U.S. patent application number 14/937724 was filed with the patent office on 2016-03-03 for lithographic apparatus and device manufacturing method.
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 Joannes Theodoor De Smit, Joeri LOF, Erik Roelof Loopstra, Hendricus Johannes Maria Meijer, Theodorus Marinus Modderman, Johannes Catharinus Hubertus Mulkens, Roelof Aeilko Siebrand Ritsema, Klaus Simon.
Application Number | 20160062248 14/937724 |
Document ID | / |
Family ID | 32715008 |
Filed Date | 2016-03-03 |
United States Patent
Application |
20160062248 |
Kind Code |
A1 |
LOF; Joeri ; et al. |
March 3, 2016 |
LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD
Abstract
A map of the surface of a substrate is generated at a
measurement station. The substrate is then moved to where a space
between a projection lens and the substrate is filled with a
liquid. The substrate is then aligned using, for example, a
transmission image sensor and, using the previous mapping, the
substrate can be accurately exposed. Thus the mapping does not take
place in a liquid environment.
Inventors: |
LOF; Joeri; (Eindhoven,
NL) ; De Smit; Joannes Theodoor; (Eindhoven, NL)
; Ritsema; Roelof Aeilko Siebrand; (Eindhoven, NL)
; Simon; Klaus; (Eindhoven, NL) ; Modderman;
Theodorus Marinus; (Nuenen, NL) ; Mulkens; Johannes
Catharinus Hubertus; (Maastricht, NL) ; Meijer;
Hendricus Johannes Maria; (Veldhoven, NL) ; Loopstra;
Erik Roelof; (Heeze, NL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASML NETHERLANDS B.V. |
Veldhoven |
|
NL |
|
|
Assignee: |
ASML NETHERLANDS B.V.
Veldhoven
NL
|
Family ID: |
32715008 |
Appl. No.: |
14/937724 |
Filed: |
November 10, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13692865 |
Dec 3, 2012 |
9195153 |
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14937724 |
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12853030 |
Aug 9, 2010 |
8344341 |
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13692865 |
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12318036 |
Dec 19, 2008 |
7795603 |
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12853030 |
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11371235 |
Mar 9, 2006 |
7482611 |
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12318036 |
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10705816 |
Nov 12, 2003 |
7193232 |
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11371235 |
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Current U.S.
Class: |
355/30 |
Current CPC
Class: |
G03F 7/7085 20130101;
G03F 9/7088 20130101; G03F 9/7003 20130101; G03F 7/70341 20130101;
G03F 7/707 20130101; G03F 9/7034 20130101 |
International
Class: |
G03F 7/20 20060101
G03F007/20 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 12, 2002 |
EP |
02257822.3 |
Jun 11, 2003 |
EP |
03253692.2 |
Claims
1-3. (canceled)
4. A method of patterning a substrate, the method comprising:
measuring a substrate while a target portion of the substrate is
submerged in a first liquid; after the measuring, removing the
first liquid from the target portion, and after the removing,
providing a second liquid in a space between a projection system
and the substrate and projecting a patterned beam through the
second liquid onto the target portion of the substrate.
5. The method of claim 4, wherein the first liquid is different
from the second liquid.
6. The method of claim 4, further comprising, after removing the
first liquid and before providing the second liquid, moving the
substrate from a first location where the substrate is measured to
a second location under the projection system.
7. The method of claim 6, wherein a measurement station is provided
at the first location.
8. The method of claim 7, wherein the measurement station is
configured to provide a level of the substrate.
9. The method of claim 4, wherein the measuring comprises measuring
a height, a tilt, or both, of each of a plurality of points on the
substrate.
10. The method of claim 4, wherein measuring the substrate at least
partially overlaps in time with exposure of a further substrate
using the projection system.
11. The method of claim 10, wherein the substrate and the further
substrate are arranged on separate substrate tables.
12. The method of claim 11, wherein the separate substrate tables
are each movable between a measurement station where the measuring
is performed and an exposure station where the projection system is
located.
13. The method of claim 4, comprising generating a map of the
substrate based on the measuring.
14. The method of claim 4, wherein the measuring is performed at a
measurement station and the projecting is performed at an exposure
station and wherein the measurement station is physically separated
from the exposure station.
