Carbon Nanotube Scanning Probes Assembled by
Nanomanipulation using Electrothermal Microgrippers
KENNETH CARLSON1, KARIN N. ANDERSEN1, VOLKMAR EICHHORN2, DIRCH. H.
PETERSEN1, KRISTIAN MØLHAVE1, IAN Y. Y. BU3, KENNETH B. K. TEO3, WILLIAM. I. MILNE3,
SERGEJ FATIKOW2 AND PETER BØGGILD1*
1NanoDTU,
Department of Micro and Nanotechnology (MIC), Technical University of Denmark, DK2800 Kgs. Lyngby, Denmark,
2Division
Microrobotics and Control Engineering, University of Oldenburg, 26111 Oldenburg, Germany
3Dept.
of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK
RECEIVED DATE
PACS: 07.07.Df; 81.05.Tp, 85.80.Fi, 87.80.Mj ??
Keywords: nanomanipulation, migrogrippers, carbon nanotubes, AFM
*
corresponding author email: boggild@mic.dtu.dk
1
ABSTRACT
Functional devices can be directly assembled using microgrippers with an in-situ electron microscope.
Two simple and compact silicon microgripper designs are investigated here. These are operated by
electrothermal actuation, and are used tp transfer a catalytically grown multiwalled carbon nanotube
from a fixed position on a substrate to the tip of an atomic force microscope cantilever, inside a
scanning electron microscope. Scanning of high aspect ratio trenches using the nanotube supertip shows
a significantly better performance than standard pyramidal silicon tips. Based on manipulation
experiments as well as a simple analysis, we show that shear pulling (lateral movement of the gripper) is
far more effective than tensile pulling (vertical movement of gripper) for the mechanical removal of
carbon nanotubes from a substrate.
2
Introduction
Carbon nanotubes (CNTs) and nanofibers are high-aspect ratio structures with excellent mechanical
properties. Through the use of catalytic growth, these structures can be precisely (in terms of position an
dimensions) grown on a flat substrate
1, 2.
Similarly for semiconducting nanowires, epitaxial growth
enables these structures to be engineered with atomic precision 3, 4.
Carbon nanotubes are already commercially available as scanning probe tips, and for large scale
production of such CNT probes, schemes for wafer-scale assembly or growth of CNTs directly on the
microcantilever has been explored by several groups
5.
Two-dimensional arrays of CNTs are
straightforward to create by defining catalytic metal particles with electron beam lithography or
nanoimprint, followed by plasma enhanced chemical vapour deposition (PECVD) 1. [please add a
reference here for nanoimprint]:
S.M.C. Vieira, K.B.K. Teo, W.I. Milne, O. Groening, L. Gangloff, E. Minoux, and P. Legagneux. "Investigation of
field emission properties of carbon nanotube arrays defined using nanoimprint lithography", Applied Physics
Letters 89, 022111 (2006).
The integration of CNTs and nanowires on microcantilevers and more complex three-dimensional
structures requires more elaborate schemes to maintain good control over the position, dimensions and
orientation of the CNTs 6, due to the difficulty of performing lithography on non-flat surfaces
Nanomanipulation using single or multiple tool tips provides a flexible alternative towards integration.
Although it is less suitable for mass fabrication, it is a route to transfer of a variety of structures to a
micro-electro-mechanical system (MEMS) device with a short turn-around time, which is not very
material- and process specific, and is well suited for proof-of-concept demonstrations.
Manipulation of a CNT from one location to another can be realized inside a scanning electron
microscope. Such a microscope offers nanoscale resolution with satisfactory frame rates. Also, the
‘macroscale’ working distance (few mm) allows a manipulator system to be incorporated, and
3
‘macroscale’ samples to be worked on. Complex multiprobe systems, allowing two or more scanning
probe tips to be moved independently inside a scanning electron microscope, have been used to
characterize the mechanical properties of CNTs and nanowires
7, 8
, but these tend to be expensive and
complicated.
Here, we demonstrate picking of an individual CNT from a two-dimensional, ordered array, grown by
plasma-enhanced chemical vapor deposition (PECVD) 1. Although we here term the structures CNTs,
