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Direct nanoimprinting of metal oxides by in situ thermal co-polymerizationof their methacrylates†

Ramakrishnan Ganesan,*a Su Hui Lim,a M. S. M. Saifullah,*a Hazrat Hussain,a John X. Q. Kwok,b

Ryan L. X. Tse,b Htoo A. P. Bob and Hong Yee Lowa

Received 26th November 2010, Accepted 21st January 2011

DOI: 10.1039/c0jm04105j

The use of polymerization to solidify, strengthen and imprint liquid organic materials is the basis of

ultraviolet (UV) nanoimprint lithography. In spite of these advantages, the use of polymerization to

pattern materials in thermal nanoimprint lithography is almost non-existent. In this study, we

demonstrate a facile and general method to directly imprint a host of unary metal oxides (Fe2O3, ZrO2,

TiO2, Nb2O5 and Ta2O5) at a very high resolution via in situ thermal free radical co-polymerization of

various metal methacrylates in the presence of cross-linker ethylene glycol dimethacrylate using

a silicon mold. Polymerization during nanoimprinting rigidly shapes the patterns, traps the metal

atoms, reduces the surface energy and strengthens the structures, thereby giving �100% yield after

demolding. It was found that the higher oxidation state of metal resulted in excessive cracking of

imprinted structures. This could be due to a higher degree of cross-linking of the precursor leading to

shrinkage-related stress. Optimization of the resin composition by partially replacing ethylene glycol

dimethacrylate with methyl methacrylate alleviated this problem. Heat-treatment of the imprinted

structures resulted in the loss of organics, their subsequent shrinkage and converted the patterns to their

corresponding metal oxides with line-widths as small as 25 nm.

1. Introduction

With the inherent advantages of low-cost and high-throughput,

nanoimprint lithography is emerging as an alternative patterning

technique to optical lithography that not only enables patterning

of resists, but also is capable of imprinting functional device

structures, which can lead to a wide range of applications in

electronics, photonics, data storage and biology.1 Nano-

imprinting involves pressing of a mold that contains surface relief

features onto a film on a substrate at a controlled temperature

and pressure. For obtaining a good imprint, it is desirable to have

a film with low viscosity and good adhesion on the surface. The

common polymeric materials used in nanoimprint lithography

are thermoplastics, thermosets and UV-curable polymers. In

order to directly imprint functional materials such as oxides, sol–

gel films and metal–organic materials are being increasingly

used.2–14

aInstitute of Materials Research and Engineering, A*STAR (Agency forScience Technology and Research), 3 Research Link, Singapore, 117602,Republic of Singapore. E-mail: ganesanr@imre.a-star.edu.sg;saifullahm@imre.a-star.edu.sgbNUS High School of Mathematics and Science, 20 Clementi Avenue 1,Singapore, 129957, Republic of Singapore

† Electronic supplementary information (ESI) available. See DOI:10.1039/c0jm04105j

4484 | J. Mater. Chem., 2011, 21, 4484–4492

Sol–gel techniques with their low-temperature characteristics,

good film quality, and as an economical way to obtain engi-

neered ceramics have attracted great deal of attention in

patterning technologies. Alkoxide-based sol–gel chemistry has

been extensively used to pattern oxides using optical litho-

graphy,15–21 electron beam nanolithography,21–27 electro-

hydrodynamic lithography28 and, more recently, it has been

embraced by nanoimprinting as well.2,5–7 However, direct

imprinting of oxides via the sol–gel route faces two major chal-

lenges. Firstly, due to their high surface energy, the sol–gel

imprinting requires a good mold release system preferably con-

taining long chain silanes and, secondly, the solvent in the sol–gel

film, which helps to ‘soften’ it for imprint process, gets trapped in

the imprinted structures resulting in poor demolding and

incomplete filling of the precursor material inside the mold.

These problems have been partly alleviated by using soft molds

with low surface energies such as polydimethylsiloxane

(PDMS),4–6,8–10 and perfluoropolyether (PFPE)7 to imprint

oxides. Soft molds absorb the solvent in the films, thus giving the

imprint its strength to withstand demolding forces and conse-

quently reducing the severity of temperature requirement as

required in the case of a silicon mold.2 However, soft molds are

amenable to deformation, especially when sub-100 nm features

are desired.

