U.S. patent application number 16/202170 was filed with the patent office on 2019-03-28 for vacuum system for immersion photolithography.
This patent application is currently assigned to ASML NETHERLANDS B.V.. The applicant listed for this patent is ASML NETHERLANDS B.V.. Invention is credited to Andrew John HARPHAM, Paul John Shechter, Paul Alan Stockman.
Application Number | 20190094715 16/202170 |
Document ID | / |
Family ID | 32851367 |
Filed Date | 2019-03-28 |
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United States Patent
Application |
20190094715 |
Kind Code |
A1 |
HARPHAM; Andrew John ; et
al. |
March 28, 2019 |
VACUUM SYSTEM FOR IMMERSION PHOTOLITHOGRAPHY
Abstract
A vacuum system for extracting a stream of a multi-phase fluid
from a photo-lithography tool comprises a pumping arrangement for
drawing the fluid from the tool, and an extraction tank located
upstream from the pumping arrangement for separating the fluid
drawn from the tool into gas and liquid phases. The pumping
arrangement comprises a first pump for extracting gas from the
tank, and a second pump for extracting liquid from the tank. In
order to minimize any pressure fluctuations transmitted from the
vacuum system back to the fluid within the tool, a pressure control
system maintains a substantially constant pressure in the tank by
regulating the amounts of liquid and gas within the tank.
Inventors: |
HARPHAM; Andrew John;
(Copthorne, GB) ; Shechter; Paul John; (Bromley
Cross, GB) ; Stockman; Paul Alan; (Hillsborough,
NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML NETHERLANDS B.V. |
Veldhoven |
|
NL |
|
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
32851367 |
Appl. No.: |
16/202170 |
Filed: |
November 28, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15857368 |
Dec 28, 2017 |
10168624 |
|
|
16202170 |
|
|
|
|
15362530 |
Nov 28, 2016 |
9857699 |
|
|
15857368 |
|
|
|
|
14469389 |
Aug 26, 2014 |
9507270 |
|
|
15362530 |
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|
13187166 |
Jul 20, 2011 |
8830440 |
|
|
14469389 |
|
|
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|
12340326 |
Dec 19, 2008 |
8164734 |
|
|
13187166 |
|
|
|
|
10869191 |
Jun 16, 2004 |
7481867 |
|
|
12340326 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03F 7/70341 20130101;
G03F 7/70858 20130101; G03F 7/70841 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Claims
1.-20. (canceled)
21. A lithographic apparatus, comprising: a projection device
configured to project a radiation beam onto a substrate; an inlet
configured to provide a liquid to a space between the projection
device and the substrate; an outlet configured to remove at least
part of the liquid as a multi-phase fluid; a separator configured
to receive the multi-phase fluid from the outlet, the separator
configured to separate the fluid into gas and liquid phases; a
first opening at a first side of the separator, the first opening
configured to allow the multi-phase fluid into a part of the
separator; and a second opening at a second side of the separator
opposite to the first side, the second opening configured to
exhaust separated gas from the separator.
22. The apparatus of claim 21, further comprising a pump
arrangement configured to draw the multi-phase fluid into the
separator.
23. The apparatus of claim 21, further comprising a pump
arrangement configured to extract separated liquid, via a third
opening, from the separator.
24. The apparatus of claim 21, further comprising a control system
configured to control a pressure within the separator by regulating
the gas and/or liquid therein.
25. The apparatus of claim 21, further comprising a control system
configured to control flow of gas from the separator.
26. The apparatus of claim 21, further comprising a control system
configured to control an amount of liquid within the separator.
27. The apparatus of claim 21, further comprising a flow
restriction in a fluid path from the separator to a pump fluidly
connected to the separator.
28. The apparatus of claim 21, wherein the separator comprises a
chamber having the first and second openings therein.
29. The apparatus of claim 21, wherein the first opening is located
at a different height than a third opening configured to exhaust
separated liquid from the separator.
30. The apparatus of claim 29, wherein the first opening is higher
than the third opening.
31. The apparatus of claim 21, further comprising a control system
configured to control a level of liquid within the separator.
