Hydroxyphenylbenzene derivatives as glass forming molecules for high resolution photoresists

Article abstract of DOI:10.1039/b719108a

This paper presents a fundamental study to relate molecular architecture to the amorphous properties of hydroxyphenylbenzene molecular glass derivatives. These studies demonstrates that the branched hydroxyphenylbenzene derivatives can form stable molecular glasses despite the π-π interactions present. Through this research, design concepts to stabilize the amorphous nature and to increase the glass transition temperature (T_g) based on asymmetric molecular structures, bulky substituents, conformation states and molecular size are explored. An understanding of such behavior is important for the development of hydroxyphenylbenzene derivatives as potential molecular photoresists for patterning below 50 nm. This research has helped to identify potential candidates to be evaluated as positive-tone photoresists for lithographic applications for extreme ultraviolet (EUV, λ = 13.4 nm) lithography. The first reported sub-30 nm feature sizes for hydroxyphenylbenzene derivatives were demonstrated with high T_g materials. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719108a

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Hydroxyphenylbenzene derivatives as glass forming molecules for high
resolution photoresists†
Anuja De Silva and Christopher K. Ober*b
a
Received 11th December 2007, Accepted 6th February 2008
First published as an Advance Article on the web 6th March 2008
DOI: 10.1039/b719108a
This paper presents a fundamental study to relate molecular architecture to the amorphous properties
of hydroxyphenylbenzene molecular glass derivatives. These studies demonstrates that the branched
hydroxyphenylbenzene derivatives can form stable molecular glasses despite the p–p interactions
present. Through this research, design concepts to stabilize the amorphous nature and to increase the
g
glass transition temperature (T ) based on asymmetric molecular structures, bulky substituents,
conformation states and molecular size are explored. An understanding of such behavior is important
for the development of hydroxyphenylbenzene derivatives as potential molecular photoresists for
patterning below 50 nm. This research has helped to identify potential candidates to be evaluated as
positive-tone photoresists for lithographic applications for extreme ultraviolet (EUV, l ¼ 13.4 nm)
lithography. The first reported sub-30 nm feature sizes for hydroxyphenylbenzene derivatives were
g
demonstrated with high T materials.
Introduction
photoresists as it provides thermal energy needed to undergo
acid catalyzed deprotection for positive tone or crosslinking
for negative tone resists. For successful imaging, this thermal
Amorphous molecular materials constitute a new class of func-
tional materials being developed for electronic applications.
Currently, there is a considerable interest in their use as either
process must take place below the material’s T
the PEB at a temperature above the T of the material can
have adverse effects in pattern distortion, acid diffusion, and
g
. Performing
g
1
charge transporting or photoresist materials. These molecules,
known as molecular glasses (MGs), can be designed to combine
beneficial properties of both polymers and small molecules.
These molecules can be characterized by the presence of signifi-
cant free volume and by the disorder in both intermolecular
8,9
limiting resolution.
Key photoresist properties such as
sensitivity, resolution, and PEB process range depend on Tg
and its proximity to PEB temperature. T is also a crucial factor
g
in amorphous charge transport materials, as it has been reported
2
distance and orientation. Like polymers, MGs can show high
g
that high T has been directly correlated to high electrolumines-
10
cence stability.
glass transition temperatures and be processed into stable thin
films. However, these molecules have a well-defined structure
that contributes to isotropic properties and can be purified using
standard chromatographic techniques. The use of MGs can
eliminate issues such as grain boundary effects, ill defined
structure and chain ends, and chain entanglement created by
With recent advances in the semiconductor manufacturing
field, particular attention has focused on the development of
11–14
MG photoresists (or resists).
The basic building block in
a MG resist may be monodisperse and as much as an order of
magnitude smaller in size compared to conventional polymeric
resists. The reduction in this ‘‘pixel’’ size is believed to be
a fundamental improvement in the ability to consistently obtain
high-resolution patterns. Several MG photoresists have been
3–7
using polymers in electronic applications.
A major challenge in developing MG materials is that there
are only limited types of materials that form stable amorphous
glasses above room temperature as most small molecules have
a marked tendency to crystallize. In order to realize the potential
of MGs in electronic applications, they need to demonstrate low
designed that are able to attain sub-50 nm resolution with
15–18
improved resist performance.
The development of MG
photoresists has also been supported by theoretical work that
shows how photoresist performance improves with the reduction
of molecular weight of the resist, for both classical and
minimum crystal growth velocity and T s significantly higher
g
g
than room temperature. High T is an important photoresist
property as it determines the ability to form stable thin films
and undergo essential processing steps such as post-exposure
bake (PEB), base development and etching. The PEB step is
especially important for processing of chemically amplified
19–21
chemically amplified photoresists.
Molecular topology is a dominant factor in the glass forming
4
ability of molecular systems. Common glass forming shapes
22–31
32–34
include branched or star shapes,
39–41
spiro links,
tetrahe-
35–38
dral
and twin
molecular structures. Many of the earliest
a
Department of Chemistry and Chemical Biology, Cornell University,
Ithaca, NY, 14850, USA
glass forming systems were based on branched phenylbenzene
42
systems such as 1,3,5-(a-naphthyl)benzene, or 1,3,5-triaryl-2-
Branched phenylbenzene derivatives have
b
Department of Materials Science and Engineering, Cornell University,
Ithaca, NY, 14850, USA. E-mail: cober@ccmr.cornell.edu
43
pyrazolines.
a tendency to form amorphous materials due to their bulky,
† Electronic supplementary information (ESI) available: TGA and DSC
data for HHPB-mp derivatives. See DOI: 10.1039/b719108a
44–46
dense architecture.
