Fabrication by electron beam induced deposition and transmission electron microscopic characterization of sub- 10-nm freestanding Pt nanowires

Article abstract of DOI:10.1063/1.2206996

We present a method to reduce the size and improve the crystal quality of freestanding nanowires grown by electron beam induced deposition from a platinum metal organic precursor in a dual beam system. By freestanding horizontal growth and subsequent electron irradiation in a transmission electron microscope, sub- 10-nm polycrystalline platinum nanowires have been obtained. A combined transmission electron microscopy-electron energy loss analysis has shown that the amorphous carbon, mixed to nanocrystalline platinum in the as-deposited material, is removed from the wires during irradiation. The same treatment progressively transforms nanocrystals dispersed in the amorphous matrix in a continuous polycrystalline platinum wire.

Full text of DOI:10.1063/1.2206996

Fabrication by electron beam induced deposition and transmission electron
microscopic characterization of sub-10-nm freestanding Pt nanowires
S. Frabboni, G. C. Gazzadi, L. Felisari, and A. Spessot
Citation: Appl. Phys. Lett. 88, 213116 (2006); doi: 10.1063/1.2206996
View online: http://dx.doi.org/10.1063/1.2206996
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APPLIED PHYSICS LETTERS 88, 213116 ͑2006͒
Fabrication by electron beam induced deposition
and transmission electron microscopic characterization
of sub-10-nm freestanding Pt nanowires
S. Frabboni,a͒ G. C. Gazzadi, L. Felisari, and A. Spessot
CNR-INFM S3 and Physics Department University of Modena and Reggio Emilia, Via Campi 213/A, 41100
Modena, Italy
͑Received 10 January 2006; accepted 12 April 2006; published online 25 May 2006͒
We present a method to reduce the size and improve the crystal quality of freestanding nanowires
grown by electron beam induced deposition from a platinum metal organic precursor in a dual beam
system. By freestanding horizontal growth and subsequent electron irradiation in a transmission
electron microscope, sub-10-nm polycrystalline platinum nanowires have been obtained. A
combined transmission electron microscopy–electron energy loss analysis has shown that the
amorphous carbon, mixed to nanocrystalline platinum in the as-deposited material, is removed from
the wires during irradiation. The same treatment progressively transforms nanocrystals dispersed in
the amorphous matrix in a continuous polycrystalline platinum wire. © 2006 American Institute of
Physics. ͓DOI: 10.1063/1.2206996͔
One motivation for fabricating freestanding nanowires
is their potential use as both interconnects and functional
components in future nanoelectronics.1 In addition, nano-
wires present innovative fundamental aspects in electron
transport,2 phonon transport,3 and structural properties4,5
when grown without physical influence of the substrate.
At present, there are few methods for preparing free-
standing metal nanowires uniform in diameter, sufficiently
long, and placed and aligned to desired configurations or
patterns. The most widely used top-down fabrication method
is a multistep electron beam lithography process.6 This is a
complex and time-consuming process, often resulting in poor
yield due to misalignment and incomplete metal lift-off. Very
short ͑ϳ6 nm͒ Au nanowires of excellent purity and ultimate
width of 0.6 nm have been fabricated in ultrahigh vacuum
͑UHV͒ transmission electron microscope ͑TEM͒.4 Different
successful approaches have been exploited to fabricate
very long and uniformly thick nanowires like step-edge
decoration7 and nanoimprint lithography.8 The main draw-
back of these methods is the difficult site control of free-
standing growth which is mandatory for studying fundamen-
tal properties and fabricating interconnects.
Nanowire fabrication could be greatly simplified by
combining patterning and metal deposition into a single step.
