U.S. patent application number 11/802082 was filed with the patent office on 2008-02-28 for lithographic apparatus and lithographic apparatus cleaning method.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Raymond Gerardus Marius Beeren, Richard Joseph Bruls, Hans Jansen, Martinus Hendrikus Antonius Leenders, Johannes Wilhelmus Jacobus Leonardus Cuijpers, Anthonius Martinus Cornelis Petrus De Jong, Marco Koert Stavenga, Peter Franciscus Wanten.
Application Number | 20080049201 11/802082 |
Document ID | / |
Family ID | 38181126 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080049201 |
Kind Code |
A1 |
Stavenga; Marco Koert ; et
al. |
February 28, 2008 |
Lithographic apparatus and lithographic apparatus cleaning
method
Abstract
An immersion lithographic projection apparatus having a
megasonic transducer configured to clean a surface and a method of
using megasonic waves to clean a surface of an immersion
lithographic projection apparatus are disclosed.
Inventors: |
Stavenga; Marco Koert;
(Eindhoven, NL) ; Jansen; Hans; (Eindhoven,
NL) ; Wanten; Peter Franciscus; (Mierlo, NL) ;
Leonardus Cuijpers; Johannes Wilhelmus Jacobus; (Roermond,
NL) ; Beeren; Raymond Gerardus Marius; (Dalhem,
BE) ; Bruls; Richard Joseph; (Eindhoven, NL) ;
Leenders; Martinus Hendrikus Antonius; (Rhoon, NL) ;
Petrus De Jong; Anthonius Martinus Cornelis; (Pijnacker,
NL) |
Correspondence
Address: |
PILLSBURY WINTHROP SHAW PITTMAN, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
38181126 |
Appl. No.: |
11/802082 |
Filed: |
May 18, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11437876 |
May 22, 2006 |
|
|
|
11802082 |
May 18, 2007 |
|
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Current U.S.
Class: |
355/30 |
Current CPC
Class: |
G03F 7/70925 20130101;
H01L 21/68735 20130101; G03F 7/70916 20130101 |
Class at
Publication: |
355/030 |
International
Class: |
G03B 27/52 20060101
G03B027/52 |
Claims
1. An immersion lithographic projection apparatus, comprising: a
substrate table constructed and arranged to hold a substrate; a
projection system configured to project a patterned beam of
radiation onto the substrate; a megasonic transducer configured to
clean a surface; and a liquid supply system constructed and
arranged to supply liquid between the megasonic transducer and the
surface to be cleaned.
2. The apparatus of claim 1, wherein the megasonic transducer has a
frequency of above 750 Hz.
3. The apparatus of claim 1, wherein the liquid supply system
comprises a barrier member which surrounds a lower end of the
megasonic transducer and which forms a seal between the barrier
member and the surface to be cleaned, thereby to contain liquid
between the megasonic transducer and the surface to be cleaned.
4. The apparatus of claim 3, wherein the barrier member, the
transducer, or both, is moveable relative to the surface.
5. The apparatus of claim 3, wherein the contactless seal is a gas
seal.
6. The apparatus of claim 3, wherein the surface is a top surface
of the substrate table.
7. The apparatus of claim 1, wherein the liquid supply system is
constructed and arranged to provide a flow of liquid over the
surface.
8. The apparatus of claim 1, wherein the megasonic transducer is
positioned, in a cleaning mode, to face the surface such that sonic
waves are directed in a line of sight path to the surface to be
cleaned.
9. The apparatus of claim 1, wherein the surface is a surface of
the substrate table, a surface of a liquid confinement system
constructed to provide liquid between the projection system and a
substrate to be exposed, or both.
10. The apparatus of claim 1, wherein the liquid supply system
comprises a bath and a controller configured to fill the bath with
liquid to a certain level.
11. The apparatus of claim 10, wherein the megasonic transducer is
positioned in a bottom of the bath to induce sonic waves, directed
away from the bottom of the bath, in liquid in the bath.
12. The apparatus of claim 10, wherein the bath is moveable to a
cleaning position under the projection system and to at least
partly enclose a liquid confinement system constructed to provide,
in use, liquid between the projection system and a substrate to be
exposed, and the certain level is below the bottom of the
projection system but above a bottom surface of the liquid
confinement system.
13. The apparatus of claim 1, wherein the liquid supply system
comprises a barrier which surrounds a top surface of the substrate
table, such that liquid can be provided on the top surface and be
prevented from escaping by the barrier.
14. The apparatus of claim 13, wherein the barrier is moveable
between a cleaning position and a non-cleaning position.
15. The apparatus of claim 1, wherein the megasonic transducer is
moveable relative to the surface.
16. The apparatus of claim 1, further comprising a shield to shield
an optical element of the projection system from megasonic waves,
liquid, or both.
17. The apparatus of claim 1, further comprising a gas supply
configured to introduce gas into the liquid.
18. The apparatus of claim 17, wherein the gas comprises N.sub.2,
CO.sub.2, O.sub.2, O.sub.3, H.sub.2 containing water or a mixture
of two or more of these gases.
19. The apparatus of claim 1, further comprising a surfactant
supplier to supply surfactant to the liquid and/or a hydrogen oxide
supplier to supply H.sub.2O.sub.2 to the liquid.
20. The apparatus of claim 1, further comprising a pump configured
to create a flow of the liquid between the megasonic transducer and
the surface to be cleaned.
