nano-sized patterns derived from a sicn preceramic polymer: fabrication and their characterization

6
Journal of Physics and Chemistry of Solids 69 (2008) 2131–2136 Technical note Nano-sized patterns derived from a SiCN preceramic polymer: Fabrication and their characterization Hong-Joo Lee a, , Tae-Ho Yoon b , Dong-Pyo Kim b a Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of Korea b School of Applied Chemistry and Biological Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea Received 5 February 2007; received in revised form 2 January 2008; accepted 26 January 2008 Abstract A microfabrication of ceramic materials with a low shrinkage has created inevitable demands in the fields of micro total analysis systems (m-TAS) and microfluidic devices with thermal, chemical, and mechanical stability. In this study, nano-sized patterns were prepared using a soft lithographic technique, followed by the UV and thermal curation of a non-oxide SiCN preceramic polymer. The characterization of the thermal and mechanical properties of the cured preceramic polymer revealed that it becomes ceramic-like after the heat treatment at higher temperature than 600 1C. The heat-treated nano-sized patterns showed a low linear shrinkage as well as good chemical stability and optical properties. The results showed the feasibility of enormous potentials of the cured preceramic polymer because of the low shrinkage, the high optical transparency, and the thermal and chemical stability. r 2008 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; A. Nanostructures; A. Non-crystalline materials 1. Introduction Demands on the microfabrication of ceramic materials have increased in the fields of micro total analysis systems (m-TAS) and microfluidic devices, which can be applied in the environments requiring a tolerance to high tempera- tures and a resistance to corrosion. The preceramic polymer for various SiC-based non-oxide ceramics such as SiC, SiCN, and SiBCN can be easily shaped by various techniques and then cross-linked by heat or UV exposure to form an infusible solid [1,2]. Among non-oxide ceramics, SiCN is considered to be one of the most interesting materials because of high thermal conductivity, excellent thermal stability, high mechanical strength, and chemical inertness [3]. The transformation of the preceramic polymer to the ceramic phase by the heat treatment is of importance for the application of ceramic materials [4,5]. Raj et al. [6–8] demonstrated the preparation of SiCN ceramic MEMS devices using a commercially available preceramic polymer via heat or UV-curing-induced trans- formation from a liquid state to an infusible solid state Soft lithography, the fabrication of microstructures by using an elastomeric silicone rubber, provides one of the valuable tools for the low-cost microstructuring and microfabrication of channels derived from a preceramic polymer [9,10]. Microcontact printing (m-CP) is used to stamp self-assembled monolayers acting as a resist or a functional layer. Replica molding is an efficient method of duplicating information (i.e., shape, morphology, and structure) present on the surface of a mold. In micro- transfer molding (m-TM), a thin layer of liquid preceramic polymer is applied to a patterned surface of a polydi- methylsiloxane (PDMS) mold, and the excess liquid is removed by scraping using a flat PDMS block or by blowing off with a stream of nitrogen. A fluid with a low viscosity is patterned by the spontaneous filling of PDMS microfluidic channels as a result of capillary action in micromolding in capillaries (MIMIC). After curing the preceramic polymer into a solid, the PDMS mold is removed to reveal the patterned microstructures of the polymer [11,12]. ARTICLE IN PRESS www.elsevier.com/locate/jpcs 0022-3697/$ - see front matter r 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2008.01.018 Corresponding author. Tel.: +82 62 970 3284; fax: +82 62 970 2434. E-mail address: [email protected] (H.-J. Lee).

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ARTICLE IN PRESS

0022-3697/$ - se

doi:10.1016/j.jp

�CorrespondiE-mail addre

Journal of Physics and Chemistry of Solids 69 (2008) 2131–2136

www.elsevier.com/locate/jpcs

Technical note

Nano-sized patterns derived from a SiCN preceramic polymer:Fabrication and their characterization

Hong-Joo Leea,�, Tae-Ho Yoonb, Dong-Pyo Kimb

aDepartment of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST),

1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of KoreabSchool of Applied Chemistry and Biological Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea

Received 5 February 2007; received in revised form 2 January 2008; accepted 26 January 2008

