U.S. patent application number 10/581934 was filed with the patent office on 2007-11-29 for vacuum pump.
Invention is credited to Stephen Dowdeswell, Michael Chung Kau Liu, Nigel Paul Schofield.
Application Number | 20070274822 10/581934 |
Document ID | / |
Family ID | 30776382 |
Filed Date | 2007-11-29 |
United States Patent
Application |
20070274822 |
Kind Code |
A1 |
Liu; Michael Chung Kau ; et
al. |
November 29, 2007 |
Vacuum Pump
Abstract
A vacuum pumping arrangement is described for evacuating a load
lock chamber. A booster pump comprises a molecular drag stage and a
multi-stage centrifugal compressor mechanism. A backing pump
comprises a multi-stage centrifugal compressor mechanism exhausting
pumped fluid at atmospheric pressure. Such arrangements can reduce
noise, size and vibration levels associated with conventional load
lock pumping arrangements.
Inventors: |
Liu; Michael Chung Kau;
(West Sussex, GB) ; Dowdeswell; Stephen; (West
Sussex, GB) ; Schofield; Nigel Paul; (West Sussex,
GB) |
Correspondence
Address: |
THE BOC GROUP, INC.
575 MOUNTAIN AVENUE
MURRAY HILL
NJ
07974-2064
US
|
Family ID: |
30776382 |
Appl. No.: |
10/581934 |
Filed: |
December 23, 2004 |
PCT Filed: |
December 23, 2004 |
PCT NO: |
PCT/GB04/05407 |
371 Date: |
April 2, 2007 |
Current U.S.
Class: |
415/90 ;
137/565.23 |
Current CPC
Class: |
F04D 17/168 20130101;
F04D 17/122 20130101; F04D 19/044 20130101; F04D 17/14 20130101;
F04D 27/0207 20130101; Y10T 137/86083 20150401 |
Class at
Publication: |
415/090 ;
137/565.23 |
International
Class: |
F01D 1/36 20060101
F01D001/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2003 |
GB |
0329839.5 |
Claims
1. The vacuum pump arrangement according to claim 37 wherein the
multi-stage centrifugal compressor mechanism comprises a housing
within which the drive shaft is rotatably mounted, a plurality of
fixed members disposed within the housing and defining a plurality
of interconnected fluid chambers, the rotor elements of the
compressor mechanism comprising a plurality of impellers mounted on
the drive shaft and disposed relative to the fixed members such
that each impeller delivers compressed fluid to a respective fluid
chamber, the compressor mechanism further comprising a bypass
channel extending between two of the fluid chambers to enable fluid
to pass between those chambers without compression, and means for
controlling the flow of fluid through the bypass channel.
2. The vacuum pump arrangement according to claim 1 wherein the
control means is arranged to open the bypass channel under the
influence of a pressure difference between said two of the fluid
chambers.
3. The vacuum pump arrangement according to claim 1 wherein the
control means is arranged to open the bypass channel when the
pressure in an upstream one of said two of the fluid chambers is
greater than the pressure in a downstream one of said two of the
fluid chambers.
4. The vacuum pump arrangement according to claim 1 wherein said
two of the fluid chambers are adjacent fluid chambers of the
compressor mechanism.
5. The vacuum pump arrangement according to claim 4 wherein the
bypass channel passes through the fixed member located between the
adjacent fluid chambers.
6. The vacuum pump arrangement according to claim 1 wherein the
control means comprises valve means.
7. The vacuum pump arrangement according to claim 6 wherein the
valve means comprises a valve member displaceable in use between a
closed position and an open position by pressurised fluid.
8. The vacuum pump arrangement according to claim 7 wherein the
valve member comprises a flap valve.
9. The vacuum pump arrangement according to claim 6 wherein the
valve means is located within a fluid chamber.
10. The vacuum pump arrangement according to claim 1 comprising,
for each fluid chamber, a respective bypass channel extending
between that fluid chamber and the adjacent downstream fluid
chamber, and means for controlling the flow of fluid through each
bypass channel.
11. The vacuum pump arrangement according to claim 1 further
comprising surge control means for controlling surge within the
multi-stage centrifugal compressor mechanism.
12. (canceled)
13. The vacuum pump arrangement according to claim 11 wherein the
surge control means comprises means for conveying a stream of fluid
to each fluid chamber, and means for controlling the rate of flow
of the fluid stream into each fluid chamber.
14. The vacuum pump arrangement according to claim 13 wherein the
conveying means is arranged to convey a stream of purge gas to each
fluid chamber.
15. The vacuum pump arrangement according to claim 14 wherein the
purge gas comprises air or an inert gas.
16. The vacuum pump arrangement according to claim 13 wherein the
conveying means is arranged to convey a stream of compressed fluid
to each fluid chamber from a downstream fluid chamber.
17. The vacuum pump arrangement according to claim 16 wherein the
conveying means comprises, for each fluid chamber, a fluid passage
extending between that fluid chamber and the adjacent downstream
fluid chamber.
18. The vacuum pump arrangement according to claim 17 wherein the
fluid passages are co-axial.