21. The apparatus of claim 1, wherein the liquid supply apparatus
comprises a controller configured to control the pH of the liquid
and/or concentration of electrolyte in the liquid.
22. An immersion lithographic projection apparatus, comprising: a
substrate table constructed and arranged to hold a substrate; a
projection system configured to project a patterned beam of
radiation onto the substrate; a megasonic transducer configured to
clean a surface, the megasonic transducer being moveable relative
to the surface such that, in a cleaning mode, a direct straight
path through liquid exists between the megasonic transducer and the
surface; and a liquid supply system configured to provide liquid
between the transducer and the surface.
23. The apparatus of claim 22, wherein, in the cleaning mode, the
megasonic transducer and the surface face each other at a distance
of less than 50 mm.
24. The apparatus of claim 22, wherein, in the cleaning mode, the
megasonic transducer and the surface face each other at a distance
of less than 50 mm, and wherein the surface is a surface of a
liquid confinement system constructed to provide liquid between the
projection system and a substrate to be exposed.
25. The apparatus of claim 22, further comprising a gas supply
configured to introduce gas into the liquid.
26. The apparatus of claim 25, wherein the gas is selected from the
group: N.sub.2, CO.sub.2, O.sub.2, O.sub.3, H.sub.2 containing
water or a mixture of two or more of these gases.
27. The apparatus of claim 22, further comprising a surfactant
supplier configured to supply surfactant to the liquid.
28. A method of cleaning a surface of an immersion lithographic
projection apparatus, comprising: covering at least a part of the
surface to be cleaned in liquid; and introducing megasonic waves
into the liquid.
29. The method of claim 28, wherein the megasonic transducer has a
frequency of above about 750 kHz.
30. The method of claim 28, wherein the megasonic waves are
introduced using a megasonic transducer and comprising moving the
megasonic transducer relative to the surface to be cleaned.
31. The method of claim 28, wherein the megasonic waves impinge
perpendicularly to the surface to be cleaned.
32. The method of claim 28, wherein the megasonic waves impinge at
an angle to the surface to be cleaned.
33. The method of claim 28, comprising moving the liquid covering
at least part of the surface to be cleaned over the surface.
34. The method of claim 28, comprising positioning a megasonic
transducer, in a cleaning mode, to face the surface such that the
megasonic waves are directed in a line of sight path to the surface
to be cleaned.
35. The method of claim 28, further comprising introducing gas into
the liquid.
36. The method of claim 28, further comprising introducing a
surfactant into the liquid.
37. The method of claim 28, further comprising moving the liquid
between the surface to be cleaned and the megasonic transducer.
38. The method of claim 28, further comprising providing the liquid
at a certain pH and/or with a certain concentration of an
electrolyte.
39. The method of claim 38, wherein the certain pH and/or
concentration is such that the zeta potential of the surface to be
cleaned and particles on the surface to be removed is not of
different polarity.
Description
[0001] This application is a continuation-in-part of co-pending
U.S. patent application Ser. No. 11/437,876 filed May 22, 2006, the
entire contents of which is hereby incorporated by reference.
FIELD
[0002] The present invention relates to a lithographic apparatus
and a method for cleaning a lithographic apparatus.
BACKGROUND
[0003] A lithographic apparatus is a machine that applies a desired
pattern onto a substrate, usually onto a target portion of the
substrate. A lithographic apparatus can be used, for example, in
the manufacture of integrated circuits (ICs). In that instance, a
patterning device, which is alternatively referred to as a mask or
a reticle, may be used to generate a circuit pattern to be formed
on an individual layer of the IC. This pattern can be transferred
onto a target portion (e.g. comprising part of, one, or several
dies) on a substrate (e.g. a silicon wafer). Transfer of the
pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In
general, a single substrate will contain a network of adjacent
target portions that are successively patterned. Known lithographic
apparatus include so-called steppers, in which each target portion
is irradiated by exposing an entire pattern onto the target portion
at one time, and so-called scanners, in which each target portion
is irradiated by scanning the pattern through a radiation beam in a
given direction (the "scanning"-direction) while synchronously
scanning the substrate parallel or anti-parallel to this direction.
It is also possible to transfer the pattern from the patterning
device to the substrate by imprinting the pattern onto the
substrate.
[0004] It has been proposed to immerse the substrate in the
lithographic projection apparatus in a liquid having a relatively
high refractive index, e.g. water, so as to fill a space between
the final element of the projection system and the substrate. The
point of this is to enable imaging of smaller features since the
exposure radiation will have a shorter wavelength in the liquid.
(The effect of the liquid may also be regarded as increasing the
effective NA of the system and also increasing the depth of focus.)
Other immersion liquids have been proposed, including water with
solid particles (e.g. quartz) suspended therein.
[0005] However, submersing the substrate or substrate and substrate
table in a bath of liquid (see, for example, U.S. Pat. No.
4,509,852) means that there is a large body of liquid that must be
accelerated during a scanning exposure. This requires additional or
more powerful motors and turbulence in the liquid may lead to
undesirable and unpredictable effects.
