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Letter
Bottom-up Nanoconstruction by the Welding of
Individual Metallic Nanoobjects Using Nanoscale Solder
Yong Peng, Tony Cullis, and Beverley Inkson
Nano Lett., 2009, 9 (1), 91-96 • DOI: 10.1021/nl8025339 • Publication Date (Web): 10 December 2008
Downloaded from http://pubs.acs.org on January 19, 2009
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NANO
LETTERS
Bottom-up Nanoconstruction by the
Welding of Individual Metallic
Nanoobjects Using Nanoscale Solder
2009
Vol. 9, No. 1
91-96
Yong Peng,†,‡ Tony Cullis,‡ and Beverley Inkson*,†
Department of Engineering Materials, Department of Electronic and Electrical
Engineering, Sheffield UniVersity, Sheffield S1 3JD, U.K.
Received August 19, 2008; Revised Manuscript Received November 17, 2008
ABSTRACT
We report that individual metallic nanowires and nanoobjects can be assembled and welded together into complex nanostructures and conductive
circuits by a new nanoscale electrical welding technique using nanovolumes of metal solder. At the weld sites, nanoscale volumes of a
chosen metal are deposited using a sacrificial nanowire, which ensures that the nanoobjects to be bonded retain their structural integrity. We
demonstrate by welding both similar and dissimilar materials that the use of nanoscale solder is clean, controllable, and reliable and ensures
both mechanically strong and electrically conductive contacts. Nanoscale weld resistances of just 20Ω are achieved by using Sn solder.
Precise engineering of nanowelds by this technique, including the chemical flexibility of the nanowire solder, and high spatial resolution of
the nanowelding method, should result in research applications including fabrication of nanosensors and nanoelectronics constructed from
a small number of nanoobjects, and repair of interconnects and failed nanoscale electronics.
One of the central challenges for the bottom-up construction,
integration, and repair of nanoscale systems incorporating
single nanoobject building blocks is to develop reliable
methods of joining individual nanoobjects together and to
substrates.1-5 Individual nanoobjects such as nanoparticles,
nanowires, and nanotubes can now be routinely fabricated
and manipulated at the nanoscale using a range of scanning
probe-based methods.5-15 However, simply bringing nanoobjects into contact cannot be relied upon to generate secure,
functional, and long-lasting bonds that are a necessity for
devices with a long lifetime. A key challenge for joining
nanoobjects is the formation of mechanically strong bonds
whose size and chemistry can be tailored to generate the
desired functional properties such as electrical conductivity.
A number of localized joining methods suitable for
individual nanoscale objects have recently been proposed
including thermal heating,16,17 ion beam deposition of material,18 laser heating,19 ultrasonic irradiation,20 high-energy
electron beam bombardment,21 and Joule heating from
electrical currents.13,22 Of these the method of Joule heating
may hold the most potential because of its formation of
electrically conducting junctions, inherent simplicity, cleanliness, and reliability. The other methods suffer from a number
of problems for practical industrial application. For example,
high-energy electron beam systems are expensive and are
* To whom correspondence should be addressed.
[email protected].
†
Department of Engineering Materials.
‡
Department of Electronic and Electrical Engineering.
10.1021/nl8025339 CCC: $40.75
Published on Web 12/10/2008
E-mail:
 2009 American Chemical Society
not easily integrated into industrial production lines. Focused
ion beam-based deposition currently only has limited chemistries with poor control of functionality and contamination,
and thermal heating methods have very poor spatial control
of heated zones and contamination.
At the nanoscale, structurally resilient carbon nanotubes
(CNTs)23 and CNTs filled with copper13 have been successfully welded together by current-induced flow of carbon and
metal to the junction. These CNT methods require that a
very high current density (typically >1 × 1011 A·m-2 for
melting metal) has to flow through the nanotubes into the
adjacent CNT or nanostructure. This means that nanostructures bonded by this method have to be both conductive and
resistant to current-induced damage. Currently, the successful
bonding of metal nanostructures by Joule heating has been
limited to the welding of Pt wires with 650 nm diameter.22
It is well known that high current flow through nanostructured metals face a major problem in that the nanoobjects
can be severely structurally and functionally damaged due
to atom migration from Joule heating and electromigration.2,24
Here we describe a new nanoscale electrical welding
technique that radically improves the spatial resolution,
flexibility, and controllability of welds between individual
nanowires and nanoobjects. The key advance is to avoid
detrimental current flow though the nanoobjects to be joined
and instead to locally deposit nanoscale volumes of a chosen
metal at the weld site by Joule heating a sacrificial nanowire.