Polymerization of liquid resist is used extensively in UV

nanoimprint lithography to imprint liquid organic materials as

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well as oxides.1,4,13 The liquid resist used for UV imprinting has

low viscosity, enhances wider resist redistribution and flow

during the process, giving rise to printing uniformity and in some

cases almost a near-zero residual layer at the bottom of the

pattern. In spite of these advantages of liquid resists, there is

hardly any work in using them to pattern materials using thermal

nanoimprint lithography.29,30 To fill this gap, in this work we

propose to combine alkoxide-based chemistry with methacrylate

chemistry to form liquid metal methacrylate resins capable of

undergoing in situ thermal free radical co-polymerization to

directly pattern oxides using silicon molds. Our preliminary work

on imprinting of TiO2 via this route has shown considerable

amount of promise.14 In this work we demonstrate a facile and

general method to directly imprint a host of unary metal oxides,

viz., Fe2O3, ZrO2, TiO2, Nb2O5 and Ta2O5 from their respective

metal methacrylates. Except for iron methacrylate, all others

were prepared by reacting their respective metal alkoxides with

methacrylic acid. Our approach incorporates the benefits of

a rigid mold and liquid precursor to achieve very high resolution

over areas >1 cm� 1 cm but at a lower temperature and pressure

to obtain yields of almost 100% after imprinting. It was observed

that the quality of imprint was dependent on the oxidation state

of the metal atom in metal methacrylate resins. Therefore, in this

paper we will only discuss the imprinting of representative resins

corresponding to the oxidation states +3 (Fe), +4 (Zr), and +5

(Ta).

2. Experimental details

Materials

Aluminium(III) tert-butoxide (97%, Aldrich), yttrium(III) n-but-

oxide (99.9+%, 0.5 M in toluene, Sigma Aldrich), zirconium(IV)

n-butoxide (80 wt% in 1-butanol, Sigma Aldrich), titanium(IV)

n-butoxide (97%, Sigma Aldrich), hafnium(IV) tert-butoxide

(99.9%, Alfa Aesar), tin(IV) tert-butoxide (99.99%, Aldrich),

niobium(V) ethoxide (99.5%, Sigma Aldrich), vanadium(V) trii-

sopropoxide oxide (95–99%, Alfa Aesar), tantalum(V) n-but-

oxide (98%, Sigma Aldrich), iron(III) chloride (anhydrous, 98%,

Alfa Aesar) and sodium methacrylate (98%, Sigma Aldrich)

were used without purification.

Ethylene glycol dimethacrylate (EDMA), methacrylic acid

(MAA) and methyl methacrylate (MMA) were purchased from

Sigma Aldrich, and were used after removing the stabilizer using

an alumina column. Azobisisobutyronitrile (AIBN) was purified

by recrystallization in methanol. 1H,1H,2H,2H-Per-

fluorodecyltrichlorosilane (96%, Alfa Aesar) was used as

received.

This journal is ª The Royal Society of Chemistry 2011

Preparation of metal methacrylates

Iron(III) methacrylate was prepared by double decomposition of

iron(III) chloride and sodium methacrylate in an aqueous

medium. The product formed was deep brown in colour. It was

washed a few times in water and dried overnight in a vacuum

oven. The dried product was purified by dissolving it in

dichloromethane and the solution was filtered to remove any

unreacted material. Iron(III) methacrylate was obtained by

removing the solvent under reduced pressure in a rotary evapo-

rator.

Rest of the metal methacrylate derivatives were prepared

inside a glove box (<5% relative humidity) by mixing the corre-

sponding metal alkoxides with stoichiometric quantity of MAA

needed to remove all the alkoxy groups. Instantaneous charac-

teristic colour change was observed, indicating formation of the

metal methacrylates.