19. The vacuum pump arrangement according to claim 17 wherein each
fluid passage passes through a respective fixed member.
20. The vacuum pump arrangement according to claim 16 wherein the
control means comprises valve means in fluid communication with
said conveying means.
21. The vacuum pump arrangement according to claim 20 wherein the
valve means comprises a spool valve.
22. The vacuum pump arrangement according to claim 1 wherein each
fixed member comprises a disc mounted on, or integral with, a
respective part of the housing.
23. The vacuum pump arrangement according to claim 1 comprising
means for cooling the fixed members.
24. The vacuum pump arrangement according to claim 23 wherein the
cooling means comprises a plurality of cooling fins located on one
side of each fixed member.
25. The vacuum pump arrangement according to claim 23 wherein the
cooling means comprises means for supplying a flow of coolant to
each fixed member.
26. The vacuum pump arrangement according to claim 1 comprising a
cooling jacket extending about at least part of the multi-stage
centrifugal compressor mechanism.
27. (canceled)
28. (canceled)
29. (canceled)
30. (canceled)
31. (canceled)
32. A vacuum pump comprising a multi-stage centrifugal compressor
mechanism comprising a plurality of rotor elements mounted on a
rotatably mounted drive shaft, and, upstream therefrom, a molecular
drag mechanism comprising at least one rotor element mounted on the
drive shaft, wherein the at least one rotor element of the
molecular drag mechanism at least partially surrounds a motor for
rotating the drive shaft.
33. The vacuum pump according to claim 32 wherein said at least one
rotor element of the molecular drag pumping mechanism comprises a
cylinder mounted for rotary movement with the rotor elements of the
compressor mechanism.
34. The vacuum pump according to claim 32 comprising means for
monitoring the temperature of the pump, and means for controlling
the speed of rotation of the shaft in dependence on the monitored
temperature.
35. (canceled)
36. (canceled)
37. A vacuum pumping arrangement comprising a vacuum pump in series
with a backing pump, wherein the vacuum pump comprises a
multi-stage centrifugal compressor mechanism comprising a plurality
of rotor elements mounted on a rotatable mounted drive shaft, and,
upstream therefrom, a molecular drag mechanism comprising at least
one rotor element mounted on the drive shaft, wherein the at least
one rotor element of the molecular drag mechanism at least
partially surrounds a motor for rotating the drive shaft.
38. (canceled)
39. (canceled)
40. The vacuum pumping arrangement according to claim 37 comprising
a bypass conduit connected between an exhaust from the booster pump
and an exhaust from the backing pump, and means for controlling the
flow of fluid through the bypass conduit.
41. A vacuum pump according to claim 37 wherein the molecular drag
mechanism defines a plurality of flow channels that each receive
fluid from a pump inlet and exhaust pumped fluid to a common
exhaust port.
42. A vacuum pump according to claim 37 wherein the molecular drag
mechanism is a multi-stage molecular drag mechanism.
43. A vacuum pump according to claim 42 wherein the stages of the
drag mechanism are arranged in parallel.
Description
[0001] The invention relates to a vacuum pump.
[0002] Vacuum processing is commonly used in the manufacture of
semiconductor devices to deposit thin films on to substrates.
Typically, a processing chamber is evacuated using a vacuum pump to
a very low pressure, which, depending on the type of process, may
be as low as 10.sup.-6 mbar, and feed gases are introduced to the
evacuated chamber to cause the desired material to be deposited on
one or more substrates located in the chamber. Upon completion of
the deposition, the substrate is removed from the chamber and
another substrate is inserted for repetition of the deposition
process.
[0003] Significant vacuum pumping time is required to evacuate the
processing chamber to the required pressure. Therefore, in order to
maintain the pressure in the chamber at or around the required
level when changing substrates, transfer chambers and load lock
chambers are typically used. The capacity of the load lock chamber
can range from just a few litres to several thousand litres for
some of the larger flat panel display tools.
[0004] The load lock chamber typically has a first window, which
can be selectively opened to allow substrates to be transferred
between the load lock chamber and the transfer chamber, and a
second window, which can be selectively opened to the atmosphere to
allow substrates to be inserted into and removed from the load
lock-chamber. In use, the processing chamber is maintained at the
desired vacuum by the processing chamber vacuum pump. With the
first window closed, the second window is opened to the atmosphere
to allow the substrate to be inserted into the load lock chamber.
The second window is then closed, and, using a load lock vacuum
pump, the load lock chamber is evacuated until the load lock
chamber is at substantially the same pressure as the transfer
chamber, typically around 0.1 mbar. The first window is then opened
to allow the substrate to be transferred to the transfer chamber.
The transfer chamber is then evacuated to a pressure at
substantially the same pressure as the processing chamber,
whereupon the substrate is transferred to the processing
chamber.
