Base-amplifying silicone resins with photobase-generating side chains and their application to negative-working photoresists

Article abstract of DOI:10.1002/pola.27551

To accomplish high photosensitivity of resist systems including photobase generators, we have proposed the concept of base-proliferation reactions that generate base molecules in a nonlinear manner by the action of a catalytic amount of base; however, exc

Full text of DOI:10.1002/pola.27551

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Base-Amplifying Silicone Resins with Photobase-Generating
Side Chains and Their Application to Negative-Working Photoresists
Masahiro Furutani, Hiroshi Kobayashi, Takahiro Gunji, Yoshimoto Abe,
Koji Arimitsu
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science,
2641 Yamazaki, Noda, Chiba 278–8510, Japan
Correspondence to: K. Arimitsu (E-mail: arimitsu@rs.noda.tus.ac.jp)
Received 14 October 2014; accepted 6 December 2014; published online 00 Month 2015
DOI: 10.1002/pola.27551
ABSTRACT: To accomplish high photosensitivity of resist sys-
tems including photobase generators, we have proposed the
concept of base-proliferation reactions that generate base mole-
cules in a nonlinear manner by the action of a catalytic amount
of base; however, excessive diffusion of generated base mole-
cules is still a problem. We have designed novel functional sili-
cone resins bearing both base-amplifying units and photobase-
generating units, and synthesized resins with various composi-
tion ratios. The synthesized resins are decomposed autocatalyti-
cally after UV irradiation and subsequent heating at 100 ꢀC,
which indicates progression of base-proliferation reactions.
High photosensitivity (8.1 mJ cm22) was recorded, and it was
found that the photosensitivity is about 490 times enhanced by
replacing
a base-catalytic reaction system with the base-
proliferation reaction system. Furthermore, a 4 3 10 mm line-
and-space pattern has been successfully fabricated using the sil-
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icone resin. 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2015, 00, 000–000
KEYWORDS: base-amplifying unit; base-proliferation reaction;
crosslinking;
negative-working
photoresist;
photobase-
generating unit; photoresists; polysiloxanes; silicone resin
not only will such infection be stopped, but also the mixing
process of BA polymer with PBG molecules will be pre-
vented. Furthermore, a siloxane polymer was applied as the
main chain of the polymer, anticipating enhancement of the
plasma-etching resistance by the silicone skeleton.30–36 How-
ever, the synthesized resins are apt to show low molecular
weight because of repulsion between the bulky 9-fluorenyl-
methyloxycarbonyl groups of the BA units or (2-nitro-4,5-
dimethoxy)benzyloxycarbonyl groups of the PBG units. To
increase the molecular weight of the resin, tetraethyl orthosi-
licate (TEOS) was added during the synthetic process as a
spacer.
INTRODUCTION The photolithography process is utilized for
high-volume production of micropatterns for circuits or elec-
trodes in electronic devices. Photosensitive polymers are
indispensable for this process as photoresists to protect
metal surfaces while being subjected to an etching pro-
cess.1–5 To accomplish high photosensitivity, various chemical
amplification systems have been developed in which photo-
generated acid6,7 or base8–23 catalyses a chemical reaction in
a resist film. While photosensitive materials based on base-
catalysed reactions have merit for preventing erosion of
metallic substrates, photobase generators generally have low
quantum yields, which leads to low photosensitivity. To over-
come this problem, we have proposed the novel concept of
base-proliferation reactions of base-sensitive compounds,
referred to as base amplifiers, that generate base molecules
in a non-linear manner by the action of a catalytic amount of
base.24–28 Photosensitive polymers bearing base-amplifying
(BA) units have also been designed for photoresist formula-
tions, taking into account the prevention of both evaporation
and excessive diffusion of base molecules from resist films
during post-exposure baking (PEB), referred to as
“air infection.”29 Nevertheless, air infection does produce
amines from low-molecular-weight photobase generators. If
photobase-generating (PBG) units are immobilized in a resin,
In this study, a novel photoreactive silicone resin 1 was
synthesized having both BA units and PBG units, and it was
found that the resins successfully functioned as negative-
working photoresists (Scheme 1). In this system, PBG units
at the side chains generate primary amines that are trig-
gered by UV irradiation. The amines catalyse autocatalytic
decomposition reactions of BA units under PEB, liberating
carbon dioxide and dibenzofulvene as by-products. Free
amines accelerate condensation reactions between ethoxy
silyl groups, leading to gelation or insolubilization of the
resin.
