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Journal ArticleDOI

Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process

01 Apr 2010-Superconductor Science and Technology (IOP Publishing)-Vol. 23, Iss: 4, pp 045015
TL;DR: In this article, a focused ion beam is used to implant Ga 3+ ions and defects instead of etching the film, which could be of interest to engineer high-Tc superconducting devices (SQUIDS, SIS/SIN junctions, Josephson junctions), as well as treat other sensitive compounds.
Abstract: Manufacturing superconducting circuits out of ultrathin films is a challenging task when it comes to pattern complex compounds, which are likely to be deteriorated by the patterning process. With the purpose of developing high-Tc superconducting photon detectors, we designed a novel route to pattern ultrathin YBCO films down to the nanometric scale. We believe that our method, based on a specific use of a focused ion beam, consists in locally implanting Ga 3+ ions and/or defects instead of etching the film. This protocol could be of interest to engineer high-Tc superconducting devices (SQUIDS, SIS/SIN junctions, Josephson junctions), as well as to treat other sensitive compounds.

Summary (2 min read)

1. Introduction

  • They present a good quantum efficiency (QE > 10%), low dark-count rate (DK < 10 Hz), and high operating frequency (> GHz), outperforming in a number of cases InGaAs Avalanche PhotoDiodes (APDs) [1].
  • Their underlying mechanism is based upon the formation of a hotspot in a current-biased superconducting stripe [2].
  • Whereas such devices have been successfully produced with low-Tc superconductors such as NbN operated at 2.6 K [3], their realization with high-Tc compounds remains a challenge.
  • Nanobridges have been fabricated with a Focused Ion Beam [10].

2.1. Overview of the FIB-based protocol

  • The authors designed a 2-step protocol, involving a preliminary chemical etching followed by a focused ion beam (FIB) managed nanostructuration.
  • An overview of the scheme used to create YBCO circuits embedding paths to characterize their transport properties with 4-point measurements is given in figure 1.

2.2. Growth of YBCO films

  • C-axis YBCO films were deposited by RF magnetron sputtering on (100) SrTiO3 substrates.
  • Along with the Ar flow a complementary O2 flow (1:5 ratio) accounts for the growth of a tetragonal, non-superconducting phase, under a total pressure P=8.10-2 mbar.
  • The fringes of X-ray spectra allow to determine a film’s thickness with unit cell accuracy.
  • The sputtering process also deposits CuO2 surface particles located on the top of the YBCO phase, with a typical diameter estimated at 250 nm; those might indeed be the origin of shortcuts after patterning.
  • No reproducible way of getting particle-free samples or eliminating them has been found, however most of the time these particles are not a real obstacle to the patterning process, since the core structure of the devices can most of the times be chosen to be located in a clear area .

2.3. Photolithography and chemical etching

  • The patterning of a 50 µm wide stripe is done by optical lithography and chemical etching with orthophosphoric acid H3PO4 (1%).
  • Four independent structures are patterned in a single run .
  • One of them, top-left on the figure, is specifically designed to electrically characterize the sample at this point of the protocol.

2.4. FIB writing

  • As the second step of the protocol, a dual beam (FEI Nova 600 Nanolab scanning electron microscope / 30 kV focused Ga3+ ion beam) was used to carry out a finer patterning of the YBCO films into meandering stripes suitable for optical characterization of SSPDs.
  • In the context of YBCO thin layers, some precautions to make a relevant use of the FIB are necessary since the standard manner of working is too destructive for the samples.
  • Accordingly, the authors devised a specific modus operandi to satisfy the needed requirements.
  • Lateral contamination is a crucial parameter to consider since it directly addresses both the issues of smallest reachable dimensions and current homogeneity.
  • The supporting stripe has to be manufactured at 1nA because of window size considerations.

