U.S. patent application number 10/536779 was filed with the patent office on 2006-06-29 for vacuum pumping arrangement.
Invention is credited to Nigel Paul Schofield.
Application Number | 20060140794 10/536779 |
Document ID | / |
Family ID | 9949817 |
Filed Date | 2006-06-29 |
United States Patent
Application |
20060140794 |
Kind Code |
A1 |
Schofield; Nigel Paul |
June 29, 2006 |
Vacuum pumping arrangement
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.
Inventors: |
Schofield; Nigel Paul;
(Horsham, West Sussex, GB) |
Correspondence
Address: |
THE BOC GROUP, INC.
575 MOUNTAIN AVENUE
MURRAY HILL
NJ
07974-2064
US
|
Family ID: |
9949817 |
Appl. No.: |
10/536779 |
Filed: |
December 9, 2003 |
PCT Filed: |
December 9, 2003 |
PCT NO: |
PCT/GB03/05375 |
371 Date: |
December 1, 2005 |
Current U.S.
Class: |
417/423.4 |
Current CPC
Class: |
F04D 19/048 20130101;
F04D 17/168 20130101; F04D 23/008 20130101; F04D 29/058
20130101 |
Class at
Publication: |
417/423.4 |
International
Class: |
F04B 17/00 20060101
F04B017/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 17, 2002 |
GB |
0229356.1 |
Claims
1. A vacuum pumping arrangement comprising a drive shaft, a motor
for driving the drive shaft, a molecular pumping mechanism and a
regenerative pumping mechanism, wherein the drive shaft is arranged
for simultaneously driving the molecular pumping mechanism and the
regenerative pumping mechanism and the drive shaft is supported by
a lubricant free bearing associated with the molecular pumping
mechanism.
2. The arrangement as claimed in claim 1, wherein the lubricant
free bearing is a magnetic bearing.
3. The arrangement as claimed in claim 1, wherein the lubricant
free bearing and the molecular pumping mechanism are substantially
axially aligned.
4. The arrangement as claimed in claim 1, wherein the drive shaft
is additionally supported by a lubricated bearing associated with
the regenerative pumping mechanism.
5. The arrangement as claimed in claim 4, wherein the lubricated
bearing is a rolling bearing.
6. The arrangement as claimed in claim 4, wherein the lubricated
bearing and the regenerative pumping mechanism are substantially
axially aligned.
7. The arrangement as claimed in claim 4, wherein the 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 the respective circumferential
pumping channels.
8. The arrangement as claimed in claim 7, wherein the rotor of the
regenerative pumping mechanism is connected to the drive shaft so
as to be sufficiently close to the lubricated bearing so that
radial movement of the drive shaft at the lubricant free bearing
translates substantially to axial movement of the rotor blades
relative to the respective circumferential pumping channels.
9. The arrangement as claimed in claim 7, wherein the lubricated
bearing and the circumferential pumping channels are substantially
axially aligned.
10. The arrangement as claimed in claim 7, wherein the lubricated
bearing is housed in the stator of the regenerative pumping
mechanism.
11. The arrangement as claimed in claim 1, wherein the molecular
pumping mechanism comprises a molecular drag pumping mechanism.
12. The arrangement as claimed in claim 1, wherein the molecular
pumping mechanism comprises turbomolecular pumping means.
13. The arrangement as claimed in claim 1, comprising a housing
which houses the molecular pumping mechanism, the regenerative
pumping mechanism, the drive shaft and the motor.
14. A vacuum pumping arrangement comprising a drive shaft, a motor
for driving the drive shaft, and a regenerative pumping mechanism,
the drive shaft being supported towards one end thereof by a
lubricant free bearing and towards the other end thereof by a
lubricated bearing, the 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 the respective circumferential pumping channels, the rotor
being connected to the drive shaft so as to be sufficiently close
to the lubricated bearing so that radial movement of the drive
shaft at the lubricant free bearing translates substantially to
axial movement of the rotor blades relative to the respective
circumferential pumping channels.
15. The arrangement as claimed in claim 2, wherein the molecular
pumping mechanism comprises a molecular drag pumping mechanism.
16. The arrangement as claimed in claim 15, wherein the molecular
pumping mechanism comprises turbomolecular pumping means.
17. The arrangement as claimed in claim 16, comprising a housing
which houses the molecular pumping mechanism, the regenerative
pumping mechanism, the drive shaft and the motor.
Description
[0001] The present invention relates to a vacuum pumping
arrangement comprising a molecular pumping mechanism and a
regenerative pumping mechanism.
[0002] 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 1.times.10.sup.-6 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.
[0003] 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.
[0004] It is desirable to provide an improved vacuum pumping
arrangement.
[0005] 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.
[0006] 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.
[0007] Other aspects of the present invention are defined in the
accompanying claims.
[0008] 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:
[0009] FIG. 1 is a cross-sectional view of a vacuum pumping
arrangement shown schematically;
[0010] FIG. 2 is an enlarged cross-sectional view of a portion of a
regenerative pump of the arrangement shown in FIG. 1;
[0011] FIG. 3 is a diagram of a control system;
[0012] FIG. 4 is a schematic representation of a vacuum pumping
system;
[0013] FIG. 5 is a schematic representation of another vacuum
pumping system; and
[0014] FIGS. 6 to 8 are cross-sectional views of further vacuum
pumping arrangements all shown schematically.
[0015] Referring to FIG. 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.
[0016] 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.
[0017] 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 arrangement 10
distal from the inlet of the pumping arrangement. The inlet of the
pumping arrangement may be in fluid connection with a semiconductor
processing chamber in which a clean environment is required.
[0018] 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 at 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 arrangement can be controlled. A suitable control
system diagram for controlling speed of the motor 34 is shown in
FIG. 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.
[0019] 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 body 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.
[0020] An enlarged cross-section of a single stage of the
regenerative pumping mechanism is shown in FIG. 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 drives 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.
[0021] 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.
[0022] The drag pumping mechanism 20 shown schematically is a
Holweck 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 structure of a Holweck drag pumping
mechanism is well known. The gas flow follows a tortuous path
flowing consecutively through the drag stages in series.
[0023] 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.
[0024] 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. FIGS.
6 to 8 show an active magnetic bearing.
[0025] 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.
[0026] 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.
[0027] 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 200 w motor, which is typically used for a molecular
pumping mechanism, is significantly less powerful than motor 34
which preferably is a 2 kw motor. In the prior art, 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.
[0028] A typical turbomolecular pumping means is evacuated to
relatively low pressures before it is started 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 arrangement 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, 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 FIGS. 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. FIG. 4 shows the arrangement 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 arrangement 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 arrangement 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.
[0029] Alternatively, vacuum pumping arrangement 10 can be started
up as described with reference to FIG. 5. The additional evacuation
means comprises a high pressure nitrogen supply which is connected
to an ejector pump 90 via valve 88. Valve 88 is opened so that high
pressure nitrogen is ejected to evacuate arrangement 10 and
therefore turbomolecular pumping means 16. Nitrogen is a relatively
inert gas and does not contaminate the system.
[0030] Although the pumping arrangement 10 may be evacuated prior
to start up, it is also possible to evacuate the arrangement after
start up, since the arrangement can be started but will not reach
suitable rotational speeds until evacuation is performed.
[0031] 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.
[0032] FIG. 6 shows a vacuum pumping arrangement 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 arrangement 100 is reduced as compared
with the first embodiment.
[0033] In FIG. 7, a vacuum pumping arrangement 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.
[0034] In FIG. 8, a vacuum pumping arrangement 300 is shown in
which molecular drag pumping mechanism 20 has been omitted.
* * * * *