materials that are compatible with high-performance immersion lithography
and Kaushal Patel, IBM; and Mark Slezak and Gary Dabbagh, JSR
use of immersion lithography asa technique to enhance resolution
and depth of focus was described as early as 1987, but it moved to the
forefront for 45- and possibly 32-nm-node patterning options with the
publication of the latest roadmap.1,2 Recently, the technology's
viability as a manufacturing tool for 90-nm devices as well was demonstrated
by two firms. IBM built a critical level of a 64-bit power processor,
while TSMC fabricated electrically functioning SRAM chips.3,4
Both companies maintain that the process yield was overlay,
resist materials, and water handling.
it is clear that immersion lithography is moving rapidly toward the
production line, many challenges related to resist materials and processes
remain to be addressed to realize its potential advantages and to prevent
defects. Potential defect sources associated with the process are numerous.
Bubbles and particles may be created during imaging, material residues
may remain following the removal of topcoats, and image collapse may
be caused by swelling of the resist from water infusion.
of the most important challenges to be overcome is the leaching of resist
constituents into the immersion fluid. Resist extraction studies have
shown that both the photoacid generator (PAG) and photogenerated acids
are present in water after 30 seconds or less of contact.5–8
Although recent modeling data suggest that low levels of PAG components
may not contaminate the scanner lens, there are still concerns that
PAG leaching will affect resist performance by changing dissolution
rates and resist profiles and by degrading CD control.9,10
Consequently, decreasing the amount of extractable products is a priority.
the application of a topcoat is the primary approach to preventing extraction
products from entering the immersion fluid. Such coatings have been
shown to be effective.11 They can also be engineered to be
antireflective and have the added advantage of preventing airborne contaminants
from reaching the resist surface, thus mitigating postexposure delay
effects.12,13 Topcoat design is critical in other respects
as well. Because the resist profile and performance must be maintained,
topcoat-resist interactions must be avoided. The added layer of material
also raises concerns about additional defects. Ultimately, the goal
is to eliminate the need for topcoats by developing materials that resist
leaching and contamination, but in the meantime topcoats are an important
part of immersion processing.
gain an understanding of how various resist chemistries and formulations
behave under immersion conditions, the study reported in this article
compared the lithographic performance of resists containing different
polymer platforms, protecting groups, and formulations under immersion
and dry process conditions. The compatibility of several developer-soluble
topcoat materials with a variety of resists was also studied from the
standpoint of profile control issues and defect reduction. In addition,
parallel efforts focused on understanding and eliminating defect sources.
The comparison of the lithographic performance of various resists was
accomplished using polymers, PAGs, and formulations supplied by JSR
Microelectronics (Sunnyvale, CA). Methacrylate polymer platforms with
high-, low-, and hybrid-activation-level protecting groups were used
in the immersion formulations, and the resists also contained two sulfonium
PAGs in combination with one of four solvent mixtures. In all cases,
the primary solvent was propylene glycol monomethyl ether acetate (PGMEA),
while the different cosolvents used were gamma-butyolactone (GBL), ethyl
lactate (EL), and cyclohexanone. The compatibility of several developer-soluble
topcoat materials with a variety of resists was also studied using three
materials supplied by JSR. The bake process for all topcoats was 90°C
for 60 seconds.
Coating was performed on one of two tracks. An ACT 12 track from TEL
(Kumamoto, Japan) was used for experimental resist coatings in both
tool- and hand-apply modes. Using a chemically filtered FOUP, the sample
wafers were then transported to an immersion tool cluster for exposure,
postexposure bake (PEB), and development. For comparison testing, a
TEL Lithius track in a linked configuration with a 0.75-NA 1150i immersion
scanner from ASML (Veldhoven, The Netherlands) was used with several
commercially available photoresists, antireflective coatings, and topcoats
plumbed in. Critical dimension (CD) measurements were done using a VeraSEM
(Applied Materials; Santa Clara, CA), while an Applied Complus defect
inspection tool was used for defect analysis in conjunction with an
Applied G2 SEM Vision for E-beam inspection and EDX analysis. Leaching
experiments were performed using an apparatus developed in-house that
has been described elsewhere.5 The resulting samples were then analyzed
for trace amounts of perfluorosulfonates using liquid chromatography/mass
spectrometry/mass spectrometry by Exygen Research (State College, PA).
The detection limit for this technique is 0.2 ppb.