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The first MG resist developed by Shirota et al. was also based
commercially available PAG, triphenylsulfonium perfluoro-1-
butanesulfonate (5% with respect to resist) and trioctylamine
(0.3 wt% with respect to resist) was added and the solution
was filtered through a 0.2 mm membrane filter. Then the
solutions were spin coated onto a 4 inch HMDS primed silicon
wafer (2000 rpm, 30 s) leading to approximately 100 nm thick
films. This was subjected to a post-application bake (PAB) at
on a branched tri(hydroxy-phenyl)benzene (TriHPB) deriva-
12
tive. Triphenylbenzene, which is a common structural unit for
charge transporting materials, is also a suitable core for MG
resists due to its non-planar, bulky architecture and high thermal
stability. Upon protecting each hydroxy group with an acid labile
tert-butoxycarbonyl (t-BOC) unit, this material can be patterned
in the presence of a photoacid generator. Though electron beam
ꢀ
125 C for 30 s and then exposed using EUV radiation at
patterning showed the promise of these materials, their low T
g
Sematech North, Albany and Lawrence Berkeley National
Laboratory (LBNL). After exposure, the wafer was baked at
was a limiting factor to patterning performance. Recent work
by Ober and coworkers has emphasized the importance of high
Tg in MGs with the ability to perform high-resolution
ꢀ
ꢀ
80 C or 90 C for 30 s and then developed in an aqueous
solution of AZ 300 MIF developer (0.262 N TMAH) for 60s.
15,16,47
patterning.
Hence, the T of TriHPB derivatives need to
g
be sufficiently increased in order to obtain sub-50 nm pattern
feature sizes. This paper focuses on a fundamental study of
Metrology
Scanning electron micrographs (SEMs) were taken on Zeiss
Ultra 55 scanning electron microscope.
g
structural concepts to increase T in phenylbenzene derivatives,
which are versatile materials for electronic applications. Several
TriHPB based cores were synthesized to evaluate the structural
effects of isomeric states, substituents, conformational states,
intermolecular bonding and molecular size. Due to the homoge-
neous nature of MGs, precise structure–property correlations
can be made between molecular structure and thermal properties.
All tri(hydroxyphenyl)benzene (TriHPB) and tetra(hydrox-
yphneyl)benzene (TetHPB) compounds were synthesized using
a similar Suzuki coupling procedure with their respective aryl
48
bromides and phenylboronic acids. A representative example
is described for TriHPB-methoxy-mp.
1
,3,5-Tribromobenzene (1.5 g, 4.76 mmol), 4-methoxyphenyl-
boronic acid (1.45 g, 9.53 mmol) and 3-methoxyphenylboronic
acid (1.45 g, 9.53 mmol) were dissolved in degassed THF
Experimental
(
75 ml). Degassed 2 M aqueous K
the solution and then tetrakis(triphenylphosphine)palladium(0)
10 mol%) was added. The mixture was heated at reflux for
8 h with vigorous stirring under nitrogen. After cooling to
2 3
CO (70 ml) was added to
Materials
Most chemicals were purchased from Sigma-Aldrich unless
otherwise stated and used without further purification. Phenyl-
boronic acid derivatives were purchased from TCI America
and used as obtained. Anhydrous or HPLC grade solvents
were used unless otherwise stated. Commercially available AZ
(
4
room temperature, the layers were separated, and the aqueous
layer was then washed twice with ethyl acetate. The combined
4
organic layer was dried over anhydrous MgSO and filtered.
3
00 MIF was employed as a standard developer (0.26 N tetrame-
The solvent was removed in a rotary evaporator, and the crude
product was purified by column chromatography (silica gel)
using CH Cl to yield 1.3 g (70%) of viscous yellow solid.
thylammonium hydroxide in water, TMAH).
2
2
Characterization
Deprotection of TriHPB-methoxy-mp
1
H NMR spectra were recorded on a Varian Inova-300 (300
MHz) spectrometer at room temperature, using the chemical shift
To a solution of TriHPB-methoxy-mp (1 g, 2.5 mmol) in CH Cl
2
2
of a residual protic solvent (CHCl at d 7.28 ppm, acetone at
3
3 2 2
(50 mL), BBr (8.3 mL of 1 M CH Cl solution, 1.1 eq with
2
.05 ppm, or DMSO at d 2.50 ppm) as an internal reference. All
shifts are quoted in parts per million (ppm) relative to CHCl
respect to -OCH3 of TriHPB-mp) was added dropwise at
ꢀ
3
,
ꢁ78 C. The mixture was allowed to reach room temperature
acetone or DMSO. The multiplicity of the signal is indicated as
follows: s (singlet), d (doublet), t (triplet), q (quartet), m (multi-
plet), and br s (broad singlet). FT-IR spectra were recorded on
a Mattson Instruments Galaxy 2020 FT-IR spectrometer.
Thermogravimetric analysis (TGA) was performed on a TA
and stirred for 12 h. The reaction mixture was poured into 2 N
NaOH (50 mL) and extracted with CH Cl . The aqueous layer
2
2
obtained and was acidified with 5 N HCl and extracted with
ethyl acetate. The organic layer was dried with MgSO4 and
evaporated to give a light brown solid.
ꢀ
ꢁ1
Instruments Q500 at a heating rate of 10 C min under N2.
The glass transition temperatures (T ) of resist materials were
g
t-BOC protection of hydroxyphenylbenzene MGs
measured on a TA Instruments Q1000 Modulated differential
ꢁ1
ꢀ
All MG cores were partially (approximately 50%) and
scanning calorimeter (DSC) at a heat/cool rate of 10 C min
under N for three heat/cool cycles. T was determined from the
completely protected by tert-butoxycarbonyl (t-BOC) groups
2
g
49
using literature procedures. The partially protected compounds
for example denoted as TriHPB-mp-50) were purified through
column chromatography using acetone as the eluent. The
00% t-BOC protected samples (for example denoted as
second heating/cooling cycle. Powder X-ray diffraction (PXRD)
traces were obtained using a Scintag Theta-Theta XDS2000
diffractometer to examine the crystallinity of the bulk material.
(
1
TriHPB-mp-100) were purified using dichloromethane as the
eluent.
Lithographic evaluation of positive-tone molecular glasses
The resist compounds were dissolved in propylene glycol methyl
TriHPB-p: Yield ¼64%; IR (NaCl film) nmax 3385 (br), 1610,
ꢁ
1
1
1511, 1410, 1230, 1007, 844 cm ; H NMR (300 MHz,
ether acetate (PGMEA) making
a
5% wt solution.
