Novel electron-beam molecular resists with high resolution and high sensitivity for nanometer lithography

Article abstract of DOI:10.1246/cl.2004.706

A novel class of chemically-amplified, electron-beam molecular resists for nanometer lithography were created. These molecular resists functioned as positive resists in the presence of an acid generator, exhibiting a high sensitivity of ≈2 μC cm^-2

Full text of DOI:10.1246/cl.2004.706

706
Chemistry Letters Vol.33, No.6 (2004)
Novel Electron-Beam Molecular Resists with High Resolution and High Sensitivity
for Nanometer Lithography
Toshiaki Kadota, Hiroshi Kageyama, Fujio Wakaya,y Kenji Gamo,y and Yasuhiko Shirotaꢀ
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565-0871
yDepartment of Electronics and Materials Physics, School of Engineering Science, Osaka University, Toyonaka, Osaka 560-0043
(Received March 5, 2004; CL-040249)
A novel class of chemically-amplified, electron-beam mo-
lecular resists for nanometer lithography were created. These
molecular resists functioned as positive resists in the presence
of an acid generator, exhibiting a high sensitivity of ꢁ2 mC cmꢂ2
and enabling the fabrication of ꢁ25 nm line patterns.
Electronic circuits density in semiconductor devices has be-
come higher year by year, and the minimum feature size required
for device fabrication has become smaller. While 1 Gbit DRAMs
with a resolution of 130 nm have been produced at present, it is
expected that 4 Gbit DRAM chips with a resolution of 65 nm will
be fabricated in the near future. Both new lithographic processes
and new resist materials for future nanolithography are under in-
vestigation. Electron-beam lithography, which enables direct
writing of nanometer-scale high-resolution patterns without
masks, is expected to be a promising nanolithographic process.
Polymers or composite polymer materials have been used as
resist materials owing to their capability of uniform amorphous
film formation.1 Generally, low molecular-weight organic com-
pounds tend to readily crystallize and do not form uniform amor-
phous films. Since the late 1980s, we have performed a series of
studies on the creation of small organic molecules that readily
form stable amorphous glasses, which are referred to as amor-
phous molecular materials.2 Amorphous molecular materials,
which are characterized by well-defined glass-transition temper-
atures (Tgs) usually associated with polymers, readily form
smooth, uniform amorphous films by themselves by vapor dep-
osition or spin coating from solution. We have proposed a new
concept for resist materials, ‘‘molecular resists.’’3 Molecular re-
sists, namely, amorphous molecular materials with resist proper-
ties have advantages over polymer materials in their smaller
molecular size and the absence of any molecular-weight distri-
bution. On the basis of this new concept, we have created a
few electron-beam molecular resists, e.g., 1,3,5-tris[4-(4-tolu-
enesulfonyloxy)phenyl]benzene (TsOTPB) and 4,40,400-tris(al-
lylsuccinimido)triphenylamine (ASITPA). TsOTPB and ASIT-
PA permitted the fabrication of 150–70 nm line patterns on
p-BCOTPB and BCOQP were synthesized by the reaction of
pyrocarbonic acid di-tert-butyl dicarbonate with 1,3,5-tris(4-
hydroxyphenyl)benzene (HOTPB) and 50,5000-dihydroxyphenyl-
500-(4,400-dihydroxy-1,10:30,100-terphenyl-50-yl)-1,10:30,100:300,1000:
3000,10000-quinquephenyl (HOQP), respectively, in the presence of
K2CO3 and 18-crown-6 in tetrahydrofuran (THF). HOTPB and
HOQP were synthesized by the hydrolysis of the corresponding
methoxy-substituted compounds, 1,3,5-tris(4-methoxyphenyl)-
benzene (p-MeOTPB) and 50,5000-dimethoxyphenyl-500 -(4,400 -
dimethoxy-1,10:30,100 -terphenyl-50-yl)-1,10:30,100-300,1000:3000,10000-
quinquephenyl (MeOQP), with hydrobromic acid in acetic acid.