3. Results and discussion

  • The ρ vs T curves of the samples can be followed during the different steps of the protocol: after the film deposition the resistivity is measured with the Van der Pauw method [14]; after chemical etching a four-point measurement is carried out along a 50 µm wide stripe using the top-left structure of figure 1a.
  • From this point of view this method differs from EBL/IBE processes where parts of the film are physically removed to create the superconducting pattern, but it also implies caution if one desires to implement it on with thicker films.
  • Both present a negative curvature in the whole range of temperature covered, yielding the existence, in every case, of a true critical current density jc.
  • Figure 7 shows that this model fits extremely well their experimental data and allows to extrapolate jc(0K) ≈ 4.1 MA/cm 2, which is two orders of magnitude smaller than the depairing limit jd = 0.54 Bc µ0λ ≈ 3 108 A/cm2, demonstrating the good quality of the samples after the processing.
  • Some samples were characterized several times at low temperature, with more than one month gone by between the measurements.

4. Conclusion

  • To summarize, a modus operandi to create ultrathin superconducting YBCO circuits by implanting Ga3+ ions with a Focused Ion Beam was devised.
  • The consistency of the resistivity vs temperature profiles measured on the samples at the different steps of the processing ensures the reproducibility of this patterning method for superconducting films.
  • For one sample, the critical current density extrapolated to 0 K has been found to be only two orders of magnitude times smaller than the depairing limit, demonstrating its quality.
  • The most natural application of this protocol would be the manufacturing of high-Tc superconducting devices.

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Article
Reference
Patterning of ultrathin YBCO nanowires using a new
focused-ion-beam process
CURTZ, Noe, et al.
Abstract
Manufacturing superconducting circuits out of ultrathin films is a challenging task when it
comes to patterning complex compounds, which are likely to be deteriorated by the patterning
process. With the purpose of developing high- T c superconducting photon detectors, we
designed a novel route to pattern ultrathin YBCO films down to the nanometric scale. We
believe that our method, based on a specific use of a focused-ion beam, consists of locally
implanting Ga 3 + ions and/or defects instead of etching the film. This protocol could be of
interest for engineering high- T c superconducting devices (SQUIDS, SIS/SIN junctions and
Josephson junctions), as well as to treat other sensitive compounds.
CURTZ, Noe, et al. Patterning of ultrathin YBCO nanowires using a new focused-ion-beam
process. Superconductor science and technology, 2010, vol. 23, no. 4, p. 045015
DOI : 10.1088/0953-2048/23/4/045015
Available at:
http://archive-ouverte.unige.ch/unige:11804
Disclaimer: layout of this document may differ from the published version.
1 / 1

arXiv:1002.0808v1 [cond-mat.supr-con] 3 Feb 2010
Patterning of ultrathin YBCO nanowires using a new
focused-ion-beam process
N Curtz
1,2
, E Koller
2
, H Zbinden
1
, M Decroux
2
,
L Antognazza
2
, Ø Fischer
2
and N Gisin
1
1
Group of Applied Physics, University of Geneva, 1 211, Geneva 4, Switzerland
2
Department of Condensed Matter Physics, University of Geneva, 1211, Geneva 4,
Switzerland
E-mail: Noe.Curtz@unige.ch
Abstract. Manufac tur ing supe rconducting circuits out of ultrathin films is a
challenging task when it comes to pattern complex compounds, which are likely to
be deter iorated by the patterning process. With the purpose of developing high-Tc
supe rconducting photon detectors, we designed a novel route to pattern ultrathin
YBCO films down to the nanometric scale. We believe that our method, based on
a specific use of a focus ed ion beam, consists in locally implanting Ga
3+
ions and/or
defects instead of etching the film. This pr otocol could be of interest to engineer
high-Tc superconducting devices (SQUIDS, SIS/SIN junctions, Josephson junctions),
as well as to treat other sensitive compounds.
PACS numbers: 85.25.Am, 81.16.Nd, 74.78 .Bz, 81.15.Cd, 85 .25.Pb

Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process 2
1. Introduction
Superconducting Single-Photon Detectors (SSPDs) are superconducting devices
designed with the purpose of detecting light down to single-photon level. They present
a good quantum efficiency (QE > 10%), low dark-count rate (DK < 1 0 Hz), and high
operating frequency (> GHz), outperforming in a number of cases InGaAs Avalanche
PhotoDiodes (APDs) [1]. These characteristics make them a premium candidate
for single-photon telecommunication and applications like Quantum Key Distribution.
Their underlying mechanism is based upon the formation of a hotspot in a current-biased
sup erconducting stripe [2]. The creation of the hotspot is triggered by the incoming
photon whose energy locally thermalizes the stripe, confining the bias current hence
raising its density, up to the point of overcoming its critical value, resulting in a local
transition of the stripe.
To achieve that, t he circuit geometry has to fulfill drastic geometrical constraints.
First, the stripe has to be narrow enough, otherwise the variation of the current
density isn’t sufficiently significant, preventing the transition to take place and the
voltage pulse to be detectable. The section of the nanowire should also be extremely
homogeneous, since any constriction locally lowers the critical current, hence affects the
whole device with f or aftermath a drop of the QE; the closer to 1 is the r atio I
bias
/I
c
,
the closer to the dissipative state is the device, and therefore the more important is
the section homogeneity. Finally, the detector’s recovery time is g overned by the device
thickness, necessitating devices less than 15 nm thick. Whereas such devices have been
successfully pro duced with low-Tc superconductors such as NbN operated at 2.6 K [3],
their realization with high-Tc compounds remains a challenge. High-Tc SSPDs would
nevertheless allow a higher working temperature, hence a significant reduction of the
associated cryogenic costs.
Among high-T
c
materials, cuprates present the advantage of a low kinetic
inductance, leading to fa st resp onse times. From a purely structural point of view,
Nd
1+x
Ba
2-x
Cu
3
O
7+δ
presents excellent crystallog raphic and planeity properties [4],
which are interesting features given the aforementioned geometrical constraints of
SSPDs. It t urns out however that the intrinsic loose stoichiometry of Nd atoms, who
interdiffuse with Ba ones, leads to a high unstability of the oxygen content and therefore
to an important lo ss of T
c
during the patterning process. YBa
2
Cu
3
O
7-δ
(YBCO) is
much more stable and appears to be a good candidate for high-T
c
detectors. Several
routes have been carried out to create junctions or patterned structures out of high-T
c
thin films, such as Selective Epitaxial Growth [5], Electron Beam Lithography / Ion
Beam Etching (EBL/IBE) [6 ], Ion Irradiation [7], using an Atomic Force Microscope
[8] or a Focused Electron Beam Irradiation [9]. Nanobridges have been fabricated with
a Focused Ion Beam [10]. However those experiments were performed on films with
thickness d > 20 nm. Here we r eport a new method using such an apparatus to write
an arbitrary pattern upon an ultrathin (< 20 nm) film, allowing to manufacture YBCO
sup erconducting circuits. The key point of this method is that to produce the structure

Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process 3
the superconducting phase is locally altered rather than etched.
2. Experimental
2.1. Overvie w of the FIB-based p rotocol
We designed a 2-step protocol, involving a preliminary chemical etching followed by a
focused ion beam (FIB) managed nanostructuration. An overview of the scheme used to
create YBCO circuits embedding paths to characterize their transp ort properties with
4-point measurements is given in figure 1.
2.2. Growth o f YBCO films
c-axis YBCO films were deposited by RF magnetron sputtering on (100) SrTiO
3
substrates. Samples are heated to 700
C and exposed to the sputtering of a YBCO
target in a Argon plasma. Along with the Ar flow a complementary O
2
flow (1:5
ratio) accounts for the growth of a tetragonal, non-superconducting phase, under a
total pressure P=8.1 0
-2
mbar. An in-situ 2- ho ur-long annealing at 580
C in a 600 mbar
O
2
atmosphere causes this YBa
2
Cu
3
O
6
phase to undergo a tetragonal-orthorhombic
transition to optimally-doped superconducting YBa
2
Cu
3
O
7-δ
. The critical temperature
of bulk YBCO is 92 K; this critical temperature decreases with the film’s thickness, down
to T
c0
(d=12 nm) 80 K. The high crystallinity of the films was demonstrated with
X-ray diffraction measurements such as depicted in figure 2. Up to 4 degrees the gra zing
incidence scan leads to Kiessig fringes contributions fro m both films [11]. Around the
(001) YBCO Bragg peak are located secondary fringes clearly showing a finite size effect
(Laue oscillations), demonstrating the high quality of the crystallographic layers. The
fringes of X-ray spectra allow to determine a film’s thickness with unit cell accuracy. In
the following, all the films processed are at d=12 nm, with T
c0
80 K.
The sputtering process also deposits CuO
2
surface part icles located on the top of
the YBCO phase, with a typical diameter estimated at 25 0 nm; those might indeed be
the origin of shortcuts after patterning. No reproducible way of getting particle-free
samples or eliminating them has b een found, however most of the time these particles
are not a real obstacle to the patterning process, since the core structure of the devices
can most of the times be chosen to be located in a clear area (figure 1c). Even when
it’s not the case, such as in figure 1d, table 2 shows that ultimately these particles are
not an issue.
In order to improve electrical contacts, we adapted a gold evaporation system
involving a mechanical mask to deposit g old slots in-situ immediatly aft er the YBCO
deposition. In addition it was observed that the in-situ deposition of a 8 nm thick
amorphous PrBaCuO passivation cap layer over the whole sample attenuates the loss of
critical temperature occurring during the chemical etching process [12]. Therefore, we
end up with a YBCO/Au/PBCO topology.