[0005] When vacuum processing has been completed, the processed
substrate is transferred back to the load lock chamber. With the
first window closed to maintain the vacuum in the transfer chamber,
the pressure in the load lock chamber is brought up to atmospheric
pressure by allowing a non-reactive gas, such as air or nitrogen,
to flow into the load lock chamber. When the pressure in the load
lock chamber is at or near atmospheric pressure, the second window
is opened to allow the processed substrate to be removed. Thus, for
a load lock chamber, a repeating cycle of evacuation from
atmosphere to a medium vacuum (around 0.1 mbar) is required.
[0006] Load lock pumps are typically oil-free in their vacuum
chambers, as any lubricants present in the vacuum chambers might
cause contamination of the clean environment in which the vacuum
processing is performed. For example, the "iH" series of BOC
Edwards "dry" vacuum pumps comprise a dry backing, or roughing,
pump in combination with a single stage Roots mechanism booster, or
blower, pump mounted above the dry pump. Backing pumps are commonly
multi-stage positive displacement pumps employing inter-meshing
rotors. The rotors may have the same type of profile in each stage
or the profile may change from stage to stage.
[0007] For the larger flat panel display tools, the pumping speed
of the load lock pumps needs to be high, for example, up to 2000
m.sup.3/hour. Whilst a load lock pump formed from a dry backing
pump using Roots and Northey mechanisms, in series with a Roots
booster pump can provide such a pumping speed, the relatively large
foot-print of the pump combination, together with the level of
noise and vibration generated during use, typically lead to the
load lock pump being located remote from the processing tool, for
example, in a basement. As well as being inconvenient to the user,
relatively long runs of large diameter pipe work are needed to
connect the load lock pump to the load lock chamber, significantly
increasing installation costs.
[0008] It is an aim of at least the preferred embodiments of the
present invention to solve these and other problems.
[0009] In summary, in accordance with the present invention at
least one of the booster pump and the backing pump in the
conventional pumping arrangement is replaced by a vacuum pump
comprising a multi-stage centrifugal compressor system. In one
embodiment, both the booster pump and the backing pump are replaced
by a single vacuum pump exhausting to atmosphere. In a second
embodiment, the booster pump is provided by a similar vacuum pump
to the first embodiment, having a reduced number of compressor
stages, backed by a backing pump. This backing pump may be a
conventional backing pump, or, in accordance with a third
embodiment, may be a vacuum pump comprising a multi-stage
centrifugal compressor system exhausting to atmosphere. Such a
backing pump may be provided with a conventional Roots booster
pump. Thus, in one aspect, the present invention provides a vacuum
pump comprising a multi-stage centrifugal compressor mechanism for
receiving fluid to be pumped and exhausting pumped fluid
substantially at atmospheric pressure.
[0010] Due to the reduced levels of size, noise and vibration
associated with a centrifugal compressor system in comparison to
the conventional dry pumps, replacing one or both of the
conventional backing and booster pumps with a pump comprising a
multi-stage centrifugal compressor mechanism can enable at least
part of the pumping arrangement to be mounted on the processing
tool, thereby potentially avoiding the expensive long runs of large
diameter pipe work.
[0011] It is desirable to perform the evacuation of a vacuum
chamber, such as a load lock chamber, from atmospheric pressure to
a low pressure as quickly as possible. The faster that this
evacuation can be accomplished, the higher the rate of processing
substrates becomes. However, during the initial stages of the
evacuation of a chamber from atmospheric pressure using a pump
having a multi-stage pumping mechanism, the compression of fluid by
the pumping mechanism can cause the fluid pressure to increase
above atmospheric pressure. This can result in undesirable
overloading of the exhaust stages of the pumping mechanism. If such
a pump is operated for a significant period in this condition,
damage can occur in the form of seals and/or bearings failing, or
by impact between the fragile rotating impellers and pump's
housing.
[0012] In view of this, in another aspect the present invention
provides a multi-stage centrifugal compressor mechanism comprising
a housing, a drive shaft rotatably mounted within the housing, a
plurality of fixed members disposed within the housing and defining
a plurality of interconnected fluid chambers, a plurality of
impellers mounted on the drive shaft and disposed relative to the
fixed members such that each impeller delivers compressed fluid to
a respective fluid chamber, a is bypass channel extending between
two of the fluid chambers to enable fluid to pass between those
chambers without compression, and means for controlling the flow of
fluid through the bypass channel. Compressed fluid can thus be
conveyed between fluid chambers without compression, which can
enable a larger upstream pumping stage to operate at full speed
without causing the pumped fluid to be pressurised above
atmospheric pressure.
[0013] The control means is thus preferably arranged to open the
bypass channel under the influence of a pressure difference between
said two of the fluid chambers, and in particular when the pressure
in an upstream one of said two of the fluid chambers is greater
than the pressure in a downstream one of said two of the fluid
chambers.
[0014] In a preferred embodiment, said two of the fluid chambers
are adjacent fluid chambers of the compressor mechanism, although
one or more other fluid chambers may, alternatively, separate these
two fluid chambers. For example, one of the fluid chambers may be
the first, lowest pressure fluid chamber of the pumping mechanism,
and the other fluid chamber by the last, highest pressure fluid
chamber of the pumping mechanism. Where these two fluid chambers
are adjacent, however, the bypass channel may conveniently pass
through the fixed member located between the fluid chambers.
