Freestanding nanostructures for three-dimensional superconducting nanodevices

Article abstract of DOI:10.1063/1.3701283

Free-space nanostructures are the fundamental building blocks of three-dimensional (3D) nanodevices with multi-functionality beyond that achievable by planar devices. Here we developed a reliable technique for the site-specific post-growth geometrical manipulation of freestanding superconducting nanowires using ion-beam irradiation with nanometer-scale resolution to fabricate uniformly shaped and sized clean-surface 3D nanostructures. Such structures could integrate with conventional superconducting quantum interference devices to detect magnetic fields both parallel and normal to the substrate. Property characterizations suggest that our focused-ion-beam technique allows tailoring of freestanding superconducting loops for size and geometry, potentially for lab-on-chip experiments.

Full text of DOI:10.1063/1.3701283

Freestanding nanostructures for three-dimensional superconducting nanodevices
Ajuan Cui, Wuxia Li, Qiang Luo, Zhe Liu, and Changzhi Gu
Citation: Applied Physics Letters 100, 143106 (2012); doi: 10.1063/1.3701283
View online: http://dx.doi.org/10.1063/1.3701283
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APPLIED PHYSICS LETTERS 100, 143106 (2012)
Freestanding nanostructures for three-dimensional superconducting
nanodevices
Ajuan Cui,a) Wuxia Li,b) Qiang Luo, Zhe Liu, and Changzhi Gub)
Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,
Beijing 100190, China
(Received 25 October 2011; accepted 20 March 2012; published online 4 April 2012)
Free-space nanostructures are the fundamental building blocks of three-dimensional (3D)
nanodevices with multi-functionality beyond that achievable by planar devices. Here we
developed a reliable technique for the site-specific post-growth geometrical manipulation of
freestanding superconducting nanowires using ion-beam irradiation with nanometer-scale
resolution to fabricate uniformly shaped and sized clean-surface 3D nanostructures. Such
structures could integrate with conventional superconducting quantum interference devices to
detect magnetic fields both parallel and normal to the substrate. Property characterizations
suggest that our focused-ion-beam technique allows tailoring of freestanding superconducting
C
V
loops for size and geometry, potentially for lab-on-chip experiments. 2012 American Institute
of Physics. [http://dx.doi.org/10.1063/1.3701283]
Three-dimensional (3D) nanostructures and nanodevices
have attracted tremendous interest in the past few years due
to their excellent mechanical and physical properties. Optoe-
lectronic devices,1 nanosensors,2 biological information
detectors,3 plasmonics,4 and quantum devices5,6 based on 3D
nanostructures have excellent functional properties that pla-
nar nanodevices cannot achieve. Among them, 3D supercon-
ducting quantum interference devices (SQUIDs), which can
be formed by the integration of free-space multiple pick-up
loops with conventional SQUID, potentially could overcome
the present planar SQUID limitation of only being able to
detect the field perpendicular to the substrate. It has been
demonstrated that with the 3D pick-up loops, nano-SQUIDS
can be used to detect different field components (Bx, By, and
Bz) or field gradients (@Bi=@j), where i, j ¼ {x, y, z} in free-
space; however, to achieve single-spin resolution, nanoscale
3D pick-up loops with uniform size and clean surface are
essential.5 Thus, it is of great importance that we seek to
explore a technique that can be used to fabricate high-quality
superconducting 3D pick-up loops with quantized properties.
Direct deposition of 3D tungsten nanostructures with a
slowly moving ion beam has been previously reported.5,7
However, the disadvantages of this technique include: (i) it
requires sophisticated software packages and an expensive
sample stage; (ii) it is time consuming to the point that struc-
tures can only be formed one-by-one, and intensive experi-
mentation is required beforehand for optimized growth
conditions; (iii) residual deposition underneath the sus-
pended portion cannot be avoided, which could induce an
extra conduction path and ruin the designed device function-
ality; (iv) it is very difficult to obtain 3D structures that are
uniform in size due to the stray dose and the beam-matter
interactions during the growth process. Other reported stud-
ies so far involving focused-ion-beam (FIB)-induced direct-
writing include the fabrication of C,8 SiO2,9 and Pt10 3D
nanostructures using a slowly moving beam or a tilt stage.