15. The method of claim 14, further comprising supplying the first
liquid with a first liquid supply system in the vicinity of the
measurement station and supplying the second liquid with a second
liquid system.
16. The method of claim 4, wherein said second liquid is in contact
with a last element of the projection system and the substrate
during projection of the patterned beam of radiation.
17. A lithographic apparatus, comprising: a measurement station
configured to measure a substrate while a target portion of the
substrate is submerged in a first liquid; an exposure station
including a projection system configured to project a patterned
beam of radiation through a second liquid onto the target portion
of a substrate, and a liquid retrieval system configured to remove
the first liquid from the target portion after the substrate is
measured at the measurement station and before the second liquid is
provided between the projection system and the substrate.
18. The lithographic apparatus of claim 17, wherein the first
liquid is different from the second liquid.
19. The lithographic apparatus of claim 17, further comprising a
substrate table to support the substrate, the substrate table being
movable from the measurement station to the exposure station.
20. The lithographic apparatus of claim 17, wherein the measurement
station is configured to provide a level of the substrate.
21. The lithographic apparatus of claim 17, wherein the measurement
station is configured to measure a height, a tilt, or both, of each
of a plurality of points on the substrate.
22. The lithographic apparatus of claim 17, wherein a measurement
of the substrate at the measurement station least partially
overlaps in time with exposure of a further substrate using the
projection system.
23. The lithographic apparatus of claim 22, wherein the substrate
and the further substrate are arranged on separate substrate
tables.
Description
[0001] This application is a divisional of co-pending U.S. patent
application Ser. No. 12/318,036, filed Dec. 19, 2008, which is a
continuation of co-pending U.S. patent application Ser. No.
11/371,235, filed Mar. 9, 2006, now U.S. Pat. No. 7,482,611, issued
on Jan. 27, 2009, which is a continuation of co-pending U.S.
application Ser. No. 10/705,816, filed Nov. 12, 2003, now U.S. Pat.
No. 7,193,232, issued on Mar. 20, 2007, which claims priority from
European patent applications EP 02257822.3, filed Nov. 12, 2002,
and EP 03253692.2, filed Jun. 11, 2003, all the foregoing
applications herein incorporated in their entirety by
reference.
FIELD
[0002] The present invention relates to immersion lithography.
BACKGROUND
[0003] The term "patterning device" as here employed should be
broadly interpreted as referring to means that can be used to endow
an incoming radiation beam with a patterned cross-section,
corresponding to a pattern that is to be created in a target
portion of the substrate; the term "light valve" can also be used
in this context. Generally, the said pattern will correspond to a
particular functional layer in a device being created in the target
portion, such as an integrated circuit or other device (see below).
Examples of such a patterning device include: [0004] A mask. The
concept of a mask is well known in lithography, and it includes
mask types such as binary, alternating phase-shift, and attenuated
phase-shift, as well as various hybrid mask types. Placement of
such a mask in the radiation beam causes selective transmission (in
the case of a transmissive mask) or reflection (in the case of a
reflective mask) of the radiation impinging on the mask, according
to the pattern on the mask. In the case of a mask, the support
structure will generally be a mask table, which ensures that the
mask can be held at a desired position in the incoming radiation
beam, and that it can be moved relative to the beam if so desired.
[0005] A programmable mirror array. One example of such a device is
a matrix-addressable surface having a viscoelastic control layer
and a reflective surface. The basic principle behind such an
apparatus is that (for example) addressed areas of the reflective
surface reflect incident light as diffracted light, whereas
unaddressed areas reflect incident light as undiffracted light.
Using an appropriate filter, the said undiffracted light can be
filtered out of the reflected beam, leaving only the diffracted
light behind; in this manner, the beam becomes patterned according
to the addressing pattern of the matrix-addressable surface. An
alternative embodiment of a programmable mirror array employs a
matrix arrangement of tiny mirrors, each of which can be
individually tilted about an axis by applying a suitable localized
electric field, or by employing piezoelectric actuation means. Once
again, the mirrors are matrix-addressable, such that addressed
mirrors will reflect an incoming radiation beam in a different
direction to unaddressed mirrors; in this manner, the reflected
beam is patterned according to the addressing pattern of the
matrix-addressable mirrors. The required matrix addressing can be
performed using suitable electronic means. In both of the
situations described hereabove, the patterning device can comprise
one or more programmable mirror arrays. More information on mirror
arrays as here referred to can be gleaned, for example, from United
States patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193,
and PCT patent applications WO 98/38597 and WO 98/33096, which are
incorporated herein by reference. In the case of a programmable
mirror array, the said support structure may be embodied as a frame
or table, for example, which may be fixed or movable as required.
