Thin niobium superconducting film prepared by modified cylindrical magnetron
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2000
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| Zitieren: | Thin niobium superconducting film prepared by modified cylindrical magnetron / J. Langner, M. Cirillo, W. DeMasi, V. Merlo R. Russo, S. Tazzari, L. Catani, R. Sorchetti // Вопросы атомной науки и техники. — 2000. — № 3. — С. 141-143. — Бібліогр.: 8 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859933367324114944 |
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| author | Langner, J. Cirillo, M. DeMasi, W. Merlo, V. Russo, R. Tazzari, S. Catani, L. Sorchetti, R. |
| author_facet | Langner, J. Cirillo, M. DeMasi, W. Merlo, V. Russo, R. Tazzari, S. Catani, L. Sorchetti, R. |
| citation_txt | Thin niobium superconducting film prepared by modified cylindrical magnetron / J. Langner, M. Cirillo, W. DeMasi, V. Merlo R. Russo, S. Tazzari, L. Catani, R. Sorchetti // Вопросы атомной науки и техники. — 2000. — № 3. — С. 141-143. — Бібліогр.: 8 назв. — англ. |
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| container_title | Вопросы атомной науки и техники |
| first_indexed | 2025-12-07T16:09:25Z |
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Problems of Atomic Science and Technology. 2000. N 3. Series: Plasma Physics (5). p. 141-143 141
UDC 533.9
THIN NIOBIUM SUPERCONDUCTING FILM PREPARED
BY MODIFIED CYLINDRICAL MAGNETRON
J.Langner,
The Andrzej Soltan Institute for Nuclear Studies (IPJ)
, 05-400 Otwock-Swierk by Warsaw, Poland
M.Cirillo, W.DeMasi, V.Merlo R.Russo, S.Tazzari
The University of Rome „Tor Vergata”,
Via della Ricerca Scientifica 1, 00133 Roma, Italy
LCatani, R.Sorchetti
INFN, LNF, 0044 Frascati, Italy
Introduction
For future large superconducting RF-
accelerators, technology of the Nb coated cooper cavities
appears as an interesting alternative to the bulk-Nb
cavities. Coating technology is advantageous compared to
bulk niobium since copper is cheaper than niobium.
Sputtering is a well-known technique for coating copper
RF cavities with superconducting thin film [1]. The
cylindrical magnetron-sputtering configuration for
coating 500 MHz cavities with thin Nb films was
developed for the first time at CERN [2]. In this system,
the magnetic field for a plasma confinement is produced
by a coil or permanent magnets placed inside the
cylindrical niobium cathode. The sputtering discharge is
realized in a noble gas atmosphere between the central
cathode and the grounded cavity.
The technology of thin niobium film coating was
successfully used for the production of the 350MHz LEP
accelerating cavities. For high-Q, high gradient 1.5GHz
cavities, a further progress in this technology is still
needed.
For the coating of 1.3 GHz cavities that are 3
times smaller in dimension it was proposed at the
University of Roma Tor- Vergata the cylindrical
magnetron sputtering system with the magnetic field
produced by external coils placed on the outside of the
vacuum chamber [3]. Such sputtering system was realized
in early 90’s, the set-up was put into operation in the mid
90’s and since 1997, in the frame of a collaboration with
SINS Swierk, more systematic studies of the deposition of
Nb films have been undertaken. The studies are aimed on
optimization of the sputtering conditions as well as on
characterization of the produced Nb films. The paper
presents the recent results obtained with modified
magnetron set-up. Glass and sapphire samples coated
with 1-2µm niobium film were characterized by
measurement of resistance versus temperature and by x-
ray diffraction.
Sputtering set-up
The sputtering system is schematically shown in Fig.1.
The stainless steel deposition chamber has the shape of
the 1.3 GHz cavity.
Fig.1 Sputtering system scheme
The vacuum chamber is evacuated by an ultraclean
pumping system consisting of a diaphragm pump for a
primary vacuum and a 180l/s turbomolecular pump. An
ultimate pressure, of the order of 10-10 mbar and
practically total absence of water and hydrocarbons are
obtained after a 20 hours bake at 150oC. The system is
equipped with a residual gas analyzer (RGA) to study the
ultimate pressure gas composition and to monitor the
percentage of gases produced during sputtering. The RGA
142
is equipped with another pumping system and it is
connected to the cavity chamber through a 0.6 mm
diaphragm. Pure Argon (99,9999%) is introduced as
working gas for the sputtering process.
