U.S. patent number RE40,084 [Application Number 11/055,790] was granted by the patent office on 2008-02-19 for optical proximity correction.
This patent grant is currently assigned to ASML Masktools Netherlands B.V.. Invention is credited to Jang Fung Chen, John S. Petersen.
United States Patent |
RE40,084 |
Petersen , et al. |
February 19, 2008 |
Optical proximity correction
Abstract
Method for utilizing halftoning structures to manipulate the
relative magnitudes of diffraction orders to ultimately construct
the desired projected-image. At the resolution limit of the mask
maker, this is especially useful for converting strongly shifted,
no-0.sup.th-diffraction-order, equal-line-and-space chromeless
phase edges to weak phase-shifters that have some 0.sup.th order.
Halftoning creates an imbalance in the electric field between the
shifted regions, and therefore results in the introduction of the
0.sup.th diffraction order.
Inventors: |
Petersen; John S. (Austin,
TX), Chen; Jang Fung (Cupertino, CA) |
Assignee: |
ASML Masktools Netherlands B.V.
(Veldhoven, NL)
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Family
ID: |
24246316 |
Appl.
No.: |
11/055,790 |
Filed: |
February 11, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09562445 |
May 1, 2000 |
6335130 |
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Reissue of: |
09840307 |
Apr 24, 2001 |
06541167 |
Apr 1, 2003 |
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Current U.S.
Class: |
430/5 |
Current CPC
Class: |
G03F
1/26 (20130101); G03F 7/70125 (20130101); G03F
7/70283 (20130101); G03F 7/70441 (20130101); G03F
1/32 (20130101); Y10T 428/24802 (20150115) |
Current International
Class: |
G03F
1/00 (20060101) |
Field of
Search: |
;430/5,30,322
;355/53,73,76 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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WO 99/27420 |
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Jun 1999 |
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WO |
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WO 99/47981 |
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Sep 1999 |
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WO |
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Primary Examiner: Rosasco; S.
Attorney, Agent or Firm: McDermott Will & Emery LLP
Parent Case Text
This application is a continuation-in-part of application Ser. No.
09/562,445, which was filed on May 1, 2000 now U.S. Pat. No.
6,335,130.
Claims
What is claimed is:
1. A device manufacturing method comprising the steps of: (a)
providing a substrate that is at least partially covered by a layer
of radiation-sensitive material; (b) providing a projection beam of
radiation using a radiation system; (c) using a pattern on a mask
to endow the projection beam with a pattern in its cross-section;
(d) projecting the patterned beam of radiation onto a target
portion of the layer of radiation-sensitive material, wherein, the
step (c): use is made of a phase-shifting mask comprising at least
one unattenuated, halftoned, phase-shift feature; the mask is
off-axis illuminated by the radiation system.
2. A device manufactured using a method according to claim 1.
3. A device manufacturing method comprising the steps of: (a)
providing a substrate that is at least partially covered by a layer
of radiation-sensitive material; (b) providing a projection beam of
radiation using a radiation system; (c) using a pattern on a mask
to endow the projection beam with a pattern in its cross-section;
(d) projecting the patterned beam of radiation onto a target
portion of the layer of radiation-sensitive material, wherein, in
step (c): use is made of a phase-shifting mask comprising at least
one feature, wherein said at least one feature includes halftoned,
phase-shifted, transparent features; and the mask is off-axis
illuminated by the radiation system.
4. A device manufactured using a method according to claim 3.
5. A computer program product for controlling a computer comprising
a recording medium readable by the computer, means recorded on the
recording medium for directing the computer to generate at least
one file corresponding to a phase shifting mask capable of
transferring an image including 0.sup.th diffraction order and
.+-.1.sup.st diffraction orders, onto a material, said generation
of said file comprising the step of: generating a phase-shifting
mask comprising at least one unattenuated, halftoned, phase-shift
feature; said phase-shifting mask being utilized in conjunction
with off-axis illumination such that radiation traverses said mask
and impinges on said material.
6. A computer program product for controlling a computer comprising
a recording medium readable by the computer, means recorded on the
recording medium for directing the computer to generate at least
one file corresponding to a phase shifting mask capable of
transferring an image, including 0.sup.th diffraction order and
.+-.1.sup.st diffraction orders, onto a material, said generation
of said file comprising the step of: generating a phase-shifting
mask comprising at least one feature, wherein said at least one
feature includes halftoned, phase-shifted, transparent features;
and said phase-shifting mask being utilized with off-axis
illumination such that radiation passes through said mask onto said
material.
7. The computer program product of claim 6, wherein said at least
one feature further includes semi-transparent features.
8. The computer program product of claim 6, wherein said at least
one feature further includes opaque features.
9. A computer program product for controlling a computer comprising
a recording medium readable by the computer, means recorded on the
recording medium for directing the computer to generate at least
one file corresponding to a mask capable of transferring an image
onto a material, said generation of said file comprising the step
of: generating a phase-shifting mask comprising at least two
unattenuated, halftoned, phase-shift features having a width w,
said features separated by a width substantially equal to w,
wherein said mask provides an image including 0.sup.th diffraction
order and .+-.1.sup.st diffraction orders, when illuminated.
10. The computer program of claim 9, wherein a focus-exposure
process window for maintaining a predetermined resist line-width
sizing of said mask is substantially common to an attenuated,
phase-shift mask of a similar pitch.
11. A computer program product for controlling a computer
comprising a recording medium readable by the computer, means
recorded on the recording medium for directing the computer to
generate at least one file corresponding to a mask capable of
transferring an image onto a material, said generation of said file
comprising the step of: generating a phase-shifting mask comprising
at least two halftoned, phase-shifted, transparent features having
a width w, said features separated by a width substantially equal
to w, wherein said mask provides an image including 0.sup.th
diffraction order and .+-.1.sup.st diffraction orders, when
illuminated.
12. The computer program of claim 11, wherein said at least two
features further include semi-transparent features.
13. The computer program of claim 11, wherein said at least two
features further include opaque features.
14. The computer program of claim 11, wherein a focus-exposure
process window for maintaining a predetermined resist line-width
sizing of said mask is substantially common to an attenuated,
phase-shift mask of a similar pitch.
.Iadd.15. A method of designing a phase-shifting mask for
transferring an image onto a material, said image including first
features having a first pitch and second features having a second
pitch different than said first pitch, said method comprising the
step of: modifying in said mask at least one of said first and
second features to include at least one unattenuated, halftoned
phase-shift feature that emulates an arbitrary percentage
transmission between 6% and 100%, whereby said first and second
features in said mask emulate different percentage transmissions
such that said first and second features share a common exposure
latitude and a corresponding depth of focus..Iaddend.
.Iadd.16. A method according to claim 15 wherein at least one of
said first and second features comprise an equal line/space
chromeless pattern and said step of modifying is carried out on
said equal line/space chromeless pattern such that said pattern
produces a 0.sup.th order having a non-zero amplitude when
illuminated..Iaddend.
.Iadd.17. A method according to claim 15, wherein said step of
modifying is carried out such that the modified features produce a
diffraction pattern and aerial image corresponding to a partially
transparent, attenuated phase-shaft mask, having an arbitrary
percentage transmission..Iaddend.
.Iadd.18. A method according to claim 15, wherein said
unattenuated, halftoned, phase-shift feature is a primary
feature..Iaddend.
.Iadd.19. A method according to claim 15, wherein said
unattenuated, halftoned, phase-shift feature is an assist
feature..Iaddend.
.Iadd.20. A method of fabricating a phase-shifting mask for
transferring an image onto a material, the method comprising
designing said mask according to the method of claim 1, and
fabricating a mask according to the design..Iaddend.
.Iadd.21. A device manufacturing method comprising the steps of:
providing a substrate that is at least partially covered by a layer
of radiation-sensitive material; providing a projection beam of
radiation using a radiation system; using a pattern on a mask to
endow the projection beam with a pattern in its cross-section; and
projecting the patterned beam of radiation onto a target portion of
the layer of radiation-sensitive material; characterized in that:
said mask includes first features having a first pitch and second
features having a second pitch, at least one of said first features
having been modified to include at least one unattenuated,
halftoned phase-shift feature that emulates an arbitrary percentage
transmission between 6% and 100%, whereby said first and second
features in said mask emulate different percentage transmissions
such that said first and second features share a common exposure
latitude and a corresponding depth of focus..Iaddend.
.Iadd.22. A device manufacturing method according to claim 21,
wherein said mask is off-axis illuminated by said radiation
system..Iaddend.
.Iadd.23. A device manufacturing method according to claim 21,
wherein at least one of said first and second features comprise an
equal line/space chromeless pattern and said step of modifying is
carried out on said equal line/space chromeless pattern such that
said pattern produces a 0.sup.th order having a non-zero amplitude
when illuminated..Iaddend.
.Iadd.24. A device manufacturing method according to claim 21,
wherein said step of modifying is carried out such that the
modified features produce a diffraction pattern and aerial image
corresponding to a partially transparent, attenuated phase-shaft
mask, having an arbitrary percentage transmission..Iaddend.
