Novel polymeric nonionic photoacid generators and corresponding polymer Langmuir-Blodgett (LB) films for photopatterning

Article abstract of DOI:10.1016/j.jphotochem.2011.01.015

A series of new polymeric nonionic photoacid generators (PAGs) and PAG-bound polymers designed for photoresist materials in Langmuir-Blodgett (LB) films have been synthesized and characterized. The novel polymer could form a stable and condensed monolayer on water surface, which could be transferred successfully onto solid substrate. Upon deep UV irradiation, the acid generated by the photoacid generator catalyzed the naphthyl moiety to liberate naphthol and regenerate carboxyl in the exposed region. The rent moiety could dissolve in alkaline aqueous, resulting in a fine positive tone resist patterns with a resolution of 0.75 μm. Sensitivity curves and TGA studies revealed that the high sensitivity in 248 nm irradiation was attributed to the present of PAG units incorporated in the polymer chains. The result of translated gold pattern with the same resolution as the resist pattern also demonstrated that the resist LB films had sufficient resistance to wet etching process.

Full text of DOI:10.1016/j.jphotochem.2011.01.015

Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Research paper
Novel polymeric nonionic photoacid generators and corresponding polymer
Langmuir–Blodgett (LB) films for photopatterning
Wenjian Xua, Tiesheng Lia,∗, Gaojian Lia, Yangjie Wua,∗, Tokuji Miyashitab
a Department of Chemistry, Zhengzhou University, The Key Lab of Chemical Biology and Organic Chemistry of Henan Province, The Key Lab of Advanced Nano-information Materials
of Zhengzhou, Zhengzhou 450052, PR China
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira Aoba-ku, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new polymeric nonionic photoacid generators (PAGs) and PAG-bound polymers designed
for photoresist materials in Langmuir–Blodgett (LB) films have been synthesized and characterized. The
novel polymer could form a stable and condensed monolayer on water surface, which could be transferred
successfully onto solid substrate. Upon deep UV irradiation, the acid generated by the photoacid generator
catalyzed the naphthyl moiety to liberate naphthol and regenerate carboxyl in the exposed region. The
rent moiety could dissolve in alkaline aqueous, resulting in a fine positive tone resist patterns with a
resolution of 0.75 ␮m. Sensitivity curves and TGA studies revealed that the high sensitivity in 248 nm
irradiation was attributed to the present of PAG units incorporated in the polymer chains. The result of
translated gold pattern with the same resolution as the resist pattern also demonstrated that the resist
LB films had sufficient resistance to wet etching process.
Received 26 September 2010
Received in revised form 19 January 2011
Accepted 21 January 2011
Available online 1 February 2011
Keywords:
Photoacid generator
Langmuir–Blodgett films
Polymer resist
Photolithography
Photodegradation
© 2011 Published by Elsevier B.V.
1. Introduction
film. It has been reported that the PAG blended CA resist mate-
rials have inherent incompatibility between polymer matrix and
The microelectronic industry has made remarkable progress
with the development of integrated circuit (IC) technology, which
depends on the research efforts in a variety of high resolution
lithography techniques, including exposure tools (electron beam,
X-ray and deep UV technologies), sensitive resist materials system
and appropriate film-forming methods. The major requirement for
photoresists is that upon exposure to irradiation they undergo a
structural change to provide solubility differentiation between the
exposed and unexposed regions (controlled by mask). Chemically
amplified (CA) resists is a promising class of materials for achieving
high sensitivity and resolution [1–3]. In designing a resist system
based on the chemical amplification concept, it has to incorpo-
rate two basic components, namely, an acid-labile compound or
polymer and a photoacid generator (PAG) which should generate a
strong acid by UV irradiation in solid state [4,5]. Recently, tremen-
dous efforts have contributed towards the design and synthesis of
new PAG for application in CA resist systems. Ionic PAGs enjoys
several advantages such as high thermal stability, easily modified
strength and size of the photogenerated acid [6,7]. Nonionic PAGs
have a wide range of solubility in organic solvents and polymer
PAG, which can lead to PAG phase separation, PAG aggregation,
nonuniform initial PAG and photoacid distribution, as well as acid
migration during the post exposure baking (PEB) processes [8,9].
The best way to eliminate the problems associated with physically
blended PAG/polymer resists is, perhaps, to incorporate the PAG
units in the polymer chain to make PAG-bound resist, rather than
physically blending the PAG with the polymer [10–12]. Wang et al.
have reported that the incorporation of anionic PAG units into the
main chain of the adamantly methacrylate based polymers showed
faster photo speed, higher stability, lower outgassing and lower line
edge roughness (LER), comparing with the PAG blended polymer
resists [13–16]. Wu et al. have reported that the CA resists with
cationic PAG unit in the polymer chain exhibited excellent film for-
mation behavior due to absence of phase separation, which would
often be observed in CA resists formulated by mixing a polymer
matrix with small molecule PAGs [17,18]. Chung et al. attempted
incorporation of camphorsulfonyl groups into a polymer chain by
means of N-substitution in the maleimide monomer to provide high
efficiency in photoacid generation and thermal stability [6]. Ahn
et al. have reported the nonionic maleimide (MI) bound polymer
has special physicochemical properties such as high thermal sta-
bility, high chemical resistance and low absorption in the deep-UV
(DUV) region [6,19,20].
∗
Ultra-thin films of photoresist less than 150 nm in thick-
ness may be necessary to implement single layer photoresist
Corresponding authors. Tel.: +86 371 67766667; fax: +86 371 67766667.
E-mail addresses: lts34@zzu.edu.cn (T. Li), wyj@zzu.edu.cn (Y. Wu).
