U.S. patent application number 09/903196 was filed with the patent office on 2002-01-17 for low thermal distortion extreme-uv lithography reticle.
Invention is credited to Gianoulakis, Steven E., Ray-Chaudhuri, Avijit K..
Application Number | 20020006556 09/903196 |
Document ID | / |
Family ID | 22485325 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020006556 |
Kind Code |
A1 |
Gianoulakis, Steven E. ; et
al. |
January 17, 2002 |
Low thermal distortion extreme-UV lithography reticle
Abstract
Thermal distortion of reticles or masks can be significantly
reduced by emissivity engineering, i.e., the selective placement or
omission of coatings on the reticle. Reflective reticles so
fabricated exhibit enhanced heat transfer thereby reducing the
level of thermal distortion and ultimately improving the quality of
the transcription of the reticle pattern onto the wafer. Reflective
reticles include a substrate having an active region that defines
the mask pattern and non-active region(s) that are characterized by
a surface that has a higher emissivity than that of the active
region. The non-active regions are not coated with the radiation
reflective material.
Inventors: |
Gianoulakis, Steven E.;
(Albuquerque, NM) ; Ray-Chaudhuri, Avijit K.;
(Livermore, CA) |
Correspondence
Address: |
Charles H. Jew
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Family ID: |
22485325 |
Appl. No.: |
09/903196 |
Filed: |
July 10, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09903196 |
Jul 10, 2001 |
|
|
|
09139149 |
Aug 24, 1998 |
|
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Current U.S.
Class: |
430/5 ; 355/18;
430/311 |
Current CPC
Class: |
G03F 1/24 20130101; B82Y
10/00 20130101; B82Y 40/00 20130101; G03F 7/2004 20130101 |
Class at
Publication: |
430/5 ; 430/311;
355/18 |
International
Class: |
G03F 009/00 |
Goverment Interests
[0001] The U.S. Government has a paid-up license in this invention
and the right in limited circumstances to require the patent owner
to licence others on reasonable terms as provided for by the terms
of Contract No. DE-AC04-94AL85000 awarded by the Department of
Energy.
Claims
What is claimed is:
1. A reflective reticle comprising: a substrate having an active
region on a first surface of the substrate; and at least one
non-active region on a second surface of the substrate wherein each
non-active region is characterized by having a surface that is
formed of material that has an emissivity that is higher than that
of the materials forming the active region surface.
2. The reflective reticle of claim 1 wherein the active region
includes a surface that comprises a radiation reflective material
and wherein each non-active region does not have a layer of
radiation reflective material on its surface.
3. The reflective reticle of claim 1 wherein the active region
defines a pattern comprising a radiation absorbing material formed
on the surface of a radiation reflective material.
4. The reflective reticle of claim 1 wherein the radiation
reflective material has a multilayer structure formed of material
that is selected from the group consisting of (a) molybdenum and
silicon, and (b) molybdenum and beryllium.
5. The reflective reticle of claim 4 wherein the layer radiation
reflective material has a thickness of between 280 nm to 320
nm.
6. The reflective reticle of claim 3 wherein radiation absorbing
material is selected from the group consisting of tungsten,
titanium, titanium nitride, aluminum and mixtures thereof.
7. The reflective reticle of claim 1 wherein each non-active region
is characterized by having a surface that is formed of material
that has an emissivity of at least 0.25.
8. The reflective reticle of claim 1 wherein the at least one
non-active region has a surface area that is at least 25% of the
surface area of the reflective reticle.
9. A photolithography system comprising: a source of extreme
ultraviolet radiation; means for collecting the radiation emitted
from the source of extreme ultraviolet radiation and forming a
light beam therefrom that is directed to an active region of a
reflective reticle, wherein the reflective reticle comprises; (i) a
substrate having an active region on a first surface of the
substrate; and (ii) at least one non-active region on a second
surface of the substrate wherein each non-active region is
characterized by having a surface that is formed of material that
has an emissivity that is higher than that of the materials forming
the active region surface; and a wafer disposed downstream from the
reflective reticle.
