Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP48

DEVELOPMENT OF NIOBIUM SPUTTERING APPARATUS AT KEK
R. Katayama∗ , T. Saeki, T. Kubo, H. Hayano, H. Ito,
High Energy Accelerator Research Organization, Tsukuba, Japan
Y.Iwashita, Kyoto University, Osaka, Japan,R.Ito, T. Nagata, ULVAC inc., Chiba, Japan
Abstract
We developed a novel Nb sputtering apparatus to coatthe interior of a 3-GHz elliptical cavity and study the S′ ISstructure. This DC magnetron sputtering apparatus allowed
the synthesis of Nb and NbN thin films on the inner surfacesof the 3-GHz SRF cavity. The theory predicts that a bi-layer
structure comprising dirty Nb or NbN on pure bulk Nb as
an S′S structure, i.e., the special case of the thickness of
the I layer 0, can also enhance the maximum acceleration
gradient better than the ordinal Nb SRF cavity. Accordingly,
we report the characteristics of this apparatus and details of
its development.

INTRODUCTION
The maximum acceleration gradient of a superconducting
radiofrequency cavity is limited by the surface magnetic field
at which a vortex avalanche occurs (effective Hc1). This
theory predicts that the effective Hc1 can be pushed up by
the superconductor–insulator–superconductor structure(S′ I-S structure) [1–4]. Experimental results demonstrate thatan S′IS structure comprising NbN, SiO2 , and pure bulk Nb
significantly enhances the effective Hc1 [5–7]. The film
formation technique is the key to realizing a next-generation
SRF cavity coated with an S′IS structure. Hence, the KEKULVAC collaboration developed a film formation techniquefor S′ IS structures [8–10]. In addition, we developed a novel
Nb-sputtering apparatus to coat the interior of an ellipticalcavity. This DC magnetron sputtering apparatus enabled thesynthesis of Nb and NbN thin films on the inner surfaces of
the SRF cavity. The theory predicts that even if such bi-layer
structure comprises dirty Nb or NbN on pure bulk Nb as
an S′ S structure, i.e., the special case of the thickness ofI layer 0, the maximum acceleration gradient is enhanced
better than the ordinal Nb SRF cavity [3]. We report thecharacteristics of this apparatus, the details of the system’sdevelopment, and the obtained results.

APPARATUS
DC magnetron sputtering was employed as the film formation technique. The schematic of the apparatus is presented
in Fig. 1. This apparatus comprises a vacuum chamber, aniobium cathode with a permanent magnet, and an inlet forargon and nitrogen gas flow. The cathode rod was mounted
at the center of the vacuum chamber. Plasma was induced
around the rod by supplying argon gas and applying a DC
voltage to the rod. The ions attracted by the negative DC∗ ryo.katayama@kek.jp

voltage kick out Nb particles from the cathode into the interior of the SRF cavity surface. We can also supply a gas
mixture of argon and nitrogen instead of argon. Here, NbN
thin films can be synthesized on cavity surfaces. In general,
DC magnetron sputtering depends on several film-formation
parameters, such as the incident angle of the niobium particles scattered onto the cavity surface, gas flow rate, gaspressure, electric and magnetic fields, and temperature ofthe substrate. However, the film formation parameters inside
the cavity continuously change depending on their position.
Hence, the optimization of the DC magnetron sputtering
process inside the cavity should be considered more difficult than that of the ordinal-plane-shaped substrate. Hence,
we prepared a 3-GHz coupon cavity to investigate the dependence of the film thickness and film quality on various
positions inside the cavity (see Fig. 2) [11, 12]. We adopted
two models, Nb and Cu, for the coupon cavities. In addition, we prepared coin samples made of Nb and Cu (𝜙-20mm), which could be attached to holes on the surface of thecoupon cavity [11, 12]. Using a coupon cavity with coinsamples, we can evaluate the position dependence of thequality of the synthesized film via several surface analyses.
For example, we can evaluate the effective Hc1 using thethird harmonic voltage method for a sample taken from acoupon cavity after synthesizing the films [7].

Figure 1: Schematic diagram of the niobium sputtering apparatus.

EXPERIMENTAL RESULT
We conducted tests on DC magnetron sputtering against
coupon cavities and coin samples in 2020 and 2023.5.17 to
5.19. Glass plate samples were employed to evaluate thethicknesses of the Nb-sputtered films. The obtained results
are described in the following section.

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August 29 - September 1, 2023, Funabashi

PASJ2023 FRP48

Figure 4: Comparison of the NbTi flange after DC magnetron sputtering before and after roughening the surface
with the sand paper.
Figure 2: Photograph of 3-GHz coupon cavity made of Cu
and Nb coin sample.