32. A lithographic apparatus, comprising: a projection device
configured to project a radiation beam onto a substrate; an inlet
configured to provide a liquid to a space between the projection
device and the substrate; an outlet configured to remove at least
part of the liquid as a multi-phase fluid; a separator configured
to receive the multi-phase fluid from the outlet, the separator
configured to separate the fluid into gas and liquid phases; a
first opening configured to allow the multi-phase fluid into a part
of the separator; and a second opening configured to exhaust
separated liquid from the separator, the first opening being
located at a different height than the second opening.
33. The apparatus of claim 32, further comprising a pump
arrangement configured to draw the multi-phase fluid into the
separator.
34. The apparatus of claim 32, further comprising a pump
arrangement configured to extract separated liquid, via the second
opening, from the separator.
35. The apparatus of claim 32, further comprising a control system
configured to control a pressure within the separator by regulating
the gas and/or liquid therein.
36. The apparatus of claim 32, further comprising a control system
configured to control flow of gas from the separator.
37. The apparatus of claim 32, further comprising a control system
configured to control an amount of liquid within the separator.
38. The apparatus of claim 32, further comprising a flow
restriction in a fluid path from the separator to a pump fluidly
connected to the separator.
39. The apparatus of claim 32, wherein the separator comprises a
chamber having the first and second openings therein.
40. A lithographic apparatus, comprising: a projection device
configured to project a radiation beam onto a radiation-sensitive
substrate; an inlet opening located above the stage, the inlet
configured to provide a liquid to a space between the projection
device and the substrate; an outlet opening located above the
stage, the outlet configured to remove at least part of the liquid
as a multi-phase fluid; a stage configured to support the substrate
and moveable relative to the inlet and outlet; a separator
configured to receive the multi-phase fluid from the outlet, the
separator configured to separate the fluid into gas and liquid
phases; a first opening at a first side of a chamber of the
separator, the first opening configured to allow the multi-phase
fluid into a part of the separator; a second opening at a second
side of the chamber opposite to the first side, the second opening
configured to exhaust separated gas from the separator; and a third
opening configured to exhaust separated gas from the chamber, the
second opening located at a different height than the second
opening.
Description
[0001] The present application is a continuation of co-pending U.S.
patent application Ser. No. 15/857,368, filed Dec. 28, 2017, now
allowed, which is a continuation of U.S. patent application Ser.
No. 15/362,530, filed Nov. 28, 2016, now U.S. Pat. No. 9,857,699,
which is a continuation of U.S. patent application Ser. No.
14/469,389, filed Aug. 26, 2014, now U.S. Pat. No. 9,507,270, which
is a continuation of U.S. patent application Ser. No. 13/187,166,
filed Jul. 20, 2011, now U.S. Pat. No. 8,830,440, which is a
continuation of U.S. patent application Ser. No. 12/340,326, filed
Dec. 19, 2008, now U.S. Pat. No. 8,164,734, which is a continuation
of U.S. patent application Ser. No. 10/869,191, filed on Jun. 16,
2004, now U.S. Pat. No. 7,481,867. The entire content of each of
the foregoing applications is herein fully incorporated by
reference.
FIELD
[0002] This invention relates to a vacuum system for extracting a
multi-phase fluid, and more particularly for extracting a
multi-phase fluid from an Immersion photolithography exposure
tool.
BACKGROUND
[0003] Photolithography is an important process step in
semiconductor device fabrication. In photolithography, a circuit
design is transferred to a wafer through a pattern imaged onto a
photoresist layer deposited on the wafer surface. The wafer then
undergoes various etch and deposition processes before a new design
is transferred to the wafer surface. This cyclical process
continues, building up multiple layers of the semiconductor
device.
[0004] The minimum feature that may be printed using
photolithography is determined by the resolution limit W, which is
defined by the Rayleigh equation as:
W = k 1 .lamda. N A ( 1 ) ##EQU00001##
where k1 is the resolution factor, .lamda. is the wavelength of the
exposing radiation and NA is the numerical aperture. In
lithographic processes used in the manufacture of semiconductor
devices, it is therefore advantageous to use radiation of very
short wavelength in order to improve optical resolution so that
very small features in the device may be accurately reproduced.
Monochromatic visible light of various wavelengths have been used,
and more recently radiation in the deep ultra violet (DUV) range
has been used, including radiation at 193 nm as generated using an
ArF excimer laser.