p-MeOTPB was prepared by the acid-catalyzed condensation re-
ation of 4-methoxyacetophenone in the presence of trifluorome-
thanesulfonic acid in toluene. MeOQP was synthesized by the
Suzuki coupling reaction of 1,3,5-tris(3,5-dibromophenyl)ben-
zene with 4-methoxyphenylbononic acid in the presence of tet-
rakis(triphenylphosphine)palladium(0) catalyst in THF/2N
K2CO3 (aq). These new compounds were identified by mass
1
spectrometry, FT-IR, H and 13C NMR spectroscopic methods,
and elemental analysis.4
While p-BCOTPB was obtained as polycrystals by recrys-
tallization from mixed solvents of THF/ethanol, BCOQP was
obtained as an amorphous glass despite attempted recrystalliza-
tion from solution. p-BCOTPB readily formed an amorphous
glass when the melt sample was cooled on standing in air. The
formation of the amorphous glasses was confirmed by differen-
tial scanning calorimetry (DSC), polarizing light microscopy
and X-ray diffraction. The Tgs of p-BCOTPB and BCOQP were
67 and 139 ꢃC, respectively, as determined by DSC.
p-BCOTPB and BCOQP readily formed uniform, amor-
phous films by spin coating from solution. The films of p-
BCOTPB and BCOQP containing a 5 wt % acid generator, di-
phenyliodonium triflate (DPI), were prepared by spin coating
from THF solution onto a silicon wafer. Thickness of the films
was ca. 0.2 mm. These spin-coated films were prebaked at 30–
55 ꢃC for 2 min for p-BCOTPB and at 60–100 ꢃC for 1 min for
BCOQP, and then irradiated with a focussed electron-beam by
the use of a scanning electron microscope (SEM) at 50 keV.
The irradiated p-BCOTPB film was developed with mixed sol-
exposure to 50-keV-electron beam; however, the sensitivity of
these molecular resists were in the range of mCcmꢂ2
3
.
We report here the creation of a novel class of electron-
beam molecular resists with high sensitivity and high resolution
and show that molecular resists can be promising candidates for
future nanolithography. The created molecular resists are chemi-
cally amplified, positive-type molecular resists, 1,3,5-tris(4-tert-
butoxycarbonyloxyphenyl)benzene (p-BCOTPB) and 50,5000-bis-
(tert-butoxycarbonyloxy)phenyl-500-[4,400-bis(tert-butoxycarbon-
yloxy)-1,10:30,100-terphenyl-50-yl]-1,10:30,100:300,1000:3000,10000-quin-
quephenyl (BCOQP).
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.6 (2004)
707
vents of tetramethylammonium hydroxide (TMAH):isopropyl
alcohol (IPA) = 30:7 (v/v) for 120 s, rinsed with deionized wa-
ter, and air-dried. The irradiated film of BCOQP was post-baked
at 100–120 ꢃC for 0–120 s, and then developed with mixed sol-
vents of TMAH:IPA = 30:6 (v/v) for 60 s, rinsed with deionized
water, and air-dried.
The synthesized materials functioned as positive-type re-
sists. That is, the exposed area of the resist films became soluble
in the developer solvent described above. It is thought that the
solubility change caused by electron-beam irradiation in the
presence of DPI resulted from the transformation of the tert-bu-
toxycarbonyloxy group in p-BCOTPB and BCOQP to the hy-
droxy group. The resulting hydroxy-substituted compounds are
soluble in the developer solvent. The transformation of the
tert-butoxycarbonyloxy group to the OH group in p-BCOTPB
was confirmed by FT-IR spectroscopy.
The p-BCOTPB resist film exhibited a sensitivity of
4.4 mCcmꢂ2 on exposure to 50-keV-electron beam. BCOQP with
a higher Tg than that of p-BCOTPB permitted post-exposure
bake at 100 ꢃC, and showed higher sensitivity and higher resolu-
tion than p-BCOTPB. While the sensitivity without any post-ex-
posure bake process was ca. 10 mC cmꢂ2, the sensitivity increas-
ed with the increasing post-exposure bake time, giving
2 mC cmꢂ2 after the bake treatment for 120 s, as shown in
Figure 1. It has been reported that the post-exposure bake proc-
ess usually enhances sensitivity, since the diffusion of Brꢀnsted
acid generated by the chemical transformation of the tert-butox-
ycarbonyloxy group to the hydroxy group, which is accompa-
nied by the formation of isobutene, carbon dioxide, and proton,
is thermally assisted.5 The sensitivities obtained for the present
chemically-amplified molecular resists, p-BCOTPB and
BCOQP, well meet the requirements of practical use as elec-
tron-beam resist materials.
