Biomedical Engineering Reference
In-Depth Information
ion-beam conditions used. Pore expansion was attributed to ion sputter erosion at the
pore edge, dominant at low temperatures and high ion flux. Pore closure was
attributed to the reflow of a stressed viscous surface layer into the nanopore. The
reduced viscosity and/or enhanced stress in this layer caused relaxation, thereby
filling the nanopore. Feedback control was used to precisely control nanopore size.
1.3.1.3 Track-Etch Method
Conical nanopores are typically formed in
m thick polymer films using the
track-etch method [
10
,
75
,
76
]. The fabrication process involves first bombarding
a thin sheet of polymer material (polyethylene terephthalate, polyimide or polycar-
bonate) with a high energy beam of nuclear fission fragments or with a high energy
ion beam from a MeV accelerator at normal or near normal incidence angle. The
irradiated polymer membrane is then placed between two chambers of a conductiv-
ity cell and etched chemically from one side. Chemical-etching of the damage track
is done in a strong alkaline solution (pH
m
13) with high chlorine content at elevated
temperatures (~50
C) using a solution such as sodium hypochlorite (NaOCl) [
77
].
The other compartment of the conductivity cell is filled with 1 M potassium iodide
(KI) solution as a stopping medium for the OCl
ions of the etchant. As soon as the
etchant completely penetrates the polymer film, iodide ions reduce OCl
to Cl
ions
thereby halting the etch process. The result is a tapered, individual conical nanopore
with pore diameter as low as ~10 nm in the polymer membrane.
1.3.1.4 Electron Beam Induced Sputtering
Electron beam induced sputtering offers a rapid and reliable method to prototype
nm
sized pores in the TEM. This method involves the use of a focused convergent
electron beam with sufficiently high current density to decompositionally sputter
nm
sized pores in thin oxide or nitride membranes (thickness
60 nm). An added
benefit of this method is that it allows the operator to inspect pore size during
fabrication and avoids the need for electron beam lithography involving e-beam
resists and reactive ion etching (RIE) pattern transfer steps. Kim et al. used high-
resolution TEM to study nanopore formation kinetics in Si
3
N
4
. Nanopore formation
was a balance between two competing processes: (a) material sputtering and (b)
surface-tension-induced shrinking [
46
,
48
]. Nanopores, 4-8 nm in diameter were
directly drilled using a JEOL 2010F field emission TEM with an accelerating
voltage of 200 keV and a beam current density of 10
8
-10
9
enm
2
. Nanopore
contraction was achieved by slightly defocusing the e-beam, effectively reducing
the beam intensity to ~10
6
enm
2
. TEM tomography was used to map the three-
dimensional structure of these solid-state nanopores. It was observed that the
sidewalls of the sputtered pores were angled (approximately 65
to the horizontal),
attributed to the intensity distribution of the e-beam around its focal point. Post-
drilling, pores formed an 'hourglass' structure with pore width being determined by
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