Titania Hybrid Resists via a Sol

Jan 14, 2010 - After these reactions, HCl, H2O, acac, and the byproduct, ethanol, were ... Alpha-Step 500) to measure the residual resist thickness af...
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J. Phys. Chem. C 2010, 114, 2179–2183

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Preparation of Epoxy/Silica and Epoxy/Titania Hybrid Resists via a Sol-Gel Process for Nanoimprint Lithography Chun-Chang Wu and Steve Lien-Chung Hsu* Department of Materials Science & Engineering, Center for Micro/Nano Science and Technology, National Cheng-Kung UniVersity, Tainan, 701-01, Taiwan, Republic of China ReceiVed: August 24, 2009; ReVised Manuscript ReceiVed: December 10, 2009

In this study, the solvent-free thermocurable epoxy/inorganic hybrid resists were prepared for low-pressure and moderate-temperature imprint lithography. Epoxy/silica and epoxy/titania hybrid resists were synthesized via a sol-gel process from a diglycidyl ether of bisphenol A prepolymer with a metal alkoxide precursor and a coupling agent, 3-glycidyloxypropyltrimethoxysilane. The introduction of the coupling agent results in the reinforced interfacial interaction between the epoxy resin and inorganic nanoparticles. Transmission electron microscopy analyses showed that the silica or titania particles were well-dispersed in the epoxy resin matrix on a nanometer scale without the formation of aggregates. The thermooxidative stability and the glass-transition temperature (Tg) of the epoxy/silica nanocomposite showed improvement compared with the pure epoxy resin. When 10 wt % inorganic nanoparticles were added, the etching resistance of the epoxy/silica and epoxy/ titania nanocomposites increased 4 and 3.2 times, respectively, compared with the pure epoxy resin. The epoxy/inorganic hybrid resist can be imprinted to obtain high-density patterns with a resolution of 110-500 nm on a flexible indium tin oxide/poly(ethylene terephthalate) substrate. The shrinkage of the epoxy/silica and epoxy/titania hybrid resist imprinted patterns decreased to 1.5% and 1.3%, respectively. Introduction Nanoimprint lithography (NIL) is a promising technique for generating nano- and microstructures. It does not require sophisticated tools to undergo nanoscale replication and has the advantages of low cost and high throughput in production.1-4 This technique has been proved to be able to produce sub-10 nm structures and large area patterning.5,6 Thus, the NIL technique is regarded as a promising candidate for nextgeneration lithography. However, its progress is closely linked to the availability of suitable resist materials.7-11 Organic-inorganic nanocomposites, with a sub-100 nm resolution and a large throughput, can act as optimum resists for a given lithographic technology.12 They are usually prepared via a sol-gel process. The sol-gel technique has been known as an economical and quick way to prepare organic/inorganic hybrid materials. The low-temperature process and good filmforming characteristics have attracted much interest in recent years. The sol-gel process consists of hydrolysis and condensation reactions. The metal alkoxide system is most frequently used for the reaction precursor.13 Nanocomposite materials prepared by the sol-gel process are usually solvent-based systems due to the typical sol-gel reaction conditions.14,15 In this study, we have prepared solventfree epoxy/silica and epoxy/titania hybrid resists using the metal alkoxide precursor via a sol-gel process in a liquid epoxy system. To avoid phase separation, some papers have reported that a coupling agent could be added to bond the organic moiety and the inorganic phase.16-19 To achieve this objective, 3-glycidyloxypropyltrimethoxysilane (GLYMO) was used as a coupling agent in our epoxy/silica and epoxy/titania hybrid systems. The solvent-free epoxy/silica and epoxy/titania hybrid resists prepared from a sol-gel reaction have many advantages, * To whom correspondence should be addressed. Tel: +886 62757575, ext. 62904. Fax: +886 6 2346290. E-mail: [email protected].