Resin formulation

Resin formulation of each metal methacrylate was prepared by

mixing it with a known amount of EDMA and 2 wt% of AIBN,

the latter corresponding to the total amount of EDMA and

MAA. All the formulations were purged with argon gas for 2–3

minutes to remove dissolved oxygen. Hereafter, the formulations

will be referred to as their corresponding metal oxide resins. For

example, zirconium methacrylate, EDMA and AIBN mixture

will be designated as ZrO2 resin.

Imprint lithography

Silicon substrates and silicon molds were cleaned using hot

piranha solution (3 : 7 by volume of 30% H2O2 and H2SO4.

Caution: Piranha solution reacts violently with most organic

materials and must be handled with extreme care!) for 2 hours,

followed by washing with deionized water and blow drying using

a nitrogen gun. The molds were silanized with 1H,1H,2H,2H-

perfluorodecyltrichlorosilane for 5 hours to decrease their

surface energy, which in turn facilitates a clean demolding after

imprinting.

The metal oxide resins were spin-coated onto the pre-cleaned

silicon substrates at two spin speeds, 3000 and 6000 rpm, in order

to control the film thickness. Imprint lithography was carried out

in an Obducat imprinter (Obducat, Sweden). Imprinting condi-

tions for various resins were determined using differential scan-

ning calorimetry (DSC, TA Instruments Q100). A Nicolet� 6700

Fourier transform infrared (FTIR) spectroscope was used to

analyze the change in structure of the resin during polymeriza-

tion. Thermogravimetric analysis (TGA, TA Instruments Q500)

J. Mater. Chem., 2011, 21, 4484–4492 | 4485

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was used to determine the degradation temperature of organic

constituents and formation of oxide. This information was used

for the heat-treatment of imprinted samples in a furnace heated

at the rate of 10 �C min�1.

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. High resolution images of as-

imprinted as well as heat-treated imprinted structures were

acquired using a JEOL JSM6700F field-emission scanning elec-

tron microscope (FE-SEM). For X-ray diffraction (XRD)

analysis, imprinting of metal oxide resins was done using a blank

silicon substrate as the mold, followed by the heat-treatment at

different temperatures. A Bruker D8 General Area Detector

Diffraction System (GADDS) equipped with a Cu-Ka source

was used to do XRD analysis of heat-treated metal oxide films.

3. Results and discussion

Metal methacrylates are either insoluble or sparingly soluble in

organic solvents. Earlier studies have suggested that when they

are formed by reacting stoichiometric amounts of their respective

alkoxides with MAA, their solubility seems to increase enor-

mously and the product formed is a clear solution.14,31 Metal

alkoxides are strong bases and may rapidly react with MAA, an

acidic monomer, and are neutralized. At [MAA]/[Mx+] ¼ 1 the

following reaction proceeds rapidly, and an Mx+ complex and

alcohol are generated.

M(OR)x + MAA / M(OR)x�1(MA) + ROH

R and MA denote alkyl group and deprotonated MAA, i.e.,

methacrylate, respectively. Furthermore, this reaction is

exothermic and is accompanied by a colour change of the solu-

tion. Complexes of the metal ions with methacrylate are gener-

ated with alcohols as byproducts.31 Table 1 gives

a comprehensive listing of reaction between liquid metal alk-

oxides and MAA, and the potential usefulness of the obtained

product in direct imprinting of oxides. It was found that

alkoxides of aluminium, yttrium, tin, hafnium and vanadium

either precipitated or formed gels when reacted with MAA. On

the other hand, reaction between MAA and alkoxides of

Table 1 Imprintability of various metal methacrylates formed by reacting th

AlkoxideColour chacid addi

Aluminium(III) tert-butoxide TransparYttrium(III) n-butoxide TransluceTitanium(IV) n-butoxide Dark broTin(IV) tert-butoxide White prZirconium(IV) n-butoxide Pale yelloHafnium(IV) tert-butoxide Pinkish, b

minuteNiobium(V) ethoxide YellowTantalum(V) n-butoxide Faint yelVanadium(V) triisopropoxide oxide Faint yel

when e

4486 | J. Mater. Chem., 2011, 21, 4484–4492

titanium, zirconium, niobium and tantalum gave clear solutions.

These liquid solutions were found to be very stable, with no

precipitate generated during several weeks of storage. They were

chosen for further studies involving direct imprinting of oxides.

