U.S. patent application number 10/177134 was filed with the patent office on 2002-11-14 for lithography apparatus.
This patent application is currently assigned to ASML Netherlands B.V.. Invention is credited to Mulkens, Johannesq Catharinus Hubertus, Rider, Gavin Charles, Ten Cate, Jan Wietse Ricolt.
Application Number | 20020167653 10/177134 |
Document ID | / |
Family ID | 8233575 |
Filed Date | 2002-11-14 |
United States Patent
Application |
20020167653 |
Kind Code |
A1 |
Mulkens, Johannesq Catharinus
Hubertus ; et al. |
November 14, 2002 |
Lithography apparatus
Abstract
An illumination system for a microlithographic exposure
apparatus comprises an adjustable axicon, a variable zoom element,
and a multipole illumination mode generating element. By
controlling the optical components, the illumination mode can be
varied continuously between conventional, annular, and
multipole.
Inventors: |
Mulkens, Johannesq Catharinus
Hubertus; (Geldrop, NL) ; Rider, Gavin Charles;
(Eindhoven, NL) ; Ten Cate, Jan Wietse Ricolt;
(Staphorst, NL) |
Correspondence
Address: |
Intellectual Property Group
Pillsbury Winthrop LLP
1600 Tysons Boulevard
McLean
VA
22102
US
|
Assignee: |
ASML Netherlands B.V.
|
Family ID: |
8233575 |
Appl. No.: |
10/177134 |
Filed: |
June 24, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10177134 |
Jun 24, 2002 |
|
|
|
09287014 |
Apr 6, 1999 |
|
|
|
6452662 |
|
|
|
|
Current U.S.
Class: |
355/67 ; 355/53;
355/69; 355/71 |
Current CPC
Class: |
G03F 7/70108 20130101;
G03F 7/701 20130101 |
Class at
Publication: |
355/67 ; 355/53;
355/69; 355/71 |
International
Class: |
G03B 027/54 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 8, 1998 |
EP |
98 201110.8 |
Claims
1. A lithography apparatus comprising: a radiation system for
supplying a projection beam of radiation; a first object table
provided with a mask holder for holding a mask, and connected to
first positioning means; a second object table provided with a
substrate holder for holding a substrate, and connected to second
positioning means; a projection system for imaging an irradiated
portion of the mask onto a target portion of the substrate, wherein
said radiation system comprises an illumination system which
comprises: an adjustable axicon; and a variable zoom element;
characterized by further comprising an adjustable element for
generating a multipole illumination mode, whereby at least one
spatial parameter of said multipole illumination mode can be
continuously varied.
2. An apparatus according to claim 1, further characterized in that
the illumination mode is quadrupole.
3. An apparatus according to claim 1 or 2, further characterized in
that the multipole mode generating element (40) comprises one or
more blades insertable into the beam path of the illumination
system following the axicon and zoom element.
4. An apparatus according to claim 3, further characterized in that
said blades comprise a Maltese cross.
5. An apparatus according to claim 3, further characterized in that
said blades comprise a pair of triangular blades.
6. An apparatus according to claim 3, 4 or 5, further characterized
in that the effective width of at least one of said blades is
continuously variable.
7. An apparatus according to claim 6, further characterized in that
said blade is a composite blade comprising a stack of blades
moveable with respect to each other to vary said effective
width.
8. An apparatus according to claim 1 or 2, further characterized in
that said element for generating a multipole illumination mode
comprises a diffractive and/or refractive component.
9. An apparatus according to claim 8, further characterized in that
said component comprises at least one pyramidal block.
10. An apparatus according to claim 8 or 9, further characterized
in that said component comprises a single pyramidal block.
11. An apparatus according to claim 8 or 9, further characterized
in that said component comprises an array of pyramidal blocks.
12. An apparatus according to claim 8, further characterized in
that said component comprises at least one wedge-shaped block.
13. An apparatus according to claim 8, further characterized in
that said component comprises at least one pair of orthogonally
oriented wedge-shaped blocks.
14. An apparatus according to claim 13, further characterized in
that the two blocks of the said pair of wedge-shaped blocks are
disposed in series in the beam path.
15. An apparatus according to claim 13 or 14, further characterized
by comprising an array of said pairs of wedge-shaped blocks.
