|
By Adam Hartman, Zyvex Instruments
Download a printable copy here.
Introduction
Carbon nanotubes’ (CNTs) unique physical properties
of high strength, low weight, high thermal and electrical
conductivity, and high surface area have made them a primary
focus of nanotechnology research,, But before CNTs can be
utilized in engineering systems, their fundamental material
properties must be well understood. Previously, nanotubes
have been mechanically characterized as bulk bundles, or as
composites with polymers. With the S100 Nanomanipulator System
and custom designed micro electromechanical systems (MEMS)
test structures, mechanical investigation of individual CNTs
is possible. Nanotubes can be manipulated in 3D, attached
to MEMS devices, and tested in-situ using the S100 system
and a scanning electron microscope (SEM). This application
note reviews the methods for attaching CNTs to MEMS devices
and measuring their force-deflection characteristics.
Materials
Many possibilities exist for measuring the mechanical
response of CNTs. For this application note, multi-walled
nanotubes were manipulated with tungsten probes and attached
to surface micromachined polysilicon MEMS devices.
Multi-Walled CNTsNanotubes were received in suspension from
the University of Kentucky (www.mrsec.uky.edu/direct/grulke.htm).
To provide accessible free ends, the CNTs were rinsed using
detergents and stored in high-purity methanol. A drop of the
methanol/CNT mixture was placed on a glass slide, and allowed
to dry. A separate glass slide with a small amount of carbon
tape adhesive at its edge was prepared and dragged over the
dried CNTs, creating available tubes as shown in Figure
1.
Figure 1. Multi-walled CNTs extending
over the edge of a glass slide, with a probe approaching at
the bottom
Tungsten Probes
Standard microelectronic testing probes were installed as
end-effectors on the S100 nanomanipulator System. However,
finer tipped Zyvex NanoSharp™ probes will enhance dexterity
for CNT samples that are densely packed.
Polysilicon MEMS Devices
Many types of MEMS test structures can be used for characterizing
CNTs. A simple approach is to use calibrated atomic force
microscope (AFM) cantilevered beams. In this custom application,
commercially fabricated polyMUMPs (www.memscap.com/memsrus/crmumps.html)
devices were used. A 3D model of one such tensile testing
device is shown in Figure 2.
Figure 2. Three-dimensional model
of a thermally actuated MEMS device for tensile testing of
CNTs
Current supplied through the connection pads created differential,
resistive heating in the thermal actuator. Thermal expansion
caused the released actuator to deflect toward the right of
the image. With a CNT connected between the actuator and movable
gauge flexure, relative displacements of the tube ends were
measured in the SEM against the stationary deflection metric.
Zyvex MEMS engineers can assist in designing other types
of micromechanical test beds, utilizing methods like electrostatics
for actuation and integrated sensing. This application note
presents a simpler technique, using one S100 nanomanipulator
in place of the thermal actuator.
Setup
The 3D nature of this application requires that the
installation and setup of the S100 System, CNT sample, and
MEMS device be considered carefully. The nanomanipulator’s
large sample area allows great flexibility in mounting the
CNT and MEMS samples. A standard SEM stub may be modified
to provide access to both samples by simply filing one edge
of the stub at 45 degrees. The nanotube sample may be bonded
to the top of the stub using carbon tape or epoxy, and allowed
to extend over the filed edge. The MEMS sample can then be
attached to the inclined surface, directly below the overhanging
CNTs. Silver filled epoxy may be used at the corners to ground
samples to the stub.
Due to the S100’s large travel area, multiple MEMS
and CNT samples can be used without breaking vacuum and exchange
mounts. The probe was installed into the S100 quadrant facing
the MEMS chip, and the manipulator was moved in all three
dimensions to ensure that the samples were placed within the
work volume of the system. After this check, the probe was
centered near the desired CNT sample, the vacuum chamber was
closed, and the SEM activated.
CNT Attachment, Manipulation, and Characterization
Using the S100 System, it was possible to select an individual
CNT from the prepared sample, attach it to the probe using
electron beam induced deposition (EBID), remove that tube
from the sample, and attach it to a tensile testing structure.
