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

P Muñoz,Y S Yong,S M García-Blanco,F B Segerink,M Dijkstra

doi:10.1364/ome.9.002046

Abstract

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 NIR 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.

Highlights

Since the first report of surface plasmons by Ritchie in 1957 [1], 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 [2], which are routinely utilized in analytical laboratories to monitor the kinetics of surface binding effects [3] and surface-enhanced Raman spectroscopy (SERS) [4,5], which is a mature technique for which commercial substrates are available
The damage consisted of bubbling and delamination of the PMMA layer [Fig. 6(b)] with the subsequent deposition of metal on undesired locations on the substrate [Fig. 6(c)]. This effect was never observed in the areas where the antennas and other patterns had been defined by e beam lithography
The fabrication process is based on a double metal layer lift-off process that permits decoupling the contradicting requirements of metal thickness for the anti-charging layer and for the adhesion layer

Summary

Introduction

Since the first report of surface plasmons by Ritchie in 1957 [1], 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 [2], which are routinely utilized in analytical laboratories to monitor the kinetics of surface binding effects [3] and surface-enhanced Raman spectroscopy (SERS) [4,5], which is a mature technique for which commercial substrates are available. Hahn et al proposed the fabrication of gold plasmonic structures on insulating substrates using a PMMA/Al bilayer [32] In their approach, a thick (30 nm) aluminum layer is used to provide anti-charging. The damage consisted of bubbling and delamination of the PMMA layer [Fig. 6(b)] with the subsequent deposition of metal on undesired locations on the substrate [Fig. 6(c)] This effect was never observed in the areas where the antennas and other patterns had been defined by e beam lithography. In a conventional lift-off process, in which either a conductive polymer or metal layer is applied on top of the PMMA to prevent charging, residual fragments of gold will remain attached all over the substrate [Fig. 6(c)]. In the proposed process flow, wet-etching of the anti-charging metal layer lifts the metallic residue, producing perfectly clean samples after the completion of the process [Fig. 6(d)]

Optical characterization of plasmonic nano-antenna arrays

Conclusion