A universal scheme for patterning of oxides via thermal nanoimprint lithography

Article abstract of DOI:10.1002/adfm.201202577

Direct patterning of oxides using thermal nanoimprint lithography is performed using either the sol-gel or methacrylate route. The sol-gel method results in resists with long shelf-life, but with high surface energy and a considerable amount of solvent that affects the quality of imprinting. The methacrylate route, which is limited to certain oxides, produces polymerizable resists, leading to low surface energy, but suffers from the shorter shelf-life of precursors. By combining the benignant elements from both these routes, a universal method of direct thermal nanoimprinting of oxides is demonstrated using precursors produced by reacting an alkoxide with a polymerizable chelating agent such as 2-(methacryloyloxy)ethyl acetoacetate (MAEAA). MAEAA possesses β-ketoester, which results in the formation of environmentally stable, chelated alkoxide with long shelf-life, and methacrylate groups, which provide a reactive monomer pendant for in situ copolymerization with a cross-linker during imprinting. Polymerization leads to trapping of cations, lowering of surface energy, strengthening of imprints, which enables easy and clean demolding over 1 cm × 2 cm patterned area with ≈100% yield. Heat-treatment of imprints gives amorphous/crystalline oxide patterns. This alliance between two routes enables the successful imprinting of numerous oxides including Al₂O₃, Ga₂O₃, In ₂O₃, Y₂O₃, B₂O ₃, TiO₂, SnO₂, ZrO₂, GeO ₂, HfO₂, Nb₂O₅, Ta₂O ₅, V₂O₅, and WO₃. Copyright

Full text of DOI:10.1002/adfm.201202577

www.afm-journal.de
www.MaterialsViews.com
A Universal Scheme for Patterning of Oxides via Thermal
Nanoimprint Lithography
Saman Safari Dinachali, Mohammad S. M. Saifullah,* Ramakrishnan Ganesan,*
Eng San Thian, and Chaobin He
To Professor Vikram Jayaram, for his contribution to ceramic engineering
optoelectronics, photonics, data storage,
and biology.[1–4] In comparison to tradi-
Direct patterning of oxides using thermal nanoimprint lithography is per-
tional lithography techniques in which
chemical and physical properties of resist
materials are modified by photons or
electrons, NIL utilizes mechanical defor-
formed using either the sol-gel or methacrylate route. The sol-gel method
results in resists with long shelf-life, but with high surface energy and a
considerable amount of solvent that affects the quality of imprinting. The
methacrylate route, which is limited to certain oxides, produces polymerizable
resists, leading to low surface energy, but suffers from the shorter shelf-life
of precursors. By combining the benignant elements from both these routes,
a universal method of direct thermal nanoimprinting of oxides is demon-
strated using precursors produced by reacting an alkoxide with a polymeriz-
able chelating agent such as 2-(methacryloyloxy)ethyl acetoacetate (MAEAA).
MAEAA possesses β-ketoester, which results in the formation of environmen-
tally stable, chelated alkoxide with long shelf-life, and methacrylate groups,
which provide a reactive monomer pendant for in situ copolymerization with
a cross-linker during imprinting. Polymerization leads to trapping of cations,
lowering of surface energy, strengthening of imprints, which enables easy
and clean demolding over 1 cm × 2 cm patterned area with ≈100% yield.
Heat-treatment of imprints gives amorphous/crystalline oxide patterns. This
alliance between two routes enables the successful imprinting of numerous
oxides including Al2O3, Ga2O3, In2O3, Y2O3, B2O3, TiO2, SnO2, ZrO2, GeO2,
HfO2, Nb2O5, Ta2O5, V2O5, and WO3.
mation of a resist material using a mold
containing surface relief features at a
controlled temperature and pressure to
achieve higher resolution. In 2005, the
International Technology Roadmap for
Semiconductors (ITRS) listed NIL as one
of the contender technologies for future
generation chip manufacturing.[5]
Thermal NIL is the most versatile of
all the imprint techniques. This tech-
nique began as a way to replicate mold
patterns into a thermoplastic material by
heating the polymer above its glass tran-
sition temperature and applying pressure
on the mold. Variations of this technique
extended its adaptability to utilize mate-
rials such as thermosets, sol-gel films,
metal-organic materials and polymeriz-
able liquid metal methacrylate resists
to pattern organic as well as inorganic
materials, especially oxides.[6–18] Direct
NIL of oxides has generated tremendous interest due to the
fact that their physical properties can be tailored by nanoscale
patterning. One- or two-dimensional nanostructured oxides
are expected to play an important role as functional units in
electronic, optoelectronic and photonic devices.[16] Since oxides
are hard materials and do not have a workable softening point,
they are imprinted using a “soft” and “moldable” precursors.