[0015] The control means preferably comprises valve means, for
example, a valve member displaceable in use between a closed
position and an open position by pressurised fluid. Such a valve
member may be conveniently provided by a flap valve, which can be
conveniently positioned within a fluid chamber to control the flow
of fluid into that fluid chamber from the bypass channel.
[0016] Preferably, the mechanism comprises, for each fluid chamber,
a respective bypass channel extending between that fluid chamber
and the adjacent downstream fluid chamber, and means for
controlling the flow of fluid through each bypass channel.
[0017] Centrifugal compressor mechanisms are susceptible to surging
of pumped fluid when the specific flow rate of the pumped fluid
through a stage of the compressor mechanism is relatively low. The
surging manifests itself in a backflow of fluid into the compressor
impeller, and adversely affects the efficient operation of the
vacuum pump, and in extreme conditions, may actually damage the
pump. In view of this, the mechanism preferably comprises surge
control means for controlling surge within the compressor
mechanism. Therefore, in a further aspect the present invention
provides a multi-stage centrifugal compressor mechanism comprising
a housing, a drive shaft rotatably mounted within the housing, a
plurality of fixed members disposed within the housing and defining
a plurality of interconnected fluid chambers, a plurality of
impellers mounted on the drive shaft and disposed relative to the
fixed members such that each impeller delivers compressed fluid to
a respective fluid chamber, and surge control means for controlling
surge within the multi-stage centrifugal compressor mechanism.
[0018] The surge control means preferably comprises means for
conveying a stream of fluid to each fluid chamber, and means for
controlling the rate of flow of the fluid stream into each fluid
chamber. In one embodiment, the conveying means is arranged to
convey a stream of gas, such as air, nitrogen or an inert gas, to
each fluid chamber. In another embodiment, the conveying means is
arranged to convey a stream of compressed fluid to each fluid
chamber. In either case, the rate of flow through the compressor
mechanism can be maintained at a value above that at which surging
will occur.
[0019] Where the conveying means is arranged to convey a stream of
compressed fluid to each fluid chamber from a downstream fluid
chamber, the conveying means preferably comprises, for each fluid
chamber, a fluid passage (separate from the previously-mentioned
bypass channel) extending between that fluid chamber and the
adjacent downstream fluid chamber. These fluid passages are
preferably co-axial.
[0020] The means for controlling the rate of flow of the fluid
stream into each fluid chamber preferably comprises valve means.
The valve means may comprise a series of valves for controlling
fluid flow through respective fluid passages or a spool valve for
controlling fluid flow through each fluid passage. The valve means
is preferably located at least partially within the chamber,
thereby avoiding the need to provide external pipe connections. The
valve means may be controlled by a separate controller. In order to
control the valve means, a pressure sensor may be provided to
monitor the pressure of fluid passing through a pump inlet, a
signal from the inlet sensor being supplied to a control system
which controls the opening and closing of the valve means. In
addition, or alternatively, pressure sensors may be provided within
the pumping mechanism to monitor pressure fluctuation within the
pumping mechanism, and thus detect the onset of surging.
[0021] Each impeller preferably has on one side thereof a plurality
of vanes or blades extending between the inner periphery and the
outer periphery thereof. Each blade preferably follows a curved
path. To facilitate manufacture, each fixed member preferably
comprises a disc integral with a respective part of the
housing.
[0022] Fluid that is compressed by the compressor mechanism
typically becomes hot. In order to cool fluid pumped by the
compressor mechanism, particularly at the exhaust stages, the
mechanism preferably comprises means for cooling each fixed member.
For example, a plurality of cooling fins may be provided on one
side thereof. Alternatively, or in addition, the cooling means may
comprise means for supplying a flow of coolant to each fixed
member. This can provide direct cooling of both the cooling fins
(where provided) and the fixed plate. The cooling fins may be
located between the fixed plate and a diffuser plate for directing
a stream of compressed fluid from an impeller to a fluid chamber so
that the fins can also provide for cooling of the diffuser
plate.
[0023] The present invention also provides a vacuum pump comprising
a compressor mechanism as aforementioned.
[0024] Excessive heating of the compressor mechanism may occur if
the pump is operated over a relatively long period at a relatively
high pressure, for example, if a door to a load lock chamber
evacuated by the pump has been inadvertently left open. In order to
prevent excessive heating of the pump, the temperature of the pump
may be monitored, and the speed of rotation of the compressor
mechanism varied in response to the monitored temperature. This can
enable the speed of the pump to be reduced in the event of
overheating, thereby reducing the temperature within the pump, and
preventing the pump from being unduly operated at a high speed for
a relatively long period.
[0025] Therefore, the pump preferably comprises means for
monitoring the temperature of the pump, and means for controlling
the speed of rotation of the shaft in dependence on the monitored
temperature. The monitoring means may be conveniently provided by
any suitable temperature sensor, such as a thermocouple, located
within or in close proximity to the housing. A controller for
controlling a motor driving the drive shaft may provide the control
means.