such as fast thermal curing using moderate temperature, good adhesion on various substrates, good thermal properties, good etching resistability, reduced shrinkage, improved dimensional stability, etc. Therefore, the epoxy/silica and epoxy/titania hybrid resists have attractive potential for use in NIL. Experimental Section Materials. An epoxy resin, diglycidyl ether of bisphenol A (DGEBA) resin with the epoxy equivalent mass of 185-192 g mol-1, was obtained from Chang-Chun Plastics Co. (Taiwan). The curing agent poly(oxypropylene) diamine 930 was supplied from Yun-Ten Industrial Co. (Taiwan). The coupling agents 3-glycidyloxypropyltrimethoxysilane (GLYMO) and tetraethyl orthotitanate (Ti(OEt)4) were purchased from Aldrich. Tetraethyl orthosilicate (TEOS, Si(OEt)4) was obtained from Acros. The hydrochloric acid (HCl, 12N) was purchased from Merck. Acetylacetone (acac) was available from Fluka. A commercial poly(ethylene terephthalate) (PET) film with a transparent conductive layer of indium tin oxide (ITO) from Join Well Technology Co. (Taiwan, ROC) was used as the substrate. The thickness of the ITO sputtered PET is 0.2 mm, and the thickness of the ITO is 150 nm. The Young’s modulus is 2900 MPa, and the tensile strength is 65 MPa of the film. Its elongation is 90%. Preparation of Epoxy/Silica and Epoxy/Titania Hybrid Resists. The solvent-free epoxy/silica and epoxy/titania hybrid resists were synthesized via a sol-gel process with a precursor system. The theoretically calculated content of the metal oxides, such as SiO2 or TiO2, in the hybrid resists is 10 wt %. The weight percent was calculated under the assumption that all the metal alkoxides (Si(OEt)4 and Ti(OEt)4) were converted to metal oxides (SiO2 and TiO2). A representative 10 wt % silica loading epoxy/silica nanocomposite was prepared as follows: The DGEBA (20 g), GLYMO (5 g), and TEOS (12.65 g) were completely mixed in

10.1021/jp908141f  2010 American Chemical Society Published on Web 01/14/2010

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SCHEME 1: Preparation Procedures of Glymo/Inorganic Hybrid Materials and Chemical Structures of the Chemicals

a three-necked round-bottom flask fitted with a mechanical stirrer. The molar ratio of TEOS to H2O was fixed to 1:4. The mixture of acid catalyst HCl (0.22 g) and H2O (4.38 g) was added dropwise into the epoxy/silica precursor with vigorous stirring to avoid local inhomogeneities. A representative 10 wt % titania loading epoxy/titania nanocomposite was prepared as follows: The molar ratio of Ti(OEt)4 to acac was fixed to 1:4. The mixture of Ti(OEt)4 (10.4 g) and acac (18.25 g) was added dropwise into the DGEBA (20 g)/GLYMO (5 g) mixture with vigorous stirring to produce a homogeneous solution. The epoxy/silica and epoxy/titania precursor underwent hydrolysis and condensation reactions at room temperature for 12 h. The solutions were kept at 120 °C for an additional 2 h for complete condensation reaction. After these reactions, HCl, H2O, acac, and the byproduct, ethanol, were removed by vacuum heating. Preparation procedures of GLYMO/inorganic hybrid materials and chemical structures of the chemicals are shown in Scheme 1. To cure the epoxy/silica and epoxy/titania precursor, the curing agent 930 was added to the epoxy/silica and epoxy/titania precursor in a stoichiometric ratio of 27 phr (parts per one hundred resin). Characterization. The epoxy/silica and epoxy/titania nanocomposite films for TEM study were microtomed with a diamond knife into 50 nm thick slices. Next, they were placed on a 200-mesh copper grid and examined with an Hitachi HF2000 TEM (equipped with an energy-dispersive X-ray (EDX) spectrometer) using an acceleration voltage of 200 kV. The thermooxidative stability was analyzed with a TA Instrument 2050 thermogravimetric analyzer (TGA) at a heating rate of 10 °C/min under air. The glass transition temperature (Tg) was measured by a TA Instruments differential scanning calorimeter (DSC) 2920 at a heating rate of 10 °C/min. The RIE resistability test of the resists was carried out by using plasma collision in a chamber (OMNI reactive ion etch equipment from Duratek Inc.). Etching was carried out using a power of 100 W. The inlet gas was a mixture of oxygen (40 sccm) and argon (4 sccm) at a pressure of 58 mTorr, and the etching rate was calculated from the depth-profile changes at a constant time interval (every 5 min) by using the Alpha-Step Profilometer (Tencor AlphaStep 500) to measure the residual resist thickness after RIE. The adhesion of the resist to the ITO/PET substrate was evaluated by a Scotch tape peeling test according to ASTM3359B. The imprinted patterns were inspected by the Phillips

