This Application Note describes an editing process for: (1) precise nanometer-scale linear etching operations, including carbon nanotube (CNT) cutting, shortening, cleaning, and other operations involving individual CNTs, and (2) precise micron-scale area etching operations, including cleaning entire areas of unwanted nanotube overgrowth. All of these operations were achieved using the NanoBot® nanomanipulator equipped with Xidex’s Parallel Multi-Precursor Gas Delivery (Parallel MPGD) system. Xidex manufactures and sells the NanoBot® system, an easy-to-use, highly versatile, user programmable nanomanipulator featuring specialized end-effectors for nanodevice fabrication and testing inside scanning electron microscopes (SEMs) and focused ion beam (FIB) tools. Our mission is to enhance the R&D productivity of nanoscientists and nanotechnologists in both industry and academia. We offer best-in-class turnkey solutions to customers with well-defined requirements that are stable over time, and highly adaptable solutions to customers who need a system that can easily be augmented with additional plug-and-play nanopositioners and end-effectors as their needs evolve.

1. Xidex Vapor Phase Editing of Carbon
Nanotube Based Nanodevices:
Using the NanoBot ® System with Gas Delivery
Application Note
Vladimir Mancevski, Xidex Corporation
Precise Site Selective CNT Editing
This Application Note describes an editing process for: (1)
An extra CNT
precise nanometer-scale linear etching operations, including Lateral CNT to be removed
carbon nanotube (CNT) cutting, shortening, cleaning, and emitter
other operations involving individual CNTs, and (2) precise
micron-scale area etching operations, including cleaning
entire areas of unwanted nanotube overgrowth. All of these
operations were achieved using the NanoBot ® Si Post
nanomanipulator integrated with a gas delivery injection
module.
Si Post 500 nm
Applications that motivated this work include fabrication and
repair of CNT-based scanning probe microscope (SPM) tips Figure 2 – Example of a lateral (horizontal) CNT device
and CNT-based electron emitters. Figures 1-3 show examples fabricated by Xidex for use as a lateral field emitter.
of CNT cutting and area cleaning experiments performed in
related research. Figure 1 shows SEM images of two sets of
carbon nanotubes that have been cut using electron beam
induced etching process [1]. The CNTs in image a) were
etched using a line scan, and the CNTs in image c) were cut
in a box scan. Image d) illustrates the com petitive
contamination that can accompany the process. Figure 2 500 nm
shows an example of a lateral (horizontal) CNT device
fabricated by Xidex for use as a lateral field emitter. Figure 3 Figure 3 – An excess CNT strung from a silicon post
(viewed top down) and the surface, before (left) and after
shows the result of using selective CNT etching to remove an (right) it was removed using vapor phase editing.
extra CNT extending from a silicon post to the substrate.
Nanomanipulator-Based Gas Delivery Nozzles
It has been shown that operating a system of gas delivery
nozzles with the NanoBot ® unit results in an optimized,
localized precursor pressure and flux, while at reduced
chamber pressures. One important outcome with the use of
this gas delivery system is that the etching time and rate both
improve by minimizing the separation distance between the
sample surface and the gas nozzle. It has also been
observed that, for a smaller sample-nozzle gap, the probe
current decreases. This decrease in probe current is due to
ionization and competitive positive current flow, which
increases with decreased spacing because of enhanced local
pressure. The end-result is that the higher local pressure is
responsible for an increased and optimal etching rate. These
Figure 1 - SEM images of two sets of carbon nanotubes results demonstrate that a smaller nozzle gap leads to a
that have been cut using an electron beam induced faster etch rate and a lower sample current.
etching (EBIE) process.