Abstract

A microfabrication of ceramic materials with a low shrinkage has created inevitable demands in the fields of micro total analysis

systems (m-TAS) and microfluidic devices with thermal, chemical, and mechanical stability. In this study, nano-sized patterns were

prepared using a soft lithographic technique, followed by the UV and thermal curation of a non-oxide SiCN preceramic polymer. The

characterization of the thermal and mechanical properties of the cured preceramic polymer revealed that it becomes ceramic-like after the

heat treatment at higher temperature than 600 1C. The heat-treated nano-sized patterns showed a low linear shrinkage as well as good

chemical stability and optical properties. The results showed the feasibility of enormous potentials of the cured preceramic polymer

because of the low shrinkage, the high optical transparency, and the thermal and chemical stability.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; A. Nanostructures; A. Non-crystalline materials

1. Introduction

Demands on the microfabrication of ceramic materialshave increased in the fields of micro total analysis systems(m-TAS) and microfluidic devices, which can be applied inthe environments requiring a tolerance to high tempera-tures and a resistance to corrosion. The preceramicpolymer for various SiC-based non-oxide ceramics suchas SiC, SiCN, and SiBCN can be easily shaped by varioustechniques and then cross-linked by heat or UV exposureto form an infusible solid [1,2]. Among non-oxide ceramics,SiCN is considered to be one of the most interestingmaterials because of high thermal conductivity, excellentthermal stability, high mechanical strength, and chemicalinertness [3]. The transformation of the preceramicpolymer to the ceramic phase by the heat treatment is ofimportance for the application of ceramic materials [4,5].Raj et al. [6–8] demonstrated the preparation of SiCNceramic MEMS devices using a commercially available

e front matter r 2008 Elsevier Ltd. All rights reserved.

cs.2008.01.018

ng author. Tel.: +8262 970 3284; fax: +82 62 970 2434.

ss: [email protected] (H.-J. Lee).

preceramic polymer via heat or UV-curing-induced trans-formation from a liquid state to an infusible solid stateSoft lithography, the fabrication of microstructures by

using an elastomeric silicone rubber, provides one of thevaluable tools for the low-cost microstructuring andmicrofabrication of channels derived from a preceramicpolymer [9,10]. Microcontact printing (m-CP) is used tostamp self-assembled monolayers acting as a resist or afunctional layer. Replica molding is an efficient method ofduplicating information (i.e., shape, morphology, andstructure) present on the surface of a mold. In micro-transfer molding (m-TM), a thin layer of liquid preceramicpolymer is applied to a patterned surface of a polydi-methylsiloxane (PDMS) mold, and the excess liquid isremoved by scraping using a flat PDMS block or byblowing off with a stream of nitrogen. A fluid with a lowviscosity is patterned by the spontaneous filling of PDMSmicrofluidic channels as a result of capillary action inmicromolding in capillaries (MIMIC). After curing thepreceramic polymer into a solid, the PDMS mold isremoved to reveal the patterned microstructures of thepolymer [11,12].

ARTICLE IN PRESSH.-J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2131–21362132

Among soft lithographic techniques, the imprint litho-graphy is considered to be an alternative of the micro-fabrication techniques. Using the imprint lithography,micro- or nano-sized patterns are fabricated at highresolution, high throughput, and low cost. The imprintlithography is a straightforward and high-throughputtechnique for producing patterns onto a wide range ofmaterials [13–15]. In addition, high-resolution polymericfeatures are fabricated by the irreversible deformation ofthe surface morphology of a thermoplastic film [16,17].This is achieved by heating the polymeric layer above itsglass transition temperature and by applying externalpressure. As a consequence of curation, the preceramicpolymer viscosity decreases dramatically, and the polymerfills the void spaces between the template and the film. Theschematic diagram of the fabrication of micro- or nano-

Glass substrate

PDMS maskPDMS mask

Substrate

Press

PDMS mold

Spin-coated preceramic polymer

Cured preceramic patterns

Heat-treated patterns

Heat Treatment(600-800 °C)

Thermal Curation(~150 °C)