Figure 2. SEM image of a positive tone line pattern obtained
for BCOQP resist film containing 5 wt % of DPI on exposure
to a 50-keV-electron beam at 66 mCcmꢂ2. Post-exposure bake
treatment was carried out at 100 ꢃC for 90 s.
electron-beam molecular resists for nanometer lithography,
p-BCOTPB, and BCOQP, were designed and synthesized. They
were found to readily form stable amorphous glasses with well-
defined Tgs and to form uniform amorphous films by themselves
by spin coating from solution. They functioned as positive-type
electron-beam resists in the presence of an acid generator such as
DPI. The BCOQP molecular resist was found to exhibit a high
sensitivity of 2 mC cmꢂ2, enabling the fabrication of 25 nm line
patterns on exposure to 50-keV-electron beam. The present
study shows a new paradigm for resist materials for future nano-
lithography, paving the way for further development of molecu-
lar resists.
References and Notes
1
a) E. Reichmanis, O. Nalamasu, and F. M. Houlihan,
Macromol. Symp., 175, 185 (2001). b) K. Arimitsu, K. Kudo,
H. Ohmori, and K. Ichimura, J. Mater. Chem., 11, 295
(2001). c) H. Okamura, S. Toda, M. Tsunooka, and M.
Shirai, J. Polym. Sci., Part A: Polym. Chem., 40, 3055
(2002). d) M. Shirai, A. Kawaue, H. Okamura, and M.
Tsunooka, Chem. Mater., 15, 4075 (2003).
Figure 2 shows the SEM image of a positive-tone line pat-
tern obtained for the BCOQP resist film containing DPI as an
acid generator on exposure to a 50-keV-electron beam. p-
BCOTPB and BCOQP permitted the fabrication of line patterns
of 40 and 25 nm, respectively. These sensitivity and resolution
results show that molecular resists can be promising candidates
for electron-beam resist materials for future nanolithography.
In summary, a novel class of chemically amplified,
2
3
4
Y. Shirota, J. Mater. Chem., 10, 1 (2000) and references
cited therein.
M. Yoshiiwa, H. Kageyama, Y. Shirota, F. Wakaya, K.
Gamo, and M. Takai, Appl. Phys. Lett., 69, 2605 (1996).
p-BCOTPB: Yield: 18%. mp: 100 ꢃC. Mass (FAB): m=z 655
1
(MHþ). H NMR (600 MHz, CDCl3, 25 ꢃC): ꢀ 7.70 (3H, s),
7.66 (6H, d), 7.28 (6H, d), 1.59 (27H, s). 13C NMR
(150 MHz, CDCl3, 25 ꢃC): ꢀ 151.9, 150.8, 141.6, 138.6,
128.3, 125.1, 121.7, 83.7, 27.7. FT-IR (KBr, cmꢂ1): 3033,
2981, 2934, 1759, 1606, 1509, 1371, 1279, 1221, 1147,
1014, 885, 834. Anal. Calcd for C39H42O9: C, 71.54; H,
6.47; O, 21.99. Found: C, 71.52; H, 6.47%. BCOQP: Yield:
54%. Mass (FAB): m=z 1460 (MHþ) 1H NMR (600 MHz,
CDCl3, 25 ꢃC): ꢀ 7.95 (3H, s), 7.85 (6H, d), 7.76 (3H, t),
7.70 (12H, d), 7.28 (12H, d), 1.58 (54H, s). 13C NMR
(150 MHz, CDCl3, 25 ꢃC): ꢀ 152.3, 151.3, 143.1, 142.7,
142.3, 139.1, 128.9, 128.8, 126.4, 126.0, 125.4, 122.2,
84.2, 28.2. FT-IR (KBr, cmꢂ1): 3033, 2980, 2931, 1760,
1592, 1510, 1371, 1279, 1224, 1149, 1016, 891, 836. Anal.
Calcd for C90H90O18: C, 74.06; H, 6.21; O, 19.73. Found:
C, 73.96; H, 6.33%.
Figure 1. Sensitivity curves of BCOQP resist films containing
5 wt % of DPI. (a) without the post-exposure bake process. (b)
with the post-exposure bake process at 100 ꢃC for 120 s.
´
C. G. Willson, H. Ito, J. M. J. Frechet, T. G. Tessier, and
F. M. Houlihan, J. Electrochem. Soc., 133, 181 (1986).
5
Published on the web (Advance View) May 17, 2004; DOI 10.1246/cl.2004.706