1

2. Limitations of FIB Processing Xidex’s Parallel MPGD Module
The ability to edit materials at the nanoscale level is critical Xidex’s Parallel Multi-Precursor Gas Delivery (MPGD) module is
for the ongoing nanotechnology revolution. While standard available as an end-effector for the NanoBot® nanomanipulator, and
and emerging lithographic techniques will continue to play a can deliver water vapor locally to a sample inside an SEM. This
critical role in nano-fabrication processes, nano-fabrication enables a gas precursor-assisted electron beam-based process that
also requires highly directed materials editing techniques can be used for precise site-selective carbon nanotube (CNT)
which are site-selective. As geometries shrink and wafer editing. The result is a simple, effective, and damage-free way to
cost-of-ownership increases, nanoscale re-manufacturing and control fabrication and repair of CNT-based nanodevices.
repair techniques will be increasingly important. Currently Details of Xidex’s Parallel MPGD system can be downloaded in the
adopted methods for selectively depositing or etching micro- “Products” section at www.Xidex.com.
and nanoscale features utilize ion beam deposition and
The Parallel MPGD system can be configured with internally
etching, laser ablative etching (using far field and near field
mounted reservoirs, or with externally mounted reservoirs, as was
optics), and mechanical abrasion using a fine micro-tip. Of
done in this study (Figure 4). A multi-nozzle fixture is attached to a
these techniques, selective focused ion beam (FIB)
NanoBot X,Y,Z nanopositioning end-effector, located within the SEM
processing is probably the most mature technology that has
or FIB sample chamber. Up to four different gas precursors can be
been extended into the nanoscale. While suitable for some
accommodated. Examples include Pt, W, Au, TEOS, and O2, in
applications, FIB processing has several drawbacks that
addition to water vapor. Key capabilities of this MPGD system are:
make it difficult to extend into many other applications. The
most severe drawback when using a Ga+ FIB, is Ga+ (i) Valves mounted on the nanopositioner enable fast on-off control
implantation into the substrate material, which can of the gas flow within each individual nozzle.
deleteriously change the properties (optical, electrical, (ii) Each gas travels through a separate tube and nozzle, thereby
mechanical, and biological) of the material [2]. Additionally, precluding contamination from residual traces of a previously
charging inherent to the ion-solid interaction causes proximity used process gas. This separation eliminates unwanted
effects that can lead to “riverbed effects” which erode nearby reactions between incompatible gasses, as may be the case
features while the heavy ion beam is scattered, and induce when multiple gasses share the same tubing.
sputtering. Consequently, although focused ion beam (iii) Availability of separate delivery tubes for each gas enables fast
processing is a very effective technique in many nanoscale switching between multiple gasses; i.e., without having to wait
applications [3], an alternative damage-free site-selective to purge the previous gas that was used.
nanomaterials editing technique is needed for fabrication and
(iv) The gas delivery nozzle assembly can be moved in three
repair of CNT-based devices used in many emerging
orthogonal directions, using the X,Y,Z nanopositioning end
applications – enter the Xidex NanoBot system!
effector, providing access to larger areas of the sample. This
also allows better control of the nozzle-sample gap.
Figure 4 – Parallel MPGD System with External Reservoirs
2

3. System for Vapor Phase Editing 4 minutes. A summary of the beam energies, currents, and
A NanoBot system with an externally mounted water reservoir SEM settings investigated is provided in Table 2.
was used to for site selective vapor phase editing of CNTs (as
per Figure 4). The NanoBot unit and MPGD nozzle assembly
were mounted inside a Hitachi S 4000 SEM. This model is not
an environmental SEM, and many other SEM brands and
models are also compatible with the NanoBot system.
Water vapor was delivered to a Gauge 26 metal nozzle with 254
μm ID. With the help of the nanomanipulator, the nozzle could
be placed 1 mm - 10 μm from the sample, depending on the
angle of the nozzle with respect to the SEM stage. Relationship Between Nozzle-Sample Distance and
Etching Time and Rate
Relationship Between Nozzle-Sample Distance and The relationship between the nozzle-sample distance and the
Background Operating Pressure etching time and rate was investigated by measuring the
Improved etching of carbon nanotubes has been correlated with etching time and the sample current. The etching rate was
the small distance between the nozzle and the sample, proving computed by knowing the size of the etched CNT. For this
that the small distance between the nozzle and the sample experiment, the needle-sample distance was changed from 87
results in an increase of the localized gas pressure which in turn μm to 328 μm by doubling the gap each iteration, and then
is responsible for the improved etching of carbon nanotubes. returning back to the smallest gap that could be used for that
sample, 76 μm, to verify that some bias was not being built up.