XL-40FEG high-resolution field emission scanning electron microscopy (FE-SEM). Results and Discussion Preparation of the Epoxy/Silica Nanocomposite and Epoxy/Titania Nanocomposite Resists. Solvent-free organicinorganic hybrid resists based on DGEBA and metal alkoxide precursors were prepared via a sol-gel process, using 3-glycidyloxypropyltrimethoxysilane (GLYMO) as a coupling agent. The epoxy/silica and epoxy/titania hybrid resists were cured with a curing agent, poly(oxypropylene) diamine 930. The prepolymer components were mixed without adding any solvent, and the mixture was subsequently inserted into a vacuum chamber to remove bubbles trapped in the resists. Our goal was to formulate a liquid mixture of reactive components with a viscosity as low as possible without diluting it with organic solvents. The lower the viscosity of the liquid resist, the lower the pressure is needed for imprinting. However, in this study, we tried to add as much inorganic nanoparticles (silica or titania) as possible to further reduce the shrinkage and improve dimensional stability. It was more difficult to blend the DGEBA/GLYMO mixtures with higher silica or titania contents up to 10 wt % because the viscosity of the entire mixtures increased greatly, making it difficult to stir the mixtures. Although the viscosities of the resists were increased, we still kept the resists in a liquid state with viscosities suitable for nanoimprinting at room temperature. The viscosity of the liquid epoxy resists was 3.5 P. The viscosities of the epoxy/ silica and epoxy/titania resists at 10 wt % were 6 P, which were measured by a Brookfield viscometer with 20 rpm rotational speed at 25 °C. The low resist viscosity is desirable in nanoimprinting because it allows imprinting at low pressure and renders a thin residual layer due to the high flow ability. Morphology of the Epoxy/Silica Nanocomposite and Epoxy/Titania Nanocomposite. The dispersion of the inorganic nanoparticles in the polymer matrix was one critical issue in the successful preparation of hybrid resists. In this work, GLYMO was used as a coupling agent to provide covalent bonding between the organic and inorganic phases, which reinforced the interfacial force of the hybrid material to avoid phase separation. GLYMO is an organofunctional alkoxysilane monomer that can react with the metal alkoxide precursor by sol-gel polymerization of the alkoxy groups. The optical

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Figure 1. Optical microscope images of the epoxy/silica composite without (a) and with (b) the addition of a coupling agent, GLYMO.

microscope image (Figure 1a) shows the epoxy/silica composite material without the coupling agent, GLYMO. It has obvious phase separation occurring between the organic epoxy resin and the inorganic silica. Figure 1b shows the composite material with the coupling agent, GLYMO. The homogeneous and transparent appearance suggests that the inorganic components are well-dispersed in the epoxy matrix. The TEM was used to study the morphology of the organicinorganic hybrid nanocomposites. TEM micrographs of epoxy/silica and epoxy/titania nanocomposites are shown in Figure 2a,b. The gray part is the epoxy resin matrix; the dark parts are the silica or titania nanoparticles that were confirmed by using EDX measurement, as shown in Figure 2c,d. Well-dispersed silica or titania particles are observed in the epoxy resin matrix without the formation of aggregates. The agglomeration could be avoided because of the strong chemical interactions between the organic and inorganic moieties. This is due to the presence of epoxy groups on the surface of inorganic particles through the coupling agent, GLYMO, and gives a better compatibility between the two phases.15 Figure 2b shows a uniform dispersion of spherical-shaped TiO2 particles in the epoxy/titania hybrid film. The average particle size of titania in the 10 wt % titania/epoxy nanocomposites is about 22 nm, as shown in Figure 2e. In the sol-gel process, the catalyst influenced the morphology of the precursor. Under an acid catalyst (HCl) condition, the structure of the epoxy/silica hybrid can exist in an interpenetrating network (IPN).18 Thus, particle size distributions of the epoxy/silica nanocomposite were not calculated. Thermal Properties of Epoxy Resin and Epoxy/Silica and Epoxy/Titania Nanocomposites. The Tg of the epoxy resin and the epoxy/silica and epoxy/titania nanocomposites are listed in Table 1. The neat epoxy resin exhibits a Tg at 90 °C. The hybrid materials have a higher Tg than that of the pure epoxy resin. When 10 wt % of inorganic particles were added, the Tg of epoxy/silica and epoxy/titania nanocomposites increased to 125 and 128 °C, respectively. The significant increase of Tg could be due to the introduction of the inorganic nanoparticles, which limits the movement of polymer chains.18 The thermooxidative stability of the epoxy resin and the epoxy/silica and epoxy/titania nanocomposites was studied using TGA in air. The results are shown in Figure 3. The polymer decomposition temperature (Td) was defined as the temperature with 5% weight loss. The decomposition temperatures of the epoxy resin and the epoxy/silica and epoxy/titania nanocomposites are listed in Table 1. The neat epoxy resin exhibits a Td at 312 °C. When 10 wt % of silica was added, the Td of the epoxy/silica nanocomposite increased by 19 °C. The increase in thermal stability could be attributed to the high thermal stability of silica. However, the introduction of TiO2 causes a decrease in thermal stability of the epoxy/titania hybrid com-

