Download Nanolithography based contacting method for electrical

INSTITUTE OF PHYSICS PUBLISHING
NANOTECHNOLOGY
Nanotechnology 16 (2005) 2936–2940
doi:10.1088/0957-4484/16/12/036
Nanolithography based contacting
method for electrical measurements on
single template synthesized nanowires
S Fusil1 , L Piraux2,4 , S Mátéfi-Tempfli2 , M Mátéfi-Tempfli2 ,
S Michotte2 , C K Saul3,5 , L G Pereira3 , K Bouzehouane3 , V Cros3 ,
C Deranlot3 and J-M George3
1
LMN, Université d’Evry Val d’Essonne, Boulevard F Mitterrand, 91025 Evry cedex, France
Unité de Physico-Chimie et de Physique des Matériaux, Place Croix du Sud, 1,
1348-Louvain-la-Neuve, Belgium
3
Unité Mixte de Physique CNRS/Thales and Université Paris Sud XI, Route départementale
128, F-91767 PALAISEAU Cedex, France
2
E-mail: [email protected]
Received 1 August 2005, in final form 7 September 2005
Published 25 October 2005
Online at stacks.iop.org/Nano/16/2936
Abstract
A reliable method enabling electrical measurements on single nanowires
prepared by electrodeposition in an alumina template is described. This
technique is based on electrically controlled nanoindentation of a thin
insulating resist deposited on the top face of the template filled by the
nanowires. We show that this method is very flexible, allowing us to
electrically address single nanowires of controlled length down to 100 nm
and of desired composition. Using this approach, current densities as large
as 109 A cm−2 were successfully injected through a point contact on a single
magnetic multilayered nanowire. This demonstrates that the technique is
very promising for the exploration of electrical spin injection in magnetic
nanostructures.
1. Introduction
Over the last decade, many groups have been exploring the
concept of using the pores in nanoporous media as templates
to prepare arrays of nanowires or nanotubes by various
synthesis techniques. This so-called ‘template method’ makes
it possible to fabricate various nanowires that are difficult to
form using a conventional nanolithographic process. Though
there now exists a huge range of hosts, most studies in
this area have entailed the use of two types of templates:
track-etch polymer membranes and nanoporous aluminas.
Under optimized fabrication conditions, these two types of
templates exhibit well defined nanoporosity with geometrical
parameters controllable to a large extent [1, 2]. Also,
depending on the thickness of the nanoporous media, above
or below a few micrometres, these templates are either
4 Author to whom any correspondence should be addressed.
5 Present address: Departamento de Fisica, Universidade Federal do Parana,
CP 19044, 81531-990, Curitiba, PR, Brazil.
0957-4484/05/122936+05$30.00 © 2005 IOP Publishing Ltd
self-supported or supported on a substrate [3, 4]. This
templating method has proven to be reliable for the synthesis
of a variety of nanowired materials including noble metals,
ferromagnets, superconductors, semimetals, alloys, oxides,
carbon nanotubes, conducting polymers and different types
of multilayers. The resulting cylinders have diameters ranging
between a few nanometres and a few microns and the aspect
ratio (length to diameter) can be controlled to a large extent
up to 103 .
Using different approaches leading to measurements on
both single wires and/or arrays, a variety of novel physical
properties and potential applications have been identified in
relation to the nanowire composition and to their nanoscopic
dimensions [5]. Fundamental transport properties such as
localization and electron–electron interaction effects have
been studied in ultrathin electrodeposited Au nanowires [6].