[0006] A programmable LCD array. An example of such a construction
is given in United States patent U.S. Pat. No. 5,229,872, which is
incorporated herein by reference. As above, the support structure
in this case may be embodied as a frame or table, for example,
which may be fixed or movable as required.
[0007] For purposes of simplicity, the rest of this text may, at
certain locations, specifically direct itself to examples involving
a mask and mask table; however, the general principles discussed in
such instances should be seen in the broader context of the
patterning device as hereabove set forth.
[0008] Lithographic projection apparatus can be used, for example,
in the manufacture of integrated circuits (ICs). In such a case,
the patterning device may generate a circuit pattern corresponding
to an individual layer of the IC, and this pattern can be imaged
onto a target portion (e.g. comprising one or more dies) on a
substrate (silicon wafer. LCD, mask etc) that has been coated with
a layer of radiation-sensitive material (resist). In general, a
single wafer will contain a whole network of adjacent target
portions that are successively irradiated via the projection
system, one at a time. In current apparatus, employing patterning
by a mask on a mask table, a distinction can be made between two
different types of machine. In one type of lithographic projection
apparatus, each target portion is irradiated by exposing the entire
mask pattern onto the target portion at one time; such an apparatus
is commonly referred to as a wafer stepper. In an alternative
apparatus--commonly referred to as a step-and-scan apparatus--each
target portion is irradiated by progressively scanning the mask
pattern under the projection beam in a given reference direction
(the "scanning" direction) while synchronously scanning the
substrate table parallel or anti-parallel to this direction; since,
in general, the projection system will have a magnification factor
M (generally<1), the speed V at which the substrate table is
scanned will be a factor M times that at which the mask table is
scanned. More information with regard to lithographic devices as
here described can be gleaned, for example, from United States
patent U.S. Pat. No. 6,046,792, incorporated herein by
reference.
[0009] In a manufacturing process using a lithographic projection
apparatus, a pattern (e.g. in a mask) is imaged onto a substrate
that is at least partially covered by a layer of
radiation-sensitive material (resist). Prior to this imaging step,
the substrate may undergo various procedures, such as priming,
resist coating and a soft bake. After exposure, the substrate may
be subjected to other procedures, such as a post-exposure bake
(PEB), development, a hard bake and measurement/inspection of the
imaged features. This array of procedures is used as a basis to
pattern an individual layer of a device, e.g. an IC. Such a
patterned layer may then undergo various processes such as etching,
ion-implantation (doping), metallization, oxidation,
chemo-mechanical polishing, etc., all intended to, finish off an
individual layer. If several layers are required, then the whole
procedure, or a variant thereof, will have to be repeated for each
new layer. Eventually, an array of devices will be present on the
substrate (wafer). These devices are then separated from one
another by a technique such as dicing or sawing, whence the
individual devices can be mounted on a carrier, connected to pins,
etc. Further information regarding such processes can be obtained,
for example, from the book "Microchip Fabrication: A Practical
Guide to Semiconductor Processing", Third Edition, by Peter van
Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4,
incorporated herein by reference.
[0010] For the sake of simplicity, the projection system may
hereinafter be referred to as the "lens"; however, this term should
be broadly interpreted as encompassing various types of projection
system, including refractive optics, reflective optics, and
catadioptric systems, for example. The radiation system may also
include components operating according to any of these design types
for directing, shaping or controlling the projection beam of
radiation, and such components may also be referred to below,
collectively or singularly, as a "lens". Further, the lithographic
apparatus may be of a type having two or more substrate tables
(and/or two or more mask tables). In such "multiple stage" devices
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 exposures. Dual stage lithographic
apparatus are described, for example, in United States patents U.S.
Pat. No. 5,969,441 and WO 98/40791, incorporated herein by
reference in their entirety.