10. The photolithography system of claim 9 wherein the active
region includes a surface that comprises a radiation reflective
material and wherein each non-active region does not have a layer
of radiation reflective material on its surface.
11. The photolithography system of claim 9 wherein the active
region defines a pattern comprising a radiation absorbing material
formed on the surface of a radiation reflective material.
12. The photolithography system of claim 9 wherein the radiation
reflective material has a multilayer structure formed of material
that is selected from the group consisting of (a) molybdenum and
silicon, and (b) molybdenum and beryllium.
13. The photolithography system of claim 9 wherein the reflective
reticle of claim 4 wherein layer radiation reflective material has
a thickness of between 280 nm to 320 nm.
14. The photolithography system of claim 12 wherein the radiation
absorbing material is selected from the group consisting of
tungsten, titanium, titanium nitride, aluminum and mixtures
thereof.
15. The photolithography system of claim 9 wherein each non-active
region is characterized by having a surface that is formed of
material that has an emissivity of at least 0.25.
16. The photolithography system of claim 9 wherein the at least one
non-active region has a surface area that is at least 25% of the
surface area of the reflective reticle.
17. A process for fabrication of a device comprising at least one
element having a dimension .ltoreq.0.25 .mu.m, such process
comprising construction of a plurality of successive levels,
construction of each level comprising lithographic delineation, in
accordance with which a subject active region of a reflective
reticle is illuminated to produce a corresponding pattern image on
the device being fabricated, ultimately to result in removal of or
addition of material in the pattern image regions, in which
illumination used in fabrication of at least one level is extreme
ultra-violet radiation, characterized in that the process employs a
reflective reticle comprising: a substrate having an active region
on a first surface of the substrate; and at least one non-active
region on a second surface of the substrate wherein each non-active
region is characterized by having a surface that is formed of
material that has an emissivity that is higher than that of the
materials forming the active region surface.
18. The process of claim 17 wherein the active region includes a
surface that comprises a radiation reflective material and wherein
each non-active region does not have a layer of radiation
reflective material on its surface.
19. The process of claim 17 wherein the active region defines a
pattern comprising a radiation absorbing material formed on the
surface of a radiation reflective material.
20. The process of claim 17 wherein the radiation reflective
material has a multilayer structure formed of material that is
selected from the group consisting of (a) molybdenum and silicon,
and (b) molybdenum and beryllium.
21. The process of claim 19 wherein the layer radiation reflective
material has a thickness of between 280 nm to 320 nm.
22. The process of claim 19 wherein the radiation absorbing
material is selected from the group consisting of tungsten,
titanium, titanium nitride, aluminum, and mixtures thereof.
23. The process of claim 17 wherein each non-active region is
characterized by having a surface that is formed of material that
has an emissivity of at least 0.25.
24. The process of claim 17 wherein the at least one non-active
region has a surface area that is at least 25% of the surface area
of the reflective reticle.
Description
FIELD OF THE INVENTION
[0002] The invention relates to projection lithography employing
soft x-rays and in particularly to reticles that exhibit minimum
thermal distortion during scanning. The invention is particularly
suited for systems that use a camera that images with acuity along
a narrow arc or ringfield. The camera employs the ringfield to scan
the reflective reticle and translate a pattern onto the surface of
a wafer.
BACKGROUND OF THE INVENTION
[0003] In general, lithography refers to processes for pattern
transfer between various media. A lithographic coating is generally
a radiation-sensitized coating suitable for receiving a projected
image of the subject pattern. Once the image is projected, it is
indelibly formed in the coating. The projected image may be either
a negative or a positive of the subject pattern. Typically, a
"transparency" of the subject pattern is made having areas which
are selectively transparent, opaque, reflective, or non-reflective
to the "projecting" radiation. Exposure of the coating through the
transparency causes the image area to become selectively
crosslinked and consequently either more or less soluble (depending
on the coating) in a particular solvent developer. The more soluble
(i.e., uncrosslinked) areas are removed in the developing process
to leave the pattern image in the coating as less soluble
crosslinked polymer.
