Application of arc plasma for a deposition of superconducting films
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2002
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| Цитувати: | Application of arc plasma for a deposition of superconducting films / J. Langner, R. Russo, L. Catani, S. Tazzari, M. Cirillo, K. Czaus, V. Merlo, F. Tazzioli, D. Proch, N.N. Koval, I.V. Lopatin // Вопросы атомной науки и техники. — 2002. — № 4. — С. 161-164. — Бібліогр.: 4 назв. — англ. |
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irk-123456789-803242015-04-15T03:01:55Z Application of arc plasma for a deposition of superconducting films Langner, J. Russo, R. Catani, L. Tazzari, S. Cirillo, M. Czaus, K. Merlo, V. Tazzioli, F. Proch, D. Koval, N.N. Lopatin, I.V. Low temperature plasma and plasma technologies 2002 Article Application of arc plasma for a deposition of superconducting films / J. Langner, R. Russo, L. Catani, S. Tazzari, M. Cirillo, K. Czaus, V. Merlo, F. Tazzioli, D. Proch, N.N. Koval, I.V. Lopatin // Вопросы атомной науки и техники. — 2002. — № 4. — С. 161-164. — Бібліогр.: 4 назв. — англ. 1562-6016 PACS: 52.77.-j http://dspace.nbuv.gov.ua/handle/123456789/80324 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
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English |
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Low temperature plasma and plasma technologies Low temperature plasma and plasma technologies |
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Low temperature plasma and plasma technologies Low temperature plasma and plasma technologies Langner, J. Russo, R. Catani, L. Tazzari, S. Cirillo, M. Czaus, K. Merlo, V. Tazzioli, F. Proch, D. Koval, N.N. Lopatin, I.V. Application of arc plasma for a deposition of superconducting films Вопросы атомной науки и техники |
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Article |
| author |
Langner, J. Russo, R. Catani, L. Tazzari, S. Cirillo, M. Czaus, K. Merlo, V. Tazzioli, F. Proch, D. Koval, N.N. Lopatin, I.V. |
| author_facet |
Langner, J. Russo, R. Catani, L. Tazzari, S. Cirillo, M. Czaus, K. Merlo, V. Tazzioli, F. Proch, D. Koval, N.N. Lopatin, I.V. |
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Langner, J. |
| title |
Application of arc plasma for a deposition of superconducting films |
| title_short |
Application of arc plasma for a deposition of superconducting films |
| title_full |
Application of arc plasma for a deposition of superconducting films |
| title_fullStr |
Application of arc plasma for a deposition of superconducting films |
| title_full_unstemmed |
Application of arc plasma for a deposition of superconducting films |
| title_sort |
application of arc plasma for a deposition of superconducting films |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2002 |
| topic_facet |
Low temperature plasma and plasma technologies |
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http://dspace.nbuv.gov.ua/handle/123456789/80324 |
| citation_txt |
Application of arc plasma for a deposition of superconducting films / J. Langner, R. Russo, L. Catani, S. Tazzari, M. Cirillo, K. Czaus, V. Merlo, F. Tazzioli, D. Proch, N.N. Koval, I.V. Lopatin // Вопросы атомной науки и техники. — 2002. — № 4. — С. 161-164. — Бібліогр.: 4 назв. — англ. |
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Вопросы атомной науки и техники |
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LOW TEMPERATURE PLASMA AND PLASMA TECHNOLOGIES
APPLICATION OF ARC PLASMA FOR A DEPOSITION
OF SUPERCONDUCTING FILMS
J.Langner1, R.Russo2,3, L.Catani2, S.Tazzari2,3, M.Cirillo4, K.Czaus1), V.Merlo4, F.Tazzioli5,
D.Proch6, N.N.Koval7, I.V.Lopatin7
1)The Andrzej Soltan Institute for Nuclear Studies, Swierk, Poland
2)INFN Roma2, Via della Ricerca Scientifica 1, 001333 Roma, Italy
3)University Tor Vergata, Via della Ricerca Scientifica 1, 001333 Roma, Italy
4)INFM Roma 2, Via della Ricerca Scientifica 1, 001333 Roma, Italy
5)INFN, LNF, P.O. Box 13, 00044 Frascati, Italy
6)DESY, Notkestr. 85, 22 603 Hamburg, Germany
7)IHCE, Tomsk,Russia
PACS: 52.77.-j
INTRODUCTION
Large (TeV regime) linear coliders like TESLA
require accelerating fields higher than 25MV/m at a
quality factor of RF cavities Qo ≈1010. Recently it was
shown that such gradients are achieved in
superconducting accelerating RF cavities based on the
bulk niobium technology [1]. The resonators are made
from 2.8mm thick sheet, very pure (RRR 300) niobium.