A
1
904 | J. Mater. Chem., 2008, 18, 1903–1910
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acetone-d ): d ¼ 6.96–7.02 (d, 6 H, Ar–H), 7.64–7.69 (d, 6 H,
acetone-d ): d ¼ 1.53 [s, (CH ) C], 2.03–2.17 (d, 6 ꢂ CH ),
6
6
3 3
3
Ar–H), 7.71 (s, 3 H, Ar–H), 8.55 ppm (s, 3 H, Ar–OH).
6.61 (s, 3 H, Ar–H), 6.78–6.95 (m, Ar–H) 8.13 ppm (br, s,
Ar–OH).
ꢀ
12
TriHPB-p-100: T (DSC) 64 C (lit., 68 C); T (TGA) 156 C;
ꢀ
ꢀ
g
d
ꢀ ꢀ
weight loss calcd for 100% t-BOC protection 46%, found 45%; IR
TriHPB-DM-100: T (DSC) 108 C; T (TGA) 179 C; weight
g d
loss calcd for 100% t-BOC protection 41%, found 39%; IR (NaCl
ꢁ
1 1
(
NaCl film); nmax 1761 cm ; H NMR (300 MHz, acetone-d
d ¼ 1.532 [s, 27 H, 3 ꢂ (CH C], 7.28–7.32 (d, 6 H, Ar–H),
.86 –7.90 (d, 6 H, Ar–H), 7.91 ppm (s, 3 H, Ar–H).
6
):
ꢁ
1 1
3
)
3
film); nmax 1760 cm ; H NMR (300 MHz, acetone-d
[s, 27 H, 3 ꢂ (CH C], 2.16 (s, 18 H, 6 ꢂ CH ), 6.94 (s, 3 H,
Ar–H), 6.96 ppm (d, 6 H, Ar–H).
6
): d ¼ 1.54
7
3
)
3
3
ꢀ
ꢀ
TriHPB-p-50: T
calcd for 50% t-BOC protection 30%, found 32%; IR (NaCl
g d
(DSC) 38 C; T (TGA) 115 C; weight loss
1
H
TriHPB-AD-methoxy:
NMR (300 MHz, CDCl
3
):
),
ꢁ
1
1
film); nmax 3220, 1760 cm ; H NMR (300 MHz, acetone-d
6
):
d ¼1.77(br, s, 18 H, 9 ꢂ CH
2
), 1.89 (br, s, 18 H, 9 ꢂ CH
2
d ¼ 1.56 [s, (CH ) C], 6.85–7.02 (m, Ar–H), 7.19–7.36 (m, Ar–H),
2.10 (br, s, 9 H, 9 ꢂ CH), 3.83 (s, 9 H, 3 ꢂ OCH ), 6.81–7.03
3
3
3
7
.62–7.80 (m, Ar–H), 7.83–7.95(m, Ar–H), 8.52 ppm (br, s,
(br, m, 3 H, Ar–H), 7.271–7.34 (br, m, 3 H, Ar–H), 7.64 ppm
(s, 3 H, Ar–H).
Ar–OH).
TriHPB-m: Yield ¼62%; IR (NaCl film) n
3321 (br), 1584,
1
494, 1418, 1325, 1190, 788, 701 cm ; H NMR (300 MHz,
TriHPB-AD: Yield ¼58%; IR (KBr pellet) n
3386(br),
max
max
ꢁ1
1
2902, 2848, 1702, 1603, 1514, 1445, 1343, 1248, 1095, 810,
ꢁ
1
1
acetone-d
6
): d ¼ 6.79–6.95 (q, 3 H, Ar–H), 7.09 (t, 3 H,
750cm ; H NMR (300 MHz, acetone-d
H, 9 ꢂ CH ), 1.89 (br, s, 18 H, 9 ꢂ CH ), 2.23 (br, s, 9 H, 9 ꢂ
), 6.81–7.03 (br, m, 3 H, Ar–H),
6
): d ¼1.78(br, s, 18
Ar–H), 7.21–7.40 (m, 3 H, Ar–H), 7.09 (t, 3 H, Ar–H), 7.81
s, 3 H, Ar–H), 8.42–8.57 ppm (broad, m, 3 H, Ar–OH).
2
2
(
CH), 3.83 (s, 9 H, 3 ꢂ OCH
3
ꢀ
ꢀ
TriHPB-m-100: T
calcd for 100% t-BOC protection 46%, found 48%; IR (NaCl
g
(DSC) 41 C; T
d
(TGA) 163 C; weight loss
7.271–7.34 (br, m, 3 H, Ar–H), 7.64 ppm (s, 3 H, Ar–OH);
+
MALDI calcd for (M + H) 756.4, found 756.2.
ꢁ
1
1
film); nmax 1760 cm ; H NMR (300 MHz, acetone-d
1
TriHPB-DH-methoxy: H NMR (300 MHz, CDCl
6
): d ¼
3
): d ¼3.87
1
7
.55 [s, 27 H, 3 ꢂ (CH ) C], 7.21–7.28 (d, 3 H, Ar–H), 7.53–
(s, 18 H, 6 ꢂ OCH ), 6.52 (s, 3 H, Ar–H), 6.82 (s, 6 H, Ar–H),
3
3
3
.59 (m, 3 H, Ar–H), 7.71 (br,s, 3 H, Ar–H), 7.76–7.81 (d, 3
7.74 ppm (s, 3 H, Ar–H).
H, Ar–H), 7.98 ppm (s, 3 H, Ar–H).
ꢀ
TriHPB-DH: Yield ¼48%; IR (NaCl film) n
3442 (br),
ꢁ1
max
ꢀ
1
TriHPB-m-50: T (DSC) 33 C; T (TGA) 123 C; weight loss
d
2983, 1599, 1465, 1370, 1255, 1142, 858, 779, 692 cm ;
NMR (300 MHz, acetone-d
H
g
calcd for 50% t-BOC protection 30%, found 28%; IR (NaCl film);
6
): d ¼ 6.42 (s, 3 H, Ar–H), 6.77
ꢁ
1 1
n
max 3225, 1760 cm ; H NMR (300 MHz, acetone-d
s, (CH C], 6.58–8.01 (m, Ar–H), 8.18 ppm (d, Ar–OH).