As iron methacrylate was difficult to prepare via the reaction of

its alkoxide with MAA, it was made as described in the earlier

section.

For high resolution nanoimprinting, low viscosity liquid

precursors are preferable since they fill into the mold cavities

easily upon applied pressure and enable printing uniformity. In

this work, nanoimprinting was performed in a two-step process

by placing the mold on a thin film of metal methacrylate con-

taining metal oxide resin spin-coated onto a silicon substrate: (i)

applying pressure to fill the monomers into the mold cavities, and

(ii) heating the set-up to the polymerization temperature, whilst

the applied pressure was kept constant. After imprinting, the

assembly was cooled down to 65 �C before releasing the pressure,

followed by demolding. We have successfully applied this

method to the resins of five different metal oxides of iron, tita-

nium, zirconium, niobium and tantalum, and demonstrated the

ability of imprinting their oxides on a sub-50 nm scale. Fig. 1

depicts the steps involved in direct nanoimprint lithography of

oxides. It was observed that the group of metal oxide resins

containing same oxidation state of metal behaves in a similar

way. Therefore, we will only discuss the imprinting of represen-

tative metal methacrylates corresponding to the oxidation states

of +3 (Fe), +4 (Zr), and +5 (Ta).

Fe2O3 resin

Iron was chosen as a trivalent candidate and its methacrylate

derivative, iron methacrylate, was synthesized from double

decomposition of iron(III) chloride and sodium methacrylate in

an aqueous medium. The FTIR spectrum of the final product

resembled that of sodium methacrylate, since the organic part

remained unchanged. Iron methacrylate (3 mmol) was dissolved

in a 1 : 1 mixture of iso-propanol–butanol (1 ml). To this,

4.5 mmol of EDMA (ideally to saturate all the iron methacrylate

double bonds) and 2 wt% of AIBN were added to give Fe2O3

resin.

The TGA and DSC studies of Fe2O3 resin were done in order

to understand its mass loss behaviour and to identify the poly-

merization exotherm, respectively. The TGA measurement in air

showed a two step mass loss: evaporation of the solvent from

eir respective alkoxides with methacrylic acid

ange after methacryliction Imprintability

ent gel Nont, turbid yellow gel Nown Yesecipitate Now Yesut precipitated in a fews

No

Yeslow Yeslow, solution turns turbidxposed to air

No

This journal is ª The Royal Society of Chemistry 2011

Fig. 1 Schematic representation of process steps for direct imprinting of

oxides from metal methacrylates. Inset: A �2 cm � 1 cm imprinted area

of ZrO2 using a mold containing 250 nm line gratings. Notice the

appearance of uniform blue colour due to interference.

Fig. 2 (a) TGA and (b) DSC scans of Fe2O3, ZrO2 and optimized Ta2O5

resins.

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30 �C to 140 �C, and the loss of organic component from 250 �C

to 490 �C (Fig. 2(a)). Above this temperature, nearly a constant

weight of the residue (�11% of the starting mass) was seen due to

the formation of Fe2O3. The DSC scan revealed that the free

radical co-polymerization of methacrylate and EDMA in Fe2O3

resin occurred at �125 �C (Fig. 2(b)). This temperature was used

This journal is ª The Royal Society of Chemistry 2011

to initiate in situ free radical co-polymerization during the

imprinting of Fe2O3 resin. The polymerization reaction was

further monitored by using time resolved-FTIR studies

(Fig. 3(a)). It was observed that after polymerization, the

intensity of the methacrylate double bond at 1640 cm�1 largely

decreased. Furthermore, peaks corresponding to EDMA at

1155, 1295, and 1724 cm�1 also decreased significantly in inten-

sity, thus showing the participation of EDMA in the polymeri-

zation process giving rise to a cross-linked network. On the other

hand, the carbonyl group peaks of the chelating carboxylate at

1538 and 1422 cm�1 were seen remaining after the polymeriza-

tion, clearly suggesting that the metal atoms remain chelated,

and therefore, stay trapped inside the polymer structure.