16. An apparatus according to claim 1 or 2, further characterized
in that said element for generating a multipole illumination mode
comprises at least one array of lenses or diffractive optical
elements.
17. An apparatus according to claim 16, further characterized in
that said at least one array comprises an array of Fresnel lens
segments.
18. An apparatus according to claim 16 or 17, further characterized
by comprising a plurality of arrays, interchangeably positionable
in the radiation path.
19. An apparatus according to any one of the preceding claims,
further characterized in that the multipole mode generating element
is rotatable about an axis parallel to the principal optical axis
of the system.
20. An apparatus according to any one of the preceding claims,
further characterized by further comprising a light pipe having a
quadrilateral cross-section.
21. An apparatus according to claim 20, further characterized in
that the multipole mode generating element is disposed adjacent to
the light pipe entrance.
22. An apparatus according to claim 20, further characterized in
that the multipole mode generating element is disposed at an
intermediate position along the light pipe.
23. An apparatus according to claim 20, 21 or 22, further
characterized in that the light pipe comprises a glass, quartz or
calcium-fluoride rod.
24. An apparatus according to any one of claims 20 to 23, further
characterized in that the principal transverse axes of the
multipole mode generating element lie along directions angularly
displaced with respect to the principal transverse axes of the
light pipe.
25. An apparatus according to any one of the preceding claims,
wherein the radiation system further comprises an excimer laser
source, and wherein the adjustable multipole mode generating
element is locatable in the collimated beam path of the source.
26. An apparatus according to any one of the preceding claims,
wherein the multipole illumination mode comprises an on-axis pole
and at least one off-axis pole.
27. A device manufacturing method comprising the steps of:
providing a substrate which is at least partially covered by a
layer of energy-sensitive material; providing a mask containing a
pattern; using a beam of radiation to project at least part of the
mask pattern onto a target area of the layer of energy-sensitive
material, characterized by generating a multipole illumination mode
from said radiation before projection, using an adjustable axicon,
variable zoom element, and further adjustable element, whereby at
least one spatial parameter of said multipole illumination can be
continuously varied.
28. A method according to claim 27, further comprising generating
an on-axis illumination pole in addition to off-axis poles of the
multipole illumination mode.
29. A device manufactured in accordance with the method of claim 27
or 28.
Description
[0001] The present invention relates to an illumination system, in
particular for a microlithographic exposure apparatus in which the
illumination mode can be varied. More particularly, the invention
relates to the application of such a device in a lithographic
projection apparatus comprising:
[0002] a radiation system for supplying a projection beam of
radiation;
[0003] a first object table provided with a mask holder for holding
a mask, and connected to first positioning means;
[0004] a second object table provided with a substrate holder for
holding a substrate, and connected to second positioning means;
[0005] a projection system for imaging an irradiated portion of the
mask onto a target portion of the substrate.
[0006] 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 elements operating according to any of these principles for
directing, shaping or controlling the projection beam of radiation
and such elements may also be referred to below, collectively or
singularly, as a "lens". Any refractive, reflective or catadioptric
elements in the radiation or illumination systems may be based on a
substrate of glass or other suitable material, and may be provided
with either single- or multi-layer coatings as desired. In
addition, the first and second object tables may be referred to as
the "mask table" and the "substrate table", respectively. Further,
the lithographic apparatus may be of a type having two or more mask
tables and/or two or more substrate 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 stages while
one or more other stages are being used for exposures. Twin stage
lithographic apparatus are described in International Patent
Applications WO98/28665 and WO98/40791.
[0007] Lithographic projection apparatus can be used, for example,
in the manufacture of integrated circuits (ICs). In such a case,
the mask (reticle) may contain a circuit pattern corresponding to
an individual layer of the IC, and this pattern can then be imaged
onto a target area (die) on a substrate (silicon wafer) which has
been coated with a layer of photosensitive material (resist). In
general, a single wafer will contain a whole network of adjacent
dies which are successively irradiated through the reticle, one at
a time. In one type of lithographic projection apparatus, each die
is irradiated by exposing the entire reticle pattern onto the die
in one go; such an apparatus is commonly referred to as a
waferstepper. In an alternative apparatus--which is commonly
referred to as a step-and-scan apparatus--each die is irradiated by
progressively scanning the reticle pattern under the projection
beam in a given reference direction (the "scanning" direction)
while synchronously scanning the wafer table parallel or
anti-parallel to this direction; since, in general, the projection
system will have a magnification factor M (generally .ltoreq.1),
the speed .nu. at which the wafer table is scanned will be a factor
M times that at which the reticle table is scanned. More
information with regard to lithographic devices as here described
can be gleaned from International Patent Application WO
97/33205.