.Iadd.25. A device manufacturing method according to claim 21,
wherein said unattenuated, halftoned, phase-shift feature is a
primary feature..Iaddend.
.Iadd.26. A device manufacturing method according to claim 21,
wherein said unattenuated, halftoned, phase-shift feature is an
assist feature..Iaddend.
.Iadd.27. A computer program product for controlling a computer
comprising a recording medium readable by the computer, means
recorded on the recording medium for the directing the computer to
generate at least one file corresponding to a phase-shift mask for
transferring an image onto a material, said image including first
features having a first pitch and second features having a second
pitch different than said first pitch, said generation of said file
comprising the step of: modifying in said mask at least one of said
first and second features to include at least one unattenuated,
halftoned phase-shift feature that emulates an arbitrary percentage
transmission between 6% and 100%, whereby said first and second
features in said mask emulate different percentage transmissions
such that said first and second features share a common exposure
latitude and a corresponding depth of focus..Iaddend.
.Iadd.28. The computer program product of claim 27, wherein at
least one of said first and second features comprise an equal
line/space chromeless pattern and said step of modifying is carried
out on said equal line/space chromeless pattern such that said
pattern produces a 0.sup.th order having a non-zero amplitude when
illuminated..Iaddend.
.Iadd.29. The computer program product of claim 27, wherein said
step of modifying is carried out such that the modified features
produce a diffraction pattern and aerial image corresponding to a
partially transparent, attenuated phase-shaft mask, having an
arbitrary percentage transmission..Iaddend.
.Iadd.30. The computer program product of claim 27, wherein said
unattenuated, halftoned, phase-shift feature is a primary
feature..Iaddend.
.Iadd.31. The computer program product of claim 27, wherein said
unattenuated, halftoned, phase-shift feature is an assist
feature..Iaddend.
Description
FIELD OF INVENTION
The present invention generally relates to lithography, and more
particularly to the design, layout and fabrication of
phase-shifting masks that can be used in the manufacture of
semiconductor and other devices.
The present invention also relates to the use of such masks in a
lithographic apparatus, comprising for example: a radiation system
for supplying a projection beam of radiation; a mask table for
holding a mask; a substrate table for holding a substrate; and a
projection system for projecting at least part of a pattern on the
mask onto a target portion of the substrate.
BACKGROUND OF THE INVENTION
Lithographic apparatus can be used, for example, in the manufacture
of integrated circuits (ICs). In such a case, the mask may contain
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) 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 one type of lithographic
projection apparatus, each target portion is irradiated by exposing
the entire mask pattern onto the target portion in one go; 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 U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection
apparatus, a mask pattern 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.
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. Twin stage lithographic apparatus are
described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791,
incorporated herein by reference.
Although specific reference may be made in this text to the use of
lithographic apparatus and masks in the manufacture of ICs, it
should be explicitly understood that such apparatus and masks have
many other possible applications. For example, they may be used 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.
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) and EUV (extreme ultra-violet radiation, e.g. having a
wavelength in the range 5-20 nm).
A lithographic mask typically may contain opaque and transparent
regions formed in a predetermined pattern. The exposure radiation
exposes the mask pattern onto a layer of resist formed on the
substrate. The resist is then developed so as to remove either the
exposed portions of resist for a positive resist or the unexposed
portions of resist for a negative resist. This forms a resist
pattern on the substrate. A mask typically may comprise a
transparent plate (e.g. of fused slicia) having opaque (chrome)
elements on the plate used to define a pattern. A radiation source
illuminates the mask according to well-known methods. The radiation
traversing the mask and the projection optics of the lithographic
apparatus forms a diffraction-limited latent image of the mask
features on the photoresist. This can then be used in subsequent
fabrication processes, such as deposition, etching, or ion
implantation processes, to form integrated circuits and other
devices having very small features.
As semiconductor manufacturing advances to ultra-large scale
integration (ULSI), the devices on semiconductor wafers shrink to
sub-micron dimension and the circuit density increases to several
million transistors per die. In order to accomplish this high
device packing density, smaller and smaller feature sizes are
required. This may include the width and spacing of interconnecting
lines and the surface geometry such as corners and edges of various
features.
As the nominal minimum feature sizes continue to decrease, control
of the variability of these feature sizes becomes more critical.
For example, the sensitivity of given critical dimensions of
patterned features to exposure tool and mask manufacturing
imperfections as well as resist and thin film process variability
is becoming more significant. In order to continue to develop
manufacturable processes in light of the limited ability to reduce
the variability of exposure tool and mask manufacturing parameters,
it is desirable to reduce the sensitivity of critical dimensions of
patterned features to these parameters.
As feature sizes decrease, semiconductor devices are typically less
expensive to manufacture and have higher performance. In order to
produce smaller feature sizes, an exposure tool having adequate
resolution and depth of focus at least as deep as the thickness of
the photoresist layer is desired. For exposure tools that use
conventional or oblique illumination, better resolution can be
achieved by lowering the wavelength of the exposing radiation or by
increasing the numerical aperture of the lithographic exposure
apparatus.
The skilled artisan will appreciate that the resolution varies in
proportion to the exposure wavelength and varies in inverse
proportion to the numerical aperture (NA) of the projection optical
system. The NA is a measure of a lens' capability to collect
diffracted radiation from a mask and project it onto the wafer. The
resolution limit R (nm) in a photolithography technique using a
reduction exposure method is described by the following equation:
R=k.sub.1.lamda./(NA) where: .lamda. is the wavelength (nm) of the
exposure radiation; NA is the numerical aperture of the lens; and
k.sub.1 is a constant dependent inter alia on the type of resist
used.
It follows that one way to increase the resolution limit is to
increase the numerical aperture (high NA). This method, however,
has drawbacks due to an attendant decrease in the depth of focus,
difficulty in the design of lenses, and complexity in the lens
fabrication technology itself. An alternative approach is to
decrease the wavelength of the exposure radiation in order to form
finer patterns, e.g. to support an increase in the integration
density of LSI (Large Scale Integration) devices. For example, a
1-Gbit DRAM requires a 0.2-micrometer pattern while a 4-Gbit DRAM
requires a 0.1-micrometer pattern. In order to realize these
patterns, exposure radiation having shorter wavelength can be
used.
However, because of increased semiconductor device complexity that
results in increased pattern complexity, and increased pattern
packing density on a mask, distance between any two opaque mask
areas has decreased. By decreasing the distances between the opaque
areas, small apertures are formed which diffract the radiation that
passes through the apertures. The diffracted radiation results in
effects that tend to spread or to bend the radiation as it passes,
so that the space between the two opaque areas is not resolved; in
this way, diffraction is a severe limiting factor for optical
photolithography.
A conventional method of dealing with diffraction effects in
optical photolithography concerns the use of a phase shift mask,
which replaces the previously discussed mask. Generally, with
radiation being thought of as a wave, phase shifting is a change in
timing (phase) of a regular sinusoidal pattern of radiation waves
that propagate through a transparent material. Although the rest of
this discussion will generally concentrate on transmissive phase
shift masks, it should be realized that reflective phase shift
masks can also be contemplated (e.g. for use with the wavelengths
associated with EUV radiation). The current invention encompasses
both these concepts.
Typically, phase-shifting is achieved by passing radiation through
areas of a transparent material of either differing thickness or
through materials with different refractive indexes, or both,
thereby changing the phase or the periodic pattern of the radiation
wave. Phase shift masks reduce diffraction effects by combining
both diffracted radiation and phase-shifted diffracted radiation so
that constructive and destructive interference takes place
favorably. On the average, a minimum width of a pattern resolved by
using a phase shifting mask is about half the width of a pattern
resolved by using an ordinary mask.
There are several different types of phase shift structures. These
types include:
alternating aperture phase shift structures, sub-resolution phase
shift structures, rim phase shift structures, and chromeless phase
shift structures. "Alternating Phase Shifting" is a spatial
frequency reduction concept characterized by a pattern of features
alternately covered by a phase shifting layer. "Sub-resolution
Phase Shifting" promotes edge intensity cut-off by placing a
sub-resolution feature adjacent to a primary feature and covering
it with a phase shifting layer. "Rim Phase Shifting" overhangs a
phase shifter over a chrome mask pattern.
In the case of transmissive masks, these phase shift structures are
generally constructed in masks having three distinct layers of
material. An opaque layer is patterned to form blocking areas that
allow none of the exposure radiation to pass through. A transparent
layer, typically the substrate plate (e.g. of quartz or calcium
fluoride), is patterned with transmissive areas, which allow close
to 100% of the exposure radiation to pass through. A phase shift
layer is patterned with phase shift areas which allow close to 100%
of the exposure radiation to pass through, but phase-shifted by
180.degree. (.pi.). The transmissive and phase-shifting areas are
situated such that exposure radiation diffracted through each area
is canceled out in a darkened area therebetween. This creates the
pattern of dark and bright areas, which can be used to clearly
delineate features. These features are typically defined by the
opaque layer (i.e. opaque features) or by openings in the opaque
layer (i.e. clear features).