For imprinting Fe2O3 resin, typically a pressure of 30 bar was

used. Cross-linking between the monomers gave good mechan-

ical strength to the imprinted features. This enabled a clean

demolding and an excellent imprint yield (�100%) was obtained.

The imprinted patterns were observed with SEM, and an AFM

was used to analyze the heights of the structures (Fig. 4). SEM

studies indicate that the width of the imprinted features was

slightly smaller than the actual dimensions of the mold—a result

most likely due to the polymerization-induced shrinkage of the

patterns.32 It was observed that 250 nm and 100 nm grating

molds gave 225 nm and 70 nm pattern widths, respectively, while

200 nm dimple mold gave �130 nm wide pillars (Fig. 4 and

Table 2). The imprinted features were heat-treated at 450 �C for 1

hour to remove the organics and convert the patterns to Fe2O3.

Pattern shrinkage of �70% with respect to the original mold size

was observed. This gave rise to sub-100 nm patterns when the

original molds with 200 nm and 250 nm features were used for

imprinting. When a mold containing 100 nm gratings was used to

J. Mater. Chem., 2011, 21, 4484–4492 | 4487

Fig. 3 Time-resolved FTIR spectra of (a) Fe2O3, (b) ZrO2 and (c)

optimized Ta2O5 resins. The broad vibration bands corresponding to

particular bonds are indicated on top. The arrow indicating a shoulder at

1698 cm�1 belongs to MAA, suggesting that the reaction between

alkoxide and MAA may have been incomplete.

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make imprints, the final pattern size shrunk to 25 nm after heat-

treatment. Interestingly, the AFM analysis showed that the

aspect ratio remained very close to 1 for all the patterns even

after heat-treatment, which suggests that the shrinkage was

nearly uniform over the entire imprinted structure. It is worth

noting that although there is a reduction in the pattern size, the

center-to-center pattern distance remains the same, while the

edge-to-edge pattern distance increases.

XRD study was performed to understand the crystallization

behaviour of Fe2O3 as a function of temperature. It was found

4488 | J. Mater. Chem., 2011, 21, 4484–4492

that the heat-treatment at 450 �C for 1 hour led to the appearance

of the hematite phase (see ESI†).

ZrO2 resin

The imprinting characteristics of ZrO2 and TiO2 resins were

found to be remarkably similar. This may be due to the fact that

they both have same oxidation number +4, and therefore, may

give rise to similar polymerization behaviour, ceteris paribus.

Since direct imprinting of TiO2 has been discussed in our

preliminary study,14 zirconium has been chosen as the tetravalent

candidate metal for imprinting studies. Its methacrylate was

synthesized by a simple reaction of zirconium n-butoxide

(3 mmol) with stoichiometric amount (12 mmol) of MAA. The

resultant liquid was mixed with 6 mmol of EDMA and 2 wt% of

AIBN to form ZrO2 resin.

The reaction between zirconium n-butoxide and MAA was

confirmed by FTIR measurement, which showed the appearance

of characteristic carbonyl peaks of the chelating carboxylate

group at around 1538–1541 and 1422–1427 cm�1 (data not

shown). In addition, the methacrylate stretching peak was

observed at around 1640 cm�1.

The TGA and DSC analyses of ZrO2 resin showed a similar

behaviour to that of Fe2O3 resin. In the TGA study, it was seen

that butanol present in the formulation evaporated below 130 �C

and the decomposition of the organic portion occurred in

a temperature range of 300–500 �C, leaving behind �12% of the

original mass (Fig. 2(a)). From the DSC study, the polymeriza-

tion exotherm of ZrO2 resin was found to be at 100 �C—a value

which was used for imprinting of ZrO2 resin (Fig. 2(b)). The

FTIR study of free radical co-polymerization of ZrO2 resin

revealed a similar behaviour to that of Fe2O3 resin (compare

Fig. 3(a) and (b)).