UV Curation

Fig. 1. Schematic illustration of the imprint lithographic technique for the

fabrication of micro/nano-structures.

structures by the imprint lithography is shown in Fig. 1. APDMS mold is pressed into the layer of the viscouspolymer film on a substrate, conforming to the mold underpressure. The polymer is cured thermally after the UVcuration. Then, the cured polymer is heat-treated toconvert it to a monolithic ceramic part.It is reported that the preceramic polymer undergoes a

linear shrinkage of approximately 30% during heattreatment. In addition, adhesion to a substrate during heattreatment can cause cracking or fracture, as well asdecrease in density [8]. Because of the low level ofshrinkage compared with ceramic materials, the conversionof preceramic polymers into cured polymers can be usedfor the fabrication of micro- or nano-structures. However,the microfabrication of a cured preceramic polymer with alow shrinkage has been rarely reported till now [18,19].This study utilized the unique hydrophobic nature of the

cured preceramic polymers after fabricating the nano-structures derived from the polymers with a low shrinkage.The feasibility of the fabrication of nano-sized patternsprepared by thermal and UV treatment of a preceramicpolymer, polyvinylsilazane, was studied in this study.

2. Experimental

The silanized masters on a Si wafer were prepared usingSU-8 photoresist (MicroChem Corporation, USA) by thephotolithographic method. As an elastomeric siliconerubber mold, PDMS mold was fabricated by casting itsprepolymer, Sylgard 184 (Dow Corning, USA), against acomplementary relief structure. The mold was cured bythoroughly mixing a 1:10 ratio of the curing agent and theprepolymer after air bubbles in the mixture were removedin a vacuum chamber for 30min to ensure complete mixingof the two components. The mixture was then castedagainst a silanized master and cured at 60 1C for 5 h toform the PDMS mold. In order to investigate the influenceof the cross-linking on the amount of polymer shrinkage,DVD was utilized as an economic nano-sized master. Itcontains the line features with an approximate width of500 nm and a height of 140 nm.For the transferred patterns of the cured preceramic

polymer, 20% of solid-type polyvinylsilazane, Ultra IIs

(KiON Corp, USA) in THF as a SiCN preceramic polymerwas used. Fig. 2 shows the chemical structure ofpolyvinylsilazane. A thermal initiator, dicumyl peroxide(Aldrich, USA), and a photo-initiator, Irgacure 369 (CibaSpecialty, Japan), were used for the thermal and UV curingof the preceramic polymer, respectively. A precleaned glasssubstrate was first coated with a liquid precursor polymerunder a nitrogen atmosphere. After the PDMS mold wasspin-coated with an appropriate mold release agent, it wasplaced on the preceramic polymeric layer on the glasssubstrate. The preceramic polymer was cured by UV andheat for the two-step curing of the imprinted patterns.The thermal stability of the preceramic polymers

was examined using a thermogravimetric equipment, TA

ARTICLE IN PRESS

H Si

Si

HN

Si N

Si or R

N

H3C

R

CH3

R

( )

R CH3

R = H, CH=CH2

n=1~20

Fig. 2. Chemical structure of the SiCN preceramic polymer (polyvinylsi-

lazane).

060

70

80

90

100

Temperature (°C)

Rel

ativ

e w

eigh

t (%

)

1000800600400200

Fig. 3. Weight change of the preceramic polymer in the themogravimetric

plot.

4000

N-H C-H (vinyl) Si-H

Wavenumber (1/cm)

a

b

c

C=C

100015002000250030003500

Fig. 4. Influence of the thermal curation on the chemical structures for the

preceramic polymer (polyvinylsilazane (a), thermally cured polymer at

100 1C (b) and 150 1C (c)).