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Chemical structure of silicone resin 1 synthesized in this study and its chemical reactions, including photodecomposi-
tion. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
EXPERIMENTAL
88% yield as a pale-yellow solid. In the case of recrystalliza-
tion with cosolvents of hexane and THF, 2 was obtained in a
General
78% yield as a white solid. 1H NMR (500 MHz, CDCl3, d,
The reagents used were purchased from Tokyo Chemical
ppm): 0.63 (t, J 5 8.4 Hz, 2H; ACH2Si), 1.22 (t, J 5 7.0 Hz, 9H;
Industry (Tokyo, Japan), Shin-Etsu Chemical (Tokyo, Japan),
Si(OCH2CH3)3), 1.64 (quint, J 5 8.4 Hz, 2H; ACH2CH2CH2A),
and Wako Pure Chemical Industries (Osaka, Japan). (3-Iso-
3.19 (t, J 5 6.5 Hz, 2H; ANHCH2A), 3.81 (q, J 5 7.0 Hz, 6H;
cyanato)propyltriethoxysilane, TEOS, tetrahydrofuran (THF),
Si(OCH2CH3)3), 4.20 (t, J 5 6.5 Hz, 1H; CH of 9-fluorenyl), 4.38
benzene, and ethanol (EtOH) were used after distillation.
(d, 2H; fluorenylACH2), 5.13 (br, 1H; AOACOANHA), 7.2–7.7
Other chemicals were used without further purification.
(m, 8H; aromatic). 29Si NMR (99 MHz, CDCl3, d, ppm): 245.5.
1H (29Si) NMR spectra were obtained using a JEOL JNM-
EX500 (JNM-EX400) spectrometer. FT-IR spectra were
FT-IR (KBr): m 5 1690 (m(C5Ocarbamate)), 2970 (m(CHalkyl)),
3320 (m(NH)) cm21. mp: 72–74 ꢀC.
measured using
a JASCO FT/IR-410 spectrophotometer.
Measurements of melting point were performed using a
Yanaco MP-S3. UV–vis spectra were measured using a JASCO
V-570 spectrophotometer. Gas phase chromatography meas-
urements were performed using a Shimadzu RID-10A detec-
tor employing THF as an eluent, and the molecular weight
data were obtained relative to polystyrene standards. Photo-
irradiation was carried out using a San-ei Electric Hg–Xe
lamp with a glass filter (UV-35 or UV-D35; AGC Techno
Glass). The amount of light was measured using an Advant-
est TQ8210 or a Hamamatsu Photonics C8026. The film
thickness was measured using a Dektak3ST (Ulvac, Japan).
