U.S. patent application number 10/261591 was filed with the patent office on 2003-06-19 for apparatus and method of image enhancement through spatial filtering.
This patent application is currently assigned to ASML NETHERLANDS B.V.. Invention is credited to Smith, Bruce W..
Application Number | 20030112421 10/261591 |
Document ID | / |
Family ID | 26839676 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030112421 |
Kind Code |
A1 |
Smith, Bruce W. |
June 19, 2003 |
Apparatus and method of image enhancement through spatial
filtering
Abstract
An image enhancement apparatus and method are disclosed. The
apparatus consists of a spatial frequency filter where zero order
mask diffraction information is reduced in an alternative pupil
plane of the objective lens, specifically just beyond the mask
plane. By introducing an angular specific transmission filter into
this Fraunhofer diffraction field of the mask, user accessibility
is introduced, allowing for a practical approach to frequency
filtering. This frequency filtering is accomplished using a
specifically designed interference filter coated over a transparent
substrate. Alternatively, filtering can also be accomplished in a
complimentary region near the wafer image plane or in both
near-mask and near-wafer planes.
Inventors: |
Smith, Bruce W.; (Webster,
NY) |
Correspondence
Address: |
PILLSBURY WINTHROP, LLP
P.O. BOX 10500
MCLEAN
VA
22102
US
|
Assignee: |
ASML NETHERLANDS B.V.
De Run 1110, 5503
LA Veldhoven
NL
|
Family ID: |
26839676 |
Appl. No.: |
10/261591 |
Filed: |
October 2, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10261591 |
Oct 2, 2002 |
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09606613 |
Jun 29, 2000 |
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60142010 |
Jul 1, 1999 |
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60196621 |
Apr 11, 2000 |
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Current U.S.
Class: |
355/71 ; 355/53;
355/67 |
Current CPC
Class: |
G03F 7/70308 20130101;
G03F 7/70125 20130101 |
Class at
Publication: |
355/71 ; 355/53;
355/67 |
International
Class: |
G03B 027/72 |
Claims
What is claimed is:
1. A filter comprising a substrate and one or more thin films
coated on the substrate, wherein the substrate and the films are
highly transparent to a selected wavelength of radiation and are
relatively different in their respective indexes of refraction for
angularly filtering incident radiation.
2. The filter of claim 1, wherein the substrate and the films have
extinction coefficients less than 0.1.
3. The filter of claim 2, wherein the extinction coefficient is
less than 0.1.
4. The filter of claim 1, wherein the difference between the
respective indexes of refraction is in the range of >0.2 at the
wavelength of illumination.
5. The filter of claim 1, wherein the difference between the
respective indexes of refraction of the films is in the range
>0.2 at the wavelength of illumination.
6. The filter of claim 5, wherein the indexes differ by 0.84 or
0.34.
7. The filter of claim 3, wherein the indexes differ by 0.84 or
0.34.
8. The filter of claim 1, wherein near normal radiation is reduced
proportionately more than oblique radiation.
9. The filter of claim 4, wherein the difference in filtering
between normal radiation and oblique radiation is in the range of
5% to 50%.
10. The filter of claim 9, wherein the difference in filtering
between normal radiation and oblique radiation is in the range of
16% to 30%.
11. The filter of claim 1, wherein the substrate and the films
comprise organic or inorganic materials.
12. The filter of claim 11, wherein the inorganic materials are
selected from the group consisting of the group shown in Table
1.
13. The filter of claim 11, wherein the organic materials comprise
polymers.
14. The filter of claim 13, wherein the polymers are selected from
the group consisting of a fluoropolymer or nitrocellulose polymers.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is application is a division of U.S. patent application
of Bruce W. Smith, application Ser. No. 09/606,613, filed Jun. 29,
2000, which claims priority from U.S. Provisional Application Nos.
60/142,010, filed Jul. 1, 1999 and 60/196,621, filed Apr. 11, 2000.
The contents of these applications are hereby incorporated into the
present application by reference in full.
FIELD OF THE INVENTION
[0002] The present invention relates to a method to form
microlithographic images using a projection exposure system for
fabricating semiconductor devices.
BACKGROUND OF THE INVENTION
[0003] Semiconductor lithography involves the creation of small
three dimensional features as relief structures in a photopolymeric
or photoresist coating. These features are generally on the order
of the wavelength of the ultraviolet (UV) radiation used to pattern
them. Currently, exposure wavelengths are on the order of 150 to
450 nm, and more specifically 157 nm, 293 nm, 248 nm, 365 nm, and
436 nm. The most challenging lithographic features are those which
fall near or below sizes corresponding to 0.5.lambda./NA, where X
is the length and NA is the objective lens numerical aperture of
the exposure tool. As an example, for a 248 nm wavelength exposure
system incorporating a 0.60NA objective lens, the imaging of
features at or below 0.25 micrometers is considered state of the
art. FIG. 1 shows the configuration of a projection exposure
system. Such an exposure system can be used in a step-and-repeat
mode (referred to a stepper tool) or in a step-and-scan mode
(referred to as a scanner tool). A UV or vacuum ultraviolet (VUV)
source 1 is used to pass radiation through the illumination system
2 using a condenser lens system 3 and a fly's eye microlens array
4. An aperture 5 shapes the illumination profile to a defined area
and radiation is reflected from a mirror 6 to pass through an
illumination lens 7 to illuminate a photolithographic mask 8. Upon
illumination of the photomask 8, a diffraction field 11 distributed
as spatial frequency detail of the photomask 8 is directed through
the objective lens system 9 to be imaged onto the photoresist
coated semiconductor substrate 10. Such an exposure system forms an
image by collecting at least more than the 0.sup.th-order of the
diffraction field from the photomask 8 with the objective lens 9.