1010-6030/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.jphotochem.2011.01.015

W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
51
O
Et3N
S
O
+ ClO2S
CH3
H3C
OH
Br
15h
O
AOTs
CH CN
O
O
3
TsCl
Et3N
OH
HO
+
K2CO 80
3
OH
Ts
O
AOP
AOPTs
Cl
O
O
O
O
O
Et3N
10h
TsCl
HO
+
OH
10h
Et3N
OH
OTs
HOPMA
TsPMA
O
Ts:
S
CH3
O
Fig. 1. Synthetic scheme of PAG monomers.
strategies for vacuum ultra-violet (VUV) and extreme ultravio-
let (EUV) lithography [21,22]. In these wavelength regimes, light
is strongly absorbed by most organic materials. The thickness
of the resist film must therefore be reduced to maintain suffi-
cient transparency to obtain high quality, anisotropic patterns.
Langmuir–Blodgett (LB) technique is more advanced than the spin-
coating and self-assembly in preparing a thin film with a controlled
thickness at a molecular size and well-defined molecular ori-
entation as well as the desquamation of resists from substrate
[23,24]. So, just because of their superior features, polymer LB
films have been recently investigated in the application to high-
resolution lithographic technology [25–27]. However, CA resists
LB films with PAGs in their chains have not received wide atten-
tion in the imaging industry, despite their potential advantages
over their counterparts that are formulated by traditional spin-
coating.
In this paper, we report a series of new polymeric nonionic
PAGs and CA resists LB films with PAG units in the polymer
main chain. The design of photoresist molecular was based on
a methacrylate polymer backbone due to minimal deep UV
absorbance: N-dodecylmethacrylamide (DDMA) was included in
the microstructure to improve the film-forming [26]; ␤-naphthyl
methacrylate (NPMA) was incorporated as the acid-cleavable
bulky aromatic substituent groups to improve resolution, sensi-
tivity and dry etch resistance [28]; allyl-4-methylbenzensulfonate
(AOTs), 4-(allyloxy)phenyl-4-methbenzenesulfonate (AOPTs) and
4-(tosyloxy) phenyl methacrylate (TsPMA) (the synthesis schemes
were outlined in Fig. 1) were bound in polymer chain as photoacid
generator (PAG). The microstructure of terpolymer is outlined in
Fig. 2. Considering these superior properties, we expect to improve
not only the sensitivity but also the imaging quality of this new
kind of resist system.
2. Experimental
2.1. Synthesis of the PAG monomer
Three kinds of PAG monomer were synthesized by multistep
procedure. The synthesis schemes were outlined in Fig. 1.
2.1.1. Synthesis of allyl-4-methylbenzensulfonate (AOTs)
AOTs was synthesized from p-toluenesulfonic chloride with
allylalcohol in the presence of triethylamine and chloroform at
0 ◦C. After 15 h, the reaction mixture was washed three-times with
NaCl aqueous solution. Then the organic layer, which was dried
with anhydrous Na2SO4 overnight, was filtrated and concentrated
to give the colorless oil. The crude product was purified by col-
umn chromatography (200–300 mesh of silica gel, eluted: acetic
ether/petroleum ether = 1/6). Colorless oil was obtained after the
solution was removed and dried under vacuum overnight. The
yield was 78.3%. The 1H NMR (400 MHz) data was given as follows:
(CDCl3, ppm) ı 2.44 (s, 3H), 4.47–4.48 (d, 2H), 5.29–5.30 (s, 1H),
5.40–5.41 (s, 1H), 5.98–6.03 (d, 2H), 6.78–6.79 (d, 2H), 6.85–6.86(d,
2H), 7.29–7.31 (d, 2H), 7.67–7.69 (d, 2H). FT-IR (cm−1): 3031 (ꢀC–H
of -benzene), 2929 (ꢀC–H of ane), 1494, 1598 (ꢀC of benzene),
1421 (ꢀas of –CH3), 1363 (ꢀ–SO2– of ester), 10782 (ꢀS–O–C of ester).
MS (EI) m/z: 233 (M + H).
C
2.1.2. Synthesis of 4-allyloxyphenol (AOP)
A mixture of hydroquinone allyl bromide, anhydrous potassium
carbonate and acetonitrile was refluxed for 12 h and cooled. The
reaction mixture was filtrated and concentrated. It was diluted with
50 mL dichloromethane, and washed once with 1 N HCl aqueous
solution and twice NaCl aqueous solution. Then the organic layer,
which was dried with anhydrous Na2SO4 overnight, was filtrated
and concentrated to give the brown oil. The crude product was
purified by column chromatography (200–300 mesh of silica gel,
eluted: acetic ether/petroleum ether = 1/5). Brown oil was obtained
after the solution was removed and dried under vacuum overnight.
The yield was 31.6%. The 1H NMR (400 MHz) data was given as fol-
lows: (CDCl3, ppm) ı 4.47–4.48 (d, 2H), 5.26–5.29 (s, 1H), 5.37 (s,
1H), 5.42 (d, 1H), 5.99–6.07 (m, 1H), 6.75–6.84 (q, 2H). FT-IR (cm−1):
3403 (ꢀC–H of O–H), 3027 (ꢀC–H of benzene), 2983, 2925 (ꢀC–H of
ane), 1644 (ꢀC C of ene), 1453, 1506, 1608 (ꢀC C of benzene), 1359
(ꢀas of –CH3), 1092 (ꢀC–O–C of ether). MS (EI) m/z: 151 (M + H).
2.1.3. Synthesis of 4-(allyloxy) phenyl-4-methbenzenesulfonate
(AOPTs)
A mixture of 4-allyloxyphenol (AOP), p-toluenesulfonic chlo-
ride, triethylamine and chloroform was stirred for 10 h at 0 ◦C. A
Fig. 2. Microstructure scheme of p(DDMA/NPMA/PAG).

52
W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
Table 1
Polymerization results of terpolymer.