[0004] Projection lithography is a powerful and essential tool for
microelectronics processing. As feature sizes are driven smaller
and smaller, optical systems are approaching their limits caused by
the wavelengths of the optical radiation. "Long" or "soft" x-rays
(a.k.a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG.
("Angstrom") are now at the forefront of research in efforts to
achieve the smaller desired feature sizes. Soft x-ray radiation,
however, has its own problems. The complicated and precise optical
lens systems used in conventional projection lithography do not
work well for a variety of reasons. Chief among them is the fact
that there are no transparent, non-absorbing lens materials for
soft x-rays and most x-ray reflectors have efficiencies of only
about 70%, which in itself dictates very simple beam guiding optics
with very few surfaces.
[0005] One approach has been to develop cameras that use only a few
surfaces and can image with acuity (i.e., sharpness of sense
perception) only along a narrow arc or ringfield. Such cameras then
scan a reflective mask across the ringfield and translate the image
onto a scanned wafer for processing. Although cameras have been
designed for ringfield scanning (e.g., Jewell et al., U.S. Pat. No.
5,315,629 and Offner, U.S. Pat. No. 3,748,015), available
condensers that can efficiently couple the light from a synchrotron
source to the ringfield required by this type of camera have not
been fully explored. Furthermore, full field imaging, as opposed to
ringfield imaging, requires severely aspheric mirrors in the
camera. Such mirrors cannot be manufactured to the necessary
tolerances with present technology for use at the required
wavelengths.
[0006] The present state-of-the-art for Very Large Scale
Integration ("VLSI") involves chips with circuitry built to design
rules of 0.25 .mu.m. Effort directed to further miniaturization
takes the initial form of more fully utilizing the resolution
capability of presently-used ultraviolet ("UV") delineating
radiation. "Deep UV" (wavelength range of .lambda.=0.3 .mu.m to 0.1
.mu.m), with techniques such as phase masking, off-axis
illumination, and step-and-repeat may permit design rules (minimum
feature or space dimension) of 0.18 .mu.m or slightly smaller.
[0007] To achieve still smaller design rules, a different form of
delineating radiation is required to avoid wavelength-related
resolution limits. One research path is to utilize electron or
other charged-particle radiation. Use of electromagnetic radiation
for this purpose will require x-ray wavelengths.
[0008] A variety of x-ray patterning approaches are under study.
Probably the most developed form of x-ray lithography is proximity
printing. In proximity printing, object:image size ratio is
necessarily limited to a 1:1 ratio and is produced much in the
manner of photographic contact printing. A fine-membrane mask is
maintained at one or a few microns spacing from the wafer (i.e.,
out of contact with the wafer, thus, the term "proximity"), which
lessens the likelihood of mask damage but does not eliminate it.
Making perfect masks on a fragile membrane continues to be a major
problem. Necessary absence of optics in-between the mask and the
wafer necessitates a high level of parallelism (or collimation) in
the incident radiation. X-ray radiation of wavelength
.lambda..ltoreq.16 .ANG. is required for 0.25 .mu.m or smaller
patterning to limit diffraction at feature edges on the mask.
[0009] Use has been made of the synchrotron source in proximity
printing. Consistent with traditional, highly demanding, scientific
usage, proximity printing has been based on the usual small
collection arc. Relatively small power resulting from the 10 mrad
to 20 mrad arc of collection, together with the high-aspect ratio
of the synchrotron emission light, has led to use of a scanning
high-aspect ratio illumination field (rather than the use of a
full-field imaging field).
[0010] Projection lithography has natural advantages over proximity
printing. One advantage is that the likelihood of mask damage is
reduced, which reduces the cost of the now larger-feature mask.