Nanowires of pure Bi and of n-type-doped Bi were fabricated
either by pressure injecting liquid melt into the nanochannels
of anodic alumina templates or by electrodeposition. These Bi
Printed in the UK
2936
Nanolithography based contacting method on single nanowire
nanowires show finite size effects, promising thermoelectric
properties and semimetal–semiconductor transition when the
diameter is reduced [7, 8]. Other interesting results were
recently obtained in 1D superconducting nanowires such
as the destruction of superconductivity by the formation
of phase slip centres [9, 10]. Nanotubules of conducting
polymers have also been prepared either electrochemically
or by chemical synthesis. These nanotubes show enhanced
conductivity compared to bulk materials and controllable
dimensionality of conduction [11, 12]. For electrodeposited
magnetic nanowires, magneto-transport measurements have
enabled us to investigate field induced magnetization reversal
mechanisms [13, 14] and electronic transport through
domain walls [15]. In addition, the transport properties
of nanowires have attracted a great deal of research
interest because the current is flowing naturally along the
axis of segmented nanowires, i.e. in the so-called CPPGMR geometry (current perpendicular to the plane–giant
magnetoresistance). Various magnetic multilayered nanowires
have been successfully fabricated using a single bath and
a pulse-plating method [16]. CPP-GMR studies on a large
set of nanowires with different materials and structures have
allowed us to extract a large bunch of important parameters
for spintronics such as spin diffusion length, spin asymmetry
coefficients in the bulk and at the interface, and interface
resistance [17]. Current induced magnetization has been
reported in Co/Cu multilayered wires [18] as well as quantized
spin transport in nanoconstrictions based on electrodeposition
in nanopores [19].
In most cases, the electrical measurements are performed
by attaching leads on either side of the nanoporous media,
so that the number of wires contributing to the conduction
measurements is unknown. However, it is often highly
desired to perform electrical transport measurements on single
nanowire. Till now, two different experimental approaches
have been developed to address electrically one single
nanowire. In the former the wires are kept embedded in the
nanoporous media, while in the latter the wires are removed
from the host nanoporous material and electrodes are next
attached to some of them.
Considering the first approach, a self-contacting technique
has been successfully employed by using polymer templates
with relatively low pore density (typically less than
In this method, prior to
109 pores cm−2 ) [13, 14].
electrodeposition, a metallic film is deposited on the top face of
the membrane. This film is thin enough to ensure that the pores
are not closed, so they can thereafter be filled by the solution.
Since the wires in the array do not grow exactly at the same
rate, the first emerging wire (detected by a sharp increase of
the plating current) stops the growth of the others. Using this
procedure, a single nanowire is contacted reliably during the
growth process and electrical contacts are therefore established
on the two extremities of a 6–20 µm long nanowires, allowing
two-probe measurements. Due to the extremely large aspect
ratio of the nanowire, the contact resistance is usually small
compared to the sample resistance. However, this method is
not suitable using anodic porous alumina as template because
of the much higher pore density (up to 1011 pores cm−2 ), that
makes it difficult to contact a single wire. This drawback of the
porous alumina template was partially solved [20] using a pair
of crossed narrow electrodes patterned on the opposite faces
of a 60 µm thick alumina membrane, allowing us to contact
more easily a single or a few nanowires.
A different way was followed by using a singlepore membrane [21], in which a Bi nanowire was grown
electrochemically and contacted using two macroscopic
electrodes on either side of the membrane.
The second approach is based on the use of optical
or e-beam lithography technique to connect a single
nanowire [22–24]. The process requires several steps prior to
the lithography process, which have been carefully optimized,
including the dissolution of the membrane, the dispersion
of an appropriate density of wires on a substrate and the
cleaning of the wires to remove all residues from the surface.
The advantage of the lithography technique is that four-point
electrical measurements can be performed on an isolated
nanowire and that segments as small as 500 nm can be
probed [22]. However, the extremely high surface to volume
ratio of the nanowires makes them more sensitive to oxidation
when removed from the template. Therefore, it is often
difficult to prevent nanowire burn-out by static discharge due
to high resistance electrical contacts between the nanowires
and the finger-like electrodes (mainly caused by oxidation and
imperfect cleaning of the nanowires).
In this paper, we propose a new reliable method that
allows the electrical connection of one single nanowire in a
large assembly of wires embedded in a nanoporous template
with a high pore density (such as anodic porous alumina
template). Nanocontacts on single wires are performed
by indentation of an ultrathin insulating photoresist layer
deposited on the top face of a thin porous alumina layer
(1 µm) supported on a silicon substrate in which the lengths
of the electrodeposited nanowires are uniform and equal to the
thickness of the template. A modified atomic force microscope
(AFM) designed for local resistance measurements [25] is used
as a nanoindenter, yielding an access point onto individual
nanowires at the surface of the template. The indented holes
are subsequently filled by metal to establish contacts on single
nanowires.
