Using a liquid metal alloy ion source for FIB patterning of noble metal plasmonic nanostructures

Plasmonic devices based on noble metal nanostructures have been extensively investigated in the last 20 years and nowadays their applications cover a wide spectrum of sectors. They are typically based on regular arrays of metal structures with sizes ranging from few to several hundreds of nm’s, sub-µm periodicity, precise shape and positioning. Their fabrication requires extremely high-resolution patterning techniques like UV and deep-UV optical lithography, electron beam lithography (EBL) and focused ion beam (FIB) patterning, among others. In particular, FIB patterning can be an excellent prototype fabrication solution, offering the advantages of maskless-direct writing, flexibility in design, and functionalization by ion beam-solid interactions. Gallium based liquid metal ion sources (LMIS) have been for several decades the main choice for focused ion beam instruments (FIB). More recently, FIB columns equipped with liquid metal alloy ion sources (LMAIS) were implemented for nanopatterning. LMAIS working principle is essentially the same as the one for Ga-LMIS, exploiting the Taylor cone formed by applying an electric field to a tip wetted by a liquid alloy melted at low temperature. For this reason, eutectic alloys are usually exploited, e.g. the gold based ones Au-Si, Au-Ge or Au-Ge-Si. The latter can also offer more flexibility on ion choice, allowing selecting adequate species and charge depending on the needs of lateral resolution, sputtering yield and possible surface functionalization. In this work, two examples of LMAIS FIB patterning for plasmonic applications will be reported. In the first, regular arrays of silver nanostructures were milled by Au through 110 nm thick Ag films, deposited on silicon photodiodes. Aim of the experiment was the exploitation of the surface plasmon polaritrons induced by the Ag gratings to excite highly-confined modes by irradiation, enhancing in this way the absorption of near-infrared radiation photons close to the active depth of the photodiodes. A FIB process was defined after calibrating the milling rate by atomic force microscopy and characterizing the contamination depth of the Au ions in the active area of photodiodes by secondary ion mass spectrometry. Electro-optical characterization proved the success of nanofabrication. In the second example, high-resolution patterning was carried out in order to produce regular (566 nm period) inverted honeycomb arrays through 20 nm thick Au films that work as efficient refractive index sensors. Focused beams of Si , Ge , Au and Au were tested in order to identify the best fabrication process, in particular to find the best combination of current and ion dose for species to mill lines with the required depth and lateral width.