WO2004055376A2

WO2004055376A2 - Vacuum pumping arrangement

Info

Publication number: WO2004055376A2
Application number: PCT/GB2003/005375
Authority: WO
Inventors: Nigel Paul Schofield
Original assignee: The Boc Group Plc
Priority date: 2002-12-17
Filing date: 2003-12-09
Publication date: 2004-07-01
Also published as: US20060140794A1; TW200417690A; ATE388328T1; EP1576292A2; JP2006509952A; DE60319585T2; EP1576292B1; DE60319585D1; GB0229356D0; AU2003295101A1; WO2004055376A3

Abstract

A vacuum pumping arrangement comprises a drive shaft, a motor for driving said drive shaft, a molecular pumping mechanism and a regenerative pumping mechanism. The drive shaft is arranged for simultaneously driving the molecular pumping mechanism and the regenerative pumping mechanism. The drive shaft is supported by a lubricant free bearing associated with the molecular pumping mechanism.

Description

VACUUM PUMPING ARRANGEMENT

The present invention relates to a vacuum pumping arrangement

comprising a molecular pumping mechanism and a regenerative pumping

mechanism.

A known vacuum pumping arrangement for evacuating a chamber

comprises a molecular pump which may include: molecular drag pumping

means; or turbomolecular pumping means; or both molecular drag pumping

means and turbomolecular pumping means. If both pumping means are included the turbomolecular pumping means are connected in series with the molecular drag pumping means. The pumping arrangement is capable of

evacuating the chamber to very low pressures in the region of 1x10^" mbar. The compression ratio achieved by the molecular pump is not sufficient to

achieve such -low pressures^ whilst at the same time exhausting to atmosphere and therefore a backing pump is provided to reduce pressure at the exhaust of

the molecular pump and hence permit very low pressures to be achieved at the

inlet thereof.

When the chamber to be evacuated is part of a semiconductor

processing system it is generally desirable that the chamber is kept free from

contamination and therefore, the drive shaft of the molecular pump, which is

disposed at the vacuum side of the vacuum pumping arrangement, is

supported by a lubricant free bearing, since lubricant can be the cause of

contamination. It is possible to use a magnetic bearing, which is lubricant free, because even though a magnetic bearing's construction allows some

radial movement of the drive shaft, operation of the molecular pump is not

significantly affected by such radial movement. The drive shaft of a

regenerative pump, which may be used as a backing pump, is typically

supported by a lubricated bearing such as a lubricated rolling bearing because

of the high loads and level of precision required with a regenerative pump. In

other words, in a regenerative pump, the radial clearances between the rotor

blades and the stator have to be very tightly controlled, and in some cases,

restricted to no more than 80 microns. It has been considered therefore that the use of a regenerative pump in a clean environment is not appropriate

because generally it is driven by a drive shaft supported by a lubricated

bearing.

It is desirable to provide an improved vacuum pumping arrangement.

The ' present' invention provides a vacuum pumping arrangement!

comprising a drive shaft, a motor for driving said drive shaft, a molecular pumping mechanism and a regenerative pumping mechanism, wherein said

drive shaft is arranged for simultaneously driving said molecular pumping mechanism and said regenerative pumping mechanism and said drive shaft is

supported by a lubricant free bearing associated with said molecular pumping

mechanism.

The present invention also provides a vacuum pumping arrangement

comprising a drive shaft, a motor for driving said drive shaft, and a

regenerative pumping mechanism, said drive shaft being supported towards one end thereof by a lubricant free bearing and towards the other end thereof

by a lubricated bearing, said regenerative pumping mechanism comprising a

stator comprising a plurality of circumferential pumping channels disposed

about a longitudinal axis of the drive shaft and a rotor comprising a plurality

of arrays of rotor blades extending axially into respective said circumferential

pumping channels, said rotor being connected to said drive shaft so as to be

sufficiently close to said lubricated bearing so that radial movement of said

drive shaft at said lubricant free bearing translates substantially to axial

movement of said rotor blades relative to respective said circumferential

pumping channels.

Other aspects of the present invention are defined in the accompanying

claims.

