Focused Ion Beam Processing at the Nano-Scale
rate Y (Y ~ sputtered atoms/primary ion) is found to depend on possible to achieve good quality and reproducible fabrication
both the sample and primary ion species. of free-standing graphene structures using an ultra-sharp
Local material fluence and swelling. It has been pencil of ions. Furthermore, the potential of this technique
observed that low-dose spot impacts exhibit reproducible may be considerable for many applications in the field of
bumps or volcano shapes. This effect of swelling originates electromechanical device fabrication.
from structural changes at the surface due to ion beam-induced
2 - Nano-Pore Containing Devices
damage and the addition of implanted ions.
Ultra-Thin Membranes as an Ideal Template for FIB
Nano-pores in thin solid-state membranes, individual or
Nano-Processing FIB nano-engraving of a membrane is
as arrays, have found a growing number of applications, for
interesting if the membrane can be made homogeneous,
example as stencils or masks to grow or deposit nano-structures
conductive, and thin enough, that is, with a thickness
or to fabricate single-molecule electronic detectors or sensors.
comparable to the projected range of the incoming ions. Such
The latter is probably the more important and consists of using
thin targets allow a better ion-deposited energy localization
the membrane as a dividing wall in an electrolytic cell and
and a reduction of straggling effects with a membrane thickness
drawing charged molecules by an electric field through the
between 10 nm and a few tens of nanometers. In addition, FIB
pore. The resulting current blockage signal reveals information
processing of ultra-thin membranes allows some interesting
about the passing molecule so that, for example, DNA or
technical features because there is a clear separation between
proteins can be manipulated and studied at a single-molecule
the upper face impacted by the incoming ions and the lower
level in native conditions.
face that remains preserved. These advantages are illustrated in
We have successfully demonstrated the possibility of
Figure 2A.
applying FIB technology for the direct fabrication of devices
Most of the energy deposited by incoming ions and
with drilled pores having diameters down to a few nanometers
collision cascades remains located at the upper side of the
(Figure 4) while reaching very interesting throughput
patterned membrane, thus preserving the backside. Moreover,
capabilities. Indeed identical devices requiring individual
scattering effects of the incoming ions allow efficient guiding
FIB processing times around 100 ms can be fabricated with
effects, that is, auto-focusing through the nano-pore itself.
quantities compatible with Lab-on-a-Chip device research and
Finally, the engraved material is efficiently ejected, thus allowing
commercialization requirements.
very sharp edges to be obtained. In addition, there is almost no
Conclusion
possibility for sputtered material to redeposit on the backside
The examples here suggest a major paradigm shift for FIB
of the foil because these particles are ejected via a scattering
processing because before these achievements, nano-fabrication
phenomenon with highly directive transfer of kinetic energy.
with scanned focused ion beams, as a sequential process, was
All these advantages can be used to pattern thin self-supported
not expected to provide a mass-production capability of devices.
membranes (SiN or SiC) within patterns as diverse as pores or
The success of our FIB approach in this application is due to the
nano-wires.
very high performance of our instrument, its capability to work
1 - Nano-Wires Direct Fabrication
“in-line” with standard lithography techniques, and to the high
We have demonstrated the possibility of fabricating added value of the final device. Owing to these performance
nano-wires within thin SiC membranes (Figure 3A), but a
characteristics, there is now a promising future for Focused Ion
new effort is aimed at assessing the promise of FIB direct
Beam processing at the nano-scale.
engraving of graphene sheets using our finely focused pencil of
gallium ions. We have shown that graphene deposited within
References
atomic-thin layers could be engraved within structures such
[1]
http://ec.europa.eu/research/growth/pdf/
as rings or nano-wires down to a few nanometers (Figure
nanotechnology-conference/nanofib-27may_en.pdf
3B) while preserving the interesting physical characteristics
[2] J Gierak, Semicond Sci Tech 24 (2009) 43001-45000.
of the materials. In light of our experiments, it now appears
Figure 4: (A) TEM image of an array of perfectly circular pores drilled in a 50 nm thick Si
x
N
y
membrane. (B-C) TEM images of pores of different sizes drilled respectively
in a 20 nm thick SiC and 50 nm thick Si
x
N
y
with a dose ~10
6
ions/pt. The smaller pores have a similar aspect ratio length/diameter ~ 7.
2009 September •
www.microscopy-today.com 17
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