The disadvantages of using a tilt stage include that such
method permits only very simple structures to be obtained; it
lacks controllability, flexibility, efficiency, and repeatability;
and it is not easy to make a solid contact between two wires
to avoid residual deposition.10
In this work, superconducting tungsten nanowires were
chosen as targets for exploring a technique that can be used
for post-growth shape manipulation. The advantages of using
FIB-induced deposition to grow tungsten nanowires for FIB-
induced shape manipulation include: (i) the electrical resis-
tivity of the freestanding tungsten nanowires is comparable
to that of nanowires laterally grown on the substrate surface,7
making it useful to construct 3D conducting nanodevices;
(ii) the nanowires have a much-higher superconducting tran-
sition temperature (Tc ꢀ 5.2 K) than the bulk counterparts,11
making them a valuable option for constructing 3D super-
conducting nanodevices; and (iii) FIB techniques offer the
means to grow true nanoscale features site-specifically
with controllable size tunability, allowing us to construct
free-space nanodevices with increased flexibility and
controllability.
In order to develop a technology for the controllable
shape manipulation of tungsten nanowires, we systematically
investigated the general bending effect, including the influ-
ence of the ion beam current and the ion incident angle. 3D
superconducting nanostructures were formed along with
temperature-dependent measurements down to 1.8 K for ba-
sic electrical property probing. The results proved that post-
growth manipulation by focused-ion-beam irradiation could
be a potential method to fabricate 3D SQUIDs.
The tungsten nanowires used in this study were grown
by FIB-induced deposition using a 1 pA ion beam current
with W(CO)6 as the gas precursor. The system used is a
dual-beam FIB/scanning electron microscope (SEM) system.
The ion source is a singly charged liquid gallium ion, which
is placed at a 52ꢁ angle to the vertically oriented electron
a)E-mail: cuiajuan@aphy.iphy.ac.cn.
b)Authors to whom correspondence should be addressed. Electronic
addresses: liwuxia@aphy.iphy.ac.cn and czgu@aphy.iphy.ac.cn.
C
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0003-6951/2012/100(14)/143106/4/$30.00
100, 143106-1
2012 American Institute of Physics
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143106-2
Cui et al.
Appl. Phys. Lett. 100, 143106 (2012)
beam column. For nanowire deposition, first, the gas precur-
sor molecules were introduced to the substrate surface
through a gas nozzle, and then an FIB with a nominal diame-
ter of 7 nm was kept stationary in a particular position; this
beam-scanning strategy was called spot-mode. The back-
ground pressure was about 5.5 ꢂ 10ꢃ5 to 1.0 ꢂ 10ꢃ6 mbar
during deposition.
bending angle (U, the angle between the beginning of the
wire and its angle from vertical) and of the incident direction
of the irradiation ion beam (h, the angle between the incident
ion beam and the long axis of the wire) are schematically
shown in the inset of Fig. 1(b). The height of the as-
deposited nanowires is 1.25, 2.62, and 3.79 lm, respectively,
and the radius is about 65 nm. The resultant images were
taken after each ion beam scanning with a current of 100 pA
and an incident angle of 40ꢁ, in the strategy of raster scan-
ning under magnification of 15 000ꢂ. It can be seen from
Fig. 1(a) that shorter nanowires bent more slowly; however,
the time required for nanowires with different lengths to be
aligned with the incident ion beam is more or less the same.
Fig. 1(b) shows the bending angle as a function of the ion
sweep number; it can be seen that the bending occurs toward
the incident ion beam until the wires become parallel to it,
and then the bending angle saturates upon further irradiation.
This is similar to the observed bending of various nano-
objects previously reported.12–17 The different colors in Fig.
1(b) stand for three different nanowires of the same length,
showing that this technique has very good repeatability.
To examine the effect of the incident angle of the ion
beam, three groups of nanowires with identical dimension
and interspacing were used, the stage was tilted to vary the
ion beam incident direction, and an ion beam current of
100 pA was scanned by raster scanning under magnification
of 15 000ꢂ. Fig. 2(a) shows the ion beam incident angle-
dependent bending of tungsten nanowires (around 2.3 lm in
height and 60 nm in radius) occurring at the initial irradiation
stage. Again, the pillars bent until the angle became aligned
with the ion beam direction, which is 10ꢁ, 30ꢁ, and 50ꢁ with
To explore the general bending phenomena, vertical
nanowires with various heights were grown on SiO2/Si sub-
strates. Ion beam currents in the range of 30–100 pA with an
ion energy of 30 keV were recorded. One ion beam sweep
loop took 163 s under magnification of 15 000ꢂ, correspond-
ing to a total area of 20.237 ꢂ 17.470 lm2. Thus, the dwell
time during scanning is 1.81 ꢂ 10ꢃ4 s, and the beam step
size is 19.8 nm. Successive repeat scanning, different inci-
dent angles, and ion beam current were employed to study
the bending and alignment phenomena of nanowires. After
each irradiation scan, the stage was tilted to 45ꢁ off-normal
to the electron beam for measuring bending by in situ SEM.