[0006] One of the solutions proposed is for a liquid supply system
to provide liquid on only a localized area of the substrate and in
between the final element of the projection system and the
substrate using a liquid confinement system (the substrate
generally has a larger surface area than the final element of the
projection system). One way which has been proposed to arrange for
this is disclosed in PCT patent application publication no. WO
99/49504. As illustrated in FIGS. 2 and 3, liquid is supplied by at
least one inlet IN onto the substrate, preferably along the
direction of movement of the substrate relative to the final
element, and is removed by at least one outlet OUT after having
passed under the projection system. That is, as the substrate is
scanned beneath the element in a -X direction, liquid is supplied
at the +X side of the element and taken up at the -X side. FIG. 2
shows the arrangement schematically in which liquid is supplied via
inlet IN and is taken up on the other side of the element by outlet
OUT which is connected to a low pressure source. In the
illustration of FIG. 2 the liquid is supplied along the direction
of movement of the substrate relative to the final element, though
this does not need to be the case. Various orientations and numbers
of in- and out-lets positioned around the final element are
possible, one example is illustrated in FIG. 3 in which four sets
of an inlet with an outlet on either side are provided in a regular
pattern around the final element.
[0007] A further immersion lithography solution with a localized
liquid supply system is shown in FIG. 4. Liquid is supplied by two
groove inlets IN on either side of the projection system PL and is
removed by a plurality of discrete outlets OUT arranged radially
outwardly of the inlets IN. The inlets IN and OUT can be arranged
in a plate with a hole in its center and through which the
projection beam is projected. Liquid is supplied by one groove
inlet IN on one side of the projection system PL and removed by a
plurality of discrete outlets OUT on the other side of the
projection system PL, causing a flow of a thin film of liquid
between the projection system PL and the substrate W. The choice of
which combination of inlet IN and outlets OUT to use can depend on
the direction of movement of the substrate W (the other combination
of inlet IN and outlets OUT being inactive).
[0008] In European patent application publication no. EP 1420300
and United States patent application publication no. US
2004-0136494, each hereby incorporated in their entirety by
reference, the idea of a twin or dual stage immersion lithography
apparatus is disclosed. Such an apparatus is provided with two
tables for supporting a substrate. Leveling measurements are
carried out with a table at a first position, without immersion
liquid, and exposure is carried out with a table at a second
position, where immersion liquid is present. Alternatively, the
apparatus has only one table.
[0009] Immersion liquid may lift debris or particles (e.g. left
over from the manufacturing process) from parts of the lithographic
apparatus and/or substrates or erode components so as to introduce
particles. This debris may then be left behind on the substrate
after imaging or may interfere with the imaging while in suspension
in the liquid between the projection system and the substrate.
Thus, the issue of contamination should be addressed in an
immersion lithographic apparatus.
SUMMARY
[0010] It is desirable, for example, to provide a lithographic
apparatus which can easily and effectively be cleaned as well as to
provide a method for effectively cleaning an immersion lithographic
apparatus.
[0011] According to an aspect of the invention, there is provided
an immersion lithographic projection apparatus, comprising: a
substrate table constructed and arranged to hold a substrate; a
projection system configured to project a patterned beam of
radiation onto the substrate; a megasonic transducer configured to
clean a surface; and a liquid supply system constructed and
arranged to supply liquid between the megasonic transducer and the
surface to be cleaned.
[0012] According to an aspect of the invention, there is provided
an immersion lithographic projection apparatus, comprising: a
substrate table constructed and arranged to hold a substrate; a
projection system configured to project a patterned beam of
radiation onto the substrate; a megasonic transducer configured to
clean a surface, the megasonic transducer being moveable relative
to the surface such that, in a cleaning mode, a direct straight
path through liquid exists between the megasonic transducer and the
surface; and a liquid supply system configured to provide liquid
between the transducer and the surface.
[0013] According to an aspect of the invention, there is provided a
method of cleaning a surface of an immersion lithographic
projection apparatus, comprising: covering at least a part of the
surface to be cleaned in liquid; and introducing megasonic waves
into the liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which corresponding reference symbols indicate
corresponding parts, and in which:
[0015] FIG. 1 depicts a lithographic apparatus according to an
embodiment of the invention;
[0016] FIGS. 2 and 3 depict a liquid supply system for use in a
lithographic projection apparatus;
[0017] FIG. 4 depicts a another liquid supply system for use in a
lithographic projection apparatus;
[0018] FIG. 5 depicts, in cross-section, another liquid supply
system for use in an immersion lithographic apparatus;
[0019] FIG. 6 illustrates, in cross-section, a first embodiment of
the present invention to clean a substrate table;
[0020] FIG. 7 illustrates, in cross-section, a second embodiment of
the present invention to clean a liquid supply system; and
[0021] FIG. 8 illustrates, in cross-section, a third embodiment of
the invention to clean a substrate table.
DETAILED DESCRIPTION
[0022] FIG. 1 schematically depicts a lithographic apparatus
according to one embodiment of the invention. The apparatus
comprises:
[0023] an illumination system (illuminator) IL configured to
condition a radiation beam B (e.g. UV radiation or DUV
radiation);
[0024] a support structure (e.g. a mask table) MT constructed to
support a patterning device (e.g. a mask) MA and connected to a
first positioner PM configured to accurately position the
patterning device in accordance with certain parameters;
[0025] a substrate table (e.g. a wafer table) WT constructed to
hold a substrate (e.g. a resist-coated wafer) W and connected to a
second positioner PM configured to accurately position the
substrate in accordance with certain parameters; and
[0026] a projection system (e.g. a refractive projection lens
system) PS configured to project a pattern imparted to the
radiation beam B by patterning device MA onto a target portion C
(e.g. comprising one or more dies) of the substrate W.