The cathode of magnetron is located on the axis of the
system. It consists of a vacuum tight stainless steel tube
surrounded by a high purity (RRR=150) niobium tube
(20/24mm inner/outer diameters). The magnetron cathode
is cooled by distilled water.
It is well known that sputtered films contain atoms of
the noble gas used in the sputtering discharge. Energetic
neutrals reflected from the cathode may be trapped in
growing film. It was also shown in many papers [4], [5]
that a discharge gas trapped in film leads to a
deterioration of superconducting film properties. For this
reason a working gas pressure and a discharge voltage
should be kept as low as possible during of the deposition.
To reduce this effect the study of a proper magnetic field
configuration in magnetron system is needed.
Two coils placed outside of the cavity chamber
produce the magnetic field in our magnetron. The coils
are surrounded by soft iron shield of 4mm thickness. The
magnetic field, well known from thermonuclear
researches as “magnetic bottle configuration” was
produced.
In order to obtain a high efficiency of magnetic
confinement of electrons in magnetron the magnetic field
lines have to die onto the cathode. The mirror ratio of the
presented magnetic bottle configuration is about 2 and
this value was limited by the dissipated power in the coils
since the system of the coils cooling was inefficient.
For improving of the magnetic configuration of the
magnetron, 2 SamCo permanent magnets (small cylinders
8mm diameter 16mm long) have been introduced into the
stainless steel tube. This mixing of magnetic fields
produced by coils and permanent magnet has improved
the electron confinement leading to an increase of
maximum discharge current of about a factor 2 respect to
the previous configuration. Obtained I-V characteristics
with improved configuration of magnetic field are shown
in Fig.2.
Fig.2. Discharge characteristics at different gas
pressure
Film deposition
Small (14x19mm) sapphire or glass samples are
placed on two sample holders located on the equator of
the cavity. Prior the mounting on the sample holder,
substrates are cleaned in an ultrasonic bath with acetone
and rinsed with de-ionized water.
Due to heaters located inside the holders it is possible
to keep samples during deposition at higher temperature,
up to 300oC.
After the 20 hours bake-out at 150oC the ultimate
pressure of 10-10mbar is reached at room temperature.
Argon is then injected at a pressure of about 7x10-3
Torr to start the discharge and then set at the desired
value, usually between 1 and 4 mtorr. The discharge
voltage is usually kept constant, while current in the coils
is adjusted in order to obtain the maximum discharge
current at the fixed pressure.
The cavity and the system are at room temperature
when discharge starts, while samples can be heated and
kept at higher temperatures (up to 300C) before and
during the coating. In order to compare films deposited
under different conditions, the sputtering time is chosen to
obtain films of 1µm thickness.
Table 1 lists the deposition parameters: working gas
pressure, cathode voltage, discharge current, time of
deposition, substrate temperature and measured values of
Residual Resistivity Ratio.
Table1
Samp.
#
Press.
[mTr]
Volt.
[V]
Curr.
[A]
Time
[min]
Temp.
[C]
RRR
1.99 1.7 430 1.0 60 8.6
3.99 2.0 430 1.0 60 9.3
4.99 2.0 430 1.0 60 10.8
5.99 2.0 420 0.98 60 275 27
6.99 2.0 420 0.98 60 11.1
7.99 1.6 410 1.0 30 7.0
11.99 1.6 410 2.1 30 300 28
12.99 1.6 410 2.1 30 18.9
Film characterization
The produced samples are mainly characterized by
measurement of resistance versus temperature and by x-
ray diffraction pattern.
The R (T) curves are carried out with a standard four-
lead technique and most of them are obtained in a
cryocooler reaching 12K as minimum temperature, not
low enough to measure the critical temperature of
produced films, but sufficient to obtain the Residual
Resistivity Ratio (RRR). RRR (also called β10) is defined
as the ratio of the resistivity at room temperature ( ρ300K)
and the resistivity at 10 K (ρ10K). This ratio gives an
estimation of impurity and lattice defect content in the
film.
143
Our β10 value range from 7 to 30 among the best
reported in literature produced by sputtering. Few
samples are measured in a cryostat to check the
superconducting critical temperature. The measures
showed a very sharp transition (∆Tc<0.1K) and Tc
between 9.5 and 9.6K. The fig. 3 shows R (T)
measurements. Fig.4 shows the detail of the transition
region.
Fig.3. R (T) characteristics
Fig.4.R (T) characteristics (transition region)
Data on critical temperature are in agreement with
existing data on niobium films of high quality in presence
of a compressive stress [6-7].