[0008] In one form of microlithography, a mask defining features is
illuminated with radiation from an effective source having an
intensity distribution at a pupil plane corresponding to a
particular illumination mode. An image of the illuminated mask is
projected onto a resist-coated semiconductor wafer.
[0009] One method to reduce feature size, i.e. increase resolution,
in optical lithography, is off-axis illumination. With this
technique, the mask is illuminated at non-perpendicular angles
which may improve resolution, but particularly improves the process
latitude by increasing the depth of focus and/or contrast. One
known illumination mode is annular, in which the conventional zero
order spot on the optical axis is changed to a ring-shaped
intensity distribution. Another mode is multipole illumination in
which several spots or beams are produced which are not on the
optical axis. The spatial intensity distribution at the pupil plane
is converted into an angular distribution at the mask plane.
[0010] Problems with the prior art include lack of flexibility of
the illumination system such as only having fixed illumination
modes or a limited range of modes or a difficulty in selecting or
mixing desired modes. Some prior systems also have a high loss of
energy by blocking parts of the illuminating radiation.
[0011] It is an object of the present invention to alleviate, at
least partially, at least some of the above problems.
[0012] According to the present invention, this and other objects
are achieved in a lithographic projection apparatus as described in
the opening paragraph, wherein the radiation system comprises an
illumination system which comprises:
[0013] an adjustable axicon; and
[0014] a variable zoom element;
[0015] characterized by further comprising an adjustable element
for generating a multipole illumination mode, whereby at least one
spatial parameter of said multipole illumination mode can be
continuously varied.
[0016] The illumination system according to the invention enables a
range of illumination modes to be produced including conventional,
annular and quadrupole. The axicon, zoom and multipole generating
element allow the spatial intensity distribution of the
illumination mode to be continuously varied. The spatial intensity
distribution results in angular or oblique illumination of the
reticle which improves the process latitude of the lithographic
exposure apparatus.
[0017] Although specific reference may be made in this text to the
use of the apparatus according to the invention 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 term "reticle", "wafer" or "die" in
this text should be considered as being replaced by the more
general terms "mask", "substrate" and "target area",
respectively.
[0018] Embodiments of the invention will now be described, by way
of example only, with reference to the accompanying drawings in
which:
[0019] FIGS. 1 and 2 illustrate prior illumination systems;
[0020] FIG. 3 illustrates some illumination intensity distributions
obtainable with prior systems;
[0021] FIG. 4 shows an illumination system for use in the
invention;
[0022] FIG. 5a shows an embodiment of a multipole mode generating
element of the system of FIG. 4;
[0023] FIG. 5b shows an illumination intensity distribution
obtained with the embodiment of FIG. 5a;
[0024] FIGS. 6 and 7 show further alternative embodiments to that
of FIG. 5a;
[0025] FIG. 8 shows in cross-section another embodiment of an
illumination system for use in the invention and the resulting
illumination intensity distribution;
[0026] FIG. 9 illustrates the effect of a wedge-shaped optical
element on a light cone;
[0027] FIG. 10 shows a pyramidal block on the entrance window plane
of a quartz rod according to another embodiment of the
invention;
[0028] FIG. 11 illustrates a double wedge element located at the
entrance window of a quartz rod according to another embodiment of
the invention;
[0029] FIG. 12 shows an array of Fresnel lenses for a conventional
or annular illumination profile;
[0030] FIG. 13 illustrates an array of Fresnel lens segments for
quadrupole illumination according to a further embodiment of the
invention;
[0031] FIGS. 14, 15 and 16 illustrate a further array of lens
segments for another embodiment of the invention;
[0032] FIG. 17 illustrates an example of a quadrupole illumination
mode intensity distribution;
[0033] FIG. 18 illustrates an illumination intensity distribution
after transmission through a quartz rod;
[0034] FIG. 19 illustrates a quadrupole mode generating element
rotated with respect to the orientation of a quartz rod;
[0035] FIG. 20 shows the illumination intensity distribution before
transmission through a quartz rod in an illumination system
according to a further embodiment of the invention;
[0036] FIG. 21 shows the resulting illumination intensity
distribution after transmission through a quartz rod of the
incident distribution shown in FIG. 20; and
[0037] FIG. 22 is a plot of depth of focus against resolution for
different illumination modes;
[0038] FIGS. 23(a) and (b) show diffracted beam for on- and
off-axis illumination modes;
[0039] FIG. 24 shows diffracted beams for larger features for
on-axis illumination;
[0040] FIGS. 25(a) and (b) show mixed illumination mode intensity
distributions for relatively small and larger features,
respectively;
[0041] FIG. 26 shows an apparatus for imaging a mask on a
substrate, in which apparatus the invention can be embodied.