[0027] The illumination system may include various types of optical
components, such as refractive, reflective, magnetic,
electromagnetic, electrostatic or other types of optical
components, or any combination thereof, for directing, shaping, or
controlling radiation.
[0028] The support structure holds the patterning device in a
manner that depends on the orientation of the patterning device,
the design of the lithographic apparatus, and other conditions,
such as for example whether or not the patterning device is held in
a vacuum environment. The support structure can use mechanical,
vacuum, electrostatic or other clamping techniques to hold the
patterning device. The support structure may be a frame or a table,
for example, which may be fixed or movable as required. The support
structure may ensure that the patterning device is at a desired
position, for example with respect to the projection system. Any
use of the terms "reticle" or "mask" herein may be considered
synonymous with the more general term "patterning device."
[0029] The term "patterning device" used herein should be broadly
interpreted as referring to any device that can be used to impart a
radiation beam with a pattern in its cross-section such as to
create a pattern in a target portion of the substrate. It should be
noted that the pattern imparted to the radiation beam may not
exactly correspond to the desired pattern in the target portion of
the substrate, for example if the pattern includes phase-shifting
features or so called assist features. Generally, the pattern
imparted to the radiation beam will correspond to a particular
functional layer in a device being created in the target portion,
such as an integrated circuit.
[0030] The patterning device may be transmissive or reflective.
Examples of patterning devices include masks, programmable mirror
arrays, and programmable LCD panels. Masks are well known in
lithography, and include mask types such as binary, alternating
phase-shift, and attenuated phase-shift, as well as various hybrid
mask types. An example of a programmable mirror array employs a
matrix arrangement of small mirrors, each of which can be
individually tilted so as to reflect an incoming radiation beam in
different directions. The tilted mirrors impart a pattern in a
radiation beam which is reflected by the mirror matrix.
[0031] The term "projection system" used herein should be broadly
interpreted as encompassing any type of projection system,
including refractive, reflective, catadioptric, magnetic,
electromagnetic and electrostatic optical systems, or any
combination thereof, as appropriate for the exposure radiation
being used, or for other factors such as the use of an immersion
liquid or the use of a vacuum. Any use of the term "projection
lens" herein may be considered as synonymous with the more general
term "projection system".
[0032] As here depicted, the apparatus is of a transmissive type
(e.g. employing a transmissive mask). Alternatively, the apparatus
may be of a reflective type (e.g. employing a programmable mirror
array of a type as referred to above, or employing a reflective
mask).
[0033] The lithographic apparatus may be of a type having two (dual
stage) or more substrate tables (and/or two or more support
structures). In such "multiple stage" machines the additional
tables may be used in parallel, or preparatory steps may be carried
out on one or more tables while one or more other tables are being
used for exposure.
[0034] Referring to FIG. 1, the illuminator IL receives a radiation
beam from a radiation source SO. The source and the lithographic
apparatus may be separate entities, for example when the source is
an excimer laser. In such cases, the source is not considered to
form part of the lithographic apparatus and the radiation beam is
passed from the source SO to the illuminator IL with the aid of a
beam delivery system BD comprising, for example, suitable directing
mirrors and/or a beam expander. In other cases the source may be an
integral part of the lithographic apparatus, for example when the
source is a mercury lamp. The source SO and the illuminator IL,
together with the beam delivery system BD if required, may be
referred to as a radiation system.
[0035] The illuminator IL may comprise an adjuster AD for adjusting
the angular intensity distribution of the radiation beam.
Generally, at least the outer and/or inner radial extent (commonly
referred to as .sigma.-outer and .sigma.-inner, respectively) of
the intensity distribution in a pupil plane of the illuminator can
be adjusted. In addition, the illuminator IL may comprise various
other components, such as an integrator IN and a condenser CO. The
illuminator may be used to condition the radiation beam, to have a
desired uniformity and intensity distribution in its
cross-section.
[0036] The radiation beam B is incident on the patterning device
(e.g., mask) MA, which is held on the support structure (e.g., mask
table) MT, and is patterned by the patterning device. Having
traversed the patterning device MA, the radiation beam B passes
through the projection system PS, which focuses the beam onto a
target portion C of the substrate W. With the aid of the second
positioner PW and position sensor IF (e.g. an interferometric
device, linear encoder or capacitive sensor), the substrate table
WT can be moved accurately, e.g. so as to position different target
portions C in the path of the radiation beam B. Similarly, the
first positioner PM and another position sensor (which is not
explicitly depicted in FIG. 1) can be used to accurately position
the patterning device MA with respect to the path of the radiation
beam B, e.g. after mechanical retrieval from a mask library, or
during a scan. In general, movement of the support structure MT may
be realized with the aid of a long-stroke module (coarse
positioning) and a short-stroke module (fine positioning), which
form part of the first positioner PM. Similarly, movement of the
substrate table WT may be realized using a long-stroke module and a
short-stroke module, which form part of the second positioner PW.