In order that the present invention may be well understood, some embodiments thereof, which -are given by way of example only," will- now be • .>^•,

described with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a vacuum pumping arrangement

shown schematically;

Figure 2 is an enlarged cross-sectional view of a portion of a

regenerative pump of the arrangement shown in Figure 1 ;

Figure 3 is a diagram of a control system;

Figure 4 is a schematic representation of a vacuum pumping system;

Figure 5 is a schematic representation of another vacuum pumping

system; and Figures 6 to 8 are cross-sectional views of further vacuum pumping

arrangements all shown schematically.

Referring to Figure 1, a vacuum pumping arrangement 10 is shown

schematically, which comprises a molecular pumping mechanism 12 and a

backing pumping mechanism 14. The molecular pumping mechanism

comprises turbomolecular pumping means 16 and molecular drag, or friction, pumping means 18. Alternatively, the molecular^' pumping mechanism may

comprise turbomolecular pumping means only or molecular drag pumping

means only. The backing pump 14 comprises a regenerative pumping mechanism. A further drag pumping mechanism 20 may be associated with

the regenerative pumping mechanism and provided between drag pumping mechanism 18 and regenerative pumping mechanism 14. Drag pumping mechanism 20 comprises three drag pumping stages in series, whereas drag

pumping mechanism 18 comprises two drag pumping stages in parallel.

Vacuum pumping arrangement 10 comprises a housing, which is formed in three separate parts 22, 24, 26, and which houses the molecular

pumping mechanism 12, drag pumping mechanism 20 and regenerative

pumping mechanism 14. Parts 22 and 24 may form the inner surfaces of the

molecular pumping mechanism 12 and the drag pumping mechanism 20, as

shown. Part 26 may form the stator of the regenerative pumping mechanism

14.

Part 26 defines a counter-sunk recess 28 which receives a lubricated

bearing 30 for supporting a drive shaft 32, the bearing 30 being at a first end portion of the drive shaft associated with regenerative pumping mechanism

14. Bearing 30 may be a rolling bearing such as a ball bearing and may be

lubricated, for instance with grease, because it is in a part of the pumping

aiTangement 10 distal from the inlet of the pumping arrangement. The inlet of

the pumping aiTangement may be in fluid connection with a semiconductor

processing chamber in which a clean environment is required.

Drive shaft 32 is driven by motor 34 which as shown is supported by

parts 22 and 24 of the housing. The motor may be supported at any convenient position in the vacuum pumping arrangement. Motor 34 is adapted to be able to drive simultaneously the regenerative pumping

mechanism 14, and the drag pumping mechanism 20 supported thereby, and also the molecular pumping mechanism 12. Generally, a regenerative

pumping mechanism requires more power for operation than a molecular pumping mechanism*, the- regenerative pumping mechanism - operating -.a

pressures close to atmosphere where windage and air resistance is relatively

high. A molecular pumping mechanism requires relatively less power for operation, and therefore, a motor selected for powering a regenerative

pumping mechanism is also generally suitable for powering a molecular

pumping mechanism. Means are provided for controlling the rotational

speeds of the backing pumping mechanism and the molecular pumping

mechanism so that pressure in a chamber connected to the aiTangement can be

controlled. A suitable control system diagram for controlling speed of the

motor 34 is shown in Figure 3 and includes a pressure gauge 35 for measuring pressure in a chamber 33, and a controller 37 connected to the pressure gauge

for controlling the pump's rotational speed.

Regenerative pumping mechanism 14 comprises a stator comprising a

plurality of circumferential pumping channels disposed concentrically about a

longitudinal axis A of the drive shaft 32 and a rotor comprising a plurality of

arrays of rotor blades extending axially into respective said circumferential

pumping channels. More specifically, regenerative pumping mechanism 14

comprises a rotor fixed relative to drive shaft 32. The regenerative pumping

mechanism 14 comprises three pumping stages, and for each stage, a circumferential array of rotor blades 38 extends substantially orthogonally

from one surface of the rotor body 36. The rotor blades 38 of the three arrays extend axially into respective circumferential pumping channels 40 disposed concentrically in part 26 which constitutes the stator of the regenerative pumping' mechanism 14. During operation, drive shaft 32 rotates rotor- ody

36 which causes the rotor blades 38 to travel along the pumping channels,

pumping gas from inlet 42 in sequence along the radially outer pumping

channel, radially middle pumping channel and radially inner pumping channel

where it is exhausted from pumping mechanism 14 via exhaust 44 at pressures

close to or at atmospheric pressure.