For semiconductor (and other device) manufacture, alternating
aperture phase shift masks may typically be used where there are a
number of pairs of closely packed opaque features. However, in
situations where a feature is too far away from an adjacent feature
to provide phase shifting, sub-resolution phase shift structures
typically may be employed. Sub-resolution phase shift structures
typically may be used for isolated features such as contact holes
and line openings, wherein the phase shift structures may include
assist-slots or outrigger structures on the sides of a feature.
Sub-resolution phase shift structures are below the resolution
limit of the lithographic system and therefore do not print on the
substrate. One shortcoming of sub-resolution phase shift structures
is that they require a relatively large amount of real estate on
the mask.
Rim phase shifting masks include phase shift structures that are
formed at the rim of features defined by opaque areas of the mask.
One problem with rim phase shift structures is that they are
difficult to manufacture. In the case of rim phase shift
structures, multiple lithographic steps must be used to uncover the
opaque layer so that it can be etched away in the area of the rim
phase shifter. This step is difficult, as the resist used in the
lithographic step covers not only the opaque layer but also
trenches etched into the substrate.
In general, improvement of the integration density of semiconductor
integrated circuits in recent years has been achieved mainly
through a reduction in size of the various circuit patterns. These
circuit patterns are presently formed mainly by lithography
processes using a wafer stepper or step-and-scan apparatus.
FIG. 1 shows the structure of such a prior art lithographic
apparatus. Mask 108 is illuminated by the radiation emitted from
illumination system 102. An image of mask 108 is projected onto a
photoresist film coated on wafer 120, which is the substrate to be
exposed through projection system 110. As shown in FIG. 1,
illumination system 102 includes a source 100, condenser lens 104,
and aperture 106 for specifying the shape and size of the effective
source. Projection system 110 includes a projection lens 112, pupil
filter 114, and aperture 116 arranged in or near the pupil plane of
focussing lens 118 to set the numerical aperture (NA) of the
lens.
As discussed earlier, the minimum features size R of patterns
transferable by an optical system is approximately proportional to
the wavelength .lamda. of the radiation used for exposure and
inversely proportional to the numerical aperture (NA) of the
projection optical system. Therefore, R is expressed as R=k.sub.1
.lamda./NA, where k.sub.1 is an empirical constant and k.sub.1=0.61
is referred to as the Rayleigh limit.
In general, when the pattern dimensions approach the Rayleigh
limit, the projected image is no longer a faithfull reproduction of
the mask pattern shape. This phenomenon is caused by so-called
optical proximity effects (OPEs) and results in corner rounding,
line-end shortening, and line width errors, among other things. To
solve this problem, algorithms have been proposed that can be used
to predistort the mask pattern so that the shape of a projected
image takes on the desired shape.
Moreover, approaches have been described which improve the
resolution limit of a given optical system, resulting effectively
in a decreased value of k.sub.1. Adoption of a phase shifting mask,
such as described above, is a typical example of this approach. A
phase shifting mask is used to provide a phase difference between
adjacent apertures of a conventional mask.
A chromeless phase shifting mask method is known as a phase
shifting method suitable for the transfer of a fine isolated opaque
line pattern, which is needed, for example, for the gate pattern of
a logic LSI device.
Off-axis illumination and pupil filtering are methods additionally
known for improving images. According to the off-axis illumination
method, the transmittance of aperture 106 is modified in the
illumination system 102 of FIG. 1 (prior art). One particular
embodiment of this method changes the illumination intensity
profile so that the transmittance at the margin becomes larger than
that of the central portion, which is particularly effective to
improve the resolution of a periodic pattern, as well as the depth
of focus. The pupil filtering method is a method of performing
exposure through a filter (pupil filter) located at the pupil
position of a projection lens to locally change the amplitude
and/or phase of the transmitted radiation. For example, this
approach makes it possible to greatly increase the depth of focus
of an isolated pattern. Furthermore, it is well known that the
resolution of a periodic pattern can further be improved by
combining the off-axis illumination method and the pupil filtering
method.
Nonetheless, an inherent problem with a conventional transmission
mask, such as the ones described above, is that the mask substrate
(plate) generally undergoes a decrease in transmissivity as the
wavelength of radiation emitted from an exposure radiation source
is decreased so as to obtain finer patterns. For example, a quartz
material substrate becomes more opaque as the wavelength of the
radiation source decreases, particularly when the wavelength is
less than 200 nm. This decrease in transmissivity affects the
ability to obtain finer resolution patterns. For this reason, a
material for a transmission phase shifting mask that can obtain a
high transmissivity with respect to radiation having a short
wavelength is needed. It is, however, difficult to find or
manufacture such a material having a high transmissivity with
respect to short-wavelength exposure radiation.
An example of a photomask pattern is shown in FIG. 2 (prior art).
Passage of radiation around the illustrated features causes
diffraction of the radiation into discrete dark and bright areas.
The bright areas are known as the diffraction orders and the
collective pattern they form is mathematically describable by
taking the Fourier transform of the collective opaque and
transparent regions. The pattern that is observed in its simplest
personification has an intense diffraction order, called the
0.sup.th order, surrounded in a symmetrical fashion by less intense
diffraction orders. These less intense orders are called the
plus/minus first (.+-.1.sup.st) order; plus/minus second
(.+-.2.sup.nd) order; and so on into an infinity of orders. For the
same feature width, different diffraction patterns are formed for
dense and isolated features. FIG. 3(A) (prior art) shows the
magnitudes of relative electric fields and respectively pupil
positions (X) of diffraction orders for a dense feature, while FIG.
3(B) (prior art) shows the magnitudes of diffraction orders for an
isolated one. The center peak observed in each plot is the 0.sup.th
order.
The 0.sup.th order contains no information about the pattern from
which it arose. The information about the pattern is contained in
the non-zero orders. However, the 0.sup.th order is spatially
coherent with the higher orders so that, when the beams are
redirected to a point of focus, they interfere, and in doing so
construct an image of the original pattern of opaque and
transparent objects. If all the diffraction orders are collected, a
perfect representation of the starting object is obtained. However,
in high-resolution lithography of small-pitch features, where pitch
is the sum of the width of the opaque and transparent objects, only
the 0.sup.th and the .+-.1.sup.st orders are collected by the
projection lens to form the image. This is because higher orders
are diffracted at higher angles that fall outside of the lens pupil
as defined by the numerical aperture (NA).
As depicted in FIG. 4(A) (prior art), the 0.sup.th order 402 and
the .+-.1.sup.st orders 404 lie within the lens pupil 406. As
further depicted in FIG. 4(A), the .+-.2.sup.nd orders 408, lie
outside the lens pupil 406. Further, as seen in FIG. 4(B) (prior
art), a corresponding aerial image is formed during exposure (I
indicates intensity, and H indicates horizontal position). The
photoresist pattern is then delineated from this aerial image.
It has long been known that it is only necessary to collect two
diffraction orders, such as either with the 0.sup.th order and at
least one of the higher diffraction orders, or simply two higher
orders without the 0.sup.th order, to form the image.
As depicted in FIG. 5(A) (prior art), radiation transmitted through
a focussing lens 502 is represented by that which is normal 504 to
the object (not shown), and that which transmits through the edges
506, 508 of the focussing lens 502. Although radiation is
continuously transmitted throughout the entire surface of lens 502,
the three radiation paths 504-508 are represented to illustrate
phase matching of different radiation paths. At point 510, the
three radiation paths 504-508 focus and are in phase together. When
the three radiation paths 504, 512, and 514 focus together at point
516, however, they are not in phase.
The phase error from a change in path-lengths of 512 and 514 from
respective path-lengths 506 and 508 results in a finite depth of
focus, DoF, of the system.
One may improve the tolerance to variations in relative phase error
caused by aberrations like defocus as depicted in FIG. 5(A). FIG.
5(B) (prior art) represents how, by eliminating the radiation path
that is normal to the object, variations to the phase error may be
reduced. Again, although radiation is continuously transmitted
throughout the surface of lens 502, the two radiation paths 506 and
508 are represented to illustrate phase matching of different
radiation paths. At point 510, the two radiation paths 506 and 508
focus and are in phase together. When the two radiation paths 512
and 514 focus together at point 516, they are in phase. Without the
radiation path 504 as seen in FIG. 5(A), the phase error from the
increased path-lengths of 512 and 514 over respective path-lengths
506 and 508 is eliminated, resulting in an infinite depth of focus,
DoF, of the system. Eliminating the radiation path normal to the
object may be accomplished by placing an obscuration in the center
of the radiation source, thus eliminating radiation normal to the
object and allowing only oblique illumination, as depicted for
example in FIG. 6(A).