In the case of a stepper (as opposed to a scanner) the support
structure. MT may be connected to a short-stroke actuator only, or
may be fixed. Patterning device MA and substrate W may be aligned
using patterning device alignment marks M1, M2 and substrate
alignment marks P1, P2. Although the substrate alignment marks as
illustrated occupy dedicated target portions, they may be located
in spaces between target portions (these are known as scribe-lane
alignment marks). Similarly, in situations in which more than one
die is provided on the patterning device MA, the patterning device
alignment marks may be located between the dies.
[0037] The depicted apparatus could be used in at least one of the
following modes:
[0038] 1. In step mode, the support structure MT and the substrate
table WT are kept essentially stationary, while an entire pattern
imparted to the radiation beam is projected onto a target portion C
at one time (i.e. a single static exposure). The substrate table WT
is then shifted in the X and/or Y direction so that a different
target portion C can be exposed. In step mode, the maximum size of
the exposure field limits the size of the target portion C imaged
in a single static exposure.
[0039] 2. In scan mode, the support structure MT and the substrate
table WT are scanned synchronously while a pattern imparted to the
radiation beam is projected onto a target portion C (i.e. a single
dynamic exposure). The velocity and direction of the substrate
table WT relative to the support structure MT may be determined by
the (de-)magnification and image reversal characteristics of the
projection system PS. In scan mode, the maximum size of the
exposure field limits the width (in the non-scanning direction) of
the target portion in a single dynamic exposure, whereas the length
of the scanning motion determines the height (in the scanning
direction) of the target portion.
[0040] 3. In another mode, the support structure MT is kept
essentially stationary holding a programmable patterning device,
and the substrate table WT is moved or scanned while a pattern
imparted to the radiation beam is projected onto a target portion
C. In this mode, generally a pulsed radiation source is employed
and the programmable patterning device is updated as required after
each movement of the substrate table WT or in between successive
radiation pulses during a scan. This mode of operation can be
readily applied to maskless lithography that utilizes programmable
patterning device, such as a programmable mirror array of a type as
referred to above.
[0041] Combinations and/or variations on the above described modes
of use or entirely different modes of use may also be employed.
[0042] Another immersion lithography solution with a localized
liquid supply system solution which has been proposed is to provide
the liquid supply system with a barrier member which extends along
at least a part of a boundary of the space between the final
element of the projection system and the substrate table. Such a
solution is illustrated in FIG. 5. The barrier member is
substantially stationary relative to the projection system in the
XY plane though there may be some relative movement in the Z
direction (in the direction of the optical axis). In an embodiment,
a seal is formed between the barrier member and the surface of the
substrate and may be a contactless seal such as a gas seal.
[0043] The barrier member 12 at least partly contains liquid in the
space 11 between a final element of the projection system PS and
the substrate W. A contactless seal 16 to the substrate may be
formed around the image field of the projection system so that
liquid is confined within the space between the substrate surface
and the final element of the projection system. The space is at
least partly formed by the barrier member 12 positioned below and
surrounding the final element of the projection system PS. Liquid
is brought into the space below the projection system and within
the barrier member 12 by liquid inlet 13 and may be removed by
liquid outlet 13. The barrier member 12 may extend a little above
the final element of the projection system and the liquid level
rises above the final element so that a buffer of liquid is
provided. The barrier member 12 has an inner periphery that at the
upper end, in an embodiment, closely conforms to the shape of the
projection system or the final element thereof and may, e.g., be
round. At the bottom, the inner periphery closely conforms to the
shape of the image field, e.g., rectangular though this need not be
the case.
[0044] The liquid is contained in the space 11 by a gas seal 16
which, during use, is formed between the bottom of the barrier
member 12 and the surface of the substrate W. The gas seal is
formed by gas, e.g. air or synthetic air but, in an embodiment,
N.sub.2 or another inert gas, provided under pressure via inlet 15
to the gap between barrier member 12 and substrate and extracted
via outlet 14. The overpressure on the gas inlet 15, vacuum level
on the outlet 14 and geometry of the gap are arranged so that there
is a high-velocity gas flow inwards that confines the liquid. Those
inlets/outlets may be annular grooves which surround the space 11
and the flow of gas 16 is effective to contain the liquid in the
space 11. Such a system is disclosed in United States patent
application publication no. US 2004-0207824, hereby incorporated in
its entirety by reference.
[0045] As noted above, an immersion lithographic apparatus is one
in which a substrate is imaged through liquid. That is, an
immersion liquid is provided between a final element of the
projection system PS and the substrate. This arrangement may pose
one or more particular problems. In particular, the liquid should
be confined in the apparatus and also the liquid should be kept as
free as possible of foreign object particles which may cause
defects during imaging and/or by being left on the substrate
surface after imaging and prior to downstream processing. Sometimes
the immersion liquid includes particles in suspension
deliberately.
[0046] One or more embodiments of the present invention addresses
the problem of foreign object particles by providing an apparatus
and a method to clean an immersion lithographic projection
apparatus in which a cleaning liquid is applied to the surface to
be cleaned and megasonic waves are introduced into the liquid to
clean the surface. The cleaning liquid may or may not be the same
as the immersion liquid. It could, for example, be ultra pure
water.
[0047] Compared to ultrasonic waves, megasonic waves produce
cavitation bubbles (which implode or vibrate) which are very small
and therefore may go very close to the surface to be cleaned.