Polymer
Compositions (in polymer*)
mol.%
Mn
Mw
PDI
Yield
%
DDMA
NPMA
TsPMA
(×104)
(×104)
a
b
c
80(58.2)
70(63.9)
60(67.5)
50(50.0)
10 (32.3)
20 (19.6)
20 (17.1)
20 (25.0)
10 (9.5)
0.84
1.23
1.09
1.38
3.04
3.26
5.12
3.24
3.62
2.56
4.68
2.35
78.1
75.3
86.2
80.1
10 (16.5)
20 (15.4)
30 (25.0)
d
*
Measured with 1H NMR.
series of purification procedure were done as the method of AOP.
Brown solid was obtained at a yield of 99.2%. mp: 47–48 ◦C. The
1H NMR (400 MHz) data was given as follows: (CDCl3, ppm) ı
2.44 (s, 3H), 4.47–4.48 (d, 2H), 5.29–5.30 (s, 1H), 5.40–5.41 (s, 1H),
5.98–6.03 (m, 1H), 6.78–6.79 (d, 2H), 6.85–6.86 (d, 2H), 7.29–7.31
(d, 2H), 7.67–7.69 (d, 2H). FT-IR (cm−1): 3070 (ꢀC–H of benzene),
2983, 2925 (ꢀC–H), 1704 (ꢀC of ester), 1648 (ꢀC of ene), 1461,
ture [26,28]. A typical procedure was as follow: the monomers and
1 wt% initiator respect to monomer (including PAG monomer) were
dissolved in dioxane. The solution was filtered through a 0.45 ␮m
Teflon micro filter into a sealed flask. N2 in-pump-out cycles were
run three times to remove oxygen. The clear solution was then
placed in a 60 ◦C water bath under stirring. The reaction was main-
tained under nitrogen for 24 h. The polymer was isolated by pouring
the reaction solution into sufficient amount of n-hexane with vig-
orous stirring, filtered and washed thoroughly with hexane. Then
the polymer was purified by redissolving in acetone and reprecipi-
tating from distilled and deionized water or methanol. Finally, the
polymer was dried in vacuum at 50 ◦C over night. In this way, PAG
homopolymers pTsPMA and terpolymer p(DDMA/NPMA/TsPMA)
were prepared respectively. Table 1 lists the characteristic data of
p(DDMA/NPMA/TsPMA).
O
C
1502, 1596 (ꢀC of benzene), 1421 (ꢀas of –CH3), 1639 (ꢀ–SO2– of
C
ester), 1092 (ꢀC–O–C of ester), 1013 (ꢀC–O–C of ether). MS (EI) m/z:
327 (M + Na).
2.1.4. Synthesis of 4-hydroxyphenyl methacrylate (HOPMA)
HOPMA was prepared by reacting methacryloyl chloride with
the hydroquinone in the presence of triethylamine and chloroform
at 0 ◦C. ␣-Methacryloyl chloride in 15 mL of anhydrous chloro-
form was dropwisely added in 50 mL of hydroquinone chloroform
with 20 g hydroquinone solution at 0 ◦C. The mixture was stirred
at room temperature for 10 h and poured into 1 N HCl ice water
containing NaCl. The organic layer, which was dried with anhy-
drous Na2SO4 overnight, was filtrated and concentrated to give the
pale-yellow solid. The crude product was purified by column chro-
matography (200–300 mesh of gel, eluted: acetic ether/petroleum
ether = 1/5). A pale-yellow crystalloid was obtained after the solu-
tion was removed and dried under vacuum overnight. Yield: 35.7%,
mp: 119–120 ◦C. The 1H NMR (400 MHz) data was given as fol-
lows: (CDCl3, ppm) ı 2.06 (s, 3H), 2.19 (s, 1H), 5.76 (s, 1H), 6.35
(s, 1H), 6.72–6.75 (d, 2H), 6.90–6.93 (d, 2H). FT-IR (cm−1): 3459
2.3. Materials and characterization
Hydroxylamine, acetone, methanol, hexane, THF (tetrahy-
dronfunan), p-Hydroquinone (HQ), and maleic anhydride
were purchased from Tianjin Chemical Reagents. Furan, 2,
2ꢀ-azobisisobutyronitrile (AIBN), rhodamine
B base (RB) and
p-toluenesulfonic chloride were purchased from China National
Medicine Co. Other chemicals were purified by conventional
methods.
1H NMR was measured with NMR DPX-400 (Bruker, German) in
CDCl3. UV–visible absorption spectrum was obtained on Lambda 35
UV–Visible spectrophotometer (Perkin Elmer Inc., USA). Infrared
spectra were obtained with Bruker Vector-22. Deep UV irradia-
tion was carried out with EX250 UV light source (Honya-Schott
Ltd., Japan). The molecular weight was determined by PL-GPC
50 in tetrahydrofuran (THF) with polystyrene standards. Thermo-
gravimetric analysis (TGA) was obtained on NETSCH TG 209 in
N2 atmosphere. Photopatternings were observed by microscope
(OLYMPUS BX51, OLYMPUS Co., Japan). The thickness of thin film
was determined with an Optical Thin Film Analysis System NKD-
8000v (Aquila Instruments Ltd., UK).
(ꢀO–H), 3033 (ꢀC–H of benzene), 2955, 2924 (ꢀC–H), 1708 (ꢀC of
O
ester), 1633 (ꢀC C of ene), 1441, 1507, 1588 (ꢀC C of benzene), 1009
(ꢀC–O–C of ester). MS (EI) m/z: 176.8 (M − 1).