Technology of Nb coated cooper cavities is an
interesting alternative to bulk-Nb cavities. Copper
cavities coated with thin niobium film have many merits
if compared to bulk ones: a lower material cost, the
higher thermal conductivity, much less sensitivity to
external magnetic fields and also a simpler fabrication
procedure. Since the late 80s the magnetron sputtering
technology has been applied for coating copper RF
cavities with superconducting thin niobium films. This
technology is based on the deposition of pure niobium,
in UHV conditions by means of a cylindrical magnetron
[2]. Unfortunately, despite past and recent efforts in the
magnetron spattering technology, a problem of fast
degradation of the quality factor for coated cavities at
higher fields has not been solved yet. Also the reason
for the observed degradation of the Qo is not completely
understood.
In 2000 a new approach to the coating of
cooper cavities was proposed – the vacuum arc
deposition. The cathodic arc deposition technology,
known since the 70 s, offers an interesting approach to
producing, at very high rates, pure metal, alloy and
compound films with excellent adhesion and density.
Advantages of this technique are given by some
characteristic features of arc metallic plasma, e.g.
higher energy of ions in comparison with the magnetron
sputtering technique, high ionization ratio of metallic
plasma and also a higher purity of the deposition
process due to absence of a working gas. These
conditions result usually in the formation of denser
films without voids. Molecular dynamics calculations
indicate that the energies of ions generated in a cathodic
arc discharge are within an optimal range for producing
dense coatings. A possible problem of this technique is
a production and a deposition of micro-droplets.
In order to study possibilities of the vacuum
arc technique to form high quality superconducting thin
films for a coating of RF cooper cavities some effort
has been undertaken in the end of 2000. Within the
framework of collaboration between the University of
Rome "Tor Vergata" and the A.Soltan Institute for
Nuclear Studies at Swierk and under the INFN grant
ARCO the prototype set-up with planar arc source was
designed, constructed and put into operation. Obtained,
preliminary results were very promising [3].
In 2001 the new UHV set–up with two planar
arc sources was assembled and put into operation at
Rome. Also, an experimental device with the linear arc
source was constructed and put into operation, at
Swierk, in the frame of the cooperation of "Tor
Vergata" University of Rome – IPJ Swierk, DESY
Hamburg and IHCE Tomsk.
UHV ARC SET-UP
The crucial role during formation of thin
superconducting niobium layer plays a purity of a
deposition process [5].
The all set-up was designed and realized in
accordance with UHV technology specifications. The
arc source was fabricated using only high purity
materials: stainless steel, OFHC copper and high
quality ceramics. The conical cathode was fabricated
from a 50mm diameter high purity Niobium rod (RRR
300) and fastened to a water-cooled Cu support. For
better thermal contact between the Nb cathode and Cu
support an eutectic mixture Ga-In has been used. A
floating potential screen (Nb) surrounds the cathode to
prevent the discharge from moving downwards,
towards the bottom part of the arc source. The water-
cooled conical chamber plays the role of the arc anode.
The magnetic field component perpendicular to the
planar cathode surface is in the range of 10÷20 mT. For
10 mT magnetic field the threshold arc current was in
the range 80-90 A. To trigger of arc discharges we used
Problems of Atomic Science and Technology. 2002. № 4. Series: Plasma Physics (7). P. 161-164 161
a simple system, based on evaporation of a thin
metallic film on dielectric surface. Construction details
of the planar arc source are shown in Fig.1, and photo
of the new experimental system is presented in Fig.2.
Fig.1 Layout of the planar arc source.
Our new experimental apparatus consists of
two vacuum chambers (10 dcm3) and two identical
planar arc sources. One of them is equipped with a 900
magnetic filter in order to compare deposition by
means of no filtered and filtered Nb plasma.
