Fabrication of a roll imprint stamp using zirconia for the UV roll imprinting process

Abstract

In the present work, we have developed a new method for fabricating a roll imprint stamp containing three dimensional micro- and nanosized patterns for the ultraviolet (UV) roll imprinting process, using a flexible poly-dimethylsiloxane (PDMS) mold and a UV-curable zirconium oxide (ZrO₂) nanoparticle (NP) dispersion resin. By employing the UV-curing imprinting process, the micro- and nanopatterns on the PDMS mold were successfully transferred to the ZrO₂ NP dispersion resin film, attached to the surface of the roll. The obtained ZrO₂ roll imprint stamp has two main advantages: a high hardness after annealing at 600 °C and a good anti-sticking surface after the surface treatment carried out using a hydrophobic self-assembled monolayer (SAM). As a result, the ZrO₂ roll imprint stamp effectively transferred its micro- and nanopatterns to a large area of the flexible polymer substrate, maintaining a high quality throughout the whole UV roll imprinting process.

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1. Introduction

Roll nanoimprint lithography (RNIL) has recently attracted a great deal of attention as an advanced technology for fabricating micro- and nanoscale patterns, due to its low cost, continuous process, high throughput, and capability of providing high resolution nanopatterning over a large area.^1,2 Thus, RNIL has been developed in a variety of modifications and employed in numerous applications in the fields of semiconductors and flexible electronic devices such as organic light-emitting diode (OLED),³ organic photovoltaic (OPV),⁴ and organic thin film transistors (OTFTs).^5,6 In the RNIL process, the fabrication of high quality micro- and nanopatterns over a large area without defects is crucially dependent on the roll imprint stamp, and thus, extensive research efforts have been devoted to the investigation of a suitable material and of a method for optimizing the roll imprint stamp.^7–9 The ideal material for a good roll imprint stamp must be flexible enough to be installed on a curved surface yet hard enough to enable the imprinting of the micro- and nanopatterns on a polymer resist. Furthermore, roll imprint stamp must have good wear resistance and anti-sticking surface properties to imprint over a large number of polymer resists in repetition without any defects and damage of roll imprint stamp. Fabricating the roll imprint stamp is thus a difficult task, because the micro- and nanopatterns must be constructed on the roller surface paying care on fulfilling all the requirements mentioned above.

Thin foils of metals such as nickel^10–12 and aluminum¹³ were used for fabricating roll imprint stamps, due to their flexibility and sufficient mechanical strength. However, manufacturing micro- and nanopatterns on thin metal foils requires an expensive equipment and the use of complex processes, thus implying high costs. Flexible polymers like poly-dimethylsiloxane (PDMS)⁵ were considered as alternative materials due to the ease of fabrication of the roll imprint stamp compared to the one obtained using thin metal foils. However, the low mechanical strength of PDMS limits the size and aspect ratio of the patterns to imprint. Recently, ZrO₂ has emerged as an important engineering ceramic material due to its high hardness, excellent wear resistance, and good chemical and thermal stability, properties all required for the roll imprint stamp used in the RNIL process.¹⁴ In the present study, we propose a simple and efficient method for fabricating the roll imprint stamp with ZrO₂. By employing a UV-curing imprinting process, ZrO₂ micro- and nanopatterns were formed directly on a curved surface using a flexible PDMS mold and a ZrO₂ nanoparticle (NP) dispersion resin as an imprint resin. This simple method can effectively form a variety of micro- and nanosized patterns on a curved surface; further, the fabricated ZrO₂ roll imprint stamp is characterized by a high hardness and a good anti-sticking surface. Finally, we show that the ZrO₂ roll imprint stamp can transfer high quality micro- and nanopatterns to a large area of the flexible polymer substrate through the UV roll imprinting process.