2. Experimental details
Figure 1 illustrates the fabrication process used to create
a nanocontact onto a single wire embedded in a thin
alumina template. The process begins with the anodization
of a ∼0.5 µm thick Al layer evaporated onto the
Ti (5 nm)/Au (100 nm)/Nb (20 nm)/Si substrate. While the
Ti and the Nb layers serve as buffer and adhesion layers,
the gold layer was used as electrode for the subsequent
electrochemical and measurement processes. The nanopores
in the template are formed by complete anodic oxidation of
the Al layer, performed in 0.3 M oxalic acid solution at 2 ◦ C
under a constant voltage of 60 V. At the end of the anodization
process, residual alumina was eliminated completely from the
bottom of all the nanopores, by etching with a solution of
5 wt% H3 PO4 at 30 ◦ C, in order to fill conducting material
into the nanopores electrochemically using the gold layer as a
working electrode on which electrodeposition takes place. The
interpore distance and pore length increase in proportion to the
applied anodization voltage and time, respectively. The use
2937
S Fusil et al
Figure 1. Schematic illustration of the single-wire contacting
process on an array of nanowires electrodeposited in a supported
nanoporous alumina template.
of supported alumina templates prevents one from breaking
the fragile template during the handling and nanocontact
fabrication process. Such a type of templates also shows
improved thermal stability (up to 1000 ◦ C) compared to thick
self-supported alumina membranes. The final thickness of the
alumina template was about 0.75 µm and the pore density was
close to 1010 cm−2 . The pore diameter used in this study was
85 nm and the interpore distance was about 140 nm.
Electrodeposition into the pores was carried out in a
conventional three-electrode cell with a platinum counterelectrode and a Ag/AgCl reference electrode. Arrays of
Co/Cu multilayered nanowires were fabricated using a pulseplating method in which the two metals are deposited from a
single sulfate bath (with copper kept in dilute concentration)
by switching between the deposition potentials of the two
constituents [16]. Co (14 nm)/Cu (7 nm) nanowires were
electrodeposited within nanopores and the deposition process
was stopped as soon as the first nanowires emerged at the
surface, as detected by a rapid increase of the electrochemical
current. This corresponds to an increase of the effective
electrode area caused by the formation of caps on the
template surface.
The surface of the filled alumina template at this stage
was examined by conductive tip atomic force microscopy (CTAFM): when a tip–sample bias is applied, both topographical
and local resistivity data are acquired simultaneously. As
the growth rate is inhomogeneous in different pores, only a
few pores show a complete filling followed by hemispherical
metallic cap growth at the end of the deposition process; see
figure 2(a). Prior to the nanolithography step, the template
was thinned by mechanical polishing using colloidal silica
2938
(Syton® ) in such a way that a large proportion of the nanowires
end at the template surface, figure 2(b). The thinning procedure
of the alumina/nanowire layer is controlled with a resolution
of few tens of nanometres. This makes it possible to adjust
the length of the nanowire over a wide range (from a few
micrometres down to ≈100 nm). In figures 2(a) and (b), we
show the top view (topography and resistance) images of the
alumina pore structure filled with Co/Cu nanowires before (a)
and after (b) polishing. The conductive surface ratio increases
from 2% for the as-grown sample up to 37% after the second
polishing step. At this stage, the measured wire length is about
350 nm, corresponding to an aspect ratio of 4, and about 50%
of the pores are filled with metal. During the mechanical
polishing, an intrinsic roughness of 20 nm corresponding to
the abrasive particle size is created. As a consequence, the
nanowires emerge from about 20 nm above the alumina top
surface (figure 2(c)).
In the fabrication process of the nanocontact, we first
deposit by spin coating a thin (60 nm) insulating polymer
resist. Then a second thick (1.2 µm) photoresist is spin
coated and designed by a classical optical lithographic process:
a 30 × 30 µm2 aperture is opened on the thin base resist.
The electrical insulation realized by the thin resist inside the
aperture is checked by CT-AFM. The CT-AFM is then used to
create a hole by indentation of the 60 nm resist until a good
electrical contact between the tip and the underlying nanowire
is obtained. More details about this nanoindentation step can
be found in [26]. The bottom of the hole is enlarged and
cleaned from resist residues by a soft O2 –Ar plasma etching.