The absolute limitation to the smallest feature that can be imaged
in any optical system corresponds to 0.25.lambda./NA. Furthermore,
the depth of focus (DOF) for such an exposure tool can be defined
as +/-k.sub.2.lambda./NA.sup.2 where k.sub.2 is a process factor
that generally takes on a value near 0.5
[0004] As geometry sizes continue to shrink further below
0.5.lambda./NA, methods of resolution enhancement are being
required to ensure that intensity images (also known as aerial
images) are produced with adequate fidelity and captured within a
photoresist material. Such methods of resolution enhancement
developed over recent years can allow for improvement in addition
to those made possible with shorter exposing wavelengths and larger
numerical apertures. These methods in effect work to control the
weighting of diffraction energy that is used for imaging. This
diffraction energy corresponds to the spatial frequency detail of a
photomask. Phase-shift masking (PSM), off-axis illumination (OAI),
and optical proximity correction (OPC) all lead to image
enhancement through control of the weighting of diffraction energy
or spatial frequency that is collected by an objective lens. As an
example, an attenuated phase shift photomask accomplishes a phase
shift between adjacent features with two or more levels of
transmission [see B. W. Smith et al, J. Vac. Sci. Technol. B 14(6),
3719, (1996)]. This type of phase shift mask is described for
instance in U.S. Pat. Nos. 4,890,309 and 5,939,227. Radiation that
passes through clear regions of the such a mask possess a phase
(.PHI.) that is dependant on the refractive index and thickness of
the mask substrate. Radiation is also transmitted through dark
features formed in the attenuating material by proper choice of a
material that has an extinction coefficient value generally less
than 1.0. The radiation that passes though these dark features
possesses a phase that depends upon the refractive index and
extinction coefficient values of the mask substrate and of the
attenuating material. It is chosen so that a 180 degree phase shift
(.DELTA..PHI.) is produced between clear regions and dark regions.
Selection of a masking material with appropriate optical properties
to allow both a 180 degree phase shift and a transmission of some
value greater than 0% will reduce the amplitude of the zero
diffraction order produced by illumination of the mask. Comparison
of the resulting frequency plane distribution with that of a
conventional binary mask can demonstrate this effect. The
normalized zero diffraction order amplitude for a binary mask is
0.5 and the first order amplitude is 1/.pi. as seen from FIG. 2A.
Using a 10% attenuated phase shift mask, the normalized zero
diffraction order amplitude is 0.4 and the first order amplitude is
1.1/.pi. shown in FIG. 2B. This reduction in the zero diffraction
order reduces the amplitude biasing of the higher order frequency
components and produces an image amplitude function that has
significant negative electric field energy. This leads to an aerial
image intensity (which is the square of the amplitude image) that
retains zero values at edges of opaque features. This edge
sharpening effect leads to higher resolution when imaging into high
contrast photoresist materials.
[0005] An attenuated phase shift mask requires a complex
infrastructure of materials, deposition, etching, inspection, and
repair techniques to replace the mature chromium on quartz binary
photomask process. This field has been investigated for over ten
years and it is not yet certain if suitable materials will exist
for 248 nm, 193 nm, or shorter wavelengths. shift masking is
difficult because of geometry, materials, and process issues and
scattering artifacts produced during imaging. As a result, it is
questionable phase shift masking will be for use in integrated
circuit (IC) manufacturing.
[0006] Off-axis or modified illumination of a photomask can produce
a similar ifying effect [see B. J. Lin, Proc. SPIE 1927, 89, (1993)
and see B. W. Smith, hy: Science and Technology, Marcel Dekker: New
York, Ch. 3, 235 (1998)]. s an example of the prior art, depicting
illumination with an annulus and lumination profiles. Definition of
the shape of illumination can be carried out the position of the
shaping aperture 5 shown in FIG. 1. Other methods of clude the use
of beam sputters, diffractive optical elements, or other optical
rough the use of such annular or quadrupole illumination,
diffraction orders uted in the objective lens 9 of FIG. 1 with
minimal sampling of the central objective lens pupil. An example,
is shown in FIGS. 4a through 4c for upole, and weak quadrupole
illumination. The impact is similar to the he zero diffraction
order or frequency produced with phase shift masking. In ero order
takes on the shape of the illumination distribution in the
condenser ppropriately designed, the central portion of the
objective lens pupil can be fraction energy.
[0007] Modified or off-axis illumination can suffer exposure
throughput, orientation, effect problems. Contact mask features,
for example, exhibit little llumination. Implementation is
therefore limited for many applications, also cality.
[0008] The use of optical proximity correction (OPC) can also
result in a reduction of fraction energy in the frequency plane.
Methods of proximity effect reduction have been introduced which
are comprised of additional lines, sometimes referred to as OPC
assist features, into a mask pattern. This was first disclosed in
U.S. Pat. No. 5,242,770. The patterning is such that an isolated
line is surrounded by sub-resolution OPC assist line features on
either side of the line, better matching edge intensity gradients
of isolated features on the mask to more dense features on the
mask. FIG. 5A shows the frequency plane distribution in the
objective lens pupil for semi isolated lines (1:7 duty ratio)
compared to the frequency plane distribution of dense lines (1:1
duty ratio), FIG. 5B. In addition to the increase in the number of
diffraction orders present for the more isolated features
(resulting from a larger pitch value than that for the dense
features) a significant increase in the zero order term exists.
Through the use of small assist features on each side of the
isolated line, the amplitude of the zero diffraction order can be
reduced as higher frequency content is increased, seen in FIG. 5C
for one pair and for two pairs of assist features respectively.
[0009] OPC methods are limited by mask making capability for ultra
small geometry and by limitations imposed by neighboring geometry.
Implementation is especially difficult for geometry below 180 nm in
size, limiting practicality.
[0010] Direct reduction of zero order diffraction energy within the
lens and specifically in the lens pupil can be carried out by
physically obscuring the central axial portion of the pupil. The
concept of using such in-pupil filters (also known as
pupil-filtering) has been applied to various optical applications,
where the result is a spatial frequency filtering effect. This has
also been studied by various workers for application to
semiconductor lithography (see W. T. Welford, J. O. S. A., Vol. 50,
No. 8 (1960), 749 and see H. Fukuda, T. Terasawa, and S. Okazaki,
J. Vac. Sci, Tech. B 9 (1991) 3113 and see R. M. von Bunau, G.
Owen, R. F. Pease, Jpn. J. Appl. Phys., Vol. 32 (1993) 5850.). This
in-pupil filtering has also been proposed in patents U.S. Pat. No.