1. Introduction
Due to its facile, cost-effective and high throughput produc-
tion process, nanoimprint lithography (NIL) has garnered
increasing attention as a next-generation patterning technique
that allows the fabrication of structures from micro- to nano-
scale at high resolution and precision with device applications in
S. S. Dinachali, Dr. M. S. M. Saifullah, Prof. C. He
Institute of Materials Research and Engineering
A*STAR (Agency for Science, Technology and Research)
3 Research Link, Singapore 117602, Republic of Singapore
E-mail: saifullahm@imre.a-star.edu.sg
Dr. R. Ganesan
Department of Chemistry
Birla Institute of Technology & Science
Pilani–Hyderabad Campus,
Jawahar Nagar, Shameerpet Mandal,
Hyderabad–500 078, Andhra Pradesh, India
E-mail: ram.ganesan@bits-hyderabad.ac.in
S. S. Dinachali, Dr. E. S. Thian
Department of Mechanical Engineering
National University of Singapore
Prof. C. He
9 Engineering Drive 1, Singapore 117576, Republic of Singapore
Department of Materials Science & Engineering
National University of Singapore
21 Lower Kent Ridge Road, Singapore 119077, Republic of Singapore
DOI: 10.1002/adfm.201202577
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2201

www.afm-journal.de
www.MaterialsViews.com
The patterned precursor is then heat-treated to yield struc-
tured oxide. The two popular methods for direct patterning
of oxides using NIL are the sol-gel and methacrylate routes.
The sol-gel route with its low-temperature characteristics,
good film quality, and as an economical way to obtain engi-
neered ceramics has attracted a great deal of attention in the
imprint technology. In this route, usually an alkoxide or ace-
tate is used as an oxide precursor.[6–15] If an alkoxide is used,
it is stabilized against hydrolysis by a chelating agent, usually
a β-diketone or β-ketoester, in an alcohol medium. The che-
lated alkoxide or acetate solution is spin-coated on a substrate
and patterned using soft stamps such as polydimethylsiloxane
(PDMS) and perfluoropolyether (PFPE) at a set pressure and
temperature. However, direct imprinting of oxides via this
route faces two major challenges. Firstly, due to high surface
energy of the precursors, the sol-gel imprinting requires a good
mold release system,[6] and secondly, the solvent in sol-gel film,
which helps to “soften” it for imprint process, may get trapped
in the imprinted structures leading to incomplete filling of
the precursor material inside the mold and resulting in poor
demolding. Moreover, soft molds are amenable to deforma-
tion, especially when sub-100-nm features are desired. On the
other hand, the methacrylate resists are prepared by reacting
metal alkoxides with methacrylic acid. The reaction leads to the
formation of liquid metal methacrylate, i.e., a clear solution of
metal methacrylate in an alcohol byproduct. This polymerizable
precursor is mixed with a cross-linker such as ethylene glycol
dimethacrylate (EDMA) to form the resist for imprinting. In
situ free-radical thermal copolymerization during imprinting
leads to the reduction of surface energy and strengthening of
patterned structures thereby giving yields close to 100%.[17,18]
Furthermore, this approach incorporates the benefits of a rigid
silicon mold and liquid precursor to achieve very high resolu-
tion over areas >1 cm × 1 cm but at a lower temperature and
imprinting pressure. However, this route has limited applica-
bility due to the instability of many metal methacrylates.[18]
In this work, we show an alternative method and a universal
route to the direct nanoimprint lithography of oxides by har-
nessing the advantages of the sol–gel and polymerizable liquid
methacrylate resist routes and at the same time alleviate the
disadvantages associated with both these methods. This syn-
ergy can be achieved when alkoxides are complexed with a poly-
merizable chelating agent such as 2-(methacryloyloxy)ethyl ace-
toacetate (MAEAA). MAEAA possesses β-ketoester and meth-
acrylate groups, the former can lead to the formation of envi-
ronmentally stable, chelated alkoxide complex while the latter
provides the reactive methacrylate group for in situ copolymeri-
zation with a cross-linker during imprinting. Our preliminary
work on imprinting of Al2O3 via this route has shown a con-
siderable amount of promise.[19] Using the synergistic effect of
β-ketoester and methacrylate groups in the chelating monomer
MAEAA, we demonstrate a universal scheme for direct thermal
NIL of various oxides such as Al2O3, Ga2O3, In2O3, Y2O3, B2O3,
TiO2, SnO2, ZrO2, GeO2, HfO2, Nb2O5, Ta2O5, V2O5, and WO3.
Furthermore, our approach also incorporates the benefits of
a rigid mold and liquid precursor to achieve very high resolu-
tion over areas 1 cm × 2 cm but at a lower temperature and
pressure to obtain yields of almost 100% after imprinting. For
the sake of better understanding of the NIL process, our study
has divided patterning process based on the oxidation state of
the above-mentioned cations, i.e., +3, +4, and +5 (and above).
Although examples of imprinting of all the oxides will be given,
only two cations from each category will be used as representa-
tive candidates for discussing the resist behaviour. They are Al
and Ga for +3; Ti and Sn for +4; and Nb and Ta for +5 valencies
and above.
2. Results and Discussion
Alkoxides are very reactive compounds due to the presence of
electronegative alkoxy groups, which make the cations highly
prone to nucleophilic attack. Due to their high affinity with
water, hydrolysis results in the formation of molecular aggre-
gates of hydrated alkoxides. However, the hydrolytic reactivity
of alkoxides can be controlled by complexation/chelation with
β-diketones, β-ketoesters, acetic acid among others.[20] This
chemical modification leads to the alteration of the whole
hydrolysis-condensation process, enabling an easy and eco-
nomic way to process sol-gel materials to obtain ceramics.
However, an addition of extra pendant group to chelating
agents such as β-diketones and β-ketoesters offers exciting
possibilities to increase the latitude of ceramic processing.