However, there is a limit to the amount of energy which may be
introduced into the liquid using megasonics. Generally whereas
ultrasonic energy may be introduced into a liquid anywhere and will
be distributed throughout the liquid, megasonic energy is only
locally high and must therefore be directed directly to the surface
to be cleaned. That is, a direct path (line of sight/straight) must
be present between the transducer introducing the megasonic waves
and the surface to be cleaned. The whole length of that path should
be filled with liquid.
[0048] Megasonic frequencies are generally regarded to be between
750 kHz to 3 MHz. For the present purposes frequencies of above
about 750 kHz, above 1 MHz or above 1.5 MHz are used.
[0049] A stagnant boundary layer in the cleaning fluid near the
surface of an object to be cleaned becomes thinner as frequency of
the introduced sonic energy is increased. At megasonic frequencies
cleaning is partly accomplished by acoustic streaming with high
velocity pressure waves in the cleaning fluid as well as bubble
vibration and, to a lesser extent cavitation and bubble
bursting.
[0050] At megasonic frequencies, particles of less than 0.5 .mu.m
in diameter may be removed without damage to the surface being
cleaned. As mentioned above, there must be a clear path from the
transducer to the surface being cleaned (line of sight). In order
to further increase the cleaning efficiency, gas may be dissolved
into the liquid to promote cavitation (bubble formation). Suitable
gases are nitrogen, carbon dioxide or oxygen but other gases may
also be suitable such as ozone, or hydrogen (containing water). The
use of a surfactant in the liquid can further enhance cleaning
efficiency. Other possibilities to enhance cleaning efficiency
include using a detergent or a solvent in the cleaning liquid or
adding H.sub.2O.sub.2 solution.
[0051] Objects which one might want to clean in an immersion
lithographic apparatus include, but not limited to, the substrate
table WT which supports the substrate W (its top surface), the
final element of the projection system PS which is immersed in
immersion liquid during imaging and parts of the liquid confinement
system (for example those illustrated in FIGS. 2-5) which provides
liquid between the final element of the projection system PS and
the substrate W during imaging.
[0052] In an embodiment, a liquid supply system is provided to
provide liquid between a megasonic transducer and the surface to be
cleaned. In the embodiment, the liquid supply system provides a
flow of liquid so that liquid is removed as the surface is cleaned
such that particles removed from the surface are carried away. One
suitable liquid is water in an ultra pure form. However, other
types of liquid may be suitable. Furthermore, an addition to the
liquid such as a surfactant as mentioned above may also have an
advantage. Other cleaning liquids are water/hydrogen peroxide,
water/ethanol, water/iso-propylalcohol (IPA), water/ammonia or
water/acetone mixtures, for example. Other chemicals which may be
useful as an addition include TMAH and SC-1 or SC-2.
[0053] One reason for introducing gas (or some solvents) into the
liquid is that this promotes stable cavitation. This results in
stable bubbles being formed in the liquid. These bubbles are then
vibrated by the megasonic waves which results in cleaning which is
likely to do less damage to the surface being cleaned than so
called transient cavitation which is cavitation where a solvent
evaporates into a bubble and then implodes or collapses. These
violent implosions may lead to damage of the surface and are
typically seen at ultrasonic frequencies and are less significant
at megasonic frequencies where the bubbles produced tend to be
smaller than those produced at ultrasonic frequencies. However, as
noted above, the megasonic waves need to be supplied in line of
sight of the surface they are to clean.
[0054] A process time up to 100 seconds may lead to a particle
removal efficiency of up to 100% at a frequency of about 1 MHz. If
the acoustic frequency exceeds much more than 3 MHz the particle
removal efficiency is dramatically decreased over a frequency of
just above 1 MHz. The introduction of gas into the liquid has a
major effect on the particle removal efficiency. Removal of 34 nm
diameter SiO.sub.2 particles may increase from zero removal
efficiency to 30% removal efficiency with the introduction of
oxygen at a level of 20 ppm into the liquid. Thus, a gas
concentration of above about 5 ppm can be useful.
[0055] Temperature may also be important and a balance should be
drawn between faster reaction time at a high temperature (say
55.degree. C.) over less gas being dissolved at a high
temperature.
[0056] There is also an effect of the pH of the liquid. At low pH,
there are many H.sup.+ ions in the liquid which results in a
positive surface charge. Similarly, at high pH the liquid contains
many OH ions which results in a negative surface charge. Therefore
ensuring that the pH of the liquid is distant from pH 7 ensures
that re-deposition of particles after they have been removed does
not occur. Moreover, the electrostatic repulsion between the
particle and the surface when both are charged equally (either
positively or negatively) assists in lifting the particle from the
surface.
[0057] The power of the transducer should be between 0.2 and 5
W/cm.sup.2, the irradiation distance should be between 5 and 20 mm
and a cleaning time should be between 10 and 90 seconds. For the
acoustic waves from the megasonic transducer to travel a direct
path to the surface to be cleaned from the megasonic transducer,
several designs are proposed to clean different parts of the
immersion lithographic apparatus.
[0058] A first embodiment is illustrated in FIG. 6 which may be
used, for instance, to clean a top surface of a substrate table WT.