5,595,857, U.S. 5,863,712, U.S. 5,396,311, and U.S. 5,677,757. The
imaging characteristics for fine geometry (generally features at or
below 0.5 .lambda./NA) have been shown to be enhanced through the
use of various pupil filters. A simple example of the prior art is
shown in FIG. 6, where a pupil filter 38 has a radiation-blocking
portion 39 that blocks zero-order diffraction energy from passing
through a central area of the filter and a radiation or radiation
or light transmitting portion 40 that transmits diffraction energy
at a peripheral area surrounding the radiation or light blocking
portion 39. With respect to the projection imaging system of FIG.
1, such a prior art filter 14 is inserted into the pupil plane of
the objective lens of the exposure system. By obscuring the central
portion of the objective lens pupil, zero diffraction order energy
is reduced. Opaque or partially transmitting (gray) obscuration of
up to 70% of the pupil as well as more complex pupil filters have
also been used where the spatial frequency filtering of the filter
is customized for specific illumination and masking situations to
meet certain imaging objectives. Implementation of such in-pupil
filtering for semiconductor lithography is not practical or
feasible because access to the objective lens pupil is difficult
given the strict requirements placed on the objective lens
performance. The filtering or obscuration of the objective lens
pupil requires access to the pupil and a lens design robust enough
to tolerate any phase, absorption, or flatness variations. It is
unlikely that a permanent filtering value would be chosen for any
lithography lens.
[0011] A practical solution of spatial frequency filtering is
needed that can lead to the resolution and focal depth improvement
that is difficult with other resolution enhancement methods. The
ideal solution is one that could preserve most attributes of
current manufacturable methods of lithography and allow the
flexibility for application with many applications. Furthermore, a
spatial frequency filtering solution that could be used together
with other resolution enhancement methods could reduce demands on
those methods to allow for their application.
SUMMARY OF THE INVENTION
[0012] The present invention is a unique approach to reducing zero
diffraction order ial frequency filtering in an alternative pupil
plane, near the mask or the wafer essible to the user in a
conventional lithography system. A conventional binary sed with
this approach. A conventional full circular pupil can be also used
as information is not filtered within the lens pupil. Furthermore,
if combined with sking, modified illumination, or optical proximity
correction, further becomes feasible.
[0013] A goal of the present invention is to provide a practical
projection exposure paratus that is capable of providing image
improvement for a variety of fine eatures, including
one-dimensional and two-dimensional geometry, in terms of n and
depth of focus by reducing the amount of zero order diffraction
energy ed from a photomask and through the objective lens of a
stepper or scanner
[0014] A second goal of the present invention is to provide a
practical projection od and apparatus that is capable of improving
the performance of small a openings that are otherwise difficult to
image using other resolution techniques.
[0015] A further goal of the present invention is to provide a
practical projection od and apparatus that is capable of providing
means to improve the image teristics of features that are different
in their optical performance from one t images in photoresist can
be created that are most favorable for all features at ure value or
across a small range of exposure.
[0016] An additional goal of the present invention is to provide a
practical projection od and apparatus that is capable of providing
imaging improvement that can be in an exposure system without
significant modification to the system.
[0017] A still further goal of the present invention is to provide
a practical projection od and apparatus that is capable of
providing for custom image modification ask feature type, mask
type, illumination, or other imaging condition and can ted or
removed from an exposure system.
[0018] A still further goal of the present invention is to provide
a practical projection od and apparatus that does not introduce
significant, measurable, or sources of error to an exposure tool,
including uncontrollable effects from tortion, polarization, and
defocus.
[0019] A still further goal of the present invention is to provide
a practical projection od and apparatus that can be specifically
designed according to desired image ties of a mask feature type,
mask type, illumination, or other imaging
[0020] A still further goal of the present invention is to provide
a practical projection od and apparatus that allows for imaging
improvement at one or more de of the objective lens, where a single
location or multiple locations may be aging.
[0021] In order to accomplish the above described goals, spatial
frequency filtering is h the present invention at locations outside
of the objective lens and at planes he objective lens pupil. This
allows access to diffraction field energy or rmation without
requiring access to the physical lens pupil. Alternative pupil ted
close to object and image planes, or mask and semiconductor
substrate vely. At predetermined distances from these positions,
separation is so that angular specific spatial frequency filtering
devices placed at these electively act on frequency content of an
image rather than spatial content (i.e. stance). A filter is
provided that controls transmission as a function of angular e
diffraction field from the photomask geometry. The filter consists
of determined transparent materials at predetermined thicknesses on
a transparent t normal or near normal incident radiation passes
through the filter attenuated ined amount while radiation at more
oblique angle passes unattenuated or ed. The angular transmission
properties of the filter act on the spatial nt of the photomask
patterns and resulting images, where zero order d energy is
attenuated to the largest degree.
[0022] Additionally, the present invention provides a method to
hold the filtering es at the predetermined separation distances.
One aspect of the present des a spacing element which consists of a
fixed ring of a width corresponding separation distance which is
fixed to both the photomask and to the filter, er at the required
separation distance. Alternatively, the projection exposure vided
with an optical changing member for selecting one of a number of a
ers that provide preferred image modification for at least one
optical f the photomask features. Alternatively, the filter is
secured at the bottom most objective lens column, closest to the
image plane of the exposure system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic of a projection exposure system used
for lithography.
[0024] FIGS. 2A , 2B, and 2C show the distribution of diffraction
energy for (A) lumination, (B) strong phase shift mask, and (C)
attenuated phase shift mask.
[0025] FIG. 3 shows several prior art arrangements for modified
off-axis or a projection lithography system including annular and
quadrupole
[0026] FIGS. 4A, 4B, and 4C show the distribution of diffraction
energy for (A) uadrupole, and (C) weak quadrupole illumination.
[0027] FIG. 5A shows the diffraction patterns for semi-isolated
lines.