[0011] The lithographic industry is constantly trying to reduce
feature sizes on silicon substrates in order to manufacture ever
more complex integrated circuits. The feature sizes are limited by
the effect of diffraction and thus the resolution of a particular
system of numeral aperture NA using a wavelength .lamda. is:
W = k .lamda. NA ##EQU00001##
[0012] where k is a pre-factor. The numerical aperture NA is n sin
.theta. where n is the refractive index of the transmissive
substance.
[0013] Hence to decrease the resolution, the wavelength can either
be reduced or the numerical aperture increased. It has been
proposed to immerse the substrate 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).
[0014] However, submersing the substrate or substrate and substrate
table in a bath of liquid (see for example United States patent
U.S. Pat. No. 4,509,852, hereby incorporated in its entirety by
reference) may mean that there is a large body of liquid that must
be accelerated during a scanning exposure. This may require
additional or more powerful motors and turbulence in the liquid may
lead to undesirable and unpredictable effects.
[0015] One of the solutions proposed is for a liquid supply system
to provide liquid in a localized area between the final element of
the projection system and the substrate (the substrate generally
has a larger surface area than the final element of the projection
systems). One way which has been proposed to arrange for this is
disclosed in PCT patent application WO 99/49504, hereby
incorporated in its entirety by reference. As illustrated in FIGS.
4 and 5, 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. 4 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. 4
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. 5 in which four sets of an inlet with an outlet
on either side are provided in a regular pattern around the final
element to form a liquid reservoir.
SUMMARY
[0016] Immersion lithography is an embryonic technology and there
remain many problems in its practical application. This patent
application is concerned in particular with alignment and leveling
of a substrate. Conventionally alignment and leveling is performed
with the substrate in the field of view of the projection system
(i.e. at an exposure station). However there is not a lot of space
for alignment or level measurement apparatus in and around an
immersion liquid reservoir so the adaptation is likely to be
complex or the accuracy can be compromised. Furthermore, the
presence of liquid near the alignment and level measurement
apparatus can degrade the performance of the apparatus.
[0017] Accordingly, it may be advantageous to provide, for example,
a method and apparatus for accurately aligning and/or leveling a
substrate in an immersion lithography apparatus.
[0018] According to an aspect, there is provided a lithographic
projection apparatus comprising:
[0019] a support structure configured to hold a patterning device,
the patterning device configured to pattern a beam of radiation
according to a desired pattern;
[0020] a substrate table configured to hold a substrate;
[0021] a projection system configured to project the patterned beam
onto a target portion of the substrate;
[0022] a liquid supply system configured to provide a liquid,
through which said beam is to be projected, in a space between said
projection system and said substrate; and
[0023] a measurement system configured to measure, not through said
liquid, locations of points on said substrate.
[0024] The position of points on the substrate are thus measured
without the presence of liquid and, in an embodiment, outside the
immersion system. Alternatively, the measurements could take place
while a target portion of the substrate is submerged in liquid,
i.e. the measurements take place through liquid, but not the same
liquid as supplied by the liquid supply system to fill a space
between the projection system and the substrate. The position of
points on the substrate would therefore be measured with liquid
between the measurement system and the substrate, the liquid would
then be removed before moving the substrate (and substrate table)
to the focal point of the projection system where the liquid supply
system would supply liquid to fill a space between the projection
system and the substrate prior to exposure taking place. A second
liquid supply system may be present in the vicinity of the
measurement system.
[0025] A possible advantage is that there is better flow in the
liquid reservoir because the measurement system is no longer in or
around the reservoir crowding the projection system and the
performance of the measurement system is not degraded by the
presence of liquid. Furthermore smooth flow conditions in the
liquid reservoir are desired as there is no change in the apparatus
leading to rough edges. Using this method, measurement systems not
specifically adapted for immersion lithography can be used without
complex adaptation. A further advantage of this measurement system
is that any improvements to such measurement systems used outside
of the immersion lithography field can easily and automatically be
incorporated into the immersion system.