pared with the pure epoxy resin. The dramatic decrease in thermal stability of the epoxy/titania hybrid could probably be attributed to metal-catalyzed oxidative decomposition pathways in the polymer/titania composite.20,21 Although the thermal stability of the epoxy/titania nanocomposite is inferior to that of pure epoxy resin, it is still good enough for nanoimprinting applications. The actual content of the inorganic nanoparticles in the organic-inorganic nanocomposites can be determined from TGA analysis.22 Because most polymers are pyrolyzed before 700 °C in air atmosphere, while inorganic materials usually remain intact over 1000 °C in air atmosphere, the remaining residue should be the actual inorganic content. The residual weight percentages of epoxy/silica and epoxy/titania nanocomposites above 700 °C were approximately proportional to the theoretical silica or titania loadings, as shown in Table 1. This indicates that the conversion of the metal alkoxide precursor to silica or titania was complete during the sol-gel process. Etching Resistability of Epoxy Resin and Epoxy/Silica and Epoxy/Titania Nanocomposites. The good etching resistability of the resist is required during the dry etching pattern-transfer process. The resistability difference between the pure epoxy resin, the epoxy/silica nanocomposite, and epoxy/titania nanocomposite is shown in Figure 4. The etching rates of the pure epoxy resin, epoxy/silica nanocomposite, and epoxy/titania nanocomposite are 100, 25, and 31 nm/min, respectively, according to the Alpha-Step Profilometer measurement. When 10 wt % inorganic nanoparticles were added, the etching resistance of the epoxy/silica and epoxy/titania nanocomposites increased 4 and 3.2 times, respectively, compared with the pure epoxy resin films. The significant improvement of etch resistance from the addition of silica or titania could provide good pattern transfer. Thus, the epoxy/silica and epoxy/titania hybrid resists could be excellent candidates for imprinting resist materials. Thermocuring Imprinting by Using Epoxy Resin and Epoxy/Silica and Epoxy/Titania Hybrid Resists. In this experiment, we used a thermocuring imprint lithography technique. Thus, the solvent-free thermocurable liquid resist was dispensed on the ITO/PET substrate using a syringe. A silicon mold was then pressed into the resist to fill the cavities of the mold at room temperature and then cured at 130 °C for 4 min. The isothermal curing condition was obtained from the isothermal DSC kinetic analysis. Using a higher curing temperature or changing the type of hardener could further reduce the imprinting time. The imprinting pressure was 0.25 kg/cm2. After being cooled to room temperature and separated, the desired patterns had been transferred completely on the resist film. A short cycle time of about 10 min could be achieved, making the resist promising for industrial applications. The shrinkage

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Figure 2. TEM micrographs of the (a) epoxy/silica nanocomposite and (b) epoxy/titania nanocomposite. EDX spectra of the (c) epoxy/silica nanocomposite and (d) epoxy/titania nanocomposite. (e) Particle size distributions of the epoxy/titania nanocomposite.