we note that such large (100+nm diameter, 5um tall), CVD-grown nanostructures are partly graphitized
but hollow, and contain residues of the catalytic material at its tip [1]. The advantage of such a
“component bank” with well-defined positions of all the CNTs is that a robotic manipulation system,
once calibrated, could locate a specific nanotube automatically, which would then allow for automatic
construction of prototype devices. Given suitable development of a special control system architecture
for process automation, such an approach has perspectives for becoming a routine or even smallscale
production process for integration of nanotubes, -fibers and -wires in microelectromechanical systems
(MEMS) devices.
One key problem is to provide a suitable tool, small enough to be aligned to a sub-100 nm diameter
CNT, while mechanically sufficiently strong to detach and pick up the CVD-grown CNT from its
substrate. This tool must then be able to release the CNTs again at the desired position. Although,
several groups have previously used microfabrication to make microgrippers for manipulation
9-11
and
several commercial vendors exist today, little work has been reported on practical uses of such tools.
The concept of nanotweezers was first demonstrated by Kim and Lieber 12, who gripped nanowires and
nanoparticles between two electrostatically biased CNTs, which were attached to a glass capillary. Using
a five-electrode microcantilever layout9, submicron grippers, capable of both opening and closing
without applying a voltage directly between the arms, were used to manipulate silicon nanowires
Recently, we demonstrated a compact, electro-thermal 3-beam microgripper fabricated in gold
11,
13.
with
the added possibility of a piezoresistive readout of the deflection.
4
We present here two monolithic high-aspect ratio electrothermal microgrippers and demonstrate the
removal of as-grown CNTs from an ordered 2D array, followed by the transfer and controlled placement
of the CNT onto the tip of an AFM probe. The supertip-enhanced AFM probe is capable of resolving
deep trenches significantly better than standard pyramidical Si AFM cantilevers. We discuss the
prospects of developing a feasible prototyping process based on pick-and-place manipulation techniques
using a dedicated tool.
The microgrippers are fabricated in single-crystalline, highly n-doped silicon, as this material offers an
attractive combination of mechanical and electrical properties. The 5 µm thick silicon device layer is
electrically isolated from the rest of the chip by a 1-2 µm silicon dioxide layer. (Is this SOI???) With a
photolithographic line width of 2 µm, we can fabricate high aspect ratio microcantilevers (see Figure 1)
designed to avoid out-of-plane deflection of the final microgripper structures.
After defining the silicon microgripper structures via lithography and reactive ion etching (SF6/O2
based chemistry), a thin, conformal low-stress silicon nitride layer is deposited on both sides of the
wafer using low pressure chemical vapour deposition (LPCVD). The silicon nitride on the back side of
the wafer is then lithographically patterned and used as an etch mask for anisotropic potassium
hydroxide etching of the substrate. The etch-stop here is the 1-2 µm buried oxide on the front side of the
wafer. The nitride mask is then removed in phosphoric acid, and then the gripper is releasedusing
buffered hydrofluoric acid. Two microgripper designs were tested: the symmetric 3-beam microgripper
(Figure 1 a) described in Ref
11,
and an asymmetric ribcage (ARC) microgripper (Figure 1 b). The 3-
beam microgripper can open and close the two arms individually, and has the possibility of in-situ
measurement of the strain during manipulation due to the piezoresistive properties of the material
11.
The 3-beam design furthermore allows actuation of each arm by electrothermal expansion, electrostatic
force or a combination of these, and hence provides a several options for operation. Electrothermal
actuation is achieved by passing a current through the two outer beams of each arm, which causes these
5
to expand more than the inner beams, resulting in deflection of the arm. Electrostatic actuation is
achieved by electrically biasing the arms.
