Reproducibility and control of solid-state nanopore production by fast helium ion beam milling

Abstract text (incl. figure legends and references)

Solid-state nanopores are of great interest for usage as an instrument for scalable and robust single-molecule measurements in life sciences. The nanopores are mechanically stable, and can withstand a wide range of environments, due to the materials used [1]. In order to further enhance the potential and usability of these nanopores, one of the main goals of our research is the fast and reliable fabrication.

Measurements of analytes with nanopores are typically done using simple flow cells, which incorporate a membrane chip. The membrane includes a single nanopore, therefore it forms the only possible ion conduction path between two bath chambers with electrodes, allowing the measurement of single molecules passing through the pore. The usage and further research of the solid-state nanopores as an instrument, requires fast fabrication methods. So far, nanopores were produced with a Transmission Electron Microscope (TEM), which requires manual control over the electron beam. Additionally a very frequent vacuum transfer of the single sample chip after each milling step is necessary.

Here we will present solid-state nanopores fabricated by a Helium Ion Microscope in silicon-nitride membranes. The HIM allows the etching of features smaller than 10 nm [2]. Furthermore, the depth of field is in the range of micrometres, which gives a better control over the aspect ratios of the pores. For high throughput production, this property is also useful, because the focus of the Helium Ion Beam does not need to be adjusted over a large field, which increases efficiency. We expect milling times 10-100x faster than TEM. Moreover, fast HIM scanning speeds allow low dwell times of the milling procedure (Fig. 3). Total dose each pore and dwell time - the time the Helium Ion Beam parks at one position - make up the precisely controllable parameters. The choice of these gives good control and reproducibility over the shapes of the pores, which was previously difficult to achieve by the fabrication using TEM.

Precise measurements of the sizes of the pores were done using a TEM (Fig. 1, 2). Electron energy loss spectroscopy (EELS) allows the measurement of the thicknesses, therefore aspect ratios are precisely determinable. We will present results on the reproducibility of different aspect ratios and sizes of silicon nitride nanopores.

Acknowledgements

We thank Christoph Blattert (Hahn-Schickard, Villingen-Schwenningen, Germany) for provided silicon nitride membranes. This work is supported by the project TechPat nano (Translational platform for nanosensor-based medical diagnostics, funding code 35 4223.10/10). We acknowledge financial support from the State Ministry of Baden Wuerttemberg for Economic Affairs , Labour and Tourism.

References

Xue, L. et al., doi: 10.1038/s41578-020-0229-6 Shixuan He et al., doi: 10.1088/2631-7990/abc673

Figure 1. TEM (80kV) phase-contrast image of a nanopore array fabricated by HIM. Different total doses were applied from right to left (10 nC/µm² .. 100 nC/µm²) and the dwell time was varied from top to bottom (1µs .. 10µs).

Figure 2. TEM (80kV) phase-contrast images of single nanopores, field of view 30 nm. With a dwell time of 1 µs, the total dose was varied from left to right: 40 nC/µm², 50 nC/µm², 100 nC/µm².

Figure 3. Milling procedure done by HIM. Diameter d sets the geometry. Repeats of the spiral milling pattern from inside to outside are dependent on the total dose and dwell time.