[0005] The value of NA is determined by the acceptance angle
(.alpha.) of the lens and the index of refraction (n) of the medium
surrounding the lens, and is given by the equation:
NA=n sin .alpha. (2)
[0006] For clean dry air (CDA), the value of n is 1, and so the
physical limit to NA for a lithographic technique using CDA as a
medium between the lens and the wafer is 1, with the practical
limit being currently around 0.9.
[0007] Immersion photolithography is a known technique for
improving optical resolution by increasing the value of NA. With
reference to FIG. 1, in this technique a liquid 10 having a
refractive index n>1 is placed between the lower surface of the
objective lens 12 of a projection device 14 and the upper surface
of a wafer 16 located on a moveable wafer stage 18. The liquid
placed between lens 12 and wafer 16 should, ideally, have a low
optical absorption at 193 nm, be compatible with the lens material
and the photoresist deposited on the wafer surface, and have good
uniformity.
[0008] These criteria are met by ultra pure, degassed water, which
has a refractive index n.apprxeq.1.44. The increased value of n, in
comparison to a technique where the medium between lens and wafer
is CDA, increases the value of NA, which in turn decreases the
resolution limit W, enabling smaller features to be reproduced.
[0009] Due to outgassing from the photoresist layer and the
generation of particulates during photolithography, it is desirable
to maintain a steady flow of water between the lens 12 and the
wafer 16. For example, as described in US 2004/0075895 the lens and
wafer could be immersed in a bath of water supported by the wafer
stage, with a pump used to recirculate the water within the bath.
However, due to the weight of the water bath acting on the wafer
stage, this technique is generally considered undesirable.
[0010] An alternative technique, as shown in FIG. 1, is to use a
nozzle or showerhead device 20 connected to a water source and a
vacuum system, shown generally at 22, to produce a localized stream
of ultra-pure, degassed water between the lens 12 and the wafer 16.
To prevent the ingress of water into other parts of the tool, for
example, the mechanism used to move the wafer stage 18, one or more
differential air seals 24 are used. As a result, the vacuum system
22 extracts from the tool a multi-phase mixture of water and CDA.
However, the extraction of such a multi-phase mixture from the tool
using a single vacuum pump, especially in slug or churn regime
flows, can generate undesirable pressure and flow fluctuations
upstream of the pump, which could be transmitted back to the tool.
This could lead to errors in the photolithography process, for
example, through variations in the refractive index of the medium
located between the lens and the wafer, or through the transmission
of mechanical vibrations to the tool.
SUMMARY
[0011] It is an object of the present invention to provide a vacuum
system for extracting a stream of a multi-phase fluid from a
photolithography tool and which can minimize any pressure
fluctuations imparted thereby to fluid within the tool.
[0012] In a first aspect, the present invention provides a system
for extracting a stream of multi-phase fluid from a
photo-lithography tool, the system comprising a pumping arrangement
for drawing the fluid from the tool, separating means located
upstream from the pumping arrangement for separating the fluid
drawn from the tool into gas and liquid phases, the pumping
arrangement comprising a first pumping unit for extracting gas from
the separating means and a second pumping unit for extracting
liquid from the separating means, and a pressure control system for
controlling the pressure within the separating means by regulating
the amounts of gas and liquid therewithin.
[0013] In order to minimize any pressure fluctuations transmitted
from the system back to the fluid within the tool, the pressure
control system can maintain a substantially constant pressure in
the separating means by regulating the amounts of liquid and gas
within the separating means.
[0014] In order to control the amount of gas within the separating
means, the pressure control system preferably comprises means for
supplying gas to the separating means from a source thereof, and
control means for controlling the flow of gas to the separating
means. For example, where there is a variation in the flow of fluid
into the separating means, and/or a variation in the flow of gas
from the separating means, gas can be introduced into the
separating means from the external source to compensate for such
variations. In a preferred embodiment, the pressure control system
comprises a variable flow control device, such as a butterfly or
other control valve, through which gas is supplied to the
separating means, with the control means being configured to vary
the conductance of the valve to control the pressure within the
separating means. For example, a controller may be configured to
receive a signal indicative of the pressure within the separating
means, and to control the conductance of the valve in dependence on
the received signal. This signal may be received from a pressure
sensor, capacitance manometer or other form of sensor of sufficient
sensitivity to achieve the required level of pressure control.
