduit 48 may be dispensed with, with the second radial openings 46 being located to the left (as illustrated) of the shaft seal 30 so that the purge gas is discharged from the bore 42 towards the pumping chamber 16. In either arrangement, the gas passageway is designed to have a relatively high conductance during normal operation of the pump 10, that is, when there is a relatively low pressure differential between the first and second openings in the gas passageway, so that purge gas may pass relatively freely along the gas passageway from the gearbox 26. Due to the gas passageway linking the gearbox 26 to one of the exhaust line 18 and the pumping chamber 16, a relatively high pressure differential would exist between the first and second openings in the event that the pump 10 is switched off during the evacuation of a chamber. For example, in the illustrated embodiment, the pressure at the exhaust line 18 would be relatively low, whilst the pressure in the gearbox 26 would be relatively high. As the gas passageway is designed to have a relatively high conductance, this could lead to gas racing from the gearbox 26 towards the exhaust line 18. The high speed of the gas may substantially reduce the efficiency of the oil filter 45, and so a result may be that the exhaust line 18 and pumping chamber 16 become contaminated with oil.

In order to avoid this, a flow control device 50 is located within the bore 42 of the shaft 22, and between the first and second radial bores 44, 46, for reducing the conductance of the gas passageway in the event that pressure differential between the first and second openings increases. With reference to Figure 2, the flow control device 50 comprises a cylindrical body 52 having a gas flow passage passing axially through the body 52. The gas passage comprises a first portion 54 of relatively large diameter for receiving gas flowing through the bore 42 from the gearbox 26, and a second portion 56 of reduced diameter from which gas is discharged towards the second radial bores 46. The gas passage tapers from the first portion 54 towards the second portion 56 to define a conical valve seat 58. An annular elastomeric valve member or diaphragm 60 is mounted in the first portion 54 adjacent to the valve seat 58. The valve member 60 has an aperture 61 located in the centre thereof through which gas passes from the first portion 54 of the gas passageway towards the second portion 56 of the gas passage. The diameter of the aperture 61 is smaller than the diameter of the second portion 56 of the gas passage through the body 52.

A sintered metal plug 62 is located in the second portion 56 of the gas passage. With reference also to Figure 3, the second portion 56 of the gas passage includes a plurality of channels 64 which are located about the sintered metal plug 62. The sintered metal plug 62 thus provides a relatively low conductance path for gas passing through the flow control device 50, whilst the channels 64 provide a second, relatively high conductance path for gas passing through the flow control device 50.

In use, when the pressure differential across the flow control device 50, and thus between the first and second openings in the gas passageway, is relatively low, the valve member 60 is in the position illustrated in Figure 2, so that gas entering the second portion 56 of the gas passage through the body 52 passes through the channels 64 located about the sintered metal plug 62. However, when the pressure differential across the flow control device 50, and thus between the first and second openings in the gas passageway, is relatively high, the valve member 60 moves against the valve seat 58, as illustrated in Figure 4, to close the channels 64, so that gas entering the second portion 56 of the gas passage through the body 52 has to pass through the sintered metal plug 62.

Consequently, the conductance of the gas passageway extending between the gearbox 26 and the exhaust line 18 is lowered when there is a relatively high pressure differential between the openings in the gas passageway. · · •

Claims

CLAIMS A vacuum pump comprising: a pumping chamber having a chamber inlet and a chamber outlet leading to an exhaust line, the pumping chamber housing at least one rotor located on a shaft extending from the pumping chamber into a gearbox; means for conveying a purge gas to the gearbox; and a purge gas passageway for conveying purge gas from the gearbox to one of the pumping chamber and the exhaust line; wherein the purge gas passageway extends at least partially along the shaft, the shaft comprising a first opening for receiving purge gas from the gearbox, a second opening for discharging purge gas from the shaft towards said one of the pumping chamber and the exhaust line, and a pressure actuated flow control device located between the openings for varying the conductance of the passageway dependent on a pressure differential between the openings. A vacuum pump according to Claim 1 , wherein the flow control device defines, at least in part, a first, relatively low conductance path and a second, relatively high conductance path for gas passing through the passageway, and comprises pressure actuated means for selectively closing one of the paths depending on the pressure differential between the openings. A vacuum pump according to Claim 2, wherein the flow control device comprises a porous body, and wherein at least part of the first path passes through the porous body and at least part of the second path passes around the porous body. A vacuum pump according to Claim 3, wherein the porous body comprises a sintered metal plug. A vacuum pump according to Claim 3 or Claim 4, wherein the flow control device comprises at least one channel for conveying gas around the porous body. A vacuum pump according to any of Claims 2 to 5, wherein the pressure actuated means comprises a valve body moveable against a valve seat dependent on the pressure differential between the openings to close the second path. A vacuum pump according to Claim 6, wherein the valve body comprises an annular diaphragm. A vacuum pump according to any preceding claim, comprising shaft seals located along the shaft for isolating the gearbox from the pumping chamber, and first and second annular plenum chambers spaced along the shaft and located between adjacent shaft seals. A vacuum pump according to Claim 8, wherein the means for conveying purge gas to the gearbox is configured to convey purge gas to the first plenum chamber. A vacuum pump according to Claim 8 or Claim 9, wherein purge gas is discharged from the second opening into the second plenum chamber, from which purge gas is conveyed to said one of the pumping chamber and the exhaust line, the second plenum chamber being located closer to the pumping chamber than the first plenum chamber.