A less versatile, but more powerful (in terms of generated force) design is the asymmetric ribcage
design shown in Figure 1b. While only offering a single operation mode, the ARC design provides
significantly more gripping force than the 3-beam microgripper. The asymmetric layout with one rigid
end-effector also facilitates controlled detachment of CNTs from their fixed position on the surface,
since the rigid side arm provides a stable base against which the CNT can be pressed. When a current is
passed through the ribs, they expand, and push the wide central beam outwards. Due to the anchoring
via the narrow beam, the left arm closes the gap.
Figure 1. a) SEM image of the 3-beam microgripper and b) asymmetric ribcage microgripper (ARC).
The nanohandling station
14
which incorporates the grippers is mounted in the vacuum chamber of a
scanning electron microscope (SEM). The nanohandling station consists of two different manipulators
for coarse and fine positioning of microgripper and CNT sample. The first manipulator is a three-axis
6
Kleindiek micromanipulator equipped with a microgripper holder. This micromanipulator offers a
incremental positional movements of 5 nm for the rotational axes and 0.25 nm for the linear axis. It is
used for the coarse positioning of microgripper to the CNT sample, bringing the microgripper close to
the CNT and thus into the workspace of the nanopositioning stage.
The second manipulator is a Physik Instrumente nanopositioning stage with three degrees of freedom
carrying the CNT-sample. The resolution of the nanopositioning stage is presently limited by the
electronics (16-bit D/A converter of the control module) and is 1.55 nm for each linear axis. The full
range of the nanopositioning stage is 50 µm. The nanohandling station is placed onto the SEM
positioning stage of a LEO 1450 SEM. The SEM positioning stage is used to bring various parts of the
nanohandling station into the view of the SEM.
The CNT samples were produced using plasma enhanced chemical vapour deposition as described in
Ref. 1.
The actuation characteristics (ie. gap change versus bias voltage) of the microgrippers was measured
using a high-magnification Navitar optical lens system. Filtering and image recognition algorithms make
it possible to continuously track the relative motion of the end-effectors from the optical images, with an
accuracy down to roughly 2 nm. In Figure 2, actuation curves are shown for 200 µm long 3-beam
grippers in ambient air. The deflection of the electrothermal microgrippers is approximately parabolic at
low actuation voltages 11, approaching linear for larger voltages.
The two arms of the 3-beam microgripper can be open as well as closed individually, depending on
the configuration of applied potentials on the six gripper arms. Closing by electrothermal actuation is
achieved with the configuration (V,0,0,0,0,V), indicating a bias voltage applied to the outer beams,
while the four inner beams are grounded. This causes twice the current density to flow in the outer two
beams, leading to resistive heating which then expands the outer beams more than the inner beams
11.
The actuation curves for two similar 3-beam microgrippers are shown. Typically, a 1 µm gap change is
achieved with a bias voltage of 15-20 V. A deviation of 10-30% of the maximal actuation from gripper
7
to gripper is typical, and is due to the variations in linewidth (due to lithography and etch) which
ultimately affects the electrical resistance and temperature distribution of the final structure. Note that
thermal expansion and thermal conductivity are temperature dependent properties, which must be taken
into accouint in the analysis15. Opening of the microgrippers (not shown) is achieved by applying a
voltage to the inner beams, while grounding the outer 4 beams, (0,0,V,V,0,0).
The two actuation curves for ARC grippers, in comparison, show a deflection of 1 µm at a bias
voltage of 5 V, much lower than the 3-beam microgripper; the current used by the ARC gripper (11mA),
however, is much larger than for the 3-beam actuator.
Perhaps you wish to compare the power of the two gripper systems to achieve the same 1um
deflection
For manipulation of delicate structures with dimensions far smaller than 1 µm, one highly important
