An electron beam can drill nanopores in SiO 2 or silicon nitride membranes and shrink a pore to a smaller diameter. Such nanopores are promising for single molecule detection. The pore formation in a 40 nm thick silicon nitride/SiO 2 bilayer using an electron beam with a diameter of 8 nm ͑full width of half height͒ was investigated by electron energy loss spectroscopy with silicon nitride facing toward and away from the source. The O loss shows almost linear-independent of which layer faces the source, while N loss is quite complicated. After the formation of a pore, the membrane presents a wedge shape over a 70 nm radius around the nanopore. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2043247͔One can locally change a material with an intense electron beam. For instance, one can amorphousize 1 or drill a hole in a crystalline material. 2 Likewise, one can change an amorphous material, such that it crystallizes 3 or that material is removed thus creating a pore. 4 Also, as we have recently shown, 4,5 it is possible to shrink a pore in a SiO 2 membrane to a smaller size. This process is driven by a minimization of the surface energy and is facilitated by a glasslike behavior that can occur at low temperature due to bond breaking by the electron beam. Thus, the electron beam allows a variety of changes, but its possibilities in nanofabrication have so far been explored minimally.The capability of making nanopores with accurate size in an insulating material, such as silicon nitride or SiO 2 , is very useful for the manufacture of single molecule sensors. [4][5][6][7][8][9][10][11] By measuring the ionic current through a voltage-biased nanopore, one can detect individual biopolymers that are pulled through the pore by the electric field. 6 When a molecule enters the channel, it displaces its own volume of solution, thereby it modifies the channel electrical impedance and a change in the ionic current can be recorded. Rapid oligonucleotide discrimination on the single molecule level has been demonstrated using ␣-hemolysin pores in lipid bilayers 7,8 and, more recently, solid-state nanopores were used for a first study of folding effects in double-stranded deoxyribonucleic acid ͑DNA͒ molecules. 5,11 Future applications of this technique may include DNA size determination, haplotyping, and sequencing.A variety of methods have been developed by different groups to obtain single nanopores in insulating membranes. Nuclear track etching was used to fabricate nanochannels in polymer foils. [12][13][14] This technique relies on the fact that single ion bombardment of polycarbonate foils produces a track that can be subsequently chemically etched in a basic medium, such as NaOH. Although channels as small as a few nanometers can be formed, they have the disadvantage of possessing a high aspect ratio since the used foils have thicknesses of the order of 10 m. Ralls et al. 15 fashioned 10 nm pores in free-standing Si 3 N 4 membranes using electron-beam lithography and time-controlled reactive ion etching that is stop...

Solid-state nanopores fabricated by a high-intensity electron beam in ceramic membranes can be fine-tuned on three-dimensional geometry and composition by choice of materials and beam sculpting conditions. For similar beam conditions, 8 nm diameter nanopores fabricated in membranes containing SiO(2) show large depletion areas (70 nm in radius) with small sidewall angles (55 degrees ), whereas those made in SiN membranes show small depletion areas (40 nm) with larger sidewall angles (75 degrees ). Three-dimensional electron tomograms of nanopores fabricated in a SiO(2)/SiN/SiO(2) membrane show a biconical shape with symmetric top and bottom and indicate a mixing of SiN and SiO(2) layers up to 30 nm from the edge of nanopore, with Si-rich particles throughout the membrane. Electron-energy-loss spectroscopy (EELS) reveals that the oxygen/nitrogen ratio near the pore depends on the beam sculpting conditions.

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