[0028] FIG. 5B shows the diffraction patterns for dense lines
[0029] FIG. 5C shows the comparison of the diffraction patterns of
semi-isolated pair (right) and two pairs (left) of OPC assist
lines.
[0030] FIG. 6 is an example of a prior art in-pupil radiation
blocking filter.
[0031] FIG. 7 shows the arrangement of the projection exposure
system according invention.
[0032] FIG. 8A, 8B, 8C, and 8D show the angular transmission
spatial frequency pes.
[0033] FIG. 9 is a graph showing performance of a frequency filter
designs for 5 nm.
[0034] FIG. 10 is a graph showing performance of a fused silica
angular frequency to operate with large angles.
[0035] FIG. 11 is a linewidth linearity plot comparing results with
a without an ncy filter.
[0036] FIGS. 12a and 12b show aerial image simulations for 150 nm
1:1.5 lines avelength and 0.63NA. Results in FIG. 12a are for
conventional imaging and 12b are for imaging with a 25% filter.
[0037] FIG. 13 shows aerial image simulations for strong off-axis
illumination h the filtering of the present invention. Results on
the left are for conventional esults on the right are for imaging
with a 25% filter.
[0038] FIG. 14 shows aerial image simulations for weak off-axis
illumination h the filtering of the present invention. Results on
the left are for conventional esults on the right are for imaging
with a 25% filter.
[0039] FIG. 15 shows aerial image simulations for 160 nm 1:1.5
contacts using ngth and 0.63NA. Results on the left are for
conventional imaging and results e for imaging with a 25%
filter.
[0040] FIG. 16 is a graph that shows the image plane distribution
for contact with ltering.
[0041] FIG. 17 is a graph that shows the polarization effects of a
typical frequency tz substrate.
[0042] FIG. 18 is a plot of the transmission verses illumination
angle of a fabricated ncy filter.
[0043] FIG. 19 illustrates a mounting method used for frequency
filtering near the
[0044] FIGS. 20a and 20b are graphs comparing a is a comparison of
dense contact t) the filtered results and (right) the control
(reference).
[0045] FIG. 21 is a comparison of semi-isolated contact F/E matrix
(left) the filtered ght) the control (reference).
[0046] FIG. 22 is a comparison of semi-isolated contact F/E matrix
(left) the filtered ght) the control (reference).
[0047] FIG. 23 illustrates an angular frequency filter design for
use in the second f the invention.
[0048] FIG. 24 is a graph that shows the transmission
characteristics of a fused frequency filter design for use with the
second embodiment of the invention.
[0049] FIG. 25 shows a lens element coated with an angular
frequency filter coating e third embodiment of the invention.
[0050] FIG. 26 is a graph that shows a transmission plot of a fused
silica angular er coating for use with the third embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0051] Features on an IC photomask are generally near the
diffraction limit and may gth in size for lens numerical aperture
values near or above 0.6. Spatial o diffraction order energy will
effectively reduce the amplitude biasing of the r of diffraction
orders that are captured by the objective lens of a stepper or the
zero diffraction order is reduced, the amplitude at the boundaries
of s is forced to zero and edges of opaque features can be
enhanced. This effect ned through image defocus or with other lens
wavefront aberration.
First Embodiment of the Invention
[0052] Spatial frequency filtering is carried out in a Fourier
transform plane of an plane is defined where the electric field
M(u,v) takes on the form in spatial frequency as: 1 M ( u , v ) = -
.infin. .infin. - .infin. .infin. m ( x , y ) - 2 ( u x + v y ) x
y
[0053] s the mask object function. Although the objective lens
pupil is in the Fourier e of the photomask, it shares this
characteristic with other alternative positions within the system.
An equivalent plane for mask diffraction for instance is in the far
field or Fraunhofer diffraction region of the photomask. Although
the Fraunhofer diffraction region is defined as an infinite
separation distance (z) from an object, more practically it exists
at a sufficient distance from the photomask object plane defined
as: 2 z >> L 1 2
[0054] where L, is the maximum radial extent of the object features
(x, y): {square root}{square root over
(x.sup.2+y.sup.2)}>L.sub.1
[0055] A factor of 10 is commonly used to satisfy the "much greater
than" condition for separation distance. For the case of IC
photomask geometry on the order of a few hundred nanometers, the
requirements of the minimum distance for Fraunhofer diffraction are
met by a factor of 103 or more at a distance of a few millimeters.
Filtering at this location therefore operates in the Fraunhofer
diffraction field of the mask object and any filtering done in this
region will act on the angular spatial frequency content of the
object. By placing an angular specific optical frequency filter at
this location, the desired effect is produced where zero order
diffraction energy is reduced while higher order diffraction energy
passes. The consequence to imaging is image sharpening, improved
resolution, and increased focal depth.
[0056] One embodiment of the present invention provides a
projection exposure system as shown in FIG. 7. A UV or VUV source 1
is used to pass radiation through the illumination system 2 by a
condenser lens system 3 and a fly's microlens array 4. An aperture
5 shapes the illumination profile to a defined area and radiation
is reflected from a mirror 6 to pass through an illumination lens 7
to illuminate a photolithographic mask 8. Upon illumination of the
photomask 8, a diffraction field 11 is directed through the
objective lens 9 to be imaged onto the photoresist coated
semiconductor substrate 10. The projection exposure her provided
with a filter 12 that lies outside of the objective lens 9 and at a
distance from the photomask 8.
[0057] The filter 12 of this invention comprises a coating or
coatings on a transparent that transmission is controlled though
the filter based on angle of incidence or near normal illumination
is attenuated greater than more oblique The filter is placed in the
optical train of a lithographic stepper or scanner in a form plane
of the object and image planes. These object and image planes
photomask and wafer planes respectively in a lithographic exposure
system.