property of the microgripper is minimal out-of-plane deflection during operation of the microgripper, to
avoid the need for continuous and laborious realignment of the microgripper during closing or opening.
The high-aspect ratio of the microgripper structure presented here, in combination with a simple,
compact design, effectively eliminates out-of-plane deflection reported for previous electrothermal
microgrippers 11, 16.
8
Figure 2. Gap change as a function of bias voltage for two 3-beam microgrippers (top) and two ARC
microgrippers (bottom), with indications of the configurations of bias voltage on the electrodes. The
dark areas on the illustrations show the areas that are heated most in the structures.
The three-beam high aspect ratio silicon microgrippers (see Figure 1a) were used to detach CNTs
from a 2-dimensional ordered array with a spacing of 10 µm. Figure 3 shows a picking sequence of such
a manipulation attempt. First, the CNT substrate and the microgripper are brought into close proximity
(Figure 3a). The microgripper is then moved such that it could grab a CNT by the base (Figure 3b), and
is closed around the CNT. A large force is then applied to the microgripper and translated in order to
pull off the CNT, by shear pulling. The optimal strategy for picking off rigid nanotubes or nanowires
was found to be “shear pulling” where the microgripper is inducing a bending moment in the CNTs by
moving it sideways, as opposed to ”tensile pulling”, as predicted theoretically in Ref 17.
Could a reason be that with tensile pulling, the CNT can break anywhere?
9
In Figure 3c the CNT has been released from the growth substrate, and the microgripper is free to
move the CNT away. When the microgripper is opened to release the CNT (Figure 3d), the CNT is seen
adhering to the sidewall of the microgripper. Due to the CNT rigidity and its strong adhesion to its
substrate, this experiment was only successful in a few cases; in most cases the CNTs were bent and not
detached. We were unable to break off any CNT by tensile pulling (vertically), regardless of the gripping
force.
Figure 3 Pick and place sequence for CNTs from a 2-dimensional ordered array using a 3-beam
electrothermal silicon microgripper. The arrows indicate the position of the CNTs which are spaced by
10 µm. a) The microgripper and the CNT array are brought in close contact to each other. b) The
microgripper is aligned to the CNT, and a voltage of 20 V is applied to the microgripper. c) The
microgripper is then moved sideways to pull off the CNT using shear pulling. d) After opening, the CNT
adheres to the end-effectors of the microgripper.
10
Figure 4. Pick and place of CNTs from an ordered array with the asymmetric ribcage silicon
microgrippers. The steps (a-h) shows the (a-b) aligning of gripper to CNT, (c ) gripping, (d) detachment,
(e) aligning of CNT to AFM probe, (f) attachment, (g-h) mechanical test.
Figure 4 shows the detachment of a CNT from a substrate, followed CNT placement onto an AFM
cantilever.
The microgripper and CNT were adjusted to be almost parallel to each other, and
perpendicular to the field of view of the SEM. In Figure 4a, positioning between microgripper and CNT
was achieved using the three-axis micromanipulator. Fine adjustments of the microgripper and CNT was
then performed using the nanopositioning stage. The alignment of the gripper with the CNT was verified
by deflecting the CNT with the microgripper (Figure 4b). The microgripper was then closed around the
CNT and moved sideways to shear the CNT from its substrate (Figure 4c). The CNT was successfully
removed from the substrate (Figure 4d) and transported to the tip of an AFM probe (Figure 4e). The end
of the CNT was then placed on top of the AFM tip (Figure 4f). Electron beam induced deposition
(EBID) of carbon was then used to weld the CNT to the AFM tip. The electron beam was focussed onto
the small contact area of CNT and AFM tip for 30 minutes to form a mechanical joint between CNT
and AFM tip. The CNT can now be released by the microgripper (Figure 4g). Finally, the strength of the
CNT joint was tested by deflecting it with the microgripper (Figure 4h).
11
Figure 5. AFM scan of an array of 2.5 m deep grooves obtained with a CNT-enhanced AFM cantilever