The development of high-performance immersion-compatible materials requires
a four-step approach. The first step involves protecting the immersion
lens from leaching materials by defining component-leaching specifications
that are conservative enough to ensure a stable and lasting lens quality
but not so stringent as to prevent valuable experiments from being performed.
Such specifications were determined by evaluating previous stray-light
test results with respect to the number of wafers run in the lithography
tool during a specific period of time in conjunction with the experimental
results of resist-leaching tests.
second step was to identify developer-soluble topcoats that are effective
as leachant barriers. Several materials using tetramethylammonium hydroxide
(TMAH) as the developer showed excellent barrier capabilities, and one
was chosen for use in evaluations with experimental resist formulations.
three involved optimizing commercial resists and processes for use with
metal- and gate-level applications so that defectivity learning related
to tooling, materials, and processes could proceed in parallel with
new resist studies.
step four, materials research and development focused on 45-nm-node
lithography without topcoats, comparing results for various polymers,
protection levels, and solvents.
a Specification. The initial component-leaching specifications
used in this study were determined by examining historical data on the
number of wafers previously exposed on the lithography tool, the resist
and topcoat combinations used during those exposures, and the stray-light
data collected for the same period of time. If extractable materials
migrating to the lens surface had either been deposited on or attacked
the lens surface during those runs, the stray-light tests would have
provided evidence of degradation. The historical data showed that the
level of stray light had been constant during the exposure of >11,000
wafers, indicating that the level of leaching in the resist and resist-topcoat
combination was sufficiently low to prevent lens degradation.
the amount of extractable components in resist-film samples was quantified,
it was found that the level of extricated perfluorosulfonates was ~12
ppb when the resist was used without a topcoat and <0.2 ppb when
a topcoat was used. Since the exact ratio of wafers with resist alone
compared with wafers with resist and a topcoat was unknown, a conservative
approach to specifying leaching levels was employed. The initial acceptance
level of extractable perfluorosulfonates was set at 12 ppb. If an experimental
resist film exhibited >12 ppb of extractable material, a topcoat
was required to bring the level down to 5 ppb or less. For resists in
the 5–12-ppb range, it was considered safe to expose a limited
number of wafers. Additional information revealing the effects of water-soluble
resist components on lens performance is expected through experimentation
and modeling and will be taken into account in future specification
Tests. The in-house technique employed for extracting resist
components from coated films involves depositing a known volume of water
on the resist film and subsequently collecting the water for analysis
after a specific contact-residence time. When 10 different commercially
available resists were evaluated for leaching, significant variations
of extracted perfluorosulfonates were observed. Figure 1a shows the
variation among these samples, which yielded extraction levels ranging
from approximately 10 to >70 ppb.
1: Comparison of the results of PAG extraction tests of 10 commercially
available resist films: (a) perfluorosulfonate extraction levels
of resists without topcoats and (b) perfluorosulfonate levels of
resists used with topcoat TC-2 (green bars). Topcoat efficiency
is represented by the line.
amount of variation was not surprising because the resists evaluated
contained a cross section of polymers, protecting groups, PAGs, and
solvents. With the application of topcoat TC-2, there was a significant
reduction in perfluorosulfonate extraction, as shown in Figure 1b. This
topcoat's efficiency at reducing leaching varied with the resist, but
it was >90% in all cases. Many topcoated samples had levels below
1 ppb, and all were below 3 ppb. With respect to leaching specifications,
all of these resist and topcoat combinations would be considered acceptable
on the immersion scanner.
1 also reveals inconsistencies in the topcoat's ability to prevent leaching
from sample to sample. While resist C had the highest level of perfluorosulfonate
extractables when used without a topcoat, there was a 99% reduction
in extractable components when the topcoat was used. In comparison,
while the untopcoated resist H had fewer extractables (approximately
30 ppb) than resist C, that level was reduced by only 90% when it was
used with the topcoat, resulting in a resist-topcoat stack with a perfluorosulfonate
content of 3 ppb, the highest of any topcoated resist. These inconsistencies
must be understood in order to progress toward more-efficient topcoats
and ultimately a topcoat-free process.
matrix of experimental polymers with varying activation levels and cosolvents,
along with a commercially available resist, was also evaluated in terms
of perfluorosulfonate leaching. In this test, PAG chemistry and loading
were kept constant for all polymers, and postapplication bake (PAB)
temperature was set at 110°C for 1 minute. The two high-activation-level
polymers and the two hybrid-activation-level polymers had similar leaching
values, while two of the three low-activation-level polymers showed
significantly greater amounts of extractable materials. A comparison
of PAG extraction with the hydrophobicity of the polymer as determined
by contact angle revealed no correlation.
another set of tests, perfluorosulfonate extraction from polymer F was
evaluated using various solvent systems. Again, the PAB temperature
was kept constant at 110°C for 1 minute. The solvent systems evaluated
included PGMEA with cosolvents of GBL, EL, and cyclohexanone. The results
indicated that the use of the cosolvents resulted in minor variations
in contact angle and a slight indication that increasing hydrophobicity
yields lower leaching levels. These results suggest that the most important
factors in controlling leaching are the polymer and its activation level.