One such technique, direct-write nanopatterning by electron
beam induced deposition9 ͑EBID͒ uses a metal organic pre-
cursor which is vaporized and injected into the path of the
beam. Precursor molecules adsorbed on the substrate are de-
composed by beam induced surface reactions, resulting in
localized deposition of material. Many studies have been car-
ried out using scanning electron microscopes ͑SEMs͒ for
various depositions showing excellent growth control in both
vertical ͓W,9 Pt,10 Au, Cu, Ru, and tetraethyl orthosilicate11
͑TEOS͔͒ and lateral geometries.12 Using this method and a
Pt precursor, interesting transport properties have been
demonstrated13,14 and tilted pillars separated by a 5 nm gap
have been fabricated under direct visual control.15
Despite these improvements, the minimum size of EBID
objects ͑wires and dots͒ fabricated on bulk substrate is lim-
ited to a few tens of nanometers. It was argued that the size
of the deposited objects was constrained by the distribution
of secondary electrons generated by primary beam interac-
tion with the substrate.16 This fact limits the minimum fea-
ture size to be larger than the beam diameter and increases
the leakage current due to metallic deposition spread.17
Strong efforts have been done to reduce the minimum
feature size performing EBID in scanning TEM on ultrathin
electron transparent substrates to limit the secondary
electron emission volume, thus obtaining sub-10-nm wide
features.18,19 An additional step to extend the usefulness of
EBID in nanowire fabrication is to improve the crystal qual-
ity ͑which in turn affects the electrical conductivity͒ of the
deposited metal. For example, the conventional Pt metal or-
ganic precursor, used for metallic interconnect and in the
present work, results in a highly resistive deposit, composed
of a carbon-rich mixture of amorphous C ͑a-C͒ and nano-
crystalline Pt.10,13–15
In this letter, we present a different approach to reduce
the minimum size and improve the crystal quality of EBID
freestanding wires, grown from a Pt metal organic precursor
in a dual beam focused ion beam ͑FIB͒-SEM system. Im-
provement of the minimum size has been achieved by setting
up a method to laterally grow suspended nanowires, about
10 nm wide and hundreds of nanometers long, with selectiv-
ity of the growth site linked to SEM resolution. To test the
spatial resolution of the overall process, freestanding nano-
wires were grown in between two pillars with ending tip less
than 50 nm wide. Irradiation by the electron beam of a con-
ventional 200 keV TEM has been used to improve the crystal
quality of the deposit and to decrease wire width. As shown
by detailed TEM analysis of both as-deposited and irradiated
samples, freestanding polycrystalline Pt nanowires, about
5 nm wide, were formed by simultaneous Pt grain growth
and a-C removal.
Nanowire growth was performed using a solid metal or-
ganic precursor, ͑trimethyl͒methylcyclopentadienylplatinum
͑IV͒, ͑CH3͒3CH3C5H4Pt vaporized by heating to 44 °C. The
a͒
Author to whom correspondence should be addressed; electronic mail:
frabboni@unimo.it
0003-6951/2006/88͑21͒/213116/3/$23.00
88, 213116-1
© 2006 American Institute of Physics
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213116-2
Frabboni et al.
Appl. Phys. Lett. 88, 213116 ͑2006͒
FIG. 1. SEM top view ͑a͒ and side view ͑b͒ of a typical nanowire. From
these images width, thickness, and length can be directly determined with
good accuracy.
gas was injected into the path of the electron beam in a FEI
Strata 235 M dual beam system. The nominal electron beam
spot size was 5 nm, probe current was 90 pA, and accelerat-
ing voltage was 15 kV. The chamber pressure was 6
ϫ10−6 mbar during deposition, which was performed at
room temperature. Before metal deposition, the Si substrate
was prepared with FIB patterned slits in order to enable TEM
measurements during irradiation. The wires were deposited
on the tip of tilted pillars grown from the same metal organic
precursor on silicon substrate, using a procedure similar to
the one reported in literature for nanogap fabrication,15
where, in order to increase the control of the growth rate and
to avoid anisotropic growth effects,20 we introduce the gas
precursor without needle insertion. Freestanding horizontal
wires were obtained by setting up a computer controlled pro-
cedure that, starting from a pillar, laterally shifts the electron
beam in spot mode by a fixed step of 5 nm. The dwell time
of the electron beam was varied in the range of 60–200 ms
in order to obtain thickness and width uniformity and desired
wire length. With this method only a few seconds are needed
to grow 200–300 nm long wires with a very high spatial
resolution.
TEM analysis and irradiation experiments were per-
formed with a JEOL 2010 ͑point resolution of 0.19 nm, LaB6
emitter͒ high resolution electron microscope ͑HREM͒
equipped with a Gatan imaging filter ͑GIF200͒ corrected for
image distortion ͑maximum distortion better than 1%͒ for
image recording, energy filtering, and electron energy loss
spectroscopy ͑EELS͒ analysis. All the EELS spectra were
acquired with the TEM operating at 200 keV in diffraction
mode, focusing the beam on the wire. The spectrometer
dispersion was 0.3 eV/pixel, the acceptance angle was
10 mrad, and the beam convergence angle was 6 mrad. En-
ergy dispersive x-ray spectroscopy ͑EDS͒ was performed us-
ing a Link Inca 100 system attached to the same microscope.