[0058] The angular specific spatial frequency filter for the
filtering of the present rises a substrate with one or more
coatings of inorganic, organic, or combined ials. Filter designs in
UV wavelengths ranging from 150 nm to 450 nm require l types and
designs, however the present invention is not necessarily limited
to gths and solutions for shorter or longer wavelengths are
possible. The filter is ovide maximum transmission at angles
corresponding to first diffraction orders olvable with the highest
numerical aperture (NA) of a given system. This .lambda./p.about.NA
or about 44 degrees for a 0.70NA system. When the optical reduction
stepper or scanner is taken into account, this angle may be 1/4 or
1/5 of this
Angular Transmission Designs
[0059] The frequency filters of and for use with the present
invention require the of interference properties within a thin film
or thin film stack across a spectral uired components of a self
supporting filter include a transparent substrate and arent optical
thin film coating materials. In some instances, the substrate may
optical interference layer if its thickness is taken into account
during design and if the wavelength or spectral bandwidth of the
illumination source is sufficiently small. The simplest type of
periodic system is a quarter wave stack, a periodic structure of
alternately high (H) and low (L) refractive index materials with
low absorbance. Designs generally take on a form denoted as
s(HL).sup.N where s is the substrate material and N is the
periodicity factor, an integer equal to or greater than 1. This
unique variation of the periodic design for this application is the
angular interference filtering, where transmission is separated in
angle. The filter is designated for instance as s[HLH (LL) HLHJ
where the central portion of the stack (LL) corresponds to the
interference cavity. The thickness of the cavity is designed so
that interference at normal incidence leads to minimum transmission
values and transmission at oblique angles is maximized. FIGS. 8A
through 8D show several designs. FIG. 8A shows a simple angular
interference filter design 10 based on high index coatings 11 on
each side of a thick low index material 12. In this case, the low
index material is the substrate itself of a predetermined
thickness, which can be an amorphous or crystalline glass, fused
silica, or fluoride-based plate or a transparent organic polymeric
coating, such as a fluoropolymer or other transparent polymeric
material. FIG. 8B shows a dielectric design 13 that could be
utilized with inorganic dielectric high index materials 14 and low
index materials 15 coated over a fused silica substrate 16. FIG. 8C
is a double sided design 17 where the front side coating 18 is
replicated on the back side of a substrate 19 to further reduce
transmission at normal incidence, and FIG. 8D is a hybrid
multilayer design 20 using a low index dielectric interference
cavity 21.
[0060] The angular differences in optical path distances give rise
to the desired transmission characteristics of the interference
filters. To accomplish the goals of these filters, the optical
properties of materials and coatings must be well characterized at
the wavelength of operation. Additionally, since interference
relies on material optical thickness; be controlled to within
0.02.lambda./n.sub.i (where n.sub.i is refractive index). This
correlates to ness values of few nm for lithographic
wavelengths.
[0061] Fabrication of the filter coatings requires the use of UV
and VUV ellipsometry to characterize organic and inorganic
materials for use as high and rials. A Woollam WVASE VUV
spectroscopic ellipsometer was used to properties of a variety of
transparent materials suitable for use in these filters ganic oxide
dielectrics such as hafnium oxide (HfO.sub.2), silicon dioxide
(SiO.sub.2), oxide (Al.sub.2O.sub.3) and fluoride dielectrics such
as magnesium fluoride (MgF.sub.2), oride (LaF.sub.3), strontium
fluoride (SrF.sub.2), hafnium fluoride (HfF.sub.4), barium ,
gadolinium fluoride (GdF), lithium fluoride (LiF), yttrium fluoride
(YF.sub.3), .sub.2O.sub.3), chromium fluoride (CrF.sub.3),
neodymium fluoride (NdF.sub.3), phosphorous and ytterbium fluoride
(YbF). The refractive index (n.sub.i) and extinction properties are
shown in Table 1 at lithographic wavelengths (365 nm, 248 nm, 57
nm) below.
1 TABLE 1 157 nm 193 nm 248 nm 365 nm Material n k n k n k n k
Al2O3 2.070 0.0321 1.8942 0.0040 1.8283 0.0000 1.7927 0.0000 B2O3
1.761911 0.08833154 1.643061 0.08630061 1.687062 0.08432748
1.806547 0.08227224 BaF2 1.654013 0.03754984 1.561748 0.0153238
1.504007 0.006285364 1.468147 0.002429049 CrF3 -- -- 2.2052 0.0983
1.9831 0.0654 1.8087 0.0411 GdF3 -- -- 1.6602 0.0392 1.5957 0.0278
1.5600 0.0192 HfF4 1.870596 0.1215669 1.737489 0.04166171 1.659593
0.01436462 1.615425 0.004612696 LaF3 -- -- 1.6702 0.0907 1.6321
0.0609 1.5922 0.0398 LiF -- -- 1.4172 0.0054 1.4314 0.0043 1.4050
0.0031 MgF2 1.474716 0.06745632 1.418481 0.0465574 1.389212
0.03220084 1.375647 0.02172941 NdF3 1.715155 0.09490583 1.646496
0.02572772 1.581322 0.007026228 1.52356 0.001759475 P2O5 1.7432
0.0932 1.6433 0.0929 1.6199 0.0932 1.5877 0.0832 SrF2 1.534777
0.1191028 1.472364 0.05597903 1.422627 0.02642328 1.383426
0.0118622 YbF3 1.708709 0.07583188 1.638029 0.0451999 1.600526
0.0270207 1.582476 0.01560728 YF3 1.621849 0.0288452 1.549471
0.02145819 1.520605 0.01598972 1.515468 0.01168297 HfO.sub.2 -- --
2.62411 0.2190 2.3215 0.00028 2.14942 0.00000 SiO.sub.2 1.6702
0.00003 1.5632 0.00000 1.4800 0.00000 1.47472 0.00000
[0062] A variety of organic materials have also been characterized
including lymer materials such as fluoropolymers and
nitrocellulose.
[0063] As an example, inorganic filters have been designed and
fabricated for use at a ngth using hafnium dioxide (HfO.sub.2)
filMS with a refractive index of 2.3215 and coefficient of 0.00028
as a high index material combined with silicon dioxide w index
material with a refractive index of 1.480 and a near-zero value
ficient. Organic filters have also been designed for 248 nm and 365
nm using a film as a high index material with a refractive index of
2.007 at 248 nm and m and a fluoropolymer film as a low refractive
index material with refractive at 356 nm and 1.602 at 248 nm.