Once an optimized polymer and protecting group are chosen, additional
reductions in leaching may be possible with the judicious choice of
cosolvents that increase hydrophobicity.
use in immersion lithography, topcoats must meet multiple requirements,
including efficiency as a barrier against leaching, low defectivity,
and transparency at 193 nm. In addition, they must not intermix with
the photoresist, affect resist profiles, or result in poorer resist
performance than dry lithography. These characteristics must be combined
with high dissolution rates in TMAH developers and insolubility in water.
determine topcoat efficiency as a leaching barrier, Figure 2 compares
three formulations used with experimental polymer A, which had exhibited
a perfluorosulfonate content of ~24 ppb in previous tests without a
topcoat (see Figure 1a). TC-1, an alpha material that had earlier provided
sufficient barrier characteristics to warrant further study, exhibited
a leaching efficiency of approximately 95%; TC-2, a second-generation
topcoat, exhibited superior leachant-barrier characteristics with an
efficiency of >98%; and TC-3, an experimental material under development,
decreased PAG extraction into water by 96%.
2: Comparison of the results of PAG extraction tests using resist
A and three topcoats. Perfluorosulfonate content is represented
by the green bars and topcoat efficiency is represented by the line.
examination of resist profiles revealed that although TC-1 worked well
with some resists, it caused tee-topping in others. It was hypothesized
that such profile changes would be avoided by increasing the acidity
of the topcoat. When an acid moiety was incorporated into the polymer
of TC-2, the profile changes were eradicated, as shown in Figure
order to develop suitable topcoats for immersion lithography, it is
important to understand how materials and material-tooling interactions
contribute to defect levels. The effects of the three topcoats on defects
in blanket films were first evaluated without imaging or exposure to
the immersion fluids. As seen in Figure 4, defect levels varied after
PAB, with TC-2 yielding the lowest number of defects. After a PEB cycle
was performed under conditions compatible with the underlying resist,
the topcoats were removed during a 60-second develop step and another
set of defect counts was taken. In that test, TC-1, the alpha topcoat
material, yielded a large number of defects, most of which were polymer
blobs. Both TC-2 and TC-3 caused significantly fewer defects than TC-1,
with TC-3 displaying the fewest.
4: Comparison of defect counts in blanket films using three topcoats.
Counts were measured both prior to exposure to immersion fluids
(shown as bars) and after PEB and developer (shown as points).
studies cannot give a complete picture of defect generation in immersion
lithography. Patterned films are required to learn about material and
tooling interactions. Therefore, the next step in this study was to
evaluate defect levels using patterned films and various resists and
were performed using resist B with topcoat TC-2. Imaging involved a
metal-level mask from a 90-nm-node product. Figure 5 shows the resulting
defect counts by category from four imaged wafers. There were approximately
20 total counts on each wafer, most of which were particles. Such particles
had not been present on the blanket films and were found on top of the
resist after topcoat removal and development, suggesting that they were
tooling-related. The particles could have been generated from a combination
of sources, including the water supply, degraded track or scanner components,
or contaminated process bowls or supply lines. Many postlithography
particle defects are closely related to missing-pattern defects because
a similar generation process is involved. A particle that blocks the
light during exposure might be removed later either by the immersion
head or during the topcoat removal and development process, leaving
a missing pattern.
5: Characterization of patterned defects on four different wafers
using resist B with TMAH-soluble topcoat TC-2.
other tests, immersion bubble defects had been abundant when a hydrophobic
solvent–soluble topcoat was used, but these defects were much
less frequent when the more-hydrophilic topcoat TC-2 was used. In the
test illustrated in Figure 5, no immersion bubbles were observed. Micrographs
of various defects are shown in Figure
6. EDX analysis of the particle defect in Figure 6a revealed carbon,
oxygen, and silicon, making it indistinguishable from the resist and
substrate. The missing-pattern defect in Figure 6b was formed when a
large particle masked the resist during exposure but was later removed
during the developing process. In some cases, particles are not removed
during development, as illustrated by the defect in Figure 6c. EDX analysis
of this defect revealed carbon, oxygen, silicon, and fluorine.
