Solid State Nanopores for Selective Sensing of DNA - Nanopores: Sensing and Fundamental Biological Interactions - page 111

Fig. 5.3 Figure shows the SEM images of the SSNs drilled with the help of FIB in SiO 2
membrane. The left and right images of the SSNs show the pores with diameters of 100 and
160 nm respectively

atoms were also displaced from their positions [ 12 ]. The penetration depth R p at
depth D has the form as given below

h i

2

DR p 2

Exp D R p

(5.1)

2

where DR p is range straggle. The ejection of atoms and the number of atoms ejected
from the surface depend on the energy of the striking ions as well as the target
material of the membrane. The typical yield is 1-10 atoms/ion. The yield also
depends on the incident angle between the ion beam and normal to the surface.
Yield increases with 1 / cosy

is the incident angle.
The initial diameter of the nanopore strongly depends on the membrane thick-
ness and ion dose (ions/cm 3 ). Typically, a thicker membrane results in a bigger
initial pore. It was very challenging to reduce the thickness of the SiO 2 membrane
below 20 nm, as the membrane becomes very fragile and can break down during
drilling and further processing. Iqbal and co-workers had also used FIB for
their nanopore fabrication. They used ZEISS 1540XB FIB equipment, which
had Gallium (Ga + ) ions accelerating at a voltage of 30 KV. Typically, the milling
current used for SSN fabrication was 5-10 pA. They used the FIB in manual mode,
and they fabricated the SSNs with a diameter range of 100-200 nm in SiO 2
membranes. The SEM images are shown in Fig. 5.3 .

, where y

5.2.1 Deposition Processes for Size Reduction

In order to have a SSN which can mimic the function of a transmembrane protein
channel in a lipid bi-layer, researchers have struggled to reduce the pore diameter
to sub-ten nanometers. In previous reports, researchers have used different shrinking