2. Experimental

2.1 Preparation of ZrO₂ NP dispersion resin

Table 1 shows the composition of the ZrO₂ NP dispersion resin used for the fabrication of the roll imprint stamp. The ZrO₂ NP dispersion resin was prepared by dispersing ZrO₂ NP with an average diameter of 3 nm in a mixture of isopropyl alcohol (IPA), methanol (Ditto Technology), and dipentaerythritolhexaacrylate (DPHA) monomer to increase the viscosity of the solution. In a second time, the free-radical photoinitiator, 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184), was added to the mixture to enable the initiation of the UV-led photopolymerization process.¹⁵

Table 1 Composition of the ZrO₂ NP dispersion resin

Components	Weight
ZrO2 nanoparticles	10%
Monomer (DPHA)	5%
Intrinsic solvent (methanol + IPA)	80%
UV initiator (Irgacure 184)	5%

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2.2 Fabrication of the roll imprint stamp

Fig. 1 shows a schematic diagram illustrating the fabrication process of the roll imprint stamp. Prior to fabricating the PDMS mold, a silicon (Si) master stamp containing the micro- and nanopatterns was prepared by deep ultraviolet (DUV) lithography and reactive ion etching process (Fig. 1a). Then, the Si master stamp was coated with a hydrophobic SAM by dipping it into a heptadecafluoro-1,1,2,2-tetra-hydrodecyl trichlorosilane (HDFS) solution diluted in 0.1 wt% n-hexane and stirred for 10 minutes, in order to facilitate the following smooth detachment of the PDMS mold (Fig. 1b).¹⁶ PDMS resin obtained by mixing Sylgard 184 with its curing agent in a volume ratio of 10 : 1 was poured onto the Si master stamp covered with the hydrophobic SAM. After the PDMS was degassed in vacuum and cured at 80 °C for 2 h, the PDMS mold featuring the inverted micro- and nanopatterns was detached from the Si master stamp as shown in Fig. 1c. The prepared ZrO₂ NP dispersion resin was deposited onto the PDMS mold by spin coating at 3000 rpm for 30 seconds, as depicted in Fig. 1d, and then transferred to the surface of a stainless steel roll (Fig. 1e). Upon exposure to UV light, the photoinitiator present in the ZrO₂ NP dispersion resin initiates the photopolymerization, providing links between the polymer and the inorganic matrix.¹⁷ After the PDMS mold was peeled off, the roll imprint stamp with the ZrO₂ micro- and nanopatterns was annealed at 600 °C for 2 h, in order to increase the hardness of the ZrO₂ layer and the adhesion between the ZrO₂ and the stainless steel roll (Fig. 1f). The annealing temperature was determined on the basis of the results of the hardness measurements shown in Fig. 3a. After the annealing process, a HDFS-SAM monolayer was applied on the surface of the roll imprint stamp as an anti-sticking layer. Finally, the roll imprint stamp was rinsed with hexane and deionized water three times to remove the SAM layer and was dried under a nitrogen flow.

Fig. 1 Fabrication process of the roll imprint stamp.

2.3 Characterization

The hardness of the ZrO₂ roll imprint stamp was measured using a nanoindenter (MTS Systems, MTS-XP). X-ray diffraction (XRD) patterns of the ZrO₂ micro- and nanopatterns were obtained using a Rigaku D/MAX-2500V diffractometer with Cu Kα radiation and a scanning rate of 2 deg min⁻¹. Field emission scanning electron microscopy (FE-SEM) images of the ZrO₂ micro- and nanopatterns on the roll imprint stamp were acquired using a Hitachi S-4300 microscope. A contact angle measurement system (Phoenix Plus 300, SEO Co. Ltd.) was employed to measure the contact angle of water on the ZrO₂ roll imprint stamp surface.

3. Results and discussion

Fig. 2a shows a photograph of the fabricated roll imprint stamp with a diameter of 5 cm and a length of 10 cm, whereas Fig. 2b–e are the FE-SEM images of the different ZrO₂ micro- and nanopatterns formed on its surface. High quality inverted micro- and nanopatterns could be formed directly on the curved surface of the roll using the PDMS mold, due to the flexibility of this material. Careful examination of the FE-SEM images reveals that the integrity of the ZrO₂ micro- and nanopatterns was retained even after the annealing process at 600 °C, with no cracks appearing on its surface.

Fig. 2 (a) Photograph of a ZrO₂ roll imprint stamp. (b)–(e) FE-SEM cross-sectional image of micro- and nanopatterns on the ZrO₂ roll imprint stamp; the insets show the relative top views. (b) Micro-convex, (c) high aspect-ratio, (d) moth-eye pattern, and (e) nanopillars (scale bar: 1 μm).

