Fluorinated silsesquioxane-based photoresist as an ideal high-performance material for ultraviolet nanoimprinting

Abstract

In this study, we developed a new kind of functional photoresist based on octamethacrylated polyhedral oligomeric silsesquioxane (MAPOSS) and fluorinated monomer as an ideal material for ultraviolet nanoimprint lithography (UV-NIL). We first optimized the synthesis of MAPOSS using the hydrolysis and condensation reactions of methacryloyl oxygen propyl trimethoxysilane. The hybrid photoresist formulations with MAPOSS and fluorinated additive were found to be effective materials for high-performance UV-NIL, which exhibited a preferable curing rate, Young's modulus and thermal stability. Additionally, the low shrinkage and low surface energy of the curing film allows for easier transfer of relief features with excellent imprint reliability for UV-based NIL techniques. These characteristics of fluorinated silsesquioxane-based photoresists make them suitable as inexpensive and convenient components in UV-NIL processes.

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1. Introduction

Nanoimprint lithography (NIL) has attracted enormous attention in a wide range of areas, including high-density storage, organic thin-film transistors, photonic crystals, bio-microfluidics, solar cells, sensors and high-precision printed circuit boards. Compared with conventional nanofabrication methods, NIL technology has many advantages such as reasonable cost, high resolution and an unsurpassed micro-replication.^1–12 Among the different types of NIL technologies, ultraviolet NIL (UV-NIL) is particularly suitable for high-resolution, high-throughput, and low-cost nanofabrication processes.^13,14

An ideal UV imprintable resist should have excellent properties, such as low viscosity, fast photopolymerization with minimal shrinkage, and high thermal stability.¹⁵ However, a concern for UV nanoimprint lithography is the formation of defects caused by adhesion and friction during the separation process, which can cause deformation and breakdown of the transferred patterns on the polymer.¹⁶ There are two main interfaces in the NIL process: one is the interface between the mold and the polymer film and the other is the interface between the polymer film and the substrate. So long as the surface energies of the materials – including the photoresist, mold and substrate – are appropriately controlled, the defects from strong adhesion at the interfaces can be effectively reduced.¹⁷ Therefore, there is a need to design new chemical species that can effectively address the above two points in a hybrid photoresist system.

Recently, many functional polyhedral oligomeric silsesquioxane (POSS) derivatives have been applied as photoresists or replica molds in UV-NIL. These hybrid materials, formed by the combination of inorganic and organic components, are interesting substances for their potential to yield increased performances, such as elevated sensitivity, optical transparency, high density, high modulus and low dielectric constant.^18–27 Guo and colleagues¹⁹ developed a UV-curable formulation based on epoxysilicone and epoxydimethylsiloxanes to overcome some of the shortcomings of the conventional photoresists, such as a required inert atmosphere for its processing due to oxygen inhibition, high adhesion to the mold, and so on. Willson and colleagues introduced a bi-functional POSS with photosensitive acrylate and thermally curable benzoyl (BCB) groups to obtain ideal imprintable characteristics for UV-NIL.²⁰ They also reported POSS azide and POSS thiol for imprintable dielectric materials in UV-NIL.²¹ Based on POSS thiol, Yin and colleagues focused on the click reaction to get a high-resolution nanopattern in UV-NIL.^22,23 Meanwhile, Lee and colleagues used methacrylate multi-functionalized silsesquioxane (SSQMA) as the key material to prepare a nonstick replica mold with versatile functionality for mold-based, high-resolution nanolithography.^24,25 Although such kinds of material present high reliability for patterning nanostructures, they still face some challenges, such as compatibility, stability, and high cost.

In this study, we aim to present a new kind of fluorinated silsesquioxane-based photoresist with various desirable characteristics for UV-NIL by incorporating a fluorinated monomer of 1H,1H,7H-dodecafluoroheptyl methacrylate (FA) into octamethacrylated POSS (MA-POSS). Owing to its easy availability, low viscosity and low surface energy compared with other types of reactive monomer/oligomer materials, fluorinated surfactant is identified as a beneficial additive to optimize the overall performance of hybrid photoresists. Here we focus on the effect of the photoresist's components including MA-POSS and FA on the curing rate, surface energy, viscosity, thermal stability and mechanical performance. With the optimized formulations for UV-NIL resists, a high-resolution pattern could be obtained with its special range of dimensions from dozens of nanometers to hundreds of nanometers. These hybrid photoresist systems with high levels of performance are believed to have a great potential for inexpensive and convenient components in UV-NIL processes.