[0026] In order to cool the housing, to which heat will be
transferred by the pumped fluid, an external cooling system may
also be provided, for example, in the form of a cooling jacket
extending about at least part of the compressor mechanism.
[0027] Where the pump is to be used as a backing pump, the backing
pump may consist of such a multi-stage centrifugal compressor
mechanism, in combination with any suitable booster pump. Such a
booster pump may be provided by a pump comprising such a
multi-stage centrifugal compressor mechanism downstream from a
molecular drag mechanism, the number of stages of the compressor
mechanism (for example, two) being smaller in the booster pump than
in the backing pump (for example, six or seven). Alternatively, the
conventional combination of booster and backing pumps may be
replaced by a single pump, this pump comprising a multi-stage (for
example, six or seven stage) centrifugal compressor mechanism
downstream from a multi-stage (for example, four stage), molecular
drag mechanism. The molecular drag mechanism preferably comprises a
multi-stage Holweck mechanism having a plurality of channels
arranged as a plurality of helixes. The drag stages may be arranged
in series, in parallel for maximum pumped volume, or in a
combination of both. In order to minimise the length of the pump
the molecular drag mechanism preferably at least partially
surrounds a motor for rotating the drive shaft. For instance, where
the molecular drag pumping mechanism is a Holweck mechanism, a
rotor element of the molecular drag pumping mechanism typically
comprises a cylinder mounted for rotary movement with the rotor
elements of the compressor mechanism, which cylinder may at least
partially surround the motor. This, in a further aspect the present
invention provides a vacuum pump comprising a multi-stage
centrifugal compressor mechanism comprising a plurality of rotor
elements mounted on a rotatably mounted drive shaft, and, upstream
therefrom, a molecular drag mechanism comprising at least one rotor
element mounted on the drive shaft, wherein the at least one rotor
element of the molecular drag mechanism at least partially
surrounds a motor for rotating the drive shaft.
[0028] As discussed above, for rapid pump down of a chamber the
pump may be provided with valve means for enabling compressed fluid
to by-pass one or more of the impellers of the multi-stage
centrifugal compressor mechanism, allowing the pump to pump down at
full inlet speed even when the exhaust stages of the compressor
mechanism are somewhat smaller than the inlet stages. With such a
design, the backing pump may become a restriction to the flow of
fluid through the pumping arrangement. Therefore, in a preferred
arrangement a fluid by-pass conduit is connected between an exhaust
from the booster pump and an exhaust from the backing pump, with
means being provided for controlling the flow of fluid through the
by-pass. Such an arrangement may be provided for any combination of
booster and backing pumps.
[0029] Preferred features of the present invention will now be
described, by way of example only, with reference to the
accompanying drawings, in which:
[0030] FIG. 1 is a cross-section through a first embodiment of a
vacuum pump;
[0031] FIG. 2 is a cross-section through a second embodiment of a
vacuum pump, which is similar to that of FIG. 1 with a different
surge control mechanism;
[0032] FIG. 3 is a cross-section through an embodiment of a booster
pump, which is similar to that of FIG. 1 with a reduced number of
compressor stages;
[0033] FIG. 4 is an enlarged view of part of the cross-section of
FIG. 3;
[0034] FIG. 5 is a cross-section through an embodiment of a backing
pump, which is similar to that of FIG. 1 without a drag
mechanism;
[0035] FIG. 6 illustrates schematically an arrangement of valves in
a pumping arrangement comprising a booster pump in series with a
backing pump; and
[0036] FIG. 7 illustrates schematically an arrangement for
controlling the speed of a booster pump.
[0037] With reference to FIG. 1, a vacuum pump 10 suitable for
evacuating a load lock chamber comprises a housing 12 having three
parts 14, 16, and 18. An inlet 20 for the pump 10 is located in the
first part 14 of the housing 12, and an exhaust 21 for the pump 10
is located in the third part 18 of the housing 12.
[0038] The first part 14 of the housing 12 houses a multi-stage
molecular drag pumping mechanism 22. As illustrated in FIG. 1, in
this embodiment the molecular drag pumping mechanism is provided by
a four-stage Holweck mechanism 22, although any suitable number of
pumping stages may be provided. The rotor of the Holweck mechanism
22 comprises two carbon-fibre cylinders 24, 26, mounted
concentrically on a disc-like impeller 28 integral with or, as
illustrated, mounted on is a rotatable shaft 30. The shaft 30 is
supported at each end by lubricant free bearings (not shown),
preferably magnetic bearings, and is driven by a motor 31 housed by
the third part 18 of the housing 12.
[0039] Each cylinder 24, 26 of the Holweck mechanism 22 has smooth
inner and outer surfaces. The stator of the Holweck mechanism
comprises a plurality of cylinders 32, 34 and 36 concentrically
arranged with and surrounding the rotor cylinders 24, 26, the
outermost cylinder 36 being provided by the first part 14 of the
housing 12. Helical grooves are formed on the outer surfaces of the
innermost stator cylinder 32, the inner and outer surfaces of the
middle stator cylinder 34 and the inner surface of the outermost
stator cylinder 36 to define co-axial helical fluid channels 38,
40, 42, 44, which receive fluid from the pump inlet 20 and exhaust
pumped, compressed fluid to a common exhaust port 48 through
openings 50 formed in the disc-like impeller 28.