Fig.2 Photo of the new arc system.
The chambers are pumped down
simultaneously or separately by an oil-free pumping
system consisting of membrane pumps and drag turbo
molecular pumps (180 l/s). A base pressure of 1x10-10
Torr is reached after one night baking of the whole
system at 200oC. To check the composition of the
residual gases before and during coating the vacuum
chambers are equipped with own Quadrupole Mass
Analyzers (QMA).
A typical behavior of partial pressures of
residual gas species versus time is shown in Fig. 3. In
particular Fig.3a shows the partial pressure rise due to
triggering sparks in a situation when the arc discharge
does not start. Fig 3b shows the behavior in time of
partial pressures when a stable arc current is
established. The total pressure increases up to 10-6 Torr
as soon as the arc discharge starts, and stays almost
stable during deposition. Note though that in such
conditions the residual gas is almost exclusively
hydrogen, its partial pressure being usually more than 3
orders of magnitude higher than that of other
contaminants. This excess of hydrogen can be
understood as generated by the bulk Nb cathode this
provides a practically ‘infinite’ source of this gas. All
other gases are emitted only by the chamber walls
surface which makes their partial pressures drop below
the detection limit of our instrument after only a few
minutes of operation.
a)
b)
Fig. 3. Ion current vs time for different mass
gases: a) triggering, but no arc;
b) arc starts with the firs spark.
DEPOSITION OF SUPERCONDUCTING FILMS
Substrates (sapphire and OFHC Cu) to be coated
were mounted on a temperature controlled, massive Cu
flange placed on top of the vacuum chamber at a
distance of about 50cm from the cathode. The all
substrates before deposition were cleaned in an
ultrasonic bath, using acetone, alcohol and de-ionized
water, and dried in nitrogen.
162
Several samples have been produced in UHV
conditions with arc currents in the range from 100A to
200A and bias voltages in the range from 0 to 100V.
First samples coated in UHV conditions (mid of 2001)
showed unexpectedly good properties of the deposited
niobium. We obtained samples with RRR in the range
from 10 to 50 even for very thin (100nm)films Fig.4.
Fig.4. Resistance versus temperature for different
samples. The resistance values are normalized
to their values at room temperature.
This result was very encouraging since it is
higher by a factor of 2 to 5 than what usually obtained
under similar conditions (same thickness and coating
temperature) by cylindrical magnetron sputtering (RRR
usually between 5 and 10). After optimization of our
set-up for more stable arc discharge the thicker (1 m)μ
niobium films have been produced. The pressure in the
chamber during these depositions was less than 10-
7Torr.
Critical current density (Jc) and transition
temperature to superconducting state (Tc) of the films
were measured on Cu and sapphire substrates using an
inductive method, Fig.5.
T(K)
Fig. 5. Critical temperature measurements on Nb
samples deposited by UHV arc.
The best samples show values identical to bulk
metal, Tc=9,26 K, ∆Tc=0,02 K and Jc=3x107A/cm2 [4].
The main disadvantage of arc coating is the
production of microdroplets that are embedded in
growing film. In our case, microdroplets are made of
high purity molten Nb and, while not contaminating the
film, increase its surface roughness. The presence of
microdroplets in our films was studied by optical and
electron microscopy and by roughness measurements.
In Fig. 6 SEM pictures show a general view of a film
on Cu (Fig. 6.a)and sapphire (Fig. 6.b). SEM image at
higher amplification (Fig. 6.c) shows small (1μm)
niobium microdroplet.
a)
b)
c)
Fig.6. SEM images of the Nb film surface: a) on Cu,
b) on sapphire, c) Nb micodroplet.
The roughness measurements performed on Cu
samples indicate surface roughness similar to that of
the Cu substrate: Ra =0.15 μm for the Cu alone and
0.18 μm for 1 μm Nb film on Cu. The roughness of the
Nb coated sapphire was 0.1 μm.
LINEAR ARC SOURCE
The prototype linear arc source was also
designed and realized in accordance with UHV
technology specifications. The cathode (450mm in
length and 34mm in diameter) made from a RRR150
niobium tube is directly water cooled. Niobium /
OFHC Cu / stainless steel electron welded transitions
163
were used to prepare vacuum tight connections. To
control an arc discharge position a small magnetic coil
or the SamCo permanent magnet is placed inside the
niobium tube. For displacement a discharge along the
cathode remote controlled, water tight system is used.