Experiment (2020)
In 2020, we conducted DC magnetron sputtering tests
at Tigold Inc. against the Cu coupon cavity to which Cu
coin samples were attached. The apparatus described in theprevious section was mounted in the vacuum chamber of aTigold Inc. production machine. After vacuum pumping,argon gas (0.65 Pa) was supplied to the Nb cathode while a
0.5-kW DC power was applied to the Nb cathode. We verified that DC magnetron sputtering occurred in the chamber,
resulting in a Nb thin-film deposition on the cavity surface.
Images of the interior of the Cu coupon cavity and Cu coinsample after the film formation are presented in Fig. 3.
A Nb thin film was deposited on the entire surface of theCu coupon cavity without peeling. In addition, no peeling of
the synthesized films from the Cu coin samples wasobserved. In contrast, peeling was observed near the part
between the beam pipe and NbTi flange weld.
Subsequently, we roughed the surface using sandpaper
because there was a possibility that these peelings could
have been caused by contamina-tion and the effect of
surface roughness. Accordingly, the adhesion wassignificantly improved, and no peeling was observed.
Images of the interior of the Cu coupon cavity after thesputtering process before and after roughing withsandpaper are presented in Fig. 4.

Experiment (2023.5.17-5.19
DC magnetron sputtering tests with Cu and Nb coupon
cavities were conducted again at the Tigold facility from
17-05-2023 to 19-05-2023. 0.5-kW DC power and 0.67-Pa
argon gas pressure were employed, same as the previous
test conducted in 2020. The Nb coin samples adopted inthis test were as follows: (1) Nb coin samples as receivedand (2) Nb coin samples to which mechanical polishing,800 ∘ C for 3-h anneal, and EP were applied. On 05-172023, Nb sputtering test was conducted. It was verified
that this test reproduced previous results. Moreover, theuniformity of the film thickness distribution improved (asdiscussed later). On May 18, 2023, the plasma could not be
suddenly induced in the vacuum chamber. After inspecting
the interior of the Nb cathode, we inferred that the magnetismof the neodymium magnet was lost. Hence, we replaced theneodymium magnets with new magnets. Subsequently, wecould induce plasma sputtering again in the chamber using
a cathode with replaced magnets. On May 19, 2023, we
first performed DC magnetron sputtering of a Cu coupon
cavity using as-received Nb coin samples and two polished
Nb coin samples. Subsequently, we sputtered the Nb coupon
cavity with three glass plates and 12 as-received Nb coinsamples. No peeling was observed during the sputtering
test of the Cu coupon cavity. In contrast, in the Nb coupon
cavity, peeling was observed near the edge of the beam-pipe
tube (Fig. 5). In addition, pits were observed on the surfacesof the two polished niobium coin samples attached to the Cu
coupon cavity (Fig. 6 and Fig. 7), whereas there was no pit
on the surfaces of the as-received Nb coin samples. We
finally performed DC magnetron sputtering of a Cu coupon
cavity using 12 as-received Nb coin samples and three glass
plates under 0.3 kW DC power and 0.67-Pa argon gas
pressure for 10 min.

Comparison of Film Thickness Distribution

Figure 3: Photograph of the interiors of the Cu coupon cavityand Cu coin sample after DC magnetron sputtering.

A comparison of the Nb film t hickness d istribution atseveral points on the 3-GHz coupon cavity is summarized inFig. 8. The uniformity obtained on May 17, 2023 is improved
compared to that obtained in 2020. The thickness of thesynthesized films increased as the position changed fromthe equator to the iris. The grown rate of Nb film thickness
increased as DC power becomes higher.

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Proceedings of the 20th Annual Meeting of Particle Accelerator Society of Japan
August 29 - September 1, 2023, Funabashi

PASJ2023 FRP48

Figure 5: Photograph of the position near the beam pipe tubeof the Nb coupon cavity after DC magnetron sputtering.

Figure 8: Comparison of film-thickness distribution on various positions of Cu coupon cavity between the experiments
in 2020 and 2023. A and E correspond to the iris position,
whereas C corresponds to the equator.

and Fig. 13. Almost all the Nb signals were observed;
however, a small oxygen peak appeared.

Figure 6: (TOP) Photograph of Nb coin Sample 1 after DC
magnetron sputtering. (Bottom) Enlarged image near thepits on the Nb coin surface.

Figure 9: SEM-EDS spectrum of the sample collected frompeeled films of Nb coupon cavity in Fig. 5.