FIG. 6(A) (prior art) depicts a lithographic "on-axis" projection
system ("C" indicates conventional) wherein the illumination
configuration 602 is such as to permit transmission of radiation
normal to the object. In the figure, radiation passes through the
reticle, comprising a quartz substrate 604 and chrome patterns 606,
through the lens aperture 608, into lens 610, and is focused into
area 612. FIG. 6(B) (prior art) depicts exemplary lithographic
"off-axis" projection systems wherein an annular (A) illumination
configuration 614, or quadrupole (Q) illumination configuration
616, prohibits transmission of radiation normal to the object. In
the figure, radiation passes through the vitreous substrate 604,
past the chrome patterns 606, through the lens aperture 608, into
lens 610, and is focused into area 618. Comparing FIGS. 6(A) and
6(B), it is noted that the Depth of Focus (DoF) of FIG. 6(A) is
smaller than that of FIG. 6(B).
Lowering the 0.sup.th order's magnitude to be the same as or less
than that of the 1.sup.st order improves the imaging tolerance of
this two-beam imaging system. One method for tuning the magnitude
of the diffraction orders is to use weak phase shift masks. Strong
phase shift masks and weak phase shift masks differ in operation
and effect.
Strong phase shift masks eliminate the zero-diffraction order and
double the resolution through a technique of frequency doubling. To
understand how strong phase shifters work, it is useful to think of
the critical pitch as having alternating clear areas adjacent to
the main opaque feature. Because of the alternating phase regions,
the pitch between same-phase regions is doubled. This doubling
halves the position at which the diffraction orders would otherwise
pass through the projection lens relative to the critical pitch,
thus making it possible to image features with half the pitch
allowed by conventional imaging. When the two opposing phase
regions add through destructive interference, to build the final
image, their respective zero-order radiation is equal in magnitude
but of opposite phase, thus canceling. Imagining is done only with
the frequency-doubled higher orders. On the other hand, weak phase
shift masks dampen the zero-order radiation and enhance the higher
orders. Weak phase shift masks from their phase shift between
adjacent features by creating electric fields of unequal magnitude
and of opposite phase, with the field immediately adjacent to a
critical feature having the lesser of the magnitudes. The net
electric field reduces the magnitude of the zero order while
maintaining the appropriate phase.
Weak phase shift masks permit an amount of exposure radiation to
pass through objects in a fashion that creates a difference in
phase between coherently linked points while having an imbalance in
the electric field between the shifted regions. FIG. 7(A) (prior
art) depicts a substrate 702 and a mask pattern 704 that does not
permit phase shifting. FIG. 7(C) (prior art) is a graph
illustrating how the 0.sup.th order's magnitude is larger than that
of the .+-.1.sup.st orders' magnitude from a non-phase shifting
mask as depicted in FIG. 7(A). FIG. 7(B) (prior art) depicts a
substrate 702 and a mask pattern 706 that permit phase shifting (in
the Figure, .PHI. is phase, t is thickness, n is index of
refraction and .lamda. is wavelength). FIG. 7(D) (prior art) is a
graph illustrating how the 0.sup.th order's magnitude is decreased
to be comparable to that of the .+-.1.sup.st orders' magnitude from
a phase shifting mask as depicted in FIG. 7(B).
Several types of phase-shifting masks are known in the art, such as
the rim, attenuated or embedded (or incorrectly halftone), and
unattenuated or chromeless (or transparent) shifter-shutter
phase-shifting masks.
FIG. 8(A) (prior art) is a cross-sectional view of a rim
phase-shifting mask 802, comprising radiation transmitting portions
804, and radiation inhibiting portions 806. FIG. 8(B) (prior art)
is a graph representing the amplitude (E) of the E-field at the
mask, whereas FIG. 8(C) (prior art) is a diagram representing the
magnitude of the 0.sup.th diffraction order 810, and .+-.1.sup.st
orders 812, 814, resulting from use of the mask depicted in FIG.
8(A).
FIG. 9(A) (prior art) is a cross-sectional view of an attenuated or
embedded phase-shifting mask 902 having an attenuation of 5%,
comprising a radiation attenuating portion 904. FIG. 9(B) (prior
art) is a graph representing the amplitude of the E-field at the
mask, whereas FIG. 9(C) (prior art) is a diagram representing the
magnitude of the 0.sup.th diffraction order, and .+-.1.sup.st
diffraction orders resulting from use of the mask depicted in FIG.
9(A). FIG. 9(D) (prior art) is a cross-sectional view of an
attenuated or embedded phase-shifting mask 912 having an
attenuation of 10%, comprising a radiation attenuating portion 914.
FIG. 9(E) (prior art) is a graph representing the amplitude of the
E-field at the mask, whereas FIG. 9(F) (prior art) is a diagram
representing the magnitude of the 0.sup.th diffraction order, and
.+-.1.sup.st diffraction orders resulting from use of the mask
depicted in FIG. 9(D).
FIG. 10(A) (prior art) is a cross-sectional view of an unattenuated
or chromeless (or transparent) shifter-shutter phase-shifting mask
1002, comprising a radiation-shifting portion 1004. FIG. 10(B)
(prior art) is a graph representing the amplitude of the E-field at
the mask, whereas FIG. 10(C) (prior art) is a diagram representing
the magnitude of the 0.sup.th diffraction order 1006, and
.+-.1.sup.st diffraction orders 1008, 1010 resulting from use of
the mask depicted in FIG. 10(A).
Typically, the phase-shifting masks of FIG. 8 through FIG. 10 form
their phase-shift differently, but, relative to their
non-phase-shifted counterpart, they all yield a 0.sup.th
diffraction order of smaller amplitude and a first diffraction
order of larger amplitude, as regards electric field. Which ratio
of 1.sup.st to 0.sup.th diffraction order magnitude is optimal
depends on the pitch of the feature being imaged, along with the
shape of the illumination configuration and the desired printing
size in the developed photoresist. These tuned diffraction patterns
are then used with off-axis illumination to image smaller pitches
with better tolerance to imaging process variation.
The concept of manipulation of the amplitude ratio of 0.sup.th to
1.sup.st diffraction orders has conventionally been restricted to
using certain weak phase-shifting techniques with biasing features
and sub-resolution assist features.
FIG. 11(A) (prior art) depicts a conventional biasing technique
used to resolve a desired feature. As seen in FIG. 11(A), biasing
(B) bars 1102 and 1104 are situated adjacent the mask of the
primary feature 1106. FIG. 11(B) depicts a half-tone biasing (HB)
technique known to the applicants of the instant application and
described in U.S. Pat. No. 6,114, 071 (incorporated herein by
reference), used to resolve a desired feature. As seen in FIG.
11(B), half-tone biasing bars 1108 and 1110 are situated adjacent
the mask of the desired feature 1112. FIG. 12 (prior art) depicts a
conventional photoresist mask 1202. The photoresist mask 1202
comprises a plurality of scatter bars 1204, serifs 1206, and chrome
shields 1208.
For conventional attenuated phase shifters, transparency of the
shifter materials typically may be adjusted, and used along with
biasing and sub-resolution assist features. Transparency of the
shifters typically ranges from 3% to 10%, wherein higher
transmissions such as from 10% to 100% are reported to be optimal
for pitches where the space between the features is larger than the
phase-shifted line. FIG. 13 (prior art) shows the dependence of
image contrast, as defined by the Normalized Image Log Slope
(NILS), with respect to varying transmittance (T) of its
phase-shifted material for a 175 nm line on a 525 nm pitch (FIG.
13A) and a 1050 nm pitch (FIG. 13B). Each curve in the figure
represents a different focus (F) setting. The curve with the
largest NILS is the most focussed, and has an F-value of zero;
further, with each change in focus, the NILS of each respective
curve decreases. FIG. 13A shows that the best transmission for the
175 nm line with the 525 nm pitch structure is 0.35 to 0.45. FIG.
13B shows that the best transmission for the 175 nm line with the
1050 nm pitch structure is 0.25 to 0.35.
An example of a 100% transparent attenuated phase-shifting
technology is the previously mentioned, chromeless shifter-shutter,
such as depicted in FIG. 10. Using a chromeless shifter-shutter,
phase-edges of a pattern typically may be placed within an area
that is 0.2 to 0.3 times the exposing wavelength .lamda. divided by
the numerical aperture NA of the projection lens. For lines larger
or smaller than this, the destructive interference is insufficient
to prevent exposure in an area not be exposed. Printing features
larger than this is accomplished in one of two ways. The first
places an opaque layer in the region that is to stay dark with the
feature edges being opaque or rim-shifted (FIG. 14; prior art). The
second, as depicted in FIG. 15 (prior art), creates a dark grating
1502 by placing a series of features 1504 whose size meets the
criteria for printing an opaque line 1506 using chromeless
technology. In FIGS. 14 and 15, "IM" denotes image, "CPSM" denotes
a chromeless phase shift mask, "OP" indicates opaque and "PS"
denotes phase shift.