Synthesis of PBG Monomer 3
(3-Isocyanato)propyltriethoxysilane (7.6 g, 31 mmol) in ben-
zene (28 mL) was dropped into a solution of 6-nitroveratryl
alcohol (5.4 g, 26 mmol) with dibutyltin dilaurate (0.12 g,
0.19 mmol) in benzene (32 mL) at 70 ꢀC. The reaction mix-
ture was stirred at 70 ꢀC for 2 h, and then the solvent
was evaporated. The residue was purified using column
chromatography (TEOS/chloroform 5 1/100, then chloro-
form) to give the PBG monomer 3 in a 78% yield (9.1 g) as
a pale-yellow crystal. 1H NMR (500 MHz, CDCl3, d, ppm):
0.65 (t, J 5 8.0 Hz, 2H; ACH2Si), 1.23 (t, J 5 7.0 Hz, 9H;
Si(OCH2CH3)3), 1.66 (quint, J 5 8.0 Hz, 2H; SiACH2CH2), 3.22
(t, J 5 7.0 Hz, 2H; SiACH2CH2CH2), 3.73 (q, J 5 7.0 Hz, 6H;
Si(OCH2CH3)3), 3.8 (s 1 s, 6H; OCH3), 5.24 (br, 1H; NH), 5.50
(s, 2H; ArACH2), 7.10 (s, 1H; aromatic), 7.70 (s, 1H; aro-
matic). 29Si NMR (99 MHz, CDCl3, d, ppm): 245.8. FT-IR
(KBr): m 5 1330 (m(CAOAC)), 1520 (m(NAO)), 1690
Synthesis of BA Monomer 2
(3-Isocyanato)propyltriethoxysilane (6.3 g, 26 mmol) in
benzene (24 mL) was dropped into
a
solution of
9-fluorenylmethanol (5.00 g, 26 mmol) with dibutyltin dilau-
ꢀ
rate (0.12 g, 0.19 mmol) in THF (32 mL) at 70 C. The reac-
tion mixture was stirred at 70 ꢀC for 2 h, and then the
solvent was evaporated. The residue was dried in vacuo and
recrystallized from EtOH to give the BA monomer 2 in an
(m(C@Ocarbamate)), 2970 (m(CHalkyl)), 3330 (m(NH)) cm21
.
Anal. calcd for C19H32N2O9Si: C, 49.54; H,ꢀ 7.02; N, 6.08.
Found: C, 49.49; H, 7.41; N, 6.39. mp: 56–59 C.
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TABLE 1 Syntheses of copolycondensation resins 1 using monomers 2, 3 and TEOSa
b
Monomer [g (mmol)]
Yield
(g)
Mw
PDIc
Ratiod
Run
Resin
2
3
TEOS
(103)
l
m
n
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
2.27 (5.12)
2.27 (5.12)
2.27 (5.12)
2.27 (5.12)
2.27 (5.12)
1.90 (4.28)
1.80 (4.06)
1.60 (3.60)
0.02 (0.05)
0.07 (0.15)
0.12 (0.27)
0.28 (0.60)
0.59 (1.28)
0.10 (0.23)
0.21 (0.45)
0.42 (0.90)
0
1.6
1.8
1.9
1.8
2.1
1.9
1.9
1.9
2.4
2.1
2.5
2.1
2.2
6.0
6.3
5.6
1.2
1.2
1.2
1.2
1.3
1.4
1.4
1.4
99
97
95
90
80
33
30
32
1
3
0
0
0
0
0
5
0
10
20
1
0
0
0
0.94 (4.51)
0.94 (4.51)
0.94 (4.51)
66
66
61
1g
1h
4
7
a
c
Mole ratios were as follows: runs 1–5: HCl/(2 1 3) 5 0.12–0.15, H2O/
(2 1 3) 5 0.25–0.29, runs 6–8: HCl/(2 1 3) 5 0.105, H2O/(2 1 3) 5 8.0.
Polystyrene standard.
Polydispersity index.
Each ratio was estimated using 1H NMR and/or UV–vis spectroscopy.
d
b
General Polycondensation Procedure with BA Monomer
2, PBG Monomer 3, and TEOS to form Silicone Resin 1
BA monomer 2, PBG monomer 3 and TEOS were dissolved
in EtOH (4.2 g for runs 1–5 and 3.3 g for runs 6–8) and
stirred for 10 min at 0 ꢀC. Aqueous hydrochloric acid was
added to the solution, stirred at room temperature for
ꢀ
another 10 min, and then stirred at 70 C for 1–1.5 h under
flowing N2 (300–360 mL min21). The residue was dissolved
in THF, and dropped into hexane or cyclohexane to give the
silicone resins as a pale-yellow or white solid.
FIGURE 1 Time courses for the decrease in peak area (carbonyl groups): (a) 1e with (25 or 10 mJ cm22) and without irradiation
and (b) 1a–1e with 10 mJ cm22 of irradiation. [Color figure can be viewed in the online issue, which is available at wileyonlineli-
brary.com.]