TriHPB-methoxy-mp: H NMR (300 MHz, CDCl
), 6.86–7.06 (br, m, Ar–H), 7.10–7.56 ppm
6
): d ¼ 1.55
(s, 6 H, Ar–H), 7.71 (s, 3 H, Ar–H), 8.40 ppm (br, s, 6 H,
+
Ar–OH). MALDI calcd for (M + H) 402.1, found 402.2;
[
3
)
3
1
3
): d ¼ 3.84–
Anal. calcd for C24
H 4.78%.
18 6
H O : C 71.64; H 4.51. Found: C 71.82;
3
.91 (d, 9 H, 3 ꢂ OCH
3
ꢀ ꢀ
g d
(DSC) 49 C; T (TGA) 123 C; weight
(
br, m, Ar–H), 7. 58–7.79 (br, m, Ar–H).
TriHPB-DH-50: T
loss calcd for 50% t-BOC protection 43%, found 52%; IR
TriHPB-mp: Yield ¼60%; IR (KBr pellet) nmax 3235 (br),
ꢁ
1
1
586, 1515,1409, 1228, 829 cm ; H NMR (300 MHz,
ꢁ1
1
1
(NaCl film); nmax 3442, 1760 cm ; H NMR (300 MHz,
acetone-d ): d ¼ 1.53 [s, (CH ) C], 6.59–6.82 (m, Ar–H),
acetone-d ): d ¼6.86–7.06 (br, m, Ar–H), 7.22–7.39 ppm (br, m,
6
6
3 3
Ar–H), 7.65–7.85 (br, m, Ar–H), 8.41–8.55 ppm (br, m, 3 H, 3 ꢂ
7.10–7.26 (m, Ar–H), 7.84–8.05 (br, s, Ar–OH), 8.12–8.19 ppm
(br, s, Ar–OH).
+
Ar–OH); MALDI calcd for (M + H) 354.1, found 354.2.
ꢀ
ꢀ
TriHPB-mp-100: T
g
(DSC) 50 C; T
d
(TGA) 160 C; weight
g g d
TriHPB-DH-100: T (DSC) No T observed; T (TGA)
ꢀ
loss calcd for 100% t-BOC protection 46%, found 46%; IR
1
NaCl film); nmax 1761 cm ; H NMR (300 MHz, acetone-d
163 C; weight loss calcd for 100% t-BOC protection 60%, found
1
ꢁ1
ꢁ1
(
6
):
59%; IR (NaCl film); nmax 1760 cm ; H NMR (300 MHz,
CDCl C], 7.18 (t, 3 H, Ar–H),
d ¼ 1.55 [s, 27 H, 3 ꢂ (CH
Ar–H).
3
)
3
C], 7.21–7.99 ppm (br, m, 15 H,
3
): d ¼ 1.55 [s, 54 H, 6 ꢂ (CH
3 3
)
7.68 (d, 6 H, Ar–H), 8.05 ppm (s, 3 H, Ar–H).
ꢀ
ꢀ
TriHPB-mp-50: T
calcd for 50% t-BOC protection 30%, found 34%; IR (NaCl
g
(DSC) 38 C; T
d
(TGA) 123 C; weight loss
TetHPB-p: Yield ¼64%; IR (NaCl film) nmax 3332 (br), 1607,
ꢁ
1
1
1514, 1483, 1441, 1365, 1232, 1171, 1112, 827 cm ; H NMR
ꢁ
1
1
film); nmax 3350, 1760 cm ; H NMR (300 MHz, acetone-d ):
(300 MHz, acetone-d ): d ¼ 6.74 (d, 8 H, Ar–H), 7.07 (d, 8 H,
6
6
d ¼ 1.52 [s, (CH ) C], 6.58–7.99 (m, Ar–H), 8.17 ppm (br, s,
Ar–H), 7.32 (s, 2 H, Ar–H), 8.36 ppm (s, 4 H, Ar–OH).
3
3
ꢀ
Ar–OH).
TetHPB-p-100: T (DSC) No T observed; T (TGA) 180 C;
g
g
d
1
TriHPB-DM-methoxy: H NMR (300 MHz, CDCl ): d ¼2.10
weight loss calcd for 100% t-BOC protection 46%, found 47%;
3
ꢁ
1 1
(
s, 18 H, 6 ꢂ CH
3
), 3.81 (s, 9 H, 3 ꢂ OCH
3
), 6.66 (s, 6 H, Ar–H),
IR (NaCl film); nmax 1760 cm ; H NMR (300 MHz, CD
d ¼ 1.54 [s, 36 H, 4 ꢂ (CH C], 7.07 (d, 8 H, Ar–H), 7.25
(d, 8 H, Ar–H), 7.52 ppm (s, 2 H, Ar–H).
TetHPB-m: Yield ¼61%; IR (NaCl film) nmax 3253(br), 1694,
1587, 1449, 1317, 1283, 1235, 1190, 867, 779, 706 cm ; H NMR
(300 MHz, acetone-d
2 2
Cl ):
6
.87 ppm (s, 3 H, Ar–H).
3 3
)
TriHPB-DM: Yield ¼56%; IR (NaCl film) nmax 3386 (br),
3
9
035, 2923, 2977, 1691, 1598, 1471, 1311, 1261, 1154, 1026,
ꢁ
1
1
76, 854, 730 cm ; H NMR (300 MHz, acetone-d
ꢁ1
1
6
): d ¼1.95
(
s, 18 H, 6 ꢂ CH
3
), 6.48 (s, 6 H, Ar–H), 6.66 (s, 3 H, Ar–H),
+
6
): d ¼ 6.68–6.80 (br, m, 12 H, Ar–H), 7.09
9
.15 ppm (s, 3 H, 3 ꢂ Ar–OH); MALDI calcd for (M + H)
(t, 4 H, Ar–H), 7.39 (s, 2 H, Ar–H), 8.30 ppm (s, 4 H, Ar–OH).