7: Depth of focus on a line-and-space feature processed using (a)
immersion scanner and (b) dry scanner.
Performance. The major advantage of immersion lithography over
dry lithography is that it improves the depth of focus. Figure 7 illustrates
the 2X improvement achieved in focus latitude for a 90-nm line-and-space
feature when a commercially available resist was used with TC-2 topcoat.
By verifying immersion performance, the study progressed toward developing
new resist polymer formulations for 90-nm metal-level applications.
An evaluation of resist performance was conducted using the seven polymers
and four solvents studied in the leaching tests. Bake conditions typical
for the polymers' activation levels were chosen, and the evaluation
criteria concentrated on mask error factor (MEF) performance for dense
and semi-isolated features and common process windows for dense features
and semi-isolated and isolated spaces and lines.
I shows the performance of the seven polymers with solvent system
C (PGMEA and GBL) compared with the commercially available resist G.
Figure 8 presents micrographs of features created using polymer A. Without
undergoing optimization, several of the experimental materials exhibited
excellent performance characteristics that were on a par with the commercially
available resist. With some fine-tuning, it is expected that the experimental
formulations will perform better than commercial resists formulated
for dry lithography.
8: SEM cross sections of features formed using polymer A, with various
pitches and CD targets.
performance was also evaluated using different solvent systems with
polymer F. The results, shown in Table II, indicate that while differences
in MEF and process window performance did arise, those differences were
difficult to distinguish from problems encountered during coating. When
solvents B and D were used, there were problems with severe pullback
on the antireflective coating, while with solvent C topcoat uniformity
was a serious issue. A detailed evaluation is needed to better understand
II: Comparison of the performance of polymer F with various cosolvents.
similar comparison focusing on MEF performance and process windows was
performed using a gate-level mask. Those results are presented in Table
III. As was the case in the evaluation of metal-level materials,
the bake processes were not optimized, leaving room for improvements.
In addition, photospeeds were slower than anticipated, which may be
improved with some formulation adjustments. Generally, MEF was higher
for the low-activation-level polymers than for those with high or hybrid
activation levels. In contrast to dry lithography, the process window
enhancement in immersion lithography was less substantial at the gate
level than at the metal level, with the formulation containing polymer
A being comparable to dry exposures on a 90-nm product.
lithography promises to achieve a substantial improvement in the depth
of focus. To move the technique into manufacturing, however, it will
be necessary to develop materials and processes that will not degrade
lens performance over time as a result of component extraction in the
immersion fluids. In the study reported in this article, historical
data were used to determine a specification for leaching components.
Then several commercially available resists and six experimental polymers
with various activation levels and cosolvents were evaluated in terms
of extractables. The results of that evaluation indicated that component
extraction was lowest for the polymers with high activation levels.
polymers' leaching performance was also evaluated when a topcoat was
applied to help contain the extractables within the film. Tests were
run using three TMAH-soluble topcoats, all of which were found to be
>90% efficient as leachant barriers.
the experimental topcoat TC-2 generated low levels of defects and did
not affect the resist profiles, it was used in subsequent evaluations
of several polymer platforms for metal- and gate-level applications.
Lithography characterization included MEF and process window analysis.
Test results revealed several promising candidate materials. Process
and formulation optimization are needed to enable these candidates to
outperform current dry lithography materials. Moving forward to the
45-nm node, the ultimate goal is a topcoat-free immersion process. Further
studies including the use of PAGs and additives will be needed to reach
that important milestone.
article is a revised version of a paper originally presented at the
SPIE International Symposium on Microlithography, held February 28–March
4, 2005, in San Jose. Used with permission. The authors would like to
thank Rex Chen, Wenjie Li, Ranee Kwong, Peggy Lawson, Rao Varanasi,
Chris Robinson, Steven Holmes, Dario Gil, and Kurt Kimmel of IBM. They
also wish to acknowledge Takashi Chiba and Tsutomu Shimokawa of JSR
Micro. Thanks also go to Carl Larson and Greg Wallraff for providing
the leaching apparatus and procedures and to Robin Keller for her SEM
support. The entire IBM Albany team also provided valuable process and
SEM support. In particular, the authors would like to thank Jon Orth,
Dan Kraft, Ed Couillard, Rich Conte, and Carol Boye. Finally, they gratefully
acknowledge the help of Mike Della Selva, John Weeks, Lior Huli, Darren
Brookhart, and Jerry Goldberg from the SUNY Albany facilities staff.