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FIGURE 2 Time courses for the decrease in peak area (carbonyl groups): (a) 1h with (50, 25, or 10 mJ cm22) and without irradia-
tion and (b) 1f–1h with 10 mJ cm22 of irradiation. [Color figure can be viewed in the online issue, which is available at wileyonline-
library.com.]
For resin 1h, remaining silanol groups were capped with tri-
methylchlorosilane. Trimethylchlorosilane (0.23 g, 2.1 mmol)
in THF (10 mL) was dropped into a solution of 1h (0.30ꢀg)
with triethylamine (0.21 g, 2.1 mmol) in THF (10 mL) at 0 C.
The reaction mixture was stirred at 0 C for 20 min. After fil-
tration, the solvent was evaporated. The residue was dissolved
In the cases of 1a–1e using BA monomer 2 and PBG mono-
mer 3, molecular weights of 2100–2500 were obtained
(runs 1–5). It was found that the copolycondensation ratio
was approximately proportional to the amount of load of the
monomers, estimated by 1H NMR and/or UV–vis spectros-
copy. The resins were found by 29Si NMR to take T2 and T3
structures.37 H2O and HCl were added as accelerating
reagents for the hydrolysis polycondensation; however, the
molecular weight was not increased sufficiently. In contrast,
it was found that introduction of TEOS units led to an
increase in the resulting molecular weight of 1f–1h to 5600–
6300 (runs 6–8). The TEOS units could act as a spacer
between the bulky BA and PBG units. These polymers also
had T2 and T3 structures, recorded by 29Si NMR.37
ꢀ
in THF, and dropped into cyclohexane to give the capped 1h.
Base-Catalyzed Decomposition Behavior
A solution of silicone resin 1 (0.1 g) in chloroform (1 mL)
was spin cast onto a hydrophobized Si substrate, and pre-
baked at 100 ꢀC for 1 min. The time course of the FT-IR
spectra of the film of 1 was monitored with heating at 100
ꢀC after 365 nm light irradiation. The normalized peak inten-
sity was calculated as the ratio of the peak intensity of the
carbonyl group absorption band to the intensity of the peak
before UV irradiation.
Decomposition Behavior of the Copolymers
The synthesized photobase-sensitive resin should fulfil the
following requirements. First, the resin should be readily
decomposed by base catalyst to produce amino groups in its
side chain. Second, these amino groups are strong enough
bases to catalyse the b-elimination reaction at BA units to
form more amino groups, leading to autocatalytic decomposi-
tion. Third, the resin should be thermally stable in the
absence of base, at least under the same reaction conditions,
to enhance the autocatalytic decomposition initiated by pho-
togenerated base. Among the synthesized resins, 1e, which
contained 20% PBG units, was spin cast and a film of 0.9
mm in thickness was prepared on a hydrophobized Si wafer.
Irradiation with 365 nm light (25 mJ cm22) did not influ-
ence the peak intensity of the carbonyl band very much in
the FT-IR spectral measurement (SupportingꢀInformation Fig.
S1). In contrast, subsequent heating at 100 C decreased the
intensity. This result implied that BA units in the resin were
decomposed by amines photogenerated from PBG units. The
normalized peak intensity plotted against heating time is
shown in Figure 1 for resin 1e. The induction period, defined
as the heating time required to begin decomposition of the
urethane bonds, was about 30 min with 25 mJ cm22 of irradia-
tion energy [Fig. 1(a)]. Lowering the irradiation energy to 10
mJ cm22 slightly increased the period. Without UV irradiation,
decomposition occurred after an induction period of about
40 min. The decomposition behaviour was sigmoidal, which
Sensitivity Determination
A solution of silicone resin 1 (0.1 g) in chloroform (1 mL) or
in propylene glycol monomethyl ether acetate (0.5 mL) was
spin ꢀcast onto a hydrophobized Si substrate, and prebaked at
100 C for 1 min. The film of 1 was exposed to 365 nm light
ꢀ
irradiation and subsequent heating at 100 or 120 C. Develop-
ment of the film was performed with chloroform for 30 s to
obtain photosensitivity curves. The normalized film thickness
was calculated as the ratio of the film thickness after the
developing process to the thickness before UV irradiation.