An enlarged cross-section of a single stage of the regenerative

pumping mechanism is shown in Figure 2. For efficient operation of the

regenerative pumping mechanism 14, it is important that the radial clearance

"C" between rotor blades 38 and stator 26 is closely controlled, and preferably kept to no more than 200 microns or less, and preferably less than 80 microns,

during operation. An increase in clearance "C" would lead to significant

seepage of gas out of pumping channel 40 and reduce efficiency of

regenerative pumping mechanism 14. Therefore, regenerative pumping

mechanism 14 is associated with the lubricated rolling bearing 30 which

substantially resists radial movement of the drive shaft 32 and hence rotor

body 36. However, if there is radial movement of the drive shaft at an end

thereof distal from the lubricated bearing 30, this may also cause radial

movement of the rotor of the regenerative pumping mechanism, resulting in loss of efficiency. In other words, bearing 30 may act as a pivot about which some radial movement may take place. To avoid loss of efficiency, the rotor

36 of the regenerative pumping mechanism is connected to the drive shaft 32 so as to be sufficiently close to the lubricated bearing 30 (i.e. the pivot) so that

radial movement of the drive shaft translates substantially to axial movement of the rotor blades relative to respective circumferential pumping channels 40.

Preferably, the bearing 30 is substantially axially aligned with the

circumferential pumping channels so that any radial movement of the rotor

blades 38 does not cause significant seepage. As shown, the stator 26 of the

regenerative pumping mechanism 14 defines the recess for the bearing 30 and

the rotor body 36 is, as it will be appreciated, adjacent the stator 26.

Accordingly, the bearing 30, which resists radial movement, prevents

significant radial movement of the rotor body 36 and also hence of the rotor blades 38. Therefore, clearance "C" between the rotor blades 38 and stator 26

can be kept within tolerable limits.

Extending orthogonally from the rotor body 36 are two cylindrical

drag cylinders 46 which together form rotors of drag pumping mechanism 20.

The drag cylinders 46 are made from carbon fibre reinforced material which is

both strong and light. The reduction in mass when using carbon fibre drag

cylinders, as compared with the use of aluminium drag cylinders, produces

less inertia when the drag pumping mechanism is in operation. Accordingly, the rotational speed of the drag pumping mechanism is easier to control.

The drag pumping mechanism 20 shown schematically is a Hoi week type drag pumping mechanism in which stator portions 48 define a spiral

channel between the inner surface of housing part 24 and the drag cylinders 46. Three drag stages are shown, each of which provides a spiral path for gas flow between the rotor and the stator. The operation and stracture^of a

Holweck drag pumping mechanism is well known. The gas flow follows a

tortuous path flowing consecutively through the drag stages in series.

The molecular pumping mechanism 12 is driven at a distal end of drive shaft 32 from the regenerative pumping mechanism 14. A back up

bearing may be provided to resist extreme radial movement of the drive shaft

32 during, for instance, power failure. As shown, the lubricant free bearing is

a magnetic bearing 54 provided between rotor body 52 and a cylindrical

portion 56 fixed relative to the housing part 22. A passive magnetic bearing is

shown in which like poles of a magnet repel each other resisting excessive radial movement of rotor body 52 relative to the central axis A. In practice,

the drive shaft may move about 0.1 mm.

A small amount of radial movement of the rotor of a molecular

pumping mechanism does not significantly affect the pumping mechanism's

performance. However, if it is desired to further resist radial movement, an

active magnetic bearing may be adopted. In an active magnetic bearing,

electro magnets are used rather than permanent magnets in passive magnetic bearings. Further provided is a detection means for detecting radial

movement and for controlling the magnetic field to resist the radial movement. Figures 6 to 8 show an active magnetic bearing.

A circumferential array of angled rotor blades 58 extend radially outwardly from rotor body 52. At approximately half way along the rotor blades 58 at a radially intermediate portion of the array, a cylindrical support

ring 60 is"provided, to -which is connected drag cylinder 62 of drag pumping

mechanism 18. Drag pumping mechanism 18 comprises two drag stages in parallel with a single drag cylinder 62, which may be made from carbon fibre

to reduce inertia. Each of the stages is comprised of stator portions 64

forming with the tapered inner walls 66 of the housing 22 a spiral molecular

gas flow channel. An outlet 68 is provided to exhaust gas from the drag

pumping mechanism 18.