(A) and a pyramidal silicon AFM cantilever (B). The inset (C) shows a SEM image of the grooves on
the same lateral scale. The sidelength of the AFM scans is 10 m. The lateral scale on the AFM images
is scaled by a factor of two compared to the lateral axes and to the inset.
In order to test the CNT supertip, AFM scans of high-aspect ratio etched Si-grooves were performed.
The grooves are roughly 2.5 m deep. From SEM images, the width of the grooves was determined to
be about 570 nm and the periodicity approximately 1 µm. Figure 5B shows an AFM scan acquired using
a standard AFM probe (Budget Sensors Tap300). Figure 5A shows the same grooves measured using the
CNT supertip. The Nanotec AFM was running in dynamic mode with a scanning rate of 0.493 Hz and
the scan area was about 10 x 10 µm with a resolution of 512 x 512 pixel for both scans. The CNT
supertip clearly improves AFM imaging of structures due to its high aspect ratio. The standard AFM
probe provides a groove-depth of only 300 nm whereas the CNT supertip scans the grooves to 1.2 µm,
and also measure the 400 nm width of the grooves correctly. The periodicity of the grooves determined
12
from the AFM images is 1.12 µm. This is slightly more than from the SEM images due to calibration
errors in the used SEM and AFM systems.
The ability of a microgripper to successfully break off nanotubes or nanofibers depends on the fracture
strength of the nanostructure as well as the method of gripping.
Figure 6. Detachment force estimates for pull (vertical) and shear (lateral) as a function of the CNT
radius. For both types the required detachment force is plotted for strong (100 GPa) and fragile (1 GPa)
nanorods as a function of the radius. The two dashed lines are the available gripping force with the two
investigated microgrippers. The insets show SEM images of tensile and shear pulling.
13
As discussed in Ref.
17
a CNT can be detached by a gripper by pulling in the longitudinal direction,
referred to as “tensile pulling”, or by pulling in the lateral direction, referred to as “shear pulling”. In
both cases, detachment occurs when any part of the CNT reaches a stress corresponding to the fracture
strength  f . The force required to induce a fracture in the CNT is referred to as its detachment force.
While the fracture strength of the CNTs investigated in this work is not known, analysis of images
such as Figure 4c, which was recorded immediately prior to breaking, gives radii of curvature
Rcurv  4  8 μm and CNT radii of R  150 nm - 300 nm . The strain at the edge of the nanotube prior to
breaking is then given by  f  R / Rcurv , which can be rewritten as Rcurv  RE /  f , where E   /  is
the Youngs modulus. Taking E to be in the range of 0.1 – 1 TPa 18, 19, the fracture strength is between 2
GPa and 90 GPa, which is comparable to the wide range of values found in literature. Ruoff and coworkers
20
reported fracture strengths between 11 and 63 GPa for CVD grown multiwalled CNTs
assuming that only the outermost shells carry the load. Barber et al
21
surprisingly found that CVD-
grown CNTs exhibit an average fracture strength of more than 100 GPa, while arc-discharge multiwalled carbon nanotubes and single-walled carbon nanotube bundles were found to be about 3 times
weaker, despite the CVD-grown multiwalled CNTs are known to have a significant amount of structural
defects.
To cover the full range of CNT mechanical properties, we consider two limiting cases of fragile CNTs
with a relatively small fracture strength f = 1 GPa and strong CNTs with a fracture strength of
 f  100 GPa , and compare the gripping force delivered by the two microgrippers, with the detachment
force as depending on fracture strength and diameter
17.
In comparison, silicon nanowires have a yield
strength of the order of 7-12 GPa 17, 22, and is between the limits we are investigating here.
As illustrated in the inset of Figure 5, the nanotube being detached by shear pulling corresponds to the
case of a guided cantilever in Ref. 23. A relation describing the detachment force Fd at which the bending
14
moment of the CNT of radius R reaches the maximal possible value, M f  Fd L / 2 at the base of a
guided cantilever, can be written:
Fd , shear 
 R 3 f  2
4
1
 