The ZrO2 resin was diluted with n-butanol in a 1 : 1 ratio by

volume for imprinting. Nanoimprinting was done by applying

the pressure between 10 and 30 bar, with hardly any detectable

change in the quality of imprint. The cross-linked polymer

network formed due to in situ thermal free radical co-polymeri-

zation of ZrO2 resin gave good mechanical strength to the

imprinted features that enabled clean demolding. The patterning

yield was close to �100%. Heat-treatment of the imprinted

features resulted in pattern size reduction. Fig. 5 shows 75 nm

wide imprinted lines obtained from a 100 nm grating mold. When

these lines are heat-treated at 450 �C for 1 hour, ZrO2 is formed

and the pattern size is reduced to 43 nm. Their aspect ratio

remains close to the original value of �1.

Ta2O5 resin

The imprinting behaviour of pentavalent metal oxide resin differs

considerably from their tetravalent and trivalent metal oxide

counterparts mainly due to the observance of cracking of the

resin after imprinting. Tantalum and niobium have been chosen

as the pentavalent candidate metals in this study; with the oxide

resin made for the former was imprinted using the experimental

conditions optimized for the latter.

Tantalum and niobium methacrylates were synthesized by

reacting their respective alkoxides (3 mmol) with stoichiometric

amounts (15 mmol) of MAA. The resultant liquids were mixed

This journal is ª The Royal Society of Chemistry 2011

Fig. 4 Composite SEM images of various as-imprinted and heat-treated structures of Fe2O3 using different molds. The insets show the structures at

higher magnification. The AFM line traces of the corresponding heat-treated imprinted structures are shown on the right.

Table 2 Summary of the approximate feature size reduction at every step of the Fe2O3 resin patterning using imprint lithography

Moldshape/size

Feature size of the imprint afterfree radical polymerization

Metal oxide feature size after the heat-treatment of imprinted structures

Total feature size reduction withrespect to mold feature size (%)

Width ofimprint/nm

Feature sizereduction (%)

Width of the metaloxide feature/nm

Feature sizereduction (%)

Dimples, 200 nm 130 35 65 50 68Lines, 250 nm 225 10 60 73 76Lines, 100 nm 70 30 25 64 75

Fig. 5 Composite SEM images of as-imprinted and heat-treated structures of ZrO2 using 100 nm line and space grating mold. Structures at higher

magnification are shown as insets. The AFM line trace of the heat-treated imprinted structures is shown on the right.

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with 7.5 mmol of EDMA (ideally to saturate all the methacrylate

double bonds) and 2 wt% of AIBN to give Ta2O5 and Nb2O5

resins, respectively. The formation of methacrylates was

confirmed by the FTIR spectroscopy and they were found to be

similar to that of Fe2O3, ZrO2 and TiO2 resins.

Nanoimprint lithography carried out with this formulation

of Nb2O5 resin film showed excessive cracking in the residual

layer (Fig. 6(a)). We speculate that this behaviour could be due

This journal is ª The Royal Society of Chemistry 2011

to the higher degree of cross-linking. In the cases of Fe2O3

resin, which has three methacrylate arms, and ZrO2 and

TiO2 resins, which have four methacrylate arms, the degree of

cross-linking should be lower than the pentavalent metal atom

containing Nb2O5 resin. Since the methacrylates are known to

shrink during polymerization,32 the cracking could be due to

excessive stress caused by the shrinkage along five

directions.

J. Mater. Chem., 2011, 21, 4484–4492 | 4489

Fig. 6 (a) Optimization of Nb2O5 resin by changing the amounts of EDMA and MMA with respect to Nb(OEt)5 to give a good quality imprint without

cracks. It was noticed that the ratios of Nb(OEt)5 : EDMA : MMA corresponding to 1 : 1.0 : 2 gave the best quality of imprint using a 250 nm line and

space grating mold. (b) The data from optimization of Nb2O5 resin were also used to imprint pentavalent metal containing Ta2O5 resin. The insets show

structures at higher magnification. The AFM line trace of the heat-treated imprinted structures is shown on the right.