The principle of the process is illustrated in Figure 1.
Figure 1. Schematic of nanoconstruction by the welding of
individual nanoobjects (nanowires) using nanoscale solder. (a)
Assembly of individual nanoobjects into a desired pattern using a
nanomanipulator probe. (b) Placement of a sacrificial nanowire in
contact with the nanostructure to be welded. (c) Nanowelding the
nanoobjects together by an electrical signal. (d) Completed nanoweld.
Importantly, the use of nanoscale “solder” to bond the
nanoobjects together also offers the opportunity to tailor the
weld’s mechanical and functional properties by controlling
the chemistry, structure, and volume of solder material used.
Furthermore the entire process can be carried out within a
standard SEM system.
The soldering of nanoobjects into complex networks is
demonstrated here by two examples which demonstrate the
wide applicability and performance of the technique. First
55 nm Au nanowires are welded with gold solder into
complex patterns, including triple junctions, which cannot
be achieved by direct Joule heating of the nanowires due to
morphological instabilities induced by current flow. The
ability to weld Au nanowires is important since, due to their
excellent electrical conductivity and stability in oxidizing
environments, they are widely used for electrodes and
interconnects in the semiconductor industry and nanoelectronics research. In the second example, we demonstrate that
dissimilar nanowires can be welded together, using a third
material as solder. SnAu alloy solder, widely used in
macroscopic welding technique due to its excellent conductivity, low melting point, and large corrosion resistance,25 is
specifically chosen to result in an ultralow electrical resistance join between nanowires. The ability to integrate
functional nanowires into circuits via high-performance
welded joins represents a significant advance for nanoscale
sensor development.
A range of nanowires were fabricated by electrodeposition
into porous templates, and then dispersed onto Si/SiO2(100
nm) wafers. To construct nanowire networks, individual
nanowires were assembled into complex patterns by mechanical manipulation using nanomanipulators within an
SEM. SEM nanomanipulators are particularly suitable for
industrial applications of localized bottom-up integration and
repair nanotechnology due to their nanoscale spatial resolution, intrinsic long manipulating distance,26 no-specialrequirement-of-sample,27 large working areas,27 speed effi92
ciency of manipulation and observation,26,27 and proven
assimilation into production lines.27 First the nanowires were
released from the strong bonding to the substrate wafer by
a mild mechanical push from a SEM nanomanipulator
(Figure 2a). The chosen nanowire is then lifted from the
substrate using Van der Waals force (Figure 2b), followed
by precise release at the desired location adjacent to another
nanowire (Figure 2c). The whole SEM mechanical manipulation process is clean, simple, quick, and reliable. It also
avoids the increased time, expense, and contamination of
methods involving nanoobject pick-up via e-beam and ionbeam gas deposition.19 By means of this clean manipulation
method, complex structures, like “NANO” shown in Figure
2d, can be easily assembled.
Once nanowires have been positioned into a required
design, the goal is to securely join them together ensuring
mechanically strong and electrically conducting junctions.