2. Experimental

2.1 Materials

3-Methacryloxy propyl trimethoxy silane (KH570) was obtained from Anhui silicon Baoxiangfei Organic Silicon New Material Co., Ltd., 1H,1H,7H-dodecafluoroheptyl methacrylate (FA) was purchased from Harbin Xuejia Chemical Co., Ltd, and 1,6-hexamethylene diacrylate (HDDA) was provided by Sartomer Co., Ltd. The photoinitiator Darocure 1173 was obtained from Tronly (Changzhou, China) Co., Ltd. Isopropanol (IPA) and a 25% aqueous solution of tetramethylammonium hydroxide pentahydrate (TMAH) was purchased from Beijing Chemical Reagent Company.

2.2 Synthesis of MAPOSS

Synthesis of MA-POSS was similar to that previously reported by H. Saito,²⁸ but the preparation process was further optimized in this study. For 30 min with slow stirring, 40 mL of isopropanol solution including KH570 (38.07 g) was slowly dropped into 120 mL of 25% aqueous isopropanol solution including 2.0 g TMAH. Then the mixture was stirred at room temperature for 2 h. The solvent was removed through vacuum filtration to obtain a colorless viscous liquid. To achieve a neutral solution, the crude product solution was dissolved in toluene and then washed with a saturated NaCl solution. The toluene phase was dried with anhydrous MgSO₄ and concentrated to obtain a gummy liquid (20.64 g). The hydrolytic recondensation of the resulting product with 10% aqueous solution of TMAH (2.0 g) in toluene (100 mL) was carried out for 6 h at 130 °C. After a series of processes including concentration, washing and drying, the functionalized viscous silsesquioxane (18.07 g) was obtained, which could be dissolved in most polar organic solvents, in acrylate and methacrylate monomers, and in aromatic and aliphatic resins.

2.3 Nanoimprinting

A Nanonex 2000 imprinting tool (Monmouth Junction, NJ, USA) with vacuum capability and wavelength for UV curing at 365 nm and a light curing system (ELC-430) from Electro-Lite Corporation (Bethel, CT, USA) were used for UV-NIL. The imprinting pressure was typically 300 psi. The silica substrate was washed with a 1 : 1 weight ratio of H₂SO₄/H₂O₂ for 15 min and then treated with O₂ plasma. The selected photoresist was dispensed as droplets onto the modified substrates, and then coated as the transfer film at 1000 rpm for 40 s and 3000 rpm for 20 s. After baking at 100 °C for 2 minutes, the thickness of the obtained film was measured to be about 600 nm. Then the imprinting process was performed under UV light for 40 s at room temperature. Finally, SiO₂ or the polyethylene terephthalate (PET) mold was removed without further treatment. The detailed nanoimprinting process is shown in Fig. 1.

Fig. 1 Schematic illustration of the UV-NIL process and the corresponding photoresist materials.

2.4 Measurements

¹H NMR and ²⁹Si NMR spectra were recorded using an AV-600 NMR spectrometer (600 MHz) with CDCl₃ as the solvent. Chemical shifts are shown in ppm from TMS with the residual protonated solvent as an internal standard (CDCl₃, 1H 7.24 ppm). FT-IR spectra were recorded on a PerkinElmer Paragon 1000. Gel permeation chromatography (GPC) measurements in THF were performed using a Waters 515-241 GPC system. The GPC was calibrated with monodisperse poly-(methyl methacrylate) (PMMA) standards with PDIs < 1.04. X-ray diffraction (XRD) patterns of the fabrics were recorded with a D/max-2500 PC X-ray diffractometer (Rigaku Corporation, Japan) under the operating conditions: Cu–K target at 40 kV 200 mA, k = 1.541 Å. Scanning electron microscopy (SEM) (Hitachi SU8000) and atomic force microscopy (AFM) (Veeco NanoMan) were used to characterize the surface morphologies of the nanopatterns produced by UV nanoimprinting on the glass and PET substrates.