Fabrication of Micropatterns
A solution of silicone resin 1e containing 20% PBG units in
chloroform was spin cast onto a hydrophobized Si substrate,
and prebaked at 100 ꢀC for 1 min. UV exposure was con-
ducted through a photomask with 365 nm light (110 mJ
cm22). After PEB at 100 ꢀC for 25 s and subsequent devel-
opment with benzene, a micropatterned film on the sub-
strate was obtained.
RESULTS AND DISCUSSION
Syntheses of Copolymers 1 Using 2, 3, and TEOS
Table 1 shows the synthetic results for resins 1a–1h. Com-
position ratios were varied between BA, PBG and TEOS units.
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FIGURE 3 Illustration of the procedure for photosensitivity determination and chemical changes in a film of 1. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
indicated exponential increase of amino groups in the resin, as
becomes a base-proliferation reaction, during PEB. The propor-
tion of PBG units also influenced the decomposition behaviour
of the resins, comparing between 1a and 1e. The induction
period decreased in the order 1% > 20% > 10% > 5% > 3%
[Fig. 1(b)]. In the case of the 1% PBG unit, the amount of pho-
togenerated amines would not be adequate to trigger the base-
proliferation reaction. Except for this case, decreasing the
amount of PBG units increased the decomposition rate.
Because PBG units are not decomposed by amine catalysts,
they do not contribute to base-proliferation reactions. The
density of the BA units was found to be a key factor for effec-
tive reaction during PEB. A similar decomposition behavior
was observed in measurements using films of resins 1f–1h
synthesized from BA monomer 2, PBG monomer 3, and TEOS
(0.7–0.9 mm in thickness, Fig. 2). The induction periods of
these films (50–120 min) were longer than those of films of
1a–1e, and a higher heating temperature (110 ꢀC) was
required. Nevertheless, the carbonyl groups decomposed in a
non-linear manner, and the induction period decreased with
increasing irradiation energy. Resin 1g containing 4% PBG
units reacted more sensitively than resin 1h containing 7% of
FIGURE 4 Photosensitivity curves (a) of films of 1c–1e after heating for 15 min and (b) of a film of 1d. The PEB temperature was
100 ꢀC.
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FIGURE 5 (a) Photosensitivity curves of films of 1h after heating at 120 ꢀC for 60 min, or at 100 ꢀC for 15 min. (b) Photosensitivity
curve of a film of 1h after capping silanol groups with trimethylchlorosilane, heating at 120 ꢀC for 60 min. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
the units. This shows that the ratio of BA units to PBG units
was crucial for an effective decomposition reaction.
A film having a lower fraction of PBG units showed higher
sensitivity. The sensitivity was improved by increasing the
heating time [Fig. 4(b)]. These results would be because of
promotion of the base-proliferation reaction. With heating
Photosensitivity Determination
for 30 min at 100 ꢀC, high photosensitivity (8.1 mJ cm22
)
Evaluation of photosensitivity was performed with films of
resins 1c–1e, and 1h (0.9 mm, Fig. 3). After 365 nm light
was accomplished using resin 1d containing 10% PBG
units.38 The normalized film thickness was about 0.4, prob-
ably because carbon dioxide and dibenzofulvene could be
removed during PEB and subsequent development.
ꢀ
irradiation and PEB at 100 C, the film was developed using
chloroform for 30 s. The thickness of the remaining film was
measured and plotted as normalized film thickness against
UV irradiation energy. Figure 4(a) shows photosensitivity
curves for films of resins 1c, 1d, and 1e. These curves indi-
cate that the resins functioned as negative-working resists
that were insolubilized by increasing irradiation energy.