Electrical currents can be directly and controllably applied
to individual nanowires and through sections of nanowire
circuits using two SEM nanomanipulators (Figure 3a). Using
SEM nanomanipulators, the 55 nm Au nanowires here
exhibited a maximum transport current of 11.73 mA at
applied voltage 0.72 V (Figure 3b), giving a calculated
maximum current density Jmax of 4.94 × 108 A·cm-2.28 Figure
3 illustrates the reason why successful bonding of ,100 nm
metallic structures by direct Joule heating is technologically
exceptionally difficult. During current flow, Joule heating
and electromigration causes diffusion of atoms along the
length of nanowires. If not carefully controlled, high and
persistent current flow causes morphological instabilities to
develop along the length of the nanowires, which eventually
causes their catastrophic failure (Figure 3c,d). Failure of
nanowires due to electromigration is a well-documented
problem in electronic interconnects.2,24
Attempts to electrically weld overlapping metal nanowires
together by directly running a current through their junction,
using up to 30% of maximum current density Jmax, were
found to be unpredictable, uncontrollable, and mostly unsuccessful. Although the local increase in resistance at the
nanowire junction enhances Joule heating and thus material
from the individual wires diffuses to the junction to form a
weld, the unpredictable development of morphological
instabilities close to the weld and along the lengths of the
nanowires (particularly at grain boundaries) frequently causes
thinning, break-up, and failure of the wires (Figure 3e, inset,
before nanowelding). Consistent with this, successful bonding
of touching metal nanowires by electrical Joule heating has
only been reported for thick Pt nanowires 650 nm in
diameter, which are more resistant to morphological failure
by virtue of their increased size.22
Dramatic advances in electrical nanowelding of nanowires
are achieved here by an approach that avoids significant
current flow and associated morphology changes along the
nanowires to be joined. Joule heating of a sacrificial nanowire
positioned at the required weld point enables nanovolumes
of conductive solder to be applied directly to the junction,
forming a controllable, secure, and conductive weld (Figures
1and 4).
Nano Lett., Vol. 9, No. 1, 2009
Figure 2. Manipulation and assembly of individual gold nanowires. (a) Release of a gold nanowire from a Si/SiO2(100 nm) wafer by using
a nanomanipulator probe to mechanically push the chosen nanowire. (b) Lift-up of the released individual gold nanowire by Van der Waals
force. (c) Assembly of the picked-up nanowire adjacent to another nanowire. (d) Nanoscale word NANO written by individual gold nanowires.
Figure 3. Electrical characterization of individual gold nanowires.
Au nanowire (55 nm) before (a) and after (b) 2.5 mA current for
2 min, showing current-induced shape change and failure. Inset:
magnified view of failure. (c) Overlap of two nanowires N1 and
N2 (arrowed) prior to welding. (d) Nanowire failure (N2, arrowed)
after directly running a current through two overlapping nanowires.
Figure 4 shows representative experimental images of the
nanoweld process, showing how a sacrificial Au nanowire
was used to deposit solder material onto an assembled Au
nanowire pattern resulting in a welded nanowire configuration in the form of the Chinese character meaning human
being. Prior to welding, a 0.7-0.85 mA current is run
through the Au solder nanowire for 1∼2 min to improve
the controllability and reliability of the nanowelding procedure. The moderate current density causes a softening of the
solder nanowire due to Joule-heating and limited diffusion
of the solder material onto the nanostructures at their contact
point induces the first join to the patterned nanowires (Figure
4a). A rectangular pulse signal of 0.7-1.0 V and 100 ms
width for Au is then applied to the solder nanowire to form
the weld. The short voltage pulse causes significant Joule
Nano Lett., Vol. 9, No. 1, 2009
heating and associated rapid material diffusion of the solder
nanowire onto the chosen junction (Figure 4b). The pulse
magnitude and time is controlled to provide just adequate
heating and diffusion of the nanosolder, and to minimize
heat dissipation into the adjacent nanostructure. For the 55
nm Au solder wires the optimum current density typically
ranges from about 1.8 × 108 to 2.5 × 108 A·cm-2 to obtain
well-controlled, reproducible, and reliable nanowelds.
The nanowelding technique can be used to weld complex
crossed structures with high angles between the connecting
nanowires, for example, the Chinese character and the letter
“N” in Figure 4. Once formed the welds joining two
nanowires are strong and very conductive. If the solder
nanowire remains attached to the junction after welding
(Figure 4b), mechanical force can be applied to separate them
by pulling the probe away (Figure 4c). Furthermore, if any
unwanted residues of solder nanoparticles and rods exist,
which are conductive and therefore dangerous and potential
short-circuit sites for real nanoelectronics devices, the
nanomanipulators can easily and precisely mechanically
remove the welding residues. The welded letter N of Figure
4f is the third letter of NANO in Figure 2d, which was
cleaned by nanomanipulators after the whole word NANO
had been accomplished.
