Neutral but Impactful: Gallium Cluster-Induced Nanopores from Beam-Blanked Gallium Ion Sources

TL;DR

The paperAddressed a key limitation in focused ion beam microscopy: neutral gallium species, including clusters, can bypass electrostatic beam blanking and damage samples. The authors demonstrate neutral Ga cluster emission from a Ga LMIS and form ~2 nm nanopores in ultrathin membranes by exposing them under beam blanking, using HR-TEM, multislice ptychography, STEM-EELS, and STEM-EDS for comprehensive characterization. They show most Ga does not implant deeply, but forms through-holes with a narrow size distribution, and that high-dose electron irradiation in TEM can remove amorphous clogging and expand pores to tune pore size, enabling potential nanofluidic applications. These findings highlight the impact of neutral species in FIB workflows and offer a method to create tunable nanopores in thin membranes for size-selective transport.

Abstract

Neutral atoms emitted from liquid metal ion sources are an often-overlooked source of contamination and damage in focused ion beam microscopy. Beyond ions and single atoms, these sources also emit atom clusters. While most studies have investigated charged clusters, here we demonstrate that neutral clusters are also emitted. These neutral clusters bypass the electrostatic beam blanking system, allowing them to impinge on samples even when the ion beam is blanked. We investigate this phenomenon using thin (<20 nm) freestanding membranes of hexagonal boron nitride, silicon, and silicon nitride as targets. Randomly dispersed nanopores that form upon neutral cluster exposure are revealed. The average nanopore diameter is ~2 nm with a narrow size distribution, suggesting that the atom clusters emitted from the source have a preferred size. Various electron microscopy techniques are used to characterize the nanopores, including high-resolution transmission electron microscopy, multislice ptychography, and electron energy-loss spectroscopy. Finally, we show how electron irradiation in the transmission electron microscope can be used to both remove any amorphous material that may clog the pores and to controllably grow the pores to specific sizes. Tunable nanopores such as these are interesting for nanofluidic applications involving size-selective membranes.

Figures (5)

• Figure 1: Bright-field TEM images of multilayer hBN samples showing the spatial distribution of pore-like structures obtained after (a) 20 minutes and (b) 20 hours of exposure to neutrals under the electrostatically blanked Ga LMIS. (c) Schematic illustrating the transit of Ga neutral clusters though the FIB column (here, assuming neutralization at the source), showing how the neutrals are unaffected by the blanker field and other electrostatic beam elements. The clusters then impact the specimen forming nanopores by local sputtering at the individual impact sites.
• Figure 2: (a) HAADF-STEM image of multilayer hBN sample exposed to neutrals from the electrostatically blanked Ga LMIS for 20 hours. Dark contrast corresponds to pore-like structures. (b) Corresponding STEM-EDS signal integrated over this region with counts plotted on a log scale. Additional labeling of low-energy X-ray peaks in Supplementary Fig. S4.
• Figure 3: (a) HR-TEM image of a single nanopore formed in the multilayer hBN sample showing amorphous contamination around the perimeter. (b) HR-TEM image of another nanopore in the same sample that appears to be filled with amorphous material. (c) Histogram of nanopore diameters calculated by measuring 106 nanopores. The curve shows a kernel density estimate overlay.
• Figure 4: (a) Middle slices from two ptychographic depth-sectioning reconstructions for an empty pore (left) and a filled pore (right). (b) Corresponding vertical structure schematics with the intersecting mid-plane slices outlined in black. (c) Correlative STEM-EELS low-loss signals extracted from the regions inside the respective empty and filled pores, compared to the signals extracted from the bulk hBN around each pore in each case. Labels I, II, and III indicate plasmons and interband transitions from bulk hBN, discussed further in the main text.
• Figure 5: (a) HR-TEM inspection of nanopores in hBN that had been subjected to electron irradiation at a dose rate of ∼4e6e^-/nm^2/s to remove hydrocarbon contamination and expand pores to larger sizes. ($\Delta$t = 150s). (b) Zoomed-in images of the highlighted nanopore in (a) showing its expansion over time.