Fabrication of Nano-precision PDMS Replica Using Two-photon Photopolymerization and Vacuum Pressure Difference Technique

Abstract

It is undoubtingly believed that a revolution in miniaturization, in the field of microelectromechanical systems (MEMS), nano- and biotechnology etc., is under way. Multidimensional micro structures are fabricated by mainly two methods, such as laser rapid prototyping and lithography. Since it is a process that builds features layer by layer, rapid prototyping is a very slow process and is difficult to fabricate smaller objects. 1 On the other hand, the conventional lithographic technique requires an etching process that limits to build complicated figures. Recently, two-photon photopolymerization has attracted increasing attention as an innovative tool for micro- and nanofabrication due to its intrinsic multi-dimensional fabrication capability and subdiffraction limited spatial resolution. 2,3 Two-photon absorption probability strongly depends quadratically on excitation beam intensity confining the process at the vicinity of the focus. Thus, by using two-photon excitation one can activate chemical and physical processes only for a tightly focused spot in the bulk medium with high spatial resolution. This substantial increase in structural complexity was only possible by using chromophores with high two-photon absorption cross-sections (σ ). It is apparent that the synthesis of chromophores with even larger two-photon activity will unleash improvements in the capability and performance of a variety of two-photon absorption application including multi-dimensional nano- and microfabrication. In this paper, we describe the synthesis of newly designed highly efficient two-photon chromophore and fabrication of three-dimensional nano-precision PDMS replica using vacuum pressure difference technique (VPDT) and twophoton photopolymerization. The two-photon absorbing chromophore 3 with long alkyl chain for improved solubility was synthesized using palladium catalyzed coupling reaction designed by Heck 4 from compound 1 and 2. The compound 1 was obtained from Vilsmeier-Haack fomylation of triphenylamine followed by Wittig raction, and the 2,7dibromofluorene was reacted with excess amount of ethylhexylbromide to give compound 2. The resulting twophoton chromophore 3 was extensively purified before use as a light sensitizer/photoinitiator in two-photon photopolymerization. 5 The optical transparency of a chromophore at the wavelength of operation is crucial for successful 3D microfabrication with two-photonpolymerzation. As shown in steady-state absorption and emission spectra (Figure 1), the two-photon chromophore 3 has no absorption beyond 475 nm making itself ideal for two-photon application at the wavelength of 780 nm which is wavelength of our interest. Two-photon absorption cross-section of chromophore 3 at 780 nm was found to be σ = 4.7 × 10 −48 cm 4 s/photon with