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Integrated optical waveguides operating at this wavelength are often fabricated using dielectric materials, including silicon dioxide (SiO 2), silicon nitride (Si 3N 4), aluminum oxide (Al 2O 3) and titanium oxide (TiO 2). Given the subwavelength dimensions of plasmonic nano-antennas and considering the inverse relationship between the Raman scattering cross-section and wavelength used, 785 nm is one the most used wavelengths to perform this technique since it offers a compromise between performance and technological requirements. Plasmonic nano-antennas can be integrated onto optical waveguides, leading to the realization of integrated plasmonic sensors, which combine the signal enhancement of the plasmonic structures with the miniaturization, scaling and interfacing provided by the integrated waveguides. The intense near-field in such regions can be used to boost the weak intensity of signals generated by the analytes of interest, such as Raman scattering in SERS. Plasmonic antennas, which receive such name in direct analogy with their RF counterparts, are nanostructures capable of concentrating light in specific regions of the space known as hot-spots. Since the first report of surface plasmons by Ritchie in 1957, research in the field of plasmonics has paved the way to multiple applications, some of them having already led to commercial products, such as surface plasmon resonance (SPR) sensors, which are routinely utilized in analytical laboratories to monitor the kinetics of surface binding effects and surface-enhanced Raman spectroscopy (SERS), which is now a mature technique for which commercial substrates are available. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement 1. Additionally, the proposed method permits eliminating any metal residue during the lift-off process, leading to a perfectly clean device outside the nanostructured region, which is instrumental when the nanostructures are to be integrated with other photonic functions on the chip. In this work, we present a nanofabrication process that decouples the contradicting requirements for the metal layers, permitting to independently optimize both the thickness and type of the metal used as anti-charging layer and as adhesion layer underneath the nanostructures.
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A possible solution to this problem is the use of an anti-charging layer under the electron sensitive resist, which should be thick enough to offer high conductivity. Moreover, the use of EBL on non-conductive substrates leads to charge accumulation in the isolating materials (i.e., charging effect), which degrades the resolution of the lithography. Thus, the thickness of the adhesion layer should be kept as thin as possible. The presence of this layer has a negative influence on the plasmonic behavior of the resulting nano-antennas. Due to its inert nature, gold offers very poor adhesion to dielectric layers, therefore often requiring the deposition of a thin metallic layer as adhesion promoter. One of the preferred metals for the realization of such nanostructures is gold given both its corrosion resistance and favorable refractive index in the visible and NIR regions. Integrated plasmonic sensors often require the nanofabrication of metallic structures on top of dielectric substrates by nanolithographic methods such as electron beam lithography (EBL).
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