[0042] Two prior illumination systems are illustrated schematically
in FIGS. 1 and 2. Referring to FIGS. 1 and 2, these systems have:
light collecting/collimating optics 10; an axicon/zoom module 12;
and light integrating and projecting optics 14. The systems define
an optical axis 16, a pupil plane 18, and reticle plane 20. The
axicon/zoom module 12 comprises a pair of axicons 22, one concave
and one convex, whose separation can be varied. The module 12 also
comprises a zoom lens 24.
[0043] For the case of conical axicons, some examples of the
illumination intensity distributions achievable at the pupil plane
18 are shown in FIG. 3. The spot size can be varied between states
A and B by changing the zoom lens position. Similarly, the
annularity can be changed between states A and C by varying the
axicon opening (separation between the axicons).
[0044] To improve the illumination homogeneity, an optical
integrator is used. In FIG. 1 this takes the form of a light pipe
26, such as a glass, calcium fluoride or quartz rod. A coupler 28
couples the illumination at the pupil plane 18 into the rod 26, and
rod exit imaging optics 30 are also provided. In FIG. 2 a fly's eye
element 32 acts as the integrator. The fly's eye element 32 is a
composite lens comprising an array or honeycomb of small lenses.
Further objective lenses 34, 36 complete the projection optics.
[0045] Further details of such illumination systems are disclosed
in EP-A-687 956. The present invention can be embodied in
illumination systems as described above, and in the following
description like items are given like reference numerals.
[0046] The embodiments of the invention described below relate to
quadrupole illumination modes as a particular example of multipole
illumination. Other modes are of course possible with the
invention, such as dipole.
[0047] A first embodiment of the invention is shown in FIG. 4. The
illumination system has light collecting/collimating optics 10; an
axicon/zoom module 12; multipole mode generating element 38; and
light integrating and projecting optics 14. The components lie
along optical axis 16 and are used to illuminate a reticle (not
shown) located at reticle plane 20 which then produces an exposure
pattern in etch resist on a wafer (not shown), via a projection
system (also not shown). The system illustrated in FIG. 4 includes
a quartz rod light integrator 26, although the invention can be
embodied in other systems such as that illustrated in FIG. 2. FIG.
4 shows the multipole mode generating element 38 located between
the axicon/zoom module 12 and the integrating/projecting optics 14
at the pupil plane 18 of the system. Several embodiments will be
described later in which the element is located elsewhere in the
system, for example before the axicon/zoom module 12, interposable
within the axicon/zoom module 12, and at the entrance window of the
rod 26. The location is related to the particular multipole mode
generating element 38 that is being used, as described in the
following embodiments. The optical axis 16 shown in FIG. 4 can of
course be folded to produce a more compact illumination system.
[0048] An embodiment of the multipole mode generating element 38 is
shown in FIG. 5a. The element 38 has four triangular blades 41, 42,
43, 44 insertable into the beam path at the pupil plane and which
form a Maltese cross 40, which is also referred to herein as a
Maltese aperture blade (MAB). Each blade has an apex angle .beta..
FIG. 5b shows the illumination intensity distribution resulting
from the combination of an annular illumination mode produced by
the axicon/zoom module and the MAB. The distribution has four light
beams or poles 45. This embodiment enables continuously variable
quadrupole illumination modes to be produced. The radial position
of each pole can be varied by adjusting the axicon optics, the
radial width of each pole can be varied by adjusting the zoom lens,
and the tangential pole width can be changed by inserting another
set of blades having a different apex angle .beta., such as Maltese
cross 40 shown in FIG. 6. By removing the blades altogether, the
illumination system can be used for conventional and/or annular
modes, again with continuous variation.
[0049] Interposing blades of different angle .beta. permits the
tangential pole width to be changed in discrete steps. According to
a further embodiment of the invention, the tangential pole width
can be continuously varied by each arm of the Maltese cross
comprising a stack of n blades, rotatable with respect to each
other about the optical axis of the system where their vertices
lie. If the angle of each separate blade is .beta., the overall
segment angle can be continuously varied from .beta. to n.beta.,
thus the tangential width of each pole can be varied between the
angles .pi./2-.beta. and .pi./2-n.beta.. The rotation of the blades
to vary the effective width of each arm of the Maltese cross can be
automated. A simple embodiment is shown in FIG. 7 in which each
stack consists of two blades. FIG. 7 shows the blades of each stack
spread out. When the blades are aligned, the Maltese cross 40 will
look the same as that shown in FIG. 6. Another variation is to have
blades rotatable about radial axes to permit their effective width
to be varied, for example two blades hinged in the form of a
butterfly.
[0050] According to a further embodiment, just two blades are used
as the multipole mode generating element 38 in an optical system
which includes a light pipe, such as a rectangular quartz rod 26,
as shown in the illumination system of FIG. 4. One of the blades is
oriented parallel to the short side of the rectangular
cross-section of the light pipe and the other blade parallel to the
long side. Due to the multiple reflections in the pipe, the
resulting illumination mode is a mixture of annular and quadrupole.
The two-blade system can produce an illumination mode including a
quadruple component with lower energy-loss than the Maltese cross
arrangement, as there are fewer blades obstructing the light beam.
In one example the blades are triangular and are like two
perpendicular arms of a Maltese cross, e.g. blades 41 and 42 shown
in FIG. 5a. One or both of the blades in this embodiment can be a
composite blade comprising a stack of smaller rotatable blades as
described above.
[0051] Typically the blades are positioned along directions
corresponding to orthogonal lines on the reticle, so that the light
poles are located in each quadrant with centres forty five degrees
from the orthogonal lines. This orientation can produce optimal
projection of the lines, particularly for dense structures, such as
for DRAM-like structures. The orthogonal lines are generally
referred to as horizontal and vertical.
[0052] A further variation on the above embodiments using blades is
to make all the blades rotatable about the optical axis of the
illumination system so that the position of the poles can be
rotated.
[0053] The next embodiment of the invention has an axicon/zoom
module with a pyramidal prism as the multipole mode generating
element. This also enables conventional, annular and quadrupole
illumination to be produced with continuous variations of the
modes. FIG. 8 shows the optical components of an axicon/zoom
module. The right hand column in FIG. 8 shows the illumination
intensity distributions at the pupil plane 18 for various positions
of the axicon pair 22a, 22b and zoom lens 24. The axicon pair 22
comprises a pair of elements having conical surfaces, one concave
22a, one convex 22b, to produce circular and annular illumination
patterns. The fourth row shows the effect of separating a
pyramid-shaped prism 50 from element 22b. The side of the element
22b facing the pyramid 50 is concave pyramidal for receiving the
pyramid 50. Element 22b and pyramid 50 comprise a second axicon
also known as a pyramidal axicon or pyramidon. The pyramid-shaped
prism 50 has a four-sided base, which consequently produces
quadrupole mode illumination patterns, such as the four spots
illustrated at the bottom in the right hand column in FIG. 8.
[0054] The illumination system of FIG. 8 is extremely versatile in
that the illumination mode can be varied continuously from
conventional to annular or quadrupole. The zoom lens 24 sets the
spot size or partial coherence factor, the axicon 22 determines the
annularity, and a pyramidon 50 determines the quadrupolarity. In
addition, since light flux is redistributed rather than blocked,
there is virtually no light loss, so that a high throughout can be
maintained.
[0055] Referring to the system of FIG. 1, as discussed earlier,
spatial intensity distribution at the pupil plane 18 corresponds to
an angular distribution at the entrance and exit planes of the
quartz rod 26. Angular or off-axis illumination at the reticle can
improve process latitude. FIG. 9 shows one way of altering the
angular distribution of the illumination using wedge-shaped optical
elements. A pair of incident light cones 52 with axes parallel to
the optical axis emerge from the wedges 51 as light cones inclined
at an angle to the optical axis.
