Double metal layer lift-off process for the robust fabrication of plasmonic nano-antenna arrays on dielectric substrates using e-beam lithography

Integrated plasmonic sensors often require the nanofabrication of metallic structures on top of dielectric substrates by nanolithographic methods such as electron beam lithography (EBL). 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 MR regions. 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. The presence of this layer has a negative influence on the plasmonic behavior of the resulting nano-antennas. Thus, the thickness of the adhesion layer should be kept as thin as possible. 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. 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. 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. 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. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement