A simple and reliable catalyst patterning technique combined with electric-field-guided growth is utilized to synthesize a sharp and high-aspect-ratio carbon nanocone probe on a tipless cantilever for atomic force microscopy. A single carbon nanodot produced by an electron-beam-induced deposition serves as a convenient chemical etch mask for catalyst patterning, thus eliminating the need for complicated, resist-based, electron-beam lithography for a nanoprobe fabrication. A gradual, sputtering-induced size reduction and eventual removal of the catalyst particle at the probe tip during electric-field-guided growth creates a sharp probe with a tip radius of only a few nanometers. These fabrication processes are amenable for the wafer-scale synthesis of multiple probes. High resolution imaging of three-dimensional features and deep trenches, and mechanical durability enabling continuous operation for many hours without noticeable image deterioration have been demonstrated

Chen, I-Chen

Chen, Li-Han

Daraio, C.

Jin, Sungho

Lal, Ratnesh

Orme, Christine A.

Quist, Arjan

Ye, Xiang-Rong

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS 88, 153102 2006Extremely sharp carbon nanocone probes for atomic force
microscopy imaging
I-Chen Chen, Li-Han Chen, Xiang-Rong Ye, Chiara Daraio, and Sungho Jina
Materials Science and Engineering, University of California, San Diego, California 92093-0411
Christine A. Orme
Chemistry and Material Science, Lawrence Livermore National Laboratory, Livermore, California 94550
Arjan Quist and Ratnesh Lal
Neuroscience Research Institute, University of California, Santa Barbara, Santa Barbara, California 93106
Received 25 October 2005; accepted 13 March 2006; published online 10 April 2006
A simple and reliable catalyst patterning technique combined with electric-field-guided growth is
utilized to synthesize a sharp and high-aspect-ratio carbon nanocone probe on a tipless cantilever for
atomic force microscopy. A single carbon nanodot produced by an electron-beam-induced
deposition serves as a convenient chemical etch mask for catalyst patterning, thus eliminating the
need for complicated, resist-based, electron-beam lithography for a nanoprobe fabrication. A
gradual, sputtering-induced size reduction and eventual removal of the catalyst particle at the probe
tip during electric-field-guided growth creates a sharp probe with a tip radius of only a few
nanometers. These fabrication processes are amenable for the wafer-scale synthesis of multiple
probes. High resolution imaging of three-dimensional features and deep trenches, and mechanical
durability enabling continuous operation for many hours without noticeable image deterioration
have been demonstrated. © 2006 American Institute of Physics. DOI: 10.1063/1.2193435The key component of atomic force microscopy AFM
is the probe tip, as the resolution and reliability of AFM
imaging is determined by its sharpness, shape, and the nature
of materials. Standard commercial probes made of silicon or
silicon nitride have tips of a pyramid shape, which do not
allow easy access to narrow or deep structural features, and
generally have a relatively blunt tip radius on the order of
10 nm. The high-aspect-ratio geometry and excellent me-
chanical strength of the carbon nanotubes CNTs offer ad-
vantages for imaging as an AFM tip. Due to their excellent
physical and chemical properties,1–3 CNTs have been at-
tached onto pyramid tips by various approaches4–8 as well as
directly grown using thermal chemical vapor deposition
CVD.9–11 The attachment methods are manual and time
consuming, and often result in nonreproducible CNT con-
figuration and placement. While the thermal CVD approach
can potentially lead to the wafer-scale production of AFM
tips, the number, orientation, and length of CNTs are difficult
to control.
An important aspect to consider in utilizing CNT probes
is that the single-walled nanotube probes with a desirable
small diameter tend to exhibit an inherent thermal vibration
problem if the length is made reasonably long, and hence
they cannot be used to trace deep structural profiles. On the
other hand, multiwalled nanotubes such as those synthesized
in dc plasma enhanced CVD dc-PECVD12–15 have a larger
diameter in the regime of 20–100 nm and hence exhibit im-
proved mechanical and thermal stability, but the catalyst par-
ticle at the nanotube probe tip or the natural dome structure
in a nanotube grown by a base growth mechanism has a
finite radius of curvature, which limits the AFM resolution.
Recently, two approaches have been employed to fabri-
cate multiwalled nanotube probes on tipless cantilevers by
aAuthor to whom correspondence should be addressed; electronic mail:
jin@ucsd.edu
0003-6951/2006/8815/153102/3/$23.00 88, 15310
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to dc-PECVD.16,17 These approaches, however, require some-
what complicated, multiple patterning steps. The catalyst
dots in both approaches are patterned by the lift-off of the
spin-coated polymethyl methacrylate PMMA layer follow-
ing typical electron- e- beam lithography. A reliable and
uniform spin coating of a resist layer generally requires a
relatively large area, and is difficult to achieve for a tipless
cantilever, which has a narrow and elongated geometry. In
one of these reports,16 patterned catalyst dots were formed
before the fabrication of the cantilevers, but the catalyst had
to be protected by the PECVD-deposited Si3N4 layer in order
for the catalyst dots to survive and keep catalytic activity
throughout the subsequent microfabrication steps. In the
other report,17 the e-beam lithography steps had to be used
twice to pattern a catalyst dot on the commercial tipless can-
tilever in order to remove the extra Ni catalyst on the canti-
lever. The probe tip radii reported are also relatively large.
In this letter, we have fabricated high-aspect-ratio carbon
nanocone CNC probes with very sharp tips on tipless can-
tilevers by employing a resist-free e-beam induced deposi-
tion EBID of carbon masks combined with electric-field-
controlled CVD growth. A high resolution AFM imaging of
nanoscale features and deep grooves are demonstrated using
CNC probes.
The fabrication process for a CNC probe is schemati-
cally illustrated in Fig. 1. First, the top surface of the canti-
lever NSC/tipless, MikroMasch, USA was coated with a
10 nm thick Ni film by e-beam evaporation. For the EBID
of carbon dots, a JEOL IC845 scanning electron microscope
SEM with the NPGS software J. C. Nabity lithography
system was used. The acceleration voltage was 30 kV, and
the beam current was 50 pA. The carbon dot deposition time
was varied between 8 and 30 s depending on the intended
size of the dot. No special carbonaceous precursor molecules
were introduced as the residual carbon-containing molecules
naturally presenting in the chamber were sufficient for the
© 2006 American Institute of Physics2-1
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
153102-2 Chen et al. Appl. Phys. Lett. 88, 153102 2006EBID processing to form amorphous carbon dots on the can-
tilever surface. A single carbon dot with a selected diameter
of 300 nm was deposited near the front end edge of the
cantilever by the EBID as illustrated in Fig. 1b. The carbon
dot serves as a convenient etch mask for chemical etching.
The Ni film was then etched away by using a mixture
of H3PO4 : HNO3 : CH3COOH : H2O=1:1 :1 :2 except
the portion underneath the mask. The removal of the carbon
dot mask after the catalyst patterning was performed with an
oxygen reactive ion etch RIE for 1 min, which exposed the
Ni island as illustrated in Fig. 1c. The cantilever with the
Ni island was then transferred to the dc-PECVD system for
subsequent growth of the CNC, Fig. 1d. The growth of the
CNC probe was carried out at 700 °C for 10–20 min using a
mixture of NH3 and C2H2 gas ratio 4:1 at 3 mTorr pres-
sure. An applied electric field was utilized to guide the
growth of the nanocone along the desired direction.
The EBID of the carbon nanodots is a simple writing
technique to directly fabricate nanoscale patterns on the sub-
strate bypassing the use of any e-beam resist layer related
steps.18 The carbon deposition is caused by the dissociation
of the volatile molecules adsorbed on the substrate into a
nonvolatile deposit via a high-energy focused electron. Com-
pared with the typical e-beam lithography approach of pre-
paring a single dot pattern on a small cantilever, the EBID
process can more accurately pattern the catalytic island at the
desired position via in situ control under high magnification
of 10 000 or higher in the SEM.
While the use of the EBID carbon patterns have been
demonstrated as dry etching masks,18 there has been no re-
port for their use as wet etching masks to the best of our
knowledge. We investigated the chemical etchability of the
carbon dots in various acids and other chemicals such as
HCl, HF, HNO3, H2O2 and acetone, and found that the car-
bon dots were very stable and remained adherent on the sub-
strate after immersing into these chemicals. The carbon dots
have a unique advantage in that while they are resistant to
chemical etching, they are easily removable by oxygen RIE.
We find that the oxygen RIE process does not affect the Ni
film and reduces its catalytic activity for CNT/CNC nucle-
ation and growth. Experimental studies on CNC growth in-
dicate that the diameter of the 10 nm thick Ni catalyst island
FIG. 1. Color online Schematic of the resist-free fabrication technique for
a single CNC based AFM tip. a Catalyst deposition by e-beam evapora-
tion. b E-beam induced deposition of a carbon dot mask. c Metal wet
etching and the removal of a carbon dot. d CVD growth of a CNC probe.should be kept smaller than 300 nm to avoid the undesir-
Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to able nucleation and growth of multiple CNCs. Our carbon
island chemical etch mask technique is generally useful for
creating a pattern on any small samples such as a prefabri-
cated tipless cantilever, on which the resist layer cannot be
uniformly coated for reliable lithography.
By adjusting the applied bias in the dc-PECVD system,
the morphology of CNTs can be controlled. At a low applied
voltage of 450 V, the size of the catalyst particles does not
change during growth and the resultant CNTs are equidiam-
eter nanotubes as shown in Figs. 2a and 2b. When the
applied voltage is increased, e.g., 550 V, the diameter of the
catalyst particle on the tip can be gradually reduced due to a
plasma etching sputtering effect, as indicated in Figs. 2c
and 2d. The gradually diminishing catalyst size causes the
nanotube diameter to change with the growth time, resulting
in a nanocone configuration and the eventual complete elimi-
nation of the catalyst particle at the CNC tip, which is the
