Biomedical Engineering Reference
In-Depth Information
demonstrated [
3
,
13
,
72
]. The sputtering process in Al
2
O
3
is attributed to the
Coulomb explosion displacement of atoms based on the Knotek-Feibelman elec-
tron-stimulated desorption mechanism. This decompositional sputtering mechanism
was exploited to form nanopores ranging in diameter from 2 to 30 nm [
89
,
90
].
Figure
1.5
shows the Gaussian/Lorentzian intensity profiles of various focused
electron probes used in the pore formation process. Probe diameters of 2.3, 2.7,
3.2 and 3.9 nm full width at half maximum (FWHM) were investigated,
corresponding to beam current-densities of 2.6
10
6
A/m
2
,4.2
10
6
A/m
2
,
6.1
10
6
A/m
2
and 1.2
10
7
A/m
2
respectively. The inset of Fig.
1.5a
is a TEM
image of a 3.2 nm probe, light areas indicating regions of maximum electron intensity
located at the center of the probe and darker areas indicating less intense regions located
at the tail of the probe. Larger probe sizes exhibited higher peak intensities and a broader
Gaussian/Lorentzian profile andwere well suited to form large nanopores with diameters
in the range of 10-30 nm, applicable for single molecule protein analysis and the
detection of “large” analytes. Smaller probes (2.7 and 3.2 nm) exhibited lower peak
intensity and a narrower profile, ideal for the high precision fabrication of 2-10 nm pores
in Al
2
O
3
, well suited for ssDNA, dsDNA and RNA analysis.
Three stages were identified during nanopore formation in Al
2
O
3
, I, Pore Nucle-
ation, II, Rapid Expansion and III, Controlled Growth as shown in Fig.
1.5b
.
A critical beam current density in excess of 2.6
10
6
A/m
2
was required for
nanopore nucleation in Al
2
O
3
membranes. This is in good agreement with threshold
current densities extracted by Salisbury et al. in experiments involving electron
beam sputtered anodized alumina [
72
]. Below this threshold, topographical
damage corresponding to the cleaving of Al-O bonds (bond dissociation energy of
513 kJ/mol) [
67
], was observed but electron momentum was insufficient to induce
an embryonic nanopore structure. Pore contraction mechanisms were also seen to
dominate at low beam current densities, possibly due to surface tension driven oxide
reflow, generation/recombination of closely spaced Frenkel pairs [
71
] and mass
transport of mobile atoms into the nucleation site. This is consistent with the
nanopore contraction phenomenon observed previously in SiO
2
[
11
,
84
], and
Al
2
O
3
systems [
89
]. The sputter rate transition observed at the boundary of the
Rapid Expansion and Controlled Growth stage was attributed to electron beam
induced crystallization and metallization of the nanopore region.
1.3.3.2 Electron Beam Induced Crystallization
Structural phase transformations in the membrane material around the pore region
was observed during electron beam induced decompositional sputtering of Al
2
O
3
[
89
,
90
]. Discrete spot reflections of
phase Al
2
O
3
were initially
identified, confirming the formation of nanocrystalline clusters of preferred phases.
In
a
,
g
,
d
and
k
-Al
2
O
3
,Al
3+
cations are octahedrally coordinated with average Al-O bond
lengths of 1.92
˚
a
-Al
2
O
3
typically exhibits a cubic defect-spinel
type structure with average Al-O bond lengths of 1.89
˚
[
5
]. The presence of
multiple heterogeneous phases with varying bond lengths and co-ordinations,
[
6
]. However,
g
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