TABLE 1: Properties of the Pure Epoxy Resin, Epoxy/Silica Nanocomposite, and Epoxy/Titania Nanocomposite code

Tg (°C)a

Td (°C)b

measured (wt %)c

shrinkage (%)

etching rate (nm/min)

epoxy resin epoxy/silica epoxy/titania

90 125 128

312 331 260

0 10.6 11.36

4 1.5 1.3

100 25 31

a Measured by DSC in N2 atmosphere. b Temperature of 5% weight loss, as determined by TGA in air. c The silica or titania content was determined by TGA at 800 °C in air atmosphere. The theoretical content is 10 wt %.

was calculated from the original mold and the transfer patterns. The liquid epoxy resist imprinted patterns have a low shrinkage of 4%. When 10 wt % inorganic nanoparticles were added, the shrinkage of the epoxy/silica and epoxy/titania hybrid resist imprinted patterns decreased to 1.5% and 1.3%, respectively. The shrinkage of the epoxy/silica and epoxy/ titania hybrid imprinted patterns was low compared with the existing materials. Their shrinkages are around 10-20, 5, and 3-6%, respectively.23-25 The significant decrease of shrinkage from the addition of silica or titania could provide good pattern transfer. Lower shrinkage allows faithful pattern replication during the nanoimprinting process. Also, the tape peeling test shows that

the resists have the highest adhesion rating of 5B on the ITO/PET substrate, according to ASTM-3359B. Figure 5a shows the microscale epoxy/silica patterns transferred onto a 4 in. flexible ITO/PET substrate through imprinting lithography. It demonstrates a large area patterning ability. Figure 5b shows the overall area of the high-density equal line/ space patterns with a resolution of 200-500 nm epoxy/silica patterns. Excellent uniformity and smooth pattern profiles were obtained on the imprinted samples. The crack-free nanoline replications without abrupt volume shrinkage have resolutions of 110/50/350 nm (line/space/height), as shown in Figure 5c. The nanopillar replications have resolutions of 285/285/280 nm

Epoxy/Silica and Epoxy/Titania Hybrid Resists

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Figure 3. TGA thermograms of the pure epoxy resin, epoxy/silica nanocomposite, and epoxy/titania nanocomposite in air.

In summary, solvent-free epoxy/silica and epoxy/titania hybrid resists based on DGEBA and a metal alkoxide precursor were successfully prepared via a sol-gel proces, using GLYMO as a coupling agent. The introduction of the coupling agent results in the reinforcing interfacial interaction between epoxy chains and the silica or titania nanoparticles. The addition of inorganic nanoparticles can enhance the thermal properties and etching resistability of the epoxy resin. It can also reduce the shrinkage and improve dimensional stability. The thermocurable epoxy/ inorganic hybrid resists have the potential for use in low-pressure and moderate-temperature nanoimprint lithography. Acknowledgment. The financial support provided by the Ministry of Economic Affairs (Taiwan, ROC) through project 97-EC-17-A-07-S1-0018 is greatly appreciated. The authors are also grateful to the NanoTechnology Research Center (NTRC) and the Industrial Technology Research Institue (ITRI), Hsinchu, Taiwan, for assistance in the nanoscale mold fabrication.

Figure 4. Etching rates of the epoxy resin, epoxy/silica nanocomposite, and epoxy/titania nanocomposite.

References and Notes

Figure 5. (a) The epoxy/silica patterns transferred onto a flexible ITO/ PET substrate through imprinting lithography. (b) The overall area of the nanoscale epoxy/silica patterns. (c) FE-SEM image of 110/50/350 nm (line/space/height) imprinted epoxy/silica patterns. (d) FE-SEM image of 280 nm imprinted nanodot patterns.

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(diameter/space/height), as shown in Figure 5d. The patterns show well-defined edges and are compliant with the mold definition. Without particular precaution, no voids or bubbles were observed on the imprinted samples. The key element for nanoimprint lithography is the resist material. Epoxy systems, such as SU-8 and mr-NIL 6000 resists, were common materials for NIL.26,27 Our epoxy/inorganic hybrid resists were thermocurable resins for low-pressure and moderatetemperature NIL. The thermocurable epoxy/inorganic hybrid resists are different significantly from the SU-8 and mr-NIL 6000 in the solvent aspect. The epoxy/inorganic hybrid resists are easily processed without a solvent. In our case, our solventfree organic-inorganic hybrid resists do not need softbaking to eliminate the solvent. The SU-8 and mr-NIL 6000 resists need to be softbaked to eliminate the solvent after spin-coating. The bubbles were obviously produced in the resist film after coating or during the prebake process. Residual solvent could

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