Resizable nanopores

Nanopores in solid-state membranes attract considerable interest due to their applicability in single molecule detection, identification and DNA/RNA sequencing. As this requires the ability to create holes on the molecular... [ view full abstract ]

Nanopores in solid-state membranes attract considerable interest due to their applicability in single molecule detection, identification and DNA/RNA sequencing. As this requires the ability to create holes on the molecular size-scale, significant fabrication challenges arise. Currently, focused ion beam (FIB) with helium ions enables the creation of single-nanometer sized openings with depth/diameter aspect ratios ranging in the hundreds. However, this type of milling is too slow to be practicable, hence, as a commercially viable option Ga FIB with subsequent hole resizing has to be pursued.

FIB milling was used to open 30-100 nm diameter holes in Si3N4 membranes of comparable thickness. Holes and their arrays were milled in tens-of-seconds on the Si3N4-membrane on Si samples. Typically, the exit diameter was up to two times smaller than that at the beam entrance. Negligible resputtering of membranes took place and high fidelity holes were obtained. Thin metal coating sto prevent surface charging were implemented, however, through-holes can be milled even on as-received membranes at the smallest 6 pA ion currents. The smallest diameter holes of 30 nm were fabricated with a 10 nm spot-size beam (IonLiNE Raith).

The opened holes were, next, subjected to electron beam for down-sizing. Following scaling rules were determined: (i) for the identical scan speed, the closure speeds in nm-per-second increase with the increasing of the magnification or the decreasing of the scan area size, (ii) hole closure speed in nm/s increases with the scan speed, and (iii) the closure speed in nm per area-scan decreases with scan speed. This indicates that material redeposition due to resputtering [1] is the most probable cause of hole rescaling. Hence, by material re-deposition it should be possible to fully close holes at the cost of thickness changes.

Possible applications of nanoholes will include ion-transfer through cell membrane where the location and contact area can be controlled to unprecedented few-nm resolution. Frequency lifetime imaging (FLIM) technique will be employed for future ion transport experiments and provides a super-resolution capability due to precise knowledge of the hole position and geometry.

[1] G. Seniutinas, et al., Beilstein journal of nanotechnology 4 (1), 534-541. 2013.