Comparative study of plasmonic antennas fabricated by electron beam and focused ion beam lithography

Michal Horák,Jiří Jaskowiec,Tomáš Šikola,Vojtěch Švarc,Vlastimil Křápek,Kristýna Bukvišová

doi:10.1038/s41598-018-28037-1

Michal Horák, Jiří Jaskowiec + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-28037-1

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Abstract

We present a comparative study of plasmonic antennas fabricated by electron beam lithography and direct focused ion beam milling. We have investigated optical and structural properties and chemical composition of gold disc-shaped plasmonic antennas on a silicon nitride membrane fabricated by both methods to identify their advantages and disadvantages. Plasmonic antennas were characterized using transmission electron microscopy including electron energy loss spectroscopy and energy dispersive X-ray spectroscopy, and atomic force microscopy. We have found stronger plasmonic response with better field confinement in the antennas fabricated by electron beam lithography, which is attributed to their better structural quality, homogeneous thickness, and only moderate contamination mostly of organic nature. Plasmonic antennas fabricated by focused ion beam lithography feature weaker plasmonic response, lower structural quality with pronounced thickness fluctuations, and strong contamination, both organic and inorganic, including implanted ions from the focused beam. While both techniques are suitable for the fabrication of plasmonic antennas, electron beam lithography shall be prioritized over focused ion beam lithography due to better quality and performance of its products.

Highlights

Localized surface plasmons (LSP) are collective oscillations of free electrons in metallic nanostructures coupled to the local electromagnetic field
We present a comparative study of gold plasmonic disc-shaped antennas fabricated by electron beam lithography (EBL) and focused ion beam (FIB) lithography with gallium ions
We have studied in detail four individual disc-shaped gold plasmonic antennas: two series prepared by both EBL and FIB consisting of antennas with the designed diameter of 120 and 140 nm and with the designed height of 25 nm

Summary

Introduction

Localized surface plasmons (LSP) are collective oscillations of free electrons in metallic nanostructures (plasmonic antennas) coupled to the local electromagnetic field. Plasmonic antennas are often fabricated by electron beam lithography (EBL)[12] or using focused ion beam (FIB) lithography[13]. Focused ion beam that locally sputters off the metal (see Fig. 1) Both the EBL process and the FIB milling are capable to create sub-10-nm structures[14,15]. There are no chemicals involved in the process but the ions in the milling beam and atoms sputtered from the metal and substrate can still contaminate the fabricated structures. Fabricated antennas were characterized using the transmission electron microscopy (TEM) including chemical analysis by energy dispersive X-ray spectroscopy (EDS) and characterization of LSP resonances and thickness measurement by EELS, and using the atomic force microscopy (AFM) to complement information about the morphology of the structures. Spectral and spatial characteristics of LSP resonances obtained from experiment were compared with numerical simulations with the MNPBEM toolbox[22] which relies on the boundary element method (BEM) approach[23,24]

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Results

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