Conventionally, chromeless phase shifting masks have not worked
with off-axis exposure as the shifter (feature) sizes and shutter
(space) sizes approach one another. FIGS. 16(A) through 16(C)
depict a conventional chromeless phase shifting mask. In FIG. 16(A)
(prior art), 1602 is a cross-sectional view of a portion of a
conventional chromeless phase shifting mask, comprising shifters
1604, and shutters 1606, wherein the shifter length is
substantially equal to the shutter length. FIG. 16(B) (prior art)
is a graph representing the amplitude of the E-field at the mask
1602. FIG. 16(C) (prior art) is a diagram representing the
magnitudes of the .+-.1.sup.st diffraction orders 1608 and 1601 for
the mask of FIG. 16(A). As seen in FIG. 16(C) there is no 0.sup.th
diffraction order. The functional limit of the relative sizes of
the shifter and shutters of conventional chromeless phase shifting
masks results from the integrated electric fields of the two
opposing phase-shifted regions being equal. This balanced condition
cancels the 0.sup.th diffraction order, making it impossible to get
the prerequisite 0.sup.th diffraction order needed for using
off-axis illumination.
To summarize, each of the above-described, conventional, weak
phase-shifting techniques solves certain imaging problems. However,
each technique has accompanying drawbacks. For example, the rim,
attenuated or embedded, and unattenuated or chromeless (or
transparent) shifter-shutter phase-shifting masks provide large
ratios of the 0.sup.th to .+-.1.sup.st diffraction orders.
Prior-art attempts to manipulate these ratios include using biasing
techniques coupled with an attenuated phase shifting mask. However,
these prior art attempts include complex manufacturing steps and
yield inefficient masks as a result of the attenuation.
Furthermore, unattenuated shifter-shutter phase-shifting mask
additionally fail to yield accurate images with off-axis
illumination as the shifter and shutter sizes approached one
another.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a simple system and
method for fabricating an efficient phase shifting mask that is
able to manipulate the ratios of the 0.sup.th to .+-.1.sup.st
diffraction orders.
It is another object of this invention to provide a system and
method for fabricating a non-attenuated phase shifting mask that is
able to manipulate the ratios of the 0.sup.th to 1.sup.st
diffraction orders.
It is yet another object of this invention to provide a system and
method for fabricating a chromeless (or transparent)
shifter-shutter phase-shifting mask that is usable with. off-axis
illumination when the shifter and shutter sizes approach one
another.
It is still another object of this invention to provide a system
and method for halftoning primary features to achieve the correct
ratio of 0.sup.th to higher diffraction order radiation for optimal
imaging.
It is yet another object of this invention to provide a system and
method for halftoning assist features to achieve the correct ratio
of 0.sup.th to higher diffraction order radiation for optimal
imaging.
The present invention provides an alternate method for effectively
manipulating the amplitude ratio of the 0.sup.th to 1st diffraction
order by using halftoning of opaque and phase-shifted
transparent/semi-transparent features within the primary feature
and as sub-resolution assist features. The relative magnitudes of
the 0.sup.th and higher diffraction orders are formed as the
exposing wavelength passes through the plurality of zero and
180.degree. phase-shifted regions. Subsequently, some of the
diffraction orders are collected and projected to form the image of
the object.
Methods in accordance with the present invention further make use
of halftoning structures to manipulate the-relative magnitudes of
diffraction orders to ultimately construct the desired projected
image. At the resolution limit of the mask marker, this is
especially useful for converting strong-shifted,
no-0.sup.th-diffraction-order, equal-line-and-space chromeless
phase edges to weak phase shifters that have some 0.sup.th order.
Halftoning creates an imbalance in the electric field between the
shifted regions, and therefore results in the introduction of the
0.sup.th diffraction order. As such, with halftoning, these
previously strong-shifted features convert to weak phase-shifters
and are compatible with the other shifter-shutter chromeless
features typically found amongst the plurality of objects used in
making a conventional semiconductor circuit.
Decreasing the size of the primary feature for the very dense
features, as in the conventional mask fabrication technique, can
achieve a limited extent of modifying diffraction order. Because of
the interference effects, it is not possible to ensure that a mask
width less than the sub-resolution assist feature can be reliably
made using conventional mask fabrication methods. However, in
accordance with the present invention, by biasing the primary
features, the feature width can be reduced to less than the
sub-resolution assist features.
Further, use of a chromeless phase-shifting mask is known to be a
powerful imaging method when combined with using off-axis
illumination, but it has serious optical proximity effects. This
invention provides an effective optical proximity solution.
In general, in one aspect, the invention features a method of
transferring an image, including 0.sup.th diffraction order and
.+-.1.sup.st diffraction orders, onto a material, wherein the
method comprises the steps of fabricating a phase shifting mask
comprising at least one unattenuated, halftoned, phase-shift
feature, and off-axis illuminating the mask such that radiation
passes through the mask onto the material.
In another aspect, the invention features a method of transferring
an image, including 0.sup.th diffraction order and .+-.1.sup.st
diffraction orders, onto a material, wherein the method comprises
the steps of fabricating a phase shifting mask comprising at least
one feature, said at least one feature including halftoned
phase-shifted transparent features; and off-axis illuminating the
mask such that radiation passes through the mask onto the material.
Preferably, said one feature further includes semi/transparent
features. Further, said at least one feature preferably includes
opaque features.
In yet another aspect, the invention features a phase shifting mask
comprising at least two unattenuated, halftoned, phase-shift
features having a width w, wherein the features are separated by a
width w, such that the mask provides an image including 0.sup.th
diffraction order and .+-.1.sup.st diffraction orders, when
illuminated.
In still yet another aspect, the invention features a phase
shifting mask comprising at least two halftoned phase-shifted
transparent features having a width w, wherein the features are
separated by a width w, such that the mask provides an image
including 0.sup.th diffraction order and .+-.1.sup.st diffraction
orders, when illuminated. Preferably, the at least two features
further includes semi/transparent features. Still preferably, the
at least two features further includes opaque features. Even more
preferably, a focus-exposure process window for maintaining a
predetermined resist line-width sizing of the mask is common to an
attenuated, phase-shift mask of a similar pitch.
As described in further detail below, the present invention
provides significant advantages over the prior art. Most
importantly, the unattenuated phase-shift photomask of the present
invention allows for the printing of high-resolution features,
while manipulating the 0.sup.th diffraction order and .+-.1.sup.st
diffraction orders.
In addition, the unattenuated phase-shift mask of the present
invention provides a focus-exposure process window for maintaining
an increased line-width sizing over that of the prior art.
Additional advantages of the present invention will become apparent
to those skilled in the art from the following detailed description
of exemplary embodiments of the present invention. The invention
itself, together with further objects and advantages, can be better
understood by reference to the following detailed description and
the accompanying schematic drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying schematic drawings, which are incorporated in and
form a part of the specification, illustrate embodiments of the
present invention and, together with the description, serve to
explain the principles of the invention. In the drawings:
FIG. 1 depicts a prior-art wafer stepper system.
FIG. 2 depicts a photomask pattern provided with optical proximity
correction features such as scattering bars and serifs.
FIG. 3(A) depicts a diffraction spectrum for equal lines and
spaces. FIG. 3(B) depicts a diffraction spectrum for an isolated
line.
FIG. 4(A) depicts a diffraction spectrum of objects whose size is
near the wavelength of the exposing energy. FIG. 4(B) depicts an
aerial image of the diffraction spectrum of FIG. 4(A).
FIG. 5(A) depicts the effects of three-beam exposure in a
conventional mask fabrication system. FIG. 5(B) depicts the effects
of two-beam exposure in a convention mask fabrication system.
FIG. 6(A) depicts a conventional on-axis exposure technique for
mask fabrication. FIG. 6(B) depicts conventional off-axis exposure
techniques for mask fabrication, wherein the illumination
configuration has an annular shape, or a quadrupole shape.
FIG. 7(A) depicts a cross-sectional view of a conventional
non-phase shifting mask. FIG. 7(B) depicts a cross-sectional view
of a conventional phase shifting mask. FIG. 7(C) depicts the
corresponding diffraction spectrum for the conventional non-phase
shifting mask of FIG. 7(A). FIG. 7(D) depicts the corresponding
diffraction spectrum for the conventional phase shifting mask of
FIG. 7(C).
FIG. 8(A) depicts a cross-sectional view of a conventional rim-type
phase shifting mask. FIG. 8(B) depicts a graph of the amplitude of
the electric field at the conventional rim-type phase shifting mask
of FIG. 8(A). FIG. 8(C) depicts the corresponding diffraction
spectrum for the conventional rim-type phase shifting mask of FIG.
8(A).
FIG. 9(A) depicts a cross-sectional view of a conventional
attenuated-type phase shifting mask, having an attenuation factor
of 5%. FIG. 9(B) depicts a graph of the amplitude of the electric
field at the conventional attenuation-type phase shifting mask of
FIG. 9(A). FIG. 9(C) depicts the corresponding diffraction spectrum
for the conventional attenuation-type phase shifting mask of FIG.