[0015] As well as, or as an alternative to, controlling the supply
of gas to the separating means, the controller is preferably
configured to control the flow of gas from the separating means in
dependence on the received signal. For example, another variable
flow control device may be provided, through which gas is extracted
from the tank by the first pumping unit, with the controller being
configured to control the conductance of this variable flow control
device.
[0016] In order to control the amount of liquid in the separating
means, the pressure control system preferably comprises means for
supplying liquid to the separating means from a source thereof, and
control means for controlling the flow of liquid to the separating
means. Forexample, in orderto minimize pressure variations due to
the extraction of liquid from the separating means by the second
pumping unit, the control means is preferably configured to
maintain a substantially constant level of liquid within the
separating means. In another preferred embodiment, the liquid
supply means comprises a variable flow control device such as a
butterfly or other control valve, through which liquid is supplied
to the separating means, with the control means being configured to
vary the conductance of the valve to control the level of liquid
within the separating means. For example, a controller may be
configured to receive a signal indicative of the level of liquid
within the separating means, and to control the conductance of the
valve in dependence on the received signal. This signal may be
received from a level meter, float detector, or other form of
sensor of sufficient sensitivity to allow a substantially constant
level of liquid to be maintained within the separating means.
[0017] One or more flexible tubes are preferably used to convey
fluid (single and/or multi-phase) between the components of the
system. For example, a flexible tube may be used to convey the
multi-phase fluid to the separating means. Further flexible tubes
may also be used to convey the single phase streams from the
separating means to respective pumping units. This can minimize the
transmission of vibrations generated during use of the system back
to the fluid within the tool.
[0018] In another aspect of the present invention, a method is
provided for extracting a stream of multi-phase fluid from a
photo-lithography tool, the method comprising the steps of:
connecting a pumping arrangement to the tool via an extraction
tank; operating the pumping arrangement to draw the fluid from the
tool; separating the fluid drawn from the tool into gas and liquid
phases within the extraction tank, the pumping arrangement
extracting separately gas and liquid from the extraction tank; and
controlling the pressure within the extraction tank by regulating
the amounts of gas and liquid therewithin.
[0019] Features described above in relation to system aspects of
the invention are equally applicable to method aspects, and vice
versa.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 schematically illustrates a known system for
immersion photolithography; and
[0021] FIG. 2 schematically illustrates the present invention
vacuum system for extracting a multi-phase fluid from an immersion
photolithography tool.
DETAILED DESCRIPTION
[0022] With reference to FIG. 2, a system 30 for extracting a
multi-phase fluid from an immersion photolithography tool comprises
a separating means depicted for purposes of illustration as an
extraction tank 32 for receiving the multi-stream fluid drawn from
the tool by a pumping arrangement located downstream from the tank
32. The tank 32 is connected to the tool by flexible tubing 34 so
as to minimize the amount of mechanical coupling between the system
30 and the tool, and thereby minimize the transmission of
vibrations generated during use of the system 30 back to the
tool.
[0023] The tank 32 is configured to separate the liquid and gas
phases within the fluid received from the tool. In this example,
the fluid received from the tool comprises a mixture of clean dry
air (CDA) and ultra-pure water, and so the tank 32 contains any
suitable material and/or structure for affecting the separation of
the CDA from the water. However, the tank 32 may be configured to
separate a different liquid-gas mixture received from the tool. For
example, the liquid may comprise an aqueous or non-aqueous
solution, and the gas may be other than CDA.
[0024] The pumping arrangement comprises a first pumping unit 36
for extracting gas from the tank 32, and a second pumping unit 38
for extracting liquid from the tank 32.
[0025] The first pumping unit 36 may comprise any suitable pump for
extracting the gas from the tank 32, and is preferably chosen for
compatibility with the gas extracted from the tank 32, which is
likely to be saturated with liquid vapor, for minimum transmission
of pressure fluctuations back to the gas contained in the tank 32,
and for relatively long maintenance periods. In this embodiment,
the first pumping unit 36 may conveniently comprise an air-powered
ejector pump or a water-based liquid ring pump for extracting CDA
from the tank 32. In order to inhibit the transfer of vibrations to
the tank 32 during use, the first pumping unit 36 is connected to
the tank using flexible tubing 40. As the gas exhaust from the
first pumping unit 36 may be saturated or supersaturated with
liquid vapor, in this embodiment water vapor, a separator vessel 42
may be connected to the exhaust of the first pumping unit 36, the
vessel 42 containing any suitable material and/or structure for
affecting the separation of water vapor from the CDA. The water
extracted from the CDA is exhaust to a drain, and the CDA is vented
to the atmosphere.
