Improvement of Post-Exposure Delay Stability in Alicyclic
ArF Excimer Photoresists
Mikio Yamachikaa,, Kyle Pattersona, Sungseo Choa, Timo Ragera, Shintaro Yamadaa,
Jeffrey Byersb, P. J. Paniezc, B. Mortinid, S. Gallyc, P-O. Sassoulasc, C. Grant Willsona
a
Department of Chemistry, The University of Texas at Austin, Austin, TX 78712
b
SEMATECH, 2706 Montopolis Drive, Austin TX 78741-6499
c
France Telecom, CNET-Grenoble, DTM/TFM, BP 98, F-38243, Meylan Cedex, France
d
STMicroelectronics, 850 rue J. Monnet, F-38920, Crolles Cedex, France
This paper reports work toward designing environmentally stable alicyclic polymerbased photoresists for ArF excimer laser lithography. A design concept for improving postexposure delay stability is suggested in this paper. The polymers described here were
prepared from dinorbornene derivatives and maleic anhydride by free radical
polymerization. The relationship among polymer structure, glass transition temperature (Tg)
and post-exposure delay stability was studied. This new polymer design offers a pathway
towards good post-exposure delay stability while maintaining high resolution.
Keyword: norbornene, chemically amplified resist, cycloolefin, 193nm photoresist
1. Introduction
Microlithography at 193 nm is one approach to
extending optical lithography to sub 0.15 m
design rules. Photoresist materials for 193 nm
lithography must fulfill the usual resist
requirements such as high transparency at exposure
wavelength and good resistance to dry etching
processes. The relationship between the etch
resistance of a polymer and the ratio of elements in
its monomers is empirically described by the
Ohnishi parameter[1].
Conventional resist materials for i-line and 248
nm lithography incorporate aromatic groups, which
have relatively low Ohnishi parameters and provide
good dry etch resistance. Unfortunately, the optical
density of these materials is too high to be suitable
for use at 193 nm. Therefore, the design of suitable
photoresist materials for 193 nm lithography has
been a significant challenge.
Polymers
incorporating alicyclic norbornene derivatives have
shown great promise as candidates for 193 nm
microlithographic application[2-14]. The alicyclic
system has good transparency at 193 nm and useful
etch resistance. In 1993, our group began studies
on norbornene-derived amorphous polyolefins for
use in 193 nm photoresist formulations. The first
public description of this work was in 1996[3].
Since that time, a variety of photoresist
formulations
based on substituted alicyclic
polymers have been reported. Resists based on
polymers
prepared
by
free-radical
polymerization[3-11],
cationic
addition
polymerization[3,6,12,13], and ring opening
metathesis polymerization (ROMP)[6,12,14] have
all been presented by our group and by others.
One of the more promising materials that has
emerged from our work is the alternating
copolymer, poly(DBNC-alt-MA), of t-butyl
tetracyclo[4.4.0.12,5.17,10]
dodec-3-ene-8carboxylate (DNBC) and maleic anhydride (MA).
This polymer can be easily prepared by free radical
polymerization, Figure 1.
O
O
V601
+
O
O
O
X
p-Dioxane
O
O
O
O
O
Figure 1: Synthesis of alternating copolymer, DNBCalt-MA
Photoresist formulations based on this polymer
have demonstrated good lithographic performance
and acceptable dry etch resistance. Previous
reports have shown how the performance of this
material can be improved in several areas[8], but
post-exposure delay stability has continued to be a
problem.
These photoresists show two different modes of
failure as a result of post-exposure delay. First, we
observe an increase in linewidth with postexposure delay and the formation of “t-topping”
and skin. We believe that diffusion of the
photogenerated acid in the bulk photoresist during
the delay period leads to this variation of the final
developed linewidth, and that contamination by
airborne bases results in decreased efficiency of
the photogenerated acid. However, this only
affects a thin surface layer and therefore results in
“t-topping” and skin. In order to improve the postexposure delay stability of these resists, new
polymers have been designed which address both
of these manifestations of post-exposure delay.
A relatively large amount of free volume is
trapped within a photoresist film during the spincoating process as solvent molecules quickly
evaporate. Trapped free volume allows increased
diffusion of small molecules and leads to decreased
post-exposure delay stability.
Annealing a
photoresist by heating during the post-application
bake step to near its glass-transition temperature
(Tg) allows the polymer molecules to relax to a
more thermodynamically stable state, removing
much of the trapped free volume. However,
poly(DBNC-alt-MA) cannot be annealed without
decomposition because the Tg exceeds the thermal
stability of the t-butyl ester protecting groups. We
therefore designed a variety of analogues in an
attempt to adjust the Tg. Copolymers of the new
24O series shown in Figure 2 incorporate spacer
groups between the acid-labile protecting group
and dinorbornene units that lower their Tg allowing
annealing of the resist film during the postapplication bake step.