[0026] The measurement system, in an embodiment, comprises an
alignment system configured to measure the location (in the x, y
and R.sub.z directions) of a plurality of alignment marks on said
substrate. According to an embodiment, said substrate table has a
reference and said measurement system measures the location of said
reference not through said liquid of said supply system. The
location of the alignment marks may in an embodiment be measured
relative to said reference on said substrate table to enable a map
of alignment marks relative to the reference to be built up.
[0027] According to an embodiment, the measurement system comprises
a level sensor configured to measure the height and/or tilt (i.e.
measuring in the z, R.sub.x and R.sub.y directions) of points on
said substrate. Thus, level measurement of the substrate, which is
conventionally undertaken "on-the-fly" at the exposure station, can
be achieved outside the liquid reservoir.
[0028] In an embodiment, the lithographic projection apparatus can
have an exposure station at which said substrate may be exposed and
a separate measurement station, said measurement system being
provided at said measurement station and said substrate table being
movable between said exposure and measurement stations.
Furthermore, there can be a plurality of substrate tables, each
movable between an exposure station and a measurement station.
While one substrate table is being mapped, a second substrate table
can be exposed. Substrate throughput may therefore be increased,
making the apparatus more efficient and improving the cost of
ownership.
[0029] According to an embodiment, said reference is a transmission
image sensor.
[0030] In an embodiment, the alignment system measures displacement
in two linear perpendicular directions and rotation within the
plane defined by the two perpendicular directions.
[0031] In an embodiment, said liquid supply system is configured to
provide said liquid to a space between a final lens of said
projection system and said substrate.
[0032] According to a further aspect, there is provided a device
manufacturing method comprising:
[0033] providing a liquid in a space between a projection system
and a substrate;
[0034] measuring the locations of points on a substrate using a
measurement beam projected from a measurement system but not
projected through said liquid; and
[0035] projecting a patterned beam of radiation, through said
liquid, onto a target portion of the substrate using the projection
system.
[0036] Although specific reference may be made in this text to the
use of the apparatus described herein in the manufacture of ICs, it
should be explicitly understood that such an apparatus has many
other possible applications. For example, it may be employed in the
manufacture of integrated optical systems, guidance and detection
patterns for magnetic domain memories, liquid-crystal display
panels, thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "reticle", "wafer" or "die" in this text
should be considered as being replaced by the more general terms
"mask", "substrate" and "target portion", respectively.
[0037] In the present document, the terms "radiation" and "beam"
are used to encompass all types of electromagnetic radiation,
including ultraviolet radiation (e.g. with a wavelength of 365,
248, 193, 157 or 126 nm).
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying schematic
drawings in which:
[0039] FIG. 1 depicts a lithographic projection apparatus according
to an embodiment of the invention;
[0040] FIG. 2 depicts a detail of a lithographic projection
apparatus according to an embodiment of the invention;
[0041] FIG. 3 depicts the same details of the lithographic
projection apparatus as FIG. 2 at a different stage in the exposure
process according to an embodiment of the invention;
[0042] FIG. 4 depicts an alternative liquid supply system according
to an embodiment of the invention; and
[0043] FIG. 5 is an alternative view of the liquid supply system of
FIG. 4 according to an embodiment of the invention.
[0044] In the Figures, corresponding reference symbols indicate
corresponding parts.
DETAILED DESCRIPTION
[0045] FIG. 1 schematically depicts a lithographic projection
apparatus according to a particular embodiment of the invention.
The apparatus comprises: [0046] a radiation system Ex, IL, for
supplying a projection beam PB of radiation (e.g. UV radiation),
which in this particular case also comprises a radiation source LA;
[0047] a first object table (mask table) MT provided with a mask
holder for holding a mask MA (e.g. a reticle), and connected to
first positioning means for accurately positioning the mask with
respect to item PL; [0048] a second object table (substrate table)
WT provided with a substrate holder for holding a substrate W (e.g.
a resist-coated silicon wafer), and connected to second positioning
means for accurately positioning the substrate with respect to item
PL; [0049] a projection system ("lens") PL (e.g. a refractive lens
system) for imaging an irradiated portion of the mask MA onto a
target portion C (e.g. comprising one or more dies) of the
substrate W.
[0050] As here depicted, the apparatus is of a transmissive type
(e.g. has a transmissive mask). However, in general, it may also be
of a reflective type, for example (e.g. with a reflective mask).