The viscosities of the formulations were measured with an Anton Paar Physica MCR 301 (Anton Paar GmbH, Austria) using rotational speeds from 5 rpm to 100 rpm at 25 °C. The kinetic parameters of the photopolymerization were determined by real-time Fourier-transform infrared (FTIR) spectroscopy using a Thermo-Nicolet 5700 instrument. The sample was exposed to the UV beam to induce the polymerization for IR beam analysis. After the methacrylic group conversion, the decrease in the absorbance of the band centered at 6130 cm⁻¹ was monitored with the glass background. Based on the standard testing method ASTM D2765-84, the gel content was determined on the cured films by measuring the weight loss after a 24 hour extraction with chloroform at room temperature. According to ISO 3521, a pycnometer was used to measure the density (ρ) of resists before and after curing. The bulk volumetric shrinkage was calculated by the formula:

Thermogravimetric analysis (TGA) was performed on a TGAQ50 at a heating rate of 10 °C min under N₂. Young's moduli and hardness of coated resins were measured at room temperature with a commercial nanoindentation system (Nanoindenter XP; MTS Nano Instruments, Oak Ridge, TN). Contact angles between the surfaces and selected liquids provided the information for calculating surface energies. With distilled and deionized water as the reference liquid, contact angles were determined using a Dataphysics OCA 20 contact angle meter. The static contact angles of deionized water, diiodomethane, and glycol on the imprinted films were measured at ambient temperature. Static and sessile drops (10 μL) were delivered from a micrometer syringe with a minimum division of 2 μL. About 5 independent measurements were carried out and their average contact angle was recorded. The surface energy of the low-surface-energy materials based on the measured contact angles was calculated according to the van Oss–Good–Chaudhury (VOGC) equation of the Lewis acid–base interaction model:

Here, γ^LW is the Lifshitz van der Waals component; γ⁺ is the polar electron-acceptor (Lewis acid) component; and γ⁻ is the polar electron-donor (Lewis base) component. To get the three unknown values (i.e., γ^LW, γ⁺ and γ⁻), three liquids of known components can be used in the process of measuring contact angles, which will result in three equations that can then be solved by the built-in command in MATLAB. The three common liquids used for the contact angle measurements and their surface free energy parameters are shown in ESI (Table S1†).

3. Results and discussions

3.1 Preparation and characterization of hybrid photoresists

Herein, MAPOSS was synthesized from a commercial organotrialkoxysilane with methacrylate groups by three steps. First, as shown in Fig. 2a, the hydrolysis of alkoxy groups and the partial condensation of the resulting SiOH groups generated a functional group containing hydrolysis products with the aid of an alkaline catalyst; and then the functionalized silsesquioxane was obtained from the hydrolytic recondensation with the alkaline catalyst. As shown in Fig. 3a, a strong peak corresponding to Si–O–Si at 1099 cm⁻¹ was observed — although neither the Si–OCH₃ peak at 1162 cm⁻¹ nor the Si–CH₂ peak at 1080 cm⁻¹ was observed — indicating that the silicon cage hydrolysis reaction was completed. The C=C peak at 1640 cm⁻¹ and the C=O peak at 1718 cm⁻¹ showed little change of the reactant KH570, meaning that the functional methacrylate side chain was not involved in the reaction. It could be seen from ¹H NMR spectra in Fig. 3b that each peak can be assigned to a corresponding proton of methacrylated-POSS. The relative number of protons in methacrylated-POSS remained the same as in the reagent. The ²⁹Si NMR spectrum shows the T³ signal of the cage structure where the split was between 65 and 75 ppm (Fig. S1†). Other characterization data are also provided in ESI (Fig. S2 and S3†). All of these results demonstrate that the MAPOSS was successfully synthesized.

Fig. 2 Synthesis of MAPOSS and chemical structure of each component of the hybrid UV-NIL resist.

Fig. 3 (a) FT-IR spectra of the MAPOSS and the KH570 and (b) ¹H NMR spectra of MAPOSS in CDCl₃.

For UV-NIL, the photoresists should have the properties of low viscosity, high curing efficiency and low surface energy.^3,6 The structures of the different components in our hybrid photoresist are shown in Fig. 2b. As one of the main ingredients of the hybrid photoresists, 1H,1H,7H-dodecafluorinatedheptyl methacrylate (FA) is a reactive fluorinated monomeric diluent that reduces viscosity. More importantly, it is an ideal agent for decreasing surface energy, which was useful in demolding as mentioned in a previous study.¹⁷ 1,6-hexamethylene diacrylate (HDDA) was selected as the crosslinker because of its elevated crosslinking density and high flexibility. The MAPOSS was adopted as the oligomer to provide accelerated curing rates and good mechanical properties as mentioned above. Darocure 1173 was used as a photoinitiator for radical photopolymerization in all the photopolymer films.

