Biomedical Engineering Reference
In-Depth Information
The shot noise observed in the area identified as the pore is indicative of perfect
transmission of the electron beam through the pore in the membrane. The three-
dimensional (3D) structure can be inferred from 2D projections of the pore taken at
various tilt angles. Although it is not unique, one simple model for the structure
consists of two intersecting cones each with a cone angle of ~30
in this case. To
ensure the integrity of the capacitor, we measured the tunneling current between the
electrodes prior to and after the pore had been sputtered [
14
,
18
]. Further details
regarding the device description can be found in [
19
].
7.3 Computational Model and Nanopore Device Modeling
In order to achieve an understanding of the electrostatics at play between the
different materials, i.e. the electrolytic solution and the DNA molecules interacting
with the semiconductor/oxide structure, we have developed a computational model
of the device that reproduces realistically the bio-electronic system shown in
Fig.
7.1
. In our model the top poly-Si and bottom Si capacitor films have been
replaced by two heavily doped
n
+
- Si layers. The nanopore is placed in the center
of the membrane and assumed to have cylindrical symmetry. The
n
+
- Si layers
have conical shape above and below the nanopore as a result of the electron beam
sputtering.
A schematic of the idealized device geometry is shown in Fig.
7.2
. We chose the
coordinate system in which the
y
-axis is parallel to the nanopore axis and the
xz
plane is parallel to the device structure layers including the SiO
2
layer.
Fig. 7.2 Schematic of the
device geometry:
xy
cross-
section through the center of
the nanopore (
yz
cross-section
is similar) provides a side
view of the modeled device
and cross-sections for which
the electron, negative and
positive ion concentrations
are represented on the
following figures. The
drawing is not to scale
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