For example, ethylacetoacetate is a well-known β-ketoester for
stabilizing alkoxides against hydrolysis by chelation for sol-
gel processing.[21] If a hydrogen atom from the ethyl group is
replaced by a methacryloyloxy group, it leads to the formation of
a bi-functional molecule which can not only chelate with alkox-
ides but also undergoes polymerization in the presence of a
thermal free radical initiator.[22] The latter is due to the fact that
the methacrylate group in methacryloyloxy unit is a highly reac-
tive polymerizable monomer. This is precisely what 2-(meth-
acryloyloxy)ethyl acetoacetate (or MAEAA) does in the presence
of an alkoxide. It chelates with an alkoxide to prevent its hydrol-
ysis as well as provides the methacrylate group for polymeri-
zation. In other words, the chelated monomer-based precursor
thus formed synergistically utilizes the advantages associated
with both sol-gel and methacrylate chemistries, thereby leading
to a much wider scope for processing of materials. Similar to
other β-diketones and β-ketoesters, MAEAA is capable of keto–
enol tautomerism, as shown in Equation 1. The enol form of
MAEAA is stabilized by chelation with an alkoxide. Further, the
reaction also results in the stoichiometric replacement of an
alkoxy group (e.g., an iso-propoxy group from gallium (III) iso-
propoxide) by a β-ketoester ligand,[23–25] as shown in Equation
2. The general reaction of an alkoxide with MAEAA is shown
in Equation 3. The hydrolytic activity of the stabilized alkoxide
is substantially reduced, perhaps due to steric hindrance. This
chelation reaction, often accompanied by a color change of the
solution (Table 1), yields a clear, flowable and polymerizable
oxide precursor.
The FTIR spectroscopy was used to characterize the reaction
between alkoxides and MAEAA (Figure 1). The FTIR spectra of
the chelated precursors are similar and show a large number of
absorption bands/peaks between 750 and 1800 cm−1 (Table 2).
The pair of absorption peaks close to 1609 cm−1 and 1522 cm−1
is attributed to the bidentate character of cation-bonded MAEAA
and hence is indicative of the formation of the chelated
©
2202 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 2201–2211

www.afm-journal.de
www.MaterialsViews.com
complex.[24,25] These peaks correspond to the ν(Cꢀ ꢀ O) and
ꢁ
v(Cꢀ ꢀ C) vibrations of the chelate ring, respectively. More impor-
ꢁ
tantly, the MAEAA-functionalized oxide precursors also show
characteristic peaks of carbonyl group at 1721 cm−1 , and meth-
acrylate double bond at 1634 cm−1 , the latter clearly suggesting
the preservation of polymerizable double bond in the complexes.
A shoulder at 1746 cm−1 in the FTIR spectra that corresponds to
Figure 1. Characteristic infrared absorption peaks of oxide precursors
formed by reacting alkoxide and MAEAA in a 1:2 ratio. The general for-
mula of the precursor is (alkoxide)m(MAEAA)2. The broad vibration bands
corresponding to particular bonds are indicated on top. Table 2 shows
details of the band assignment.
T a b le 1 . Observation of color change due to the chelation reaction
between alkoxides and MAEAA.
Alkoxide
Color/Appearance after the chelation
reaction
the carbonyl group of MAEAA may indicate incomplete reaction
between MAEAA and the corresponding alkoxides (Figure 1).[25]
Stability of the chelate rings indicates the atmospheric sta-
bility of the chelated precursors. Figure 2 shows the FTIR
spectra of the representative precursors for each valency
recorded at 0, 6 and 24 h under ambient laboratory conditions
of 30 °C and at ≈40% relative humidity. It is seen that for all
the precursors, even after 24 h, the FTIR spectra shows hardly
any change in the peaks associated with the chelate rings. This
shows that they are stable in normal laboratory conditions.
Unsurprisingly, all the chelated precursors showed exceptional
stability over a storage period of more than 3 months at room
temperature. They were found to be clear and flowable.
aluminium (III) tri-sec-butoxide
gallium (III) iso-propoxide
indium (III) iso-propoxide
yttrium (III) n-butoxide
tri-iso-propyl borate
faint yellow/clear
faint yellow/clear
faint yellow/clear
faint yellow/clear
no color/clear
titanium (IV) n-butoxide
zirconium (IV) n-butoxide
tin (IV) tert-butoxide
lemon yellow/clear
light brown/clear
faint yellow/clear
light brown/clear
brown/clear
germanium (IV) ethoxide
hafnium (IV) tert-butoxide
niobium (V) ethoxide
no color/clear
Oxide resists for NIL were prepared by adding cross-linker
EDMA and free-radical initiator benzoyl peroxide (BPO) to the
corresponding chelated precursors. They were subjected to TGA
and DSC studies in order to understand their degradation behav-
iour and to identify the polymerization exotherm, respectively
tantalum (V) n-butoxide
vanadium (V) tri-iso-propoxide oxide
tungsten (V) ethoxide
no color/clear
reddish brown/clear
yellow/clear
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2203

www.afm-journal.de
www.MaterialsViews.com
T a b le 2 . Characteristic infrared absorption peaks of chelated oxide pre-
cursors. Albeit the principal peaks indicated below are for the Al2O3
precursor, other chelated precursors show similar values as seen in
Figure 1.