In this embodiment the liquid supply system comprises a barrier
member 12 which surrounds a transducer 20. The barrier member 12
could be similar to that of the liquid confinement system of FIG. 5
in that it comprises a gas seal device 14, 15 to create a seal
between the bottom of the barrier member 12 and the top surface of
the substrate table WT using a flow of gas 16. The transducer 20
which fits inside the barrier member 12 may thereby be positioned
very close to the surface of the substrate table WT. This is an
advantage because the transducer 20 should be quite small in order
to fit inside the barrier member 12 and therefore should be
positioned relatively close to the top surface of the substrate
table WT being cleaned (because it has low power). In an
embodiment, the megasonic transducer 20 is less than 1.5 mm or less
than 1.6 mm away from the surface it is cleaning. A flow of liquid
is provided across the barrier member 12 and the bottom of the
transducer 20 is covered in liquid.
[0059] The substrate table WT and/or the barrier member 12 and
transducer 20 are moved relative to the other so that all of the
top surface of the substrate table WT may be cleaned. The cleaning
could take place within the lithographic apparatus in an automated
way or could be carried out by manually bypassing the barrier
member 12 and transducer 20 over the top surface of the substrate
table WT by hand or by some tooling.
[0060] If the cleaning process is automated, one way of arranging
for this is to provide the barrier member 12 and transducer 20
arrangement to be moveable from a stationary store position to a
(stationary) clean position and to move the substrate table WT
relative to the transducer 20 when the transducer is in the
(stationary) clean position. The substrate table WT may need to be
moved in the Z axis prior to activation of the liquid supply system
of the cleaning device. Once the gas flow 16 has been created the
barrier member 12 is supplied with liquid and the substrate table
WT is moved in the X-Y plane so that the surfaces which are desired
to be cleaned can be cleaned.
[0061] FIG. 7 illustrates a second embodiment which is used in
order to clean a liquid confinement system LCS, such as one
illustrated in FIGS. 2-5, which is positioned around the final
element of the projection system PS. In FIG. 7 a liquid confinement
system in accordance with that shown in FIG. 5 is illustrated as an
example.
[0062] In this embodiment a moveable bath 50 is provided which has
on its bottom surface a megasonic transducer 20. The megasonic
transducer 20 of the second embodiment may be larger than the
megasonic transducer of the first embodiment because there are no
particular size constraints. Thus, the distance between the
megasonic transducer and the surface to be cleaned (the bottom
surface of the liquid confinement system) can be made larger and up
to 50 mm. In an embodiment, the distance is less than 40 mm or less
than 30 mm.
[0063] A controller controls the position of the bath 50 which is
moveable between a store position and a cleaning position (as
illustrated) and which controls the level of fluid in the bath 50.
In one embodiment the level of liquid is controlled so as to cover
the bottom surface of the liquid confinement system LCS but not to
cover the final element of the projection system PS. Thus, the gap
75 is not filled with liquid. This is to protect the final element
of the projection system from the sonic waves and/or the cleaning
liquid which could damage it. Additionally or alternatively, a
shield (perhaps in the form of a plate) may be used to shield the
final element of the projection system from megasonic waves and/or
liquid. In an embodiment, this bath arrangement may be used to
clean the final element of the projection system PS and in which
case the controller increases the level of liquid in the bath 50 so
that the final element of the projection system PS is covered in
liquid.
[0064] In the second embodiment illustrated in FIG. 7, the bath 50
and/or transducer 20 may be moveable relative to the projection
system PS in the cleaning position and liquid confinement system
LCS such that the whole under surface of the liquid confinement
system LCS may be cleaned. Of course, this need not be implemented
if the transducer 20 is large enough to clean the whole bottom
surface of the liquid confinement system LCS (or just a desired
portion thereof) without being moved.
[0065] In a third embodiment illustrated in FIG. 8, which is used
to clean, for example, the top surface of the substrate table WT,
the substrate table WT is provided with a retractable barrier 80
which, in its clean position extends above and around the top
surface of the substrate table WT to be cleaned. Once the barrier
80 has been raised to its cleaning position liquid can be provided
on the surface to be cleaned and a megasonic transducer 20 can be
moved over the surface of the substrate table WT (with the bottom
surface of the transducer 20 covered by liquid) and/or the
substrate table WT may be moved under the transducer 20, thereby to
clean the top surface of the substrate table WT. Of course a
similar embodiment is possible where the barrier 80 is not
retractable and is permanently attached to the substrate table WT
or is a removable part. The transducer 20 could be fixed or
moveable (particularly in the Z direction) and/or also moveable in
the X/Y axis during the cleaning operation.
[0066] When the top surface of the substrate table WT is cleaned,
it is also possible to clean at the same time one or more sensors
provided on the top surface of the substrate table WT. Examples of
types of sensors include a transmission image sensor, a lens
interferometer, and/or a spot sensor.
[0067] In an embodiment, it may be useful to ensure that the
acoustic waves produced by the transducer impinge at 90.degree.
onto the surface to be cleaned. For this purpose, a micrometer may
be provided to adjust the tilt of the transducer 20 relative to the
surface to be cleaned. In an embodiment, it may be advantageous to
provide the transducer tilted relative to the surface and again,
this can be adjusted using a micrometer. A micrometer may also be
used to adjust the distance from the transducer to the surface to
be cleaned. In all embodiments, a flow of liquid across the
distance between the transducer and the surface to be cleaned is
desirable though not essential.
[0068] The above described megasonic cleaner is well suited to
removing particles from a surface. However, once those particles
have been removed they sometimes re-attach themselves to a surface
unless the liquid in which the particles are suspended is quickly
moved away. Therefore, it is desirable to provide a flow of liquid
between the megasonic transducer and the surface being cleaned
using, for example, a pump. In particular, it is desirable to
design the apparatus such that there are no or few locations of
zero flow velocity (stagnant zones).