2.1.5. Synthesis of 4-(tosyloxy) phenyl methacrylate (TsPMA)
TsPMA was synthesized from p-toluenesulfonic chloride with
4-mathacrylocoxyphenol (MAOP) in the presence of triethylamine
and chloroform at 0 ◦C. After 10 h, the reaction mixture was washed
twice with NaCl aqueous solution. Then the organic layer, which
was dried with anhydrous Na2SO4 overnight, was filtrated and
concentrated to give the pale yellow solid. The crude product was
purified by column chromatography (200–300 mesh of gel, eluted:
acetic ether/petroleum ether = 1/5). A pale yellow crystalloid was
obtained after the solution was removed and dried under vacuum
overnight. Yield: 98.6%, mp: 78–79 ◦C. The 1H NMR (400 MHz) data
was given as follows: (CDCl3, ppm) ı 2.04 (s, 3H), 2.45 (s, 3H), 5.77 (s,
1H), 6.33 (s, 1H), 6.98–7.00 (d, 2H), 7.04–7.06 (d, 2H), 7.31–7.33(d,
2H), 7.70–7.22 (d, 2H). FT-IR (cm−1): 3070 (ꢀC–H of benzene), 2962
Measurement of surface pressure (ꢁ)–area (A) isotherms and
the deposition of monolayer were carried out with a computer
controlled Langmuir–Blodgett system (KSV-5000-3, KSV Instru-
ments, Helsinki, Finland) at a compression speed of 10 mm/min.
Distilled and deionized water with resistivity of 18.2 Mꢂ cm (MILI-
Q gradient MILIPORE CO., USA) was used for subphase. The
terpolymer was dissolved in chloroform at a concentration of
approximately 0.5 mg/mL and the solution was spread on the
water surface by a micrometric syringe. Waiting for about 30 min
to let the solvent evaporate out, the monolayer compression
started at a compression rate of 10 mm min−1. The isotherms for
every sample were measured for twice to ensure good repro-
ducibility. Quartz, glass and silicon substrate slides used for LB
films deposition were prepared by being treated with boiling
concentrated HNO3 solution, and then washed with water to
ensure its hydrophile. All measurements were carried out at room
temperature.
(ꢀC–H of ane), 1735 (ꢀC O of ester), 1633 (ꢀC of ene), 1454, 1500,
C
1596 (ꢀC of benzene), 1373(Cs=O of ester). MS (EI) m/z: 355.1
C
2
(M + Na).
2.2. Synthesis of polymers
Both homo and copolymerization were carried out by con-
ventional free radical polymerization with AIBN as initiator. The
N-dodecylmethacrylamide (DDMA) and ␤-naphthyl methacrylate
(NPMA) was synthesized as the methods mentioned in the litera-

W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
53
3.0
2.5
2.0
1.6
1.2
0.8
1.2
1.0
0.8
0.6
0.4
0.2
TsPMA
AOTs
2.0
555 nm
1.5
0
1.0
0
0.5
1.0
1.5
2.0 2.5
Acid concentration (μM)
AOPTs
0.5
0
0.4
0
0
0.2
0.4
0.6
0.8
1.0
1.2
Dose (J/cm2)
190
290
390
490
590
Fig. 4. Acid generation efficiency vs. doses for PAG monomer in acetonitrile.
Wavelength (nm)
Fig. 3. Absorption spectra of rhodamine B base without and with varying amounts
of 4-methylbenesulfonic acid in acetonitrile at room temperature. Inset: absorption
at 555 nm as a function of the acid concentrations.
ation source. Since the polymers contain varying amounts of the
acid generating units, comparison of the acid generating efficiency
for each polymer was expressed as mole of TsOH acid per mole of
the PAG units in the polymer. The number of moles of the PAG units
in each polymer was determined by 1H NMR.
3. Results and discussion
Fig. 4 showed the results obtained from the PAG monomers
of AOTs, AOPTs and TsPMA in acetonitrile. In all cases, the
acid generation efficiency increased as the doses of radiation
applied. Surprisingly, all samples followed a similar pattern, which
suggested that they had the same acid generating mechanism
depended on irradiation dose. At the forepart of irradiation, the
acid generation efficiency rose rapidly until 200 mJ/cm2, and then
underwent a slow growth. TsPMA had the highest acid yield among
of them. So a series of copolymer film contained PAG monomer
TsPMA had been used to measure the acid generation efficiency as
follow.
Fig. 5 showed the results obtained for copolymers b–d. In all
cases, the acid generation efficiency increased with doses of radia-
tion applied. All three samples followed a similar pattern, viz. there
was a down fold around 50 mJ/cm2 in the slow climbing curve of
acid generation efficiency vs. dose of radiation.
The possible explanation is generated to interpret their different
demonstration as follows. It had been well reported that the sen-
sitivity of radiation-sensitive compound usually depended on its
environment. While the radiation-sensitive groups in such a com-
pound could absorb energy directly from radiation, and they could
obtain energy indirectly from its neighbor as well, which is called
as energy transfer. Unlike a discrete PAG, the PAG units in polymer
d were probably incorporated into the polymer chains and there-
fore could absorb energy from neighboring repeating units. This
3.1. Acid generating efficiency measurements
The acid generating efficiency of the monomers and poly-
mers were measured by xanthene dyes rhodamine B base (RB)
[29]. To calibrate the absorption of RB, varying amounts of
4-methylbenesulfonic acid and RB stock solution (CRB = 2.8 ×
10−5 mol L−1) were placed in a 4 mL quartz cell. The corresponding
UV–visible spectra were recorded on a Perkin-Elmer Lambda 35
spectrophotometer, in which acetonitrile was taken as reference.
The absorption maximum at 555 nm was measured on the spectra.
In Fig. 3, bulky line showed the UV–visible spectra RB in acetoni-
trile as a function of 4-methylbenesulfonic acid concentrations. The
absorbance increasing at 555 nm was caused by successive addition
of 4-methylbenesulfonic acid to the solution in 0.022 ␮mol incre-
ments. A straight line was obtained when absorbance at 555 nm
was plotted as a function of the acid concentrations, as shown in
Fig. 3 inset, which can be used as calibration curves for determining
the acid concentrations of PAG.