[0040] The second part 16 of the housing 12 may be conveniently
provided by a plurality of co-axial rings 16a, 16b, 16c, 16d, 16e,
and 16f, and houses a multi-stage centrifugal compressor mechanism
52. In the embodiment shown in FIG. 1, the compressor mechanism 52
comprises seven pumping stages. Each of the first six pumping
stages comprises a respective fluid chamber 58, each defined
between respective discs 60 co-axially mounted on the inner wall of
the second part 16 of the housing 12. Apertures 62 in the discs 60
interconnect the fluid chambers 58 to enable fluid to be conveyed
from the exhaust port 48 of the Holweck mechanism 22 to the exhaust
21 of the pump through each of the fluid chambers 58 in turn.
[0041] Each fluid chamber 58 includes a rotor in the form of an
impeller 54 mounted on the shaft 30. Each impeller 54 has a
plurality of curved blades or vanes 56 located on the upper surface
(as shown in FIG. 1) of the impeller 54. The impellers 54 are
disposed relative to the discs 60 such that, during use, each
impeller 54 delivers compressed fluid to a respective fluid chamber
58.
[0042] Each fluid chamber 58 also includes a disc-like diffuser
plate 64, each integral with a respective ring 16a, 16b, 16c, 16d,
16e, and 16f for directing compressed fluid output from the each of
the impellers 54 radially outwardly. As a result, compressed fluid
flows within the fluid chambers 58 in a serpentine manner; within
each fluid chamber 58, compressed fluid initially flows radially
outwards between the upper surface (as shown) of the diffuser plate
64 and the facing, lower surface of the upper disc 60 defining that
fluid chamber, and subsequently flows radially inwards between the
lower surface (as shown) of the diffuser plate 64 and the facing,
upper surface of the lower disc defining that fluid chamber.
[0043] Each of the diffuser plates 64 comprises a plurality of
cooling fins 66, provided on the lower surface thereof, for cooling
the compressed fluid. In order to cool the housing 12, to which
heat will be transferred by the fluid pumped by the compressor
mechanism 52, an external pump cooling system (not shown) may also
be provided, for example, in the form of a cooling jacket extending
about at least the second part 16 of the housing 12.
[0044] In use, the motor is activated to rotate the shaft 30 at a
high speed, typically in the range from 15,000 to 80,000 rpm. Fluid
enters the pump 10 through the inlet 20, and passes in turn through
the Holweck mechanism 22 and compressor mechanism 52 before being
exhausted from the outlet of the pump 10 at a pressure at or around
atmospheric pressure. With the arrangement shown in FIG. 1, a
pressure less than 1 mbar, typically at or around 0.1 mbar, can be
generated in a load lock chamber connected to the inlet 20 of the
pump 10.
[0045] In order to inhibit surging within the compressor mechanism
52 at relatively low flow rates, the pump 10 is provided with a
surge control mechanism for selectively increasing the rate of flow
of fluid through one or more of the pumping stages of the
compressor mechanism 52. In the first embodiment shown in FIG. 1, a
fluid port 68 is provided in each of the rings 16a, 16b, 16c, 16d,
16e, 16f, each port 68 extending radially through the ring to allow
a stream of fluid to be injected into the fluid chamber 58. This
stream of fluid may be provided by any suitable source. In a first
example, the fluid ports 68 of adjacent pumping stages may be
connected via an arrangement of conduits located to one side of the
pump 10, the conduits containing one or more valves for selectively
opening the conduits to allow pumped fluid from one pumping stage
to flow from the fluid port 68 of that stage to the fluid port 68
of the adjacent upstream pumping stage, thereby increasing the rate
of flow of fluid through the inlet to the pumping stage. In a
second example, a stream of purge gas, such as nitrogen or air, may
be selectively supplied from a suitable source to one of more of
the fluid ports 68 in order to increase the rate of flow of fluid
through one or more of the pumping stages.
[0046] In a third example, as illustrated in FIG. 2, a passage 70
for compressed fluid may be provided through the pumping stages of
the compressor mechanism 52 in addition to, or as an alternative
to, providing fluid ports 68. The passage 70 is defined by a series
of co-axial apertures 72 formed in the discs 60 co-axially mounted
on the inner wall of the second part 16 of the housing 12. A spool
valve 74 is provided for selectively opening and closing the
apertures 72 to control the flow of compressed fluid through the
passage 70. As illustrated, the spool valve 74 may be shaped so
that movement of the valve 74 causes all of the apertures 72 to be
opened simultaneously to allow compressed fluid to flow through the
apertures 72 to adjacent upstream pumping stages. Alternatively,
the spool valve 74 may be shaped so that movement of the valve 74
causes each of the apertures 74 to be opened in turn, starting, for
example, with the aperture 74 connecting the exhaust pumping stages
of the compressor mechanism 52.