the formation of corresponding oxides (Figure 3a). The DSC scan
showed that the thermal free radical copolymerization of chelated
precursor and EDMA mixture in various resists occurred in a
narrow range of temperatures, i.e., 80–130 °C (Figure 3b). For
every oxide resist, their respective polymerization exotherm
was used as a guide to initiate in situ free radical copolymeri-
zation during NIL. Time-resolved FTIR studies were also used
to monitor the thermal copolymerization reaction in the oxide
resists (Figure 4). The FTIR spectrum of the oxide resists before
polymerization closely resembles their respective precursors. It
was observed that after the thermal copolymerization, the inten-
sity of the methacrylate double bond at 1634 cm−1 decreased sig-
Absorption peak [cm−1
]
Assignment
νa(CꢂO)
ν(CꢂO)
Peak number
1746
1721
1634
1609
1
2
3
4
ν(CꢂC)
ν(Cꢀ ꢀ O)
ꢁ
1522
5
ν(Cꢀ ꢀ C)
ꢁ
nificantly. It is noteworthy that the enolic ν(Cꢀ ꢀ O) at 1609 cm−1
1418, 1372
6,7
δCH3
ꢁ
and enolic ν(Cꢀ ꢀ C) complexed with cation at 1522 cm−1 remain
ꢁ
unaffected after the polymerization. This confirms the fact that
the cations remain trapped inside the polymer matrix after the
polymerization reaction.
(Figure 3). The TGA measurement in air showed a two-step
mass loss. The minor loss from 30 °C to 150 °C was due to the
loss of alcohol byproduct/solvent and the major loss from 300 °C
to 550 °C occurred due to the degradation of organic component.
Beyond 550 °C, a constant residual weight was observed due to
2.1. Nanoimprint Lithography of Oxides
The schematic of the steps involved in direct
NIL of oxides is shown in Figure 5. Imprinting
of all the oxide resists were carried out in two
steps. Firstly, at room temperature (30 °C) a
pressure of 30 bar was applied for 300 s to
ensure a complete filling of the 250 nm equal
line-and-space grating mold (aspect ratio
1) with the resist. Secondly, whilst holding the
pressure constant, the coated substrate and
mold were heated between 110 and 130 °C for
1000–1600 s in order to induce thermal free
radical copolymerization in oxide resists. This
leads to hardening of the imprint. The assembly
was cooled down to room temperature after
imprinting. The pressure was then released
which was followed by a clean demolding,
giving ≈100% yield over ≈1 cm × 2 cm area.
Cross-sectional SEM studies show that the
width and height of the imprinted features were
slightly smaller than the actual dimensions of
the mold, a result most likely due to the polym-
erization-induced shrinkage of the patterns.[26]
The minimum temperature required to
burn off the organic content completely from
the oxide resists was determined using the
isothermal TGA analysis. Table 3 provides
the heat-treatment conditions used to con-
vert imprinted resists to amorphous/crys-
talline oxide patterns. Ceteris paribus, the
shrinkage of the imprints depends upon the
metal atom, the size of the alkoxy group in
precursor, heat-treatment temperature and
the size of unit cell of metal oxide formed
after heat-treatment. With some exceptions,
generally speaking, pattern shrinkage close
to 80% with respect to the original mold size
was observed (Table 3). Except for WO3, in
Figure 2. Atmospheric stability of as-coated precursor films, after 6 and 24 h at 30 °C and 40%
relative humidity.
©
2204 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 2201–2211

www.afm-journal.de
www.MaterialsViews.com
containing alkoxide, MAEAA and EDMA
is essential. In our case, their relative ratios
were 1:2:1.5. The reason for choosing to react
an alkoxide and MAEAA in a molar ratio
of 1:2 for direct NIL of oxides is three-fold.
Firstly, the presence of two reactive methacry-
late groups chelated to the alkoxide atom
provides better probability for them to get
co-polymerized with the cross-linker, thereby
trapping the cation inside the polymer net-
work. Secondly, having more than two che-
lated MAEAA groups attached to the cation
does not provide any additional benefit except
to increase the undesirable mass loss during
final heat-treatment to convert the imprinted
structure to oxide. Thirdly, there is hardly
any difference between the atmospheric sta-
bility of complexes which have one or three
chelated MAEAA groups.[19] On the other
hand, the ratio of 2:1.5 between MAEAA and
EDMA lead to consistently high yields close
to ≈100%. This may be due to the increased
degree of polymerization. Furthermore,
this composition of resist also determines
the imprinting time. The imprinting time
can be significantly reduced if the amount
of the EDMA is increased. However, a pen-
alty is paid in the form of increased organic
losses leading to smaller amount of remnant
oxide after heat-treatment in the imprinted
patterns.