key mechanism to obtain a very sharp tip.
Figures 3a and 3b show the SEM images of our CNC
probe marked by an arrow grown on a tipless cantilever. In
Fig. 3a, a single CNC probe grown near the edge of a
tipless cantilever is shown. Figure 3b, a higher magnifica-
tion SEM micrograph, shows a CNC with 2.5 m height,
200 nm base diameters, and a cone angle 5°. The inset in
Fig. 3b is an example transmission electron microscopy
TEM image of a CNC tip, which shows the tip radius of the
curvature of only a few nanometers. The microstructure of
the nanocone appears to be a mixture of crystalline and
amorphous phases in general agreement with previous
work.13,19 It should be noted that the CNC probe in Fig. 3b
is made intentionally tilted by manipulating the electric field
direction during CVD growth. Such a tilted probe is desir-
FIG. 2. Color online a Schematic of an equidiameter carbon nanotube
growth. b SEM image of equidiameter CNTs grown on an Si substrate at a
dc bias of 450 V for 20 min. c Schematic of the gradual reduction of a
catalyst particle at the CNT tip by sputtering and accompanying reduction in
the CNT diameter. d SEM image of the CNC with the catalyst particle
completely removed at 550 V for 20 min.
FIG. 3. Color online a Top view SEM image of the single CNC probe
grown near the edge of a cantilever low-magnification, 30° tilted view. b
Side view SEM image of the CNC probe. Inset: TEM image of the CNC tip.
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
153102-3 Chen et al. Appl. Phys. Lett. 88, 153102 2006able as it compensates the operation tilt angle of the AFM
cantilever so that the probe itself is close to being vertical for
stable imaging. The tilt angle of the CNC probe is 13° with
respect to the normal direction of the cantilever surface.
The performance of the CNC probe was evaluated in the
tapping mode using a Dimension 3100 AFM with a Nano-
scope IIIa controller VEECO Instruments for imaging in
air. The surface of a copper film 300 nm thick sputter
deposited on the Si surface was imaged by using our CNC
probe, as shown in Figs. 4a–4c. These images clearly
show a well-defined and rounded grain structure, even for
the grain size of 5 nm or smaller. The sharper grain bound-
aries and image quality were well revealed due to the sharp-
ness of the CNC tip.
To demonstrate the advantage of the high aspect ratio of
a CNC tip, a 300 nm line/space, 500 nm deep PMMA pattern
was evaluated and the imaging performance of the conven-
tional Si tip versus our CNC tip was compared. The image
acquired with a Si probe, Fig. 4c, shows a misleading im-
age pattern due to the angle of the pyramid. The CNC probe,
on the other hand, reveals the true geometry of the pattern
including the configuration of the vertical walls implying
that the CNF probe is able to trace the contour of the deep
profile pattern, as presented in Fig. 4d. From Fig. 4e, the
imaged right sidewall slope of 55°–56° and a left sidewall
slope of 86°–88° of the pattern actually matches those of the
Si pyramid shape itself. From a calculation of geometry, the
Si pyramid tip cannot possibly reach the bottom of the
PMMA pattern. The experimental results also confirmed this.
As shown in Fig. 4f, the sidewall slopes of the same pattern
FIG. 4. Color online a Copper film AFM image. b Zoom-in Cu film
AFM image. c AFM image of a PMMA line pattern by a conventional Si
pyramid tip. d Image of the same PMMA pattern by a CNC tip. e The
height profile of image c. f The height profile of image d.Downloaded 28 Sep 2006 to 131.215.225.9. Redistribution subject to acquired with the CNC probe are about 87°–89°. These data
demonstrate that the CNC probe is is strong enough to trace
the abrupt step structure without breakage.
In order to evaluate the mechanical durability and adhe-
sion strength of the CNC probe, the probe was operated on a
continuous scan mode on Au or Cu film samples for as long
as 8 h. The lateral resolution of the obtained AFM image was
not noticeably changed as compared to the initially scanned
image at time zero data not shown.
In summary, the fabrication of a sharp and high-aspect-
ratio carbon nanocone probe that possesses desirable thermal
stability and mechanical toughness has been demonstrated
using resist-free patterning of catalyst nanodots and electric
field guided CVD growth. The catalyst particle on the nano-
cone tip was completely removed via time-dependent size
reduction, thus leading to an extremely sharp tip, which can
be used for AFM imaging and deep profile analysis.
The authors acknowledge the support of the work by
NSF-NIRTs under Grant Nos. DMI-0210559 and DMI-
0303790, UC Discovery Fund under Grant No. ele02-10133/
Jin, Lawrence Livermore National Lab LLNL MRI Fund
Contract No. 04-006, National Institute of General Medical
Sciences Contract No. GM056290, the Philip Morris Exter-
nal Grant Program, and Alzheimer’s Disease Program of
California Department of Health, U.S. Department of Energy
by the University of California via Lawrence Livermore Na-
tional Laboratory under Contract No. W-7405-Eng-4.
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Extremely sharp carbon nanocone probes for atomic force microscopy imaging

http://authors.library.caltech.edu/5093/1/CHEapl06b.pdf

Extremely sharp carbon nanocone probes for atomic force microscopy imaging

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