Pairs and groups of nanowires with different sizes and inter-
spacing were also deposited for free-space nanogap/joint-
contact fabrication. For this purpose, we utilized two experi-
mental setups: (i) irradiating the whole field of view by raster
scanning or (ii) scanning a selected nanopillar or part of it by
reduced raster scanning. The electrical properties of the
resulting tungsten nanostructures were measured using a
quantum design physical properties measurement system
(PPMS) with temperature sweeping from room temperature
down to 1.8 K.
Figure 1(a) shows the SEM images of tungsten nano-
wires irradiated with various ion beam sweep numbers to
identify their free-space movement. The definitions of the
FIG. 1. (a) SEM images of the as-deposited tungsten nanowires (a1) and
those irradiated with an increasing Ga ion-beam sweep number, showing the
bending sequence ((a2)–(a6)). (b) The bending angle as a function of the ion
sweep number. The scale bar is 2 lm. The inset in the upper left corner of
(b) shows the schematic of the definition of the bending angle (U), and that
in the lower right corner shows the definition of the ion beam incident angle
(h).
FIG. 2. The bending angle as a function of the ion sweep number for FIB-
grown tungsten nanowires. (a) Bending angle for nanowires irradiated by a
100 pA ion beam with an ion beam incident angle of 50ꢁ, 30ꢁ, and 10ꢁ,
respectively. (b) Bending angle for nanowires irradiated with an ion beam
incident angle of 30ꢁ and an ion beam current of 100 pA, 50 pA, and 30 pA,
respectively. The lines are provided to help guide the reader’s eye.
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143106-3
Cui et al.
Appl. Phys. Lett. 100, 143106 (2012)
respect to the vertical. Besides, it can be seen that nanowires
bent faster when irradiated with a larger ion beam incident
angle. The effect of the ion beam current was also explored,
and the bending trend is shown in Fig. 2(b). It shows that
using a higher ion beam current, it takes less time for a nano-
wire to achieve a higher bending angle. These findings are
consistent with the results previously reported.18
Conversely, nanogaps with a tunable distance, as can be
seen from Fig. 3(c), were formed by adjusting the stage-
rotation angle, the ion beam-incident angle, and the size and
interspacing of the as-grown nanowires. To achieve solid
contact between two wires with one of them remaining
undisturbed, a reduced-raster scan is performed only to irra-
diate the wire to be bent. Furthermore, multi-branched struc-
tures (joined at one point) and cross-contacted/gapped
structures can be formed by performing stage-rotation and
stage-tilting repeatedly to manipulate the leaning direction
of different wires in a combination method of irradiating a
portion of a wire.
Figure 3(a) shows the schematic process to construct 3D
nanocontacts, multi-branched structures, and nanogaps by
FIB-induced deformation. To guide the nanowire bending
process and control its ultimate shape, the optimized stage-
tilting angle is proposed based on the values of nanowires
heights and interspacing. For instance, to form free-standing
joint contact on nanowires W1 and W2 as illustrated in Fig.
3(a), where nanowire W1 should stay vertical and W2 is sup-
posed to bend to contact with W1, the value of the proposed
stage tilt angle is thus given by
However, it is very difficult to form sophisticated nano-
structures such as four-point structures with good repeatabil-
ity and efficiency. In theory, the spatial precision of this
technique would be determined by the resolution of the ion
beam system, which is about 7 nm for the FIB we used; tech-
nically, it is also largely dependent on facility operation con-
ditions. Nonetheless, based on the above results, we can
conclude that freestanding 3D nanostructures with various
geometric styles can be formed by FIB-induced deformation
with reasonably good controllability, repeatability, and spa-
tial precision.
h¼ arc tanðh1=dÞ:
(1)
The other condition required for solid contact formation is
h22 ꢄ h21 þ d2:
(2)
Figure 4 shows the temperature-dependent electrical
property of an air-bridge-like free-space nanostructure con-
structed by FIB-induced deformation (red triangles) and that
for the as-grown nanowire (black circles). In the inset is a
typical SEM image of free-standing 3D nanostructures. To
form such a structure, first, FIB was used to deposit tungsten
strips on large contact pads, which were previously formed
on a SiO2/Si substrate. Then, pairs of tungsten nanowires
were deposited on the edges of the strip and one pad, fol-
lowed by ion irradiation to bend one of the wires that has a
higher aspect ratio (the left-side one in this case) to join the
two wires. Following the method reported in our previous
work,7 the normal state resistance and resistivity of the free-
standing portion were calculated to be around 2.7 kX and
540 lX cm, respectively; the superconducting transition tem-
perature derived from the curve is about of 5.1 K. For com-
parison, ultra-low current FIB lateral milling was used to
release the as-deposited nanowire from its base for measure-
ment of its electrical properties.19
Here h1 and h2 are the heights of W1 and W2, respectively, d
is the interspacing between them, and h is the tilt angle that
the stage should situate. Thus, by tilting the stage with an
angle of h, the projection length of W2 on the substrate sur-
face is not less than d; meanwhile, the projection length of
W2 on the length direction of W1 is not less than h1. Fig.