[0047] In order to control the valves in any of these three
examples, a pressure sensor may be provided to monitor the pressure
of fluid passing through the pump inlet 20. A signal from the inlet
sensor may be supplied to a control system, which controls the
opening and closing of the, or each, valve in order to inhibit
surging. In addition, or alternatively, pressure sensors may be
provided within the pump 10 to monitor pressure fluctuation within
the pump, and thus detect the onset of 15 surging. Motor current
may also be used to indicate shaft torque and power, and thus an
estimation of inlet pressure.
[0048] As the pumps illustrated in FIGS. 1 and 2 exhaust fluid at
or around atmospheric pressure, each pump 10 would be suitable for
replacing both the conventional booster and backing pump used for
evacuating a load lock chamber. Due to the reduced size of the pump
10 relative to the size of the conventional combination of booster
and backing pumps, and due to the reduced noise and vibration
levels associated with the pump 10, the pump 10 may be conveniently
mounted on the side of the processing tool.
[0049] By reducing the number of stages of the compressor mechanism
52, the pump 10 can be suitable for use as a booster pump. FIGS. 3
and 4 illustrate an embodiment of such a booster pump 100. The
booster pump comprises a housing having three parts 102, 104, and
106. An inlet 110 for the pump 100 is located in the first part 102
of the housing, and an exhaust 112 for the pump 100 is located in
the third part 106 of the housing.
[0050] The second part 104 of the housing houses a multi-stage
molecular drag pumping mechanism 120. As illustrated in FIG. 3,
similar to the pump 10 of FIG. 1 the molecular drag pumping
mechanism is provided by a four-stage Holweck mechanism 120,
although any suitable number of pumping stages may be provided. The
rotor of the Holweck mechanism 120 comprises three carbon-fibre
cylinders 122, 124, 126, mounted concentrically on a disc-like
impeller 128 integral with or, as illustrated, mounted on a
rotatable drive shaft 130. The shaft 130 is supported at each end
by rolling bearings 132 and is driven by a motor 134 partially
surrounded by the cylinders 122, 124, 126 of the Holweck mechanism
120.
[0051] Each cylinder of the Holweck mechanism 120 has smooth inner
and outer surfaces. The stator of the Holweck mechanism 120
comprises a plurality of cylinders 136, 138 and 140 concentrically
arranged with and surrounding the rotor cylinders 122, 124, 126,
the outermost cylinder 36 being provided by, or, as illustrated,
mounted on the second part 104 of the housing. Helical grooves are
formed on the outer surface of the innermost stator cylinder 136,
the inner and outer surfaces of the middle stator cylinder 138 and
the inner surface of the outermost stator cylinder 140 to define
co-axial helical fluid channels which receive fluid from the pump
inlet 110 through one or more openings 142 formed in the disc-like
impeller 128 and exhaust pumped, compressed fluid to a common
exhaust port 144.
[0052] The second part 106 of the housing may be conveniently
provided by a plurality of co-axial rings 106a, 106b, 106c, and
106d, and houses a multi-stage centrifugal compressor mechanism
150. In the embodiment shown in FIGS. 3 and 4, the compressor
mechanism 150 comprises four pumping stages. Each of the first
three pumping stages comprises a respective fluid chamber 158, each
defined between respective discs 160 co-axially mounted on the
inner wall of the second part 106. Apertures 162 in the discs 160
interconnect the fluid chambers 158 to enable fluid to be conveyed
from the exhaust port 144 of the Holweck mechanism to the exhaust
112 of the pump through each of the fluid chambers 158 in turn.
[0053] Each fluid chamber 158 includes a rotor in the form of an
impeller 154 mounted on the shaft 130. Each impeller 54 has a
plurality of curved blades or vanes 156 located on the upper
surface (as shown in FIG. 1) of the impeller 154. The impellers 154
are disposed relative to the discs 160 such that, during use, each
impeller 154 delivers compressed fluid to a respective fluid
chamber 158.
[0054] Each fluid chamber 158 also includes a disc-like diffuser
plate 164, each integral with a respective ring 106a, 106b, 106c,
106d, for directing compressed fluid output from the each of the
impellers 154 radially outwardly. As a result, compressed fluid
flows within the fluid chambers 158 in a serpentine manner; within
each fluid chamber 158, compressed fluid initially flows radially
outwards between the upper surface (as shown) of the diffuser plate
164 and the facing, lower surface of the upper disc 160 defining
that fluid chamber, and subsequently flows radially inwards between
the lower surface (as shown) of the diffuser plate 164 and the
facing, upper surface of the lower disc 160 defining that fluid
chamber.
[0055] Each of the diffuser plates 164 may comprise a plurality of
cooling fins (not shown), provided on the lower surface thereof,
for cooling the compressed fluid. In order to cool the fins, a
coolant may be conveyed through cooling channels defined between
the lower (as shown) surface of the diffuser plate 164 and the
facing, upper surface of the lower disc 160.