ꢀ
4
38.2, found 438.3; Anal. calcd for C H O : C 82.16; H 6.89.
3
TetHPB-m-100: T (DSC) No T observed; T (TGA) 175 C;
d
0
30
3
g
g
Found: C 82.38; H 6.95%.
weight loss calcd for 100% t-BOC protection 46%, found 47%;
ꢁ1
ꢀ
ꢀ
1
TriHPB-DM-50: T (DSC) 61 C; T (TGA) 123 C; weight
g
IR (NaCl film); nmax 1762 cm ; H NMR (300 MHz, CD Cl ):
2 2
d
loss calcd for 50% t-BOC protection 25%, found 31%; IR
d ¼ 1.49 [s, 36 H, 4 ꢂ (CH ) C], 7.07–7.18 (br, m, 12 H,
3
3
ꢁ
1
1
NaCl film); nmax 3447, 1756 cm ; H NMR (300 MHz,
(
Ar–H), 7.32 (t, 4 H, Ar–H), 7.56 ppm (s, 2 H, Ar–H).
This journal is ª The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 1903–1910 | 1905

View Article Online
1
ꢁ1
1
IR (NaCl film); nmax 3397, 1756 cm ; H NMR (300 MHz,
TetHPB-methoxy-mp: H NMR (300 MHz, CDCl ): d ¼ 3.64
3
(
d, 6 H, 2 ꢂ OCH ), 3.79 (s, 6 H, 2 ꢂ OCH ), 6.72–6.88 (br, m,
acetone-d ): d ¼ 1.47 [d, 7.87–8.03 ppm (br, s, Ar–OH).
3
3
6
Ar–H), 7.10–7.21 (br, m, Ar–H), 7. 41–7.58 ppm (br, m, Ar–H).
TetHPB-mp: Yield ¼60%; IR (KBr pellet) nmax 3412 (br),
Results
1
8
708, 1608, 1591, 1514, 1481, 1445, 1378, 1259, 1224, 1170,
ꢁ
1 1
29, 794 cm ; H NMR (300 MHz, acetone-d
Synthesis of hydroxyphenylbenzene derivatives
6
): d ¼6. 61–6.86
(
br, m, Ar–H), 6.96–7.17 ppm (br, m, Ar–H), 7.30–7.41 (br, m,
The synthesis of methoxy protected phenylbenzene derivatives
was performed by a palladium catalyzed cross coupling between
48
Ar–H), 8.23–8.37 ppm (br, m, 4 H, 4 ꢂ Ar–OH); MALDI calcd
+
for (M + H) 446.2, found 446.2.
the respective phenyl bromides and phenylboronic acids. By
ꢀ ꢀ
TetHPB-mp-100: T
loss calcd for 100% t-BOC protection 47%, found 45%; IR
g d
(DSC) 78 C; T (TGA) 160 C; weight
varying the phenylboronic compound, TriHPB derivatives with
varying substituents were synthesized. Similarly, tetra(hydroxy-
phenyl)benzene (TetHPB) derivatives were also synthesized
through cross coupling reactions by using tetrabromo-phenyl-
benzene as the core molecule. Hexa(hydroxyphenyl)benzene
ꢁ
1 1
NaCl film); nmax 1761 cm ; H NMR (300 MHz, acetone-d ):
6
(
d ¼ 1.52 [s, 36 H, 4 ꢂ (CH ) C], 6.68–6.80 (br, m, Ar–H),
3
3
7
.06–7.18 (br, m, Ar–H), 7.24–7.36 ppm (br, m, Ar–H).
ꢀ ꢀ
TetHPB-mp-50: T
loss calcd for 50% t-BOC protection 31%, found 34%; IR
g d
(DSC) 48 C; T (TGA) 115 C; weight
(
HHPB) derivatives were synthesized through a trimerization
0
of N,N -dimethoxy-bisphenyl-acetylene using Co
50,51
2
(CO)
8
as
ꢁ1
1
(
NaCl film); nmax 3465, 1758 cm ; H NMR (300 MHz,
acetone-d C], 6.67–6.84 (br, m, Ar–H),
): d ¼ 1.51 [s, (CH
.03–7.18 (br, m, Ar–H), 7.24– 7.58 (br, m, Ar–H), 8.34 ppm
br, s, Ar–OH).
HHPB-p: IR (NaCl film) nmax 3535, 3256 (br), 1691, 1604,
a catalyst.
The compounds were obtained in relatively high
6
3 3
)
yields after careful purification through regular chromatographic
7
techniques. The deprotection of the methyl group with BBr was
3
(
conducted to yield hydroxyphenylbenzene derivatives as the core
compounds.
ꢁ1
1
1
511, 1430, 1402, 1359, 1227, 1173, 1104, 816 cm ; H NMR
In order to convert these compounds into positive tone photo-
resists, they were protected with tert-butoxycarbonyl (t-BOC) to
varying degrees (50%–100%) by a standard base catalyzed
reaction in the presence of 4-dimethylaminopyridine (DMAP).
(
300 MHz, dmso-d ): d ¼ 6.36 (d, 12 H, Ar–H), 6.64 (d, 12 H,
6
Ar–H), 7.89 ppm (s, 6 H, Ar–OH).
HHPB-p-100: T (DSC) No T observed; T (TGA) 165 C;
weight loss calcd for 100% t-BOC protection 49%, found 48%;
ꢀ
g
g
d
ꢁ1 1
IR (NaCl film); nmax 1759 cm ; H NMR (300 MHz, CDCl
d ¼ 1.48 [s, 54 H, 6 ꢂ (CH C], 6.73 (d, 12 H, Ar–H),
.26 ppm (d, 12 H, Ar–H).
3
):
Thermal properties
3 3
)
7
The thermal properties of the hydroxyphenylbenzene MGs were
investigated by differential scanning calorimetry (DSC) and ther-
1
HHPB-methoxy-m: H NMR (300 MHz, CDCl
s, 18 H, 6 ꢂ OCH ), 6.35–6.54 (br, m, 18 H, Ar–H), 6.79 ppm
t, 6 H, Ar–H).