In the case of 1h film having TEOS units, 400 mJ cm22 of
irradiation energy was required with heating for 15 min
(Fig. 5). In contrast, the film was insolubilized without
FIGURE 6 Sensitivity curves of films of resin 4 having blank/PBG units (blank:PBG 5 74:26, ᭺) and resin 5 having BA/PBG units
(BA:PBG 5 82:18, •). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 7 (a) Negative-tone micropatterns of a film of 1e containing 20% PBG units after UV irradiation at 110 mJ cm22 through a
photomask, PEB at 100 ꢀC for 25 s and development with benzene for 30 s. (b) 10 3 10, 6 3 10, and 4 3 10 mm line-and-space pat-
terns. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
irradiation after heating for 60 min at 120 ꢀC, probably
because of a condensation reaction between the remaining
ethoxy silyl groups or silanol groups. Trimethylchlorosilane
was then used to cap the remaining silanol groups. A photo-
sensitivity of 30 mJ cm22 for the resin film was accom-
plished by heating for 60 min at 120 ꢀC. TEOS units could
contribute to increasing the value of the normalized film
thickness to about 0.6, although the units would decrease
the density of PBG and BA units, leading to low
photosensitivity.
times enhanced compared with resin 4, even though resin 5
has a smaller amount of PBG units than resin 4. This would
strongly demonstrate that introducing base-proliferation
reactions to photoresist systems is a powerful approach for
enhancement of the photosensitivity of materials consisting
of base-reactive polymers and PBGs.
These resins could be applied to fabricate micropatterns,
and a demonstration was performed. A film of resin 1e (film
thickness: 0.9 mm) containing 20% PBG units was used. Irra-
diation with 365 nm light (110 mJ cm22) followed by PEB at
ꢀ
100 C for 25 s successfully gave 10 3 10, 6 3 10, and 4 3
To evaluate the effect of the base-proliferation reaction by
BA/PBG units on the photosensitivity, an additional experi-
ment was performed (Fig. 6).39 Instead of BA units, blank
units having amines capped by 9-fluorenyloxycarbonyl
groups were combined with PBG units. The ratio of blank/
PBG units of the resulting resin 4 was 74/26. This resin
does not undergo the base-proliferation reaction, while the
PBG units generate primary amines. The corresponding resin
5 was also synthesized in a similar manner where the ratio
of BA/PBG units was 82/18. UV irradiation towards films of
these resins (0.8–0.9 mm in thickness) was performed using
a Hg–Xe lamp without a glass filter, followed by PEB at 100
ꢀC for 20 min. The film was developed using chloroform for
30 s. It took 240 s to obtain the value of 0.31 of normalized
film thickness in the case of resin 4. On the other hand, it
took just 0.49 s in the case of resin 5, where the value was
0.47. Interestingly, the photosensitivity of resin 5 was ca 490
10 mm line-and-space patterns, where development with
benzene for 30 s was performed (Fig. 7).
CONCLUSIONS
Base-proliferation reactions in which BA units are decom-
posed autocatalytically using a base catalyst can be inte-
grated in a photoresist system with a photobase generator,
as required for enhancement of photosensitivity. Silicone res-
ins bearing both BA units and PBG units were synthesized.
TEOS was utilized as a spacer unit, resulting in relatively
high molecular weights. The BA units in these resins were
found to be decomposed abruptly after an induction period,
which indicated autocatalytic decomposition triggered by
amines generated from PBG units. High photosensitivity, 8.1
mJ cm22, was accomplished using a film of the resin
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containing 10% PBG units with PEB for 30 min at 100 ꢀC.
To our great delight, the potential ability of the base-
proliferation reaction was clearly shown by comparison
between resins having blank/PBG or BA/PBG units, which
showed about 490 times enhancement of photosensitivity.
Finally, a negative-tone relief image with 4 3 10 mm line-
and-space pattern was successfully fabricated, which was
comparable to a conventional acid-amplifying resist.
20 S. Fukuda, K. Amagai, S. Kanke, K. Sakayori, J. Photopolym.
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