[0069] Another way to prevent or at least reduce re-attachment of
the particles to the surface is to change one or more properties of
the liquid between the megasonic transducer and the surface to
ensure that the zeta potential of the particles and the zeta
potential of the surface is such that the particles are not
attracted to the surface, desirably such that they are repelled
from the surface.
[0070] The zeta potential is the potential of a surface in a
liquid. The zeta potential generally decreases with distance from
the surface. A given type of material has a given zeta potential
for a particular type of liquid. One way of varying the zeta
potential of a surface is to change the concentration of
electrolyte in the liquid and another method to change the zeta
potential is to change the pH of the liquid. By careful selection
of the concentration of electrolyte in the liquid (e.g. salt) or
the pH of the liquid, (i) the zeta potential of the surface from
which the particles are removed (and/or any other surface where
adherence is to be avoided) and (ii) the zeta potential of the
particles can be chosen. Desirably those two zeta potentials are
chosen such that they have the same polarity and thereby repel.
[0071] The pH of the liquid and/or concentration of the electrolyte
is chosen with a knowledge of the material from which the surface
which is being cleaned is made and with a knowledge of the type of
material the particles are likely to be made. If the materials are
the same, then it should easy to select a pH or electrolyte
concentration at which the zeta potential is non zero for both the
surface and the particle. In that circumstance the potential would
be either positive or negative for both the surface and the
particle such that they would repel one another and the particles
would be unlikely to re-adhere to the surface. If the materials are
different, pH or electrolyte concentration may be harder to choose,
but it is likely that there will be at least one pH and/or
concentration at which the zeta potential will have the same
polarity for both materials. A controller may be provided to
control the pH of the liquid and/or concentration of electrolyte in
the liquid based upon one or more aspects of the foregoing
knowledge.
[0072] Because changing the pH of the liquid may have a negative
impact on the solubility of materials, which can itself result in
contamination or loss of material integrity, it is desired to
change the electrolyte concentration. If this is done by adding
salt (NaCl) then this does not greatly affect the pH of the liquid
(i.e. the liquid remains substantially neutral).
[0073] The use of any or all the above techniques in combination
(in particular changing pH and electrolyte concentration to change
zeta potentials and the use of a surfactant) may be the best
approach.
[0074] Although specific reference may be made in this text to the
use of lithographic apparatus in the manufacture of ICs, it should
be understood that the lithographic apparatus described herein may
have other applications, such as the manufacture of integrated
optical systems, guidance and detection patterns for magnetic
domain memories, flat-panel displays, liquid-crystal displays
(LCDs), thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "wafer" or "die" herein may be considered as
synonymous with the more general terms "substrate" or "target
portion", respectively. The substrate referred to herein may be
processed, before or after exposure, in for example a track (a tool
that typically applies a layer of resist to a substrate and
develops the exposed resist), a metrology tool and/or an inspection
tool. Where applicable, the disclosure herein may be applied to
such and other substrate processing tools. Further, the substrate
may be processed more than once, for example in order to create a
multi-layer IC, so that the term substrate used herein may also
refer to a substrate that already contains multiple processed
layers.
[0075] The terms "radiation" and "beam" used herein encompass all
types of electromagnetic radiation, including ultraviolet (UV)
radiation (e.g. having a wavelength of or about 365, 248, 193, 157
or 126 nm).
[0076] The term "lens", where the context allows, may refer to any
one or combination of various types of optical components,
including refractive and reflective optical components.
[0077] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. For example, the invention
may take the form of a computer program containing one or more
sequences of machine-readable instructions describing a method as
disclosed above, or a data storage medium (e.g. semiconductor
memory, magnetic or optical disk) having such a computer program
stored therein.
[0078] One or more embodiments of the invention may be applied to
any immersion lithography apparatus, in particular, but not
exclusively, those types mentioned above and whether the immersion
liquid is provided in the form of a bath or only on a localized
surface area of the substrate. A liquid supply system as
contemplated herein should be broadly construed. In certain
embodiments, it may be a mechanism or combination of structures
that provides a liquid to a space between the projection system and
the substrate and/or substrate table. It may comprise a combination
of one or more structures, one or more liquid inlets, one or more
gas inlets, one or more gas outlets, and/or one or more liquid
outlets that provide liquid to the space. In an embodiment, a
surface of the space may be a portion of the substrate and/or
substrate table, or a surface of the space may completely cover a
surface of the substrate and/or substrate table, or the space may
envelop the substrate and/or substrate table. The liquid supply
system may optionally further include one or more elements to
control the position, quantity, quality, shape, flow rate or any
other features of the liquid.
[0079] The immersion liquid used in the apparatus may have
different compositions, according to the desired properties and the
wavelength of exposure radiation used. For an exposure wavelength
of 193 nm, ultra pure water or water-based compositions may be used
and for this reason the immersion liquid is sometimes referred to
as water and water-related terms such as hydrophilic, hydrophobic,
humidity, etc. may be used.
[0080] The descriptions above are intended to be illustrative, not
limiting. Thus, it will be apparent to one skilled in the art that
modifications may be made to the invention as described without
departing from the scope of the claims set out below.
* * * * *