Solid State Nanopores for Selective Sensing of DNA - Nanopores: Sensing and Fundamental Biological Interactions - page 113

In another report, Chang et al. demonstrated similar results by using a conventional
field emission scanning electron microscope (FESEM) [ 7 ]. They made the mem-
brane using SOI wafers and made initial pores of 50-200 nm in diameter. They
reported shrinking the pore diameter to 10 nm using FESEM. Chang et al. proposed a
different mechanism for pore shrinking, which they called as radiolysis. When a
high energy beam of electrons struck the surface of the porous Si or polycrystalline
Si, it caused defects and dehydrogenation due to disruption of Si/Si and Si/H bonds.
This resulted in the shrinking of the pore under FESEM. As a result, they claimed
that previous assumptions of pore shrinking due to TEM irradiation by Storm et al.
[ 4 ] could no longer applied on polycrystalline silicon.

5.2.1.2 Plasma Polymer Film Deposition

Plasma deposited polymers have an additional advantage over other deposition
processes in the area of SSN diameter reduction: we can achieve different surface
cross linker chemistries, improve surface wetability, and control surface densities
of functional groups. In previous reports, polymer films have been used to reduce
the diameter of nanopores fabricated in polycarbonated track-etched (PCTE)
membranes [ 20 ]. PCTE filters are available commercially with 50-100 nm pores.
Low pressure plasma discharge was produced using radio frequency (RF) electric
fields to accelerate the electrons for collisions with the polymer monomer and
creation of radical species. When the radical monomer species touched the surface
of the target, they got deposited there and made a thin film of the polymer [ 21 ].
Timmons and co-workers at University of Texas at Arlington have been working
on deposition of polymer films using 1-Vinyl-2-Pyrrolidone monomer [ 22 ].
They have discovered that the density of surface groups and cross linker chemistry
could be controlled by using pulsed plasma instead of continuous plasma. They
have used variable duty cycle pulsed plasma conditions and have found that
the composition of the resultant films show increased retention of the functional
groups in the starting monomer. The crosslinked densities of the polymer films
were also reduced by decreasing the duty cycle. They applied an RF power of
13.56 MHz for plasma formation. Similar results had also been reported by Zhang
et al. but they deposited pyrrole using plasma polymerization process [ 23 ].
In another report, Rinsch et al. had used a low pressure pulsed plasma poly-
merization process for deposition of allyl alcohol as monomer gas [ 24 ]. They noticed
that when the RF duty cycle is lowered, the -OH functionality of the monomer is
increasingly retained in the resulting organic polymer films. They reported increased
surface adhesion with the help of controlled density and gradient layering technique.
Chapman et al. had reduced the nanopore diameter from 50 nm to 20 nm in PCTE
membrane using RF pulsed plasma deposition [ 20 ]. They used vinyl acetic acid
(CH 2 ¼

CHCH 2 COOH) and perfluorohexane ( n C 6 F 14 ) monomers for their
process. The vinyl acetic acid (VAA) and perfluorohexane are hydrophylic and
hydrophobic respectively. Various pulse condition were applied in order to have