Solid State Nanopores for Selective Sensing of DNA - Nanopores: Sensing and Fundamental Biological Interactions - page 110

Fig. 5.2 Figure shows the confocal microscope images of the thin SiO 2 membrane. These images
were taken after 8 h etching of Si in TMAH

SiO 2 served the purpose of a hard mask. The etching rate of TMAH for Si is 1 m

per minute at 90 C while the etching rate for SiO 2 is just one Angstrom per minute
[ 8 ]. The roughness of the etched surface was improved by adding isopropyl alcohol
in TMAH solution [ 9 ]. The wafer was etched through all of its thickness until a thin
membrane of SiO 2 was left at other side. The optical microscope image of
membranes are shown in Fig. 5.2 .
After fabricating the membranes, the thickness of the SiO 2 was reduced by
using reactive ion etching (RIE). In previous reports, fluorine and chlorine have
been used as feed gases. Iqbal and coworkers had used CF 4 gas in an RIE chamber
at 100 W with a gas flow rate of 20 sccm. They had the option to use high
power, but the membranes were already very thin; high power could break the
membranes. In addition to this, a higher power yields a higher etching rate. This
makes it difficult to control the final membrane thickness.
By using RIE, Iqbal and co-workers reduced the membrane thickness to tens
of nanometers. A thin membrane was desirable, because it was essential to drill
smaller nanopores with the help of FIB. Also, the thickness of the membrane
would finally be transformed into the length of nano channel. Normally a smaller
nano channel is desirable. They have also taken into account the RIE etching
of from the back side the membrane. According to their analysis, the backside
etch rate is roughly 15% as compared to that of the front side.
Gierak et al. reported the drilling of SiC membranes using FIB [ 10 ].
They fabricated 15 nm pores by using FIB. They explicate that the limitation
of FIB is its dependability on physical and geometrical features of the surface
[ 11 ]. However, FIB allows direct patterning of the membranes and good reproduc-
ibility. The acceleration voltage used in the FIB system was in the range of
30-50 KV. When accelerated ions struck the surface of the membrane, they
sputtered the target. Energy of incident ions was transferred to the target atoms
and electrons. The colliding ions were deflected from their paths, and surface

m