Alternatively, the apparatus may employ another kind of patterning
device, such as a programmable mirror array of a type as referred
to above.
[0051] The source LA (e.g. a laser-produced or discharge plasma
source) produces a beam of radiation. This beam is fed into an
illumination system (illuminator) IL, either directly or after
having traversed conditioning means, such as a beam expander Ex,
for example. The illuminator IL may comprise adjusting means AM for
setting the outer and/or inner radial extent (commonly referred to
as .sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in the beam. In addition, it will generally comprise
various other components, such as an integrator IN and a condenser
CO. In this way, the beam PB impinging on the mask MA has a desired
uniformity and intensity distribution in its cross-section.
[0052] It should be noted with regard to FIG. 1 that the source LA
may be within the housing of the lithographic projection apparatus
(as is often the case when the source LA is a mercury lamp, for
example), but that it may also be remote from the lithographic
projection apparatus, the radiation beam which it produces being
led into the apparatus (e.g. with the aid of suitable directing
mirrors); this latter scenario is often the case when the source LA
is an excimer laser. The current invention and claims encompass
both of these scenarios.
[0053] The beam PB subsequently intercepts the mask MA, which is
held on a mask table MT. Having traversed the mask MA, the beam PB
passes through the projection system PL, which focuses the beam PB
onto a target portion C of the substrate W. With the aid of the
second positioning means (and interferometric measuring means IF),
the substrate table WT can be moved accurately, e.g. so as to
position different target portions C in the path of the beam PB.
Similarly, the first positioning means can be used to accurately
position the mask MA with respect to the path of the beam PB, e.g.
after mechanical retrieval of the mask MA from a mask library, or
during a scan. In general, movement of the object tables MT, WT
will be realized with the aid of a long-stroke module (course
positioning) and a short-stroke module (fine positioning), which
are not explicitly depicted in FIG. 1. However, in the case of a
wafer stepper (as opposed to a step-and-scan apparatus) the mask
table MT may just be connected to a short stroke actuator, or may
be fixed in the XY plane.
[0054] The depicted apparatus can be used in two different
modes:
[0055] 1. In step mode, the mask table MT is kept essentially
stationary, and an entire mask image is projected at one time (i.e.
a single "flash") onto a target portion C. The substrate table WT
is then shifted in the x and/or y directions so that a different
target portion C can be irradiated by the beam PB;
[0056] 2. In scan mode, essentially the same scenario applies,
except that a given target portion C is not exposed in a single
"flash". Instead, the mask table MT is movable in a given direction
(the so-called "scan direction", e.g. the y direction) with a speed
v, so that the projection beam PB is caused to scan over a mask
image; concurrently, the substrate table WT is simultaneously moved
in the same or opposite direction at a speed V=Mv, in which M is
the magnification of the projection system PL (typically, M=1/4 or
1/5). In this manner, a relatively large target portion C can be
exposed, without having to compromise on resolution.
[0057] In FIG. 2 the substrate table WT is at a measurement station
where alignment and/or level measurement take place. The substrate
table is provided with a reference F1, sometimes referred to as a
fiducial, which can comprise a plate etched through with a pattern
corresponding to a standard alignment mark underneath which is a
radiation sensor, also known as a transmission image sensor,
responsive to radiation, At the measurement station, the substrate
table WT is moved to detect the reference F1 using an alignment
system within the measurement system 30 and then to detect the
alignment marks on the substrate W thereby enabling the location
(in directions x, y and R.sub.z) of the substrate alignment marks
to be found. In an embodiment, the location of the alignment marks
are measured and determined relative to the reference.
[0058] Level measurement of the substrate then occurs at the
measurement station. In order to measure the level of the
substrate, a leveling beam (projected from the measurement system
30) can be used that traverses a first grating prior to reflection
by the substrate W. A second grating is then placed in the path of
the leveling beam after reflection by the substrate W. The extent
to which the images of the first and second gratings coincide is
measured by a level measurement sensor and is determined by the
height and/or tilts of the substrate W (the z, R.sub.x and R.sub.y
coordinates are thus determined). For a further description of
level measurement of substrates reference is made to European
patent application EP 0502583. Hence, using data from the alignment
of the substrate and the level measurement of the substrate, a map
of the substrate can be generated.