[0026] The second pumping unit 38 may comprise any suitable pump
for extracting the liquid from the tank 32, and is preferably
chosen for compatibility with the liquid extracted from the tank 32
and for relatively long maintenance periods. In this embodiment
where the liquid is water, the second pumping unit 38 may
conveniently comprise a water-powered ejector pump or a diaphragm
pump for extracting water from the tank 32. In order to inhibit the
transfer of vibrations to the tank 32 during use, the second
pumping unit 38 is connected to the tank using flexible tubing 44.
The internal diameter of the flexible tubing 44 may be chosen to
restrict the flow rate of liquid from the tank 32 to the second
pumping unit 38. Alternatively, or in addition, a fixed or variable
flow restrictor may be located between the tank 32 and the second
pumping unit 38.
[0027] In order to minimize any pressure fluctuations transmitted
from the system 30 back to the fluid within the tool, the system 30
includes a pressure control system for maintaining a substantially
constant pressure in the tank 32. In this embodiment, this is
achieved by regulating the amounts of liquid and gas within the
tank 32.
[0028] The amount of liquid contained in the tank 32 is maintained
at a substantially constant level by a controller 46, thereby
maintaining a substantially constant volume of gas in the tank 32.
The controller 46 is connected to a sensor 48 for detecting the
amount of liquid within the tank 32. The sensor 48 may comprise,
for example, a level meter, float meter or other form of suitable
sensor. The sensor 48 outputs a signal to the controller 46
indicative of the level of the liquid within the tank 32. In
response to this signal, the controller 46 outputs to a variable
flow control device 50 located between the tank 32 and a
pressurized external liquid source 52 connected to the tank 32 a
signal which causes the device 50 to vary the flow of liquid, in
this embodiment water, to the tank 32. For example, the device 50
may be a butterfly or other control valve having a conductance that
can be varied in dependence on, preferably in proportion to, the
signal received from the controller 46. By varying the flow rate of
the water to the tank from the external source 52, the controller
46 can compensate for any variation in the flow rate of fluid to
the tank 32 from the tool and/or any variation in the rate of
extraction of liquid from the tank 32 by the second pumping unit
38, and thus maintain the liquid in the tank 32 at a substantially
constant level. The controller 46 may be arranged to process the
signal received from the sensor 48 to compensate for any ripples
generated in the surface of the liquid during use.
[0029] With the gas occupying a substantially constant volume
within the tank 32, any variations in the amount of gas contained
in the multi-phase fluid received from the tank, and/or any in the
rate of extraction of gas from the tank 32 by the first pumping
unit 36, and any temperature fluctuations within the tank 32, could
vary the pressure of the gas within the tank 32, and impart
pressure and flow fluctuations to the fluid in the tool. The
pressure control system is therefore configured to maintain a
substantially constant pressure within the tank 32 by also
regulating the amount of gas within the tank 32.
[0030] To achieve this, the pressure control system comprises a
controller 54 connected to a sensor 56 for detecting the gas
pressure with the tank 32. The sensor 56 may comprise, for example,
a pressure sensor, a capacitance manometer or other form of sensor
of sufficient sensitivity to achieve the required level of pressure
control. The sensor 56 outputs a signal to the controller 54
indicative of the gas pressure within the tank 32. In response to
this signal, the controller 54 outputs to a variable flow control
device 58 located between the tank 32 and a pressurized external
gas source 60 connected to the tank 32 a signal which causes the
device 58 to vary the flow of gas, in this embodiment CDA, to the
tank 32. A further variable flow control device 62 may be located
between the tank 32 and the first pumping unit 36 and configured to
receive a signal from the controller 54 to vary the flow of gas
from the tank 32. For example, the devices 58, 62 may also be
butterfly or other control valves having a conductance that can be
varied in dependence on, preferably in proportion to, the signal
received from the controller. 54. By controlling the flow of gas
into and out from the tank 32, the controller 54 can maintain a
substantially constant gas pressure within the tank 32.
