Josephson junctions and superconducting quantum interference devices
made by local oxidation of niobium ultrathin films
V. Bouchiat
a)
and M. Faucher
GPEC, UMR CNRS 6631, Universite
´
de la Me
´
diterrane
´
e, Case 901, 13288 Marseille, France
C. Thirion and W. Wernsdorfer
Laboratoire Louis Ne
´
el, UPR CNRS 5051, BP 166, 38042 Grenoble, France
T. Fournier and B. Pannetier
CRTBT, UPR CNRS 5001, BP 166, 38042 Grenoble, France
͑Received 9 November 2000; accepted for publication 8 May 2001͒
We present a method for fabricating Josephson junctions and superconducting quantum interference
devices ͑SQUIDs͒ which is based on the local anodization of niobium strip lines 3–6.5 nm thick
under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID
loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link
geometries are fabricated: lateral constriction ͑Dayem bridges͒ and variable thickness bridges.
SQUIDs based on both geometries show a modulation of the maximum Josephson current with a
magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4
K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the
weak link size becomes comparable to the superconducting film coherence length
which is of the
order of 10 nm. © 2001 American Institute of Physics. ͓DOI: 10.1063/1.1382626͔
Nanolithography using scanning probe microscopes
͑SPMs͒ offers powerful methods
1
for patterning surfaces
with a resolution beyond the range of conventional lithogra-
phies based on resist exposures. During the last decade, these
techniques have brought some important features to device
fabrication, such as easy alignment and in situ control of the
device electrical characteristics during its fabrication.
2
Fur-
thermore, local probe techniques, based on near field inter-
actions, show a greatly reduced proximity effect
3
which lim-
its resolution in e-beam lithography. On the one hand,
lithography using UHV-scanning tunneling microscopes
͑STMs͒ leads to ultimate resolution ͑i.e., at the atomic
scale͒,
4,5
but the building of structures with a permanent and
stable electrical connection
6
has not been completely
achieved. On the other hand, non-UHV-SPM lithography
techniques are mainly based on the atomic force microscope
͑AFM͒. These latter techniques, based either on tip
indentation
7–9
or on a voltage biased tip,
10–19
retain nanom-
eter scale resolution and show better versatility. Among these
resistless AFM lithographies, local anodization of the surface
of a semiconductor
11,12
or of non-noble metals
13–15
by the
biased tip of an AFM is a versatile method by which to make
nanoscale quantum devices. Quantum point contacts,
11,13
nanowires,
12
single electron devices,
14
superconducting
devices
16
as well as other nanoscale devices involving
nanotubes
17
or clusters
18
have been obtained.
In order to fabricate the structure in a single step, the
film thickness must be less than the typical depth of oxidized
metal ͑i.e., 10 nm͒, thus allowing direct writing of fully in-
sulating regions. Such a process can be controlled well and is
sufficiently reproducible to control the oxide linewidth to
values defining either a complete electrical separation or, for
a single line drawn at high speed and low voltage, a metal/
insulator/metal tunnel barrier with low transparency.
15
The
intrinsic ultrasmall capacitance of such tunnel junctions has
been exploited to produce single electron devices operating
at room temperature.
14
In this letter, we present an application of this anodiza-
tion technique for fabricating superconducting nanostruc-
tures using high quality ultrathin niobium films. As an initial
demonstration of potential applications for mesoscopic su-
perconductivity, we have made and tested at low temperature
a series of dc-superconducting quantum interference devices
͑SQUIDs͒ based on microbridge technology with various
geometries.
A single crystal sapphire wafer was chosen as the sub-
strate for ultrathin film growth. It has a 11
¯
02 orientation with
an off-axis miscut as low as possible ͑about 10
Ϫ3
rad͒. After
thermal treatment at 1100 °C for1hinair,
19
the sapphire
surface is reconstructed such that 0.3–0.8
m wide, atomi-
cally flat terraces separated by 0.3 nm high steps are ob-
served ͓visible in Fig. 1͑b͔͒. A niobium layer with thickness
of either 3 or 6.5 nm is then epitaxialy grown using an elec-
tron gun evaporator in UHV conditions. The sapphire sub-
strate was in situ cleaned by Ar ion milling and heated at
550 °C during Nb deposition.
20
In order to prevent rapid ag-
ingofthefilm,a2nmthick silicon layer is deposited on top
at room temperature in the same vacuum.
21
As expected, the
films show superconducting properties slightly depressed
with respect to the bulk:
20
the critical temperatures of the
bare films are, respectively, 5 and 6.6 K, while their residual
resistivity ratios ͑Tϭ300/4.2 K͒ are 1.5 and 2.2.
22
Before proceeding to the AFM lithography, a prefabrica-
tion step is performed in order to define the electrical con-
nections. The film is patterned using standard UV lithogra-
phy techniques and is dry etched in a SF
6
plasma to define
a͒
Author to whom correspondence should be addressed; electronic mail:
bouchiat@gpec.univ-mrs.fr
APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 1 2 JULY 2001
1230003-6951/2001/79(1)/123/3/$18.00 © 2001 American Institute of Physics