Then a 100 nm Au layer is sputtered to fill this access point to
the nanowire and create the nanocontact. The overall shape of
this nanocontact can be represented by the inverted 3D image
of the indented hole obtained by tapping mode AFM using
ultrasharp tips, figure 2(d). This inverted 3D image is just a
trick to visualize the shape and size of the nanocontact to be
deposited since it is not possible to see the bottom of the hole
in the raw image (image inversion option of the AFM data
processing software: the bottom of the indented hole, i.e. the
future contact surface, is transformed to the top of the hill). The
interpore distance (140 nm, figure 2(c)) is significantly larger
than the size of the hole (≈50 nm, figure 2(d)) and thus the
probability to connect a single wire is very high. Finally, the
top electrical contact is designed in the Au layer by subsequent
technological steps such as optical lithography and Ar ion
milling. The area of each alumina template is about 1 cm 2 ,
on which are fabricated about 15 nanocontacts. The whole
process allows us to connect separately individual nanowires
at different locations on the same sample and provides an easy
way to perform single-wire magneto-transport measurements.
In the inset of figure 3, we show an I (V ) curve obtained
at room temperature on a single Co/Cu nanowire. An Ohmic
behaviour is found for dc current up to 12 mA (otherwise,
all electrical measurements were performed using dc signals),
which corresponds to a current density of 2.5 × 108 A cm −2 .
This proves the very good quality of the electrical contact on
both extremities of the wire. The wire resistance (∼10 )
is consistent with the estimated mean resistivity of the
multilayer (∼15 µ cm) and geometrical parameter (S/L ∼
1.5 × 10−6 cm). The GMR curve obtained with the field
perpendicular to the wire axis is reported in figure 3. A GMR
Nanolithography based contacting method on single nanowire
Figure 2. CT-AFM 10 µm × 10 µm images (left, topography; right, local resistivity) of as-grown sample with a few emerging nanowires
with hemispherical conducting heads (a) and polished sample with 37% of conductive surface ratio (b); tapping mode AFM 1 µm × 1 µm
image of magnetic nanowires embedded in a nanoporous alumina template after polishing (c); tapping mode AFM inverted image of a
typical nanoindented hole after plasma etch, x y scale is 1 µm, z scale is 100 nm (d).
Figure 3. Variation of the resistance as a function of the magnetic field at 300 K measured on a single Co/Cu multilayered nanowire. The
magnetic field is applied perpendicular to the wire axis. Dark squares correspond to an applied current of 100 µA and open circles to an
applied current of 15 mA. The inset shows the I (V ) curve.
ratio of 3.1% has been measured under an excitation current
of 100 µA. This GMR value is in good agreement with
that expected from previous studies on the Co/Cu nanowires
system [17, 27]. To demonstrate the stability of the system
when high current density is reached, we also add in figure 3
the magnetoresistance curve recorded with a current of 15 mA
(corresponding to a current density of 3 × 108 A cm−2 ).
A small increase of the wire resistance leading to a small
decrease of the GMR ratio (2.95%) was observed. This can
be attributed to an increase of the nanowire temperature due
2939
S Fusil et al
to Joule heating. The first effect of degradation, evidenced
by irreversible change of the wire resistance, appears for a
current density of 6 × 108 A cm −2 . Finally, burn-out of the
nanowire was observed at 1.8×109 A cm −2 . A possible reason
for the very good resistance of the nanowires to high current
densities is that heat is evacuated by conduction through the
alumina matrix surrounding the nanowire. Thanks to the very
large wire density in alumina templates, heat transfer is also
favoured by the neighbouring wires that increase the thermal
conductivity of the Al2 O3 /metal nanocomposite system.
3. Conclusions
As a conclusion, we demonstrate a new reliable method that
allows the electrical connection of one single nanowire in a
large assembly of wires embedded in a nanoporous template
with a high pore density (such as an anodic porous alumina
template). Nanocontacts on single wires are performed by
indentation of an ultrathin insulating photoresist layer by using
a modified atomic force microscope (AFM) designed for local
resistance measurements. The good quality of the contact was
evidenced by magnetoresistance measurement. CPP-GMR
was clearly visible and we have shown that current densities as
large as 6 × 108 cm −2 can pass through the nanowire without
deterioration. Our approach is very flexible and promising
to investigate the influence of high electric current density
on the magnetic state of a ferromagnet, i.e. current induced
magnetization reversal, domain wall motion and magnon
generation.