Filter designs are those of types B, C, and D in norganic materials
have been coated onto fused silica substrates. Organic filters e
types, utilizing the polymeric films themselves as support layers
and dditional support substrate.
[0064] FIG. 9 shows the angular transmission results for three
filter designs using acterized for use in the UV/VUV wavelength
region. Transmission is plotted as ngle at the mask side of the
imaging system. The designs and layer thicknesses ables 2a and 2b
below.
2TABLE 2a Angular filter designs DUV-MULTI DUV-3 I-LINE H 32 run 32
nm 52 nm L 1243 2548 2841 H 44 41 52 L 1227 H 46
[0065]
3TABLE 2b Materials for filter designs DUV (n, k) I-LINE (n, k) H
HfO2 (2.3215, 0.00028) PVN (1.704, 0.013) L SiO2 (1.48, 0.00)
Fluoro (1.363, 0.00)
[0066] For a 0.65NA 5.times. system, this angle corresponds to
sin.sup.-1 (0.65)/5 or 8.11.degree.. For a 4.times. system, this is
10.13.degree.. These values correspond to the maximum diffraction
angle that can be captured with a given NA (for coherent
illumination or for partially coherent illumination spread by a).
As the maximum oblique diffraction angle is increased, either
through higher NA or through lower reduction factors, the thickness
of the cavity layer can be decreased, thus allowing for normalized
transmission differential values above 30%. An example is shown in
FIG. 10 where a filter has been designed using a fused silica
substrate to operate with maximum transmission at angles between
35.degree. and 45.degree., which would correspond for instance to
high NA (0.57 to 0.71) in a situation where reduction does not
occur.
[0067] When an angular frequency filter is incorporated into an
exposure system, producing lower transmission at normal incidence
compared to more oblique angles, zero order diffraction energy can
be reduced. An added benefit that can be realized using this
spatial frequency filtering is a potential increase in critical
dimension (CD) linearity as feature pitch changes. It is well
understood that as mask feature pitch values are decreased, high
frequency diffraction energy is lost, leading to loss in image
fidelity. There is an additional effect that occurs that leads to
further loss in CD linearity, namely the reduction of total energy
in the lens pupil with decreasing pitch values. The linearity plot
of FIG. 11 depicts the situation for dense (1:1) line geometry
ranging from 500 nm to 150 nm in a 248 nm lithography system with a
0.61 NA and 0.7 .sigma.. As mask feature size is decreased, an
increase in resist feature sizing occurs with a positive
photoresist. By decreasing the energy in the center of the pupil,
through the angular spatial frequency filtering of the present
invention, CD linearity can be increased for a greater range of
feature values. The linearity plot of FIG. 11 also contains this
result, where the resist feature sizing remains more constant as
mask feature size is decreased.
[0068] The advantage of spatial filtering in a plane near the
photomask is the accessibility it allows the user. Where
conventional pupil plane filtering would require major
modifications to an exposure tools optical system, this approach
can immediately be put to practice. Additionally, the flexibility
of this approach allow for customizing of frequency filtering based
on mask and imaging requirements.
[0069] The benefits to imaging with this invention are realized
through the examination of lithographic results. A high-NA scalar
model was used to simulate lithographic imaging (using PROLITH/2
from FINLE Technologies, Version 6.05) for 150 nm semi-dense lines
(1:1.5) using a 248 nm wavelength and a 0.63NA. Aerial images were
evaluated by measuring normalized image log slope (NILS--the
product of the log of the slope of the aerial image and the feature
size) through focus for a range of partial coherence values from
0.3 to 0.8. Preferably, NILS values should be large. Results with a
25% frequency filter, based on the DUV-3 design shown in FIG. 9,
were incorporated into the model and compared to results without a
filter, which are shown in FIG. 12. There is improvement for all
conditions of partial coherence and maximum improvement occurs at
lower sigma values. This is expected based using coherent
illumination analysis. An added benefit of the present invention is
the improvement that it exhibits to imaging with modified
illumination, such as off-axis illumination. FIG. 13 shows results
comparing aerial images of 130 nm features produced with strong
off-axis illumination both with and without the spatial frequency
filter of this invention. For all pitch values shown, the filtered
result is superior to the non-filtered result. FIG. 14 show similar
improvement for these lines imaged with weaker or "soft" off-axis
illumination, where the circular poles of a quadrupole source
illumination have been replaced with gaussian shapes. A similar
comparison was made for 160 nm 1:1.5 contacts, shown in FIG. 15
where a 35% improvement in DOF is realized for the filtered
situation verses conventional imaging. The situation for contacts
is unique compared to line features, warranting further
description. For small contacts, the diffraction pattern overfills
the objective lens pupil. The resulting image of the contact more
closely resembles the transform of the pupil (the point spread
function) than it does the intended contact. The result is the
characteristic side-lobes or ringing seen for small contacts
especially at very low partial coherence values. The situation is
made worse however from the falloff within then lens pupil that
results from the product of the pupil with the contact diffraction
pattern. Contact t image widening results. If the falloff within
the lens pupil could be reduced, additional confinement in the
contact image size would result and features could be printed
smaller. This can be accomplished using pupil filtering with a
character that resembles the inverse of the falloff within the lens
pupil. This is precisely what is carried out by using the frequency
filtering approach described here. The falloff for a k.sub.1=0.5
contact at the edge of the pupil is 65%. A frequency filter with
near 65% attenuation at the center of the field, would therefore be
desirable. Coherent analysis of a 200 nm contact using a 248 nm
wavelength and a 0.63NA is shown in FIG. 16. Filtering with a 25%
filter leads to measurable reduction in contact size. Together with
the enhancement described earlier, the potential improvement for
contact level lithography using the frequency filter is
significant.
[0070] An interference filter placed into the optical path of a
projection system becomes an optical element and any impact on the
lithographic process may not be negligible. The challenge is to
address potential source of errors and design systems that minimize
these errors. There are four areas that may be a concern when using
filtering devices, specifically added wavefront aberration, focus
shift, polarization effects, and distortion.