Attachment
Trace gases present in the SEM vacuum chamber can be deposited
onto surfaces being struck with the electron beam. The tungsten
tip was positioned against an exposed CNT end and the electron
beam was focused, in spot mode, at the point where the tube
and tip meet. Electron beam deposits “welded”
the CNT rigidly to the probe. The tube was then pulled from
the CNT sample and manipulated freely in three dimensions.
The Zyvex application note “Attaching a Nanotube to
a Zyvex S100 Nanomanipulator End Effector” provides
an excellent, detailed description of this process. An example
of a tube being extracted from a CNT sample using the S100
is shown in Figure 3.
Figure 3. Tungsten probe end-effector
for the S100, used to pull a single tube from the CNT sample
The image has been color inverted for clarity. Figure 4 shows
a close-up image of a nanotube that has been EBID-welded onto
a tungsten probe.
Figure 4. Close-up image of the electon
beam weld used to attach a CNT to a tungsten probe
Manipulation
After the nanotube was removed from the sample, the
probe was further retracted to place it (approximately) over
the MEMS device. The focus and position of the SEM image was
changed until the desired MEMS site was clearly visible, then
the probe was lowered with the S100 Nanomanipulator until
it too came into focus. Using the S100, it was possible to
position the free end of the tube at any location on the MEMS
structure. In this application note, the tube was attached
to the movable gauge flexure.
As the tube end came into contact with the polysilicon surface,
an electrostatic attraction suddenly held the CNT against
the MEMS device. To test exactly where the tube was in contact,
the fine positioning controls of the S100 were used to move
the probe back and forth, paying attention to the region of
the CNT over the MEMS surface. As the tube was moved, it appeared
to rotate around a fixed spot, which defined the point at
which the two surfaces touched.
In spot mode, the electron beam was focused at this point,
welding it in place. The beam dwell time was equivalent to
that for the weld onto the probe. This ensured that the tube-to-MEMS
weld was as strong as the tube-to-probe weld. Figures
5 and 6 show a nanotube being positioned
on a MEMS device and welded into place, respectively.
Figure 5. Free end of a CNT in contact
with a polysilicon surface
Figure 6. CNT welded between the movable
MEMS flexure and the tungsten probe
Figure 7 shows a 3D AFM scan of the region
where a nanotube was welded to the surface.
Figure 7. AFM image showing the smooth
weld bonding a nanotube to a surface
Characterization
To measure the force-deflection characteristics of
the attached nanotube, the S100 was moved away from gauge
flexure. As the probe retracted, tension was created in the
tube. This force transferred to the flexure and began to deflect
it. The SEM was used to capture images between S100 movements,
making sure to include the stationary deflection metric in
each picture. Figures 8 a-d show a sequence
of such images captured while moving the MEMS device with
an attached CNT.
Figure 8 a-d. Image sequence showing
a MEMS device beign deflected by a CNT attached to an S100
Nanomanipulator
Beam bending analysis for approximate flexure geometries,
or finite element simulations for more precise results, can
be used to find the spring constant relating force to deflection
for your particular gauge design. For the flexure used in
this application note, analytical and simulated results agreed
on a spring constant of 0.63 µN/µm. Based on this
constant, and digital image analysis of flexure deflections,
a tensile force of 2.4 µN was applied to the tube.
Conclusions
Although carbon nanotubes are a promising material
for future nanotechnology projects, they still require some
very fundamental material properties research before they
can be included in engineered systems. However, such characterization
has mostly been done on bulk clusters of tubes, or on tubes
mixed with polymers.
The challenges to CNT measurement are numerous. Their unique
aspect ratio makes them difficult to study as individual structures.
Diameters on the order of tens of nanometers require a fine
positioning capability in three dimensions to select a single
tube, while lengths of up to tens of microns require large
travel for manipulation and deflection. Their overall size
makes optical placement and measurement of tubes impossible,
so an SEM-compatible system is essential. During the EBID
process, specimen drift would allow a misplaced weld to release
a nanotube on the surface, meaning that a very stable manipulator
must be employed. The Zyvex S100 Nanomanipulator fits these
stringent requirements, and using this system, it is possible
to conduct ground-breaking mechanical experiments on individual
nanotubes.
back to top
| ©
Copyright 2008, Zyvex Instruments. All Rights Reserved. |
|