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To circumvent this problem, we replaced part of cross-linker

EDMA with MMA, a monomer with one unsaturated –C]C, to

reduce the degree of cross-linking. The results of optimization

are shown as ratios between niobium ethoxide (Nb(OEt)5),

EDMA and MMA (Fig. 6(a)). When the molar amount of

EDMA is reduced by 1 and instead replaced by the same amount

of MMA, the cracking in the residual layer showed noticeable

reduction. However, further replacement of EDMA with MMA

(Nb(OEt)5 : EDMA : MMA ¼ 1 : 0.5 : 2) resulted in excessive

cracking. This may be due to higher degree of cross-linking

within methacrylate arms of niobium methacrylate in the absence

of sufficient monomeric cross-linkers to cushion the stress arising

due to shrinkage. Now when the amount of MMA is increased to

give a composition ratio Nb(OEt)5 : EDMA : MMA ¼1 : 0.5 : 3, the quality of the imprint increased dramatically with

cracks visible only close to the imprinted lines. The best

composition was found to be Nb(OEt)5 : EDMA : MMA ¼1 : 1.0 : 2, which was devoid of cracks after imprinting.

Similar trend was also seen in the imprinting characteristics

of Ta2O5 resin as tantalum is a pentavalent atom (see ESI†).

Unsurprisingly, Ta2O5 resin gave best imprints for the same

4490 | J. Mater. Chem., 2011, 21, 4484–4492

composition as used for Nb2O5 resin (Fig. 6(b)). SEM images

of the heat-treated samples also showed shrinkage of the

imprinted material, but to a lesser degree than what has been

seen for Fe2O3 and ZrO2 resins. For example, Ta2O5 resin

imprinted by 250 nm line and space mold gave 210 nm line-

widths, which on heat-treatment at 550 �C for 1 hour gave

�100 nm lines. AFM measurements revealed that the aspect

ratio of these lines was close to 1.5, unlike �1 obtained for

Fe2O3 and ZrO2 lines.

The TGA of optimized composition of Ta2O5 resin showed

similar behaviour to that of the other metal methacrylates

described earlier. The organics were completely degraded at

about 580–590 �C, leaving behind the metal oxide residue.

Compared to the other metal oxide resins, the pentavalent resins

required higher temperature to completely decompose the

organics, which may be due to the higher degree of cross-linking

between five-armed methacrylate and EDMA (Fig. 2(a)). DSC

analysis of this composition showed that the polymerization

took place at about 95 �C (Fig. 2(b)). The polymerization

behaviour, studied using the FTIR, was consistent with previ-

ously described metal oxide resins (Fig. 3(c)).

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4. General remarks

Direct imprinting of metal oxides by in situ thermal free radical

co-polymerization of their respective metal methacrylates with

EDMA has remarkable similarities. They all use liquid mono-

mers which undergo co-polymerization during imprinting.

Polymerization rigidly shapes the imprinted patterns, traps the

metal atoms, reduces the surface energy and strengthens the

structures, thereby giving �100% yield after demolding. Ther-

molysis of the imprinted patterns converts them to their corre-

sponding oxides with much smaller feature size than what was on

the original mold. There are, however, some significant differ-

ences as well. It was observed that the quality of an imprint and

its subsequent conversion to oxide varies with spin-coating

speed. In the case of Fe2O3 resin, after coating at 3000 rpm, an

imprint with a residual layer as thick as �600 nm showed no

cracking even after heat-treatment (see ESI†). Similar observa-

tion was also made for TiO2 resin which had a residual layer

thickness of �1200 nm. However, in the case of ZrO2 resin,

although the imprint with residual layer thickness of �1800 nm

did not show any cracking, fine cracks were seen after heat-

treatment, most likely arising due to thermal stresses. When ZrO2

resin was spin-coated at 6000 rpm and imprinted (residual layer

thickness �900 nm), the problem of cracking was somewhat

alleviated after heat-treatment (see ESI†). Not surprisingly,

crack-free imprints after heat-treatment were seen when ZrO2

resin was diluted with n-butanol in a 1 : 1 ratio to give even

thinner residual layer, slightly less than half the thickness of what

was obtained at 6000 rpm without dilution (Fig. 5). Likewise, in

the case of Ta2O5 and Nb2O5 resins, the optimized composition

gave good imprint at 3000 rpm, and the quality was improved

further at 6000 rpm.