3
): d ¼ 3.45
(
(
3
g
mogravimetric analysis (TGA). High T and good thermal
stability at elevated temperatures are prerequisites for their use
in patterning applications. TGA analysis showed that all
unprotected compounds are thermally stable until about
HHPB-m: Yield ¼60%; IR (NaCl film) n
3231(br), 1697,
max
1
6
584, 1491, 1441, 1402, 1326, 1224, 1162, 925, 858, 785, 732,
ꢁ
1 1
95 cm ; H NMR (300 MHz, acetone-d ): d ¼ 6.21–6.57 (br,
ꢀ
6
300 C. When partially protected with t-BOC groups, the
ꢀ
m, 18 H, Ar–H), 6.69 (s, 6 H, Ar–H), 7.86 ppm (t, 6 H, 6 ꢂ
thermal stability decreases up to about 120 C whereas it is
ꢀ
Ar–OH).
around 160 C for fully protected compounds. Both unprotected
ꢀ
HHPB-m-100: T
g
(DSC) 93; T
d
(TGA) 167 C; weight loss
g
and protected MGs exhibit T s that could be verified through
calcd for 100% t-BOC protection 49%, found 48%; IR (NaCl
multiple heat/cool cycles.
ꢁ1
1
film); nmax 1762 cm ; H NMR (300 MHz, CDCl
s, 54 H, 6 ꢂ (CH C], 6.63–6.71 (br, s, 6 H, Ar–H), 6.72
6.84 (br, s, 12 H, Ar–H), 6.85–6.94 ppm (br,s, 6 H, Ar–H).
3
): d ¼ 1.46
[
3 3
)
Discussion
–
ꢀ
ꢀ
HHPB-m-50: T
calcd for 50% t-BOC protection 32%, found 32%; IR (NaCl
g
(DSC) 103 C; T
d
(TGA) 113 C; weight loss
As triphenylbenzene is known as a common glass-forming core,
it served as an ideal platform to investigate molecular design
concepts that can establish guidelines for good MG design.
These molecules otherwise have a high tendency towards crystal-
lization due to p–p stacking of the benzene rings. The ability to
realize their potential in lithographic applications is based upon
identifying specific structural features that prevent molecular
ꢁ
1
1
film); nmax 3462, 1759 cm ; H NMR (300 MHz, acetone-d ):
6
d ¼ 1.49 [s, (CH ) C], 6.25–6.52(br, m, Ar–H), 6.62 –6.96
3
3
(
br, m, Ar–H), 7.73– 8.04 ppm (br, s, Ar–OH).
HHPB-mp: Yield ¼60%; IR (NaCl film) nmax 3328 (br), 1701,
1
611, 1585, 1516, 1444, 1401, 1332, 1228, 1172, 832, 786, 692
ꢁ1
1
cm ; H NMR (300 MHz, acetone-d
6
): d ¼ 6.24–6.46 (br, m,
packing and increase T . The effects of various structural
g
1
2 H, Ar–H), 6.58–6.82 (br, m, 12 H, Ar–H), 7.81–7.98 ppm
modifications such as mixed isomeric states, large substituents,
(
6 H, 6 ꢂ Ar–OH).
HHPB-mp-100: T
weight loss calcd for 100% t-BOC protection 49%, found 48%;
equal conformational states and enhanced intermolecular
bonding have also been examined. The effect of molecular size
was evaluated by expanding to tetraphenylbenzene and hexaphe-
nylbenzene molecular systems (Fig. 1).
ꢀ
g d
(DSC) No T observed; T (TGA) 160 C;
g
ꢁ
1 1
IR (NaCl film); nmax 1761 cm ; H NMR (300 MHz, acetone-
d ): d ¼ 1.30 [s, 36 H, 4 ꢂ (CH ) C], 6.26–6.52 (br, m, 12 H,
6
3 3
Phenylbenzene compounds with free hydroxyl groups can be
used as the matrix in a negative tone photoresist when combined
with a photoacid generator (PAG) and a crosslinking agent.
Upon UV exposure, acid is generated by the PAG molecules,
Ar–H), 6.62–6.81 ppm (br, m,12 H, Ar–H).
ꢀ
HHPB-mp-50: T (DSC) No T observed; T (TGA) 115 C;
d
g
g
weight loss calcd for 50% t-BOC protection 32%, found 34%;
1
906 | J. Mater. Chem., 2008, 18, 1903–1910
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Table 1 Thermal characterization of the hydroxyphenylbenzene (HPB)
derivatives
Fully t-BOC
/ C protected T / C protected T
Partially t-BOC
ꢀ
ꢀ
ꢀ
Structure
Unprotected T
g
g
g
/ C
TriHPB-p
TriHPB-m
TriHPB-mp
TriHPB-DM Not observed
TriHPB-AD 103
TriHPB-DH 142
TetHPB-p
TetHPB-m
TetHPB-mp
HHPB-p
HHPB-m
HHPB-mp
122
110
127
66
41
50
38
33
38
61
108
N/A
Not observed
Not observed
Not observed
78
Not observed
93
105
N/A
49
N/A
N/A
48
N/A
103
>120
119
119
157
Not observed
120
Not observed
the solvent. However, subsequent heating and cooling from the
melt yields kinetically trapped amorphous solids, which undergo
g
structural relaxation, and exhibit well-defined T s that can be
confirmed through multiple heat/cool cycles (Table 1).
When all isomers are meta or para, phenylbenzene molecules
form highly symmetric structures. This symmetry can be identi-
fied as the main cause for its packing ability and crystalline
nature. Hence, a mixture of meta and para isomers have been
introduced to create an asymmetric structure that hinders mole-
cular packing of p–p electron domains. The structural asymme-
Fig. 1 Hydroxyphenylbenzene derivatives.
try also helps to create several molecular conformations with
53,54
which causes an acid catalyzed crosslinking reaction in the
exposed regions during the post- exposure bake (PEB) process.
To enable molecules to be used as a negative tone photoresist,
they should be able to form thin films without any molecular
equal or extremely small potential energy barriers.