The hardness of the ZrO₂ roll imprint stamp measured as a function of the annealing temperature is shown in Fig. 3a. The annealing time was 2 h for all the stamps. As evident in Fig. 3a, the hardness of the ZrO₂ roll imprint stamp was one order of magnitude higher than that of the PDMS polymer (∼40 MPa), even before annealing.¹⁸ The hardness increased up to 3.81 GPa when the annealing temperature increased to 500 °C; above 500 °C, it reached a relatively constant value. The increase of the hardness with the annealing temperature can be attributed to the crystallization of ZrO₂,¹⁹ as shown by the XRD results in Fig. 3b. The XRD pattern of the ZrO₂ roll imprint stamp before annealing exhibits a broad hump between 15° and 35°, indicating that ZrO₂ is in the amorphous phase. Increasing the annealing temperature resulted in an increased crystallinity of ZrO₂. The formation of diffraction peaks corresponding to ZrO₂ cubic/tetragonal phase can be observed in the XRD pattern relative to the ZrO₂ roll imprint stamp annealed at 600 °C.

Fig. 3 (a) Hardness of the ZrO₂ roll imprint stamp measured at different annealing temperatures, and (b) XRD analyses before and after annealing at 600 °C.

The anti-sticking surface of the ZrO₂ roll imprint stamp is important to avoid defects during demolding process in UV roll imprinting process. Fig. 4 shows the pictures of a deionized water droplet placed on the surface of nanosized ZrO₂ line pattern. Before the anti-sticking treatment was applied on its surface, the contact angle was 85°, indicating that the surface was basically hydrophilic. However, the anti-sticking surface treatment obtained applying a hydrophobic SAM significantly increased the surface hydrophobicity, giving a contact angle of 143°, as shown in Fig. 4b.

Fig. 4 Contact angle of water on the ZrO₂ roll imprint stamp surface (a) without and (b) with anti-sticking surface treatment.

A photocurable perfluoropolyester-based (PFPE) material was used as a resist to carry out a UV roll imprinting process using the fabricated ZrO₂ roll imprint stamp.²⁰ A schematic diagram of the UV roll imprinting process is shown in Fig. 5a. The PFPE resin was dispensed on the transparent polycarbonate (PC) substrate and cured by UV light, which was transmitted through the PC substrate right after the imprinting process. The wavelength of UV light was 365 nm. Fig. 5c shows the FE-SEM image of a nanosized line pattern with 600 nm pitch and 250 nm line width on the ZrO₂ roll imprint stamp, whereas Fig. 5d shows the UV roll imprinted PFPE resin pattern on the PC substrate. Fig. 5c and d confirm that the inverse of the pattern originally formed on the roll imprint stamp was successfully transferred to the PC substrate without the occurrence of any defect by using the UV roll imprinting process and the fabricated ZrO₂ roll imprint stamp. The ZrO₂ roll imprint stamp enabled the fabrication of a scalable patterned flexible polymer film having a large area (88 cm²) employing the UV roll imprinting process schematized in Fig. 5b.

Fig. 5 (a) Schematic diagram of the UV roll imprinting process. (b) Photograph of a PC substrate with a UV roll imprinted PFPE resin pattern. FE-SEM image of (c) a nanosized line pattern on ZrO₂ roll imprint stamp and (d) a UV roll imprinted PFPE resin pattern on PC substrate.

4. Conclusions

We fabricated a ZrO₂ roll imprint stamp containing three dimensional micro- and nanopatterns for the UV roll imprinting process using a flexible PDMS mold and a ZrO₂ NP dispersion resin. By employing a UV-curing imprinting process, the ZrO₂ micro- and nanopatterns were transferred directly on the surface of the stainless steel roll. The fabricated ZrO₂ roll imprint stamp presents a high hardness and good anti-sticking properties, confirming that ZrO₂ is a suitable material for roll imprint stamps used in the UV roll imprinting process. The obtained ZrO₂ roll imprint stamp was employed to transfer a nanosized pattern to a large area of the flexible polymer substrate without the occurrence of any defect by using the UV roll imprinting process.

Acknowledgements

This research was supported by the Pioneer Research Center Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF-2013M3C1A3063597). Also this research was conducted by the R&D program for Industrial Core Technology through the Korea Evaluation Institute of Industrial Technology supported by the Ministry of Knowledge Economy in Korea (Grant no. 10040225).

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Footnote

† These two authors contribute equally to this work.

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