H.-J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2131–2136 2133

Instrument 2950, in N2 atmosphere. A UV–VIS spectro-scope, S-2000 (Sinco, Korea), was used to measure thetransmittance of the preceramic polymer and curedpolymers. Also, the influence of the thermal curing onthe chemical structures of the preceramic polymer wasinvestigated using an ATR-FTIR (Travel IR, SensIRTechnologies). The contact angles of deionized water onthe surface of the cured preceramic polymer were measuredusing the sessile drop technique on a contact anglegoniometer (DSA100, KRUSS GmbH). The solventcompatibility of the cured polymers against differentsolvents (water, ethanol, isopropyl alcohol, THF, toluene,acetonitrile, and dimethyl sulfoxide) was examined usingthe size of 5mm � 5mm. The swelling ratio of examinedsamples was estimated from the relative linear dimensionsof the cured polymer after wetting the dried cured polymer[18]. Using a Nano Indenter XP (MTS Systems Corp.,USA) with a Berkovich indenter, the mechanical propertiesof the cured polymer were estimated from the load–dis-placement data obtained by the nano-indentation. Ascanning electron microscopy (SEM) (JSM-840, JEOL,Japan) and an atomic force microscopy (AFM) (XE-100,PSIA Inc., Korea) were used to examine the morphology ofthe transferred patterns.

3. Results and discussion

Change in mass in the thermal characterization wasmeasured as a function of temperature up to 1000 1C in aN2 atmosphere and the results are shown in Fig. 3 for thepolyvinylsilazane. A small change (approximately 5%) wasobserved at 300 1C in the figure. At temperature higherthan 400 1C, the preceramic polymer was thermallydegraded. The ceramic yield was estimated to be 80% attemperature higher than 600 1C, indicating that thepolyvinylsilazane becomes ceramic-like at temperaturearound 600 1C.

The hydrophobicity of the cured polymer was analyzedby contact angle measurement. The UV and UV/thermallycured polyvinylsilazane showed around 871 and 891,respectively, showing the highly hydrophobic nature ofthe surface. For the optical transparence, the thermallycured polyvinylsilazanes were further annealed at 300 and400 1C for 1 h in a N2 atmosphere. The transmittanceresults of the annealed polymers were compared with thatof the cured polyvinylsilazane at 150 1C. All the poly-vinylsilazanes examined showed 93% transmittance even inthe range up to 280 nm. The results suggest that the curedpreceramic polymers can be used for the photochemicalreaction due to their high optical properties.For the chemical stability with respect to common

solvents, the cured polymer samples were prepared by theUV curing, followed by the thermal curing. The annealedsamples were prepared by heat treatment of the cured

ARTICLE IN PRESSH.-J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2131–21362134

polymer at 150 and 300 1C for 2 h in the N2 atmosphere.The annealed polyvinylsilazane showed good solvent com-patibility with all examined solvents. The solvent compat-

Table 1

Mechanical properties of the cured and heat-treated preceramic polymers

Treatment Elastic modulus

(GPa)

Hardness

(GPa)

Curation

UV 0.30 0.021

UV/Thermal 0.60 0.11

Heat treatment at the nitrogen

atmosphere (600 1C)

18.00 2.28

Ela

stic

mod

ulus

(GP

a)

0

20

40

60

80

100

120

140

160

180

Heat treatm0

Har

dnes

s (G

Pa)

0

2

4

6

8

10

12

14

6400200

Heat treatm0 6400200

Fig. 5. Mechanical properties of heat-treated polyvin

ibility tests indicate that the UV and thermally curedpolyvinylsilazanes have a good chemical stability [20].The influence of the curing on the chemical structures of

the cured polyvinylsilazane in the presence of the thermalinitiator was investigated and the spectra changes wereshown in Fig. 4. The absorption peak of the vinyl group(C–Hvinyl and CQC) mostly disappeared after curing at150 1C compared with the spectrum of the cured poly-vinylsilazane at 100 1C under the nitrogen atmosphere. Theresults imply that complete cross-linking in the thermalcuration occurred above or around the maximum activityof the thermal initiator at 150 1C [11].The mechanical properties are some of the critical

considerations in determining the long-term stability ofmicro- or nano-sized structures for the fabrication and

ent in N2 (°C)1200100080000

ent in N2 (°C)1200100080000

ylsilazane (elastic modulus (a) and hardness (b)).

ARTICLE IN PRESS

Fig. 6. SEM microphotograph of the transferred nano-sized patterns followed by heat treatment at 800 1C.