Figure 7: (TOP) Photograph of Nb coin Sample 2 after DC
magnetron sputtering. (Bottom) Enlarged image near thepits on the Nb coin surface.

Analyses Result of SEM-EDS
We performed SEM-EDS measurements on samples ofthe peeled films collected from the Nb coupon c avity. The
obtained results are presented in Fig. 9. In the SEM-EDS
spectrum, the horizontal and vertical axes represent the X-ray
energy and number of photons, respectively. Almost all Nb
signals were observed; however, small carbon and oxygen
peaks were also emerged. In addition, we performed SEMEDS measurements on two polished Nb coin samples. The
obtained results are presented in Fig. 10, Fig. 11, Fig. 12,Figure 10: SEM-EDS spectrum on the pit of the Nb coinsample in Fig. 6.

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August 29 - September 1, 2023, Funabashi

PASJ2023 FRP48

to prepare Nb coin samples without mechanical polishingand by baking process at 100-200 ∘ C for several hours, tosuppress scratching and gas emissions during DC magnetron
sputtering.

SUMMARY

Figure 11: SEM-EDS spectrum on the pit of the Nb coinsample in Fig. 6.

We developed a DC magnetron sputtering apparatus tocoat a 3-GHz SRF cavity for a film formation study of theS′ IS structure. A niobium thin film can be formed inside
the 3-GHz coupon cavity. Using this apparatus, we cansynthesize a bilayer structure inside a pure bulk Nb cavity.

ACKNOWLEDGMENTS
This work was performed by ULVAC-KEK Collaboration
Project Number 2018-C315 and 2019-C315. This work
was supported by Toray Science Foundation Grant No.196004 and Japan Society for the Promotion of Science (JSPS)
KAKENHI Grant Numbers JP17H04839, JP19H04395, andJP23H00125. I am grateful to Masaki Ishida of Applied
Research Laboratory Radiation Science Center at KEK forchemical analysis with measuring SEM-EDS.

REFERENCES
Figure 12: SEM-EDS spectrum on the pit of the Nb coinsample in Fig. 7.

[1] A. Gurevich, “Enhancement of rf breakdown field of superconductors by multilayer coating”, Appl. Phys. Lett. 88,012511 (2006).
[2] T. Kubo et al., “Radio-frequency electromagnetic field andvortex penetration in multilayered superconductors”, Appl.
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[3] T. Kubo, “Multilayer coating for higher acceleration fields
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[4] T. Kubo, “Superheating fields of semi-infinite superconductors and layered superconductors in the diffusive limit: structural optimization based on the microscopic theory”, Supercond. Sci. Technol. 34 045006 (2021).

Figure 13: SEM-EDS spectrum on the pit of the Nb coinsample in Fig. 7.

DISCUSSION
No surface or heat treatment was applied to the Nb couponcavity. In general, surface contamination causes film peeling problems. Hence, in the future, chemical- or electropolishing should be applied to the Nb coupon cavity prior
to DC magnetron sputtering. In addition, gas emissions,scratches, and/or dust triggered pits on the surfaces of theNb coin samples after film c oating. However, dust was most
likely eliminated from the candidates because there were nostrong peaks other than those of the Nb in SEM-EDS. The
lack of a baking process may trigger gas emission during
DC magnetron sputtering, and mechanical polishing may
cause scratches on the surfaces. Therefore, it is desirable[5] R. Katayama et al., “Evaluation of superconducting characteristics on the thin-film structure by NbN and Insulator coatings
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[6] R. Katayama et al., “Precise Evaluation of Characteristic ofthe Multi-layer Thin-film Superconductor Consisting of NbN
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[7] R. Katayama et al., “Evaluation of the superconducting characteristics of multi-layer thin-film structures of NbN andSiO2 on pure Nb substrate”, SRF2019 Proceedings, Dresden,Germany, pp.807–809.
[8] R. Ito, T. Nagata et al., “Development of Coating Techniquefor Superconducting Multilayered Structure”, IPAC2018 Proceedings, Vancouver, Canada, pp.4954–4956.
[9] R. Ito, T. Nagata et al., “Construction of Thin-film CoatingSystem Toward the Realization of Superconducting Multilayered Structure”, LINAC18 Proceedings, Beijing, China,
TUPO050, pp.445–447.

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August 29 - September 1, 2023, Funabashi

PASJ2023 FRP48[10] R. Ito, T. Nagata, H.Hayano, R.Katayama, T.Kubo,S.Takayuki et al., “Nb3 Sn Thin Film Coating Method forSuperconducting Multilayered Structure”, SRF2019 Proceedings, Dresden, Germany (2019), pp.628–631.
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