Figure 6 shows the FE-SEM images of
as-imprinted and heat-treated structures
of Al2O3, Ga2O3, In2O3, Y2O3 and B2O3 pat-
terned using their respective resists con-
taining +3 valency cations. None of these
resists were found to be imprintable using
the methacrylate route.[18] However, the sol-
gel route appears to be successful in pat-
terning Al2O3 and In2O3–the former used the
nitrate route[8] whilst the latter was imprinted
Figure 3. a) TGA analysis of the Al2O3, Ga2O3, TiO2, SnO2, Nb2O5, and Ta2O5 resists showing
mass losses during continuous heating to 800 °C. b) DSC data showing the exothermic peaks
of onset of the thermal free-radical copolymerization in Al2O3, Ga2O3, TiO2, SnO2, Nb2O5, and
Ta2O5 resists.
all the cases, the imprinted structures maintain their integrity
even after the heat-treatment. The AFM analysis showed that
the aspect ratio between 0.5 and 1.5 (as opposed to the aspect
ratio of 1 for molds used in the imprinting) was achieved for
oxide patterns after heat treatment (Figures 6–8). The reason
for such a difference of aspect ratios in various oxides may be
primarily attributed to the difference in organic content before
heat-treatment. However, the shrinkage appears to be nearly
uniform over the entire imprinted structure. Although there
is a reduction in the size of the patterns after heat-treatment,
the center-to-center pattern distance remains the same whilst
the edge-to-edge pattern distance increases, keeping the feature
density of heat-treated patterns unchanged. While the calcina-
tion shrinkage comes at the cost of pattern size, it is actually an
easy and cheaper way to access the nanoscale. As the aim is to
minimize the organic losses and maximize the oxide content
after heat-treatment in patterns without compromising their
integrity, a judicious formulation of the composition of resist
using a commercially available sol-gel-based indium-tin oxide
precursor.[12] Out of these resists, the most interesting case is
that of B2O3. The precursor for B2O3 resist was prepared by
reacting tri-iso-propyl borate with MAEAA in a 1:2 molar ratio.
However, the FTIR study of the precursor indicates that there
is hardly any chelation between the alkoxide and MAEAA (see
Supporting Information). This is not surprising as tri-iso-propyl
borate is well-known for its inability to chelate under normal
conditions.[27] However, the presence of MAEAA and cross-
linker EDMA in B2O3 resist resulted in in situ copolymeriza-
tion during imprinting leading to the trapping of tri-iso-propyl
borate in the polymer structure. This clearly suggests that alkox-
ides such as tetraethoxysilane (a precursor for SiO2), which
do not chelate easily with β-ketoesters, may also be imprinted
using this method.
TiO2, ZrO2, SnO2, GeO2 and HfO2 resists containing +4
valency cations were imprinted and heat-treated as shown
in the composite FE-SEM images (Figure 7). Among these
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2205

www.afm-journal.de
www.MaterialsViews.com
oxides, TiO2, due to its outstanding material properties, has
been extensively patterned using the sol-gel route.[9,11,15] Other
oxides may also be imprinted using the sol-gel method as they
form stable chelated alkoxides when reacted with β-diketones
and β-ketoesters. On the other hand, only TiO2 and ZrO2 are
amenable to NIL using the methacrylate route.[17,18] Among
the oxides imprinted in Figure 7 using the chelated monomer
route, SnO2 and GeO2 stand out because of their unusual
behavior. The former shows a very low shrinkage after heat-
treatment of imprinted structures whilst the latter forms unu-
sual residual layer pattern after imprinting. Reduction of the
film thickness by diluting GeO2 resist in n-butanol and sub-
sequently imprinting it did not lead to any substantial change
in the residual layer pattern. However, heat-treatment of the
imprinted GeO2 resist lead to the disappearance of this pattern.
The reason for this strange behavior is unclear.
Figure 8 shows the FE-SEM images of as-imprinted and
heat-treated structures of the resists containing +5 valency
cations and higher. The resists in this category include Nb2O5,
Ta2O5, V2O5, and WO3. No attempts have been made to pat-
tern these oxides using the sol-gel method. The methacrylate
route, on the other hand, was successfully used to imprint
Nb2O5 and Ta2O5.[18] It is interesting to note that except for
WO3, all the imprinted structures maintain their integrity
even after the heat-treatment. This characteristic behavior may
be due to excessive loss of organics and/or rapid grain growth.
Attempts to minimize the collapse of the imprinted structures
by reducing the residual layer thickness did not yield prom-
ising results.
Direct nanoimprinting of oxides by the chelated monomer
route offers significant advantages over the sol-gel and meth-
acrylate routes. The former mainly uses a soft mold for NIL
and requires a critical amount of solvent for patterning. The
chelated monomer route, just like the methacrylate route, offers
the convenience of liquid monomers which can be imprinted
at lower pressures without worrying about the removal of sol-
vent. It was observed that the presence of small amount of
solvent left over in the spin-coated film aids in imprinting at
lower pressures and its presence does not seem to hinder the
free-radical polymerization. The in situ polymerization in the
chelated monomer system gives rise to hardened pattern fea-
tures and thus facilitates clean demolding with ≈100% yield
over 2 cm × 1 cm areas. By adjusting the metal content in the
overall formulation, one should be able to control the degree
of pattern shrinkage after heat treatment. More importantly,
unlike methacrylate route, the chelated monomer route syn-
ergistically utilizes the advantages associated with both sol-gel
and methacrylate chemistries, thereby leading to an increased
latitude for processing of materials. This is evidenced by the
fact that our scheme is not only able to pattern oxides which
Figure 4. Time-resolved FTIR data of a) Al2O3, b) TiO2, and c) Nb2O5
resists heat-treated at 120 °C. Notice the peak corresponding to the
polymerizable –C=CH2 group decreases in intensity with increasing heat-
treatment time.
Figure 5. Schematic illustration of the nanoimprint lithography process that was used in direct patterning of oxide resists.