Acknowledgment
The work was partially supported by the Belgian Science
Policy through the Interuniversity Attraction Pole Program PAI
(P5/1/1).
References
[1] See Ferain E and Legras R 2001 Nucl. Instrum. Methods B 174
116 and references therein
[2] Furneaux R C, Rigby W R and Davidson A P 1989 Nature
337 147
Masuda H and Fukuda K 1995 Science 268 1466
[3] Rabin O, Herz P R, Lin Y M, Akinwande A I, Cronin S B and
Dresselhaus M S 2003 Adv. Funct. Mater. 13 631
2940
[4] Dauginet-De Pra L, Ferain E, Legras R and
Demoustier S 2002 Nucl. Instrum. Methods B 196 81
[5] For recent reviews, see He H and Tao N J 2003 Encyclopedia
of Nanoscience and Nanotechnology vol 2 (Stevenson
Ranch, CA: American Scientific Publishers) p 755
Piraux L, Encinas A, Vila L, Mátéfi-Tempfli S,
Mátéfi-Tempfli M, Darques M, Elhoussine F and
Michotte S 2005 J. Nanosci. Nanotech. 5 372 and
references therein
[6] Williams W D and Giordano N 1986 Phys. Rev. B 33 8146
[7] Zhang Z, Sun X, Dresselhaus M S, Ying J Y and
Heremans J P 1998 Appl. Phys. Lett. 73 1589
[8] Heremans J P and Thrush C M 1999 Phys. Rev. B 59 12579
[9] Dubois S, Michel A, Eymery J P, Duvail J L and
Piraux L 1999 J. Mater. Res. 14 665
[10] Michotte S 2003 Int. J. Mod. Phys. B 17 4601
[11] Martin C R 1994 Science 266 1961
[12] Spatz J P, Lorenz B, Weishaupt K, Hochheimer H D,
Menon V, Parthasarathy R, Martin C R, Bechtold J and
Hor P H 1994 Phys. Rev. B 50 14888
[13] Wegrowe J E, Kelly D, Franck A, Gilbert S E and Ansermet J P
1999 Phys. Rev. Lett. 82 3681
[14] Pignard S, Goglio G, Radulescu A, Piraux L, Duvail J L,
Dubois S and Declémy A 2000 J. Appl. Phys. 87 824
[15] Ebels U, Radulescu A, Henry Y, Piraux L and
Ounadjela K 2000 Phys. Rev. Lett. 84 983
[16] Piraux L, George J M, Despres J F, Leroy C, Ferain E,
Legras R, Ounadjela K and Fert A 1994 Appl. Phys. Lett. 65
2484
[17] For a review, see Fert A and Piraux L 1999 J. Magn. Magn.
Mater. 200 338 and references therein
[18] Wegrowe J E, Fabian A, Guittienne P, Hoffer X, Kelly D and
Ansermet J P 2002 Appl. Phys. Lett. 80 3775
[19] Elhoussine F, Mátéfi-Tempfli S, Encinas A and Piraux L 2002
Appl. Phys. Lett. 81 1681
[20] Wu W, DiMaria J B, Yoo H G, Pan S, Rothberg L J and
Zhang Y 2004 Appl. Phys. Lett. 84 966
[21] Toimil Molares M E, Chtanko N, Cornelius T W, Dobrev D,
Enculescu I, Blick R H and Neumann R 2002
Nanotechnology 15 S201
[22] Vila L, Piraux L, George J M, Fert A and Faini G 2002 Appl.
Phys. Lett. 80 3805
[23] Toimil Molares M E, Höhberger E M, Schaeflein Ch,
Blick R H, Neumann R and Trautmann C 2003 Appl. Phys.
Lett. 82 2139
[24] Tanase M, Silevitch D M, Chien C L and Reich D H 2003
J. Appl. Phys. 93 7616
[25] Houzé F, Meyer R, Schneegans O and Boyer L 1996 Appl.
Phys. Lett. 69 1975
[26] Bouzehouane K, Fusil S, Bibes M, Carrey J, Blon T, Le Dû M,
Seneor P, Cros V and Vila L 2003 Nano Lett. 3 1599
[27] Ohgai T, Hoffer X, Gravier L, Wegrowe J-E and
Ansermet J-P 2003 Nanotechnology 14 978