9(A). FIG. 9(D) depicts a cross-sectional view of a conventional
attenuated-type phase shifting mask, having an attenuation factor
of 10%. FIG. 9(E) depicts a graph of the amplitude of the electric
field at the conventional attenuation-type phase shifting mask of
FIG. 9(D). FIG. 9(F) depicts the corresponding diffraction spectrum
for the conventional attenuation-type phase shifting mask of FIG.
9(D).
FIG. 10(A) depicts a cross-sectional view of a conventional
transparent or chromeless shifter-shutter-type phase shifting mask.
FIG. 10(B) depicts a graph of the amplitude of the electric field
at the conventional transparent or chromeless shifter-shutter-type
phase shifting mask of FIG. 10(A). FIG. 10(C) depicts the
corresponding diffraction spectrum for the conventional transparent
or chromeless shifter-shutter-type phase shifting mask of FIG.
10(A).
FIG. 11(A) depicts a conventional biased photomask. FIG. 11(B)
depicts a halftone biased photomask.
FIG. 12 depicts a conventional, attenuated phase-shifting,
lithographic mask.
FIGS. 13(A) and 13(B) graphically depict image contrast as a
function of transmittance (T), for different pitches and focus
settings.
FIG. 14(A) depicts a conventional opaque feature and its
corresponding image. FIG. 14(B) depicts a conventional rim-shifted
opaque feature and its corresponding image.
FIG. 15 depicts a conventional chromeless dark grating as an opaque
feature, and its corresponding image.
FIG. 16(A) depicts a cross-sectional view of a conventional
chromeless phase-edge mask. FIG. 16(B) depicts a graph of he
amplitude of the electric field at the conventional transparent or
chromeless phase-edge mask of FIG. 16(A). FIG. 16(C) depicts the
corresponding diffraction spectrum for the conventional chromeless
phase-edge mask of FIG. 16(A).
FIG. 17(A) depicts a primary feature in a phase-edge mask. FIG.
17(C) depicts the corresponding diffraction spectrum for the
phase-edge mask of FIG. 17(A). FIG. 17(B) depicts a halftone
primary feature in a phase-edge mask in accordance with the present
invention. FIG. 17(D) depicts the corresponding diffraction
spectrum for the phase-edge mask of FIG. 17(C).
FIG. 18 depicts a halftone scattering bar assist feature in
accordance with the present invention.
FIG. 19(A) is a diagram depicting simulated resist images for a
plurality of conditions of pitch and transmission. FIG. 19(B) is a
graphical representation of the data represented in FIG. 19(A).
FIG. 19(C) is a contour plot showing the set of exposure dose
(vertical axis) and focus (horizontal axis) conditions to size a
100 nm resist image between 90 nm and 110 nm. The two process
windows in FIG. 19C are for a 400 nm pitch (upper contour) and for
a 600 nm pitch (lower contour). These process windows do not
overlap. FIG. 19D is a different analysis of the same information
shown in FIG. 19C. FIG. 19D illustrates how much exposure latitude
(vertical axis) there is within a process window for a certain
depth of focus (horizontal axis). In FIG. 19D, the top curve is for
the 400 nm pitch and the lower curve is for the 600 nm pitch.
FIG. 20(A) depicts layout for a 26% transmittance attenuated
phase-shifting mask. FIG. 20(B) depicts a layout for an
unattenuated, chromeless phase-shifting mask that has been
halftoned, in accordance with the present invention, to make its
diffraction pattern similar to that of the 26% attenuated mask of
FIG. 20(A). FIG. 20(C) is a graphical comparison of the diffraction
orders produced by the mask of FIG. 20(A) and the mask of FIG.
20(B). FIG. 20(D) is an aerial image of a portion of the pattern of
the mask of FIG. 20(B). FIG. 20(E) is a graphical representation of
the focus-exposure process window for maintaining a specified
line-width sizing for the mask of FIG. 20(A). FIG. 20(F) is a
graphical representation of the focus-exposure process window for
maintaining a specified line-width sizing for the mask of FIG.
20(B). FIG. 20(G) is a graphical representation of the percent
exposure latitude for both masks of FIG. 20(A) and FIG. 20(B)
respectively. FIG. 20(H) is an alternative graphical representation
of the percent exposure latitude for both masks of FIG. 20(A) and
FIG. 20(B), respectively.
FIGS. 21(A) and 21(B) depict conventional chromeless phase-shift
patterns. FIGS. 21(C) and 21(D) depict halftone chromeless
phase-shift patterns in accordance with the present invention,
corresponding to the phase-shift patterns of FIGS. 21(A) and 21(B)
respectively. FIGS. 21(E) through 21(H) depict diffraction patterns
(graphs of the diffraction orders) for the object pattern of the
phase-shift masks of FIGS. 21(A) through 21(D) respectively. FIG.
21(I) shows the focus-exposure process windows for a 100 nm line
with pitches of 400 nm (2134 and 2132) and 600 nm (2130) for masks
that used the appropriate halftoning (FIG. 21D for the 400 nm pitch
and FIG. 21C for the 600 nm pitch) to make the features size with
similar exposure and focus. For the 400 nm pitch the total process
window includes areas 2134 and 2132, with 2134 overlapping with the
600 nm pitch process window, 2130. FIG. 21J shows the exposure
latitude for varying amounts of depth of focus for the common
focus-exposure area 2134.
FIG. 22(A) depicts a diffraction pattern for the object pattern of
a conventional phase-shift mask. FIG. 22(B) depicts a diffraction
pattern for the object pattern of a halftone phase-shift mask in
accordance with the present invention. FIG. 22(C) is a graphical
representation of the focus-exposure process window for maintaining
a specified line-width sizing for the mask of FIG. 22(A). FIG.
22(D) is a graphical representation of the focus-exposure process
window for maintaining a specified line-width sizing for the mask
of FIG. 22(B).
FIG. 23(A) depicts a global layout of a conventional primary
feature. FIG. 23(B) depicts a global layout of a halftone primary
feature in accordance with the present invention. FIG. 23(C) is an
aerial image of the global layout of FIG. 23(A). FIG. 23(D) is an
aerial image of the global layout of FIG. 23(B). FIG. 23(E) is a
magnified portion of FIG. 23(C). FIG. 23(F) is a magnified portion
of FIG. 23(D).
FIG. 24 is a logic flow diagram describing a method for designing
patterns that emulate different phase-shift masks in accordance
with the present invention.
FIG. 25 depicts a lithographic projection apparatus.
In the Figures, like reference symbols indicate like parts.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, for the purposes of explanation,
numerous specific details are set forth in order to provide a more
thorough understanding of the present invention. It will be
apparent, however, to one skilled in the art that the present
invention may be practiced without these specific details.
Specifically, the following detailed description of the
unattenuated phase-shift mask of the present invention relates to
both the mask itself as well as a method of forming the mask. It is
noted that, in an effort to facilitate the understanding of the
present invention, the following description details how the
unattenuated phase-shift mask can be utilized to form features
contained in today's state-of-the-art semiconductor devices.
However, it is also noted that the present invention is not limited
to use in semiconductor devices. Indeed, the present invention can
be utilized in a multitude of different types of designs and
processes that include the projection of high-resolution
images.
A first exemplary embodiment of the present invention includes
halftoning primary features of a chromeless shifter shutter
phase-shifting mask. FIG. 17 shows halftoning of a primary feature
so that it has an optimal 0.sup.th to .+-.1.sup.st diffraction
order amplitude using a chromeless shifter-shutter phase-shifting
mask. FIG. 17(A) shows primary features 1702 before halftoning,
whereas FIG. 17(B) shows the primary features 1704 after
halftoning. FIGS. 17(C) and 17(D) depict diffraction patterns of an
equal line/space chromeless pattern to that of the respective
structures in FIGS. 17(A) and 17(B). As seen in FIG. 17(C), without
halftoning, there are .+-.1.sup.st diffraction orders 1706 and
1708; however there is no 0.sup.th diffraction order. On the other
hand, as seen in FIG. 17(D), because of the halftoning of the
primary features, thereby permitting off-axis illumination for
these dense features, there are .+-.1.sup.st diffraction orders
1710, 1712, and there is a 0.sup.th diffraction order 1714. In the
Figures, "CPE` denotes chromeless phase edge, and "HCPE" denotes
halftone CPE.
A second exemplary embodiment of the present invention includes
halftoning scattering bar assist features of a chromeless
shifter-shutter phase-shifting mask. FIG. 18 depicts an example of
a chromeless shifter-shutter phase-shifting mask 1802, comprising
halftoned scattering bar assist features 1804, and primary features
1806. Halftoning a scattering bar assist feature permits its
associated primary feature to have an optimal 0.sup.th to
.+-.1.sup.st diffraction order amplitude using a chromeless
shifter-shutter phase-shifting mask.
Examining features of varying pitch-size imaged using
phase-shifting masks shows a pitch dependence on the transmission
that is best suited to obtaining the same size of resist image for
a given exposure condition. FIGS. 19(A) and 19(13) show--for 100 nm
lines that are separated by spaces ranging in size from 100 nm to
800 nm--the transmission of the phase-shift required to produce a
100 nm line for each pitch.