Therefore,
these isomeric mixtures can form kinetically stable amorphous
solids. In the HHPB system, this is accomplished by introducing
a discrete asymmetric intermediate with isomers in both meta
and para positions. This yields the final product as a mixture
of two structural isomers with alternating meta and para
aggregation and demonstrate high T s that can withstand the
g
post-exposure thermal processing. Hence hydroxyphenylbenzene
derivatives that can be vitrified upon evaporating from solvent
and isolated as amorphous materials can have potential applica-
tion in this field.
51
hydroxyl groups.
However, for TriHPB and TetHPB, their mixed isomeric
structures were synthesized using a less discrete method. As the
Suzuki coupling is done in a one pot synthesis between the
phenyl bromides and phenylboronic acid, an equimolar mixture
of meta and para phenylboronic acid was simultaneously
introduced (Fig. 2). As the oxidative addition of phenyl bromide
to a palladium(0) complex is often the rate-determining step in
this catalytic cycle, each boronic acid isomer is expected to
Hydroxyphenylbenzene derivatives with acid labile t-BOC
groups were also synthesized and evaluated as positive tone
photoresists. Upon UV exposure, the PEB step (usually above
ꢀ
8
0 C) is required to remove t-BOC groups through an acid cata-
lyzed deprotection reaction. Similar to the negative tone system,
understanding of the T of these protected MG compounds is
g
crucial to their lithographic performance. T s were evaluated
g
48
have similar reactivity. Hence, an equal distribution of meta
in both fully and partially protected states and the most
promising candidates were identified for lithographic evaluation.
Introducing functional substituents to a phenylbenzene core
and para isomers is expected in each MG system. This can be
confirmed through NMR analysis of the respective methoxy
groups in the cross-coupled product. Though precise structures
for TriHPB-mp and TetHPB-mp cannot be determined, a statis-
tical mixture of meta and para isomers yield amorphous
materials that can be vitrified upon evaporating from solvent.
The XRD analysis of TriHPB derivatives (Fig. 3) show that
25,27,31
can significantly affect its properties.
Despite a non-planar
molecular structure 1,3,5-triphenylbenzene instantly crystallizes
without vitrification even when its melt samples are rapidly
cooled with liquid nitrogen. But when the hydroxyl functionality
is added, TriHPB derivatives form amorphous solids when
cooled from the melt. It has been observed that the position of
the substituents affects both thermal and electronic properties
52
of phenylbenzene derivatives. Similarly, we observed that the
amorphous character and T of the MG systems could also be
g
varied by the position of the hydroxyl group. All para (p) and
meta (m) isomers of TriHPB, TetHPB and HHPB discussed in
this paper are obtained as crystalline material upon evaporating
Fig. 2 Synthesis of TriHPB-mp.
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J. Mater. Chem., 2008, 18, 1903–1910 | 1907

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g
Fig. 4 Trends in T with increasing molecular size.
Fig. 3 Grazing angle X-ray diffraction spectra of TriHPB isomeric
compounds.
may be more suited for amorphous material design. As Fig. 4
shows, the T s of fully t-BOC protected MG systems continue
g
the meta–para mixture is an amorphous solid as characterized by
the amorphous halo. For comparison, the crystalline peaks of
TriHPB-p and TriHPB-m also appear in Fig. 3.
to increase as the molecular size is increased from TriHPB to
HHPB. As expected, this result confirms that glass formation
26,27
is facilitated with increasing molecular size.
Due to its methyl substituents that restrict molecular rotation,
TriHPB-DM-100 has the highest T recorded for a fully t-BOC
Other strategies for MG design rely on incorporating various
functionalities into phenyl rings. Dimethyl substituents are
incorporated into TriHPB not only to create a non-planar
molecular structure, but also to create the existence of different
g
protected derivative. Despite its significantly lower molecular
weight compared to HHPB-mp-100, it demonstrates a higher
25,55
conformers.
Therefore, TriHPB-DM should possess a weak-
g
T , illustrating how molecular size can be compensated by other
ened tendency towards crystallization, mainly for kinetic
reasons. Incorporating sterically demanding substituents can
also inhibit crystallization in phenylbenzene derivatives.
Adamantane side groups have been integrated into the molecular
architecture in TriHPB-AD. These large substituents can prevent
molecular packing due to the difficulty of incorporating bulky
structural attributes through careful design. In contrast TriHPB-
DH-100 is obtained as a crystalline material due to its symmetric
structure. In TriHPB-AD, adamantane acts as a dangling side
group that increases free volume in the molecular system.
Though adamantane type bulky substituents prevent crystalliza-
tion, they are also structurally unfavorable for high T . Instead
g
56
side groups into a crystalline lattice. The effect of additional
hydrogen bonding sites in the MG system was evaluated through
the inclusion of extra hydroxyl functionality in TriHPB-DH. The
result of strong attractive interactions such as hydrogen bonding
in molecular architecture is dependent upon molecular geometry.
If the molecule has a high tendency towards crystallization, the
increase in intermolecular attractive forces will promote crystal-
lization. But if the molecular system should have a weak
tendency towards crystallization the strong intermolecular
a more rigid moiety that can reduce molecular motion needs to
25,28,57
be incorporated as a side group to increase T
g
.
Hence,
further evaluation was not carried out by protection of
TriHPB-AD with acid labile t-BOC groups.