To measure the acid generation efficiency of the PAG-bound
polymers, films were casted on 30 mm × 30 mm quartz plate whose
weights were predetermined on an analytical balance, from 10 wt%
solutions in acetone. Then polymer films were baked at 90 ◦C for
5 min to remove the solvent and weighted again. The amount of
polymer in film was calculated. Then the polymer film was exposed
to 248 nm DUV radiations for varying length of time. Doses were
calculated by multiplying exposure times by lamp intensity. The
exposed film was immediately stripped from the plate with ace-
tonitrile and collected in a 20 mL measuring flask. It was added
2.8 × 10−5 mol L−1 RB stock solution. The total volume was then
raised to 20 mL by addition of acetonitrile. The UV–visible spec-
trum of the resulting solution was obtained using acetonitrile as
reference. The amount of acid generated in film by UV radiation was
determined by measuring the absorbance at 555 nm and comparing
with the calibration curve.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
b
c
d
1.1
Copolymer samples contained PAG were designed as potential
CA resists. As discussed earlier [13–16], the acid generation effi-
ciency of PAG in a polymer/PAG blend was highly dependent on
the miscibility of the polymer and the PAG. In the present case,
acid generating units were incorporated into the polymer chains
for polymers a–d. Therefore the acid generation efficiency for these
polymers was determined.
0
0
0.1
0.3
0.5
0.7
0.9
1.3
Dose (J/cm2)
The acid generation efficiency of these polymers was measured
by the rhodamine B base (RB) method using 248 nm DUV as a radi-
Fig. 5. Acid generation efficiency vs. doses for the thin film of p(DDMA/
NPMA/TsPMA).

54
W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
Fig. 8. UV absorption of PAG-copolymer b LB films as a function of the number of
deposited layers. Inset: plots of the absorbance at 225 nm vs. the number of LB films
deposited.
Fig. 6. Surface pressure–area isotherms of the PAG-copolymer on the water surface
at 25 ◦C with a compression rate of 10 mm/min.
additional energy could assist in fission of oxygen–sulfur bonds
that were necessary for acid generation. More importantly, since
the photo-induced acid generation from a sulfonic group involved
a hydrogen-abstracting process, these neighboring repeating units
provided much more abundant hydrogen resource to the sulfonic
groups. Maybe this was not the case with a polymer/PAG blend.
and the c gave the highest peak value. All of them will be prof-
itable for achieving optimum conditions for LB films deposition of
terpolymer monolayer.
The multilayer transfer behavior of the copolymer monolayer
was examined by UV–visible spectra. Fig. 8 showed the UV–visible
absorbanceofthe b LB films as afunction ofthenumber of deposited
LB films on quartz substrate at 25 mN/m (with a total average trans-
fer ratio of 0.9 in upward stroke). The absorbance at 225 nm was
proportion to the number of layers at least up to 90 layers. The lin-
ear relationship between the absorbance and the number of layers
suggested that a regular deposition of the copolymer monolayer
took place (Fig. 8 inset), resulting in a fair uniform LB films. Fig. 9
showed plots of the absorbance at 225 nm vs. the number of LB
films deposited for TsPMA copolymer a, b, c and d. All of these indi-
cated that the LB films of PAG-bound copolymer were ordered film
with well-defined architecture.
3.2. Behavior of monolayer and LB films formation at the
air/water interface for PAG-bound copolymers
Fig. 6 showed the surface pressure (ꢁ)–area (A) isotherms of
Langmuir monolayer of p(DDMA/NPMA/TsPMA) copolymers. For
comparison, the isotherm of TsPMA homopolymer (pTsPMA) was
also shown in Fig. 6. The (ꢁ)–area (A) isotherms of PAG-bound
copolymer with various compositions showed that all the copoly-
mers formed stable monolayer on water surface under a high
surface pressure (ꢁ). Compared with copolymer, the curve of PAG
homopolymer showed the best ability of forming more stable
monolayer with the highest collapse pressure (50 mN/m). This
maybe attribute to the big steric hindrance of naphthyl side group
in the copolymers.
The static elasticity (Es)–surface pressure (ꢁ) isotherm [30]
of p(DDMA/NPMA/TsPMA) with various TsPMA had been stud-
ied (Fig. 7). For each Es–ꢁ curve in Fig. 7, two peaks appeared in
the liquid-expandedand theliquid-condensed sectionrespectively.
The isotherm of PAG homopolymer demonstrated the widest peak,
3.3. Decomposition of p(DDMA/NPMMA/TsPMA) Induced by deep
UV light
Fig. 10 showed the change of UV absorption spectra of
p(DDMA/NPMMA/TsPMA) c LB films with 86 layers deposited onto
quartz following deep UV irradiation. The maximum absorbance
at 196 nm and 225 nm, which can be assigned to the characteris-
0.8
120
0.6
c
100
225 nm
homo
d
80
60
40
20
0
a
0.4
b
b
a
c
0.2
d
0
0
20
40
60
80
0
20
40
60
80
100
Surface Pressure (mN/m)
Number of layers
Fig. 7. The static elasticity as a function of surface pressure on water surface at 25 ◦
C
Fig. 9. Plots of the absorbance at 225 nm vs. the number of LB films deposited for
for p(DDMA/NPMA/TsPMA).
PAG-containing copolymer.

W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
55
Fig. 10. UV absorption spectra for the LB films of p(DDMA/NPMA/TsPMA)s c LB films
with 86 layers on a quartz at different irradiating times. Insert: relationship between
absorbance and irradiation time at 196 and 224 nm.
Fig. 12. Change of IR spectra of terpolymer p(DDMA/NPMA/TsPMA)s c casting mem-
brane, which were deposited on KBr tablet, irradiated by deep UV light for 0, 1, 3, 5,
15 and 25 min.
