Focused ion beam
A focused ion beam (FIB) is a beam of ions, most often gallium, that is focused to a spot a few nanometers to tens of nanometers across and scanned over a sample. At low current the beam images the surface, in the way a scanning electron microscope does with electrons. At high current each ion knocks atoms out of the surface, and the scanned beam cuts the material away in a pattern chosen on a screen. Laboratories use it to section chips, to repair circuits, to prepare thin samples for the electron microscope, and to drill holes in membranes, among them nanopores.
The instrument is the older and more flexible cousin of the beam methods in nanopore fabrication. It can cut a pore of any outline in minutes, but one at a time, and every pore is a separate job.
Instrument
Gallium is the usual ion because it melts at 29.8 °C, so a drop of it can be held liquid on a needle in the ion source. A strong electric field pulls the liquid into a cone and field evaporation lets ions escape from its tip, a liquid-metal ion source whose brightness Krohn and Ringo described in 1975.[1] Electrostatic lenses accelerate the ions to some tens of kilovolts and focus them, and deflection plates scan the beam. Many instruments pair the ion column with an electron column aimed at the same spot, so that the sample can be watched in the electron beam while the ion beam cuts.[2]
Side effects
The ions that mill a surface also stay in it. Gallium is implanted in the top layers of the sample, the crystal there is damaged or made amorphous, and some of the atoms that the beam removes land again on the walls of the hole, so a milled hole is rarely a clean cylinder.[2] Milling from one side leaves walls that taper, with the hole wider where the beam enters.
Nanopores
Ion beams were first used on nanopores to shrink them. In 2001 a Harvard group used a low-energy beam under feedback control to close a larger hole in silicon nitride to molecular size, a method they called ion-beam sculpting.[3] Direct milling with gallium followed. A 2006 paper used a focused ion beam followed by an electron beam to make symmetric pores smaller than 5 nm,[4] and in 2011 a scanning helium ion microscope cut pores below 4 nm, with noise comparable to that of pores drilled in an electron microscope.[5] In a 2018 test with the same instrument, pores milled with equal exposures varied in size by less than 10%.[6]
A study of pores from 7 to 200 nm wide, milled with a focused ion beam through silicon nitride 20 nm thick, found that both the ions in the bulk of the pore and the ions along its charged walls contribute to the low-frequency noise (see Flicker noise).[7]
At Molecular Reality
The pores in Molecular Reality Corporation's best early data were milled with a focused ion beam, and the founder told the story in a post of March 29, 2026 titled "Focused Ion Beam pore". The company, then demonpore, inc. on Edison Way in Menlo Park, had "a handful of the silicon pores", drilled by an outside fabricator 3,000 miles away "at a very reasonable cost (given how much effort is involved!) of about $100 each". Most of them broke in the first week or so, while the team tried to find the right wetting protocol. The founder called the problem of getting water through the pore "the biggest, ugliest, dumbest one" of what he named dumb problems.[8]
On the day in question, around 2018, "everything aligned, and we had a beautiful, fully wet, stable 200 nm underpore. It's the tiny white nipple you see in the middle of the dark circle." The current trace moved up and down as a top blade slid across the underpore, the arrangement called sliding occlusion. The founder was not in the room.[8] He ended the post with a line about the method: "there are now better, faster, cheaper ways for us to get pores like this".[8] The pore on the company's live molecular stream is 200 µm across and was made with a laser (see 200-micron laser-drilled pore).
See also
References
- ^ Krohn, V. E.; Ringo, G. R. (1975). "Ion source of high brightness using liquid metal". Applied Physics Letters 27 (9): 479-481. doi:10.1063/1.88540.
- ^ a b Volkert, C. A.; Minor, A. M. (2007). "Focused ion beam microscopy and micromachining". MRS Bulletin 32 (5): 389-399. doi:10.1557/mrs2007.62.
- ^ Li, J.; Stein, D.; McMullan, C.; Branton, D.; Aziz, M. J.; Golovchenko, J. A. (2001). "Ion-beam sculpting at nanometre length scales". Nature 412 (6843): 166-169. doi:10.1038/35084037.
- ^ Lo, C. J.; Aref, T.; Bezryadin, A. (2006). "Fabrication of symmetric sub-5 nm nanopores using focused ion and electron beams". Nanotechnology 17 (13): 3264-3267. doi:10.1088/0957-4484/17/13/031.
- ^ Yang, J.; et al. (2011). "Rapid and precise scanning helium ion microscope milling of solid-state nanopores for biomolecule detection". Nanotechnology 22 (28): 285310. doi:10.1088/0957-4484/22/28/285310.
- ^ Xia, D.; et al. (2018). "Rapid fabrication of solid-state nanopores with high reproducibility over a large area using a helium ion microscope". Nanoscale 10 (11): 5198-5204. doi:10.1039/c7nr08406d.
- ^ Wen, C.; et al. (2017). "Generalized noise study of solid-state nanopores at low frequencies". ACS Sensors 2 (2): 300-307. doi:10.1021/acssensors.6b00826.
- ^ a b c Kemmishtree (March 29, 2026). "Focused Ion Beam pore". EQBio-OS.