In practical application, MG resists are used as positive tone
photoresists in their partially protected form. The free hydroxyl
groups in the molecular system help with solubility in spinning
solvents, adhesion to the silicon wafer, and improvement of resist
sensitivity. Furthermore, the miscibility of ionic small molecule
photo acid generators (PAGs) in the resist matrix is also
enhanced due to the polarity of hydroxyl groups. Therefore, it
attraction will give rise to high T s amorphous solids.
g
In order to evaluate the effect of each design concept on the
glass transition temperature, T s of t-BOC protected phenylben-
g
is important to understand the T trends in partially protected
g
zene compounds were analyzed. When fully protected with
t-BOC groups, the structural features dominate the thermal
behavior of these compounds. In TriHPB, all three isomeric
states form amorphous solids in their fully t-BOC protected
form. However, in TetHPB derivatives only TetHPB-mp-100
forms an amorphous solid upon t-BOC protection. The TriHPB
structure maybe more conducive to forming amorphous
materials due to its 1,3,5-branched architecture. This study
also demonstrates that even a highly crystalline material like
TetHPB can be made amorphous through asymmetric molecular
design. In HHPB derivatives, HHPB-p-100 forms a crystalline
hydroxyphenylbenzene derivatives as well. In selected TriHPB
g
and TetHPB isomers analyzed in this study, the T s of partially
protected compounds are lower compared to their fully
protected counterparts. Though enhanced intermolecular inter-
actions between the free hydroxyl groups are expected to raise
T
g
, other effects such as molecular flexibility and increase of
free volume in the system can reduce T . However, comparing
g
TriHPB-m-50 with TriHPB-DH-50 shows that additional
hydroxyl functionality can increase Tg in similar amorphous
materials. In contrast, the T trends for HHPB-m and HHPB-
g
mp derivatives show the opposite effect, with partially t-BOC
solid while HHPB-m-100 forms a high T
g
amorphous solid
protected compounds showing higher T
protected counterparts. This illustrates that at the molecular
weight range of HHPB, the partially protected T is influenced
g
compared to their fully
despite its symmetric structure. Unlike para substituents that
tend to enforce linearity of arms, meta substituted phenyl groups
g
1
908 | J. Mater. Chem., 2008, 18, 1903–1910
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by enhanced hydrogen bonding of the free hydroxyl groups.
Based on this study of molecular structure and thermal
properties, HHPB derivatives have been identified as suitable
candidates for lithography evaluation.
This study has helped to establish a set of guidelines for
g
designing high T amorphous hydroxyphenylbenzene drivatives.
Design features that enable molecular conformations with equal
or extremely small potential energy barriers need to be incorpo-
rated to prevent molecular packing. Molecules are also designed
with asymmetric structures that are inherently not suited for
crystallization. Forming meta-para isomer mixtures is a key to
preventing p-p stacking and forming amorphous solids. Various
substituents, both bulky and rigid in nature, can be incorporated
to prevent molecular packing in addition to increasing T by
g
restricting molecular rotation. Enhancing intermolecular
interaction through attractive forces such as hydrogen bonding
Fig. 5 EUV contrast curve for HHPB-mp-50.
g
should exhibit a tendency towards high T molecular systems.
Molecular size can be also varied in an incremental manner to
create branched and bulky structures that form stable molecular
g
glasses with increasing T .
Lithographic evaluation
The goal of this systematic study in understanding the thermal
properties of hydroxyphenylbenzene derivatives has been to
g
develop patternable, high T MG systems. HHPB compounds
ꢀ
with T
g
s higher than 100 C have been identified as promising
candidates capable of sub-50 nm resolution. Upon evaluating
a range of protection ratios for the HHPB-mp derivative, the
5
0% protection ratio was identified as the optimal amount with
respect to T , sensitivity, contrast and solubility in aqueous
g
base developer. Therefore, for our preliminary lithographic
assessment HHPB-mp-50 was evaluated through EUV exposure
tools at SEMATECH North in Albany, NY and at Lawrence
Berkeley National Laboratory (LBNL).
Fig. 6 SEM images of 1 : 1 (top) and 1 : 5 (bottom) lines space patterns
obtained after EUV patterning of HHPB-mp-50 thin films.
HHPB-mp-50 was dissolved in PGMEA at 5 wt%, spin-cast on
ꢀ
a HMDS treated silicon wafer and prebaked at 125 C for 30s. A
The high optical contrast and sensitivity demonstrated through
the contrast curve also play important roles in high resolution
patterning of chemically amplified resists.
g
range of PEB temperatures below its bulk T such as 80, 85, 90,
ꢀ
9
5 and 100 C for 30 s were tested. The best result was obtained
ꢀ
using a post-exposure bake of 90 C. These films were developed
in 0.26 N TMAH (tetramethylammonium hydride) which is the
standard commercial developer.
Conclusion
The contrast curve was obtained through EUV exposure at
SEMATECH North. The resist film was exposed to a stepwise
In this paper, we report a fundamental study of hydroxyphenyl-
benzene MGs through the synthesis and characterization of
several molecules with variable size and structural features.
This work is aimed at understanding the relationship between
molecular structure and the ability to form amorphous materials
ꢁ
2
increase in EUV dosage from 0.2–20 mJ cm . The exposed
ꢀ
regions were developed in 0.26 N TMAH after PEB at 90 C.
The remaining film thickness was measured to determine the
resist contrast. The contrast curve shown in Fig. 5 indicates
that the resist sensitivity of HHPB-mp-50 for EUV exposure is
with high T . Effects of isomeric states, substituents, conforma-
g
tion states, hydrogen bonding and molecular size have been
studied in detail through selected hydoxyphenylbenzene
derivatives. This understanding is important from both scientific
and application points of view. HHPB-mp-50 identified as
a suitable MG photoresist has been evaluated through EUV
lithography to produce sub-30 nm feature sizes.
ꢁ2
between 10–20 mJ cm , which is within the expected range for
t-BOC protected chemically amplified resists.
EUV patterning was performed using the microexposure tool
at LBNL which is capable of fabricating fine features below
3
0 nm. This is the first report of a hydoxyphenylbenzene resist
structure able to demonstrate sub-50 nm patterns. Feature sizes
of 30 nm and below could be resolved for the HHPB-mp-50 resist
system upon EUV exposure (Fig. 6). The high resolution of this
system can be attributed to several of its promising features. The
Acknowledgements
This work was funded by the Semiconductor Research Corpora-
tion (SRC) and Intel Corporation. The Cornell Nanoscale
high T
g
before and after patterning may be the primary factor.
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Science and Technology Facility (CNF) and the Cornell Center
for Materials Research (CCMR) are thanked for use of their
facilities. Finally, we thank SEMATECH North and Lawrence
Berkeley National Laboratory for assistance with EUV
exposures and lithographic evaluation.
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