H.-J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2131–2136 2135

control on the micro/nano scale [21–23]. The elasticmodulus and hardness for UV curing followed by thermalcuring were measured in this study. Table 1 summarizes themechanical properties of the polyvinylsilazane cured byUV and UV/thermal methods and heat treatment at600 1C. For the UV-cured polyvinylsilazane, the elasticmodulus and the hardness were measured to 0.30 and0.02GPa, respectively. The elastic modulus and hardnessof the thermally cured polymer increased to 0.60 and0.11GPa, respectively, much higher than the UV-curedone. For the heat-treated polymer at 600 1C, the elasticmodulus and the hardness are considerably increased asshown in the table.

The mechanical properties of the heat-treated polyvi-nylsilazanes in the N2 atmosphere were estimated withincreasing heat-treatment temperature and the results areshown in Fig. 5. The values of the elastic modulus werelower than 10GPa as the temperature increased from 200to 400 1C as shown in Fig. 5(a). However, the modulus ofthe heat-treated SiCN at 600 1C or above increasedsignificantly, with the estimated value being 18GPa.Increase in the mechanical properties on heat treatmentat temperatures higher than 600 1C corresponds to thethermal degradation result in Fig. 3. The hardness of theheat-treated polyvinylsilazane in Fig. 5(b) significantlyincreased at 600 1C, similar to the elastic modulus change.It is considered in the characterization of the mechanicalproperties that the preceramic polymer became ceramic-like at temperatures higher than 600 1C [18].

It is accepted that the polymer shrinkage is one of theimportant considerations in the design and fabrication ofnano-sized patterns. When the microstructures are fabri-cated using soft lithographic techniques, the presence of themold and/or substrate constrains polymer shrinkage [7].After the DVD patterns were transferred via imprintlithography, the polymers were cured by UV and thermal

methods to produce the polyvinylsilazane glass patterns toinvestigate the amount of polymer shrinkage [24]. Micro-photographs of SEM and AFM images for the morphol-ogy of the DVD patterns heat-treated at 800 1C are shownin Figs. 6 and 7, respectively. Considering the dimensionalchanges, the width and height of the transferred DVDpatterns were measured to be ca. 480 and 130 nm,respectively, shown in Table 2. The average linearshrinkage relative to the size of the structure after cross-linking was estimated to be 4% and 7% for the width andthe height, respectively. The transferred patterns showedthat the preceramic polymer undergoes a low linearshrinkage during heat treatment. It is considered that themicrofabrication of the cured inorganic polymers hasenormous potentials because of high optical transparency,thermal and chemical stability [25,26].

4. Concluding remarks

Interests on the low-cost soft lithographic techniquehave increased in the fields of m-TAS and microfluidicdevices because of thermal, chemical, and mechanicalstability. The transferred patterns from a cured preceramicpolymer were prepared using the imprint lithography andthe characteristics of the thermal, chemical, mechanicalproperties were characterized in this study. The thermaland mechanical characterization showed that the curedpolyvinylsilazane becomes ceramic-like at the heat treat-ment of 600 1C or above. In addition, the cured preceramicpolymer showed good chemical stability and opticaltransparency. In the fabrication of the nano-sized patterns,the transferred patterns had a low linear shrinkage with4–7%. It was shown in this study that the cured inorganicpolymers have enormous potentials due to high opticaltransparency, thermal and chemical stability.

ARTICLE IN PRESS

Table 2

Change in dimension of the nano-sized patterns

Width

(nm)

Height

(nm)

Master (DVD) 50073 14072

Transferred PDMS mold 50272 13973

UV/thermal cured patterns (heat curing

temperature at 150 1C)

49573 13573

Heat-treated patterns (heat treatment at 800 1C) 48072 13072

Fig. 7. AFM microphotograph of the transferred nano-sized patterns followed by heat treatment at 800 1C.

H.-J. Lee et al. / Journal of Physics and Chemistry of Solids 69 (2008) 2131–21362136

Acknowledgment

This work was financially supported by the 2005National Research Laboratory (NRL) Project [M10400000320-05J0000-32010] administered by the KoreanMinistry of Science and Technology (MOST).

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