[0031] System 30 provides the capability of extracting a
multi-phase fluid from the immersion lithography tool while
minimizing any pressure fluctuations imparted thereby to the fluid
within the tool.
[0032] In an embodiment, there is provided a system for extracting
a stream of multi-phase fluid from a photo-lithography tool
comprising: a pumping arrangement for drawing the fluid from the
tool; separating means located upstream from the pumping
arrangement for separating the fluid drawn from the tool into gas
and liquid phases, the pumping arrangement further comprising a
first pumping unit for extracting gas from the separating means and
a second pumping unit for extracting liquid from the separating
means; and a pressure control system for controlling the pressure
within the separating means by regulating the amounts of gas and
liquid therein.
[0033] In an embodiment, the pressure control system comprises gas
supply means for supplying gas to the separating means from a
source thereof, and control means for controlling the flow of gas
to the separating means. In an embodiment, the gas supply means
comprises a variable flow control device for gas supply to the
separating means, the control means configured to vary the
conductance of the variable flow control device to control the
pressure within the separating means. In an embodiment, the control
means comprises a controller configured to receive a signal
indicative of the pressure within the separating means and to
control the conductance of the variable flow control device. In an
embodiment, the controller is further configured to control the
flow of gas from the separating means in response to the received
signal. In an embodiment, the pressure control system comprises a
further variable flow control device through which gas is extracted
from the separating means by the first pumping unit, the controller
being configured to control the conductance of the further variable
flow control device in response to the received signal. In an
embodiment, the pressure control system comprises control means for
controlling the flow of gas from the separating means. In an
embodiment, the pressure control system comprises a variable flow
control device through which gas is extracted from the separating
means by the first pumping unit, the control means configured to
control the conductance of the variable flow control device to
control the pressure within the separating means. In an embodiment,
the control means comprises a controller configured to receive a
signal indicative of the pressure within the separating means, and
to control the conductance of the variable flow control device. In
an embodiment, the pressure control system comprises liquid supply
means for supplying liquid to the separating means, and control
means for controlling the flow of liquid to the separating means.
In an embodiment, the control means is configured to control the
amount of liquid within the separating means. In an embodiment, the
liquid supply means comprises a variable flow control device
through which liquid is supplied to the separating means, the
control means configured to vary the conductance of the variable
flow control device to control the amount of liquid within the
separating means. In an embodiment, the control means comprises a
controller configured to receive a signal indicative of the amount
of liquid within the separating means, and to control the
conductance of the variable flow control device in response to the
received signal. In an embodiment, the pressure control system
comprises liquid supply means for supplying liquid to the
separating means from a source thereof, and liquid supply control
means for controlling the flow of liquid to the separating means.
In an embodiment, the liquid supply control means is configured to
control the amount of liquid within the separating means. In an
embodiment, the liquid supply means comprises a variable flow
control device through which liquid is supplied to the separating
means, the liquid supply control means being configured to vary the
conductance of the variable flow control device to control the
amount of liquid within the separating means. In an embodiment, the
liquid supply control means comprises a controller configured to
receive a signal indicative of the level of liquid within the
separating means, and to control the conductance of the variable
flow control device in response to the received signal. In an
embodiment, the separation means comprises an extraction tank. In
an embodiment, the system comprises a flexible tube for conveying
gas from the separating means to the first pumping unit. In an
embodiment, the system comprises a flexible tube for conveying
liquid from the separating means to the second pumping unit. In an
embodiment, the system comprises a flexible tube for conveying
liquid from the photolithography tool to the separating means.
[0034] In an embodiment, there is provided a method of extracting a
stream of multi-phase fluid from a photo-lithography tool, the
method comprising the steps of: connecting a pumping arrangement to
the tool via an extraction tank; operating the pumping arrangement
to draw the fluid from the tool; separating the fluid drawn from
the tool into gas and liquid phases within the extraction tank, the
pumping arrangement extracting separately gas and liquid from the
extraction tank; and controlling the pressure within the extraction
tank by regulating the amounts of gas and liquid therewithin.
[0035] While the foregoing description and drawings represent the
preferred embodiments of the present invention, it will be apparent
to those skilled in the art that various changes and modifications
may be made therein without departing from the true spirit and
scope of the present invention.
* * * * *