Effect of Tantalum and MgO adhesion layers on plasmonic nanostructures

Plasmonic structures have a wide variety of sensing applications because of their high field localization effect that leads to high sensitivity at lower powers. Specifically, plasmonic nanohole arrays are attractive platforms for sensing because of their easy alignment and measurement. In terms of fabricating these sensors, usually an adhesion layer is needed to ensure firm contact between the plasmonic metal layer and the substrate. Most fabrication efforts rely on titanium or chromium based metallic adhesion layers. However, the presence of the adhesion layer may hinder the plasmonic resonance by broadening the resonance and reducing the plasmonic field enhancement. This leads to degradation of sensing capabilities. We investigate the effect of tantalum, chromium, and titanium adhesion layers on plasmonic sensors made of nanohole arrays. Using the bulk refractive index data for metallic adhesion layers, we show that tantalum has the potential to show less damping effect compared to commonly used chromium and titanium. However, it still causes significant damping because of its high absorption, which becomes even larger for tantalum thin film according to our ellipsometry measurement results. We also propose here to use MgO dielectric adhesion layers to avoid the damping effect. Our investigation on MgO adhesion layers shows strong adhesion properties without scarifying sensor performance. Moreover, we will present an alternate sensor geometry that is less prone to damping by the adhesion layer and that can enhance the plasmonic resonance even if there is a metallic adhesion layer.