PFPE-based materials for the fabrication of micro- and nano-optical components
Graphical abstract
Highlights
► Synthesis of UV-curable PFPE–tetraurethane methacrylate prepolymer. ► Fabrication of diffraction gratings, microlens arrays and nanophotonic devices replicas. ► Efficient production of micro and over 100 nm features with high fidelity and resolution.
Introduction
Demands for effective processing and fabricating of micro- and nano-functional devices is addressing investigations in many different fields to develop novel flexible techniques capable of providing very small features. Especially in the case of optics and electronics, the manufacturing of nanostructures is expected to pave the way to the industrial production of a large number of functional elements through reproducible and low cost procedures [1], [2]. Even though advanced lithography techniques (e.g. e-beam writing, X-ray lithography or near-field lithography) provide reproducible nanometer-size structures, the development of scalable nanotechnology methods for the fabrication of optical and optoelectronic devices requires cost-effective procedures, such as those based on soft-lithography, imprint-lithography or replica molding [3].
Apart from the usual requirements of photoresists, relative to resolution, quality and performance, fabrication of optical components also demands a clear and transparent nature of the resist at the wavelength of operation. However first attempts of lithographic fabrication of micro and nano-optical components focused their efforts on materials inherited from the microelectronic industry, which often lacked of appropriate transparency to be used in optical applications, and even newly proposed materials (SU-8, PMMA, etc.) are due to be affected by aging processes that result in color changes and in a more general degradation of their properties and optical performance [4]. Although an increasing number of materials satisfy the transparency requirements, an elastomer, i.e., polydimethylsiloxane (PDMS), has become the material of choice. Fabrication techniques like soft-lithography [3] have contributed to its success but in particular due to a low value of the elastic modulus, replication of structures in the sub-100 nm range is difficult [5], [6]. Moreover, common organic solvents cause swelling of PDMS, as a matter of fact impeding their manipulation in microfluidics or lab-on-a-chip made of silicone [7]. To overcome these limitations some fluorinated polymers and in particular perfluoropolyether-based (PFPE) polymers have been recently proposed as UV-curable polymers for replication processes [8], [9], [10]. In this paper we shortly report the most significant results of a preliminary study on the use of an especially prepared multifunctional urethane methacrylate PFPE as liquid UV-curable material for replica molding. The polymer was chosen due its properties of extreme transparency and chemical resistance to harsh environments, as well as to its low surface energy and hydrophobic/oleophobic character that make easy to demold even from densely packed structures. In particular, the fabrication of different micro and nano-optical components, such as diffraction gratings, microlens arrays, phase plates, waveguides and photonic crystals by replica molding and UV-lithography using this PFPE elastomer is shortly described.
Section snippets
Synthesis and purification of PFPE–tetraurethane methacrylate
A facile one-pot procedure for the preparation of a tetrafunctional urethane methacrylate PFPE (PFPE–TUM) oligomer under mild reaction conditions was developed, based on the methods reported by Bongiovanni et al. and De Simone et al. for bifunctional oligomers [8], [9], [10]. The starting material is a commercial PFPE–tetraol with two alcohol groups at each end of the polymer chain, namely a poly(tetrafluoroethylene oxide-co-difluoromethylene oxide) α,ω-diol bis(2,3-dihydroxypropyl ether)
Results and discussion
Manufacturing of optical components was carried out either by UV-lithography or replica molding [11], [12]. For UV-lithography binary masks were used to fabricate integrated lenses. For replica molding no pressure or temperature was applied and the master molds were fabricated by diverse techniques such as interference lithography, ablation by laser writing and ion beam writing (FIB).
Conclusions
In summary, we have reported examples of fast replication of micro- and nano-optical components using a tetrafunctional PFPE-based UV-curable elastomer, without applying pressure or thermal treatments, demonstrating its efficiency to reproduce micro and over 100 nm features with large aspect ratios, with high fidelity and resolution. On the other hand, the replication of smaller features is still facing distortion and resolution problems, possibly due to a higher than expected viscosity of the
Acknowledgements
The authors thank the financial support by the Xunta de Galicia (PX2010/168-2 and PX2010/152-2 Matera+, Hybrid Organic-based Nanostructured Devices for Applications).
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