©
2206 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 2201–2211

www.afm-journal.de
www.MaterialsViews.com
T a b le 3 . Summary of the approximate feature size reduction at every step of the imprinting of various oxides with a 250 nm grating mold (aspect
ratio = 1) using the chelated monomer route. The imprinted patterns were subjected to isothermal heat-treatment (see Supporting Information).
Resist
Feature size of the imprint after thermal
polymerization
Heat-treatment
conditions
Oxide feature size after the heat-treatment
of imprinted structures
Total feature size reduc-
tion with respect to mold
feature size [%]
Width of imprint [nm] Feature size reduction
[%]
Width of the oxide feature Feature size reduction [%]
[nm]
Al2O3
Ga2O3
In2O3
Y2O3
225
200
185
165
200
171
147
170
162
194
176
197
131
175
10%
20%
26%
34%
20%
32%
41%
32%
35%
22%
30%
21%
48%
30%
55
43
76%
79%
30%
70%
25%
77%
56%
3%
78%
83%
48%
80%
40%
84%
74%
34%
79%
78%
77%
77%
54%
–
450 °C for 1 h
450 °C for 1.5 h
300 °C for 1 h
450 °C for 1 h
300 °C for 3 h
450 °C for 1 h
400 °C for 1.5 h
450 °C for 1 h
350 °C for 1 h
450 °C for 1 h
475 °C for 1 h
450 °C for 1 h
375 °C for 1.5 h
450 °C for 1 h
130
49
B2O3
TiO2
150
40
ZrO2
SnO2
GeO2
HfO2
Nb2O5
Ta2O5
V2O5
WO3
65
165
53
67%
72%
68%
71%
12%
–
55
57
58
115
–
can be imprinted using sol-gel or/and methacrylate routes, but
also those which are impossible to pattern with either of these
two methods. Equally important is the fact that our technique,
unlike step-and-flash imprint lithography,[28] is inexpensive,
fault-tolerant (with a dash of alcohol added, imprinting will
still work well to give ≈100% yield) and has the potential to be
extended to other types of materials as well.
A few words must be said about the long term chemical sta-
bility of the resists. Although the chelated precursors used in
the preparation of the resists were found to be exceptionally
stable, the same can’t be said about the resists. The oxide resists
for NIL were prepared by adding a cross-linker and a free-
radical initiator to the corresponding chelated precursors. The
presence of initiator appears to affect the long-term stability
of the NIL resists. For example, Al2O3, Ga2O3, In2O3 and TiO2
resists were found to be stable even after 3 months of storage.
Resists for imprinting Y2O3, Nb2O5, and ZrO2 were found to
be stable for more than a month. On the other hand, V2O5 and
WO3 resists were stable for the time period of less than a week.
Nevertheless, the chemical stability of chelated monomer-based
approach is far superior to that of the methacrylate route, the
latter’s shelf-life ranges from anywhere between a few days to
at most a month.
organic solvents. Moreover, these alkoxides can be expensive and
difficult to prepare in a laboratory. The other problem which may
arise is the lack of or weak chelation behavior of MAEAA with
alkoxides such as those of boron and silicon. As we have seen,
this problem can be circumvented by trapping their respective
alkoxides in the polymer network comprising of the chelated
monomer and cross-linker to produce imprinted structures.
A related issue with universality is whether monomers other
than MAEAA can be used to perform NIL of oxides. Prelimi-
nary studies on imprinting of TiO2 using allyl acetoacetate, a
β-ketoester with a monomer pendent, yielded satisfactory results
with ≈100% yield. It may also be surmised that β-ketoamines
with a monomer group are also potential chelating agents for
alkoxides to produce metal oxy-nitrides using NIL.[29]
3. Conclusions
We have demonstrated a universal way to pattern oxides
by thermal nanoimprinting lithography using stable and
polymerizable precursors formed by reacting MAEAA, a
chelating monomer, with alkoxides. MAEAA possesses
β-ketoester and methacrylate groups–the former helps in the
formation of environmentally stable, chelated alkoxide with
a long shelf-life whilst the latter provides a reactive methacr-
ylate group for in situ copolymerization with a cross-linker
during imprinting. The use of MAEAA leads to synergisti-
cally utilizing the advantages of the sol-gel and methacrylate
routes whilst alleviating drawbacks associated with both of
them. Imprintable oxide resists were formed by mixing poly-
merizable chelated alkoxides with EDMA, a cross-linker, and
BPO, a thermal free-radical initiator. During imprinting using
2.2. Limitations to Universality?
Of the variety of oxides imprinted in this study, the most egre-
gious absence is the cations that have valencies of +1 and +2.
This may suggest that our method of using chelatable monomer
like MAEAA to produce precursors for NIL of oxides lacks uni-
versality. On the contrary, the problem arises primarily due to
the poor solubility of alkoxides of +1 and +2 valency cations in
a
rigid silicon mold, these resists underwent thermally
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2207

www.afm-journal.de
www.MaterialsViews.com
Figure 6. Imprinting of resists containing +3 valency cations. Composite FE-SEM images of as-imprinted and heat-treated structures of Al2O3, Ga2O3,
In2O3, Y2O3, and B2O3 using a 250 nm equal line and space silicon grating mold. The insets show the structures at a higher magnification. The AFM
line traces of the corresponding heat-treated imprinted structures are shown on the right.
initiated in situ free radical copolymerization that led to the
trapping of the cation, lowering of surface energy of the resist
and strengthening of the imprint. This enabled easy and clean
demolding over 1 cm × 2 cm patterned area with ≈100% yield.