During normal operation, inlet 70 of pump arrangement 10 is

connected to a chamber, the pressure of which it is desired to reduce. Motor

34 rotates drive shaft 32 which in turn drives rotor body 36 and rotor body 52. Gas in molecular flow conditions is drawn in through inlet 70 to the

turbomolecular pumping means 16 which urges molecules into the molecular

drag pumping means 18 along both parallel drag pumping stages and through

outlet 68. Gas is then drawn through the three stages in series of the drag

pumping mechanism 20 and into the regenerative pumping mechanism

through inlet 42. Gas is exhausted at atmospheric pressure or thereabouts

through exhaust port 44.

Regenerative pumping mechanism 14 is required to exhaust gas at approximately atmospheric pressure. Accordingly, the gas resistance to

passage of the rotor blades 38 is considerable and therefore the power and

torque characteristics of motor 34 must be selected to meet the requirements of the regenerative pumping mechanism 14. The resistance to rotation encountered by the molecular pumping mechanism 12 is relatively little, since 'the ■molecular ■ pumping mechanism- operates -at relatively low-pressures,;

Furthermore, the structure of the drag pumping mechanism 18 with its only

moving part being a cylinder rotated about axis A does not suffer significantly

from gas resistance to rotation. Therefore, once power and torque

characteristics for motor 34 have been selected for regenerative pumping

mechanism 14, only a relatively small proportion of extra capacity is needed

so that the motor also meets the requirements of molecular pumping

mechanism 12. In other words, a 200w motor, which is typically used for a

molecular pumping mechanism, is significantly less powerful than motor 34

which preferably is a 2kw motor. In the prior ait, the typical motor is not powerful enough so that pressure change in a chamber can be controlled by

controlling the rotational speed of the pump. However, since a powerful

motor is selected to drive regenerative pumping mechanism 14, the additional

power can also be used to control rotational speed of the molecular pumping

mechanism and thereby control pressure.

A typical turbomolecular pumping means is evacuated to relatively

low pressures before it is stalled up. In the prior art, a backing pumping mechanism is used for this purpose. Since the backing pumping mechanism

and turbomolecular pumping means are associated with the same drive shaft

in vacuum pumping arrangement 10, this start up procedure is not possible.

Accordingly, the vacuum pumping aiTangement forms part of a vacuum

pumping system which comprises additional evacuation means to evacuate at least the turbomolecular pumping means 16 prior to start up to a predetermined' pressure:-' Preferably,¹ the turbomolecular pumping ^■ means_^is

evacuated to less than 500 mbar prior to start up. Conveniently, the whole

vacuum pumping arrangement is evacuated prior to start up, as shown in

Figures 4 and 5. The evacuation means may be provided by an additional

pump, although this is not preferred since an additional pump would increase

costs of the system. When the pumping arrangement 10 is used as part of a

semi-conductor processing system, it is convenient to make use of a pump or

pumping means associated with the system such as the pump for the load lock

chamber. Figure 4 shows the aiTangement of a semiconductor processing

system, in which the load lock pump 74 is, in normal use, used to evacuate pressure from load lock chamber 76. A valve 78 is provided between load

lock chamber 76 and load lock pump 74. Load lock pump 74 is connected to

the exhaust of pumping arrangement 10 via valve 80. A further valve 82 is

provided downstream of exhaust 44 of pumping aiTangement 10. During start

up, valve 78 and valve 82 are closed whilst valve 80 is opened. Load lock

pump 74 is operated to evacuate gas from aiTangement 10 and therefore from

turbomolecular pumping means 16. During normal operation, valves 82 and

78 are opened whilst valve 80 is closed. Arrangement 10 is operated to evacuate pressure from vacuum chamber 84.

Alternatively, vacuum pumping arrangement 10 can be started up as described with reference to Figure 5. The additional evacuation means

comprises a high pressure nitrogen supply which is connected to an ejector pump 90 via valve S8. Valve 88 is opened so that high pressure nitrogen is ejected to- evacuate aiTangement ' 10 and therefore- turbomolecular pumping

means 16. Nitrogen is a relatively inert gas and does not contaminate the

system.