3.2 Viscosity and Surface properties of hybrid photoresists

In order to optimize the photoresist components, a series of hybrid resists were prepared by changing the mass ratio between MAPOSS and FA, as shown in Table 1. The prepared resists consist of the transparent and homogeneous oil with good compatibility and stability. As for the UV-curable resist, low viscosity plays a decisive role for high efficiency and accuracy of UV-NIL. We found that by incorporating the viscosity-decreasing acrylic reactive diluents, the viscosities of the resins were drastically reduced from 1700 cP to values between 5.5 and 60.1 cP. Additionally, the viscosity decreased as the content of MAPOSS was decreased as listed in Table 1, indicating that MAPOSS content had a direct effect on the viscosity.

Table 1 The components and some physical properties of the hybrid resists. The photoinitiator 1173 makes up 4 wt% and the crosslinker makes up 30 wt% of the weight of the whole resist

Resist	F content (wt%)	Mole ratio of MAPOSS/FA	Viscosity (cP)	Surface energy (mJ m^−2)	Shrinkage (%)	Hard (MPa)	Young's modulus (GPa)
FMH1	10	0.77	60.11	34.8	2.80	220	3.13
FMH2	20	0.25	27.16	32.1	5.41	197	2.96
FMH3	30	0.07	16.94	22.9	6.14	156	2.85
FMH4	35	0.02	5.86	20.1	9.12	142	2.84

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All photopolymerization films were prepared according to the following procedure. The UV-curable resists were spin-coated onto silica substrates after a series of surface treatments, as mentioned in the experimental section. The thin films were then cured by exposure to 365 nm UV light at an intensity of 10 mW cm⁻² for an appropriate amount of time under a nitrogen atmosphere. Due to van der Waals forces, polymerization shrinkage of the photoresists occurred upon light curing: the distance between the monomers changed by up to the bond lengths of the covalent bonds that form the resultant polymers. As listed in Table 1, the bulk shrinkage was markedly reduced by adding MAPOSS into the polymer – the percentage of MAPOSS in the resulting resist was between 2.8% and 9.1%, which was much lower than that of the conventional methacrylate resists (∼11%). This reduction in shrinkage may be attributed to the rigidity and steric hindrance of the functional groups. Bulky MAPOSS molecules, each having eight methacrylate arms, could not approach each other freely and the crosslinked hybrid provides an anchor point within the polymer matrix.

Additionally, contact angles between the surfaces and three selected liquids provide the information about surface energies according to the VOGC Equation. As summarized in Table 1, surface energy clearly decreased with the increasing content of fluorinated monomer (FA), which implies that the addition of FA can lower the surface energy of the cured hybrid resist. When the mass content was above 35%, the cured film showed desirable surface and interface properties for easier release. Based on the above analysis, we think our hybrid resists have the potential to be used as photoresists for UV-NIL because of their low viscosity, low shrinkage and low surface energy.

3.3 Photopolymerization kinetics of hybrid photoresists

Kinetic curing profiles of UV photoresists were investigated by real-time FTIR spectroscopy. It was found that the FTIR traces of the induction and conversion periods of the acrylic double bond were greatly affected by the types of oligomer, photo-initiators and acrylic monomers as well as the intensity and spectral range of the irradiance. To simulate the imprint scene, glass was used as the background. The dose was launched at 5.96 s to make a comparison between the fresh and cured resins. FTIR spectra of –C=C– exposed for various reaction times are shown in Fig. 4, in which the peaks at 6130 cm⁻¹ are attributed to –C=C–. The increasing content of FA in the photocurable formulation induced a slight decrease of the photopolymerization rate. Nevertheless, when the curing was completed, the absorption peak intensity was basically unchanged. The curing rates and conversion are closely related to the MAPOSS content in weight. The addition of the MAPOSS can accelerate the reaction rate, and increase the curing efficiency. However, too much of –C=C– is not conducive to improving the conversion rate. The resist with a lower molar ratio of MAPOSS/FA can reach a 93% conversion rate.

Fig. 4 Conversion and curing rate (differential curve) of various fluorinated silsesquioxane-based photoresists.

3.4 Mechanical properties and thermal stability of hybrid photoresists

Evaluations of the mechanical integrity and transfer efficiency of the resist are key indicators of the resist's performance for NIL.⁶ Rigid materials with a tensile modulus above 100 MPa are appropriate for reproducible replication, with little collapse at the 50 nm scale. The moduli and hardness of the fully cured formulations were investigated using the nanoindentation technique. Fig. 5 shows that the Young's modulus and hardness of the cured resists are functions of the contact depth. The mechanical properties were observed to be uniform and stable between 30 and 400 nm. The Young's modulus of the curing formulation increased with the content of MAPOSS. The results show that the MAPOSS can enhance the mechanical property efficiently. The modulus of the sample with 13.4 wt% MAPOSS is 2.85 GPa, which is rigid enough for building the nanostructured patterns to resist failures for NIL.