Our approach to addressing the “t-top”
formation is exactly that which has proven so
successful in the design of 248 nm photoresists.
We simply increased the background dissolution
rate by introducing an appropriate level of
unblocked carboxylic acid comonomers.
O
O
O
x
O
O
CH2
n
O
O
Figure 2: New annealable 24O series copolymer design
2. Experimental
2.1 Preparation of materials
2.1.1 tert-butyl -bromoalkylcarboxylates
A series of tert-butyl esters of -bromo
carboxylic acids, with alkyl chains of various
lengths, were obtained according to the synthetic
scheme shown in Figure 3[15].
OH
Br(CH2)n
TFAA
O
Figure 3: Synthesis
alkylcarboxylates
TBA
O
Br(CH2)n
O
THF
of
tert-butyl
-bromo-
-Bromoalkyl carboxylic acids, trifluoroacetic
anhydride (TFAA), and tert-butyl alcohol (TBA, 2methyl-2-propanol HPLC grade) were purchased
from Aldrich Co. and used without further
purification.
A solution of -bromoalkyl
carboxylic acid in dry THF was cooled to 0 oC
under a nitrogen atmosphere, and 1.5 equivalents of
TFAA was added dropwise. The reaction mixture
was then stirred for 1 hr maintaining a temperature
below 15 oC. The reaction mixture was recooled to
0 oC, and 3 equivalents of TBA were added
dropwise, followed by stirring overnight at room
temperature under a nitrogen atmosphere. The
reaction mixture was diluted with diethyl ether and
washed with 3M NaOH, brine, and deionized
water. The organic phase was then dried over
MgSO4, and the solvents removed under reduced
pressure to give the corresponding esters as clear or
slightly yellow liquids which were used without
further purification.
2.1.2 Tetracyclo[4,4,0.12,517,10] dodec-3-ene-8carboxylate derivatives
The dinorbornene monomers were obtained by
esterification of tetracyclo[4,4,0.12,517,10] dodec-3ene-8-carboxylic acid with the tert-butyl bromoalkyl carboxylates according to the scheme
shown in Figure 4.
O
+
DBU
O
THF
Br(CH2)n
(CH2)n
O
OH
O
O
O
O
Figure 4: Synthesis of tetracyclo[4,4,0.12,517,10] dodec3-ene-8-carboxylate derivatives
The reagent, 1,8-diazabicyclo [5.4.0]undec-7ene (DBU), was purchased from Aldrich Co. and
tetracyclo[4,4,0.12,5.17,10] dodec-3-ene-8-carboxylic
acid (DNCA) was obtained by the hydrolysis of
methyl
tetracyclo[4,4,0.12,5.17,10]dodec-3-ene-8carboxylate, which was obtained from JSR
Corporation. The esters were prepared by adding
an excess of DBU dropwise to a solution of DNCA
in dry THF under a nitrogen atmosphere at room
temperature.
The tert-butyl -bromoalkyl
carboxylate was added to the solution resulting in
the formation of white crystals. The resulting
mixture was stirred at room temperature under a
nitrogen atmosphere overnight, then filtered and
diluted with diethyl ether. The filtrate was washed
with 3% HCl, 10% K2CO3, and deinonized water.
After drying over MgSO4, the solvent was removed
under reduced pressure to give esters as clear,
viscous oils which were used without further
purification.
2.1.3 Alternating copolymers and terpolymers
The alternating polymers were prepared by
radical
polymerization.
The
dinorbornene
monomer(s) and maleic anhydride were dissolved
in dry p-dioxane with dimethyl 2,2’azobisisobutyrate as a radical initiator (V601,
Wako chemicals). Polymerization was carried out
at 75 oC for 8 hrs under a nitrogen atmosphere.
After the reaction was complete, an extra portion of
p-dioxane was added to the polymer solution, and
slowly added with stirring to an excess of cold
hexane/IPA (50/50 by weight). The resulting
precipitate was isolated by filtration, rinsed with
the cold mixed solvent, air-dried, precipitated again
under the same conditions, and vacuum dried at 50
o
C overnight. The overall conversion ranged from
34-65%.
2.1.4 Triphenylsulfonium perfluorobutanesulfonate
(TPS-Nf)
The
triphenylsulfonium
perfluorobutanesulfonate was prepared by a metathesis. Equimolar
amounts of sodium perfluorobutanesulfonate and
triphenylsulfonium bromide were partitioned
between chloroform and deionized water by
vigorously stirring the two-phase mixture for three
hours. The organic phase was then seperated, dried
over MgSO4, and the solvent was removed under
reduced pressure to give triphenylsulfonium
perfluorobutanesulfate as a oily residue.