 L Lg

 .

(1)
Here L is the length of the CNT from substrate to gripper, and Lg is the length of the CNT being held by
the end-effectors, providing the necessary moment to twist the nanotube as illustrated in the inset in
Figure 6. In our experiments, both L and Lg are of order 1 µm.
In Figure 6 the force required for detaching a cylindrical rod by shear pulling is plotted as a function
of radius and yield strength of the rod for the two limiting cases (full lines). In comparison, the force
required to detach the CNF by tensile pulling, Fd , pull   f  R2 /  , is plotted as well (dotted lines). Even
by assuming a maximal friction coefficient of =1, shear pulling requires smaller forces than tensile
pulling in the range of radii investigated.
According to calculations made using the finite element simulation software (CoventorWare) with
boundary conditions matched to the experimental conditions, the gripping force is 1 N for the 3-beam
microgripper and 25 N for the ribcage microgripper, which is shown in Figure 6 as horizontal lines.
The crossing points indicate for which radii and strength of nanorods, the two grippers are capable of
detachment using shear and tensile pulling.
Hence, the 3-beam microgripper is capable of detaching fragile (   1 GPa) rods with diameters up to
140 nm by shear pulling, and strong rods (   100 GPa) with diameters up to 40 nm. The ARC
microgripper, is however capable of detaching strong rods up to 100 nm in diameter by means of shear
pulling, and should also be able to pick fragile rods with diameters up to 400 nm.
If the CVD-grown CNTs investigated here are considered to be rather fragile ( < 10 GPa), the crude
model is roughly consistent with the experimental observations, i.e. detachment diameters of 500 nm
and 100 nm for the ARC and 3-beam microgripper respectively. To obtain more reliable numbers from
15
such a model, the exact geometry of the gripping expierments and the CNT should be considered.
Moreover, precise measurements of the CNT modulus and strength should be undertaken, for instance
using the method described in Ref.
22.
Smaller-diameter CNTs were easily broken off by the ARC
microgripper, simply by pushing sideways with the side of the beam. Tensile pulling (see inset in Figure
6) was not possible in detachment of CNTs in our expierments.
We have compared here two types of silicon microgrippers and demonstrate picking of CVD-grown
CNTs from an ordered array, and placing one such CNT onto the tip of an AFM probe. The CNT was
welded to the cantilever using EBID and then used as a high-aspect ratio AFM tip for scanning of deep
trenches. A first-order analysis shows the merits of shear pulling versus tensile pulling for CNT
detachment, and indicates that the maximal gripping force for the two microgrippers is well matched to
the force required to detach CNTs.
This demonstrates for the full process flow for the assembly of a simple devices which can be
automated in the future. The CNTs were grown in an ordered array with well-known positions,
dimensions and direction, which makes it possible for a robotic system to locate, detach, move, position
and fix a nanostructure on an AFM probe or another device, inside an SEM without little or no
supervision. While automated manipulation using tools such as microgrippers will remain a slow
process in comparison to catalytic growth of CNTs directly on the target device, mechanical
manipulation does offer a great deal of flexibility in terms of prototyping: pick and place can be done
entirely as a post-process, i.e. on a finished device, allowing a wide range sizes, shapes and materials of
the nanostructures to be positioned precisely onto nanodevices., and is ideal for rapid prototyping.
I deleted the next paragraph as it is not a good idea to tell other people what you are working on
for your next publication!
16
Acknowledgement
Discussion with Jakob Kjelstrup-Hansen and Ole Hansen is appreciated. The project was supported by
EU grants NANOHAND (IP 034274), NANORAC (STREP 013680) and the talent-project NAMIC
from the Danish Technical Research Agency through grant No. 26-04-0258.
Supporting information available.
AVI movie showing the two described manipulation experiments are available on the website.
17
Additional note, for reviewer: Estimate of detachment force for shear pulling
Shear pulling. By bending a wire it is possible to break it at the base where the stress is largest. The
nanowire strain is proportional to the distance z from the center axis,    z / Rcurv , so with    E we
get at the surface the curvature at which the wire breaks ( z  R ):
Rcurv 
RE
f
where Rcurv is the radius of curvature at which the nanowire breaks. The bending moment is related to
the radius of curvature through the Youngs modulus E, and the geometrical stiffness, the plane moment
of inertia, I   d 4 / 64   r 4 / 4 :
Mf
I f  R 3 f
1
EI

Mf 


Rcurv
EI
Rcurv
R
4
If the gripper is holding the CNT firmly, and the CNT will be clamped in both ends as shown in the
inset, corresponding to the “fixed-in-one end, guided in the other end” case in (Roarks, formulas for
stress & strain), which tells us that the moment at the ends is given by M  FL / 2 , which gives the
force Fbend required to bend the CNT until it reaches the breaking point:
Fbend 
2M f
L

 R 3 f
2L
However, the gripping force is not just used to bend the CNT, it is also maintains the moment and thus
prevents the CNT from twisting the gripper open. In the inset of figure 6 the gripper has only the length
of the end-effector surfaces Lg to generate the moment M max , so force Fgrip is required simply to keep
the gripper end-effectors together:
Fgrip 
M max
Lgripper
Hence, the gripper is both deflecting the nanowire using the force Fbend , and twisting the nanowire with
a force Fgrip , guiding as opposed to merely deflecting. The total force can be estimated by adding the
two contributions:
Ftotal 
2 1

Mf  
L L
L
Lg
g

2M f
Mf
  R 3 f
 
4

2 1
 
 L Lg



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