[0056] FIG. 10 shows an embodiment of the invention employing this
principle. A pyramidal element 54 is positioned on the entrance
plane of the rod 26. The inclined faces of the pyramid act as
wedge-shaped refractive elements. Light incident on the pyramid is
refracted in four directions, so quadrupole-like illumination is
created. FIG. 11 shows an embodiment comprising a pair of
wedge-shaped elements 56 placed in series in front of the entrance
window of the rod 26. The wedges 56 are rotated by 90.degree. with
respect to each other to tilt an incident light cone in two
directions, which after multiple reflections in the rod 26 creates
quadrupole-like illumination. Thus tilting the light cone in only
two orthogonal directions can still produce quadrupole
illumination. Once again, since light flux is redistributed rather
than blocked, there is virtually no light loss, so that a high
throughout can be maintained.
[0057] A variation on the above pyramid and wedge embodiments is to
replace the single large pyramid or wedges by an array of many
small pyramids or wedges. The light deviation with small elements
can be obtained by diffractive as well as refractive effects. In
the case of an array of wedges, one can alternate the orientation
of the wedge faces within the array rather than stacking pairs of
wedges on top of each other.
[0058] Two further ways of generating desired illumination modes
are shown in FIGS. 12 and 13. For conventional and annular
illumination, a micro-lens array as shown in FIG. 12 can be used.
Each element of the hexagonal array 59 comprises a Fresnel lens or
refractive lens. For flexible quadrupole illumination, the
embodiment shown in FIG. 13 can be used. Each element of the square
array 61 of micro lenses comprises a segment or quadrant of a
Fresnel lens 60. One way of arranging the four quadrants of the
lens 60 is shown in FIG. 13. The four lens quadrants create four
illumination poles at the pupil plane. Once again, since light flux
is redistributed rather than blocked, there is virtually no light
loss, so that a high throughout can be maintained.
[0059] A further currently preferred embodiment of the multipole
illumination mode generating element is illustrated with reference
to FIGS. 14, 15 and 16. The element is an array of lens segments.
This is particularly efficient because the multipole modes are
generated without blocking any parts of the beam, so there is no
intensity loss and throughput is maintained. FIG. 14 shows where
each of four lens segments, labelled N, S, E and W, effectively
come from in a complete lens. The lens segments and half lens
segments can be tessellated to form rectangles, as shown in FIG.
15. A complete surface can be covered by lens segments, which are
preferably formed into rectangles and tiled in a staggered pattern
as shown in FIG. 16.
[0060] In practice the lens array can be formed on the surface of a
quartz substrate. The lens segments are formed of grooves etched in
the surface to provide segments of a Fresnel lens. The depth and
width of the grooves is typically of the order of micrometers, each
lens segment being of the order of millimetres in size and the
array dimensions being centimetres.
[0061] Fresnel lenses are merely used as an example. Other types of
lenses or diffractive optical elements may be used. The same or
better performance can be achieved using conventional refractive
lenses or lens segments in the array. However, Fresnel lenses may
be preferred from a manufacturing point of view.
[0062] The lens segment shape determines the pole shape. FIG. 17
shows an example of the pole pattern at the lens pupil. In this
case, each pole is a segment of an annulus. The angle .alpha. of
each pole is determined by the lens segment angle. The radii
.sigma..sub.i and .sigma..sub.o are adjustable by the axicon/zoom
module. A preferred value of .alpha. is 30.degree.. Different
.alpha. values and pole patterns, such as dipole, can be achieved
by providing several different, interchangeable, lens arrays or
diffractive optical elements. An automatic changer can be used to
swap between such different multipole mode generating elements in
the illumination system.
[0063] The optical elements discussed above can be positioned at
the rod entrance, for example as shown in FIGS. 10 and 11, but it
can be advantageous to place them at some intermediate cross
section of the rod. The intermediate positioning gives a more
homogenous angular distribution of the incoming light cones when
entering the optical refractive or diffractive elements. The
Fresnel lens arrays are particularly suited to excimer laser
illumination systems and may be placed in the collimated laser
beam, for example before the light enters the axicon/zoom
module.
[0064] The above systems for producing quadrupole illumination
result in intensity distribution patterns in which there is
substantially no light around the x and y axes. The four poles are
located at .+-.45.degree. and .+-.135.degree. from the positive x
axis of the orthogonal coordinate system. The z axis lies along the
optical axis of the system and the x and y axes are in the plane
perpendicular to the optical axis. In a system including an
integrating quartz rod (e.g. FIG. 1), the x-axis is conventionally
perpendicular to the long side of the rectangular cross section of
the quartz rod, and the y-axis is perpendicular to the short side.