An immediate check of nanoweld quality is essential for
practical nanoconstruction, and can be done in situ directly
after welding. Figure 4e shows the quantification of resistances across a “T” shaped weld by directly touching probes
to the ends of the welded nanowires (see inserts of Figure
4e, with the tip-tip circuit resistance subtracted). The
electrical characterization of a range of welded nanowires
confirms the formation of electrically conducting ohmic
junctions. Although the resistances across the Au welds are
typically higher than along single integral nanowires (Figure
4e), the welded junctions still exhibit low contact resistances
in the range ∼10-1000 Ω. This phenomenon is similar to
traditional macroscopic welds using the solder technique.
93
Figure 4. Nanowelding complex nanowire structures using nanovolumes of metal solder, using 55nm Au nanowires as an example. (a)
Preweld softening of the solder nanowire and contact to the two nanowires to be welded. (b) Nanowelding two nanowires together by a
rectangular voltage pulse through the solder nanowire, causing significant material diffusion onto the junction. (c) Mechanically removal
of the solder nanowire, leaving miniaturized Chinese character. (d) High magnification SEM image of the nanoweld. (e) Immediate quality
check of the welded nanojunction in panel d, blue curve showing the resistance of the left nanowire (left insert), and red curve resistance
through the welded nanojunction (right insert). (f) Fabrication of miniaturized letter “N” welded using nanoscale solder.
The ability to weld similar nanowires together with
matching solder material will have many applications,
particularly in the repair of failed interconnects and electronic
circuitry. However the presented welding technique has far
wider applicability in the area of integrating individual
nanoobjects into circuits where their functional properties
can be exploited. To do this, dissimilar nanoobjects have to
be joined together with welds with the desirable characteristics such as strength and low electrical resistance. Here
we demonstrate that individual 55 nm diameter (CoPt/Pt)n
multilayer nanowires, which have potential uses as nanosensors and nanoelectronics due to their GMR effect29,30 can be
successfully integrated with 55 nm Au interconnect nanowires. The two dissimilar nanowires are welded together
using Sn99Au1 solder. The use of a solder with a melting
point significantly below that of the materials to be joined
minimizes the likelihood of any structural damage due to
heat dissipation during the welding process. The welding
technique used here, carried out in vacuum with an extremely
short electrical-heat pulse, also inhibits oxidation effects
which can affect welds formed by prolonged heating in air.25
Figure 5 illustrates the successful welding of a 55 nm
(Co78Pt22/Pt)n multilayer nanowire to a 55 nm Au nanowire
using a 200 nm Sn99Au1 solder nanowire. Before assembly,
the resistances of the circuit components (consistently
reproducible for the given SEM tip-pair) were determined
to be the following: SEM manipulators tip-to-tip circuit )
74 Ω, the chosen individual (Co78Pt22/Pt)n multilayer nanowire ) 4153 Ω, and the gold nanowire ) 79 Ω. After the
chosen (Co78Pt22/Pt)n multilayer nanowire and gold nanowire
were assembled together (Figure 5a) and prior to welding,
the resistance ATD was measured to be isolating due to a
poor contact. A sacrificial 200 nm Sn99Au1 solder nanowire
was then placed at the weld point (Figure 5b), and a weld
formed by using a 0.5∼1.0 mA current for 1∼2 min,
followed by a 50 mA rectangular pulse signal with 150 ms
width (Figure 5c). It was found that to obtain well-controlled,
reproducible, and reliable nanowelds, the optimum current
density for the sacrificial 200 nm Sn99Au1 nanowires typically
94
Figure 5. Nanowelding an individual 55nm (Co78Pt22/Pt)n multilayer
nanowire to a 55 nm gold nanowire using a sacrificial 200 nm
Sn99Au1 solder nanowire. (a) Assembly of a gold nanowire (CTD)
adjacent to a (Co78Pt22/Pt)n multilayered nanowire (ATB). (b)
Placement of a sacrificial 200 nm Sn99Au1 nanowire in contact with
the nanostructure to be welded. (c) Nanowelding the nanowires
together by a rectangular voltage pulse through the solder nanowire,
causing significant material diffusion onto the junction. (d) Immediate quality check of the welded nanojunction in panel c: I-V
curves showing the resistance of SEM tip-tip circuit (left y-axis,
black curve), resistance of gold nanowire CTD (left y-axis, red
curve), resistance through (Co78Pt22/Pt)n multilayer nanowire ATB
(right y-axis, green curve), resistance through whole structure ATD
(right y-axis, blue curve).
ranged from about 0.9 × 108 to 1.6 × 108 A·cm-2. The
volume of solder in Figure 5c is larger than the nanowire
thickness; however, if required the volume of solder can be
controlled and reduced by altering the sacrificial nanowire
diameter.