Patterning of ultrathin YBCO nanowires using a new focused-ion-beam process 4
Table 1. Room-temperature resistance of a FIB-made barrier across a 20 µm wide
stripe for various numbers of passes (N) with a 50 pA beam.
N 0 1 2 5 20 50 100
R () 2k 2k 3k 700k 3M >10M >10M
2.3. Photolithography and chemical etching
The patterning of a 50 µm wide stripe is done by optical lithography and chemical
etching with orthophosphoric acid H
3
PO
4
(1%). Four independent structures are
patterned in a single run (figure 1a). One of t hem, top-left on the figure, is specifically
designed to electrically characterize the sample at this point of the pro t ocol.
2.4. FIB writing
As the second step of the protocol, a dual beam (FEI Nova 600 Nanolab scanning
electron microscope / 30 kV focused Ga
3+
ion beam) was used to carry out a
finer patterning of the YBCO films into meandering stripes suitable for optical
characterization of SSPDs. In the context of YBCO thin layers, some precautions to
make a relevant use of the FIB are necessary since the standard manner of working is
too destructive for the samples. Accordingly, we devised a specific modus operandi to
satisfy the needed requirements.
First of all, due to the extreme thickness of the involved films, the alignment routines
cannot be handled with the standard procedure, as the whole window, containing areas
destined to remain superconducting is exposed to the Ga
3+
beam during t he operation,
and irreversibly damaged. To circumvent this problem we have to synchronize the SEM
and the FIB on a non- critical area, then align the pattern to etch with the sample using
the SEM.
Secondly, to find which parameters (intensity, number of passes, numerical
aperture...) are optimal to produce an effective pattern, we tried to separate two YBCO
areas by creating FIB-made barriers and injecting current through them; results for a
50 pA beam are shown in table 1. We determined that at 50 pA, with a beam diameter
δ 20 nm, 50 passes are optimal to electrically isolate both areas, while a t 1 nA, with a
beam diameter δ 100 nm, 10 passes are optimal. The fact that for N>20 the resistance
exceeds 3 M rules out an exclusive YBCO
7
to YBCO
6
transformation due to oxygen
loss, for if it were the case, the ba rr ier transition would have resulted in a 5 k resistance
(assuming very conservatively ρ(underdoped)=10
-1
.cm at room temperature [13]) for
a 1 µm wide affected width. Even if the whole stripe had been transformed to YBCO
6
its resistance wouldn’t have exceeded 150 k. We attribute this result to the fact that
some Ga
3+
ions are implanted or give rise to columnar defects into the YBCO phase
during the exposure, turning it locally into an insulator.
Lateral contamination is a crucial parameter to consider since it directly addresses

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Frequently Asked Questions (2)
Q1. What contributions have the authors mentioned in the paper "Patterning of ultrathin ybco nanowires using a new focused-ion-beam process" ?

CURTZ et al. this paper used a focused ion beam to implant Ga 3 + ions and/or defects instead of etching the film. 

Photoresponse experiments to characterize the devices as single-photon detectors are left for future work.