Solid-State Nanopore Sensors for Nucleic Acid Analysis - Nanopores: Sensing and Fundamental Biological Interactions - page 8

1.3 Solid-State Nanopores
With advances in microfabrication technologies, focus has shifted to the solid-state
domain with numerous groups studying biomolecule transport through solid-state
nanopores. Solid-state nanopores exhibit superior chemical, thermal, and mechani-
cal stability over their biological counterparts, and can be fabricated using conven-
tional semiconductor processes, thereby facilitating mass fabrication and size
tunability. In addition, solid-state nanopores are functional over a wide range of
pH and do not exhibit any voltage gating, whilst allowing for integration with
electrical contacts and optical probes.
1.3.1 Fabrication of Single Nanopores
There are four primary techniques available for the fabrication of solid-state
nanopores in thin Si 3 N 4 , SiO 2 ,Al 2 O 3 or polymer membranes; surface tension driven
oxide reflow, ion milling, track-etch method and electron beam based
decompositional sputtering. Other lithography-free techniques for creating individ-
ual nanopores include focused ion beam (FIB) techniques coupled with ion-beam
sculpting to achieve pore sizes as low as 10 nm [ 54 ], and laser ablation methods
capable of achieving sub 100 nm pore diameters [ 99 , 101 ].
1.3.1.1 Electron Beam Induced Oxide Reflow
The oxide reflow technique involves the use of e-beam lithography to pattern large
40-100 nm holes in micromachined silicon membranes. These pores are subse-
quently oxidized and shrunk to the sub-10 nm range using a TEM. The TEM
shrinking process, discovered by Storm [ 84 ] uses a high energy electron beam to
locally fluidize the oxide surface in the vicinity of the nanopore causing the oxide to
reflow in the direction that minimizes interstitial surface energy. For nanopores
with diameter, d < t , where t is the membrane thickness, nanopore shrinking was
repeatedly observed. Figure 1.3 illustrates this electron beam induced shrinking
process. Schenkel et al. attributed this shrinking phenomenon to the build up of a
low- Z hydrocarbon layer in the nanopore during electron-beam irradiation [ 74 ].
Electron Energy Loss Spectra (EELS) from the localized nanopore region however
revealed the presence of only Si and O and the absence of C, [ 84 ] thereby confirming
that oxide reflow is indeed the mechanism responsible for nanopore shrinking.
The temporal contraction of a SiO 2 nanopore formed in an oxidized free
standing Si membrane through electron beam induced oxide reflow processes is
shown in Fig. 1.3a-d . The temporal contraction of an Al 2 O 3 nanopore is illustrated
in Fig. 1.3e-h [ 89 ]. The formation of SiO 2 nanopores in oxidized free standing Si
membranes presents some inherent limitations. Pores were oxidized (wet oxidation)