Helium focused ion beam direct milling of plasmonic heptamer-arranged nanohole arrays

We fabricate plasmonic heptamer-arranged nanohole (HNH) arrays by helium (He) focused ion beam (HeFIB) milling, which is a resist-free, maskless, direct-write method. The small He+ beam spot size and high milling resolution achieved by the gas field-ionization source used in our HeFIB allows the milling of high aspect ratio (4:1) nanoscale features in metal, such as HNHs incorporating 15 nm walls of high verticality between holes in a 55-nm-thick gold film. Drifts encountered during the HeFIB milling of large arrays, due to sample stage vibrations or He beam instability, were compensated by a drift correction technique based on in situ He ion imaging of alignment features. Our drift correction technique yielded 20 nm maximum dislocation of HNHs, with 6.9 and 4.6 nm average dislocations along the horizontal and vertical directions, respectively. The measured optical resonance spectra of the fabricated plasmonic HNH arrays are presented to support the fabrication technique. Defects associated with HeFIB milling are also discussed.