Exhaustive removal of the solvent gives the desired
product as a white solid.
2.2 Measurements
The monomers were characterized by NMR
(General Electric, QE300) and IR spectra (Nicolet,
Magna IR). Molecular weights were determined
by gel permeation chromatography equipped with a
refractive index detector (polystyrene standard,
Viscotek). The glass transition temperature (Tg) of
each alternating copolymer was measured by
differential scanning calorimetery (Perkin Elmer,
DSC7) (scan range=50-220 oC, scan rate=20
°C/min).
2.3 Resist Processing
2.3.1 Imaging Experiments
The photoresist solutions were coated over
52nm of DUV-30 ARC from Brewer Science.
After spin-coating, the wafers were baked on a
hotplate at 140 or 150 oC for 90 seconds. The
resist films were exposed using an ISI 193nm
Microstepper (0.6 NA, 0.7 , and then baked at
140 or 150 oC for 90 sec. Development was
performed using 2.38% tetramethylammonium
hydroxide (TMAH) aqueous solution (Shipley CD26) for 60 seconds in puddle mode.
250
225
200
Tg / degree
2.3.2 Etching Experiments
Blanket etch rates were determined by
measuring thickness loss observed during a fixed
etch time in a LAM TCP 9400 prototype etcher
with 300W chuck power and a gas mixture
consisting of 150 sccm of HBr and 50 sccm of Cl2.
The etch time was held constant because we have
shown[7,8] that etch rates for polymers of this type
are subject to large errors when determined by a
single thickness measurement.
175
150
125
100
75
50
0
1
2
3
4
5
6
7
8
9
10
11
Chain length / n
3. Results and Discussion
3.1 Annealing Polymer Design
The annealable copolymers, represented by the
structural formula shown in Figure 2, contain
higher amounts of oxygen in their empirical
formula. It was therefore important to determine
the extent to which we had traded reduction of the
Tg for dry etch resistance. Table 1 summarizes the
characterization results of etching experiments on
the 24O series copolymers. The etch rate of these
new alternating copolymers are shown, normalized
to that of the standard, poly(DBNC-alt-MA). We
were pleased to see that all of the 24O series
copolymers show an etch resistance very similar to
that of poly(DBNC-alt-MA). There appears to be a
slight dependence on the spacer chain length but
the differences are so small that a statistical study
would be required to clearly establish that
relationship.
Figure 5: Tg versus spacer chain length
Table 1: Characterization
polymers
3.2 Imaging Experiments
Photoresist formulations for the initial imaging
studies were prepared exactly like the standard
formulation developed for poly(DBNC-alt-MA)
photoresists[7,8] which consists of the polymer(s),
triphenylsulfonium nonaflate as the photoacid
generator, and 1-piperidine ethanol as a base
quencher. Both the post-application and postexposure bakes were fixed at 140 oC for 90
seconds. Figure 6 shows images printed using
different effective spacer chain lengths for the 24O
series polymers. When n=3, an insoluble surface
layer was observed that was not contamination
related, but was rather the result of excessive
hydophobicity.
Polymer
24O-1
24O-3
24O-5
24O-10
DNBC-altMA
Mw
3790
3340
4130
4380
4120
results
Pd
1.52
1.47
1.47
1.49
1.49
for
24O
Tg(oC)
190
125
110
100
>250*)
series
Etch
rate
1.01
1.06
1.10
1.09
1.00
24O-n, n: Spacer chain length
*)
Tg is higher than decomposition temperature of the tbutyl ester functionality
A plot of Tg vs. chain length data from Table 1
is shown in Figure 5.
The Tg of the polymer decreases with
increasing spacer chain length, but the influence of
chain length plateaus when the chain length
exceeds about n=5. Because of the bake conditions
required to process resists of this type, we did not
explore copolymers with a Tg below 125 °C,
corresponding to a chain length of n = 3. Our goal
was to produce copolymers with a Tg of
approximately 140 C.
Unfortunately, that
temperature corresponds to a chain length of
approximately 2.5. We therefore blended these
copolymers to precisely control the Tg.
Terpolymers were also prepared from a mixture of
appropriate 24O monomers with maleic anhydride.
Both methods allowed us to effectively prepare
polymers “non-integral” chain lengths.
Figure 6 also shows images obtained with
blends of copolymers 24O-1 and 24O-3 that
provided non-integral effective chain lengths. As
the effective chain length is reduced, the problem
of hydrophobicity is improved but the Tg begins to
exceed the annealing range, so there is a narrow
design window. The best images were obtained
with a 1:2 blend ratio of 24O-3:24O-1 copolymers,
giving an effective chain length of n=1.67 and a Tg
of 156 °C. A terpolymer sample with the same
effective chain length was synthesized by using a
feed ratio of 2 : 1 : 3 of 24O-3 : 24O-1 : MA. The
resulting terpolymer has the same Tg as the blend.