Metal methacrylates used in this work are multi-functional

and can act as cross-linkers. So, why use another cross-linker like

EDMA as well? It was observed that metal methacrylates have

poor film forming ability, and when imprinted, they showed a lot

of cracking as well as poor adhesion to the silicon substrate.

However, with the addition of EDMA, not only better film

adhesion to substrate was observed but also the quality of

imprint improved substantially. Furthermore, EDMA is miscible

in metal methacrylates prepared in this study and this helped in

optimization of metal oxide resins. The content of EDMA in the

resin affects the imprint quality. In most cases, the best

imprinting results were achieved when the content of EDMA was

just enough to ideally saturate all the –C]C bonds in metal

methacrylate during polymerization.

Direct imprinting of metal oxides by the metal methacrylate

route offers significant advantages over the sol–gel-based route

which mainly uses a soft mold for nanoimprint lithography.

Unlike the critical requirement of solvent in the sol–gel material

for imprinting, metal methacrylates can be dissolved in liquid

monomers which can then be imprinted at lower pressures

without worrying about the removal of solvent. In fact, 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 polymerization. The polymerization in this system is

comparable to bulk polymerization (without solvent), which

gives rise to hardened pattern features and thus facilitates clean

demolding with�100% yield over large areas. This approach has

This journal is ª The Royal Society of Chemistry 2011

the capability of producing high-throughput at a very high

resolution, which is not possible when a soft mold is used. Using

our approach, one could make sub-100 nm imprints over >2 cm

� 1 cm by using molds containing sub-micron pattern features,

which can translate into huge cost savings (Fig. 1, inset).

Furthermore, by adjusting the metal content in the overall

formulation, one should be able to control the degree of pattern

shrinkage after heat-treatment.

Unlike the conventional nanoimprint lithography using an

organic resist, where the residual layer can be easily removed by

reactive ion etching, it is difficult to remove the residual layer by

such techniques when the resin is composed of organometallics.

However, a physical bombardment method was demonstrated to

be useful to remove poly(ferrocenylmethylphenylsilane) residual

layer.33 Using this method, it may be possible to obtain isolated

patterns of metal oxides. Furthermore, our method also has the

potential to fabricate metal lines by hydrogen reduction of metal

oxides which reside above the water formation line (i.e., H2 +

½O2 ¼ H2O line) in the Ellingham diagram. In other words,

imprinted patterns containing metal oxides such as NiO, CuO,

PbO, CoO, etc., which are thermodynamically less stable than

water, can be reduced by hydrogen to their respective metals.34

5. Conclusions

Using in situ thermal co-polymerization of the metal methacry-

lates with cross-linker EDMA during imprinting and their

subsequent thermolysis, we have demonstrated a facile and

general route to direct imprint lithography of various metal

oxides. This approach enabled us to fabricate large area nano-

scale patterns of various metal oxides such as Fe2O3, TiO2, ZrO2,

Nb2O5, and Ta2O5. Resin formulations were made by mixing

individual metal methacrylates with EDMA and AIBN. All the

metal oxide resins underwent thermally initiated free radical co-

polymerization, leading to lowering of surface energy and

strengthening of the imprint, which enabled easy and clean

demolding. Since the metal atoms remain trapped inside the

polymer matrix, subsequent thermolysis in air yielded the cor-

responding metal oxide patterns. Shrinkage of pattern size was

observed in all the metal oxide resin imprints due to the loss of

organics giving rise to oxide feature sizes as small as 25 nm. While

the imprint characteristics of Fe2O3, TiO2 and ZrO2 resins were

similar, optimization of the resin composition was necessary in

the case of Nb2O5 and Ta2O5 resins to reduce the cross-linking

density in order to obtain crack-free residual layer. This was

achieved by suitably adjusting the amounts of EDMA and

MMA. By tweaking the chemistry of metal oxide resins, our

technique can potentially be extended to UV nanoimprinting of

oxides as well as to pattern multi-component oxide systems.

Acknowledgements

The authors would like to thank Lim Poh Chong of Institute of

Materials Research and Engineering for his assistance in XRD.

This work was supported by the IMRE-funded core project no.

IMRE/09-1C0319.

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This journal is ª The Royal Society of Chemistry 2011