More specifically, FIG. 19(A) shows the imaging result for
different combinations of attenuated phase-shift mask transmittance
and apace sizes between 100 nm features. As seen in FIG. 19(A),
Figures 1902 at the intersection of each condition of transmittance
and space size are simulated cross-sections of developed
photoresist images that were exposed at 22 mJ/cm.sup.2 and -0.15
microns of focus using a 0.70 NA, 248 nm exposure tool with
quadrupole illumination. The images 1904 surrounded by the boxes
have a resist image size between 90 and 110 nm. These sizes are
used here to arbitrarily derive the lower and upper limits for
acceptable sizing. Images outside of the boxed areas do not meet
this criterion. In the Figure, "PSM1T" denotes PSM Feature #1
Transmittance, and "PSM2W" denotes PSM Feature #2 Width. FIG. 19(B)
is a graphical representation of the same data as represented in
FIG. 19(A). As seen in FIGS. 19(A) and 19(B), in the range of 20 to
30% transmittance, the figures meet the sizing criteria of .+-.10%
of 100 nm for the exposure condition of 22 Mj/cm.sup.2 and -0.15
microns of focus, for 400 nm pitch sizes and 600 nm pitch sizes,
each having a transmission of 100%.
FIG. 19(C) is a graph showing the focus-exposure process window for
maintaining a specified line-width sizing for 600 nm and 400 nm
pitch phase shift masks, each having 100% transmission. As seen in
FIG. 19(C) the exposure dose (D) and focus (F) conditions for
attaining 100 nm lines for 600 nm and 400 nm pitches are totally
separate, with no common process corridor. FIG. 19(D) is a graph
showing exposure latitude (EL) verses the depth of focus (DoF) for
600 nm and 400 nm pitch phase shift masks, each having 100%
transmission. The exposure latitude is the range of exposure that
maintains .+-.10% feature sizing, divided by the exposure dose to
size the feature, times 100. It is clear that as the exposure
latitude decreases, the depth of focus increases for each pitch.
However, as seen in FIG. 19(D), there is not a point in which both
the 600 nm and 400 nm pitch phase shift mask share a common
exposure latitude and corresponding depth of focus. As such,
without correction in accordance with the present invention, a 600
nm and 400 nm pitch can not size a 100 nm resist line using the
same conditions of exposure and focus.
FIGS. 19(A)-(D) represent simulated data corresponding to
photomasks. Generally, 5-10% attenuated PSMs are conventionally
available for commercial applications, whereas higher transmissions
may be custom produced. As such, there are limited materials
available to produce masks. Further, different pitch structures may
not perform optimally on such limited-availability materials. Still
further, one material would never be optimal for the plurality of
structures occurring on a set of patterns found on a single
conventional mask. Therefore, the present invention removes these
barriers because the present invention provides a method of
halftoning primary and assist features to emulate the diffraction
pattern of any of the prior-art phase shifting masks. Specifically,
the present invention permits different pitch structures to preform
optimally on a single mask.
The following describes the invention method to solve the problem
described with respect to FIGS. 19(A) through 19(D).
Halftoning may be used to permit an unattenuated, 100% chromeless
mask to produce a diffraction pattern and resultant aerial image
that emulate a diffraction pattern and resultant aerial image
corresponding to a 26% transparent, attenuated phase-shift mask.
This is a hypothetical example, because 26% attenuated material
does not commercially exist in mass quantities. Nevertheless, it is
an optimum transmission for some features and the present invention
makes a halftoned chromeless phase-shift mask that matches the
performance of the 26% attenuated material. As such, the optimum
transmission is attainable without attenuating the image with the
prior-art attenuated masks.
FIGS. 20(A) through 20(D) represent how an attenuated
phase-shifting mask is fabricated from an unattenuated, chromeless
phase-shifting mask in order to image, in this example, a 100 nm
line of a 400 nm-pitch feature the same way as if an unattenuated,
chromeless phase-shifting mask were used. FIG. 20(A) depicts a
primary feature 2006 in a portion 2004 of a mask layout 2002 for a
26% transmittance attenuated phase-shifting mask. FIG. 20(B)
depicts halftoning objects 2012 in a portion 2010 of a mask layout
2008 for an unattenuated, chromeless phase-shifting mask that has
been halftoned, thereby rendering a diffraction pattern nearly
perfectly similar to that of the 26% attenuated mask of FIG. 20(A).
FIG. 20(C) is a graph comparing the diffraction orders produced by
both portions 2004 and 2010 of the attenuated and the halftoned,
unattenuated masks respectively (the overlap of the graphs
resulting from the two different situations is substantially
perfect); NA is the numerical aperture. FIG. 20(D) is an aerial
image of the halftone mask of FIG. 20(B), wherein the aerial image
shows no signs of the discrete halftoning objects.
FIGS. 20(E) and 20(F) illustrate graphs showing the focus-exposure
process window for maintaining 90 nm to 110 nm resist line-width
sizing in the 26% transmittance attenuated phase-shifting mask
(attPSM) of FIG. 20(A), and the halftone unattenuated, chromeless
phase-shifting mask (HTPSM) of FIG. 20(B), respectively. FIG. 20(G)
is a graphical representation of the focus-exposure process window
for maintaining a specified line-width sizing for both masks of
FIG. 20(A) and FIG. 20(B). As seen in FIG. 20(G), there is an
overlapping portion (OV) of the graph for both masks of FIG. 20(A)
and FIG. 20(B). Further, as seen in FIG. 20(H), the percent
exposure latitude for both masks is relatively similar. Therefore,
as evidenced by FIGS. 20(G) and 20(H), the halftone unattenuated,
chromeless phase-shifting mask of FIG. 20(B) may be used to emulate
a 26% transmittance attenuated phase-shifting mask of FIG.
20(A).
In this exemplary embodiment, emulating a 26% attenuated-like
phase-shift mask, such as depicted in FIG. 20(A), with a 100%
chromeless phase-shift mask, such as depicted in FIG. 20(B),
included increasing the width of the 100 nm line to 115 nm and
halftoning the line using a 180-nm halftone pitch (htp) with a 67%
duty cycle of 180.degree. shifter-to-non-shifter regions. The
halftone has a region that is shifted relative to a region that is
not. In this exemplary embodiment, a 67% halftone duty cycle means
that 67%, or 120 nm, has been modified to be 180.degree.
phase-shifted, and 33%, or 60 nm, is an unmodified 0.degree.
reference area.
FIGS. 21(A) through 21(J) show how two features that have different
optimal transmissions can be halftoned so that they have optimal
imaging capability using the same attenuated phase-shifting
material. FIG. 21(A) depicts a primary feature 2106 in a portion
2104 of a mask layout 2102 for a 600 nm pitch chromeless
phase-shifting mask. FIG. 21(B) depicts a primary feature 2112 in a
portion 2110 of a mask layout 2108 for a 400 nm pitch chromeless
phase-shifting mask.
FIG. 21(C) depicts a primary feature 2118 and halftoning scatter
bar 2120 in a portion 2116 of a mask layout 2114 for a 600 nm pitch
unattenuated phase-shifting mask (CrSB denotes chrome scattering
bar). The line-width of primary feature 2118 is increased over that
of primary feature 2106 of FIG. 21(A). Similarly, FIG. 21((D)
depicts a halftone (HT) primary feature 2126 in a portion 2124 of a
mask layout 2122 for a 400 nm pitch unattenuated phase-shifting
mask, wherein the line-width of halftone primary feature 2126 is
increased over that of primary feature 2112 of FIG. 21(B). The
masks depicted in FIG. 21(A) and FIG. 21((B) have been modified to
result in the masks depicted in FIG. 21(C) and FIG. 21(D),
respectively.
FIGS. 21(E) through 21(H) show the diffraction patterns
corresponding to the mask patterns of FIGS. 21(A) through 21(D)
respectively. FIG. 21(G) and FIG. 21(H) show the modified
diffraction patterns corresponding to the mask patterns FIG. 21(C)
and FIG. 21(D). As compared to the diffraction patterns illustrated
in FIG. 21(E) and FIG. 21(F), the diffraction patterns are modified
when the original mask patterns illustrated in FIG. 21(A) and FIG.
21((B) are modified to become the mask patterns illustrated in FIG.
21(C) and FIG. 21(D), respectively.
FIG. 21(I) sows that there is a common focus-exposure corridor for
both mask patterns of FIGS. 21(C) and 21(D). As seen in FIG. 21(I)
the focus-exposure process window 2130 for the mask of FIG. 21(C)
overlaps the focus-exposure process window 2132 for the mask of
FIG. 21(D) at a common focus-exposure process window 2134. This
simulation example shows that a halftoned unattenuated chromeless
mask may emulate an attenuated phase-shift mask of lower
transmittance. FIG. 21(J) shows the exposure latitude for varying
amounts of depth of focus for the common focus-exposure area
2134.