BJ Lin, "The Future of Subhalf-Micrometer Optical Lithography,"
Microelectronics Engineering 6, no. 31 (1989): 31–51.
The International Technology Roadmap for Semiconductors
(San Jose: Semiconductor Industry Association, 2004); available from
P Clark, "IBM Used Albany Immersion Tool to Make Power Processor,"
EE Times [on-line] (2 December 2004 [cited 1 June 2005]); available
from Internet: www.eet.com/article/showArticle.jhtml?articleId=54800014&sub_taxonomyID=.
"TSMC Achieves 90nm ICs Using Immersion Litho," Electronic
News [on-line] (22 December 2004 [cited 1 June 2005]); available
from Internet: www.reed-electronics.com/electronicnews/article/CA489760?spacedesc=latestNews.
W Hinsberg et al., "Liquid Immersion Lithography: Evaluation of
Resist Issues," in Proceedings of SPIE Microlithography
vol. 5376 (Bellingham, WA: SPIE, 2004), 21.
C Taylor et al., "Implications of Immersion Lithography on 193-nm
Photoresists," in Proceedings of SPIE Microlithography
vol. 5376 (Bellingham, WA: SPIE, 2004), 34.
RJ LeSuer et al., "Using Scanning Electrochemical Microscopy to
Probe Chemistry at the Solid-Liquid Interface in Chemically Amplified
Immersion Lithography," in Proceedings of SPIE Microlithography
vol. 5376 (Bellingham, WA: SPIE, 2004), 115.
S Robertson et al., "Characterization and Meaningful Quantification
of Resist Component Leaching into Immersion Fluid" (paper presented
at the International Symposium on Immersion and 157nm Lithography, Vancouver,
BC, Canada, August 2–5, 2004).
G Nellis et al., "Contamination Transport from Wafer to Lens"
(paper presented at the International Symposium on Immersion and 157nm
Lithography, Vancouver, BC, Canada, August 2–5, 2004).
S Kishimura, M Endo, and M Sasago, "Resist Interaction in 193-/157-nm
Immersion Lithography," in Proceedings of SPIE Microlithography
vol. 5376 (Bellingham, WA: SPIE, 2004), 44.
R Dammel et al., "The PAG Leaching Phenomenon in 193nm Immersion Lithography"
(paper presented at the International Symposium on Immersion and 157nm
Lithography, Vancouver, BC, Canada, August 2–5, 2004).
M Slezak, "45nm Node Materials Solutions and Progress" (paper presented
at the Anti-Reflective Coatings Symposium, Albany, New York, October
W Hinsberg et al., "Influence of Water Immersion on Properties of Lithographic
Materials" (paper presented at the International Symposium on Immersion
and 157nm Lithography, Vancouver, BC, Canada, August 2–5, 2004).
Petrillo is an IBM development engineer
in the lithography processes area working at Albany Nanotech in Albany,
NY. She has written numerous conference papers and holds 24 patents.
She received a BA from SUNY Oswego, NY, and an MS in quality assurance
from California State University at Dominguez Hills. (Petrillo can be
reached at 518/487-6455 or firstname.lastname@example.org.)
S. Patel, PhD, is a development engineer in the lithography
materials area at IBM in Hopewell Junction, NY. He has published several
papers for journals and conferences and holds one patent. He received
a BTech degree in chemical engineering from the Indian Institute of
Technology in Mumbai and a PhD in chemical engineering from the Georgia
Institute of Technology in Atlanta. (Patel can be reached at 845/894-3373
Slezak is a technical manager at JSR Microelectronics in Sunnyvale,
CA. He has published 10 papers and holds one patent. He received a BS
in engineering from the California Polytechnic State University in San Luis Obispo. (Slezak can be reached
at 408/543-8835 or email@example.com.)
Dabbagh is an applications engineer at JSR Micro. Previously,
he was a member of the technical staff at Bell Labs–Lucent Technologies.
He has published more than 70 technical papers and holds one patent.
He received a BA in chemistry from Hunter College in New York City and
an MS in organic chemistry from Rutgers University in New Brunswick,
NJ. (Dabbagh can be reached at 908/790-9327 or firstname.lastname@example.org.)
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