Because it is impossible to directly measure the localized
Figure 6 shows the result of this trial. To be consistent, all CNT
precursor pressure at the sample, the local pressure has been
cuts were done on the same multi micron long carbon nanotube
computed by knowing the chamber pressure and the gas nozzle
where each cut was a few microns away from the other. For
geometry. To estimate the localized pressure/flux from the
example in Figure 6, for 164 μm it can be seen that the CNT
nozzle, a program initially developed by Kohlmann et al. [4] was
was cut in a segment. During these experiments the
used. The program inputs the flow rate of precursor gas (for
magnification was also kept the same for all trials, at 35 kx for
instance in standard cubic centimeters per second, as
imaging and 100 kx during etching. Table 3 lists all the
determined from the throughput calculations). To find the
parameters for this experiment.
approximate gas spot area and associated pressures, the gas
molecular weight and temperature are also required. Finally,
geometrical factors that determine the area to which the gas is
applied are input. Table 1 lists the parameters, units and some
notes regarding the model parameters.
One important conclusion from this experiment is that the
etching time and rate improved for a smaller gap between the
sample and the nozzle, as shown in Table 3. Further, for a
smaller gap the probe current decreased, as per Table 3. The
change in the probe current as a function of distance is
interpreted as ionization and competitive positive current flow,
The next issue to be resolved was to determine the most
which increases with a decrease in gap spacing, because of an
effective beam energy for etching, which turned out to be 5 keV.
enhanced local pressure. Because the smaller nozzle distance
CNTs can also be cut with modest sample currents of 10−80
shows faster etching rates and lower sample currents, the
pA, as shown in Figure 5. Although the etching process is
results from Table 3 lead to the conclusion that the local
slower at lower currents, one benefit of a low current is that the
pressure is responsible for the increased etching rate.
etching process is more selective. With currents of less than 10
pA a CNT could not be etched in a reasonable time of less than
3

4. a) b)
Figure 5 – Cutting a CNT, before (a) and after (b)
Gap(μm) Gap Image CNT Before Cut CNT After Cut
87
328
Figure 6 – Demonstration of gas delivery system fixed to a nanomanipulator that allows precise positioning of the nozzle gap to the
sample, with a range of 50 μm to 1 mm and more. The resulting nozzle proximity results in improved CNT etching capabilities.
References
NanoBot Sales
[1] Images provided by Phip Rack, The University of Tennessee For product inquiries please contact:
at Knoxville. Dr. Ray Eby
[2] M.C. Peigon, Ch. Cardinaud, and G. Turban, J. Appl. Phys. 70(6), 15 phone: 312-545-6527
September pg. 3314-3323 (1991). e-mail: nanoray@sbcglobal.net
[3] S.J. Randolph, J.D. Fowlkes, P.D. Rack, Critical Reviews of Solid For direct contact:
State and Materials Sciences, Vol. 31, p. 55-89 (October 2006) Xidex Corporation
[4] Kohlmann, K., Thiemann, M. and Bringer, W. E-beam induced X-ray 8906 Wall Street, Suite 703 fax: 512-339-9497
mask repair with optimized gas nozzle geometry. Microelectronic Austin, Texas 78754 e-mail: info@xidex.com
phone: 512-339-0608 web: www.xidex.com
Engineering 13 (1991), 279.
Xidex Corporation © 2011, Xidex Corporation. All right reserved. NanoBot, Xidex, and
Xidex manufactures and sells the NanoBot ® system, an easy-to- the Xidex logo are trademarks of Xidex Corporation. Other
use, highly versatile, user-programmable nanomanipulator built for trademarks are property of their respective owners. Product
use inside scanning electron microscopes (SEMs) and focused ion specifications and descriptions in this document are subject to
beam (FIB) tools. The NanoBot system transforms a SEM or FIB change without notice
into a workshop for nanodevice fabrication and testing. Xidex 110313
Corporation was founded in 1997 as an Austin -based Texas
Corporation by Vladimir Mancevski, President and Chief
Technology Officer and Dr. Paul F. McClure, CEO.
4