Samples were also analyzed by X-ray Diffraction in
the Bragg-Brentano configuration (θ/2 θ) using a Cu Kα
source. A typical X-ray diffraction spectrum is reported in
fig. 5 and it shows that Niobium films grow with the
(110) plane parallel to the film surface.
For most of the explored coating conditions, the peak
positions are shifted to angles lower than the nominal
values, corresponding to a lattice constant about 0.5%
larger than the bulk (Å = 3.303Å) in the growing
direction. The produced niobium samples are in a
compressive stress in the substrate plane mainly due to
the bombardment of high energy neutral and to self-
bombardment of the sputtered material during deposition
[7][8].
This compressive stress is responsible for the observed
increase of the critical temperature.
While the changes in the lattice parameters and stress
are within the error of our measurements, we observe a
significant increase of the β10 obtained with the mixed
magnetic configuration on samples coated at room
temperature, which rise from about 7 in the previous
configuration up to 19 in the present one. Higher β10
values can be obtained by the heating of the substrate
with a maximum of β10=30 for a coating temperature of
300oC.
Conclusions
We have presented the results obtained on niobium
films produced by magnetron sputtering in a UHV
system. The use of a mixed magnetic configuration
improves the efficiency of magnetic confinement of
electron leading to higher discharge current. Very good
film quality is obtained with critical temperature of about
9.5K and RRR values up to 30.
References
[1] C. Benvenuti, Part. Accel ., 40, 43 (1992)
[2] C. Benvenuti, N. Circelli, M. Hauer, Appl. Phys. Lett.,
45, 583 (1984)
[3] M.Ferrario, S.Kulinski, M.Minestrini, S.Tazzari,
INFN / TC_93 / 16
[4] Benvenuti, et. al Proc. of the 8th Workshop on RF
Superconductivity, Oct 1997, p. 1057-1064
[5] C. Benvenuti, S. Calatroni, I.E. Campisi, P. Darriulat,
M.A. Peck, R. Russo, and A.M. Valente, Physica C,
316, 153 (1999)
[6] G. Heim and E. Kay, J.Appl.Phys. 46, 9, 4006 (1975)
[7] R. Russo and S. Sgobba, Part. Accel., 60, 135 (1998)
[8] H. Ljungcrantz et al. J.Vac. Sci. Technol. A11(3), 543
(1993)
40 60 80 100 120
2000
4000
6000
8000
10000
12000
14000
(2
2
0
)
(1
1
0
)
C
o
u
n
ts
p
er
s
ec
o
n
d
2Θ
Fig.5 X-Ray diffraction spectrum in the θ/2 θ
configuration for a typical niobium
sample on sapphire. Only the
reflections relative to the Niobium
planes (110) and (220) are present
|
| id | nasplib_isofts_kiev_ua-123456789-82403 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T16:09:25Z |
| publishDate | 2000 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Langner, J. Cirillo, M. DeMasi, W. Merlo, V. Russo, R. Tazzari, S. Catani, L. Sorchetti, R. 2015-05-29T08:59:44Z 2015-05-29T08:59:44Z 2000 Thin niobium superconducting film prepared by modified cylindrical magnetron / J. Langner, M. Cirillo, W. DeMasi, V. Merlo R. Russo, S. Tazzari, L. Catani, R. Sorchetti // Вопросы атомной науки и техники. — 2000. — № 3. — С. 141-143. — Бібліогр.: 8 назв. — англ. 1562-6016 https://nasplib.isofts.kiev.ua/handle/123456789/82403 533.9 en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Low Temperature Plasma and Plasma Technologies Thin niobium superconducting film prepared by modified cylindrical magnetron Article published earlier |
| spellingShingle | Thin niobium superconducting film prepared by modified cylindrical magnetron Langner, J. Cirillo, M. DeMasi, W. Merlo, V. Russo, R. Tazzari, S. Catani, L. Sorchetti, R. Low Temperature Plasma and Plasma Technologies |
| title | Thin niobium superconducting film prepared by modified cylindrical magnetron |
| title_full | Thin niobium superconducting film prepared by modified cylindrical magnetron |
| title_fullStr | Thin niobium superconducting film prepared by modified cylindrical magnetron |
| title_full_unstemmed | Thin niobium superconducting film prepared by modified cylindrical magnetron |
| title_short | Thin niobium superconducting film prepared by modified cylindrical magnetron |
| title_sort | thin niobium superconducting film prepared by modified cylindrical magnetron |
| topic | Low Temperature Plasma and Plasma Technologies |
| topic_facet | Low Temperature Plasma and Plasma Technologies |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/82403 |
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