[0059] As shown in FIG. 3, substrate table WT is then moved to the
separate exposure station where a liquid supply 18 is provided to
supply liquid (e.g. water) to a space between the projection system
PL and the substrate table WT to form a liquid reservoir 10. In
this example, the reservoir 10 forms a contactless seal to the
substrate around the image field of the projection system PL so
that liquid is confined to fill a space between the substrate
surface and the final element of the projection system PL. A seal
member 12, positioned below and surrounding the final element of
the projection system PL, borders the reservoir 10 and comprises
the liquid supply 18. The seal member 12 extends a little above the
final element of the projection system and has an inner periphery
that at the upper end closely conforms to the step 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. Liquid is brought into the space below the projection
system and within the seal member 12 and the liquid level rises
above the final element of the projection system PL so that a
buffer of liquid is provided.
[0060] A gas seal 16, formed between the bottom of the seal member
12 and the surface of the substrate W, confines the liquid in the
reservoir. 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 seal member 12 and the
substrate W and extracted via first outlet 14. An overpressure on
the gas inlet 15, vacuum level on the first outlet 14 and geometry
of the gap are arranged so that there is a high-velocity gas flow
inwards that confines the liquid.
[0061] In an embodiment, the liquid reservoir defined by inlet(s)
IN and outlet(s) OUT as shown in FIGS. 4 and 5 can be similarly
applied. In such a case, a measurement station can be provided as
well as an exposure station comprising inlet(s) IN and outlet(s)
OUT.
[0062] To ascertain the exact position of the substrate table WT at
the exposure station the reference F1 is scanned in three
dimensions through the aerial image of an alignment mark on the
mask MA. The maximum signal is returned when the reference is
aligned with the image of the mark on the mask in the plane of best
focus. Using the map of the substrate W generated at the
measurement station, the location, height and/or tilt of positions
on the substrate W are therefore known. In order to track the
movements of the substrate table WT, suitable position measurements
devices can be used such as an interferometer beam projected
towards one or more sides of the substrate table WT. A particular
point on the substrate table can be placed at the focal point of
the projection system PL and exposure of a target portion C of the
substrate W can take place.
[0063] Once exposure of the substrate W is completed it is then
removed for further processing and a new substrate placed on
substrate table WT. The substrate table with the new substrate
returns to the measurement station and the process can be
repeated.
[0064] Prior to the substrate table WT leaving the exposure
station, the liquid reservoir can be emptied, for example in the
case shown in FIGS. 2 and 3, by reducing the gas inlet pressure and
allowing the liquid to be sucked out by the vacuum system or, for
example in the case shown in FIGS. 4 and 5, by discontinuing flow
of liquid onto the substrate through inlet IN and allowing the
liquid to be sucked out by outlet OUT.
[0065] To ascertain the exact position of the substrate table WT,
the position of the transmission image sensor described above can
be sensed through the liquid, or alternatively not through the
liquid and a correction applied.
[0066] According to an embodiment, there are at least two substrate
tables, each bearing a reference, and while one substrate table is
at the measurement station the other is at the exposure station.
The substrate tables are movable between an exposure station and a
measurement station.
[0067] Instead of using the reference mark F1 and the projection
system to align the substrate, off-axis measurement can be used.
The reference mark F1 can be aligned using another system near the
projection system PL. Alternatively, a different reference and a
different system, for example one with an axis perpendicular to the
projection axis of the projection system can be used, Further
description of such off-axis measurement can be found in European
patent application publication EP 0906590.
[0068] Alternatively, if the substrate table is above the
projection system (i.e. the projection system is upside down
compared to FIG. 1) the liquid in liquid reservoir 10 may not need
to be completely removed and could just be refilled as
necessary.
[0069] In an alternative detection embodiment, there is no separate
measurement station. Detection and measurement of an alignment mark
takes place at the exposure station but with no liquid in reservoir
10. The liquid reservoir 10 is then filled up and exposure takes
place. Similarly level measurement can take place at the exposure
station with no liquid in reservoir 10. These measurements can be
either off-axis or on-axis.
[0070] While specific embodiments of the invention have been
described above, it will be appreciated that the invention may be
practiced otherwise than as described. The description is not
intended to limit the invention.
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