KBr tablet. The peak at 1274 cm−1 assigned to the stretching vibra-
tion of SOO moiety of sulphonate decreased with irradiation time
increasing and disappeared after UV irradiation for 15 min, which
confirmed that the destruction of the sulphonate unit happened in
side chain of polymer. The destruction of the benzene and naph-
thalene ring was confirmed by the disappearance of weak bands
at 1480 cm−1 and 1592 cm−1, which were assigned to the charac-
teristic frequency for a skeletal in-plane vibration of the benzene
and naphthalene ring. Meanwhile, the peaks at 3064 cm−1 assigned
to the stretching vibration of C-H in aromatic nucleus disappeared
after 248 nm UV irradiation 1 min, which could also be seen from
the IR spectra. All the results indicated that the sulphonate was
decomposed and new functional group was produced, such as
–COOH or –OH. Absorbance at approximately 3400 cm−1, assigned
to the stretching vibration group of N-H, free (non-associated) and
hydrogen bonded hydroxyl/hydroperoxide group, was decreased
with increasing irradiation time. The phenomenon could be seen
in the UV spectrum of terpolymer p(DDMA/NPMA/TsPMA)s c as
shown in Fig. 10. Moreover, the peaks at 1750 cm−1 assigned to
the stretching vibration of C O decreased, which was assigned to
the carbonyl group weakened by the deep UV irradiation. This also
meant that the benzene and naphthalene ring group was removed
from the side chain to give a phenol group so that the residual could
be dissolved in alkaline solution. Such decrease involved not only
the side groups but also the backbone structure, as shown by the
spectral changes in the zone of –C–H stretching of CH2/CH3 groups,
2850–3000 cm−1. Key to the chemical amplification feature of this
resist was the fact that proton that initiates the overall reaction
sequence was not lost, which then became available for additional
deprotection. PAG can generate proton when irradiated by UV light,
so it can catalyze the deprotection of photoresist.
tic absorption of benzene and naphthalene ring, decreased with
the increasing irradiation time. Apparently, upon irradiation, the
LB films underwent photodegradation of polymer side chain. The
absorbance decreasing in such a short time indicated that the LB
films of p(DDMA/NPMA/TsPMA)s were sensitive to deep UV light.
The sensitivity curves were measured for the samples of
p(DDMA/NPMA) (0 mol.% TsPMA), p(DDMA/NPMMA/TsPMA17)
(16.5 mol.% TsPMA), p(DDMA/NPMMA/TsPMA25) (25.0 mol.%
TsPMA) LB films with 39 layers. The residual thickness of the
LB films in the exposed portion was measured as a function of
the exposure time (Fig. 11). The results of the normalized film
thickness decreasing with increasing exposure time showed they
were the typical positive type resist, i.e., the irradiation region
of copolymer LB film was effectively decomposed by deep UV
irradiation and removed completely with a developer of alkaline
aqueous (TMAH). It was clear that due to the presence of the PAG,
the sensitivity of PAG containing terpolymer was higher than that
of p(DDMA/NPMA) under the same condition. It was also found
that due to the number of Ts group increased, the sensitivity was
remarkably enhanced with the mole fraction of PAG.
Infrared spectroscopy was employed to investigate the change
of the photodegradable properties of PAG terpolymer film by deep
UV irradiation. Fig. 12 showed the infrared spectrum of terpoly-
mers p(DDMA/NPMA/TsPMA)s c thin films, which was cast on a
1.2
1.0
p(DDMA-NPMA)
0.8
0.6
3.4. Thermal stability of polymer containing PAG
p(DDMA/NPMA/TsPMA25)
One basic requirement for CA resist is that it must be ther-
mally stable [19]. Fig. 13 illustrated the effect of the thermal
stability of homopolymer pTsPMA (curve 1 and 2), terpoly-
0.4
0.2
0
p(DDMA/NPMA/TsMPA17)
10
mer pDDMA/NPMA/TsPMA (curve
3 and 4) and copolymer
pDDMA/NPMA (curve 5 and 6) before and after DUV irradia-
tion. TGA was used to follow the weight loss associated with the
thermolysis of the naphthyl group from the polymer backbone.
Upon thermal decomposition, the naphthyl moiety liberated naph-
thyl alcohol and other derivant. To determine the comparative
effect, TsPMA on the thermolysis of the naphthyl group differ-
ent molar of the sulfonate ester 100 (pTsPMA), 15.4 (terpolymer
0
100
Exposure time (min)
Fig. 11. Sensitivity curves of p(DDMA-NPMA) and p(DDMA/NPMA/TsPMA) thin film
irradiated by deep UV light followed by developed with 1% TMAH for 20 s. PEB
temperature: 90 ◦C for 30 s.

56
W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
100
80
60
40
20
0
3
4
1
2
5
6
0
100
200
300
400
500
600
Temperature (ºC)
Fig. 13. TGA of terpolymer p(DDMA/NPMA/TsPMA)s c, copolymer p(DDMA/NPMA
50) and homopolymer p(TsPMA) (1) homopolymer p(TsPMA) before (2) after expo-
sure to 250 nm UV for 60 min; (3) terpolymer p(DDMA/NPMA/TsPMA)s before (4)
after exposure to UV light for 60 min; (5) copolymer p(DDMA/NPMA 50) before (6)
after exposure to UV light for 60 min.
Fig. 14. Optical micrograph of positive fine patterns with p(DDMA/NPMA/TsPMA)s
c LB film 35 layers on a glass substrate after UV irradiation following development
with 1% TMAH aqueous solution.
c) and 0 (pDDMA/NPMA50) mol.% relative to polymer were moni-
tored. The TGA result indicated that the deprotection temperature
(Tdp) was decreased with increasing the amount of TsPMA in
copolymer before exposing, which showed the sequence of Tdp
(pTsPMA) < Tdp (pDDMA/NPMA/TsPMA) < Tdp (pDDMA/NPMA50)
as shown in Fig. 13. The deprotection temperature of copoly-
mer had obvious phenomenon after irradiated by 248 nm light
for 60 min. This reduction in the thermolysis temperature of the
naphthyl group verified that acidic species was liberated upon the
decomposition of TsPMA in copolymer. For the first, the DUV irra-
diation of p(DDMA/NPM/TsPMA)s resulted in generation of the
corresponding p-toluenesulfonic acid, which catalyzed the depro-
tection of naphthyl groups. As seen from Fig. 13, the temperature
of deprotection of naphthyl functionalized was greatly reduced by
comparing with (1) and (2), (3) and (4). It had a smaller effect
on the temperature of deprotection of pDDMA/NPMA50 before
and after irradiation, because there was no TsPMA in copolymer
(i.e., no amount of acid was formed). The TGA studies showed
that the thermal stability of the terpolymers p(DDMA-NPMA-
TsPMA)s greatly decreased with an increase of the number TsPMA
units in the copolymer chain irradiated by deep UV light and
increased without irradiation. We could deduce that the sensitiv-
ity of p(DDMA/NPMA/TsPMA)s could be improved by introducing
TsPMA group into the main chain of p(DDMA/NPMA).