The resistances across a range of nanoscale welded
heterojunctions were measured in situ directly after welding.
The resistances of the individual nanowires and SEM tipto-tip circuits were found to be the same, within error, before
and after the weld process. For the dissimilar nanowire weld
Nano Lett., Vol. 9, No. 1, 2009
Figure 6. Characterization of Au weld microstructure. (a) Weld P
formed between two Au nanowires (I and II) using Au solder. The
welded nanostructure was picked up on a SEM nanomanipulator
tip. (b) TEM characterization of the weld confirms a fully integrated
join, with a θ fcc diffraction pattern of the solder material shown.
in Figure 5, the I-V curves for the SEM tip-to-tip circuit,
the individual (Co78Pt22/Pt)n multilayer nanowire ATB, the
gold nanowire CTD, and the whole welded structure ATD
are shown in Figure 5d. The difference between the sum of
the nanowire resistances (ATB plus CTD), and the resistance of the whole welded structure (ATD), gives a
nanoweld resistance of just 21.6 Ω.This extremely low weld
resistance shows that an excellent quality of weld has been
achieved using the Sn99Au1 alloy as solder material. Experimental data from further nanowelding experiments shows
that the resistances of Sn99Au1 nanowelds were reliably in
the range ∼10-100 Ω, indicating that the tin alloy is a
reliable and reproducible nanosolder material.
The quality and structure of the welded nanojunctions have
also been checked by transmission electron microscopy
(TEM). The mechanical strengths of the welds were easily
strong enough to enable welded nanowires to be directly
lifted up from the Si/SiO2 (100 nm) wafers by a SEM
nanomanipulator, and then transferred to TEM on the
nanomanipulator tip using a modified Gatan TEM tomography sample holder. Figure 6 shows an example of two Au
nanowires (I and II) welded together at a junction P using
Au solder (Figure 6a). The weld P is confirmed by TEM to
be a completely integrated Au join with polycrystalline fcc
structure (Figure 6b).
In summary, the high spatial resolution nanowelding
method presented here provides a controllable and reliable
approach for manipulating, assembling and welding <100
nm 1D metallic nanowires and nanoobjects into complex
structures by means of SEM nanomanipulators. We have
shown that the whole welding process using nanoscale solder
is clean, simple, quick, controllable, and reliable. The
technique has a number of important features relevant to
applications in industry that stem from the use of a sacrificial
nanowire to provide solder material for the weld. First the
nanoobjects to be welded do not provide the weld material,
so they retain their structural integrity. Second, a wide range
of different materials can be used as solder material, giving
the opportunity to specifically tailor the weld properties such
as chemistry, strength, and conductivity. Sn99Au1 is shown
to be a suitable solder material for low electrical resistance
Nano Lett., Vol. 9, No. 1, 2009
welds, and the integration of dissimilar materials. Third, the
technique can be applied to manipulate and weld nanoobjects down to less than 10 nm in size, depending on size
of the sacrificial solder nanowire and the resolution of the
SEM and nanomanipulator systems used.
The ability to reliably weld individual nanowires and
nanoobjects into complex geometries with controllable
junctions represents a significant breakthrough for the current
and future bottom-up localized assembly, integration, and
repair of micro/nanodevices,31 nanoelectronics,32 nanosensors,33 and nanoelectromechanical systems (NEMS).34 Because of the strength of the welded junctions, the nanowelding technique can be applied to the pick-up, relocation,
and integration of individual nanostructures and functional
components across a substrate, providing the basis of a
nanoscale fabrication line. Opportunities also exist to exploit
the ability to deposit controlled nanovolumes of solder
material at specific sites along nanowires, for example, to
fabricate novel chemically modulated structures.
Acknowledgment. We would like to thank Mr. Alan
Walker, Department of Electronic and Electrical Engineering,
Sheffield University, for technical support. This work was
supported by a Basic Technology Grant GR/S85689 from
the EPSRC, U.K.
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