Figure 1 shows two SEM images of a typical nanowire,
as viewed from the top ͑a͒ and side ͑b͒. The wire is about
250 nm long, 20 nm wide, and 30 nm thick. Since the wires
were directly grown on a perforated, prethinned silicon
sample, TEM analysis ͓bright field ͑BF͒, EDS, selected area
electron diffraction ͑SAED͒, HREM, and EELS͔ could be
performed directly on the wires, without additional TEM
specimen preparation. EDS allows a semiquantitative estima-
tion of the mean composition ͑C70% and Pt30%͒ which
roughly agrees with previously reported analysis on thicker
and longer wires deposited on the substrate.10
FIG. 2. As-deposited nanowire: ͑a͒ Morphology from the BF TEM image.
͑b͒ Structural details from higher magnification BF TEM image. ͑c͒ SAED
pattern typical of amorphous/nanocrystalline material. ͑d͒ RDF extracted
from the SAED shown in ͑c͒. The reported atomic distances corresponding
to the G͑r͒ peaks are in agreement with Pt fcc structure ones.
density ͑dark contrast͒. The SAED pattern reveals the pres-
ence of broad diffraction rings typical of amorphous/
nanocrystalline materials. Similar morphology and structure
have been already observed in thicker and wider films pre-
pared with substrate-mediated EBID.10,13–15 In order to ob-
tain quantitative data on the structure of the wires we have
performed analysis of the SAED pattern of Fig. 2͑c͒ in terms
of radial density function ͑RDF͒ G͑r͒, which is reported in
Fig. 2͑d͒. As it is well known, the RDF can be derived by
Fourier sine transform of diffracted intensities and describes
the fluctuation of the local density with respect to its average
value.21 Therefore, RDF peaks reveal the most probable
atomic distances. We measure a first neighbor distance of
0.28 0.01 nm, a second neighbor distance of 0.39 0.01 nm,
and a third neighbor distance of 0.49 0.01 nm. Since all
these distances reproduce well first, second, and third neigh-
bor distances of Pt fcc cubic unit cell ͑a=0.3919 nm͒, we
can deduce the nanocrystalline arrangement of Pt atoms in-
side dark contrast details dispersed in the a-C matrix. In
addition, we observe that the intensity of the SAED pattern is
completely dominated by Pt atoms so we cannot obtain any
information on the C atoms forming the amorphous matrix
from diffraction data ͑as expected by a simple comparison on
total elastic scattering cross section of Pt and C͒. On the
contrary, EELS is the elective tool for this kind of elemental/
structural analysis.22 In Fig. 3 is reported the typical C K
edge of an as-deposited wire. A sharp preedge peak ͑285 eV͒
In Fig. 2͑a͒ a low magnification BF TEM image of the
wire is reported; a magnified image of the central part of the
wire and the relative SAED pattern are shown in Figs. 2͑b͒
and 2͑c͒, respectively. The BF images show the morphology
and the structure of the nanowires: they are formed by a low
electro-optical density amorphous matrix ͑bright contrast͒
and nanoareas ͑ϳ2 nm wide͒ having high electro-optical
FIG. 3. C K edge extracted from EELS spectra taken in as-deposited nano-
wires and after electron irradiation steps shown in the legend. The intensity
of the edge relative to the as-deposited wire has been reduced by a factor of
5 for the sake of clarity. The vanishing of the carbon signal after 180 s
irradiation time is evident.
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213116-3
Frabboni et al.
Appl. Phys. Lett. 88, 213116 ͑2006͒
sible candidates, probably acting at the same time during
electron irradiation; discriminating among them is an impor-
tant issue and work is in progress in this field.
In conclusion, we have reported a controlled method to
fabricate sub-10-nm freestanding Pt wires exploiting hori-
zontal suspended growth by EBID and TEM irradiation. A
detailed TEM-HREM-EELS analysis has shown that a-C
matrix can be removed from the wire during high energy
electron irradiation. This process produces pure metallic
wires. We have shown that this combined approach features
site selectivity of the growth and improves minimum size
and crystal quality of the deposit, thus it seems promising for
both nanoelectronic applications and fundamental studies.
FIG. 4. Structural evolution during irradiation. ͑a͒ BF TEM image taken
after 60 s irradiation at the wire center ͑arrowed͒. ͑b͒ BF TEM image taken
after a sequence of five 180 s irradiations performed on different areas of the
wire, in order to remove a-C over the total wire length. ͑c͒ HREM image
and corresponding fast Fourier transform of a grain junction showing the
͕111͖ Pt lattice fringes.
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looks very similar to those observed in evaporated amor-
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the wires reduces their lateral dimension with respect to the
substrate-mediated case, without affecting their morphology
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