Fig. 15. Optical micrograph of etched pattern of gold film on glass substrate.
ever, in lithography process the subsequent pattern transferring
is the most important step. Thus, the etch resistance of films is
also the critical factor just as sensitivity and resolution. Therefore,
the polymer films employed in microlithography must have suffi-
ciently resistance to wet etching or dry etching (plasma). The fine
resist pattern obtained above was etched by immersing it in a mixed
solution of iodine (0.6 g), ammonium iodide (1.5 g), ethanol (20 mL)
and pure water (30 mL) for 20 s in order to remove the gold film
uncovered by the resist LB film. Finally, the p(DDMA/NPMA/TsPMA)
resists LB films were removed by chloroform for 20 s. Fine gold film
pattern with the same resolution as that of the resist pattern was
drawn as shown in Fig. 15. It can be observed that the surface of
sample was smooth and clear-cut and the gold pattern had the max-
imum resolution of 0.75 ␮m as same as the resist pattern. These
results indicated that the resist pattern was translated into gold
pattern completely and the resist LB films had a high resistance to
the wet enchant. Therefore, p(DDMA/NPMA/TsPMA)s LB films have
a potential application to microfabrication.
3.5. Photopatterning of polymers containing PAG
The p(DDMA-NPMA-TsPMA)s c LB films with 35 layers were
deposited on glass-gilded substrate, irradiated by 248 nm UV light
source in air through a photomask for 20 min, and then were devel-
oped in 1% aqueous solution of tetramethyl ammonium hydroxide
(TMAH) for 10 s. The patterns of resists LB films were obtained
as shown in Fig. 14. This microscopic image showed that the
masked region of resist LB films was not dissolved, while the irra-
diated part was dissolved in 1% (TMAH) aqueous solution due
to the chain scission, resulting in the decrease in the molecu-
lar weight as mentioned above. The maximum resolution of the
resist pattern was 0.75 ␮m line-and-space, which was the high-
est resolution of the photomask employed in this study. Obviously,
p(DDMA/NPMA/TsPMA) was a typical positive-tone photoresist,
and this copolymer LB films was effectively decomposed by deep
UV irradiation and completely removed with a developer of alkaline
solution.
4. Conclusion
From the experiment results obtained above, fine positive-type
patterns of p(DDMA/NPMA/TsPMA) LB films could be drawn. How-
A series of novel functionalized nonionic photo-acid gen-
erator, including 4-(tosyloxy)phenyl methacrylate (TsPMA),

W. Xu et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 50–57
57
allyl-4-methylbenzensulfonate (AOTs), 4-(allyloxy) phenyl-4-
methbenzene- sulfonate (AOPTs) as well as corresponding PAG
bound polymer resist, were prepared and characterized. A new
approach of acid generating efficiency measurement was also
employed. The terpolymers could form a stable, condensed mono-
layer on the water surface and could be transferred successfully
onto solid supports, due to the absence of phase separation. The
p(DDMA/NPMA/TsPMA)s functioned as novel CA resist with PAGs
incorporated in the polymer chain. Preliminary results showed
that terpolymer c LB films were successfully applied to photopat-
terning and the fine resist patterns with the resolution of 0.75 ␮m
was yielded, which was the limit resolution of the photomask
employed. High sensitivity of resist in 248 nm irradiation was
attributed to the present of PAG units incorporated in the polymer
chains. The result of translated gold pattern with maximum
resolution also demonstrated that the resist LB films had sufficient
resistance to a wet etching process. UV, IR and TGA spectra
studies revealed that the formation of the pattern was attributed
to the scission of the main chain and the cleavage of the side
chain in LB films. Subsequently, a positive-tone pattern with high
resolution was figured. The photopatterning properties and high
resistance ability of terpolymers p(DDMA/NPMA/TsPMA)s LB films
are expected to be applied as a photoacid-generating photoresist
in ultra-thin photoresist and the fabrication of nano-filter as well
as nano-tube.
[8] M. Shiral, M. Tsunooka, Photo-acid and photobase generators: chemistry and
applications to polymeric materials, Prog. Polym. Sci. 21 (1996) 1–45.
[9] M. Shiral, T. Sumino, M. Tsunooka, Polysiloxane formation at the irradiated sur-
face of polymers containing both photoacid generating units and epoxy unites,
Eur. Polym. J. 33 (1997) 1255–1262.
[10] M.D. Stewart, H.V. Tran, G.M. Schmid, T.B. Stachowiak, D.J. Becker, C.G. Will-
son, Acid catalyst mobility in resist resins, J. Vac. Sci. Technol. B 20 (2002)
2946–2952.
[11] D. He, H. Solak, W. Li, F. Cerrina, Extreme ultraviolet and x-ray resist: compar-
ison study, J. Vac. Sci. Technol. B 17 (1999) 3379–3383.
[12] D. Yang, S.W. Chang, C.K. Ober, Molecular glass photoresists for advanced
lithography, J. Mater. Chem. 16 (2006) 1693–1696.
[13] M. Wang, W. Yueh, K.E. Gonsalves, Novel ionic photoacid generators (PAGs)
and corresponding PAG bound polymers, J. Photopolym. Sci. Technol. 20 (2007)
751–755.