Although the pumping arrangement 10 may be evacuated prior to start

up, it is also possible to evacuate the aiTangement after start up, since the

arrangement can be started but will not reach suitable rotational speeds until

evacuation is performed.

There now follows a description of three further embodiments of the

present invention. For brevity, the further embodiments will be discussed only in relation to the parts thereof which are different to the first embodiment

and like reference numerals will be used for like parts.

Figure 6 shows a vacuum pumping aiTangement 100 comprising an

active magnetic bearing in which a cylindrical pole of the magnetic bearing 54

is mounted to the drive shaft 32 with a like pole being positioned on housing

22. The rotor body 52 of the turbomolecular pumping means 16 of the

molecular pumping mechanism, is disc-shaped and the overall size of the aiTangement 100 is reduced as compared with the first embodiment.

In Figure 7, a vacuum pumping aiTangement 200 is shown in which

the turbomolecular pumping means 12 comprises two turbomolecular pumping stages 16. A stator 92 extends radially inwardly from housing part 22 between the two turbo stages 16.

In Figure 8, a vacuum pumping arrangement 300 is shown in which

molecular drag pumping mechanism -20 has-been omitted. • • --

Claims

1. A vacuum pumping arrangement comprising a drive shaft, a motor for

driving said drive shaft, a molecular pumping mechanism and a regenerative

pumping mechanism, wherein said drive shaft is arranged for simultaneously driving said molecular pumping mechanism and said regenerative pumping

mechanism and said drive shaft is supported by a lubricant free bearing

associated with said molecular pumping mechanism.

2. An aiTangement as claimed in claim 1, wherein said lubricant free bearing is a magnetic bearing.

3 An aiTangement as claimed in claim 1 or 2, wherein said lubricant'free bearing and the molecular pumping mechanism are substantially axially

aligned.

4. An arrangement as claimed in any preceding claim, wherein said drive

shaft is additionally supported by a lubricated bearing associated with said

regenerative pumping mechanism.

5. An arrangement as claimed in claim 4, wherein said lubricated bearing

is a rolling bearing.

6. An aiTangement as claimed in claim 4 or claim 5, wherein said

lubricated beaiing and the regenerative mechanism are substantially axially

aligned.

7. An aiTangement as claimed in any of claims 4 to 6, wherein said

regenerative pumping mechanism comprises a stator comprising a plurality of

circumferential pumping channels disposed about a longitudinal axis of the

drive shaft and a rotor comprising a plurality of arrays of rotor blades extending axially into respective said circumferential pumping channels.

8. An aiTangement as claimed in claim 7, wherein said rotor of said regenerative pumping mechanism is connected to said drive shaft so as to be

sufficiently close to said lubricated- beaiing so that radial movement &f-*said

drive shaft at said lubricant free beaiing translates substantially to axial movement of said rotor blades relative to respective said circumferential

pumping channels.

9. An aiTangement as claimed in claim 7 or 8, wherein said lubricated

beaiing and said circumferential pumping channels are substantially axially

aligned.

10. An aiTangement as claimed in any one of claims 7 to 9, wherein said

lubricated beaiing is housed in the stator of the regenerative pumping

mechanism.

11. An arrangement as claimed in any one of the preceding claims,

wherein said molecular pumping mechanism comprises molecular drag

pumping means.

12. An aiTangement as claimed in any one of the preceding claims,

wherein said molecular pumping mechanism comprises turbomolecular

pumping means.

13. An arrangement as claimed in any one of the preceding claims, 'comprising a housing whic -houses the molecular pumpin -mechanism^-the

regenerative pumping mechanism, the drive shaft and the motor.

14. A vacuum pumping aiTangement comprising a drive shaft, a motor for

driving said drive shaft, and a regenerative pumping mechanism, said drive

shaft being supported towards one end thereof by a lubricant free bearing and

towards the other end thereof by a lubricated bearing, said regenerative

pumping mechanism comprising a stator comprising a plurality of

circumferential pumping channels disposed about a longitudinal axis of the

drive shaft and a rotor comprising a plurality of arrays of rotor blades extending axially into respective said circumferential pumping channels, said

rotor being connected to said drive shaft so as to be sufficiently close to said

lubricated bearing so that radial movement of said drive shaft at said lubricant

free bearing translates substantially to axial movement of said rotor blades

relative to respective said circumferential pumping channels.