Fig. 5 The hardness (a) and Young's modulus (b) as a function of the contact depth of various fluorinated silsesquioxane-based photoresists.

Thermal gravimetric analysis in Fig. 6 shows that the hybrid resists exhibit better thermal stability than does the pure organic polymer. The resists with higher MAPOSS content exhibited higher thermal weight-loss temperature at 5% mass loss, which is due to the MAPOSS being conducive to increasing the crosslinking density of the organic matrix during the photopolymerization. These results also suggest a homogeneous phase of the cured polymer and that the cage structure of MAPOSS can efficiently enhance the heat resistance.

Fig. 6 TGA curves of the fluorinated silsesquioxane-based photoresist.

3.5 Nanopatterns of UV nanoimprinting

To verify the applicability of the fluorinated silsesquioxane-based photoresist in the UV-NIL process, patterning of a photoresist with a MAPOSS/FA ratio of 0.25 was performed on glass substrates, as shown in Fig. 1. Two hard SiO₂ molds with feature sizes of several hundreds of nanometers were used to verify the ability of this MAPOSS/FA = 0.25 photoresist to form nanostructures. As shown in the SEM images of Fig. 7a and c, this photoresist, which is of low viscosity, was successfully transferred from the SiO₂ master mold to the nanostructures using UV irradiation at 25 °C and low pressure over a relatively short period of time (40 s). Most importantly, almost no signs of defects were observed according to the magnified images in Fig. 7b and d. These results demonstrate good coating properties and low surface energy of the hybrid resists that allow them to be easily released from the molds and obtain high-quality patterns during the demolding process.

Fig. 7 (a) and (c): SEM images of the pattern imprinted by the fluorinated silsesquioxane-based photoresist on the glass substrate; (b) and (d): the corresponding high-magnification SEM images.

Additionally, we evaluated the patterning of fluorinated silsesquioxane-based photoresists on a flexible PET substrate. Fig. 8a shows the real images of the patterns on PET, and the corresponding SEM and AFM images of the imprinted grating with 140 nm line widths are shown in Fig. 8b–d. It can be clearly seen that a fluorinated silsesquioxane-based photoresist can be easily patterned on top of a flexible substrate with high quality. Additionally, even when a mold with a higher resolution was used, the imprinted nanopattern was also obtained with good features as shown in Fig. 8e and f. Here the patterning area is just limited by the availability of the mold and the dimensions of the curing light sources. In fact, we think this kind of hybrid photoresist is also suitable for continuous roll-to-roll nanoimprinting. We also note that the patterns obtained from the fluorinated silsesquioxane-based photoresist on the PET substrate can be used as a flexible and easily replicable mold to replace the original SiO₂ or Si mold.¹⁹

Fig. 8 (a) Fluorinated silsesquioxane-based photoresist patterned on a flexible PET substrate; (b) SEM image of the imprinted grating with 140 nm line width; (c) and (d): corresponding AFM images of the patterns with 140 nm line width; (e) and (f) SEM images of dense approximately 50 nm-sized rod structures patterned on a PET substrate.

4. Conclusions

To summarize, a new kind of fluorinated silsesquioxane-based photoresist has been developed for ultraviolet nanoimprint lithography (UV-NIL). A facile route for synthesizing octamethacrylated polyhedral oligomeric silsesquioxane (MAPOSS) was first developed. Then the hybrid photoresists composed of MAPOSS, fluorinated monomer and crosslinker were fabricated, and they exhibited desirable characteristics for UV-NIL application, such as high curing rate, low surface energy, low viscosity (5.9–60.1 cP) and low bulk volumetric shrinkage (6.14%), together with increased thermal stability (342–352 °C) and high Young's modulus (2.84–3.13 GPa). With the optimized formulations for UV-NIL resists, a high-resolution pattern could be achieved with its special dimensions in the range of dozens of nanometers to several nanometers, on both glass and flexible substrates. This work shows the potential to use simple but multifunctional hybrid components of photoresists for future UV-NIL applications.

Acknowledgements

This work was supported by Beijing Municipal Natural Science Foundation (Grant no. 2102035), National Natural Science Foundation of China (Grant no. 51173013 and 51373013) and Beijing Young Talents Plan (YETP0489). We are grateful to Prof. L. Jay Guo, Dr Tao Ling and Mr Cheng Zhang of Department of Electrical Engineering and Computer Science of University of Michigan for their help of nanoimprinting experiment and related discussion.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06022a

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