[0071] The spatial frequency filter of the present invention is
essentially a parallel plate optical component, coated
appropriately to produce desired interference results. Imaging
through a parallel plate results is image displacement (described
later in detail), but its impact is not limited to this effect. By
applying aberration theory of two surfaces, imaging and aberration
results can be evaluated [see V. Mahajan, Aberration Theory Made
Simple, SPIE Press, Vol TT (1991) 30]. The wavefront aberration
function through the two surfaces of the plate can be written
as:
W(.rho.,.theta.;h)=.alpha.(.rho..sup.4-4hp.sup.3
cos(.theta.)+4h.sup.2.rho- ..sup.2
cos.sup.2.theta.+2h.sup.2.rho..sup.2-4h.sup.3.rho. cos .theta.)
[0072] Where h is the separation of an object point from the
optical axis, a is the weighted aberration coefficient, and p is
the normalized radius of the pupil. The five terms of the function
describe spherical, coma, astigmatism, defocus, and tilt
respectively. For coherent or for symmetrical illumination, all
terms of the wavefront aberration function drop out except for
spherical. For a parallel plate, the aberration coefficient a is: 3
= t ( n 2 - 1 ) 8 n 3 S 4
[0073] where t is the plate thickness, n is refractive index and S
is the separation distance form the mask. The wavefront aberration
function becomes: 4 W ( , , 0 ) = t ( n 2 - 1 ) 8 n 3 S 4 4
[0074] paration, a refractive index of 1.5, and a 2 .mu.m thick
filter, the induced Seidel ration is {fraction (1/5000)} wave. For
a 0.5 mm quartz plate, {fraction (1/20)} wave of spherical dded.
Although {fraction (1/20)} wave is not a negligible amount of
spherical wave by aphic requirements, sufficient adjustment and
compensation ability exists on o correct for this level of
aberration. This is therefore not a critical concern.
[0075] When a parallel plate is placed in an optical path, a shift
in focus occurs due to tical path length. While the lens numerical
aperture does impact this effect, its minor and the shift at the
mask plane can be approximated by:
.delta.f=t(n-1)/n
[0076] e focus shift. In general, a mask plane focus shift of about
one third of the filter of index 1.5 is produced, which is also
easily correctable in the exposure er plane.
[0077] At large angles, polarization effects by reflection (or
transmission) begin to icant. As angles approach Brewster's angle,
polarization increases. For the case nce filter, the process is
repeated through the multilayer stack. FIG. 17 is a al frequency
filter interference coating at angles to 20 degrees (equivalent to
an or a 4.times. or 5.times. system). Only at angles greater than
13.degree. (equivalent to an NA of system) do polarization effects
become noticeable, where s and p polarization different
transmission properties. This is less of a concern in a system
utilizing rtially polarized radiation. Modified filter designs can
push this maximum ar 18.degree..
[0078] For single telecentric systems, distortion will be caused as
radiation or light h a parallel plate in the mask pellicle plane.
The degree of distortion depends on the refractive index and
thickness of the plate, magnification, and the maximum ray angle
through the plate. This potential problem is eliminated with
respect to current lithographic requirements with the dominance of
modern double-telecentric steppers and scanners.
[0079] In general, the errors induced by use of this invention in a
lithographic application are all within the control of modern tools
and imaging systems and present no issues for the implementation of
this technology.
Filter Fabrication
[0080] Fabrication of spatial frequency filters has been carried
out for application at DUV (248 nm) exposure as inorganic
multilayer stacks on fused silica substrates. Substrates were 4"
diameter round high quality UV fused silica polished to a
.about.0.5 mm thickness. Optical properties (ni, k, and
transmission) were measured and fitted for these substrates using a
Woollam UV/VUV spectroscopic ellipsometer (WVASE) for wavelengths
from 190 to 500 nm, resulting in a refractive index at 248 nm of
1.480 and a zero extinction coefficient. Three layer filters were
fabricated based on the DUV-3 design shown in FIG. 9. Hafnium oxide
was chosen as the high index material based on low absorption and
thermomechanical properties. Silicon dioxide was chosen as the low
index material because of its low stress properties compared to
other alternatives. Films were deposited using a Leybold APS 1104
ion assisted evaporation system. Hafnium oxide was reactively
deposited from a hafnium metal target and silicon dioxide was
deposited from an oxide target. The targeted bottom high index
layer thickness was 32 nm, the thick low index cavity layer
targeted thickness was 2548 nm, and the target high index top layer
thickness was 41 nm. Iteration of the multilayer was carried out by
performing metrology at various points during the stack
fabrication. Initially, a two layer stack over the fused silica
substrate was created without the final top HfO.sub.2 layer.
Ellipsometry and fitting solved for the actual SiO.sub.2 thickness
value, which was 7 nm short of the target value. A commercial
optical thin film design software, TfCalc, was used together with
WVASE to redesign the multilayer stack and arrive at a new
solution. A second coating set was performed on the filter with a
small additional amount of SiO.sub.2 (7 nm) and the final HfO.sub.2
top layer. Transmission results are shown in FIG. 18.
[0081] Lithographic testing was carried out on an 248 nm ASML
5500/300 exposure tool with a NA of 0.63. Partial coherence values
chosen for testing were 0.3 and 0.5. Mask features evaluated
consisted of 250 nm contacts on duty ratios of 1:1, 1:5 and
isolated. A Shipley UV 110 resist was coated at a thickness of 0.42
.mu.m over a DUV ARC material for imaging. The quartz filter was
mounted to an aluminum 3.28" diameter ring, 1 mm thick, and with a
3 mm height or standoff distance, as shown in FIG. 19. Mounting
consisted of bonding the ring 40 to the filter 42 with a
cyanoacrylate cement and mounting the ring 40 and filter 42 to the
contact photomask 44 using removable adhesive. Although the filter
was mounted at a fixed distance onto the mask, there are other
means to place such a filter in the mask Fraunhofer diffraction
region (at a distance of at least a few mm. For instance, an
optical changing member (e.g. a turret with a plurality of filters)
can be provided for selecting one of a number of a plurality of
filters that provide preferred image modification for at least one
optical characteristic of the photomask features and placing the
selected filter at the required separation and location with
respect to the mask and optical components. The ring method
provided convenient means to mount the filter in an optical plane
with control of parallelism between the mask and filter. For
comparison during lithographic testing, an uncoated fused silica
plate was also configured for mounting onto the photomask.