Imaging or camera optics in-between the mask and the wafer
compensate for edge scattering and, so, permit use of longer
wavelength radiation. Use of extreme ultra-violet radiation
(a.k.a., soft x-rays) increases the permitted angle of incidence
for glancing-angle optics. The resulting system is known as extreme
UV ("EUVL") lithography (a.k.a., soft x-ray projection lithography
("SXPL")).
[0011] A favored form of EUVL is ringfield scanning. All ringfield
optical forms are based on radial dependence of aberration and use
the technique of balancing low order aberrations, i.e., third order
aberrations, with higher order aberrations to create long, narrow
illumination fields or annular regions of correction away from the
optical axis of the system (regions of constant radius,
rotationally symmetric with respect to the axis). Consequently, the
shape of the corrected region is an arcuate or curved strip rather
than a straight strip. The arcuate strip is a segment of the
circular ring with its center of revolution at the optic axis of
the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary
schematic representation of an arcuate slit defined by width, W,
and length, L, and depicted as a portion of a ringfield defined by
radial dimension, R, spanning the distance from an optic axis and
the center of the arcuate slit. The strip width is a function of
the smallest feature to be printed with increasing residual
astigmatism, distortion, and Petzval curvature at distances greater
or smaller than the design radius being of greater consequence for
greater resolution. Use of such an arcuate field allows
minimization of radially-dependent image aberrations in the image.
Use of object:image size reduction of, for example, 5:1 reduction,
results in significant cost reduction of the, now, enlarged-feature
mask.
[0012] Masks or reticles for EUV projection lithography typically
comprise a silicon substrate coated with an x-ray reflective
material and an optical pattern fabricated from an x-ray absorbing
material that is formed on the reflective material. In operation,
EUV radiation from the condenser is projected toward the surface of
the reticle and radiation is reflected from those areas of the
reticle reflective surface which are exposed, i.e., not covered by
the x-ray absorbing material. The reflected radiation effectively
transcribes the pattern from the reticle to the wafer positioned
downstream from the reticle. Among the problems encountered in EUV
projection lithography are point-to-point reflectivity variations.
The art is in search of techniques to reduce reticle
distortions.
SUMMARY OF THE INVENTION
[0013] The present invention is based in part on the recognition
that thermal distortion in reticles can be significantly reduced by
fabricating reticles exhibiting improved radiative cooling in
vacuum systems. For example, this can be achieved by designing the
nonactive regions of reflective reticles not to be coated with the
high reflective material that is found on the surface of the active
region where the pattern is formed. Alternatively, the nonactive
regions can be coated with a high emissivity material. By employing
emissivity engineering which involves the selective placement or
omission of coatings on the reticle, the inventive reflective
reticle fabricated will exhibit enhanced heat transfer thereby
reducing the level of thermal distortion. Ultimately, the quality
of the transcription of the reticle pattern onto the wafer is
improved.
[0014] Accordingly, in one aspect, the invention is directed to a
reflective reticle that includes:
[0015] substrate having an active region on a first surface of the
substrate; and
[0016] at least one non-active region on a second surface of the
substrate wherein each non-active region is characterized by having
a surface that is formed of material that has an emissivity that is
higher than that of the materials forming the active region
surface.
[0017] In another aspect, the invention is directed to
photolithography system that includes:
[0018] a source of extreme ultraviolet radiation;
[0019] means for collecting the radiation emitted from the source
of extreme ultraviolet radiation and forming a light beam therefrom
that is directed to an active region of a reflective reticle,
wherein the reflective reticle includes:
[0020] (i) a substrate having an active region on a first surface
of the substrate; and
[0021] (ii) at least one non-active region on a second surface of
the substrate wherein each non-active region is characterized by
having a surface that is formed of material that has an emissivity
that is higher than that of the materials forming the active region
surface; and
[0022] a wafer disposed downstream from the reflective reticle.