24O-3 (n=3)
Tg = 125 °C
24O-3 : 24O-1 = 1 : 2
Effective n = 1.67
Tg = 156 °C
24O-3 : 24O-1 = 2 : 1
Effective n = 2.33
Tg = 140 °C
240-1 (n=1)
Tg = 190 °C
Figure 6: Effect of chain length, n, on the imaging
quality of 24O series copolymers
On the basis of this data, a series of experiments
were performed to optimize the resist formulations
based on these new polymers. Several basic
additives were auditioned as possible replacements
for 1-piperidineethanol. Since this particular base
was chosen for use with the standard poly(DBNCalt-MA), it is possible that performance of the new
24O series polymers could be improved with a
different base. Figure 7 shows the structure of the
various bases tested. Although poly(DBNC-altMA) performs best with polar, cyclic amines such
as 1-piperidineethanol, photoresists based on these
new 24O-series copolymers perform best in
conjunction with non-polar, trialkyl amines. For
the remainder of imaging studies with these new
polymers, 1-piperidineethanol was replaced with
trioctylamine.
O
R
N
R2
N
R
N
R
R
Figure 7: Structures of amines tested as possible
improved basic additives
R1
3.3 Post-exposure Delay Stability
Having found a formulation based on
annealable polymers that printed about as well as
the standard formulation, we were now in a
position to address the second mode of postexposure delay failure. To that end, we explored
the effect of adding small amounts of free acid to
achieve a finite background dissolution rate. We
knew from prior work that incorporating free acid
into a polymer results in an increase in Tg. We
therefore used the 24O-3 monomer alone to
counteract the influence of the acid. Table 2 shows
that incorporation of the 7-10% of free acid
required to achieve a finite background dissolution
rate gives a Tg of 155-158 °C, almost identical to
that of the terpolymer or blend with n=1.67.
Table 2: Tg and thickness loss for acid containing
terpolymers. (thickness loss measured after 60 seconds
develop time.)
Acid feed
(%)
0
5
7.5
10
15
Thickness loss
(nm)
0.4
2.9
5.7
11.6
100.0
Tg (oC)
127
155
158
158
175
We examined the post-exposure delay stability
of all of these materials. The measurements were
made in comparison to a standard UT formulation
based on poly(DBNC-alt-MA). Each wafer was
coated, baked, and exposed without delay, followed
by storage in open cassettes. The measured NH3
concentration in the facility was ~4ppb. Both the
post-application and post-exposure bakes were
fixed at 150 oC for 90 seconds, a temperature close
to the Tg of the polymers with effective n=1.67.
Figure 8 shows the results of these studies. Note
that the standard formulation shows t-topping after
only 10 minutes, and by 40 minutes, the linewidth
has also increased dramatically. The annealable
0 delay
10 minute delay
formulations, both the blend and the terpolymer,
show little improvement in t-topping, but suffer
minimal linewidth change even after 40 minutes of
delay. As we had hoped, incorporation of a
controlled amount of background dissolution
removes the t-topping and gives excellent image
quality with storage stability in excess of 40
minutes. Delay times as high as 100 minutes are
necessary before "t-top" formation becomes
evident with formulations that incorporate 10 %
DNCA.
20 minute delay
40 minute delay
100 minute delay
I
II
III
IV
V
Figure 8: Post-exposure delay test results showing 160 nm images with a 1:1 pitch. (I)Standard formulation of
poly(DBNC-alt-MA). (II)Blend polymer of 24O-1 and 24O-3 with effective n=1.67. (III)Terpolymer of 24O-1, 24O-3,
and MA with effective n=1.67. (IV)Terpolymer of 24O-3, DNCA, and MA, acid feed concentration=7.5%.
(V)Terpolymer of 24O-3, DNCA, and MA, acid feed concentration=10%.
4. Conclusions
It is not possible or appropriate to state that we
have a full molecular-level understanding of the
processes responsible for t-topping and linewidth
change during post-exposure storage of chemicallyamplified photoresists. However, assuming that
the linewidth change is largely the result of acid
diffusion and that t-top formation is largely the
result of gettering of contaminants from the
atmosphere has allowed us to rationally modify the
design of our 193 nm photoresists to improve
storage stability. Reducing the glass-transition
temperature and adding a controlled amount of
background dissolution have allowed us to extend
the storage stability to over one hour in a typical
cleanroom environment.
5. Acknowledgements
We are indebted to both Sematech (Contract
#36041900) and SRC (Contract #96-LP-409) for
their generous support of this project. We also
would like to thank JSR Corporation for their
support of Mikio Yamachika and for generous
supply of monomer precursors.
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