After transmission through the quartz rod, the four poles each
comprise a number of small dots because of the discrete number of
internal reflections along the quartz rod. FIG. 18 illustrates
schematically each pole comprising a discrete number of light
spots.
[0065] According to a further embodiment of the invention, a new
illumination mode can be produced which is a mix between quadrupole
and annular. This is achieved by orienting the quadrupole mode
generating element such that the regions of no light intensity are
no longer centered on the x and y axes. For example, the blades of
a Maltese cross aperture are rotated about the z-axis by a suitable
angle as shown in FIG. 19. FIG. 20 shows an example of the
resulting illumination intensity distribution at the pupil plane
after the axicon/zoom module and before entry to the quartz rod.
The multiple internal reflections in the quartz rod impose a
symmetry on the intensity distribution with respect to the x and y
axes, resulting in the intensity distribution pattern shown in FIG.
21, after transmission through the quartz rod. As can be seen in
FIG. 21, the result is four regions of high light spot density
along the 45.degree. diagonal directions, one in each quadrant.
Between these are regions of low light spot density around the x
and y axes. The spot densities depend on the positions of the four
incident spots, i.e. the orientation of the quadrupole element and
the other parameters of the illumination system such as the type of
quadrupole element, the axicon and zoom positions.
[0066] Research shows that for exposing horizontal or vertical
features, quadrupole illumination results in larger depths of focus
than annular illumination. This is particularly true for dense
periodic features. However, for equivalent features oriented around
45.degree. with respect to the x and y axes, the imaging capability
with quadrupole illumination will be inferior to annular
illumination. This is illustrated in FIG. 22, where the depth of
focus (DOF) in micrometres is schematically plotted against
resolution (.lambda./NA) for (a) conventional circular
illumination, (b) annular illumination, (c) quadrupole illumination
for horizontal/vertical features and (d) quadrupole illumination
for features/lines at 45.degree.. To benefit from the improved
depth of focus, the quadrupole parameters must, of course, be
selected according to the periodicity of the pattern being
imaged.
[0067] The intensity distribution of the kind shown in FIG. 21 will
be called "soft quadrupole", in contrast to "hard quadrupole", as
shown in FIG. 17, in which there is no illumination in the vicinity
of the x and y axes. Studies indicate that soft quadrupole
illumination provides a compromise that improves on annular
illumination for vertical and horizontal features, and improves on
hard quadrupole illumination for diagonal features. In simulations,
the soft quadrupole illumination had, in the annular sections, a
relative intensity of 0.5 on the x and y axes and a relative
intensity of 1.0 on the diagonals.
[0068] Quadrupole illumination can enhance the image definition and
depth of focus of finely spaced periodic arrays. Previously it has
not been considered very suitable for use with aperiodic and widely
spaced (isolated) structures. Where such structures are used in
combination with dense periodic arrays (such as edge lines,
conductors leading to contact pads, mixed logic and memory
circuits, etc.) a compromise has to be found between the use of
quadrupole or conventional illumination conditions. Typically this
means the quadrupole is "softened" by using soft-edged illumination
poles, by enlarging the poles or by adding illumination in the
background.
[0069] A further embodiment of the present invention is to combine
two kinds of illumination in one exposure--conventional
illumination for the isolated structures and quadrupole for the
dense periodic structures. Since the quadrupole is generally tuned
to enhance structures that are at or near the diffraction limit of
the lens, conventional illumination cannot resolve these features
because the diffraction orders (+1, -1 etc.) fall outside the pupil
(70), as shown in FIG. 23(a) for conventional on-axis illumination
in contrast with FIG. 23(b) for off-axis illumination, e.g.
quadrupole.
[0070] However, for isolated features the addition of light
intensity to supplement the off-axis illumination will aid the
printing of these features. General background illumination will
overwhelm the off-axis illumination, so the proportion of off-axis
and conventional illumination needs to be controlled. Mixing a
well-defined, narrow on-axis beam of light with the off-axis
illumination in a fixed ratio can be achieved, for example with a
multipole diffractive optical element.