[0082] Focus exposure plots are shown in FIGS. 20 through 22. FIG.
20 shows results for the filtered dense contact imaging (1:1)
compared to the control "reference". The dose range for the
filtered results is larger than that for the control (1.52.times.
vs. 1.20.times. respectively). For each case, the minimum contact
size and DOF is shown for a 10% dose variation and a 10% size
variation. The contact sizing for the filtered case is measurably
smaller than for the control, 220 nm vs. 260 nm. For semi-isolated
contacts (1:5) shown in FIG. 21, the filtered contact results also
exhibit smaller sizing. For isolated contacts shown in FIG. 22,
were the most significant improvement is predicted, a smaller
sizing results with an improvement in DOF over the control. There
is also a significant improvement in process overlap between all
duty ratios when the filtered result is compared to the control.
This is summarized in Table 3 where the sizing range for the
filtered contact is 210 to 220 nm and the sizing range for the
control is 220 to 260 nm. This is a significant result.
4TABLE 3 Comparison of results for spatial filtering vs. unfiltered
(control) showing improvement in process overlap. Filtered Control
Dense sizing (1:1) 220 nm 260 nm Semi-iso sizing (1:5) 215 nm 235
nm Iso sizing 210 nm 220 nm Sizing range 10 nm 40 nm
Second Embodiment
[0083] A second embodiment of the invention provides a projection
exposure system as shown in FIG. 7 with an illumination system 2
that illuminates a photomask 8 so that diffraction field energy 11
is directed toward an objective lens 10. The projection exposure
system is further provided with a filter 13 that lies outside of
the objective lens 9 and at a predetermined minimum distance from
the photoresist coated semiconductor substrate 10 at an analogous
or complementary position to that for the mask plane. The filter 13
consists of a coating or coatings on a transparent substrate such
that transmission is controlled though the filter based on angle of
incidence where normal or near normal illumination is attenuated
greater than more oblique illumination. By placing the filter at
this location, the desired effect is also produced where zero order
diffraction energy is reduced. The design of the filter differs
from the mask side filter in its angular requirement, where a 1/4
or 1/5 reduction factor is not utilized. The benefits of this
approach to the present invention can be that a transparent
substrate may already be a component of the lens system, used to
protect bottom elements from contamination and to isolate the lens
system from atmosphere. By coating this transparent substrate to
exhibit the properties of a spatial filter, image improvement can
result.
[0084] FIG. 23 shows an angular filter design for use as a wafer
side spatial frequency filter 13 of FIG. 7. The filter 10 consists
of alternative layers of transparent high 11 and low 12 refractive
index coating materials on a transparent substrate 13. FIG. 24
shows the transmission characteristics at a wavelength of a 248 nm
fused silica filter designed and fabricated using Hafnium Dioxide
and Silicon Dioxide. Thicknesses of the layers are given. Angles
ranging from normal incidence (zero degrees) to 44 degrees are also
shown, corresponding to a 0.70 NA objective lens. The larger angles
of the high-NA side of the reduction objective lens allow for
greater attenuation of near normal incidence radiation, therefore
leading to greater attenuation of zero order diffraction energy as
needed. Furthermore, attenuation matching to that of a frequency
filter for use on the mask side of the objective lens can be
obtained with lower layer thickness values and lower total
thickness values of the filter coatings. This can lead to greater
process control during filter fabrication.
[0085] Additionally, filters can be implemented at both the
complimentary wafer and mask sides of the objective lens, that is
at both location 12 and location 13 of FIG. 7, increasing the
degree of control over spatial frequency filtering. This capability
can prove useful as some minimum amount of filtering may be
generally desired for most lithographic imaging and a
semi-permanent filter can be positioned in one of the filtering
planes. Filtering can be increased for certain desired imaging
situations by incorporating a filter in the second complimentary
filtering location. This is a also significant opportunity for
imaging improvement.
Third Embodiment
[0086] I have discovered that coating of the lens elements
themselves, located as components of the objective lens, can lead
to similar results as those described here. The thin film
interference coatings can be designed and fabricated to be used on
optical elements to filter diffraction energy at near normal
incidence and emphasize energy at large angles. This is especially
useful as high NA lenses are being made and traditional AR coatings
are more difficult to design so that low reflectance is achieved at
all angles. FIG. 25 shows this embodiment of the invention. An
optical element 10 from an objective lens 11 is coated on one or
either sides with an interference filter 12 that consists of one
layer or a periodic structure of more than one layer, which in turn
consists of alternating high refractive index films 13 and low
refractive index films 14. FIG. 26 shows a plot of transmission
verses angle for normal incidence to 44 degrees for a fused silica
filter consisting of Hafnium Oxide and Silicon Dioxide coatings.
Film thicknesses are also shown. The degree of reflection reduction
in a lens system, or anti-reflection (AR), is optimum at the most
oblique angles, corresponding to the full NA of the system.
Particular angular requirements of a lens element depend on
particular lens element properties. Using this invention, coatings
can be designed so that maximum reduction in reflection or AR is
achieved at the most oblique angles with lower transmission and low
angles. This relaxes design constraints and at the same time can
lead to improved performance, as described previously. This is a
significant result.
[0087] Spatial frequency filtering in an alternative Fourier
Transform plane has been demonstrated with the present invention as
a practical method of imaging enhancement. The designs and
fabrication results are very encouraging for this new approach to
resolution enhancement. Although the present invention has been
described it is to be understood that it is not limited to these
descriptive examples. The described embodiments are not necessarily
exclusive and various changes and modifications in materials,
designs, and placement may be made thereto without departing from
the scope of the invention, which is only limited by the following
claims.
* * * * *