Successful imprinting was demonstrated for numerous oxides
4. Experimental Section
Materials: Aluminum (III) tri-sec-butoxide (97%, Sigma Aldrich),
tantalum (V) ethoxide (99.98%, Sigma Aldrich), tin (IV) tert-butoxide
(>99.99%, Sigma Aldrich), titanium (IV) n-butoxide (97%, Sigma
Aldrich), vanadium (V) oxytri-iso-propoxide (Sigma Aldrich), niobium
such as Al2O3, Ga2O3, In2O3, Y2O3, B2O3, TiO2, SnO2, ZrO2,
GeO2, HfO2, Nb2O5, Ta2O5, V2O5, and WO3. Thermolysis of the
imprinted patterns yielded amorphous/crystalline oxide pat-
terns with good integrity.
(V) ethoxide (99.95%, Sigma Aldrich), yttrium (III) n-butoxide (0.5 M in
toluene, ≥99.9%, Sigma Aldrich), germanium (IV) ethoxide (≥99.95%,
Sigma Aldrich), zirconium (IV) n-butoxide (80 wt% solution in 1-butanol,
Sigma Aldrich), hafnium (IV) tert-butoxide (99.9%, Alfa Aesar),
©
2208 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 2201–2211

www.afm-journal.de
www.MaterialsViews.com
Figure 7. Imprinting of resists containing +4 valency cations. Composite FE-SEM images of as-imprinted and heat-treated line structures of TiO2, ZrO2,
SnO2, GeO2, and HfO2 using a 250 nm equal line-and-space silicon grating mold. The insets show the structures at a higher magnification. The AFM
line traces of the corresponding heat-treated imprinted structures are shown on the right.
tungsten (V) ethoxide (Alfa Aesar), gallium (III) iso-propoxide (mixture
of oligomers, 99%, Alfa Aesar), tri-iso-propyl borate (98%+, Lancaster
Synthesis), indium iso-propoxide (99.9%, Multivalent Laboratory),
2-(methacryloyloxy)ethyl acetoacetate (95%, Sigma Aldrich), and
1H,1H,2H,2H-perfluorodecyltrichlorosilane (96%, Alfa Aesar) were used
as received. Ethylene glycol dimethacrylate (98%) was also purchased
from Sigma Aldrich and was used after removal of stabilizer using an
alumina column. Benzoyl peroxide (BPO), an initiator for thermal free-
radical polymerization, purchased from Sinopharm Chemical Reagent
Co. Ltd., was used without any further purification.
Resists Formulation and Characterization: The alkoxides of aluminum,
yttrium, boron, titanium, zirconium, tin, germanium, hafnium, tantalum,
vanadium, niobium, and tungsten were reacted with MAEAA in 1:2
molar ratio inside a glove box (<5% relative humidity) to form the
chelated precursor. Those alkoxides that are not in liquid form such as
indium iso-propoxide and gallium iso-propoxide were first dissolved in
toluene and then reacted with MAEAA in stoichiometric proportions.
In all these chelation reactions, the byproduct was an alcohol that was
not removed as it provided fluidity to the resist film during spin-coating.
This byproduct evaporated during imprinting at temperatures >100 °C.
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2209

www.afm-journal.de
www.MaterialsViews.com
Figure 8. Imprinting of resists containing +5 valency cations and higher. Composite FE-SEM images of as-imprinted and heat-treated line structures
of Nb2O5, Ta2O5, V2O5, and WO3 using a 250 nm equal line-and-space silicon grating mold. The insets show the structures at a higher magnification.
The AFM line traces of the corresponding heat-treated imprinted structures are shown on the right.
The final resist composition for nanoimprint lithography was made
by mixing the respective chelated precursors with 1.5 equivalent of the
crosslinker ethylene glycol dimethacrylate (EDMA) and 2 wt% of BPO
(with respect to the monomers). Hereafter, the formulations are referred
to as their corresponding oxide resists. In order to study the stability
of the precursors and to monitor changes in structure of the resists
during polymerization, a Nicolet 6700 Fourier transform infrared (FTIR)
spectroscope was used. The suitable imprinting temperatures of the
resists were determined using differential scanning calorimetry (DSC, TA
Instruments Q100). Thermogravimetric analysis (TGA, TA Instruments
Q500) was performed to follow the degradation behavior of the resists
and formation of oxides.
handling the Piranha solution because it reacts aggressively with
most organic materials). Before imprinting, the molds were silanized
using 1H,1H,2H,2H-perfluorodecyltrichlorosilane in order to reduce
the surface energy and therefore enable clean demolding after NIL.
An Obducat imprinter (Obducat, Sweden) was used to carry out NIL.
Suitable temperature for thermal NIL of the individual oxide resists was
determined with the help of DSC measurement.
Characterization of Imprints: Scanning electron and atomic force
microscopies were used to study the topography and morphology of
imprinted features before and after heat-treatment. A JEOL JSM6700F
field-emission scanning electron microscope (FE-SEM) was used to
obtain high-resolution images of as-imprinted as well as heat-treated
oxide patterns. For measuring the height of the imprinted structures, a
Digital Instruments Nanoscope IV atomic force microscope (AFM) was
used.