FIGS. 22(A) through 22(D) show how the focus-exposure process
window is enhanced using scattering bars to suppress the 0th
diffraction order. FIG. 22(A) depicts the diffraction order graph
for an uncorrected attenuated phase-shift 100 nm line with a 600 nm
pitch. FIG. 22(B) depicts the diffraction orders for a corrected
halftoned unattenuated chromeless layout. FIG. 22(C) is a graph
showing the process window for the uncorrected attenuated
phase-shift 100 nm line with a 600 nm pitch of FIG. 22(A). FIG.
22(D) is a graph showing the process window for the connected
halftoned unattenuated chromeless layout of FIG. 22(B). Note that
the corrected mask has four times the depth of focus (DoF) of the
uncorrected mask.
FIGS. 23(A) through 23(F) show how an angle in a given pattern can
be accurately compensated for using halftone (HT) structures on the
primary feature. FIG. 23(A) shows a global layout (uncorrected) of
a pattern 2302 comprising primary features 2304. FIG. 23(B) shows a
halftone corrected portion 2306 of an angled primary feature. FIGS.
23(C) and 23(D) are aerial images for the respective features shown
in FIGS. 23(A) and 23(B), respectively. FIGS. 23(E) and 23(F) are
magnified views of the aerial images of the angled primary feature
and halftone-corrected angled primary feature of FIGS. 23(C) and
23(D), respectively. As seen in FIG. 23(E), the aerial image of the
primary feature includes hot spots 2310, wherein the diffraction
orders are decreased as a result of destructive interference in the
diffraction pattern. However, as seen in FIG. 23(F), the aerial
image of the primary feature does not include hot spots, thereby
resulting in a more precise aerial image of the primary
feature.
In addition, FIG. 23 shows that these halftone structures are used
to render a plurality of sizes, shapes and pitches such that the
formed images produce their respective desired size and shape with
sufficient image process tolerance. These images are typically made
under identical exposure conditions, but not limited to
single-exposure conditions. These halftoning structures can be used
exterior, as assist features, or interior to the primary feature.
These structures can range in transmission from 0% to 100% and they
can be phase-shifted relative to the primary features or not.
Variations of the unattenuated phase-shift mask of the present
invention are also possible. For example, while the hybrid
disclosed in the exemplary embodiment set forth above may emulate a
26% attenuated phase-shift mask, alternatives are possible.
In the exemplary embodiment above, 600 nm pitch and 400 nm pitch
have a common focus-exposure process window; however, such a
relationship may be generalized. A general method for developing a
focus-exposure process window that is common to multiple
predetermined pitch sizes may be accomplished as described with the
logic flow diagram of FIG. 24.
FIG. 24 is a logic flow diagram describing a method for designing
unattenuated phase-shift masks patterns, whose corresponding
diffraction patterns emulate the diffraction patterns corresponding
to attenuated phase-shift masks, and whose focus-exposure process
window is common to predetermined pitch sizes.
After an internal counter n is set to 1 (Step S1), the sizing dose,
which is the dose of exposure energy needed to make a resist image
of the target size, and line-width control for different features,
including types, sizes, and pitches of interest, for different weak
phase-shift mask transmissions are determined (Step S2). This
determination may be made, for example, using a lithography
simulator (such as Pro-Lith.sup..TM.or Solid-C.sup..TM.). Next, the
feature with the most sensitivity to exposure, focus and
aberrations is determined (Step S3). This determination
additionally may be made, for example, using a lithography
simulator. Next, it is determined whether the internal counter n is
greater than 1, thereby indicating whether Step S2 and Step S3 have
been repeated (Step S4). If n is not greater than 1 (Step S4), then
a modification to the imaging process is provided that lowers the
pattern's sensitivity to exposure, focus, and aberrations (Step
S5). The modification may include the use of different transmission
weak phase-shifting masks, exposure apparatus conditions, and
resist processes. Again, these modifications may be provided, for
example, using a lithography simulator. The internal counter is
then increased by 1(Step S6). At this point Step S2 and Step S3 are
repeated to ensure that the new process conditions provided during
Step S5 did not change that which was observed after the first
application of Step S2 and Step S3 (Step S7).
After the second run through Step S3, the internal counter n is
determined to be greater than 1, indicating that Step S2 and Step
S3 have been repeated (Step S4). As such, the results of the
determinations made during the first run through Step S2 and Step
S3 and the second run through Step S2 and Step S3 are respectively
compared (Step S8). If the comparison between the determinations
found during the two previous runs through Step S2 and Step S3 is
not within a predetermined threshold, i.e. new process conditions
introduced at Step S5 have changed that which was observed in the
first run-through of the two previous steps Step S2 and Step S3,
then a new modification to the imaging process is provided (return
to Step S5). However, if the comparison between the determinations
found during the two previous runs through Step S2 and Step S3 is
within a predetermined threshold, i.e. new process conditions
introduced at Step S5 have not changed that which was observed in
the first run-through of the two previous steps Step S2 and Step
S3, then the process proceeds to Step S9.
The mask layout conditions for the other features of interest, that
first match the amplitude of the electric field at zero frequency
of the diffraction pattern in the pupil plane of the exposure lens,
and that place the maximum amplitude of the side-lobes for each
isolated feature at the frequency of the reference geometry, are
then determined (Step S9). Non-limiting methods for accomplishing
the mask layout conditions for the other features of interest
include halftoning each feature, biasing each feature, or adding
scattering bars to isolated features. If scattering bars are used,
the scattering bars should be placed at one reference pitch away
from the feature being tuned, in order for the resulting diffracted
image of the scattering bars plus their respective primary feature
to match the diffracted image reference feature being tuned.
Further, scattering bars may be halftoned, of opposite phase and/or
biased, as prescribed by the mask fabrication technology used to
fabricate such patterns.
Next, the enveloping sinc [sinc(x)=sin(x)/x] function is tuned for
all features so that they all have the same shape (Step S10). A
sinc function relates to the non-discrete diffraction pattern for
an isolated feature or for a series of lines and spaces. Adding
scattering bars to an isolated feature modifies its sinc function
by attenuating the image at certain frequencies and amplifying the
image at other frequencies. The resultant diffraction pattern can
resemble discrete orders even though it is the result of a sinc
function. All diffraction orders of a series of lines and spaces
are separated by .lamda./pitch, and, without the sinc envelope, are
of the same magnitude. The sinc function for a single space within
the series of lines and spaces varies the amplitude of each order.
If scattering bars are used, the spacing may need adjusting to move
the side-lobe so that the maximum amplitude is placed outside of
the numerical aperture of the lithographic apparatus and only the
side of the side-lobe is inside the lens. This may be accomplished
by reducing the primary feature's scattering bar structure
pitch.
It is then determined whether all the features have the same aerial
image shape attributes such as, for example, I-MAX (maximum
intensity level), I-MIE (minimum intensity level), and Normalized
Image Log Slope (NILS), and whether the process windows of each of
the features overlap (Step S11). If all the features do not have
the same aerial image shape attributes, or the process windows of
each of the features do not overlap, then Step S9 is repeated with
a new modification to the enveloping sinc function. If all the
features have the same aerial image shape attributes, and the
process windows of each of the features overlap, then the process
stops, wherein an optimal phase shift mask is provided.
FIG. 25 schematically depicts a lithographic apparatus in which the
mask according to the invention can be employed. The apparatus
comprises: a radiation system Ex, IL, for supplying a projection
beam PB of radiation (e.g. UV radiation). In this particular case,
the radiation system also comprises a radiation source LA; 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; 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; a projection system ("lens") PL (e.g. a refractive, reflective
or catadioptric 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. As here depicted, the apparatus is of a
transmissive type (i.e. has a transmissive mask). However, in
general, it may also be of a reflective type, for example (with a
reflective mask). Alternatively, the apparatus may employ another
kind of patterning means, such as a programmable mirror array.
The source LA (e.g. a lamp or excimer laser) 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. Furthermore, the illuminator may comprise means for generating
off-axis illumination configurations, such as annular, quadrupole,
dipole and/or soft multipole configurations; these may be generated
in a variety of ways, as detailed for example in European Patent
Application EP 0 949 541 (incorporated herein by reference). In
this way, the beam PB imprinting on the mask MA has a desired
uniformity and intensity distribution in its cross-section.
It should be noted with regard to FIG. 25 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 direction mirrors); this
latter scenario is often the case when the source LA is an excimer
laser.
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 lens 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. 25. 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.
The depicted apparatus can be used in two different modes: In step
mode, the mask table MT is kept essentially stationary, and an
entire mask image is projected in one go (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; 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 positive direction at a speed
V=M.nu., in which M is the magnification of the lens 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.
Although certain specific embodiments of the present invention have
been disclosed, it is noted that the present invention may be
embodied in other forms without departing from the spirit or
essential characteristics thereof. The present embodiments are
therefor to be considered in all respects as illustrative and not
restrictive, the scope of the invention being indicated by the
appended claims, and all changes that come within the meaning and
range of equivalency of the claims are therefore intended to be
embraced therein.
* * * * *