From both ends of niobium tube the cathode is
surrounded by ceramic rings play a role of floating
potential screen. The cathode is vertically introduced
into the vacuum chamber evacuated by the oil-free
pumping system. The trigger electrode is placed in
down part of the cathode. The schematic drawing of the
system with the prototype linear arc source is sketched
in Fig. 7
Fig.7 The schematic drawing of the system with
the prototype linear arc source.
The linear arc source was put into operation in
the end of 2001. An optimization of its work has been
performed in oil-free, high vacuum conditions (10-6
Torr). The stable arc discharges have been obtained
with the arc current as low as 50-60A. Various version
of the cathode-anode system were tested. The
configurations with small diameter (75 mm) spiral and
tubular anode were checked also. The version with
tubular anode is presented in Fig.8.
Fig.8. The linear arc source with the tubular anode.
SUMMARY
We have presented the recently obtained
results on application of arc plasma for a formation of
thin superconducting films. The experimental UHV
apparatus equipped with planar arc sources to study the
deposition of superconducting niobium films has been
designed and realized. The obtained results are very
promising. Prototype linear arc source as well as
filtered arc source to study arc coating in actual cavity
geometry is under development.
ACKNOWLEDGEMENTS
This work was supported mainly by Italian
INFN grant ARCO and DESY.
The authors are grateful to Dr. S.Calatroni and
Dr. C.Benvenuti of CERN for constant support of
cathodes and samples substrates. We would like to
thank Prof. M.Sadowski for help in the arrangement of
the new laboratory in Swierk and very useful
disscusion. We are indebted to the group of Prof.
Vaglio (University of Naples) for help and support for
inductive measurements). We are also very indebted to
R.Sorchetti and G.Fuga from INFN, LNF Frascati and
A.Trembicki as well as ing.R. Mirowski from IPJ
Swierk for the constant help during the design,
construction and commissioning of our apparatus.
REFERENCES
[1]. B.Aune et al. The superconducting TESLA
Cavities", DESY 00-031, February 2000
[2]. S.Calatroni, "CERN Studies on Niobium-Coated
1.5GHz Copper Cavities", X workshop on RF
Superconductivity, September 6-11, 2001
[3]. R.Russo, J.Langner, L.Catani, M.Cirillo,
V.Merllo, S.Tazzari, F.Tazzioli, "Arco Procject
Status Report", Proc. XII Worshop on RF
Superconductivity, 6-11 September 2001
Tsukuba, Japan
[4]. J.Langner, L.Catani, R.Russo, S.Tazzari,
M.Cirillo, V.Merlo, F.Tazzioli, "Formation of
Thin Superconducting Films by Means of Ultra
High Vacuum Arc", Czechoslovak Jurnal of
Physics, Vol.52 (2002), Suppl.D
164
Arc Current
Supply
Water
Nb cathode
Sample holder
Screen
Coil
Trigger
Triggering
system
Vacuum
Substrate
bias
System
3)University Tor Vergata, Via della Ricerca Scientifica 1, 001333 Roma, Italy
4)INFM Roma 2, Via della Ricerca Scientifica 1, 001333 Roma, Italy
The main disadvantage of arc coating is the production of microdroplets that are embedded in growing film. In our case, microdroplets are made of high purity molten Nb and, while not contaminating the film, increase its surface roughness. The presence of microdroplets in our films was studied by optical and electron microscopy and by roughness measurements. In Fig. 6 SEM pictures show a general view of a film on Cu (Fig. 6.a)and sapphire (Fig. 6.b). SEM image at higher amplification (Fig. 6.c) shows small (1μm) niobium microdroplet.
a)
b)
c)
Fig.6. SEM images of the Nb film surface: a) on Cu,
The roughness measurements performed on Cu samples indicate surface roughness similar to that of the Cu substrate: Ra =0.15 μm for the Cu alone and 0.18 μm for 1 μm Nb film on Cu. The roughness of the Nb coated sapphire was 0.1 μm.
Linear arc source
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