[0023] In a further embodiment, the invention is directed to a
process for fabrication of a device comprising at least one element
having a dimension .ltoreq.0.25 .mu.m, such process comprising
construction of a plurality of successive levels, construction of
each level comprising lithographic delineation, in accordance with
which a subject active region of a reflective reticle is
illuminated to produce a corresponding pattern image on the device
being fabricated, ultimately to result in removal of or addition of
material in the pattern image regions, in which illumination used
in fabrication of at least one level is extreme ultra-violet
radiation, characterized in that the process employs the inventive
reflective reticle.
[0024] Modeling studies suggest that emissivity engineering can
effectively reduce the distortions especially for reflective
silicon reticles. For silicon reticles, simulations have shown an
82% reduction in total placement errors and a 25% reduction in
residual placement errors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a schematic of an EUV lithography device;
[0026] FIGS. 2A and 2B are plan and cross-sectional views,
respectively, of a reflective reticle;
[0027] FIG. 3 is a schematic of a dark field reticle; and
[0028] FIG. 4 is a schematic of a half dark/ half bright field
reticle.
DETAILED DESCRIPTION OF THE INVENTION
[0029] FIG. 1 schematically depicts an apparatus for EUV
lithography that comprises a radiation source 11, such as a
synchrontron or a laser plasma source, that emits x-rays 12 into
condenser 13 which in turn emits beam 14 that illuminates a portion
of reticle or mask 15. The emerging patterned beam is introduced
into the imaging optics 16 which projects an image of reticle or
mask 15, shown mounted on mask stage 17, onto wafer 18 which is
mounted on stage 19. Element 20, an x-y scanner, scans reticle 15
and wafer 18 in such direction and at such relative speed as to
accommodate the desired mask-to-image reduction.
[0030] The wafer is preferably housed in a wafer chamber that is
separated from the other elements of the photolithography system
located upstream as illustrated in FIG. 1. These other elements can
be housed in one or more chambers which are preferably maintained
in vacuum to minimize attenuation of the x-rays. EUV radiation
projected from the reticle and translated by the camera travels
through an aperture in the wafer chamber.
[0031] The EUV lithography device of the present invention is
particularly suited for fabricating integrated devices that
comprise at least one element having a dimension of .ltoreq.0.25
.mu.m. The process comprises construction of a plurality of
successive levels by lithographic delineation using a mask pattern
that is illuminated to produce a corresponding pattern image on the
device being fabricated, ultimately to result in removal of or
addition of material in the pattern image regions. Typically, where
lithographic delineation is by projection, the collected radiation
is processed to accommodate imaging optics of a projection camera
and image quality that is substantially equal in the scan and
cross-scan directions, and smoothly varying as the space between
adjacent lines varies. In a preferred embodiment, projection
comprises ringfield scanning comprising illumination of a straight
or arcuate region of a projection reticle. In another preferred
embodiment, projection comprises reduction ringfield scanning in
which an imaged arcuate region on the image plane is of reduced
size relative to that of the subject arcuate region so that the
imaged pattern is reduced in size relative to the reticle
region.
[0032] The individual elements that form the EUV lithography device
as shown in FIG. 1 can comprise conventional optical devices, e.g.,
condensers, cameras, and lens, for projection EUV lithography.
Preferably the EUVL device employs a condenser that collects soft
x-rays for illuminating a ringfield camera. A particularly
preferred EUVL device that employs a condenser having a diffraction
grating on the surface of a mirror upstream from the reflective
mask that enhances critical dimension control is described in
Sweatt et al., U.S. patent application Ser. No. ______ entitled
"Diffractive Element in Extreme-UV Lithography Condenser" filed on
Aug. 6, 1998 (Docket No. 023890-003) which is incorporated by
reference. The condenser illustrated therein has the ability to
separate the light from a line or quasi point source at the
entrance pupil into several separated lines or transform point foci
that are still superimposed on each other at the ringfield radius,
thus maximizing the collection efficiency of the condenser and
smoothing out any inhomogeneties in the source optics.
