Solid-State Nanopore Sensors for Nucleic Acid Analysis - Nanopores: Sensing and Fundamental Biological Interactions - page 11

the width of the narrowest constriction [ 46 - 48 ]. Similarly, Heng et al. used a
focused convergent electron beam to form nanopores in ultra-thin 10 nm Si 3 N 4
membranes. The nanopore structure resembled a double cone structure with a cone
angle of 10 [ 30 ]. Smeets et al. observed a cone angle of 45 for nanopores
sputtered in composite SiO 2 /SiN/SiO 2 membranes [ 81 ]. In all cases, nanopores
formed directly through electron beam induced sputtering exhibited the ability to
contract under a defocused electron beam.

1.3.2 Fabrication of Nanopore Arrays

Multiple methods exist for the formation of nanopore arrays. The track-etch method
is one which has been used to produce commercially viable nanopore arrays with
diameters as low as ~10 nm and packing densities as high as 6 10 8 pores/cm 2 [ 28 ].

Nanopore arrays can also be fabricated through an anodization process of thin
aluminum films. In one such process that we have previously explored, aluminum
foil is first anodized in a 0.3 M oxalic acid solution at 5 C at a constant applied
voltage of 40 V for 20 h [ 63 ]. The anodized aluminum is then etched in an aqueous
mixture of phosphoric/chromic acid at 60 C. Any remaining Al in the pore region is
dissolved using a saturated HgCl 2 solution. We used this process to produce
Anodized Aluminum Oxide (AAO) membranes with a highly ordered network of
nanopores (diameters 105 nm), as shown in

Fig. 1.4a . These nanopores can be further reduced in size through atomic layer
deposition (ALD). Figure 1.4b shows an array of nanopores with final diameters of
15

¼ 75 nm, center-to-center distance ¼

1 nm formed using a combination of anodization and ALD processes.
Nanopore arrays can also be formed using a serial write process using ion or
electron beams. Figure 1.4c, d show arrays of ~200 nm diameter nanopores with
1

5 nm thick, free standing Al 2 O 3 membrane using a FIB
tool employing a high energy Ga + beam. Arrays were formed in these studies using
a FEI DB235 FIB system at an accelerating potential of 30 keV and 10 pA beam
current. This process can be used to achieve pore diameters as low as 30 nm by
controlling the ion dose, accelerating potential and beam current. Nanopore arrays
with pore sizes as low as ~20 nm have also been formed in SiO 2 using electron
beam lithography processes [ 70 ]. In addition, Kim et al. demonstrated that
nanopore arrays could be fabricated by stepping a focused electron probe formed
in a TEM over the sample surface [ 47 ]. Though the process was time intensive,
arrays with pore diameters as low as ~5 nm could be produced this way.

m m pitch, formed in 45

1.3.3 Nanopore Formation in Thin Al 2 O 3 Membranes

Recently, we demonstrated the fabrication of highly sensitive, mechanically stable
nanopore sensors

in Al 2 O 3 membranes formed via electron beam based