Nanomprint Lithography: The silicon substrates and molds used
for imprinting were cleaned with the Piranha solution (3:7 by volume
of 30% H2O2 and H2SO4. Caution: Extreme care must be taken while
©
2210 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 2201–2211

www.afm-journal.de
www.MaterialsViews.com
[11] G. Shi, N. Lu, L. Gao, H. Xu, B. Yang, Y. Li, Y. Wu, L. Chi, Langmuir
2009, 25, 9639.
[12] Y. Kang, M. Okada, K.-I. Nakamatsu, K. Kanda, Y. Haruyama,
S. Matsui, J. Vac. Sci. Technol. B 2009, 27, 2805.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
[13] K.-Y. Yang, K.-M. Yoon, S. Lim, H. Lee, J. Vac. Sci. Technol. B 2009,
27, 2786.
[14] A. Revaux, G. Dantelle, D. Decanini, F. Guillemot,
A.-M. Haghiri-Gosnet, C. Weisbuch, J.-P. Boilot, T. Gacoin,
H. Benisty, Nanotechnology 2011, 22, 365701.
[15] D. A. Richmond, Q. Zhang, G. Cao, D. N. Weiss, J. Vac. Sci. Technol.
B 2011, 29, 021603.
[16] Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates,
Y. D. Yin, F. Kim, H. Q. Yan, Adv. Mater. 2003, 15, 353.
[17] S. H. Lim, M. S. M. Saifullah, H. Hussain, W. W. Loh, H. Y. Low,
Nanotechnology 2010, 21, 285303.
[18] R. Ganesan, S. H. Lim, M. S. M. Saifullah, H. Hussain, J. X. Q. Kwok,
R. L. X. Tse, H. A. P. Bo, H. Y. Low, J. Mater. Chem. 2011, 21,
4484.
Acknowledgements
The authors thank Su Hui Lim of Institute of Materials Research and
Engineering (IMRE) for her assistance in performing preliminary
studies. S.S.D. gratefully acknowledges the A∗STAR Graduate Academy
for providing him the A∗STAR Graduate Scholarship for his Ph.D.
study. This work was supported by the IMRE-funded core project no.
IMRE/09-1C0319.
Received: September 7, 2012
Revised: October 19, 2012
Published online: November 27, 2012
[19] R. Ganesan, S. S. Dinachali, S. H. Lim, M. S. M. Saifullah,
W. T. Chong, A. H. H. Lim, J. J. Yong, E. S. Thian, C. He, H. Y. Low,
Nanotechnology 2012, 23, 315304.
[1] H. Schrift, J. Vac. Sci. Technol. B 2008, 26, 458.
[2] E. A. Costner, M. W. Lin, W.-L. Jen, C. G. Willson, Annu. Rev. Mater.
Res. 2009, 39, 155.
[20] C. Sanchez, J. Livage, M. Henry, F. Babonneau, J. Non-Cryst. Solids
1988, 100, 65.
[21] R. Jain, A. K. Rai, R. C. Mehrotra, Polyhedron 1986, 5,
1017.
[3] L. J. Guo, Adv. Mater. 2007, 19, 495.
[4] J. J. Dumond, H. Y. Low, J. Vac. Sci. Technol. B 2012, 30, 010801.
[5] ITRS 2005 Edition, http://www.itrs.net/Links/2005ITRS/Home2005.
htm (accessed November 2012).
[22] R. Agarwal, J. P. Bell, Polym. Eng. Sci. 1998, 38, 299.
[23] R. Lichtenberger, M. Puchberger, S. O. Baumann, U. Schubert,
J. Sol-Gel Sci. Technol. 2009, 50, 130.
[6] M. Li, H. Tan, L. Chen, J. Wang, S. Y. Chou, J. Vac. Sci. Technol. B
2003, 21, 660.
[24] J. Méndez-Vivar, P. Bosch, V. H. Lara, R. Mendoza-Serna, J. Sol-Gel
Sci. Technol. 2002, 25, 249.
[7] S. J. Kwon, J. H. Park, J. G. Park, J. Electroceram. 2006, 17, 455.
[8] O. F. Göbel, M. Nedelcu, U. Steiner, Adv. Funct. Mater. 2007, 17,
1131.
[9] M. J. Hampton, S. S. Williams, Z. Zhou, J. Nunes, D. H. Ko,
J. L. Templeton, E. T. Samulski, J. M. DeSimone, Adv. Mater. 2008,
20, 2667.
[25] N. Miele-Pajot, L. G. Hubert-Pfalzgraf, R. Papiernik, J. Vaissermann,
R. Collier, J. Mater. Chem. 1999, 9, 3027.
[26] M. P. Patel, M. Braden, K. W. Davy, Biomaterials 1987, 8, 53.
[27] M. M. Haridas, J. R. Bellare, Mater. Charact. 1999, 42, 55.
[28] R. Ganesan, J. Dumond, M. S. M. Saifullah, S. H. Lim, H. Hussain,
H. Y. Low, ACS Nano 2012, 6, 1494.
[10] K.-Y. Yang, K.-M. Yoon, K. W. Choi, H. Lee, Microelectron. Eng. 2009,
86, 2228.
[29] D. C. Bradley, R. C. Mehrotra, D. P. Gaur, Metal Alkoxides, Academic
Press, Orlando, FL 1978 Ch. 4.
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 2201–2211
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2211