[0033] FIG. 2A shows a reflective reticle 30 having a generally
circular perimeter; it is understood that the perimeter of
reflective reticle for the present invention can have any figure
including, for example, polygons. The surface of the reflective
reticle includes an active region 39 where the mask pattern is
formed. During projection printing, EUV radiation is reflected from
the active region and onto the wafer. The surface of the reflective
reticle also includes non-active regions 32, 34, 36, and 38. EUVL
preferably employs an x-ray radiation source at about 13 nm, but
the absorption at this wavelength is very strong in essentially all
materials; therefore, EUVL employs reflective optics such as Mo/Si
multilayer mirrors. The multilayer mirror, which typically ranges
from about 280 nm to 320 nm in thickness, can also be made from,
for example, Mo/Be. The strong x-ray absorption also necessitates
the use of reflective mask or reticle patterns which are typically
made by depositing absorber patterns on top of the Mo/Si multilayer
mirror as described, for instance, in U.S. Pat. No. 5,052,033, and
D. M. Tennant et. al., Appl. Opt. 32, 7007 (1993), which are
incorporated herein by reference.
[0034] FIG. 2B shows the cross-section of inventive reflective
reticle that comprises a silicon substrate 40 having a mask pattern
that comprises a multilayer mirror structure 42 onto which absorber
patterns 48 and 50 are deposited. Silicon substrates that are
doped, e.g., by arsenic, boron, or phosphorus, are preferred since
they have higher emissivity levels than pure silicon. Absorber
materials typically comprises tungsten, titanium, titanium nitride,
or aluminum. As illustrated in FIG. 2B, non-active regions 44 and
46 do not include the multilayer reflective structure, rather, in
this embodiment, the surface of the non-active regions is bare or
exposed silicon. As described herein, it has been demonstrated that
not covering the non-active regions with the multilayer reflective
structure or any low emissivity material reduces thermal
distortion. The reflective reticle as illustrated in FIG. 2B can be
made by conventional methods. During the fabrication process, the
surface of the silicon substrate corresponding to non-active
regions 44 and 46 is covered with photoresist so that subsequent
sputtering of the Mo/Si multilayer coating will not deposit any of
the reflective substances onto these regions.
[0035] In addition to constructing the nonactive regions with high
emissivity surface materials, further reduction of reticle thermal
distortion can be achieved by choosing materials with higher
emissivity in fabricating the active regions. For example, in
selecting the absorber material, TiN is preferred because of its
high emissivity relative to most other absorber materials.
Analogous selection of high emissivity materials from among
suitable candidates, with respect to other parts of the active
regions, can be employed.
[0036] Instead of forming non-active regions having bare silicon
surfaces, the non-active regions can comprise regions of the
substrate that are covered with a suitable high emissivity
material. As is apparent, the use of any substrate compatible
material which has an emissitivity that is higher than that of the
materials forming the active region should enhance heat
dissipation. When the multilayer structure of the active region is
made Mo/Si, which as an emissivity of about 0.12, then the
substrate covering material must have an emissivity of greater than
0.12. However, preferably this substrate covering material has an
emissivity of greater than 0.25, and more preferably greater than
0.40. Suitable high emissivity materials include, for example,
metal oxides, e.g., aluminum oxide, copper oxide and molybdenum
oxide. From a practical standpoint, given that silicon has an
emissivity of 0.72, thermal dissipation will not be significantly
enhanced unless the substrate covering material has a very high
emissivity value.
[0037] As is apparent, for any reflective reticle, the higher the
ratio of the non-active region surface area to the active region
surface area, the greater the reduction in thermal distortion will
be when the non-active region is fabricated without the multilayer
reflective structure or is covered with a high emissivity substrate
covering material. While the present invention is applicable even
if this ratio is small, typically the combined surface area of the
non-active regions will range from about 50% to about 60% and
preferably at least about 25% and more preferably at least about
40% of the total surface area of the reflective reticle.