[14] M. Wang, N.D. Jarnagin, C. Lee, C.L. Henderson, W. Yueh, J.M. Roberts, K.E.
Gonsalves, Novel polymeric anionic photoacid generators (PAGs) and cor-
responding polymers for 193 nm lithography, J. Mater. Chem. 16 (2006)
3701–3707.
[15] M. Wang, W. Yueh, K.E. Gonsalves, New anionic photoacid generator bound
polymer resists for EUV lithography, Macromolecules 40 (2007) 8220–8224.
[16] M. Wang, N.D. Jarnagin, W. Yueh, J.M. Roberts, M. Tapia, N. Batina, K. EGonsalves,
Novel polymeric anionic photoacid generators (PAGs) and photoresists for sub-
100 nm patterning by 193 nm lithography, Proc. SPIE 6519 (2007) 65192C.
[17] H. Wu, K.E. Gonsalves, Preparation of a photoacid generating monomer and its
application in lithography, Adv. Funct. Mater. 11 (2001) 271–276.
[18] H. Wu, K.E. Gonsalves, A novel single-component negative resist for DUV and
electron beam lithography, Adv. Mater. 13 (2001) 195–197.
[19] K.-D. Ahn, C.-M. Chung, D.-H. Koo, Nevol functional polymers from N-(Tosyloxy)
maleimide photochemical acid ceneration in solid-state and application as
resist materials, Chem. Mater. 6 (1994) 1452–11452.
[20] K.-D. Ahn, D.-H. Koo, C.G. Willson, Synthesis and polymerization of t-
BOC protected maleimide monomers: N-(t-butyloxycarbonyloxy) maleimide
and N-[p-(t-butyloxycarbonyloxy) phenyl] maleimide, Polymer 36 (1995)
2621–2628.
[21] F.M. Houlihan, G.I. Dabbagh, R. Hutton, K. Bolan, E. Reichmanis, O. Nala-
masu, Z. Yan, A. Reiser, Fundamental studies of dissolution inhibition in
poly(norbornene-alt-maleic anhydride) based resins, Radiat. Phys. Chem. 62
(2001) 69–76.
[22] M.A. Ali, K. Gonsalves, V. Golovkina, F. Cerrina, High sensitivity nanocomposite
resists for EUV lithography, Microelectron. Eng. 65 (2003) 454–462.
[23] A. Ulman, Function and structure of self-assembled monolayers, Chem. Rev. 96
(1996) 1533–1554.
Acknowledgements
This work is supported by the Innovation Fund for Outstand-
ing Scholar of Henan Province (0621001100), the Natural Science
Foundation of Henan Province (0611020100) and the Innovation
Fund for Outstanding Youth of Henane Province (074100510015).
[24] T. Miyashita, Recent studies on functional ultrathin polymer films prepared by
the Langmuir–Blodgett technique, Prog. Polym. Sci. 18 (1993) 263–294.
[25] T.S. Li, M. Mitsuishi, T. Miyashita, Photolitho-graphic properties of ultrathin
polymer Langmuir–Blodgett films containing anthracene moieties, J. Mater.
Chem. 13 (2003) 1565–1569.
References
[1] H. Ito, Chemical amplification resists for microlithography, Adv. Polym. Sci. 172
(2005) 37–245.
[2] H. Ito, Chemical amplification resists: history and development within IBM,
IBM J. Res. Dev. 44 (2000) 119–130.
[3] S.-Y. Moon, J.-M. Kim, Chemistry of photo-lithographic imaging materials based
on the chemical amplification concept, J. Photochem. Photobiol. C: Photochem.
Rev. 8 (2008) 157–173.
[4] T. Fujimori, S. Tan, T. Aoai, F. Nishiyama, T. Yamanaka, M. Momota, S. Kanna,
Y. Kawabe, M. Yagihara, T. Kokubo, S. Malik, L. Ferreira, Structural design
of a new class of acetal polymer for DUV resists, Proc. SPIE 3999 (2000)
579–590.
[5] A.M. Azam, K.E. Gonsalves, V. Golovkina, F. Cerrina, High sensitivity nanocom-
posite resists for EUV lithography, Microelectron. Eng. 65 (2003) 454–462.
[6] C.-M. Chung, K.-D. Ahn, Photochemical acid generation from copolymers based
on camphorsulfonyloxymaleimide and acidolytic deprotection, React. Funct.
Polym. 40 (1999) 1–12.
[26] T.S. Li, M. Mitsuishi, T. Miyashita, Ultrathin polymer Langmuir–Blodgett films
for microlithography, Thin Solid Films 446 (2004) 138–142.
[27] W.J. Xu, T.S. Li, G.L. Zeng, F.F. Ren, S.H. Zhang, Y.J. Wu, T. Miyashita, Molecu-
lar arrangement and photopatterning of copolymer containing ketal-protected
group in ultrathin nanosheets, Surf. Sci. 602 (2008) 1141–1148.
[28] W.J. Xu, T.S. Li, G.L. Zeng, S.H. Zhang, W. Shang, Y.J. Wu, T. Miyashita, Studies
on photolithography and photoreaction of copolymer containing naphthyl in
ultrathin nanosheets induced by deep UV irradiation, J. Photochem. Photobiol.
A: Chem. 194 (2008) 97–104.
[29] G. Pohlers, J.C. Scaiano, R. Sinta, A novel photometric method for the determi-
nation of photoacid generation efficiencies using benzothiazole and xanthene
dyes as acid sensors, Chem. Mater. 9 (1997) 3222–3230.
[30] W.J. Xu, F.F. Ren, S.H. Zhang, G.L. Zeng, T.S. Li, Y.J. Wu, Surface behavior of poly-
mer Langmuir monolayer at the air/water interface, Adv. Technol. Mater. Mater.
Proc. J. 8 (2006) 160–165.
[7] M. Shiraia, M. Endo, M. Tsunooka, M. Endo, Highly sensitive positive surface
modification resists, Microelectron. Eng. 53 (2000) 475–478.