3(b) shows the joint-nanocontacts formed on two rows of
tungsten nanowires with heights of 2.8 and 1.5 lm and an
interspacing of 2 lm; these nanocontacts were grown on a Si
substrate by FIB-induced chemical-vapor-deposition. To
achieve such structures, the stage was rotated to guarantee
that W1, W2, and the ion beam are aligned; reduced raster
scanning was then performed to irradiate one row of nano-
wires by tilting the stage by 40ꢁ so that they bent toward the
incident ion beam and finally made contact with the as-
grown nanowires on the other row.
FIG. 3. Free-space tungsten nanostructures formed based on FIB-grown
tungsten nanowires. (a) Schematics for the construction of joint-nanocon-
tacts/nanogaps by the FIB-induced deformation of vertically grown nano-
wires. (b) SEM image of arrays of free-space joint-contacts. (c) Nanogapped
and branched structures. The scale bar is 1.0 lm.
FIG. 4. The temperature-dependent normalized resistivity of a free-space
tungsten nanostructure fabricated by the FIB-induced deformation of FIB-
grown tungsten nanopillars (red triangle) and that of the as-deposited nano-
pillar without deformation (black circle). The inset shows the SEM side
view image of the corresponding nanostructure.
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143106-4
Cui et al.
Appl. Phys. Lett. 100, 143106 (2012)
A four-terminal-configuration was formed on the fallen
nanowire, and the normalized resistivity was plotted in Fig.
4 (black), which has a slightly steeper superconducting tran-
sition since the measured structure is the nanowire itself
only. The slightly broader transition width present in the
free-space structures is mainly due to the tungsten strips
used to connect the nanostructure to the larger contact pads.
The current density is measured to be about 0.3 ꢂ 104 lA
lmꢃ2. The resistivity of the vertically grown and felled W-
wire is about 520 lX cm,7 and it is about 550 lX cm for the
as-deposited air-bridge structures.19 The resistance of the
measured part is in the range of a few kilo-Ohms. Since the
resistivity difference of the as-deposited, bent, and felled
tungsten nanowires is less than 10%, we conclude that once
a solid contact is formed, the contact resistance is negligible.
Thus, we have demonstrated that the technique of focused-
ion-beam irradiation can be used to form freestanding super-
conducting nanostructures with clean, smooth, and uniform
size as well as reliable superconductivity and mechanical
properties; we expect that this could be a potential method of
forming super-performance superconducting nanodevices.
In conclusion, we have developed a technique for
manipulating the shape and orientation of freestanding
superconducting nano-objects with nanoscale resolution and
accuracy by FIB-induced deformation. The deformation of
the freestanding structures is highly reproducible and con-
trollable. Free-space conducting and superconducting nano-
structures were constructed, and characterization of the
nanostructures’ temperature-dependent electrical properties
suggested that directly modifying the shape of freestanding
nano-objects by ion-beam irradiation could potentially be
used to fabricate versatile building blocks for free-space
multifunctional nanodevices, such as one-node multi-branch
3D functional units, freestanding electrodes, wiring, and
nanoswitches, especially in the construction of supercon-
ducting devices with performance that cannot be achieved
by planar devices (e.g., fabrication of multiple loops in three
dimensions toward the measurement of different field com-
ponents or field gradients).
the incident ion beam, which is a different bending mecha-
nism from those grown on conducting substrates. Factors
such as electrostatic interactions between the irradiated
wires, the substrate, and the impinging ions; the thermal and
electrical properties of the supporting substrate and the irra-
diated wires; and the energy, fluence, and scanning strategy
of the ion beam may all affect the bending direction and
bending rate of the irradiated nanowire. However, theoretical
studies and additional experiments are required in order to
ascertain how to distinguish the effect from each parameter
and to determine which one would dominate the whole pro-
cess; we expect to conduct such research in the near future.
The authors would like to thank Dr. Paul Warburton and
Dr. Jon Fenton (London Centre for Nanotechnology, UK) for
very valuable discussions. This work is supported by the
Outstanding Technical Talent Program of the Chinese Acad-
emy of Sciences; the National Natural Science Foundation of
China under Grant Nos. 91123004, 11104334, 50825206,
10834012, and 60801043; and the National Basic Research
Program (973) of China under Grant No. 2009CB930502.
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We would also like to point out that for ion beam current
ranges from 30 to 100 pA, the bending angle is almost line-
arly proportional to the ion fluence (irrespective of the values
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current as low as 1 pA, no bending was observed, and even
the ion fluence is increased by using a longer exposure time.
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