Solid-State Nanopore Sensors for Nucleic Acid Analysis - Nanopores: Sensing and Fundamental Biological Interactions - page 9

Fig. 1.3 TEM based nanopore contraction process: (a-d) Sequence of phase contrast images
obtained during TEM based temporal contraction of a SiO 2 nanopore. Pore contraction was
observed over a period of 30 mins from an initial pore size of ~40 nm to a final pore size
of ~8 nm. (e-h) TEM based contraction of an Al 2 O 3 nanopore [ 89 ], reprinted with permission.
A tightly focused electron beam is used to initially sputter a pore of diameter ~4 nm. Exposure to a
defocused electron beam results in material reflow into the pore, resulting in pore contraction.
Final pore size is approximately 1 nm

at 900 C for 31 min resulting in a SiO 2 layer of thickness between 800 and 900 ˚ .
This oxide layer acts as an insulating barrier, shielding the electrolyte solution from
the underlying conductive Si surface. Tilted SEM images of these thermally grown
silicon dioxide membranes illustrate high levels of compressive stress and mem-
brane buckling post oxidation [ 89 ]. The result is extremely fragile, highly stressed
membranes that frequently rupture during DNA translocation experiments. Fitch
et al. studied the intrinsic stress and strain in thin films of SiO 2 prepared by the
thermal oxidation of crystalline silicon and concluded that large intrinsic stress
gradients exist in the layers of SiO 2 at the Si/SiO 2 interface [ 25 ]. The residual
intrinsic interfacial stress was calculated at ~460 MPa and was independent of the
growth temperature and thermal annealing processes, attributed to mismatches in
the molar volumes of Si and SiO 2 at the interface. This highlights the need for low
stress, mechanically stable membranes for nanopore formation.

1.3.1.2 Ion Beam Sculpting

The ion-beam sculpting process first developed by Li et al. [ 53 ], uses an energetic
beam of Ar + ions to form nanopores with dimensions as low as 1.5 nm in thin Si 3 N 4
membranes. Contrary to what one would expect, a 3 keV Ar + ion beam rastered
continuously over a Si 3 N 4 sample at room temperature resulted in ion beam assisted
diffusion of atoms into the pore region resulting in nanopore contraction. The flow of
matter to the developing nanopore showed temperature dependence with a transition
between pore opening and pore closing consistently observed at ~5 C under the