[0071] Furthermore, larger features in the image field can be
imaged perfectly well with the conventional illumination component
of the light whose first order diffraction components do not fall
outside the pupil, as shown in FIG. 24. Since the separate
illumination sources are not coherent, the images do not interfere
with one another, and merely add to each other. Examples of pole
patterns for small and large features are illustrated in FIGS.
25(a) and (b) respectively.
[0072] Phase shift masks can be used to enhance isolated features.
To use these masks the illumination is set to low sigma (highly
coherent, close to normal incidence). According to another
embodiment of the invention, the combination of quadrupole
illumination (which does not enhance isolated features) for
enhancing dense arrays and an intense low-sigma central pole for
enhancement of isolated features, in combination with a phase shift
mask, may yield an overall improvement of depth of focus for all
features.
[0073] The apparatus of this invention is particularly flexible and
has minimal loss of light. The embodiments of the invention
described above are suitable for use in lithographic systems
operating with ultraviolet illumination, for example using mercury
arc lamps or excimer lasers as sources. Typically, mercury arc
lamps are used to produce "i-line" radiation with a wavelength of
365 nm, and excimer lasers are used to produce deep ultraviolet
radiation at wavelengths of 248 nm, 193 nm and 157 nm.
[0074] Although in the illustrated examples the illumination
radiation passes through the axicon before the zoom lens, the
sequence of these elements can be changed. This is a design choice
and can depend on the radiation source that is used.
[0075] Referring to FIG. 26, a lithographic apparatus will now be
described in which an illumination system as described above can be
used to embody the invention, for repetitive imaging of a mask M
(for example a reticle) on a substrate W (for example a
resist-coated wafer). The particular apparatus shown here is
transmissive; however, it may also be reflective or catadioptric,
for example.
[0076] The apparatus comprises an illumination housing LH
containing a radiation source and an illumination system according
to the invention for supplying an illumination beam IB. This beam
passes through a diaphragm DR and is subsequently incident on the
mask M which is arranged on a mask table MT, which is adjustable in
position. The mask table MT forms part of a projection column PC
incorporating also a projection lens system PL which comprises a
plurality of lens elements, only two of which, L.sub.1 and L.sub.2,
are shown in FIG. 26. The projection lens system images the mask M
onto the substrate W which is provided with a photoresist layer
(not shown). The substrate is provided on a substrate support WC
which forms part of a substrate table WT on, for example, air
bearings. The projection lens system has, for example a
magnification M={fraction (1/5)}, a numerical aperture NA>0.48
and a diffraction-limited image field with a diameter of, for
example 22 mm. The substrate table WT is supported, for example by
a granite base plate BP which closes the projection column PC at
its lower side.
[0077] The substrate can be displaced in the x, y and z directions
and rotated, for example about the z axis with the aid of the
substrate table. These adjustments are controlled by various
servosystems such as a focus servosystem, for example an x, y,
.phi..sub.z interferometer system cooperating with the substrate
support, and an alignment system with which mask marks can be
aligned with respect to substrate marks. These servosystems are not
shown in FIG. 26. Only the alignment beams (with their chief rays
AB.sub.1, AB.sub.2) of the alignment system are shown.
[0078] The mask must be imaged a number of times, in accordance
with the number of ICs to be formed on the substrate, each time on
a different target area of the substrate.
[0079] The depicted apparatus can be used in two different
modes:
[0080] In step mode, the mask stage MT is kept essentially
stationary, and an entire mask image is projected in one go (i.e. a
single "flash") onto a target area. The substrate stage WT is then
shifted in the x and/or y directions so that a different target
area can be irradiated by the beam IB.
[0081] In scan mode, essentially the same scenario applies, except
that a given target area is not exposed in a single "flash".
Instead, the mask stage MT is movable in a given direction (the
so-called "scan direction", e.g. the x direction) with a speed
.nu., so that the projection beam IB is caused to scan over a mask
image; concurrently, the substrate stage WT is simultaneously moved
in the same or opposite direction at a speed V=M.nu., in which M is
the magnification of the lens PL (e.g. M={fraction (1/5)}). In this
manner, a relatively large target area can be exposed, without
having to compromise on resolution.
[0082] These processes are repeated until all areas of the
substrate have been illuminated.
[0083] Whilst we have described above specific embodiments of the
invention it will be appreciated that the invention may be
practised otherwise than described.
* * * * *