[0056] In use, the motor is activated to rotate the shaft 130 at a
high speed, typically in the range from 15,000 to 80,000 rpm. Fluid
enters the pump 100 through the inlet 110, and passes in turn
through the Holweck mechanism 120 and compressor mechanism 150
before being exhausted from the outlet 112 of the pump 100 at a
sub-atmospheric pressure.
[0057] Similar to the pump described in FIG. 1, a surge control
mechanism may be provided to inhibit surging within the compressor
mechanism. For example, as shown in FIG. 4, a fluid port 168 may
provided in each of the rings 106a, 106b, and 106c, each port 168
extending radially through the ring to allow a stream of fluid to
be injected into a respective fluid chamber 158. This stream of
fluid may be provided by any suitable source. Preferably, such a
mechanism would be operated only at relatively low inlet pressures
in order to maximise throughput at relatively high inlet
pressures.
[0058] In addition to such a surge control mechanism, an additional
mechanism may also be provided to enable rapid pump down of a
chamber attached to the inlet 110 of the booster pump 100 without
overloading the exhaust stages of the compressor mechanism. As
shown in FIG. 4, one or more of the discs 160 are provided with
bypass channels 170 for enabling compressed fluid to pass to an
adjacent, downstream fluid chamber without compression by an
impeller 154. The channels 170 are normally closed by a valve
mechanism 172, which in this embodiment is in the form of a pair of
flap valves having a common mounting within the downstream fluid
chamber 158. The valve mechanism 172 is selectively opened by a
pressure differential between fluid within the adjacent fluid
chambers 158, so that when the pressure of fluid in the upstream
fluid chamber is greater than that in the downstream fluid chamber,
the valve opens to enable fluid to pass from the upstream fluid
chamber to the downstream fluid chamber without compression.
[0059] This can enable gas being pumped by the pump 100 to pass
through one or more of the smaller, exhaust stages of the
compressor mechanism 150 without compression, thereby avoiding the
gas from being compressed above atmospheric pressure by those
exhaust stages and thus preventing those stages from becoming
overloaded. The booster pump 100 may be used in combination with
any suitable backing pump. FIG. 5 illustrates an embodiment of a
backing pump 200 employing a multi-stage centrifugal compressor
mechanism, which would be suitable for use with such a booster
pump, or any conventional booster pump. The backing pump 200 is
similar to the pump 10 illustrated in FIG. 1, with the exception
that the backing pump 200 does not require a drag mechanism as the
fluid entering the backing pump 200 would be at a higher pressure
than that entering the pump 10. In other words, the backing pump
200 comprises a multi-stage compressor mechanism 252 for receiving
fluid from the pump inlet 220 and exhausting pumped fluid at or
around atmospheric pressure from pump outlet 221. The compressor
mechanism 252 of the backing pump 200 is similar to the compressor
mechanism 52 of the pump 10, and so is not described in further
detail here.
[0060] During pump down of a chamber attached to a series
combination of the booster pump 100 and backing pump 200, depending
on the pumping mechanism of the backing pump 200, the backing pump
200 may restrict the rapid evacuation of the chamber, as the
backing pump 200 may not be able to pump the fluid exhaust from the
booster pump 100 sufficiently quickly. In order to enable at least
some of the gas pumped from the chamber to by-pass the backing pump
200, as shown in FIG. 6 an external by-pass conduit 250 may be
provided in fluid communication with the exhaust 112 of the booster
pump 100 and the exhaust 221 of the backing pump 200. The by-pass
conduit 250 preferably includes a by-pass valve 252 for opening the
conduit 250 at high exhaust pressures from the booster pump 100 to
enable "excess" fluid exhaust from booster pump 100 to by-pass the
backing pump 200.
[0061] With reference now to FIG. 7, in order to prevent
overheating of, say, the booster pump 100 during pump down of a
chamber attached to the inlet thereof, a the pump 100 may be
provided with a temperature sensor 300 located, for example, within
the housing of the pump 100, for outputting to a controller 302 a
signal indicative of the current temperature within the housing of
the pump 100. In response to the received signal, the controller
302 can issue a command to the motor 134 of the pump 100 to adjust
the speed of rotation of the shaft 130. By reducing the speed of
the pump, the temperature within the housing of the pump 100 can be
reduced. As an alternative, or in addition, to the control of the
speed of the pump in dependence on the temperature of the pump, the
speed of the pump may also be controlled in dependence on the
pressure of gas being conveyed to the inlet 110 of the pump using a
pressure sensor 304 located proximate the inlet of the pump.
[0062] In summary, two vacuum pumping arrangements are described
for evacuating a load lock chamber. In the first arrangement, a
single pump comprises a multi-stage molecular drag stage and a
multi-stage centrifugal compressor mechanism exhausting pumped
fluid at atmospheric pressure. In the second arrangement, a booster
pump is provided in series with a backing pump. The booster pump is
similar to the pump of the first arrangement, but with a reduced
number of compressor mechanism stages. The backing pump also
comprises a multi-stage centrifugal compressor mechanism exhausting
pumped fluid at atmospheric pressure. Such arrangements can reduce
noise, size and vibration levels associated with conventional load
lock pumps.
* * * * *