[0038] A series of simulations were performed to examine the effect
of emissivity engineering to reduce the thermal distortions of
reflective silicon reticles during scanning. Specifically, the
simulation measured the placement errors and blurs associated with
conventional and inventive reflective reticles. Placement error is
defined as the error in position of a point on the reticle just
prior to the arrival of the illumination. Blur is defined as the
motion of a point during the time that it is illuminated.
[0039] FIGS. 3 and 4 show schematics of a 200 mm diameter by 0.75
mm thick silicon wafer and the active reticle region used for the
simulation. The dimensions of the reticle were 130 mm in the
direction of the scan and 104 mm normal to the scan. The width of
the illumination field was 6 mm and the height was 104 mm. The scan
velocity was 38.7 mm/sec. The heat flux in the illumination field
was 0.76 mW/mm.sup.2. This flux corresponds to the power required
to expose 10 wafers/hour with a resist sensitivity of 10
mJ/cm.sup.2. The simulation was started with the leading edge of
the illumination field aligned with the edge of the reticle. The
effect of framing blades was taken into account in this analysis so
that the simulated illumination exposed only the active reticle
region of the wafer and not the surrounding areas. Dark field and
half dark/ half bright field reticles were simulated. A schematic
of the half dark/half bright reticle configuration is shown in FIG.
4. The dark field reticle conservatively assumes that all incident
energy was absorbed. The half dark/ half bright field mask assumes
all incident energy is absorbed on the dark region and
approximately 42 percent is absorbed in the bright region. At the
reticle, the integrated average reflectivity for Mo/Si is
approximately 58%. The emissivities of the bright and dark fields
were based on actual measurements. The emissivity of the Mo/Si
coating was 0.122, and that of a tungsten absorber material was
0.037. Outside the active region of the reticle, the Mo/Si coating
was not deposited. This region was assumed to be bare silicon with
an emissivity of 0.72. The reticle was also allowed to expand
freely. This assumption in effect neglected the frictional and
electro-static forces on the reticle from the chuck.
[0040] The technique used in the finite element analysis to
simulate the moving illumination source included the step of
tagging all element faces in the active reticle region, and at each
time step determining which of the faces were within the bounding
area of the moving source. Simple linear equations of motion with
constant velocity were used to describe the location of the leading
and trailing edges of the source. Finite element faces which lied
fully or partially within the boundary of the moving source had an
appropriate heat flux boundary condition applied corresponding to
the flux within the source and the fraction of the face that lied
within the location the source. The silicon material properties
used for the simulations are given as follows:
1 Density (Kg/m.sup.3) 2330.0 Thermal Conductivity (W/m K) 148.0
Specific Heat (J/kg K) 712.0 Young's Modulus (GPa) 107.0 Poissons
Ratio 0.25 Coefficient of Thermal Expansion 2.5E-06
[0041] The following table provides distortion comparisons for
silicon reflective reticles with full Mo/Si coating and bare
silicon outside the active region.
2 Half Half Bright Dark Field Bright Half Dark Dark Field Bare Si
Half Dark Bare Si Condition/ Full Mo/Si Outside Full Mo/Si Outside
Distortion Coating Active Coating Active Total 1457.8 259.87 879.3
184.6 Placement (nm) Residual 6.64 4.97 7.65 7.65 Placement (nm)
Blur (nm) 0.67 0.69 0.72 0.70
[0042] These simulation results strongly suggest that for both dark
and half dark/half bright pattern densities, low distortion
reticles can be designed using emissivity engineering approaches.
In particular, it is demonstrated that by coating only the active
region of the reticle with Mo/Si and absorber, and leaving the
non-active region uncoated, a significant reduction in total
distortion can be achieved.
[0043] Although only preferred embodiments of the invention are
specifically disclosed and described above, it will be appreciated
that many modifications and variations of the present invention are
possible in light of the above teachings and within the purview of
the appended claims without departing from the spirit and intended
scope of the invention.
* * * * *