A Study of the HTS Josephson Junction Formed bya Ga Focused Ion Beam

EIIRISプロジェクト研究成果報告会
穂の国豊橋芸術劇場PLATFeb. 26, 2020

K. Hayashi, T. Ueda, R. Ohtani, S. Ariyoshi and S. Tanaka
Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
https://chem.tut.ac.jp/squid/ *email: tanakas@tut.jp

Abstract

1. BackgroundHTS SQUIDs (High TC Superconducting Quantum Interference Devices) mainly utilize
grain boundary Josephson junctions (G.B.JJ), e.g. a bi-crystal JJ. But bi-crystal JJs havesome problems, as the layout and number of JJs are restricted because they must be locatedalong the grain boundary on the substrate. Therefore, the use of Ga Focused Ion Beam (GaFIB) irradiation to make nano-brige JJs (which is HTS JJs) was investigated, as it introduced
an atomic disorder in the superconducting region.
Before we fabricated the nano-bridge JJs, Au protective layer, which thickness was 20 nm
was deposited on a YBCO layer in order to prevent from over dose during SIM (Scanning IonMicroscope) observation.
We observed decrease of Ic of nano-bridge JJs along the fluence; Shapiro steps wereobserved in 500 nm wide nano-bridge irradiated by the fluence of 2.0 ×1015 ions/cm2 under
2.0 GHz microwave irradiation.

Problems of HTS G.B.JJ
・The layout and number of JJs are restricted because they must be located along thegrain boundary on the substrate.
・Bi-crystal substrate is expensive.
Advantages of nano-bridge JJs by Ga-FIB
・Standard Substrate can be used.
・There is much flexibility in the device design, such as an arrangement
and number of JJs.
・Parameters of JJs can be controlled by adjusting irradiation conditions of Ga FIB.

2. Fabrication of nano-bridge JJ by Ga-FIB
Nano-bridge JJ

Ga-FIB

Mechanism of Tc decrease in Ga-FIB process

・ Size : nm orderr [W·cm]

・ Substrate : Standard substrate(Non bi-crystal)

FOV of SIM
VACC = 5kV(ii) Low dose

Irradiated regions(Normal conductor)

0T [K]
77 K

Nano-scale

Nano-bridge JJ

YBa2Cu3O7-d

Schematic of R-T characteristics ofirradiated YBCO films

Ga irradiation

Irradiation
regionProtected YBCOtYBCOtYBCO

Ga 5 kVGa 30 kVAu20 nm

We simulated the defect distribution in the film when Ga ion
number of 10000 were irradiated. At low VAcc of 5 kV, Ga ionsdidn’t reach the YBCO film. But when the VAcc was 30 kV, thenumber of defects enough to change the properties wasintroduced into the YBCO thin film though the Au protective layer.
Optimizing the thickness of Au layer, we determined the Au
thickness of 20 nm.

0.12 nA

VAcc

5 kV

FIBprocessingIBeam

0.07 nAVAcc

40 kV

Ic=250 mA

Irradiated trace

IC0 : IC before FIB irradiation

Ic=110 mA

0.1
W n= 500 nm

W n = 500 nm

0.0

1015

1016
Fluence [ions/cm2]

1017

W = 4000 nm

SEM observation at Wn = 500 nm,
2.0×1015 ions/cm2 （Mag. 11 k）

1015

Fluence [ions/cm2]
Decrease of Jc along the fluence was
not observed at VAcc of 5 kV. In contrast,
the Jc was decreased with an increase of
the fluence at VAcc of 30 kV.

2. By giving a 20 nm thick Au protective layer on
YBCO layer, we could prevent from over dose
during SIM observation at VAcc of 5 kV. And weconfirmed that the Jc was decreased with anincrease of the fluence at VAcc of 30 kV for JJ
fabrication.

0.2

MgO

1014

1. We investigated properties of nano-bridge JJ
irradiated by Ga-FIB introducing an atomic
disorder in the superconducting region.

W n=1000 nm

Ic=170 mA
Au 20 nm/YBCO 100 nm/MgO

1013

Measurement of Shapiro steps

Au (20 nm) /YBCO (100 nm) /MgO @77 K

0.3

IC / IC0IBeam

1.0

5. Conclusions

Dependence of Ic on bridge width and fluence

SIMobservation

VAcc = 5 kVVAcc = 30 kV

4. Properties of nano-bridge JJ
0.4

Magnification : 10 k
FOV :11.96 × 10.32 mm2Ibeam : 10 pA

0.0Focusing and positioning at low VAcc
→ Fabrication at high VAcc

11 kAu（20 nm）/YBCO（100 nm）/MgO @77 K

YBCO
80 nm

An YBCO film is damaged by irradiation even at
low acceleration voltage (VAcc). But by giving an Au
protective layer on YBCO thin film, damage of theYBCO layer can be suppressed under irradiation atlow VAcc. In addition, defects can be introduced intothe YBCO thin film through the Au protective layer athigh VAcc.

Magnification

Nano-bridge wide
W N = 500~1000 nm

0.5

Substrate

Irradiation condition

1.5

SRIM program was used to simulate binary collision model.

Au protectivelayer
YBCO layer

tprotect

VACC = 40 kV

Experimental Result

JC/JC05 kV 20 kV 40 kV

Normal conducting region
or removal

Simulation of defects formation

Focusing and Fabrication
Positioning
5 kV
20 kV 40 kV

Ga ion beam

Au protected
Superconductor

ProblemDuring a SIM (Scanning Ion Microscope)
observation, which is required for a
focusing and a positioning process, anover dose amount is given in the field ofview (FOV), resulting in excess defects.

3. Prevention from over dose by Au protective layer
No protected YBCO

Ga-FIBirradiation

Micro-channel wide
W M = 4 mm

Thinner YBCO film is required toimprove controllability because thedistribution of the ion defects isexpanded when the film is thick andas a result the ion penetration depth
becomes deeper.

Atomic disorder occurs in YBCO crystal structure
by Ga ion irradiation.
Disorder of crystallinity causes decreasing
critical temperature(TC).

SNS JJ

MgO substrate

To form a nano-bridge, Ga ion
beam is irradiated on a selected
region of an YBCO thin film, where
the superconductivity property turns
into normal conductivity.

TC~90 K

Normalconductive
thin layer

Au protected
Superconductor

WN(i) Non-doseSuperconductive
thin layer

YBCO film

Pointdefects(iii) High dose

・ Property : SNS

Patterned by Ga-FIB

Nano-bridge(Superconductor)

Ic decreased with an increase of thefluence in each nano-bridge withdifferent width. Ic for W n =500 nm
disappeared at 1016 ions/cm2, and thatfor W n =1000 nm disappeared at 1017
ions/cm2.

This shows IV-characteristic of a
500 nm wide nano-bridge irradiatedby the fluence of 2.0 ×1015 ions/cm2
under 2.0 GHz microwave irradiation.
Under microwave irradiation, clear
Shapiro steps were observed.
We confirmed that the nano-bridge
fabricated by Ga-FIB irradiationshows Josephson-like behavior.

3. We observed decrease of Ic of nano-bridge JJs
along the fluence; Shapiro steps were observed
at a 500 nm wide nano-bridge fabricated by thefluence of 2.0 ×1015 ions/cm2 under 2.0 GHz
microwave irradiation.

[1] K. Hayashi, T. Ueda and S. Tanaka, Extended
Abstracts of HTSFF2018, 56-57, 2018.
[2] K. Hayashi, T. Ueda, R. Ohtani, and S. Tanaka,
ISS2019, EDP1-1, 2019.